compressionloss

have 1991 4.3 v6 175hp just replaced heads,risers,manafolds then had to replace a piston because the motor burned whole in the top of the piston boat ran fine for couple trips out then started lossing power about half throttle so did compression test and the number3 cylinder has no comp what should i check

Joshua, the hole in the top of the piston is most likely due to detonation damage.
Detonation causes range from lean F/A ratio, excessive ignition TA to excessive cylinder temperatures.... and in particular with the Marine Gassers.

FYI: detonation and pre-ignition are two entirely different phenomena.

As part of your forensic work (prior to complete tear-down), ignition TA should have been verified....... and possibly the fuel delivery system verified.

Sounds as though you had a repeat of the original problem.
Do your forensics and home work so that this doesn't happen again.




.

What is "ignition TA"?

kimcrwbr1 said:

Or he is talking total advance or all in? these motors work hard full time and if the advance curve is not correct you will cook valves and pistons rather quick.

Click to expand...

Yes and Yes! Kim is dead on!

TA numbers will always be associated with an RPM... or should be!
IOW, if you were to strobe your timing marks, and if you were to read 26*.... you must know what RPM this 26* is to occur at.
Too soon.... and you'll risk detonation damage.
Too late.... and you'll leave performance behind.


Here's an example of a Marine ignition advance curve.
These curves are often minus initial/base advance in the line graph, of which means that Initial/Base must be added when doing the math.

However, Initial/Base will always be part of the dynamic readings while strobing timing marks.


Again... this is an example only.
Note the correlation between degrees of ignition spark "lead" and the crankshaft RPM.

Joshua Lee Bonniville said:

have 1991 4.3 v6 175hp just replaced heads,risers,manafolds then had to replace a piston because the motor burned whole in the top of the piston boat ran fine for couple trips out then started lossing power about half throttle so did compression test and the number3 cylinder has no comp what should i check

Click to expand...

Ayuh,.... When ya rebuilt the motor, did ya find the Reason ya burnt down the piston,..??

My guess is, Ya Didn't, 'n now ya burnt down Another piston,....

A Lean condition, from Bad fuel, or crud in the fuel system is My guess,...

I agree with Bill.

If we were to over-all or replace an engine, yet re-use the original carburetor, ignition system, exhaust system or sea water pump without having done any Forensics Work........ we may be destine for a repeat of the original failure.


.