hi guys, i have a set of brand new SU's carbs on my car, they ran good for a while, but now they have started acting up.

The problem i have is that the car will run great until operating temperature or a bit above. I notice it the most when I get on the highway, or sustain rpm above 2000 for any length of time. as long as my revs are up, this problem persists, if I stop and let the car idle rough for a couple minutes it smartens up and runs nice again.

at first i thought it was the stock mechanical pump not being able to keep up with the 2.9 stroker. I saw a lot of air inside the lines, it was boiling the fuel in the lines so I decided to install a carter pusher pump at the tank. this resolved the fuel boiling issue but my car still isn't running right. I've thrown a good bit of money at it now and nothing has changed.

I have checked and rechecked the carbs for dirt, engine bay fuel filter was just replaced, and last chance filters are spotless, fuel pressure is right, bowls are staying full, and float levels are correct (I checked them right after it started acting strangely the last time)

2 things that i have not had the ability to check are carb synch and timing. But I can't see those being intermittent problems. It runs so happily when it decides to run right. I did back off the timing a bit to see if that would help with the backfires (lean) but it didn't do much.

I'm contemplating selling these things and just getting a holley manifold and 390 carb. I can't stand being so frustrated with this beautiful machine.

Edited June 5, 201410 yr by azriel_strife

So I found something else contributing to my issues. The car was running good for a few days, but then it cooled off a bit and it started backfiring through the carbs again. It only backfires around town at 2000rpm or below when I slowly apply throttle. I put 91 octane in just to be sure it wasn't the 87 i have been running. The fuel made no difference.

(still running the points system from the 240 engine, my engine is an f54 with flat tops stroked to 2.9L with an N42 head, pretty high compresion)

I decided to check the vacuum advance line to the dizzy. The line itself is fine, I checked the vac advance by pulling off the Cap and watching the advance work as i sucked on the hose, i blocked the hose with my tonuge and there are no leaks in the distributor or line. I decided to disable the vacuum advance so I blocked off the carb side with a bolt in the line connected to the carb.

This completely solved all of my backfiring issues around town, the car runs amazingly, idles nicely, and runs flawlessly under all conditions at speeds below 80km/h. My guess is that it was getting too much ignition advance for the compression ratio of my engine build.

When I take it on the highway however, it will hesitate (no backfiring) if you lightly apply throttle. Hesitation appears at sustained engine speeds of 2500rpm or more in any gear, but it seems to be just fine under WOT.

Carb oil levels are good, plugs and wires are good, cap and rotor look good, Timing is set. I'm at a loss, should I be running different timing than stock for this Block/head combo?

Edited June 15, 201410 yr by azriel_strife

Sounds like the diaphragm in your vacuum advance might have a leak, thus leaning out your mixture.

The line itself is fine, I checked the vac advance by pulling off the Cap and watching the advance work as i sucked on the hose, i blocked the hose with my tonuge and there are no leaks in the distributor or line.

That is what made me think of the distributor, but I have already checked that.

That is what made me think of the distributor, but I have already checked that.

Didn't you say it ran better without the vacuum advance?

Now without it you won't get the ignition advance you normally get at cruise with light throttle advances. Not sure what numbers you are running but you might want to advance your timing with the vacuum part disconnected and see how it does.

Measure the advance with an instrument if you want to tune, otherwise it is guessing.

Didn't you say it ran better without the vacuum advance?

Now without it you won't get the ignition advance you normally get at cruise with light throttle advances. Not sure what numbers you are running but you might want to advance your timing with the vacuum part disconnected and see how it does.

Yes it runs much better without the vac advance hooked up, because I believe it was getting too much advance for my compression ratio. thus backfiring through the carbs on detonation. This is what makes me believe this distributor is not right for my engine.

Measure the advance with an instrument if you want to tune, otherwise it is guessing.

The timing is set to stock 240 specs because I am using the 240z distributor, i thought that to be the best option, since stock timing spec is based on the distributors advance amount.

Can you watch the timing with a timing light. It is easy to see the timing at any rpm with a light, especially one that has an adjustable offset.

Check at idle, 2000 rpm, 3000rpm and at max advance. (probable ~ 3000)

To Hell with stock specs - you aren't running a stock motor. Any pinging you might have had might have been from vacuum advance. Unplug it and play around with advance. If you don't have a timing light- you shouldn't own a Z

Can you watch the timing with a timing light. It is easy to see the timing at any rpm with a light, especially one that has an adjustable offset.

Check at idle, 2000 rpm, 3000rpm and at max advance. (probable ~ 3000)

So I should set timing based on full advance and not at idle? it's set for idle as the FSM states. But timing is more important at driving rpm.

To Hell with stock specs - you aren't running a stock motor. Any pinging you might have had might have been from vacuum advance. Unplug it and play around with advance. If you don't have a timing light- you shouldn't own a Z

Who said i don't have a timing light? It's set properly. I know my engine isn't stock, I know the distributor isn't even for this size of engine. It's got plenty of torque, i don't need the vac advance at all. I guess I'll just set it up for total advance instead of factory spec.

You should check your timing at all rpms up to max advance to understand what it is doing and when. Try with and without the vac advance to see what is going on.

The neat thing with the distributor is that it is independent of load. It only depends on rpm.

Guess I sounded snarky there - sorry.

Maybe not the best dizzy and they are all different as far as mechanical advance.

How much timing all in?

Might consider a ZX dizzy or electronic ignition .

You should check your timing at all rpms up to max advance to understand what it is doing and when. Try with and without the vac advance to see what is going on.

The neat thing with the distributor is that it is independent of load. It only depends on rpm.

I'll have a look when I get some extra time here soon. I would think that i can only have one or the other if the advance range doesnt meet my engine needs, correct? so if i have to choose, i should tune for operation RPM, not idle?

Guess I sounded snarky there - sorry.

Maybe not the best dizzy and they are all different as far as mechanical advance.

How much timing all in?

Might consider a ZX dizzy or electronic ignition .

I am sitting at 32 degrees total advance @ 3000rpm without advance plugged in.