carb and exhaust popping issues

[vwvanman0]

hi,
Been struggling with this for a while,

van has a newly built 1776 with twin 36 dells,the carbs came from the alfa guy on ebay 5 yrs ago,seen light use since.
changed jets etc to run 32 venturis,132 main jets,180 air and 58 idles.as reccomended by eurocarb.

engine starts and runs,ive balanced the carbs and set the timing with a strobe.

however it pops back through the carb on no4 cylinder,not so much on tick over but on a light throttle,when revved that seems to be ok,

however then i get some popping out of the exhaust.

i always thought the carb = blockages and exhaust = air leaks but ive stripped and cleaned the carbs and replaced the gaskets and seals,and now im even running a spare carb body on it and its still the same.ive checked the compressions and they are all good/similar.

Its really winding me up now and im worried it may be doing some damage so hardly use the van now

any help greatfully recieved,

cheers,steve.

 

[montypsx]

hi pal have you checked your valves i had a similar problem on number1 pot, checked valve clearance ok, compression slightly lower but not bad when i took the rocker off and put fluid in the inlet and turned it over by hand it bubbled like f**k. i regound valves all ok . i'm only a novice so i could be wrong but the bigs boys should be along to help shortly. hope this helps cheers phil

 

[Tristan71]

I agree with the air leaks but maybe it's ignition related what state colour was the plug when you took it out try swapping the lead with a one next to it obviously both ends see if you get the same that way you can rule the leads out is the contact for that cylinder in the dizzy cap clean could also be a duff plug or the little nipple could be loose if your using them

 

[Tristan71]

Also try winding the air mix screw all the way out see if there's a little o ring on it it could be sucking air in from there also did you check all the idle jets before you put them in

 

[vwvanman0]

Thanks guys, I had the lead off and got a belt there's a nice juicy spark,the plug looks similar to the 1 I removed from no2 cylinder a bit cleaner,must be a bit leaner on no4 i've tried swapping the plugs round and its made no difference,I ran out of time so will check the mixture screw tomorrow,if that's ok then I guess it will be time to investigate the valves. Aaagh!
How would I check the jets? Do you mean the hole sizes? I checked they were all stamped the same before fitting.
Thanks for the ideas so far guys.

 

[Tristan71]

I've had issues before with popping and banging through the carbs it turned out to be the idle jets yours sounds exactly the same as mine try winding the mixture screw all the way out on that barrel

 

[Tristan71]

It sounds like idle jets wind the mixture screw all the way out see if it changes anything

 

[vwvanman0]

Well I've had the jets out and tried swapping the one in no3 and no4 that made no difference, then I tried winding out the mixture screw,that also made no difference,when removed the rubber and spring are on the mixture screw as they should be and look in good condition. V. Did notice the take off for the brake servo comes from the manifold on that cylinder,could that be an issue? And is ther anything else carb related,floats etc that could be the suspect before I remove the engine to check the valves? S. Thanks for the advice so far, steve.

 

[Tristan71]

Try blocking that vacuum pipe off if the non return valve could be defective

 

[Pearls cafe]

Out of interest, what fan housing do you have?

I have twin webers fitted but with the stock fan housing it's difficult to get the manifolds really tight as the carbs are tight up against the fan housing. I have assumed that as a result there may be an air leak hence the banging and popping with mine

 

[vwvanman0]

Have a half round fan housing on to clear the carbs,tried to do everything right,that's why this is so frustrating,

hope to get some time over the weekend and I'm going to swap the manifolds side to side as the pipe fitting isn't brilliant,then if its the manifold no1 or 2 will pop!

 

[Tristan71]

Let's us know how it goes where are you out of interest

 

[vwvanman0]

Finaly got some time and a dry couple of hours to mess with the van,I've blocked off the pipe to the servo off the popping cylinder and that seems to have cured the popping thru the carb. I'm a happy chap!

But now If I route the servo pipe to another cylinder am I going to get issues on that cylinder? Is there a better way?
Or have I got a leak somewhere in my servo/master cylinder.
I don't fancy having servo less brakes!

Thanks for the replies so far,and I'm in colne lancs btw.

 

[Tristan71]

You could try blocking it of at the servo end if it's back to popping then you know it's the pipe or non return valve can't hurt having a good look at all the vacuum pipe work too

 

[vwvanman0]

Good idea,will try that,I know the pipework is ok as I replaced it with top quality stuff when I replaced the engine.

Just glad to be rid of the popping,was really bothering me.