Ignition or Jets?

[seafeye]

I having a problem with my car spitting at 4000rpm.
It idles fine. Accelerates fine. Doesn’t run hot. But under constant steady throttle at or around 4000 rpm you can hear it spitting.
Weber 40’s.
I’m running a 2.7l twin plug. Not sure of compression. No records.
JB racing twin plug distributor. MSD ignition I think they set the advance to 25? I don’t remember it was so long ago and my paperwork doesn’t mention anything.
Exhaust is headers, no heat with a stainless muffler.


Ideas on where to start?

[icarp]

Install an AEM wide band air /fuel meter just after the collector .
that is the only accurate way to measure your mixture , unless you are a weber carb expert!
Start with that. sounds like the air corrector is too big

[Steve@Rennsport]

First, you need to know what your jetting package is. This means finding out what idle jets, main jets, emulsion tubes, air corrector jets, and venturies have been installed.

It sounds lean, BUT you need to know what you have before making any changes.

Beyond that, I would do some dyno runs with a wide-band AFR to see what your WOT & PT cruise AFR's are. Once you know what jets you have, you can carry some different ones to the dyno session to make the time constructive.

[tom1394racing]

I have had intermittent problems with idle jets plugging on my 3 liter twin plug with PMO 46’s. Very similar symptoms. When I pull the idle jets, blow them out with compressed air and use a needle to clean the orifice, the problem disappears.

Worth a try. 10 minute fix that has worked for me.

[Gordo2]

If it ran correctly before / the problem has recently surfaced - I agree with the previous post recommending starting by cleaning your idle jets.

[Trackrash]

I would put a timing light on it just to make sure everything ignition wise is working correctly.

If this started after the motor sat a while, cleaning the idle jets would be prudent.

[seafeye]

Ok. Got the timing light and syncometer. Took the tops off the carbs.
Still trying to figure out how to get advance to show on my light gun. Right now it shows 0.0. RPM works great. Bought the: Innova 5568
Main Jets are 140
Venturies are 34
Air correction looks like 180

[Steve@Rennsport]

What size idle jets??

[Trackrash]

Hook the timing light's input on the #1 ignition wire.

You also need to determine what emulsion tubes you are running.

I believe 140 main jets are too lean, especially with the 180 airs. Try 150 mains, and you may be able to use 160s.

The only way to really get it right is to use an AF gauge.

Have you pulled a plug? What do they look like?

[seafeye]

Hmmm 60 or 09

[seafeye]

Number 1

[seafeye]

Cleaned out the idle jets. Seems a bit better but still getting a bit of spitting. Should I go to 150 for the main Jets?

[TimT]

Its been awhile since I fiddled with carbs.. but.. I recall the system doesn't progress from the idle circuit to the mains until around 3000-4000 rpm.

And the emulsion tube selected is a pretty key piece of the puzzle..

This should be required reading to learn about setting up Weber carbs

I have to relevant Weber documentation that I have scanned before, however its on a dead hard drive..

[ed mayo]

Did this occur after you installed that muffler ?

[seafeye]

Went to 150 for the main Jets and it seemed to have cured my spitting and popping at 3000-4000rpm.
Still got a little burping at idle waiting for the light to turn green. But that’s about it.

Thanks everyone for their input.

[Mark Henry]

Something even more basic is make sure you linkage is properly sorted. Both carbs should open at exactly the same time and position from idle to WOT.

Weber linkage isn't too bad, but I make my own drop links (better ball links and one arm has a small S bend) because I want the drop link pushing at exactly 90 degrees from the throttle shaft. Besides solving linkage opening sync issues this stops side thrust on the throttle shaft, which causes premature wear to the carb.

[seafeye]

Great idea. Gives me something to do tomorrow.

[faapgar]

F24 tubes are good for your setup.I am in Pinehurst.You can drive up and I would be glad to help you.Fred