Z therapy SU carbs not running right.

[beermanpete]

Try a thicker oil in the carbs. ATF is about 7 weight. Thin oil will allow it to lean out on throttle opening. Try 10W30 engine oil. It works better for me that ATF.

[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, 2014 by azriel_strife

[Jarvo2]

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

[azriel_strife]

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.

[madkaw]

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.

[240260280z]

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

[azriel_strife]

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.

[240260280z]

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)

[madkaw]

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

[azriel_strife]

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.

[240260280z]

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.

[madkaw]

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 .