Visa engine

[howardjs]

Hi Boggy... As far as I can remember I used 2CV pushrods 'cos didn't have the Visa items. The the rest, rockers etc were all in the Visa heads so just used them as is. (or was)!
My engine is still running and pulling very well as long as the accelerator is open, but backfires when lifting off. I've tried adjusting the tappets (the gaps were too small), checked the exhaust for leaks, advanced the timing (8deg. for 2CV 10deg. for Visa) as it was set to 8deg. and adjusted the mixture. All to no avail! Now I'm considering putting a BMW engine in, I have already made the adaptor, but no time at the mo.
But I digress.. again!!!
Howard

[Boggy]

Hi Howard
Many thanks for info , I have both so no problem sourcing. Bit of bugger the backfire , what carb are you using ?
Cheers
Boggy

[notsmartcar]

What size is the idle jet howard. too small will cause it as well

[howardjs]

Hi carbs are twin PHBH28BS from Eurocarb, came ready jetted for a 2CV 602.

There is also a flat spot tendency from tick-over. Eurocarb have recommended changing the main jets to a smaller size. I have the jets but not yet fitted as I didn't think they would help the backfiring.. Maybe I'm going in the wrong direction with this...

What does the panel think?
Howard

[Boggy]

Hi Howard ,
Little confused You say carbs are jetted for 602 but you are running 652 barrels ,pistons and heads ?
Surely you need to be upping the jet size not lowering them .
As you stated you may be going in the wrong direction .

Boggy

[cmwbob]

going wrong direction, not neccessarily!

If the backfiring is (as most instances) unspent fuel exploding within the exhaust system, then smaller jets may help. However sooty black plug colour will soon show if its overfueling.

If there are no manifold air leaks then it must be a fueling issue. To get a good loud backfire the mix of fuel and air in the exhaust has to be right, loads of fuel will just create flames out the end (world rally style).

Is the backfiring after a long throttle off downhill coast, or as a consequence of throttle lift after hard acceleration?
It could be a combination of jet size and the needle taper. too narrow at the thick end.

I am no expert just sharing observations from my low level motorsport past.

[howardjs]

Hi Bob thanks for the input.
The plugs are a nice straw colour, there is no smoke, the engine runs very well and smoothly, pulling well; as long as the accelerator pedal is being pressed. The tickover is also constant and not lumpy.

But, as soon as the accelerator is lifted or closed when the car is on the overrun we get an AK47 simulation!

I have checked everything already listed in a previous post to no avail...

I will fit the smaller jets I received from Eurocarb after the hols. and try again.
Howard

[cmwbob]

Howard.
Just given the old grey matter a tweak,
Assuming a small amount of unspent fuel is accumulating in exhaust during normal running, then on overrun you are effectively creating a vaccum in the active cylinder and as there is no expanding gas to exhaust there can be a tendency to get small suck back as the exhaust valve opens. Then the lazy spark of the dead cylinder fires that very weak fuel mix.

So smaller jet may resolve it, other option after that is to consider exhaust system layout bearing in mind the original purpose/need of the 2cv balance pipe.