Carb acceleration problem

[Zanetti]

I'm wondering if anyone here has experienced the following or if anyone has any thoughts on the subject...

From idle my S2 1300 revs freely to 6000/7000 rpms no problem, but when under load as in when out on the road she has a massive flat spot accompanied by some backfiring I think through the carb, ie. lean backfire.. my first thought would be accelerator pump but looking down the throat of all 4 barrels the is definitely a nice squirt of fuel when pushing the throttle.

I've heard the squirters can be removed, is this so as I can't see how.

Ignition system is ok with a new Bosch Blue coil...

Comments?

[lancialulu]

Valve timing? Ignition timing (retarded)? or poor condenser??

Did this just start to happen??

Is the fuel pump giving good fuel flows when under load??

Tim

[Zanetti]

I changed the carbs with another pair from another Fulvia that I have and she ran no problem, no hesitation or flat spot at all, which tells me that it is definitely a carb problem.

The timing is set at 8 degrees (it's a 303) first statically timed the checked with a timing light.

I did notice that adjusting the idle mixture changed the response a bit when pushing the throttle flat at idle. It went from no response to a smoother response but not 100%.

[Ed Levin]

I'd say it's definitely a carb issue. But where in the RPM range is the flat spot?

[Zanetti]

It appears to be anytime I accelerate hard, ie push the pedal down quickly.

When I slowly and progressively accelerate the symptoms are there but much less pronounced, which brings me back to the accelerator pump.

Is it possible that the duration of the squirts are too short? I know there is a volume figure that the plunger should deliver...

[niagra]

Could it be lack of use? When I bought my car there was a bit of hesitation under load, especially when accelerating hard from a constant speed. It wasn't used often and since I've had it I've been using it regularly and it's pretty much cleared it up.

Dario

[Zanetti]

The more I think about it the more I believe it to be an accelerator pump problem.

The question is: What is the correct way to check its stroke and when it comes into play (if this can be checked)?

[lancialulu]

I am inclined to think if the accelerator pumps are squiriting as you say they are then this is not the problem.

Tim

If these carbs are off another car then they should be opened up to see what corrosion has taken place, and also float height...

Tim

[Ed Levin]

It sounds counterintuitive, but I wonder if it's possible that the accelerator pump jets are too large. Excess fuel could be causing the mixture to go temporarily much too rich, which will indeed bog down the engine until the revs increase enough to bring up the air speed through the venturis.

[Geoff]

There are far more knowledgeable people out there on carbs and tuning, but my guess is not the accelerator pumps. They are the most direct and clear aspect of the carbs - and rarely cause big flat spots.... as whether they give a bit of fuel or a lot of fuel isn't usually the big culprit in these things. There is another part of the puzzle here to be unravelled.... I'd go through all the other parts of the carbs - maybe check the jet sizes against the book, and what came off the car. Could be that something is off, and the moment of more load and more throttle opening is just sending the whole setup off whack.