Le Jetronic Vs Carbs

[Andy 105 1 Cars]

I am sure this issue has been raised and discussed before, but I would be grateful for any input anyone has on the subject. I have rebuilt my 1.9 8v engine with new bearings, pistons, liners, balanced rotating components etc. The head is also rebuilt with mild porting, new valves and guides and a stage. One Kent cam and vernier pulley. I have a completely standard injection system with a new AFM . It all runs reasonably well, although the off throttle response is a bit disappointing and the idle, although fairly stable under normal circumstances, can get a bit nervous with electrical load. However, light throttle response is good and acceleration excellent.

Nevertheless, I am an old git so hanker after the simplicity of carbs as I have experience of them on my much beloved Alfasud( mind you, I have also had some success playing with a fully mappable system on my last car, an Integrale, so I am not paranoid about injection, I just think that the jetronic system is rather crude and inflexible. )So, apart form the lovely induction warble of twin forties, what are the real pros and cons of them. Absolute maximum power is not the most important factor. Stable tick over, smooth progression and light throttle response( in other words, being nice to drive) is the most important feature. I imagine I could also go down the route of an aftermarket ecu which is mappable and get the effect I am after, but then I would not have a nice uncluttered engine bay!!!!

Any thoughts much appreciated.

Andy

[Tom Fenton 1,542]

Have you set it up at all, or is it "as bolted on"?

 

OK there are a limited amount of adjustments, but nevertheless they can make quite a difference to how the car drives.

 

Jetronic in comparison to modern management IS crude, but at the same time set up well is nice enough to drive.

 

Electrical load does drag the idle down a bit, there is no mechanism in place to compensate for the extra load.

[Andy 105 1 Cars]

Thanks,

Yes, I have made all the usual adjustments in the correct sequence. I have also altered the spring tension in the AFM slightly to reduce the rich mixture at idle ( I used a lambda probe and gauge to monitor) . I also have a rising rate adjustable fuel pressure regulator which has given me a small amount of leeway in terms of mixture adjustment. However, in the end , to get an acceptable mixture on heavy load and light throttle and at tick over is not easy with this system because the injector pulse bandwidth is set and not adjustable and the AFM Spring tension has a linear effect across its entire range of operation. Having said all this, yes, the engine does run reasonably well, but it is not ideal.

Andy

[Reebmit 51 4 Cars]

Right for me it breaks down into 3 simple decisions. First standard Jetronic, can be okay when set up correctly, okay performance but a standard dull experience. Second is carbs, I'm old school and hear all the s*ite about how bad they are on fuel, keep needing to be set up, etc etc, this normally comes from people who haven't a clue how to set them up correctly. One of my pugs runs a nice lightened balanced 1900 8 valve with big valve head, 285 cam, pistons etc...it runs 45 side draughts which have been set up well, makes very good power with great torque, ticks over perfectly, starts from cold perfectly and sounds spot on. It hasn't been touched in 2 to 3 years and is as good as the day it was set up. Its fuel consumption is great. If you are lucky you can get second hand quite cheaply but make sure everything is there as all the little bits cost real money. Most 2 litre engines like early Lotus Esprit with carbs can easily do 30 plus mpg.... and mine is similar. Third has to be throttle bodies with management but when you think that this set up along with rolling road mapping sessions cost as much as a whole 205 car you have to think twice.... but cannot argue that they are best overall. If you've got a big budget motor then this is obviously the route to go. For your spec I would personally go carbs all day long. I have cars with Jetronic, carbs and turbo/management so have gleened a little along the way but ultimately everyone has their own views, most people on here are throttle body worshipers hope this helps a little

[opticaltrigger 52]

Hi Andy

I've got to go with everything Reebmit above has said to.

I cut my teeth on carbs to and still love them. Nothing beats that sound.

That said though I have to just stand in awe of modern standalone ECU's.The degree to which you can affect the ignition and fuel is unrivaled and the data logging features are staggering.

It's obviously the case that mapped injection / ignition will be vastly more precise. That's what hundreds of millions of pounds and decades of intense R&D will do.

 

Carbs are nice and easy to. I ran a motor years ago with twin downdraft Dellortos and once set up properly they ran on for years to,without really needing any adjustment. However I do think a quality throttle linkage is the key to that though.

They always worked well,made good power,and sounded great to.

Personally, if I could lay my hands to a nice set of Webbers or Dellortos and come up with a quality linkage for them, I'd go with them on your motor and remove the Jetronic altogether.

To me,that engine spec is screaming out for a set of well tuned side drafts and an exhaust.... Nice!

 

However, as you said Andy your not a stranger to proper ECU's so just to put a thought out there.

You could just hang a crank sensor and a TPS on it, screw a temperature sensor in the standard manifold and your going to get everything you want and more.

That's assuming you have an ECU to hand still. But as said above and I'm sure you know yourself, mapping and dynos ,etc,etc.It adds up.

 

All the best

O.T.

[Andy 105 1 Cars]

Morning,

Thank you for your helpful replies. I shall begin my search for all the parts I need, including carbs, manifold, fuel pump and air box. Bought new these could be a bit expensive but I hope to find at least some items second hand.

Andy

[welshpug 1,661]

personally, no contest, Motronic

 

second choice, throttle bodies.

[Reebmit 51 4 Cars]

Oh how I knew you wud say that WP lol....

[Andy 105 1 Cars]

Costed the whole set up for carbs. There seems no doubt that having gone to the trouble of correctly setting them up, the benefits of fully electronic, mappable ignition are huge, but that then puts the costs Ito the throttle body territory, which is what I am now also researching. Too many decisions!!!! The 205 is an awesome little car. It would be nice, having gone to the trouble of stripping the car to a bare shell and trying to rebuild it to a better than new standard with sensible upgrades where appropriate, getting it to run as well as a 21st century car is quite important. However,I really like a clean and uncluttered engine bay. The O.E system has many qualities but clean and uncluttered are not two of them in my view. That, combined with with a brave but crude cold start system, fuelling independent of ignition and a clumsy mass flow measuring system means that the le jetronic is ready for a well earned retirement. All my view you understand based on relatively little experience of the 205 ( I have only owned one for just over a year).

Andy

[Reebmit 51 4 Cars]

WP is probably right in that if you want a cheap route to having a modern running pug the Motronic is a good route to go, perfect idle etc.....propbably pick up the bits for £100 was going to do it on one of mine. I would only ever go the carb route over bodies if the carbs were second hand and cheap... as a new option bodies is the way to go... new carbs are big money.. My 45's only cost me 150 bucks and manifold was about 50...jets 50... sundries 50..

[welshpug 1,661]

I do have most of the motronic bits!

[Andy 105 1 Cars]

So there is another option? I will now display my huge ignorance by assuming that a switch to motronic would need the ecu, the loom, the coil pack, the airflow meter and perhaps a throttle switch/ potentiometer and the flywheel and sensor. Advantages include the cost but the software is locked.

I am sure I am not fully correct but equally sure someone will do the honours and correct me.

Andy

[welshpug 1,661]

no afm, that the beauty of the mp3.1 system, onboard map sensor.

 

can be mapped by chipwizards.

 

you would need the flywheel to do any 2/3d mapped system.

[petert 589]

personally, no contest, Motronic

 

second choice, throttle bodies.

I thought you'd misspelt Motec.

[Andy 105 1 Cars]

Ah. No AFM.. And mappable. This sounds interesting. Thank you.

Andy

[Andy 105 1 Cars]

O.k.Next piece of frightening ignorance on display as I ask to which vehicle(s) the motronic MP3.1 was fitted.

Andy

[Andy 105 1 Cars]

Evening welshpug,

Further to your comments about the motronic system, which bits do you have available and what parts, therefore, would I need to source elsewhere?

Andy