I have been adjusting the SU carbs only to find that at times the fuel/air mixture "backfires" and is ejected out of the throat of the carburetors. I just rebuilt the carbs and field adjusted the float tabs. Also, I went to Autozone and replaced the rubber tubing from the float bowl to the jet. The new tubing has a somewhat smaller inside diameter than the originals. Could this be my problem?

I don't think the fuel lines from the float bowls are the problem of your backfiring through the carbs. Honestly, I don't even have an answer for that one. But I do know that you need to be careful when buying fuel line for these particular areas.

The original fuel line coming off the float bowls were very flexible and allowed the line to make the radius curve it needs to make going to the carb. Most typical fuel lines from autoparts stores are stiffer and will tend to kink, cutting off some or all of you fuel supply. Not a good thing under acceleration.

Z-Therapy has the correct fuel line for the SU's and it's only a few bucks for both lines. When I received mine from them, there was a noticable difference in the flexibility of the lines. It was almost like using clear tubing, it was that flexible. And of course I have no kinks in my lines now.

Could it be the use of ATF fluid instead of 2 in1 oil?

I don't think it's the ATF in the piston dash pots either. Too many people use and recommend that fluid to often. Did you check your timing and valve clearences before you checked out the carbs? Also check to make sure your vacuum advance is working correctly.

Another thing to check is to make sure the carbs are in sync.

I had that problem on my flat tops after I'd rebuilt the head... Turned out to be the vacuum advance had been hooked up on the wrong part of the manifold. Once I disconnected it and put it on the right fitting it sorted itself out and ran smooth as.

I sounded really bad and smelt worse while it was doing it though and usually stalled the engine.

Cheers,

Ross.

It sounds like a lean pop , if you blip the throttle to wide open and let it go back to idle , is that when you get the pop ? If so just richen , turn the mixture adjustment clockwise about a 1/4 turn and try the test again and repete untill it no longer pops. As far as the fuel line get the line from therpy , it needs to be vary flexable because when you pull the choke lever the gross jet needs to drop and that is what is connected to the other end o f the line , if the line is to stiff it will bind the unit and it may stick in the down position and will dump unwanted fuel into the engine. You probibly already know all this but for those that dont I wanted to mention it all. All the best of luck.

The fuel/air mixture ejects from the carburetor throat when I "race" the throttle. Do you know the approximate number of turns the mixture nut should be backed off from it's "set" position? I have heard various information.

The standard mixture screw "Backoff" is one and one half turns from the completely closed position.

In other words: tighten the screws completely against the carb body and back them off (counter clockwise) 1 & 1/2 turns as a starting point for your sync and adjustment process...

Keith

I forgot to mention that is for the SU type Hitachi carb.

sorry something went wrong.

( new at this forum )

Edited November 29, 201014 yr by Robin260z