popping through the exhaust at 4500 rpm or so.. HELP!

[Captain Obvious]

I'm no carb expert either, but when you're talking about transition/acceleration issues with the SU's the discussion traditionally turns toward the damper function. I saw above you are running standard issue SU oil from MSA. One would hope that is suitable. I assume you have verified that the damper oil level is up to the proper level.

Have you verified that the dampers actually work? I've seen some that look fine, but don't actually "damp". The check valve has worn or the shaft has misaligned to the point where the check valve don't check. Stick a finger up your carb and lift each piston. They should be hard to lift. Hard enough that it leaves an impression dent in your finger. Let it down and lift again. It should fall instantly, and be hard to lift again. And both carbs should be about the same.

You're running SM needles in a mostly stock 240, right? Have your standard needles arrived yet? It sounds like you're lean at midrange, but it would be hard to believe you're lean with the SM's in there. They are richer than the stock needles.

Maybe a plug chop at the offending RPM? Run it at your sputtering RPM for as long as you can and then do a plug chop. I don't think it would tell you anything if you're lean, but you might see some black fluff if you are rich.

[Zedyone_kenobi]

Great advice!

Thanks Captain! Just so you know my SU's are very hard to push up and they do leave a noticeable dent in your finger. Check valve okay.

My standard needles have arrived, but I am going to try to clean the damper and dome more thoroughly today. That really seemed to help out yesterday. It was Steve91tt idea, and I think it paid off.

I ordered an innovative O2 wideband yesterday as well. I will get that welded in and the guess work will be over when it comes to what is happening with my carbs.

[Captain Obvious]

Gotcha. Check valves ok.

I don't know what cleaning the damper and dome could possibly do to help your situation, but if it helps, then more power to ya! Maybe they are binding in there or something? Only binds when engine vacuum pulls the pistons toward the engine? I know... I'm reaching here, but like I said, I'm no expert!

As for swapping needles, I've found from experience that a small change can make a noticeable difference. I'm running custom modified flat tops and have tried a couple different needles. You'd think that one thousandth in diameter wouldn't make that much difference, but from experience, I can tell you that it really can.

You should notice a difference between stock and SM needles. The SM needles are .001 to .0015 richer than the stock (N-27) needles at mid-cruise, which is where you are probably running at steady cruise 4500 RPM. In other locations on the needle, the SM's are even richer than that when compared to stock. Short story? If you end up being lean with the SM's then I don't know what to tell you. Shouldn't happen...

All will become clear once you get the wideband installed, right?

[doradox]

If you want to see if you are too lean at the popping rpm then lean on the choke while in the zone. Explore some of the boundary conditions. Go a turn (or two) rich or lean on the carbs and see what happens. Take all the oil out of the dampers and see how it responds, then try it with really thick oil. You seem to be stuck on making very small changes from what is supposed to be "correct". Your system should be insensitive to small deviations from nominal or else these cars would rarely ever have run correctly. If you want some data points that rise above the noise then make some BIG changes and see what happens.

Steve

[30 Ounce]

I agree with Steve: take the oil out of the dampers & give her a spin. You mentioned earlier that they were difficult to push up. There should be some resistance but you should be able to raise them completely in about 2 seconds. If it runs better through the range you are having trouble (it will stumble a little off idle because the oil isn't there to damp it) then try Marvel Mystery oil in them. It's the lightest oil I've found and will cure the off idle stumble. After that try 5 wt fork oil. Then 10 wt. I'll bet this will solve it.

[madkaw]

I'll throw in here again about the SM needles. Running a wideband I have run both types of needles on my MODIFIED L24. The stock needles supply more than enough fuel for my motor. I am running stock needles now and the AFR dip into the 12's WOT and the plugs look beautiful(until the headgasket blew;( ) I believe it is also critical to get the float levels correct in conjunction with the needles.

I will also agree above that the changes need to be more drastic to tell if your going in the right direction.

[Zedyone_kenobi]

about 2 seconds is what it takes to push them up.

What I did find that both my screw valves that act like a damper in the SU oil are both exactly .348" plus or minus many half a ten thousanth. Very close to each other.

I took off the first carb dome today and cleaned the begezzus out of it. I used some gun cleaning towlettes and a good thin plastic pushrod. I cleaned every where there is metal to metal in close proximity.

It went together and the piston acts like a mirror at all times to the rear carb piston with the exception of one thing. WHen I push the piston ALL the way up, it took the front piston a good 4 or 5 mississippi's to come down.. while the back one dropped right away. In an any other position, the pistons dropped identically, but all the way up, the front SERIOUSLY lagged the back.

This may explain why my car only stuttered, at high rpm, at part throttle, and while changing throttle positions. At wide open throttle it would just go open and not have to adjust...

So long story short, I took everything apart again and cleaned it up including the oil that was from the overfilled front SU oil reservoir and whala, the pistons fall at the same rate even from the very top of the stroke.

I got my stock SU needles in but have not touched them yet. Only going to change ONE Thing at a time.

I have high hopes that this may have fixed things. Perhaps my tolerances on my piston and dome are such that any liquid contamination at all was making it hang up. This is going to be interesting. If this does not work, I will let you guys know, and then get that O2 sensor installed ASAP.

I will also try the NO SU oil trick and see how it acts.

[240260280z]

"and voila, the pistons fall at the same rate even from the very top of the stroke."

"Only going to change ONE Thing at a time."

Excellent maintenance work and test plan. The pistons are supposed to be matched in "drop times" so cleaning the walls and rim and also alignment when tightening the dome screws are very important for proper piston excursion.

Hopefully this (alignment correction) will fix your problem.

[Zedyone_kenobi]

Well went for a test drive, and it is still popping through the exhaust. I will recheck everything balance wise, and mixture wise in the morning.

I will also probably change needles, even though I do not think it will make a bit of difference.

[steve91tt]

Did you try running with the choke on (Doradox's idea above)? If the stumble is caused by being slightly lean at 4500rpm then adding choke should help. Just another data point.

[Zedyone_kenobi]

nope, I forgot about that, but I have tried that before. Funny though, I have no idea if it helped or not.. I do not remember.

[Zedyone_kenobi]

Okay folks, I decided to try to swap out my SM needles for some OEM needles from Z Therapy. I got the mixture at 2.5 turns, which is close and it is actually doing a similar thing as before, maybe worse, but it is close. I was still able to pull to red line if I floor it in second, but anything part throttle just pops up a storm out of the exhaust.

I am going to pull the float bowls this afternoon after the car cools down.