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

[30 Ounce]

Have you checked the grounds to the engine & body?

[Captain Obvious]

Are there ignition phenomena that would manifest itself in a way to cause the issues you're having? Doesn't misfire at idle, doesn't misfire at WOT, but does misfire at part throttle.

What could cause that?

[Zedyone_kenobi]

The good captain asks the 45000 dollar question.

The partial throttle nature of this means it must be vacuum dependent. As Blue suggested, the vacuum inside the manifold, or rather the Delta Pressure from the inlet of the carburetors to the inlet of the cylinder head is greatly dependent on the two things.

1) The engine speed

2) The position of the butterflies in the carburetor.

If the butterflies are held constant, and the engine speed increases, the vacuum must go up as the engine is sucking more and more air into the cylinders. If this were happening normally, the SU would allow the piston to rise keeping the velocity constant over the jet needle orifice and enough fuel would come into air stream for close to stochiometric operation.

It would seem though that at high vacuum, my pistons are rising up too quickly (remember I am running 10w-30 in the damper now, but lighter fluids made this problem MUCH worse) and the velocity over the jet is not great enough to draw up the fuel. At WOT, the delta P is not so great across the carb and the piston rises more slowly up the dome.

I am going to try some thing completely crazy. I am going to put in straight 40 wt in the damper to see if I can force it to suck WAAAY too much gas into the engine. That will be done today if I can. The FSM says do not run straight 30wt. But at this point, I want to try something else.

I have not traced ALL the grounds. The connections to the coil are all taped up from the factory still and have not been messed with. I will check those out soon, but I still think this is carb related more than electrical, since it seems throttle position dependent.

[madkaw]

I think your experiment is only going to prove the transitional-not a steady cruise scenerio.

...and if your going to deviate from the FSM, then go in there and set the needles FLUSH with the groove on the piston. This can only help your cruise mixture-and try that on the road. Then go really crazy and richen up those floats. I won't tell anyone;)

Edited January 30, 2012 by madkaw

[Zedyone_kenobi]

Well my steady state cruise is actually pretty good.

What worries me is that factory settings SHOULD work for this motor. I feel like deviating from those may just be masking a deeper issue, so in a sense, I am not fixing anything, I am just covering it up. I really want to FIX this.

With that being said, I will try the oil first, as my idle mixture is perfect and scooting up the needles will richen it up, but at this point, nothing is off the table.

Thanks so much for putting up with me for so long guys.

[motorman7]

My Theory:

The needle and seat relative position at steady state condition at 4500 RPM+ is not the same for your two carbs. One is running too lean. The other may be just right. I do not think new springs will fix the problem.

So what can cause this disparity? There are a number of items. Difference in float bowl height would cause this assuming both SU pistons rose the exact same height. From earlier posts, it looks like the float bowl levels are equivalent, so this most likely is not the issue.

The piston height in the vacuum chamber can vary based on the amount of air that escapes through the piston/wall interface with uniform manifold vacuum. An SU piston with a weaker seal will ride lower than one with a tighter seal at this interface. There are no ‘piston rings’ on this interface so there should be some very real blow by. My guess is that there is too much disparity there. The piston with the tighter seal is ‘riding high’ and running lean at higher RPM’s. That’s why car still has some power, although diminished as you are running 3 nominal cylinders (either forward or aft SU set) and 3 intermittent cylinders as some fire with the higher fuel ratio and some don’t.

There is a balance tube on the manifold between the two SU carbs, however, at the higher RPM’s I do not think it has much effect on fuel mixture balancing. Actually It probably has no effect at all on fuel mixture balance. It is more for pressure/vacuum balance.

This leads us to the Oxygen sensor. The sensor measures the product of the six cylinders and cannot differentiate between the forward three cylinders and the back three cylinders. It can provide an overall result, but cannot tell you whether the front set (of 3) or rear set is too lean.

Based on the assumption of SU piston height disparity, an increase in the oil WT will improve your short term steady state RPM runs; but as the carbs manifold maintains a steady state condition, the pistons will again offset. This also helps to explain the poor performance of the low oil WT as the pistons achieve their offset condition quicker due to the reduced damping.

Solution? That’s a tough one as there is probably some difficulty in determining what piston chamber combo is running lean or rich. Maybe you can tell by plug color. Assuming this was evident from plug color, the lean SU piston edge could be lightly sanded (1000 grit) to help equalize the blow by. This would be a trial and error process. Pistons could be swapped within the chambers, this might prove easiest and easy to check. Those are just a few ideas off the top of my head.

Anyway, this is my theory. It has kept me pre-occupied a bit here so I thought I would write it down and send it forth. Please remember, this is a theory and not a law.

Best regards,

Rich

Edited January 30, 2012 by motorman7

[Captain Obvious]

I say skip to the end of the play and toss on a known good set of round tops and see what the afr says. That won't tell you exactly what's going on, but it should tell you if it's the carbs or not. Heck... You've got to be an expert at taking them off and putting them on by now, right? Half hour start to finish, max.

I'd even send you a set of round tops to try except that I've modified mine and they wouldn't fit on your car.

I know it's a little bit of a cop-out and you've got to admit partial defeat to resort to crude shotgun methods like that, but after thirteen pages, nobody's gonna give you static for taking the easy way out!

[Diseazd]

That's obvious Captain!

[Zedyone_kenobi]

HAHAHA, I needed that. I am looking into buying new damper springs, but I first want to say Thanks to each and every one of you again. I have said it before, but the words do not seem enough to show my appreciation for how much effort all of you have put into my little 'issue'.

Motorman, that theory did occur to me. I made a great video of me performing the piston drop test. The test where you install the pistons in the dome with the damper attached and plug the sense holes in the bottom of the piston and let them drop. They dropped darn near identically, but I did not time them, just that they dropped at the same time. Which means air is rushing by them at about the same time.

[motorman7]

Motorman, that theory did occur to me. I made a great video of me performing the piston drop test. The test where you install the pistons in the dome with the damper attached and plug the sense holes in the bottom of the piston and let them drop. They dropped darn near identically, but I did not time them, just that they dropped at the same time. Which means air is rushing by them at about the same time.

I think on this test you would want to run it without the dampers. The dampers just effect your ramp up and ramp down. At constant speed, the dampers don’t do much other than maybe minimize oscillations. They mask the difference in the ‘blow-by’ on your test.

I was wrong on the springs not helping, kind of. If you were to put a spring with a higher K value in the dome with the better (assumed) seal, this could compensate for the Delta P within the chambers.

This really is a GREAT thread by the way. Engineers love this stuff….seriously.

Best regards,

Rich

[Zedyone_kenobi]

I will try to crop my video short enough to post!

I agree, this thread will live on for years hopefully helping others with this problem as well. I am making a definitive 'WHAT I HAVE DONE LIST' for the next guy. Maybe I will write it as a sticky.

[Travel'n Man]

Did you call Z Therapy and run this occurance by them?