poppin noise from engine

[chrissiefiddle]

we have a 1600ct engine in our camper that has started poppin when revv'd, particularly when under load. We recently changed the carb as it was worn out and the noise seem to be evident after that. We have had it tuned and it passed its MOT easily, in fact the mixture was a bit weak.
Husband thinks it might be the exhaust or cylinder head leaking on one side of the engine as it seems less evident when the engine has warmed up Any bright ideas?

[rugchucker]

sounds like a leak on the exhaust, when cold the mixture is richer and any unburnt fuel will re-ignite when the air is sucked in at the leak. What you need to do is use a bit of tubing as a stethascope to find the leak as it will be sucking not blowing under load.

[VWCamperfan]

You will need to get that weak mixture sorted first, even if only to stop the engine overheating. A weak mixture will also cause popping.
The exhaust can do this if there are leaks anywhere but I would be looking at the carb mixture first.

[Mocki]

i would be looking at a induction air leak, which will cause weakened mixture, and popping

carb gasket, manifold gasket??????

[chrissiefiddle]

thanks
will investigate and let you know.

[chrissiefiddle]

yep, the carb was a complete recon from a carb specialist in leighton buzzard.
so we are guessing its prob exhaust leak or mixture. to strengthen the mixture do you turn the screw clockwise or anti?

[Mocki]

yep, the carb was a complete recon from a carb specialist in leighton buzzard.
so we are guessing its prob exhaust leak or mixture. to strengthen the mixture do you turn the screw clockwise or anti?

if the carb specialist is any good they will have correctly set the mixture,

have you checked for induction vac leaks?
loose bolt/nut on the manifold allowing airleak below the carb, or between manifold and heads?

[HarryMann]

to strengthen the mixture do you turn the screw clockwise or anti?

Any screws will only adjust the idle mixture on a fixed jet carb, not when under power, which is why I asked about the likelihood of any muck being in it.. backfiring/misfiring is usually a result of weakness

The fact you have just fitted it: ..

are you sure it is seated and gasketed nicely without air-leaks there?

We have had it tuned

What was involved here? What does that mean 'tuned' and by whom?

We can't really move forward here with anything else until you answer those questions...

[VWCamperfan]

The idle jet is adjusted by the adjuster screw, clockwise for leaner, anti-clockwise for richer. The idle jet will be in use up to 2400 rpm, at this point the main jet takes over and supplies the majority of fuel to the engine. So up to 2400 rpm its an issue of idle screw adjustment.
Theres no such thing as a carb being adjusted for an engine before it's fitted. There are too many variables that may require a different mixture ie. engine age and wear, compression, engine size etc. The only way to adjust a carb is with it on the engine!
As said above, check for good seals around joins espicially where the new carb has been fitted to the manifold. Can easily be done by spraying wd40 around suspect areas while engine is running. If engine revs increase when spraying then there is probably a leak in that area.

[Mocki]

Theres no such thing as a carb being adjusted for an engine before it's fitted. There are too many variables that may require a different mixture ie. engine age and wear, compression, engine size etc. The only way to adjust a carb is with it on the engine!
.

i assumed the "carb specialist" fitted the carb.... if they didnt then i retract my remark, if they did fit it, then i stand by it!!!

[HarryMann]

The idle jet will be in use up to 2400 rpm, at this point the main jet takes over and supplies the majority of fuel to the engine. So up to 2400 rpm its an issue of idle screw adjustment.

It may be in use but it will be doing damn all... flat out up a hill at 2,000, and it's on the 'idle' jet - can't agree, it'll be drawing most of its fuel through one of the intermediate jets and the emulsion tube will be controlling richness, as things progress.

There's no such thing as a carb being adjusted for an engine before it's fitted.

There is... all the fixed jets and emulsion tube can be fitted for that engine (original spec. for the country of delivery and expected altitude of use) + the idle jet can be set to a nominal 'as-delivered' state, to at least get you going.

There are too many variables that may require a different mixture ie. engine age and wear, compression, engine size etc.

Yes, that's true.
But it doesn't stop a supplier setting it up as mfr supply above to stand a damn good chance of running well enough to start with, if that carb was originally specced for those engines..

Few people go changing main and intermediate jets as their engine wears

[Mr Bean]

Bit late now probly but:
Another thing that can cause backfiring or combustion in the inlet/exhaust is retarded or intermittent ignition timing.
Cheers
Wolfie

[chrissiefiddle]

thanks for all the advice chaps
The van was set up by a friend who is a trained mechanic for Ford, BMW, and Mercedes (but not VW). However he used a timing gun, a portable sun tuner and a dwell meter so I don't believe that anything he has done would cause this problem.There is definately no leak on the carb or the gasket. It could be the manifold to engine gasket but it appears that the poppin is only coming from the left hand bank of cylinders so hubbie reckons it could be the exhaust leaking there. He has retarded and advanced the timing and it doesn't really stop the poppin.
Do you think its ok to drive from Luton to Derby and back tomorrow if we take it easy or is it going to break something?

[VWCamperfan]

It should be fine. If the popping is coming from just the left bank then it could well be the exhaust as the carb on these engines feeds all four cylinders but it should only be happening on the over-run ie. when you lift your foot off the accellerator, not when on it!

[chrissiefiddle]

thanks,VWCamperfan,
the noise is when you press on the acclerator - blip it - , when then engine is being worked.