Backfire on idle???????

[mikemango]

Hi folks
Just wondered if anybody has experienced a slight backfire on idle, it's bugging the hell out of me, I've changed the bushes in the carb as they needed doing, checked for leaks around the inlet manifolds and joining rubbers using wd40,stripped the carb blown through with airline and fitted new gaskets,strobed the timing, reset the tappets,checked the exhaust manifolds are tight, checked the vacuum lines to the brakes,adjusted the fuel and air to all possible settings.
I have a 1972 bay with a 1600 tp engine, 34 pict 3 carb, svda dizzy with electronic ignition and a bugpack aftermarket exhaust.thermostat has also been removed.
It seems to be more prevalent when the engine is hot or been driven hard.
But occasionally disappears when left to idle for a long time.
Any suggestions will be much appreciated
Kind regards
Mike

 

[Trikky2]

Well you have already checked all the things that cause this problem.

It's caused by weak mixture which is either down to incorrect adjustment of the carb or jets or down to air leaks in the induction system or the exhaust.

Provided the throttle butterfly is completely shut at idle so the engine is only running on the bypass system, does it not stop backfiring when you unscrew the small mixture screw half a turn? If not then maybe the idle jet is blocked or too small.

If on the other hand the throttle butterfly is not completely shut at idle (when warmed up) then that's your problem.

 

[A5H]

Do you have a mechanical or electric fuel pump?

 

[mikemango]

How do I check the butterfly is completely closed?
Is there adjustment?
Fuel pump is mechanical.

 

[Trikky2]

Back off the screw on the throttle arm till there is just a gap between it and the lowest step of the auto choke stepped cam.

Then screw it in until it is just in contact. In other words, the throttle stop screw hits and rests on the step of the cam but the throttle butterfly is closed.

This screw is literally a throttle stop screw in that all it does is stop the butterfly from closing too far and sticking in the body of the carb.

 

[mikemango]

Carb set like you said, doesn't look like there's any gaps around the butterfly.
Although it does appear to have some wear to the sealing edges.

 

[Trikky2]

Carb set like you said, doesn't look like there's any gaps around the butterfly.
Although it does appear to have some wear to the sealing edges.

So if you undo the small mixture screw, the one close to the big volume screw, by half a turn or more, does it have any effect on the engine?

 

[Trikky2]

Also - when did this problem first start?

 

[mikemango]

Doesn't seem to have an effect adjusting the mixture screw, I've tried an old 30 pict carb too which has the same symptoms, I'm going to do a compression test next week, as its a recon engine with 800 miles on,I suspect a valve not seating correctly.
I've also got a 009 dizzy with points to try this week which will eliminate the electronic ignition being faulty.
Runs really nice apart from idle????

 

[ozziedog]

Hi there, have you got servo brakes by any chance ?? Might be worth a little sniff (spray) around the hoses and connections etc or perhaps the servo itself.

Ozziedog,,,,,,,,,,,,, Just a thought :mrgreen:

 

[mikemango]

Yeah tried the vacuum line to servo, no apparent leaks

 

[Trikky2]

Doesn't seem to have an effect adjusting the mixture screw, I've tried an old 30 pict carb too which has the same symptoms, I'm going to do a compression test next week, as its a recon engine with 800 miles on,I suspect a valve not seating correctly.
I've also got a 009 dizzy with points to try this week which will eliminate the electronic ignition being faulty.
Runs really nice apart from idle????

Well yes, seem to be on the same track, I was going to suggest a test to check for valve leakage once the ignition, inlet and exhaust systems checked out as being definitely ok.

I assume screwing the mixture screw inwards does have an effect and that its set for max idle speed. If screwing it inwards does have an effect and out wards causes a slowing once its gone beyond max best idle it would indicate that all is well with both the carb and the intake system.

That leaves the exhaust, ignition and valves.

It's rather hard to help with this type of diagnosis like this. It's much easier to diagnose when your actually there, hearing the problem for yourself. So much can be learnt from the subtleties one only gets from being there. A vid might help in that its better than nothing.

Keep us posted.

 

[mikemango]

Also jet sizes are as follows:

Pilot jet = g55
Air correction = 60z
Main jet = x130

I think these are the correct sizes?
any thoughts?

 

[Trikky2]

Also jet sizes are as follows:

Pilot jet = g55
Air correction = 60z
Main jet = x130

I think these are the correct sizes?
any thoughts?

Those jet sizes look right to me.

 

[mintey]

hi
i had the same problem
turned out to be the intake valve not seating properly
put on a complete new cylinder head on problem solved
175 complete pricey i know but peace of mind runs sweet as now