Backfiring

[howardjs]

My engine; 2CV crankcase fitted with Visa barrels and heads, so valves are bigger that the 2CV.
Ignition is using a DG unit (similar to 123)
It is fitted with twin Dellortos

After the conversion with Visa barres the engine ran well, gradually improving as the new piston rings bedded in.

However, some time ago (2 years)! It started to backfire on the overrun, not the occasional pop but AK47 stuff!

The mixture looks OK, plugs a nice straw colour, and the exhaust system is new with no leaks.

I have just tried increasing the Static timing from 8deg. to 10deg. (10deg. is the recommended setting for a Visa) This has improved things a little, it now sounds like a silenced AK47 but it is still backfiring.

Any ideas?
Howard

[Jack602]

Howard,
I am fairly confident that bigger idle jets will fix it.
Straw is fairly weak and even then may not represent the mixture at idle .
It is probably no coincidence that around 2 years ago is when they started adding ethanol to petrol that requires a richer mixture.
I had the same problem, at the same time, slightly larger jets diminished the problem, an even bigger jet fixed it!
Regards

Jack

Pembleton SuperSport

[howardjs]

Thanks for that info Jack, Mmm could be as you suggest as this issue got a lot worse in France where maybe the petrol has moor poison in it!

Does you car have PHBH28BS Dellortos? Mine came from Eurocarb ready jetted for a 2CV and were OK initially.
If you have these carbs do you know the part number of the jet that improved your backfiring?

Thanks
Howard

[Jack602]

Howard,
I am running a standard Solex, however my symptoms & timeline (including France) were the same as yours.
For the benefit of others running the standard Solex, after trying various jets I ended up with a 47.5 idle jet (co-incidentally also from Eurocarb).

Regards
Jack
Pembleton SuperSport

[neil curson]

Hi Howard. As you saw on the run I have just fitted the Burton 652 kit, and they supply bigger jets for Solex carb.

[howardjs]

Backfiring now eliminated!
Twin Dellortos PHBH28s

Also had a flat spot from idle to 1500rpm. After consulting Eurocarb they advise fitting smaller idle jets, they sent me a pair of size 30 (the smallest). This seemed to me to be the opposite way to go, but they are the experts... this made the problem worse. So I went the other way.
The original jets were 36 so I ordered 46. This was a total success, the flat spot is gone... but so is the backfire..

Howard

[notsmartcar]

I’m working on my triumph bike at the moment with a Keihin carb it has an idle jet and an air adjusting screw ..an air screw has a working range fully in and to let’s say 3 full turns out if by the time your more than 3 and it makes no difference and you got popping on the over run or red hot header pipes it’s time for a larger idle jet to bring the adjuster screw into a working range of 3turns.

[motomallett]

Hi Guys i constantly read about the problems with twin carbs on our small cc engines. I converted mine with an SU HS2 carb red spring and gg needle the car runs great right up to max revs and returns approx 60 miles per gallon. Are we trying to hard to expect miracles from these engines. Perhaps the answer is a bigger engine ???????. just another line of thought regards Roger