four cylinders not firing, oil in each; stock 240z

[jwilliams905]

I don't really know where to start, so please bare with me.

About three months ago, I sent my carburetors off to Ztherapy to re manufacture them because the rear three cylinders quit firing, and my float bowl was overflowing. I finally got them back (2 months later), installed them, did all the necessary tuning that I could without the car running (including float level), and it will only fire up if I've got both carbs fully choked. It'll run at high rpms until I unchoke it, and then it just sputters out. Turns out, (from the front) the 4th, 5th, and 6th cylinders are for sure not firing, and I think the 1st isn't because its exhaust channel on the exhaust header is the same temp as 4, 5, and 6. When I pull 4, 5, and 6 spark plugs, the tips are drenched in oil, but they're still sparking. Could it be that my cylinders are full of oil, and it's not sparking? I adjusted the timing, distributor gap, and valve clearance properly about 800 miles before I had to send off the carbs, so I don't think it's any of those. I just don't get why those cylinders aren't firing.

Please feel free to bombard me with questions and help me figure this out!

Edited August 31, 2013 by jwilliams905

[zKars]

Not good. Sorry to have your first experience here be related to a problem.

Do a compression test on all six cylinders and see what's up. Combustion requires fuel, spark and compression.

Also test your fuel pressure and volume. Disconnect the fuel line to one carb, crank the engine and let it pump fuel into a bottle. See how much you get. Do each carb fuel line separately. replace your fuel filter too.

Let us know what you find.

[jwilliams905]

Haha, it's a learning experience, and I'm eager to work out the kinks, so I'm pretty thankful to even have a forum to post questions on! Before this, I was pretty well capable of figuring out the minor problems, but this one's had me baffled.

I'll see if I can't get a hold of a compression tester from somebody. And I never thought of checking the fuel pump that way. I'll get back to you with the results and hopefully a conclusion!

Thanks a lot, Jim!

[rcb280z]

You can usually rent a compression tester from your local parts house.

[madkaw]

If you were dumping fuel into the cylinders before you could have done several things. You have washed down the cylinder walls with fuel and the rings are not doing their job keeping the oil out. There's the possibility of hydraulic damaged from fuel in the cylinder also.

I would turn over the motor with the plugs out for a while.

I would also make sure to clean plugs well or replace oiled plugs before restarting and do compression test

[jwilliams905]

So I spent about ten hours on trial and error. I first found a friend who'd let me borrow his compression tester. So I tested each cylinder and found that all were running (dry) about 165 psi except for the rearmost which was at 175. Didn't make much sense to me, but I don't think it's a major issue. So I pulled the valve cover and adjusted my valve gaps again, hoping that might even out each cylinder, but it didn't change much. Still wouldn't crank and hold unless I was choking it, and every time I'd hit the throttle, there'd be a delay in it revving up. Spent a lot of time pondering, tinkering with my air flow and mixture nuts, and finally decided I might have adjusted the floats wrong. I originally adjusted them according to the Ztherapy video which was .55 inches (approximately 13.97 mm), but I've seen some say between 12 and 13 millimeters. So I re-adjusted them to 12.5 mm (float to float bowl lid). It made a slight difference, but not enough. Still the same issues, still pretty sure those three cylinders weren't firing. I knew fuel was pumping, I just didn't know how to measure the fuel quantity, and how much I should be getting, so my brother recommended pulling and plugging the fuel line to the front carb to see if the vehicle would start and run off of the rear (the cylinders that aren't firing up). Sure enough, fired right up, ran the same way just different cylinders. I'm pretty sure there must be a blockage in the fuel line, because I'm getting fuel, it's just going to the first carb. The filter is still good (by visual inspection), and the fuel pump is brand new (installed by the previous owner maybe 4 months ago).

Tomorrow, I'm going to blow the fuel back into the fuel tank from the filter, hopefully clearing the blockage, and then I'll just drop the fuel tank and clean it.

Feels like I wasted a lot of time, but I learned every damn inch of my carburetors, and much more. Thank you all for your help! I wouldn't have known where to start without your advice!

Edited September 1, 2013 by jwilliams905

[madkaw]

Why would you pull the fuel tank if you are getting fuel to the carb. So you think that there is only enough fuel coming up to operate 1 carb? That doesn't seem to make sense. What About the fuel rail-have you checked it? Have you visually verified how much fuel is coming from the fuel rail?

[beermanpete]

You should measure the fuel pressure and delivery rate. The factory service manual explains how to do this and provides the performance specifications.

Will the car run (at idle) on ether (starting fluid)?

Did you replace the plugs when you installed the freshly rebuild carbs?

How long did you let the car run before turning off the choke? If the preheat water in the manifolds is not flowing (disconnected or plugged up) the car takes a long time (about 10 minutes) to warm up enough to idle correctly and drive without stumbling a bit on acceleration.

[jwilliams905]

Why would you pull the fuel tank if you are getting fuel to the carb. So you think that there is only enough fuel coming up to operate 1 carb? That doesn't seem to make sense. What About the fuel rail-have you checked it? Have you visually verified how much fuel is coming from the fuel rail?

Doesn't make too much sense to me either, but If the rear three cylinders aren't firing while the front three are, and I can unplug and block the fuel line to the first carb and all of the sudden the rear three are firing up just fine, then that's the only explanation. I figured the tank was probably in need of cleaning anyway, but it seems I'd have to cut the thing open just to clean it and then get it re-welded when I'm done, so I don't know what I'll do there. I'm afraid if I just blow out the fuel line, and it clears the blockage into the fuel tank, then it'll just get clogged again. I'm assuming that the reason why there's a delay in it reving up when I open the throttle is because it's taking longer for that float bowl to fill up. the float level is to spec, and it runs fine if it's the only carb that's hooked up, so I'm assuming there is either a blockage or just not enough fuel pressure.

Could it be something to do with the return line?

[jwilliams905]

You should measure the fuel pressure and delivery rate. The factory service manual explains how to do this and provides the performance specifications.

Will the car run (at idle) on ether (starting fluid)?

Did you replace the plugs when you installed the freshly rebuild carbs?

How long did you let the car run before turning off the choke? If the preheat water in the manifolds is not flowing (disconnected or plugged up) the car takes a long time (about 10 minutes) to warm up enough to idle correctly and drive without stumbling a bit on acceleration.

I've just managed to get it to hold at idle off of only 3 cylinders (I unplugged the rear three plug wires and it didn't affect the way the engine was running).

I put brand new NGK Plugs in before I even tried to start it (they're just nickel).

I ran it choked for about 10 minutes and still when un-choked it, it sputtered out. I think resetting the float level is what caused it to finally hold at idle.

I'll look into running a fuel pressure and delivery rate test, like you said, before doing anything else.

[madkaw]

I would remove or blow out the fuel rail first and check the return orifice

[Patcon]

Check the screens in the banjo bolts, replace the fuel filter and do a flow test. The rear screen may be clogged and with the front line blanked off enough pressure can be built up to feed the rear carb. Cutting on a gas tank and welding for that matter is a very bad idea. The tank has to be absolutely clean and your welding has to be very good to not have pinhole leaks. I weld a lot for a hobbyist and I wouldn't try it. There are kits that can be used for cleaning the tank and resealing. Clean with acid, neutralize and coat with tank sealant. A much easier and safer idea...

Charles