Bad backfire

[royrender1]

HI all, after 32 years my 1968 +2 is back on the road but as you pull away it really back fires. I have done all the usual plugs, leads, cap, coil, timing. Carbs rebuilt still the same, I have a new luminition optronic which I have sent back to be tested, I have also checked all the earths. It starts well and ticks over perfect. as you can see Im a newby here, Ive been reading this site for a few weeks now and I must say how helpfull eveyone is.
ROY.

[nebogipfel]

Roy,

I'd check for vacuum leaks, likely candidates are the "O" rings in the carb mountings or probably very likely cracked or leaking plumbing on the headlamp vacuum system.

[SADLOTUS]

Hi Roy, i did all the same as you ...then found the carbs were not balanced - not opening together (in fact the man at the rolling road found it).

ran fine at tickover and full throttle.

...could be that

[leifanten]

Carb balancing and vacuum leaks would a good place to start
Check also
compression
valve timing (you got the two little notches in the sprockets pointing nicely to eachother at TDC)
coil- an overheating coil will fail you under load
quality of fuel - if it has been off the road for that long......
charge voltage- the electronic ignition can throw some gremlins in your joy if it does not get high enough voltage

good luck, you will find out what is wrong I am sure.

[billwill]

You left out ignition timing

ISTR that that can cause backfiring too.

[leifanten]

If stromberg: check mounting height of jet and needle.Jet at 2.1-2.2 mm below deck and needle shoulder flush with air valve piston works for me.

[JJDraper]

Check the fuel supply - restriction on fuel hose (collapsed?), float height (if Webers), fuel pump pressure etc. My +2 had similar symptoms - fine at tickover, but started to misfire & backfire as revs increased or you put any load on the engine, such as pulling away). I traced the problem to a kink in the fuel hose, restricting the flow rate to the carbs. If you have Webers, keep a screwdriver in the car ready to whip off the carb tops and see how much fuel is in the bowls after the car mis-behaves.

Worth finding a quiet stretch or road to conduct a test, where you can stop and carry out the examination as above without being flattened..

Jeremy