Pop, pop, popping

[Gogso1]

Hi all,

I feel I've a bit of a cheek posting here as all in all the van (2l air cooled with 105000 on the clock) is running really well and has just got its MOT for another year.

Having said that, it seems to have developed a bit of a 'popping' from the exhaust when changing up from 1st to 2nd, and sometines when changing up from 2nd to 3rd. I'm told this could be down to fuel burning off as a result of the mixture being a bit too rich. If this is the case, how would I go about stopping it without messing up how the van is running?

Any other ideas?

Thanks a lot,

Gordon

[HarryMann]

I'm told this could be down to fuel burning off as a result of the mixture being a bit too rich.

Almost certainly the opposite!

Lean mixtures take longer to burn, and mixture still burning as exhaust valve opens, hence can hear a (small) pop

Engines often too lean on idle (or exhaust too open or inlet air leak) - all lead to same thing

So when throttle shuts during gear-change, mixture weakens as transiently reverts to idle >> popping back

Also, can lead to burnt exhaust valve (though needs to be bad for along time), or popping can be a bad seating valve (burnt or otherwise)

Have never heard an overly rich running engine popping... this fallacy seems to have evolved since turbos, lifting off and silly excess fuel (kids games). Nothing to do with NA engines AFAIK

===

On the other hand if its spitting back through inlet, ask again...

[Gogso1]

Wow, that was quick, thanks!!

As I'm more than a bit rubbish when it comes to fixing, well, pretty much anything, is this the sort of thing that could wait until the van is back in the garage with its inevitable next bout of more serious ails, whenever they may strike?

Cheers.

[HarryMann]

Depends how bad, but just popping which peeps put up with for months sometimes years, yes definitely wait. Some even think its nice, good...

Chances are its an exhaust or inlet leak causing the weakness, so start thinking that way i.e. is the idle dead steady or does it hunt (up & down)

Oh! Yes one more thing could create that popping when lifiing off for a gearchange.. excessively retarded ignition. But excessive, unless on a very highly tuned engine with a good degree of overlap and high compression.. then slightly retarded (& often acceptable in race circles c.f. over-advanced, which'd be bit more dangerous re: detonation)

[BOXY]

If the van's just got its MOT chances are the garage adjusted the mixture to get it through the emissions test. Now its got a ticket see if they will adjust it back to stop it popping.

[Gogso1]

Thanks for the help folks.

[Tatty Chap]

Interestingly, mine is doing the same after getting through emissions tests, but on deceleration.
Had a play with the dizzy advancing it a tad etc, but as it only happens on the road at deceleration, its hard to assess in the driveway.

Sounds like its not worth a massive investigation, as the exhaust is blowing from the connection to the head a bit.

[HarryMann]

Interestingly, mine is doing the same after getting through emissions tests, but on deceleration.
Had a play with the dizzy advancing it a tad etc, but as it only happens on the road at deceleration, its hard to assess in the driveway.

If you read my previous reply, it should make fairly clear it is almost always on the overrun (deceleration), due to weakness and altering the ignition timing rarely a good thing to be doing (why wouldn't it be right anyway - it can't change of its own accord.)

So there you go, re-read reply above to Gogso and that'll like as not point to a leaky exhaust