Maybe not a Weber problem - popping at 3,000

[Quickstep192]

After a great deal of experimentation several years ago, I came up with a Weber configuration that worked well in my car....... until now. This setup has been in my car for several years and until now, I could adjust it to run like a top. On my inaugural drive this spring, I noticed popping when driving between 2600 and 3200 rpm. It's worst at 3,000. This happens when I'm just cruising along without too much throttle. When I hit the gas, the popping goes away, but of course the rpm goes up at that point too. I've been tinkering with it ever since, and I just can't seem to tune it out. I've cleaned and rebuilt the carbs and still have the problem. I'm reluctant to re-jet, since I know this configuration has worked perfectly in the past. I'm not the world's best mechanic to begin with and this has me stumped. Any ideas?

[jpnovak]

Popping in intake or exhaust? If exhaust, look for an exhaust leak. You might also have a stuck float or bad float valve that is sending excess fuel into the exhaust stream where it ignites.

If intake... Ethanol gas? If so, you need to jet slightly rich to account for the difference in stoichimetric. You should be able to use the idle mixture screws to adjust this out.

[Joe Bob]

Assuming the fuel delivery pressure is good, fuel filter is good, no intake/air leaks, a gas analyzer is a good idea.

Fuel is not the same as when you first set up your carbs. If you've had no issues the last couple of years, you've been lucky. Carbs aren't like FI where you can set up and forget....if you don't have the Weber carb book, it's a good read.

Good Luck.

[Quickstep192]

Thanks for the reply.

I'm pretty certain it's in the exhaust.

If the float was stuck or bad, keeping the float valve open, would the float level gauge overflow when I check levels?

Also, can you help this novice better understand how an exhaust leak would cause popping and where to check?

[1QuickS]

3000RPM is the heart of transition from progression onto the mains where progression circuit is decreasing in its fuel delivery and main circuit is beginning to become active. Since popping goes away when you accelerate it implies a lean condition since the accelerator circuit squirts raw fuel which richens mixture.

Perhaps a larger idle jet will help (readjust idle mixture screws) or you could try adjusting float levels a little bit higher which will speed up fuel delivery from mains.

[Joe Bob]

A wet hand run under a running engine can feel a puff of air.....

Look for loose bolts, blown out gaskets where the muffler meets the heat exchangers, the exhaust manifolds at the heads, holes in the muffler....holes inside the HEs are harder to find....

Reving the engine and listening for the pop can help. Sometimes the pop won't happen unless the engine is under load and mostly on decel.....popping on accel can mean a lean condition.

[1QuickS]

Floats need to be verified for correct height with engine running; this is a dynamic fuel delivery check valve and static setting will not replicate actual minute opening/closing action of the needles. If you moved the needles from one location to another then there is the real possibility that you have disrupted their "happy place" and the divots they created on the float tabs won't be correctly positioned when needles are swapped around; this causes side loading of the needles with resulting sticking. If your needles are overflowing you WILL notice this as black smoke out the exhaust. You will also notice during float level adjustment as you opined.

Exhaust leak allows air into the muffler where it can mix with unburned fuel and explode or backfire.

If you adjusted floats using the gauge and fuel level is between the top two lines and verified with the engine running then it would be rather easy to uniformly add a 0.010" shim under each needle and go for a test drive.

Check for obvious leaks around exhaust as Joe Bob mentioned; intake leaks not as probable since the manifold vacuum at idle is far greater than at 3K RPM.

[T77911S]

check for air leaks. manifold gaskets, rubber hoses.
check timing and make sure the advance is working

[Quickstep192]

An update.......

I didn't find any leaks.
Float levels seem dead on with the meniscus centered between the two lines on the gauge.

Even though this set up has run like a champ in the past, I thought it might be a good idea to run it by you guys.

Here goes:

Idle Jets are 60
Main Jets are 130
Emulsion Tubes are F-26
Air Correction are 185
Venturis are 30
Plugs are Bosch W8DCO

The engine was original a 2.4, re-built to 2.7 by the PO. Still has T cams.

Ironically, other than the popping, it sounds pretty good, but the popping sure is annoying. Again, this setup ran like a top in the past, but I sure can't seem to get rid of this popping now.

[1QuickS]

Have you synchronized air balance side-to-side at 3000 RPM?

Jetting sounds OK.

[Quickstep192]

It took me a while to get back to this, but finally some news to report.

Balance is good side to side at 3,000

I also did some other experimentation. Since the gas was old, I drained the tank and re-filled with new gas. I was thrilled to see that the gas that came out was clean and the bottom of the tank was clean too. As expected, fresh gas didn't have much impact.

One other thing I tried was to make the idle a lot richer. After getting it adjusted at idle, I backed out every idle adjustment screw almost a full turn. This made things a little bit better, but there's still popping.

Any ideas?

[1QuickS]

Pull your emulsion tubes and check for debris blocking their holes. While they are out, pull the main jets out and blow air down the emulsion tube wells so it comes out the main jet openings. I had a transition issue which I later found to be a tiny bit of paper towel that was partially blocking one of my emulsion tubes.