Had the engine out, put in a new cam, fresh rebuilt stock heads, stock lifters, new valve train otherwise, installed mini ram, stock throttle body.


Engine idles a little weird, like the valves are tight. I keep backing them off but its still the same. Hydro roller.


Rev the engine in the garage and it starts to pop through the intake at about 2500-3k rpm at 6*. Bring timing down to 0* and the popping gets quieter but still there. Distributor is not 180* off and I've checked my firing order at least 6 times.


Compression test shows 140-150 on the initial hit on all cylinders, 210-215 max on all cylinders.


7 plugs read fine, #8 plug reads overly rich. I have verified that the exact plug and wire will spark in open air. Fuel pressure is 45 dead head and holds pressure hours after being shut off.


I'm really at a loss right now. The only thing I can think of is that the lifters are pumping up and holding the valves open but I have almost no preload on them right now. I was hoping you guys might have some ideas before I just throw some new lifters in this thing. Any ideas on the rich cylinder?

 

More checks. Vacuum is at 15" with a slight bounce. After setting timing I plug the wire back in and timing is advanced off the plate, a good 6 degrees before it. I have the valves backed off until there was play in the pushrod, it seems to idle better but still doesn't sound like it should from the exhaust. Idle is pretty high at ~900 rpm.


I would hate to think it could be an improperly ground cam. Maybe the tune? Its a mail order from PCMofSC.

 

The backfiring at upper RPM symptom is common with incorrect valve timing.

It's usually a good idea to degree a cam when installing in case the grind, pinning, or timing set are off. In your case, it may be good to degree several lobes on both the intake and exhaust sides to verify the timing. The worst situation would be if the lobes were not properly indexed so that only SOME of the lobes are a few degrees off.

The presumed rich cylinder may have an injector problem. It could be exchanged with another to see if the problem moves. Apparently it is not leaking down since the residual pressure maintains after shutdown, but that says nothing about its flow rate, spray pattern, or propagation delays.

If the custom tune is in question, can you install an OEM PROM for testing? While it may not be optimal for the engine, it would eliminate one variable at least for diagnosis.

 

I did degree cylinder one when I installed the cam, I didn't think to degree the rest. I can get degree tape for the balancer and check the rest.

I may have fixed the rich cylinder, I found a burn spot on my plug wire that I'm assuming is arching to the valves cover. I haven't been able to reproduce is but the plug looks to be staying cleaner after I isolated the wire. New wire set is on the way.

I'll try running the stock prom and see how it does, that's a good idea.

On a positive note I tried adding some timing and it seems to run a lot better, 14* btdc and it seems to need more. I bought a timing light with an advance dial because I ran out of room on the timing cover marker, will report back on that front.

Thanks for the reply, I really appreciate the help. I'm still stumped on why it seems like the valves are tight at idle, it smells rich and idles poorly. I guess the next step is to adjust the valves while its running.

 

210-215 psi cylinder pressure is ALOT. That is no where near a stock type engine. That is in fact is high compression, mild race engine pressures. 140-175 psi is more inline with stock type build.

So it sounds like something is a mist.. Compression much higher than you think (heads or block milled a lot) or smaller than stock cam or cam timing way off.

 

The heads have been milled twice but I can't imagine the compression being crazy high. It makes me think the cam timing is off but I know I degreed it properly and it was 1* retarded from the cam card. I even did it twice because I went back and adjusted my timing marks on the front cover to be sure 0* was at tdc for setting timing. Same result both times.

I pulled the chip and ran the factory prom but nothing changed. I even called comp to see what they recommend for base timing and they said 12-14*, runs poor in that range. Best I had it running so far was at 18* but it had 50* total at that setting and that scared me into backing it down.

I'll wait for the degree tape to show up and check the cam lobes then.

 

are you off a tooth on distbuter

 

I really don't think so, the timing seems to go in the spot I set it.