Engine will only run above 4000 RPM. Carbs or timing?

[Diseazd]

Low compression (leaking valves) were my first choice.....his characteristics are exactly what a car with a couple bent valves or too tightly adjusted valves exhibit. He said the only thing he did was adjust the valves.....well maybe that did it. Also, he did a compression check.....do it again....you've tried everything else. In all the higher compression ratio engines I've built, I've never gotten a reading of over 185 lbs., he got 215....on a stock engine.....wow...maybe a bad gauge? Regardless, I'd run another compression check. IMO

[Stanley]

A bad PCV valve could cause vac leak but it's restricted so I don't think it would cause such extreme symptoms. If the leak was on the exterior of the engine (like the big hole in a vacuum hose on my Chevy truck) you would hear it hissing.

[Patcon]

I thought the compression numbers were very high too. I would block off every manifold port I could and all the ports on the engine I could. Adjust the valves a little looser even if they tap a little bit. It should idle fine with the gap a little wide and a little lower lift a few thousands wont hurt anything. I wouldn't rev it way up like that, but at this point I would just try to get it running first.

[grannyknot]

Low compression (leaking valves) were my first choice.....his characteristics are exactly what a car with a couple bent valves or too tightly adjusted valves exhibit. He said the only thing he did was adjust the valves.....well maybe that did it. Also, he did a compression check.....do it again....you've tried everything else. In all the higher compression ratio engines I've built, I've never gotten a reading of over 185 lbs., he got 215....on a stock engine.....wow...maybe a bad gauge? Regardless, I'd run another compression check. IMO

That's interesting, I did check the valve tolerances and a few were too tight, I loosened 4-5 and set them to the cold setting. That's certainly worth a second look in case a screwed something up in the adjustment but I should say again, it's not a stock engine. It was built by Whitehead Performance here in Ontario, they build street, strip and race engines almost exclusively for Nissan cars. F54 block,N42 head, 10:1 compression w/flat top pistons ...

I have another pressure gauge head that I can swap, that should tell me if my first compression readings were off.

Chris

[doradox]

Have you actually measured the fuel pressure instead of relying on the "setting" on the regulator?

Steve

[grannyknot]

Well I didn't find anything out of order. I checked the valve clearances, they are right where I left them .20mm IN,

.25mm EX. Did a leak down test, all cylinders register very low, compression test was within 5lbs of the last test.

I couldn't find the alternate compression gauge head so I tested my trusty Subaru Impresa and the gauge is accurrate.

Besides, it's not the actual numbers on the compression test that matters but how close they are to one another.

Just going to wait until I have the new carbs installed and I have a second person on hand.

Chris

[doradox]

You have a fuel oversupply problem. IF you actually have fuel dripping from your carbs at the end of the test that is a physical indicator that the carb bowls are overflowing. There is only one place that too much fuel comes from and it's not the ignition system or anything mechanical in the engine. MEASURE the fuel pressure.

[LeonV]

You have a fuel oversupply problem. IF you actually have fuel dripping from your carbs at the end of the test that is a physical indicator that the carb bowls are overflowing. There is only one place that too much fuel comes from and it's not the ignition system or anything mechanical in the engine. MEASURE the fuel pressure.

I think this is the most logical idea at this point...

[grannyknot]

How can fuel pressure be a factor when many of the attempts to get an idle where done with NO power to the fuel pump?

Using fuel in the bowls only resulted in the same symptoms.

Fuel dripping out the front of the throttle body happened with the triple carbs which were replaced with SU's about a month ago and has not occured with the SU's.

I know this thread has gone on too long and the facts are getting lost, it does need a spreadsheet sating what has worked and what hasn't but I don't have those kind of computer skills. Once ZTherapy actually ships the carbs there is a retired mechanic in our local Z club that will come by my place and we will have a go at it. I'll let you know.

Thanks,

Chris

[doradox]

One thing to consider. If you were running rich/flooding you may have a lot of fuel in the crankcase. You should check your oil and change it if there is any question about that. Won't solve your problem but fuel thinned oil won't do your engine any good. Good luck.

[bartsscooterservice]

Reading this topic again, I still thinks it's carbs flooding the engine. When you have the Hitachi carbs back from Ztherapy, use the stock mechanical fuel pump !

[bartsscooterservice]

Any news from ZTherapy ?