When backing off the acceleration, what causes a car to backfire? (or in my case a semi backfire - more like popping out the exhaust).

Is it a vacuum leak?

Idle seems a little erratic, if that helps diagnose.

?In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where is fails to ignite consistently. This, in turn allows some un-burnt fuel to get into the exhaust pipes. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust pipe, causing the backfire. Newer Virago carbs have an 'enricher circuit' which cuts in on deceleration to help this problem. Earlier carbs do not have this. I would say check the pilot circuits, and set them a bit richer. It is hard to do this right without some equipment to test where you are. Aftermarket pipes often increase backfiring, probably because they may the vehicle run leaner.?


Hope this helps.

Backfiring from the carb = too rich, backfiring from the exhaust pipe = too lean

I'd double check my last statement though as it's been a while since I had this problem

It could be caused by many things. Is this on Pops?? if so and he is running a standard/standard setup then I'd have a look at rebuilding his carb. Could it also be fuel starvation? maybe the fuel pump needs an inspection? just some ideas...

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Lillee - 1969 Morris Mini K

hmmm.....interesting. That's kind of the opposite to what I was thinking.

Too lean ey?

Appart from that popping out the exhaust on deceleration, he drives A1. Carb 'seems' to function really well (ie: doesn't need rebuild)

Have you taken the dash pot out and inspected the jet seat?

I just thought of another thing, is your dashpot oil too thin? (or thick... I can't remember which). Basically if it's wrong (I think too thin) the spring "returns" the piston back to idle too soon (as in before the engine has had a chance to rev out of decelleration) and hence there is not enough fuel for the decelleration revving and hence a lean situation. This is because with thin oil there is not enough dampenning effect of the dashpot piston and the piston moved too freely.

I'd clean it all out and try a thicker oil like maybe 15W or 20W?

I'm no expert but last time I had an SU (seams like yonkers ago) dashpot oil made heaps of difference to the way the car behaved. Hence why I went over to Dellorto, can't stand chaos

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Lillee - 1969 Morris Mini K

hmmm...dashpot oil...must check and clean. It's prolly got the same OLD stuff in there from 1988 (last service)

I DID check the level however.,..I am not THAT stoopid

Backfire in exhaust is often due to exhaust leaks near the front- when you back off it sucks air in and it mixes with the unburned fuel...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

you should leave it.... it'll make naming him pop's all the more appropriate

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LOL

There you go Anton I am sure one of these things will fix him!

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Lillee - 1969 Morris Mini K

also could be a crack in the exhaust sucking in cold air causeing it to pop
makka

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Anonymous moderation means the agro that is built up towards the mods isn't aimed at an individual (fewer "Makk" attacks). Less stress for mods means less moderator "retirements".

if there is a zorst leak, wouldn't you HEAR it? I can't hear any leak....

depends

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Anonymous moderation means the agro that is built up towards the mods isn't aimed at an individual (fewer "Makk" attacks). Less stress for mods means less moderator "retirements".

Yeah not necessarily, it only has to bring in a small amount of air to cause the problem. Go block the exhaust pipe temporarily with your hand, and you will see if it has an exhaust leak or not. My car hardly drops the idle at all when I do it, yet when driving you can bearly hear a leak (I just get a headache if I drive with the window closed)

should the idle go UP when blocking the exhaust?

No, hopefully it will stall. Otherwise the exhaust will just be diverted out the leak, idle may remain uneffected if the leak is bad enough. Just be careful, if the car is old enough you may inadvertantly cause a new leak!