Carb acceleration problem

[Ed Levin]

It's entirely possible it has nothing to do with the accelerator pump jets. But I do think the flat spots are being caused by too much fuel causing the mixture to go much too rich. When a "flat spot" happens with sharp acceleration at any RPMs, it could be accelerator pumps, but it could also be the float levels, as Tim suggests. In any case, I think it's too much raw fuel, rather than too little.

[Geoff]

+1

[bart]

I used to have the same, and also thought is was too much fuel.

Then I welded a M18x1.5 boss in the exhaust and (temporarily) fitted a wide band lambda sensor. A whole new world unfolds itself! I had too little fuel! I also had a mixture that was too lean at low rpm part load and too rich a high rpm part load. Full load high rpm (which off course you rarely use) was ok!
A modern lambda sensor, fitted temporarily, solves all these questions instantly and is a great help with tuning!

Bart

[ncundy]

I'm interested in your solution Bart. How did you measure the output of a wideband sensor? I've always been led to believe that you needed a translator with a wide band (5 wire) sensor, but with a 3 wire sensor you can use a standard oscilloscope.

Did you test it on the road?

[lancialulu]

broad band fulvia tuning now theres a mouth watering concept....

Tim

[Geoff]

In dyno testing (rolling chassis) they sometimes stick a probe in the exhaust pipe to measure fuel:air ratios. Is that the same as the Lambda sensor up by the motor? Its pretty cool information - tells rich vs. lean along the rev range.

[Ed Levin]

A tailpipe probe is a lambda sensor. The only difference is how far downstream you're sampling the % Ox.

[Geoff]

thanks for this. Clearer now....

[Zanetti]

I also had considered that... and there seems to be something to it! So after a fair bit of tuning (idle screws) she is running substantially better B), not that she wasn't running "well" prior to that. The flat spot seems to be mostly gone... and yes it did mean turning the screws in about a 1/4 to 1/2 turn in, so I'd say definitely was too rich. Strange though how when she idling previously she was smooth and revved well...hmmm...

Thanks for all the replies!

[Ralph deMasi]

Zanetti,

So on May 5th you post a tutorial on taking apart and cleaning Solex carburetors and then on the 12th you say you are having problems with the carbs.

Did you take apart the carbs prior to having the problem?