Engine synchronization on hp 500's
 

My boat was perfect when I put it up last year. This year the port engine runs 2-300 rpm more at the same throttle position. what in the hell could have changed?? I'm gonna check the timing this week but it has me puzzled. Any ideas would be appreciated...

Fogging oil is a killer on plugs. Check starboard for a fouled plug, or a gummed up carb jet , etc

I didn't fog it but I will go through the carbs...

could be fuel system related to ignition related. I would look at your distrib cap and rotor first then move on to other ignition components like coil, spark plug wires and so on - checking spark plugs and condition along with examining them for lean or rich conditions. Possible carb overhaul, Also comp test and fuel pressure test would nt hurt either.

300rpm drop is usually a good sign of being down a cylinder

You mentioned 300 more rpm at same position however is it at full throttle? Ck cables and throttle position vs plates on ea also.

I hear you that's why I mentioned examine the spark plugs and a comp testing wouldn't hurt either. You know really a forgotten test that not many do anymore is a vacuum gauge testing and power balancing testing. I would a do vacuum testing, they can determine a lot.

Weak or a problem with the valve springs too.

What is power balancing testing?

The jist is we use a digital rpm gauge, and ground out spark at each cylinder one by one and look for rpm drop - if no rpm drop that is your problem cylinder. We use 3 inch vacuum hose lead that connected to the distrib terminals and spark leads and ground each by a grounded test lite.

I just did all the heads and springs last spring. I bought cap and rotors so that will be first....

Ah, good plan!

Yes sir. Thats how I used to do it back in the day when I worked as a tech. Very common procedure on the old HEI ignitions. As well as turning the lights off, and spraying windex on the plug wires looking for arcs.

This was the fun days.. And actually still good knowledge that works to date that seems to be long forgotten at times. Thumbs up!

We still do a power balance testing on MPI engines but the kicker is you have to catch any RPM drop very quickly because the IAC will over compensate pretty fast for the drop then help pick the rpms back up ( inputs to ECM - ECM adjusts RPM's) . Another reason why I like the digital rpm gauge.

You ever get this figured out?

I got everything set perfect timing wise. Went through the carb on the slow motor.....changed cap and rotors on both and it's still that way. Going to look closer at the linkage and WOT... not sure how that could have changed but.....

How does the slower engine sound compared to the other? Should be a noticeable sounds difference at idle if it is dropping a cylinder on those motors.

They both sound good. No miss. The linkage looks good... going to swap props next and see if it goes with the prop. Like I said. I put it up perfect last fall... Got me puzzled.