Weber Dcom

[Guest ashnicholls]

Hi,

 

Are the DCOM's better than the DCOE's.

 

Are they easy to get parts for.

 

As I know a few people I can get some DCOM's off.

 

But my local engine tuner.

 

Advised against getting DCOM's and said it was better to look for DCOE's

 

As the parts for the DCOM's are difficult to find.

 

And what does fast road cars mean by

 

The air jet is built into the holder, but this is not a problem & will be discussed with the purchaser so any jet may be fitted.

 

Is this a problem, does the carb need machining?

 

Cheers Ash

[pugrallye 0]

no differences really except for accelerator pump positioning, some parts are peugeot specific, most are weber DCOE parts

[Grahamrally 0]

no differences really except for accelerator pump positioning, some parts are peugeot specific, most are weber DCOE parts

 

There is actually a relatively large difference in fueling. There are more progression holes on the DCOM making the car leaner over the initial progression phase from idle. I think this was done so that on cruising the carbs wouldn't drink as much fuel as the DCOE's. The DCOM was only ever made specifically for the 1294 205 rallye and things like Alfa road cars so I guess they needed the cars to have a decent MPG. I did have a 1294cc Euro rallye engine that ran the DCOM which I later changed to DCOE purely because they were very lean on progression. Not ideal for optimum performance from a carburettor!

DCOM are cheaper to buy new and as long as you dont mind a small loss in power on progression then they're great!

[Beastie 1]

Aren't both of them infinitely adjustable for progression by means of the idle tube, idle jet and idle air jet anyway?

[puginfo 0]

Aren't both of them infinitely adjustable for progression by means of the idle tube, idle jet and idle air jet anyway?

 

i think what he was saying is that, on the dcom, the engine stays on the idle cct way longer before moving on to the main cct as compared with the dcoe. Hence, its lean running. Progression time from the idle cct to main cct would also be a function of the emulsion tube type and the float set on the dcoe, but i guess the additional progression holes in the dcom gives a finer control over mixture for wider rev range.

[Beastie 1]

i think what he was saying is that, on the dcom, the engine stays on the idle cct way longer before moving on to the main cct as compared with the dcoe. Hence, its lean running. Progression time from the idle cct to main cct would also be a function of the emulsion tube type and the float set on the dcoe, but i guess the additional progression holes in the dcom gives a finer control over mixture for wider rev range.

 

Beautifully expressed. Thank you

[Grahamrally 0]

i think what he was saying is that, on the dcom, the engine stays on the idle cct way longer before moving on to the main cct as compared with the dcoe. Hence, its lean running. Progression time from the idle cct to main cct would also be a function of the emulsion tube type and the float set on the dcoe, but i guess the additional progression holes in the dcom gives a finer control over mixture for wider rev range.

 

As far as I understand the operation of the carbs, the emulsion tubes and jets and air correctors obviously set up the mixture that is delivered. As the butterfly sweeps past the progression holes it lets a certain amount of air in to mix with the fuel to lean the mixture off slightly when you are off idle. On normal DCOE's there are 3 progression holes to do this process, on the DCOM's i think there are more like 5. Obviously you can cover the lean phase of progression by upping the idle jet selection, but you would have to increase the mixture on the DCOM's higher to cover the lean progression phase but you'd probably end up with a very rich idle.

 

Beastie, carbs are very rudementary mechanical devices to input fuel into an engine, they are certainly not infinately adjustable. If they were infinately adjustable we wouldn't have throttle bodies and engine management!

Edited September 7, 2006 by Grahamrally

[christopher 5 1 Cars]

Let me clear a few things here..

 

Are the DCOM's better than the DCOE's.

 

The DCOM uses a diaphram accelerator pump in the DCOM vs. a piston type in the DCOE. It is am improved design of the earlier DCOE design

 

As the parts for the DCOM's are difficult to find.

 

They are not as well known as the DCOE, as far as I know they are basically the same. The only part I needed was an idle screw that I bought from webcom in UK.

 

And what does fast road cars mean by

The air jet is built into the holder, but this is not a problem & will be discussed with the purchaser so any jet may be fitted.

 

Basically this is talking about the main jet assembly. On a DCOE it is a mix of:

 

1) The main jet

2) The emulsion tube

3) Air correctot jet

4) Emulsion tube holder

 

This you can see if you screw of the top cover, and screw out the main yet assembly

 

Now on a Euro Rallye for some reason they used a part that has the air corrector jet incorporated with the air corrector jet. This is the top part and the only difference. Since the jets from the DCOE are interchangeable with the DCOM you can just use the whole assembly from that. Alternatively you just buy two new emulsion tube holders. This is no big deal like Fast Road Cars says.

 

 

Is this a problem, does the carb need machining?

 

No, the jets are interchangeable.

 

 

no differences really except for accelerator pump positioning

 

The Pump diaphram is located underneath.

 

some parts are peugeot specific, most are weber DCOE parts

 

Sorry, this is not correct. Ther are no peugeot parts

 

 

There is actually a relatively large difference in fueling. There are more progression holes on the DCOM making the car leaner over the initial progression phase from idle.

 

The progression from the idle phase to the acceleration phase is caused by the butterfly opening more to allow the signal to draw fuel from these in addition to the idle jet. Shortly after the pump jet squirts more fuel in. When the throttle is opened further, the pressure differential between the idle progression holes and and the air pressure acting on the fuel in the bowl tapers off until the pressure is not enough to push the fuel up the idle jet and the idle system ceases to supply fuel. Shortly before the the supply ceases fuel begine to be delivered through the main system that is used for high speed operation and cruising.

 

Carburetors with more progression holes require bigger idle jets then carburetors with fewer holes(the increased number of holes was produced in order to have a smoother transition fase from idle to main.An increased number of progression holes and an adjustable pump lever arm provide you with plenty of adjusting possibilities to get the transition right, requiring a larger idle jet. These two elements are found in all the later types of Italian made carburetors-dellorto,weber (dcom and dcoe) and solex.if factories went out of their way to change the production lines for these last two element changes-there is a good reason for it.technology goes forward in time!

Edited September 7, 2006 by christopher

[Grahamrally 0]

Let me clear a few things here..

The DCOM uses a diaphram accelerator pump in the DCOM vs. a piston type in the DCOE. It is am improved design of the earlier DCOE design

They are not as well known as the DCOE, as far as I know they are basically the same. The only part I needed was an idle screw that I bought from webcom in UK.

Basically this is talking about the main jet assembly. On a DCOE it is a mix of:

 

1) The main jet

2) The emulsion tube

3) Air correctot jet

4) Emulsion tube holder

 

This you can see if you screw of the top cover, and screw out the main yet assembly

 

Now on a Euro Rallye for some reason they used a part that has the air corrector jet incorporated with the air corrector jet. This is the top part and the only difference. Since the jets from the DCOE are interchangeable with the DCOM you can just use the whole assembly from that. Alternatively you just buy two new emulsion tube holders. This is no big deal like Fast Road Cars says.

No, the jets are interchangeable.

The Pump diaphram is located underneath.

Sorry, this is not correct. Ther are no peugeot parts

The progression from the idle phase to the acceleration phase is caused by the butterfly opening more to allow the signal to draw fuel from these in addition to the idle jet. Shortly after the pump jet squirts more fuel in. When the throttle is opened further, the pressure differential between the idle progression holes and and the air pressure acting on the fuel in the bowl tapers off until the pressure is not enough to push the fuel up the idle jet and the idle system ceases to supply fuel. Shortly before the the supply ceases fuel begine to be delivered through the main system that is used for high speed operation and cruising.

 

Carburetors with more progression holes require bigger idle jets then carburetors with fewer holes(the increased number of holes was produced in order to have a smoother transition fase from idle to main.An increased number of progression holes and an adjustable pump lever arm provide you with plenty of adjusting possibilities to get the transition right, requiring a larger idle jet. These two elements are found in all the later types of Italian made carburetors-dellorto,weber (dcom and dcoe) and solex.if factories went out of their way to change the production lines for these last two element changes-there is a good reason for it.technology goes forward in time!

 

I stand corrected! I have a book by Des Hammill which suggests the more progression holes is less desirable for tuning purposes. But everyone has their own opinion!

[Beastie 1]

As far as I understand the operation of the carbs, the emulsion tubes and jets and air correctors obviously set up the mixture that is delivered. As the butterfly sweeps past the progression holes it lets a certain amount of air in to mix with the fuel to lean the mixture off slightly when you are off idle. On normal DCOE's there are 3 progression holes to do this process, on the DCOM's i think there are more like 5. Obviously you can cover the lean phase of progression by upping the idle jet selection, but you would have to increase the mixture on the DCOM's higher to cover the lean progression phase but you'd probably end up with a very rich idle.

 

 

I think that you have rather misunderstood the operation of the carbs then: The emulsion tubes are to some degree submerged in fuel which lies in the emulsion tube well. This fuel is metered by the main jet and fed into the well from the centre of the emulsion tube outwards. Fuel needs to be drawn from this well and mixed with air. In the case of the main circuit the fuel is drawn into the main venturi from the auxillary venturi by the depression caused by the narrowing of the venturi area. Unfortunately as the velocity of air passing through the venturi increases the depression does not increase in a linear fashion so unless something is designed to correct this the carburettor will richen the mixture as the speed of the engine increases. This is why there is an air correction jet: Air is allowed through the air correction jet into the opposite end of the emulsion tube from where the fuel is allowed in. As engine speed increases and consequently velocity of air increases the level drops in the fuel well and some of the holes in the emulsion tube begin to be exposed, allowing thereby correcting the mixture so that it does not become any richer. During acceleration there needs to be some temporary enrichment of mixture. This is catered for by the lag in time during which the level in the fuel well lowers and uncovers holes in the emulsion tube.

 

The arrangement described occurs twice for each barrel: There is a main jet, main emulsion tube and main air correction jet assembly. There is also an idle jet, idle tube and idle air jet assembly. Each circuit has built in progression capability and the primary progression must overlap the secondary progression. The idle assembly has two jobs to fulfil: Low speed running before there is sufficient air velocity for fuel to be drawn from the auxillary venturi and secondly the primary progression function. The holes which you describe fall into 2 categories: The idle holes which are adjacent to the butterfly and are fed with fuel air mixture from the primary circuit. (these holes occupy a slightly different location on models with bypass idle function) and the progression holes which are also fed with fuel air mixture from the primary circuit. The idle screws adjust the amount of mixture at the idle drillings and not the progression drillings. If you need to adjust progression by fiddling with the idle screws BTW then it means there is an error in the primary jet settings.

 

The progression holes dont let air in. They let fuel air mixture in during progression. When the butterfly is open wide enough and velocity past the holes drops as a consequence they are no longer needed and stop working. The secondary progression has nothing to do with these holes at all. If you put your foot down at 20 or 30 mph and the car accelerates properly then you can assume that the secondary progression is working - and those holes aren't having any effect!

 

Beastie, carbs are very rudementary mechanical devices to input fuel into an engine, they are certainly not infinately adjustable. If they were infinately adjustable we wouldn't have throttle bodies and engine management!

 

There's nothing special about something being infinitely adjustable. The tap in your kitchen sink is infinitely adjustable because it's an analog not a digital assembly. The reason carbs are the same is because there are so many overlapping and interchanging factors to vary air fuel ratio and progression. Richening mixture for example can be acheived either by enlarging the fuel jet or reducing the main jet. However which way you choose effects at which end of your targetted speed range you will be having most effect. In addition to this the type of emulsion tube you use effects the relationship between main jet and air correction jet and in turn how the complete assembly responds to the progressive demands of the individual engine. The behaviour of the emulsion tube / jet assembly can be likened to the tapered needle in an SU carburettor which is of course (like the kitchen tap) infinitely adjustable.

 

Engine management on the other hand is not infinitely adjustable. It is digital and adjusts in small increments. Fortunately the reason we have throttle bodies and engine management has nothing to do with this phenomenon but everything to do with the fact that as engine requirements change they are better at metering the fuel and responding quickly to those changing requirements. Carburettors such as the DCOE (when you understand fully how they work) are far from rudimentary!

[Grahamrally 0]

I think that you have rather misunderstood the operation of the carbs then: The emulsion tubes are to some degree submerged in fuel which lies in the emulsion tube well. This fuel is metered by the main jet and fed into the well from the centre of the emulsion tube outwards. Fuel needs to be drawn from this well and mixed with air. In the case of the main circuit the fuel is drawn into the main venturi from the auxillary venturi by the depression caused by the narrowing of the venturi area. Unfortunately as the velocity of air passing through the venturi increases the depression does not increase in a linear fashion so unless something is designed to correct this the carburettor will richen the mixture as the speed of the engine increases. This is why there is an air correction jet: Air is allowed through the air correction jet into the opposite end of the emulsion tube from where the fuel is allowed in. As engine speed increases and consequently velocity of air increases the level drops in the fuel well and some of the holes in the emulsion tube begin to be exposed, allowing thereby correcting the mixture so that it does not become any richer. During acceleration there needs to be some temporary enrichment of mixture. This is catered for by the lag in time during which the level in the fuel well lowers and uncovers holes in the emulsion tube.

 

The arrangement described occurs twice for each barrel: There is a main jet, main emulsion tube and main air correction jet assembly. There is also an idle jet, idle tube and idle air jet assembly. Each circuit has built in progression capability and the primary progression must overlap the secondary progression. The idle assembly has two jobs to fulfil: Low speed running before there is sufficient air velocity for fuel to be drawn from the auxillary venturi and secondly the primary progression function. The holes which you describe fall into 2 categories: The idle holes which are adjacent to the butterfly and are fed with fuel air mixture from the primary circuit. (these holes occupy a slightly different location on models with bypass idle function) and the progression holes which are also fed with fuel air mixture from the primary circuit. The idle screws adjust the amount of mixture at the idle drillings and not the progression drillings. If you need to adjust progression by fiddling with the idle screws BTW then it means there is an error in the primary jet settings.

 

The progression holes dont let air in. They let fuel air mixture in during progression. When the butterfly is open wide enough and velocity past the holes drops as a consequence they are no longer needed and stop working. The secondary progression has nothing to do with these holes at all. If you put your foot down at 20 or 30 mph and the car accelerates properly then you can assume that the secondary progression is working - and those holes aren't having any effect!

There's nothing special about something being infinitely adjustable. The tap in your kitchen sink is infinitely adjustable because it's an analog not a digital assembly. The reason carbs are the same is because there are so many overlapping and interchanging factors to vary air fuel ratio and progression. Richening mixture for example can be acheived either by enlarging the fuel jet or reducing the main jet. However which way you choose effects at which end of your targetted speed range you will be having most effect. In addition to this the type of emulsion tube you use effects the relationship between main jet and air correction jet and in turn how the complete assembly responds to the progressive demands of the individual engine. The behaviour of the emulsion tube / jet assembly can be likened to the tapered needle in an SU carburettor which is of course (like the kitchen tap) infinitely adjustable.

 

Engine management on the other hand is not infinitely adjustable. It is digital and adjusts in small increments. Fortunately the reason we have throttle bodies and engine management has nothing to do with this phenomenon but everything to do with the fact that as engine requirements change they are better at metering the fuel and responding quickly to those changing requirements. Carburettors such as the DCOE (when you understand fully how they work) are far from rudimentary!

 

lol that's a far better explanation than what I have in my book! Fair point well presented :-)

[Beastie 1]

lol that's a far better explanation than what I have in my book! Fair point well presented :-)

 

Thank you but um.. on the last course I went on the lecturer swore blind that he'd have a whip round in order to buy me an anorak. I'm worried that sort of describes me

[Guest ashnicholls]

So what do people recon?

 

Get a set of DCOM's or stick to DCOE's

 

Cheers Ash

[pugrallye 0]

Sorry, this is not correct. Ther are no peugeot parts

 

there are.. as i have just rebuilt my carbs, well they are not peugeot specific, but you can still buy them through peugeot if you wished, but weber sales is getting better at sourcing, all they used to say was 'go see peugeot'

Edited September 8, 2006 by pugrallye

[Guest ashnicholls]

I still dont understand what people are on about with the jets being different?

 

Can you fit the air corrector jet, emulsion tube and main jet out of a dcoe in then.

 

Dont you just put the jets in the emulsion tube and then just screw the emulsion tube in the top like on the dcoes?????

 

Cheers Ash

[Guest ashnicholls]

Also, where do you get the Weber DCOM service kits from?

 

Cheers Ash

[Guest ashnicholls]

Where has everyone gone, that was answering the questions?

[SPGTi 2]

Ash,

 

Try contacting Aldon Automotive ( they are 5 mins from my house ). They always use to be the official Weber dealers round here. See what they advise. I take it Mick at Hi-tech said stick with DCOE's, if the cost isn't much different and you are going to take the car to be set up on the rolling road, why not just go with these ?

 

Steve

 

Hurry up and get it running, I would like to see the finished article .

[Guest Nuno205Rallye]

There are more differences between the Weber 40DCOE and the 40 DCOM besides the ones already stated over here.

 

 

But first, I think that something must be explained...

 

 

The DCOE carb is arround for many years, and the Dellorto DHLA is nothing more than a re.design of the DCOE concept, with many improvements.

 

The DCOM is in fact a cross breed between the DCOE and the DHLA concept, top part of the carb is a complete DCOE, but the acceleration pump is a copy from the Dellortto. It uses a Rubber diafragm.

 

Anyways, the Webber 40 DCOE has got the venturis held in place by the air horns, the 40 DCOM and the 45 DCOE have got a nut and stud arregement on the lower part of the carb to secure the venturis.

 

If you measure the intake side of a 40 DCOM and a 45 DCOE without air horns, you will get in both cases a diameter of 47mm .

 

This is the reason that the 40 DCOM must use air horns from a 45 DCOE. Horns from a 40 DCOE just dangle loose on them.

 

 

In the Euro Rallye, you have a pair of 40 DCOM's (type 10 and 11, ie master and slave), wich have 34mm chokes (huge for a 1.3 liter engine).

 

In the Citroen AX Sport you have 40 DCOM's Type 8 and 9, with 32mm chokes but a very different choke tube design, rendering them impossible to fit air horns in them.

 

The Idle mixture screws are also different, on the 40 DCOM you have a towered idle mixture screw just like the ones in the DHLA, on the DCOE's that I have seen they are the fully exposed type (with the spring in view). The screw thread is the coarse type like in the DCOE, instead of the fine type in the DHLA (fortunately!!).

 

 

The DCOM is as capable or even superior to the DCOE, it is a de-poluted version of the DCOE with finer progression mixture control and in they where used in the 205 Rallye Grp. A rally car, capable of producing 140 bhp at 7800 rpm with standart size chokes.

 

In France they race these litle tu24 engined cars with screaming 9500 rpm and still using the 40 DCOM.

 

The rebuild and service kits are exactly the same as the DCOE carb, except that the DCOM ones bring a new pump diafragm.

 

The DCOM is if fact a modernized and converted 45 DCOE with 40mm throtle plates and 40mm bore diameter on the manifold side.

[Guest ashnicholls]

Ok thanks Nuno205Rallye,

 

I think I will get DCOMs then?

 

All the chokes are same the DCOE though, ie 30 32 34mm?

 

Mick and Hi-Tech just said parts were easier to find for DCOE's

but said are the same except for the pumps.

 

The car is up and running.

 

Just the accelerator pumps in my carbs do not work,

 

so has a major flat spot, so need new carbs.

 

And I know a few people with some DCOM's.

 

Do the DCOE emulsion tube/jet holders fit the DCOM ok then?

 

Cheers Ash

[Guest Nuno205Rallye]

Ok thanks Nuno205Rallye,

 

I think I will get DCOMs then?

 

All the chokes are same the DCOE though, ie 30 32 34mm?

 

Mick and Hi-Tech just said parts were easier to find for DCOE's

but said are the same except for the pumps.

 

The car is up and running.

 

Just the accelerator pumps in my carbs do not work,

 

so has a major flat spot, so need new carbs.

 

And I know a few people with some DCOM's.

 

Do the DCOE emulsion tube/jet holders fit the DCOM ok then?

 

Cheers Ash

 

In fact, the Euro Rallye version is a strange case. My car has got emulsion tubes exactly the same as a DCOE carb.

 

Cristopher is right though, because in DENMARK and in other North European coutries the 205 Rallye had different emulsion tube and jet arrangement to comply with running conditions in those countries and to deter the car owner to change jets. in those cases you can still replace the all assembly by a DCOE one.

 

In France and over here in Portugal the jets and emulsion tubes are the DCOE ones.

 

For Japan the car was setup with a leaner arrangement (better gas over there). For England... eh eh ... no car at ALL

 

The DCOM runs bigger jets all round, my car has got 155 Main and 190 Air Correctors from the factory.

[Guest ashnicholls]

Can the fuel Banjo's on the DCOM's fit in the top and side of the carbs.

 

As the one I am looking at fit in the top of the carb, but this will foul mine and many other linkages.

 

Cheers Ash

[christopher 5 1 Cars]

Sorry, this is not correct. Ther are no peugeot parts

 

there are.. as i have just rebuilt my carbs, well they are not peugeot specific, but you can still buy them through peugeot if you wished, but weber sales is getting better at sourcing, all they used to say was 'go see peugeot'

 

PugRallye no offence intended hoper none taken. What I mean is that they all seem to be weber parts not peugeot. I suppose these are available through peugeot like you say

 

 

Nuno205Rallye

 

A very accurate account.

 

Here are some pictures to help show what they look like:

 

These are the main and idle jet assemblies in mine

 

 

This is the Jet holder. The difference here is the small aperature inside. In a normal DCOE jet this would be hollow

 

 

 

Can the fuel Banjo's on the DCOM's fit in the top and side of the carbs.

 

 

As you can see here mine fit in the side and not in the top. The filter is a mesh that fits around.

 

 

I have also seen some DCOM's (although not the ones in Rallyes) that have the banjo on top like you describe. Here is a link that shows it link

 

Here is a pic of where the jets screw in, same as a DCOE AFAIK

 

 

Here is a pic of the intake as nuno describes

 

 

Here is a pic of the pump diaphram and adjustable lever

 

 

Mick and Hi-Tech just said parts were easier to find for DCOE's

but said are the same except for the pumps.

 

This is because the gasket set is basically the same as the DCOE apart from the pump jets

 

Also, where do you get the Weber DCOM service kits from?

 

Ash I got mine from germany. I gave up looking in UK it was hard for a full set. Let me know if you need to know where. To be fair te DCOM is not well known in UK. When I needed a idle screw (which is not teh same as a DCOE as numo correctly says) they didn't know how to help. I was stuck until I rang Southern Carburretors, who are brilliant I might add... I worked out that the screw is the same as another weber sidedraught DCOP or something I forget..anyway they sent one off the shelf

Edited September 13, 2006 by christopher

[Guest ashnicholls]

So if the banjo comes out of the top, that is it, there is nothing you can do about it????????

 

So the ones you showed on ebay. Could not have a top mounted linkage on them then?????/

Edited September 13, 2006 by ashnicholls

[christopher 5 1 Cars]

So if the banjo comes out of the top, that is it, there is nothing you can do about it????????

 

So the ones you showed on ebay. Could not have a top mounted linkage on them then?????/

 

To be honest I don't know about these, since I have only seen them in pictures. But my guess is no they cannot. Since you can see in mine that you cannot fit a banjo to the top. You would have to see them to be sure to see if there is thread there.

 

I'n not sure about teh weber style linkage since it bolts onto teh screw at the top. It would be very close to that usion in any case. The Euro Rallyes have a balljoint in teh centre that is connected to