Weber Backfiring

[jhirasak]

Jim,

Before you start changing emulsion tubes to correct your over rich idle and main circuit transition situation, I would suggest that you make sure that the idle circuit is properly jetted since it affects the fuel/air ratio during the transition to the main circuit.

You mentioned that it appeared to run fine while it was cold but began to backfire and pop after it warmed up. If the idle circuit was jetted correctly, it would cough and spit back out of the carbs when cold and then behave normally after the engine has fully warmed up. The first thing that I would try was to incrementally decrease the size of the idle fuel jet until it idled without any hickups when hot. Having done this, I think that your rich main circuit transition situation will have improved. Once the idle circuit performs properly, then you should go to work on the main circuit and emulsion tubes.

By the way, it is a lot cheaper and easier changing idle fuel jets than emulsion tubes.

Just my 2 cents.

John

[James Stern]

John,

I'm sorry I am not responding all that quickly. I was away for 2 weeks and didn't see your response. I got a 2nd set of idle jets (55) and main jets (117). More importantly I looked on Jim Inglese's site and turned my fuel pressure down to 2.7 lbs. Now my main problems are;

1. It spits back through the carbs

2. It feels very sluggish until it hits the main jets 2700 rpm and away you go.

Any thoughts?

Jim

[jhirasak]

Jim,

If you have made as large of change in idle jets and main jets sizes as you indicate in your last message, it is no wonder that you are having tuning problems. It does not take much a jet change to get significant difference in response at various ranges of engine operations. Sometimes, you can just make an adjustment to the idle mixture screw and clear up problems in your transition performance. You need to systematically make one change at a time and note how it affects your air/fuel ratio as well as engine performance. Based on those results, you should make the next incremental change to determine whether you have improved the situation or made it worse. Without a record of each change that you have made along with the performance difference associated with that change, I cannot provide you with any guidance on how to correct your problem.

John

[zrayr]

Originally Posted by James Stern

John,

I'm sorry I am not responding all that quickly. I was away for 2 weeks and didn't see your response. I got a 2nd set of idle jets (55) and main jets ....."

Jim

That seems way lean to me. I'd try 65 idle with a 100 or 110 idle jet holder and a 135 main jet ( at least, if not bigger)

It's a BIG HELP to get a decent wideband A/F meter, then you see exactly where the problem is. That will save you a lot of time. I use the AEM unit.

Z

[Gaz64]

Jim,

You have dropped your idle jet from 70 to 55 in one hit?

Way too much percentage change.

A 55 jet is 62% of a 70 jet.

Obviously a 70 was rich, a 55 is lean. A 60 to 65 is what you need for fuel.

So get a number drill set and drill your 55s to #74 and work up, until you find your driveability is acceptable, watch your A/F meter while tuning.