SU's got me stumped

[steve91tt]

I got the new dome and slide from Z Therapy today (excellent customer service!). Swapped the needle over to the new unit and the assembly installed without drama. The only difference that I noticed was that the new slide seems to drop with a louder clank than the old one. I would say the old one make more of a click than a clank as it hit the bottom of the carb. It's not a big difference at all but there is a difference.

The car runs great, exactly as it did 90% of the time before the swap. If I can put a couple of hundred miles on it without the slide sticking then I'm going to call it solved.

I've got my first track day coming up in January so hopefully I can get all the bugs worked out before then.

[Gary in NJ]

A solid "thunk" is a good thing. Hopefully you're on your way to trouble-free SU's.

[steve91tt]

Looks like I spoke too soon.:disappoin

After I wrote my last post I went out to put a few more break in miles on the motor. I warmed it up and accelerated fairly briskly to 60 MPH. The car ran very strong, perfect. However, as soon as I pushed the clutch in to down shift for a traffic light it stalled. I stopped for the light, started the car but it wouldn't keep going without throttle input. I drove it a mile or so to the next parking lot, pulled over and the car was idling fine again. It then ran perfectly all the way home.

Basically the same symptoms as before. Random stalling mixed with long runs of perfect operation.

Looks like the new dome and slide did not solve my problem.:paranoid:

Edited December 16, 2010 by steve91tt

[olzed]

Amazing service from Z Therapy. Sure hope your cars fixed Steve. My 72 runs great most of the time and pulls strong, except on hot summer days, which we are experiencing at the moment. The car runs OK on a journey,and if it's raining , but when it's hot and sunny, doesn't like tooling through the city and stopping at traffic lights. It will give a couple of coughs when I move off and then smooth out again. Could this be fuel vapourising.? When I touch the carbs they are very hot. Is this normal. ?

Brian

OOPS We must be sitting at the keyboard at the same time.

Sorry to here theres still a problem. Might be spark issues . Do you run origional coil/ distributor?

Edited December 16, 2010 by olzed

[steve91tt]

Do you run origional coil/ distributor?

I am running the original distributor with a pertonix coil and pickup. This system is hooked to an MSD box.

I know that the slide on the front carb was sticking (I saw it, during a recent stalling event). I won't know if the new slide and dome has solved this problem until I can get it to stall and get under the hood before it corrects itself. I had assumed that this was the only issue but I guess that it is possible that there is something else going on like an intermittent spark that is making the problem hard to diagnose.

When the car acts up I am always able to keep the car going with throttle or choke. Would this be the case if it were a spark issue?

[olzed]

No, you are right Steve. If it was a spark issue , using the choke or throttle would not keep it running. At least I wouldn't think so.

If the slide is stuck open that carb will be running richer than the other one,

so would not the revs increase, or that carb be flooding? Perhaps not, as the butterflys are still shut.

I'm no help with electronic ignition. I'm still running the points setup in my 240.

Brian

[LeonV]

No, you are right Steve. If it was a spark issue , using the choke or throttle would not keep it running. At least I wouldn't think so.

If the slide is stuck open that carb will be running richer than the other one,

so would not the revs increase, or that carb be flooding? Perhaps not, as the butterflys are still shut.

I'm no help with electronic ignition. I'm still running the points setup in my 240.

Brian

That's incorrect.

FWIW, for those unfamiliar with CV carbs, if the piston stays up you lean out the mixture. With more cross-sectional area velocity decreases, therefore deltaP decreases and less fuel is drawn from the jet.

A sticky piston will surely stall the car out, you just have to figure out why it sticks. Make sure the dome screws are equally tightened and not over-tight.

EDIT: here is one link describing the operation of SU carbs.

Edited December 16, 2010 by LeonV

[olzed]

Well thats something I never knew. The saying 'never too old to learn is true'

An interesting link.

Edited December 16, 2010 by olzed

[LeonV]

Well thats something I never knew. The saying 'never too old to learn is true'

An interesting link.

Definitely interesting, SU-style carburetors are a simple yet ingenious design. They take the opposite approach when compared with your typical fixed venturi carbs, using one "jet" and varying the choke size instead of utilizing different jets (circuits) with a constant choke size.

[FastWoman]

So if you're replaced the slide assembly with no luck, perhaps the needle is hanging on an irregularity in the hole? Or perhaps the hole misaligns with the needle? (I'm not sure I'm using the correct terms here.)

[steve91tt]

So if you're replaced the slide assembly with no luck, perhaps the needle is hanging on an irregularity in the hole? Or perhaps the hole misaligns with the needle? (I'm not sure I'm using the correct terms here.)

Could be. Or maybe dirt in the nozzle? The needle is not bent as far as I can tell. What ever is causing the problem seems to come and go. It only acts up maybe once ever 50 miles or so.

I'm going to try to get a hold of Steve a Z Therapy this afternoon as see what he suggests as a next course of action.

[FastWoman]

If there's such a misalignment, I wonder whether it happens only when the piston is forced in the direction of air flow. Perhaps you could find the problem by sliding the piston up and down while gently applying sideways pressure to it with your finger through the carb's throat.