Backfire Through Carb

[mickie 3]

hi, i have an annoying backfire through only 1 of my twin 45's. i thought it may have been plugs/HT leads but ive changed the plugs, i havent changed HT leads yet but if i un-plug the lead off cylinder 1, the same as the backfiring carb you can tell its running on 3 cylinders which must mean that spark plug is ok?

 

when i took it too a tuner a few months back he tightened the carbs right up, saying they were too slack but i thought that was the idea ? to stop the fuel throfing up ?

 

another thing i havent done is connect the little pipe up from the dizzy, but the weird thing is its only started popping. it was fine after the tune..

 

the other thing is though it was tuned with the exhaust gaskets on the wrong way around, halving the exhaust outlet size, totally my fault so it was tuned like that and now ive fixed that gesket problem im sure it will be all out of tune again, the car doesnt feel fast..

 

but being sick of spending money is there something i can do? should that dizzy pipe be connect, should i slacken the carbs back off?

 

can a backfiring carb damage anything ? ive drove it 60 miles today, slowly but the constant bomb going off underload makes me think something has to go wrong..

 

cheers

[projectpug 0]

I cant help you too much with your issue but i will have a go. You shouldn't connect the dizzy vac advance when you have carbs. The mounting should be fairly firm but as you say not too tight so as the fuel doesn't froth about. Which type of inlet mountings are you using for the carbs? I have seen some pretty bad ones from certain "pattern" manufacturers. If all the carbs are in balance then that wont be the issue and you will probably have to inspect further.

[Baz 421]

Carbs should be mounted on proper misab plates and spring washers, allowing them to 'wobble' although still sealed, so no they shouldn't be tight.

 

The dizzy doesn't need, or should i say won't be working with the vac. advance, so no it doesn't need to be plumbed in, and you could actually remove the vacuum capsule completely if you liked.

 

Is it a re-curved dizzy to suit the carbs? If not i'd recommend getting one, makes a difference, especially if still running on the 8v advance curve.

 

Popping or backfiring regularly is usually a sign of running lean and i'd say they're not set-up that well, it doesn't sound right from what you're saying at all!

 

One thing i will say from experience of carbs & pouring good money after bad, make sure everything's perfect and exactly as you want before getting it set up.

[mickie 3]

cheers for the help, i guess the carbs arent all set up right now the exhaust gaskets have been fitted the right way, more flow ?

 

the inlet manifold is a cast ali item that came with the carbs as a kit, cant remember who from.

i have them misab plates and spring washers, thats why i thought it was wrong the idiot spent 30 minutes tightening them up and charging for it :S

 

i have the original dizzy, ive never heard of a re-curved dizzy untill you said, do you have any more info on this ? or where i can buy one baz.

[Baz 421]

cheers for the help, i guess the carbs arent all set up right now the exhaust gaskets have been fitted the right way, more flow ?

 

the inlet manifold is a cast ali item that came with the carbs as a kit, cant remember who from.

i have them misab plates and spring washers, thats why i thought it was wrong the idiot spent 30 minutes tightening them up and charging for it :S

 

i have the original dizzy, ive never heard of a re-curved dizzy untill you said, do you have any more info on this ? or where i can buy one baz.

 

Never let anyone touch carbs, including yourself unless you trust that that person is completely capable of what they're doing!

 

H&H Ignition solutions re-build distibutors to your spec, google should find them!

 

What is the cam/engine spec as one of mine may actually be suitable & spare now i've gone to bodies.

Edited October 31, 2010 by Baz

[mickie 3]

i have a 1.9 cam, sharp edges or atleast i think its a 1900, 1600 has rounded right ? i read most uprated ones dont have much of a lobe difference ?

 

im going on a bit, i have a 1900 standard apart from twin 45s and 1600 box and 4 branch manifold

Edited October 31, 2010 by mickie

[Cameron 16]

As Baz said backfiring through the inlet is a sign of it running lean, I'd get it checked out before you drive it too much more.

[mfield 20]

If it's just the mixture which I doubt as a quick fix just screw the idle mixture screw in until it's JUST nipped and then unwind it 2 1/2 - 3 turns, this should help. The main mixture is dictated by the jets which should all be the same.

 

The fact it doesn't seem quick and is backfiring unload and when letting off I would check to make sure the venturi is still secured properly, now the carbs have been tightened up the vibrations can shake the grub screws out. This will make the main fueling leak into the carb rather than spray.

 

The fact it's just started and seems quite bad points to something like that rather than settings or balance, although checking the balance regulary is always a good practice for keeping them sweet

[PVFCpug 6]

If they have been tightened up that is quite possibly the cause of your issue. I had exactly the same one time when my car went in for some paint work and when I picked it up the young guy said "oh it was running lumpy so I tightened your carbs up, did you know they were loose?" Could have throttled him, the rubbers were crushed so you have to renew and get your carbs set up again.

 

Its correct what Baz say's only give your carbs over to someone with a known reputation of setting them up properly. Also worth thinking about getting them re-furbed. Suprising what a differance this can make. Mine used to need balancing or tuning once a year, after a good re-furb they have now been in good tune for three years.

Edited November 1, 2010 by PVFCpug

[mickie 3]

 

the carbs are new but look what ive just found, i thought it was cylinder 1 but its 2.. all the other 3 suck air were as cylinder 2 constantly blows back. #2 has spark and fuel becuase its blowing back fuel at the same time..

 

a bit of the socks missing so all i can think is that its been sucked into the cylinder and now its backfiring constantly..

 

this has never happened to me before so does this mean part of the engine may be f***ed ? bit gutted about this.. stopping off work today to pull the carbs/manifold off

[mfield 20]

Would blowing back air be a sign that your valve isn't closing any more

[mickie 3]

this is going to be a head off job isnt it ? just took off the manifold/carbs there and cant see anything, i think you're right about the bloody valves

 

just when i finish this car its broke again, s*ity socks

Edited November 1, 2010 by mickie

[mickie 3]

here is the rusult of a sucked in sock

 

 

one bent exhaust valve

 

compaired to 1 good exhaust valve

this is the inlet valve on cylinder 2, looks in good shape

 

this explains how it was ok up until on load

[mfield 20]

Gutted

 

At least you know what was causing the misfire

[Baz 421]

Ahh sh!t!! Bad news!

[mickie 3]

well i assumed the valve would be bent but turns out it isnt

 

 

one camshaft lob worn away and a lifter worn out..

 

im now thinking the ITG filter has kept the valve open long enough for the little collet, that little thing under the lifter to move

 

ive took the valve right out and its fine, that lifter obviously doesnt like being too high and its worn against the camshaft

Edited November 1, 2010 by mickie