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.

[*13*]

I'm running 60-110 on a pretty mild build(small block)

[James Stern]

Thank you to all who have responded. I have ordered the two intermediate idle jets (60 & 65). I expect to get them next week. I will report on any progress made.

[James Stern]

I ordered a set of .060 and .065 idle jets and put them in for an immediate difference. Jim Inglese is spot on about the fuel psi being between 2.5 lbs and 3 lbs. Really important. A little polite burping and I can live with that until I get it to my mechanic. He has webers on his Mustang and never had a problem. Thanks to all of you have contributed.

[James Stern]

Does anyone have a spark plug recommendation for webers on an small block?

E303 cam
48 IDA's
302/331

[*13*]

I'd have to look at what I'm running. Don't remember now. Nothing over the top fancy or anything, I know that. You having issues?

[James Stern]

13,

The current plugs are showing some carbon build up with just a little running. We think we have everything pretty well set with the carbs. I am currently running Champion RC 12YC. Any thoughts?

Jim

[*13*]

I only have experience with my setup. Carbon build up seems to indicate that maybe the carbs maybe aren't quite dialed in? Have you cleaned the plugs & checked gaps since you got everything set?

[zrayr]

I used Autolite 45's. They worked fine.

Z

[carmine]

What fuel pump pressure are you running

[James Stern]

Carmine,

We have gone through 4 gauges so far. Currently we have a dry gauge that default reads 2+ lbs. We have a Jim Inglese fuel pressure regulator that should give a max of 4 lbs . The current reading is 5 lbs (deduct the 2 that are the default reading) for the adjusted reading of 3 lbs. I hope this makes any kind of sense.

Jim

[James Stern]

13,

We have cleaned the plugs and replaced them.

Jim

[carmine]

Yes good. 3 lbs min. 3.5 max
What idle rpm you running?

[*13*]

I set mine at 2.5 & have a shut off valve instead of a return line to the tank. I was going to replace my gauge last year but, decided to take it apart to find out what made it tick. There really wasn't anything that can go wrong with the gauge unless it flat out broke. I reinstalled it & reset the pressure at that time. Dropping from 3.5 to 2.5 & having the shut off valve cured a lot of problems I was having.

[James Stern]

Originally Posted by carmine

Yes good. 3 lbs min. 3.5 max
What idle rpm you running?

Carmine,

We are running 800rpm.

[James Stern]

13,

Please tell me more about your shut off valve vs the return line. We are running neither a return line or shut off valve. When I get to the point of engine shut down I turn off the electric fuel pump and run the engine to reduce pressure in the line.