350bayliner

Engine is 350 chevy 86+ Hyd. Roller, 9.0:1 CR at most, new engine, had same problem with old engine (not why I replaced it) Anyway, here's the deal. I can retard the timing back enough to where it will quit pinging (which is observed by running the boat and someone sitting back there listening at the motor for ping) but when retarded this far, doesn't have enough power to break about 3500rpms. Checking it with a timing light, it looks like I'm getting only about 22* total advange before it starts to ping, maybe less!! It just started doing it one day. I've replaced entire ignition system, cleaned carb, etc. etc. Distributor is old style HEI with mechanical advance only. Tried it with 87 and 89 octane, no difference. I can't believe I would have to run anything higher with such low compression.

HELP!!!

Kent Perroux

Is this a new engine? An older engine could have carbon buildup on the pistons.

Is it carb? If a carb, did you remove the electric choke from a Holley carb? IF yes, you may have introduced additional air into the carb making it run lean.

What is your fuel pressure? Should be 6-8 carb, higher for EFI.

Sounds like the pinging did not follow the engine, not ignition which leaves carb/fuel.

Kent

350bayliner

Kent, it's a new short block, but there could be maybe just a little carbon on the heads. It's carbbed with an edlebrock that I've been through a few times, but maybe I should try a different carb. Haven't checked fuel pressure yet, I'll check it tonight.

Thanks
Drew

350bayliner

Something else I should have mentioned, If I set the timing to 32* total advance I can start it out of the water and rev it and I can hear it ping some as the rpms are climing. Weirdest thing I've ever seen, and yes, I'm sure it's a ping and not a bottom end or valvetrain noise.

Drew

Kent Perroux

If it pings with no load, you definately have a problem. Richen the carb a few notches and see if it still pings with no load (on the trailer). On Holley I would go up about 4 jet sizes to start.

However, the question remains as to what has changed to get you to this condition. Make sure you did not change anything on the fuel system that might change the fuel pressure, fuel flow, or fuel/air mixture.

350bayliner

Allright, here's an update. Did a compression test, 130psi on all cylinders except #4, it was 90. I pulled the head, and it looked ok, so I pulled the Valves out of #4 and the intake valve doesn't appear to have been seating properly. Do you think this could have possibly been the problem?

Drew

Kent Perroux

It would have been good to do another compression test with a teaspoon of oil in that cylinder also. That will isolate that you do not have a ring problem.

130 seams a bit low for a 9:1 compression engine. I usually got around 145-155.

Since you have the head off, a good way to check if the valves are seated good is to leave the valves and springs assembled on the head, prop the head with the intake ports up, the pour some light liquid (WD40 works good) into the intake port. Watch around the valves from the combusion chamber. If they are seated good, the valves will not leak the fliud. I do this every time I pull heads off. Always an easy check. Do the same on the exhaust ports.

I would do this before you make any corrections. If you have a bad leak, you will know immediately.

blue thunder

What do the spark plugs look like after the ping? Sounds like a carb tuning problem, maybe float set too low or wrong size needle and seat. If you still have the problem when put back together a vacuum gage may tell the story.

Good luck,

BT

350bayliner

Update again- Pulled heads, found one valve that needed work, lapped it in, now have balanced compression but same problem. Definitely is not a carb problem (verified with different carb) Is it possible that the noise I'm hearing is not detonation, but a similar noise that only occurs when the timing is advanced? The plugs look good with no sign of aluminum specks. Checked with vacuum gauge, looks normal. I'm beginning to wonder if my old, possibly weak valve springs could have something to do with it.

Drew

cuda

It could be hot spots on the pistons, either a carbon build up, or any little piece of protruding metal on a piston (nick, etc).