1983 7.5 low full throttle power

[islander9]

Hello folk's, I would appreciate some thought's regarding the compression of a little 7.5 that I have gone over from end to end and it still lacks power at the top end. It runs real nice at idle and spins up the rpm in a test tank, but under load it will not put out.

As part of the maintenance I have done on it I replaced the head gasket with a Sierra gasket, after which the compression was only 95lbs/100lbs. Figuring it might be the aftermarket gasket, I installed an OEM head gasket and the compression did not increase.

I know 100psi is quite low, would anyone happen to know what I should be expecting for a compression reading from this model engine? I'm thinking a minimum of 120 should be the reading, but not sure on this model engine.
I have many accurate compression gauges and have tried a few of them and they all read the same.

The cylinders are in good shape (no scoring), but that said they do not show any of the original cross hatching and yet they don't looked glazed.


A bit more background....
The engine had been running with a stuck open thermostat when I got it, replaced it of course. Have soaked the carb and thoroughly cleaned it including popping the welch plugs to check the passages. Reeds are in good shape.

I am prepared to hone the cyl's and replace the rings if the engine is capable of producing a higher compression reading.

Anyone have an opinion on the compression reading I am getting and any suggestions on where the full throttle power might be going?

Regards

 

[racerone]

I do not believe compression is the issue here.-----Does spark jump a gap of 3/8" on both leads, yes or no ?----They run very well on just 1 cylinder.

 

[islander9]

Yes it has a consistent blue spark on both cyl's from low idle up to as fast as I dare run it in a test tank.

 

[racerone]

Spark on both sparkplugs or a 3/8" gap on a testing device ?----There is a difference !----Checked fuel pump diaphragm ?

 

[islander9]

Yes an adjustable spark tester on each plug one at a time and then one each plug at the same time. I have also watched the spark on each cylinder with my battery powered inductive timing light. It will run equally on either of the top, or bottom cyl's with a plug disconnected/shorted.

I rebuilt the fuel pump, replaced the fuel lines, verified the throttle/spark advance timing, checked the lower unit for a bind, replaced the water pump. I have given the engine a good treatment with Mercury Powertune and run it extensively since. New spark plugs of course.

They don't get much simpler than this little engine....when they are working perfectly.

I was considering pulling the powerhead to see if it was carbon blocked when I posted this request for help. Not much point if I am going to have to address the low - er compression.

Thought I might have read some time ago about these being very low compression models to start with???

Anyone know what the compression potential is??

Thanks gent's

 

[F_R]

I don't know the compression answer, but just wondering---you say it doesn't put out at full throttle. What does that mean? Is the motor large enough for the boat? Have you received satisfactory performance from this or similar motors on the same boat?

If it is just a bit less than you expect, you can always test it with a "Poor Man's Dynamometer" otherwise known as a test prop of the proper size for the motor. May have to go to a properly equipped shop and pay a fair price to have that done.

 

[islander9]

At full throttle it doesn't have enough power to get the rpm anywhere in the range it should be, or to crest a little 10 footer onto plane which in past years it has had no trouble doing with the young fella in it.
There is no noticeable bog, or ignition type sound/failure.

I have access to test prop's which would save me a bit of hustling the engine back and forth to the water for sure. Ha, my buddy who owns a local dealership was trying to give me his old test props because they haven't been off the shelf in 10 years. I mean how difficult can a little 1983 7.5 be?????..ha

Ironically I have worked on outboards/snowmobiles, ATV's for many years and this is what I do all summer from a small shop on a lake. Every once in a while you get a real challenge and this is one for me.

If someone should see this post and know what I can expect for a compression spec it would be much appreciated.

 

[Chinewalker]

Is the plastic roller bit that activates the carb in place and operating properly? Had one that the little arm broke and it wouldn't allow full advance.

 

[islander9]

Thanks Chinewalker, it is and the advance timing is where it should be relative to the throttle advance. The throttle butterfly is also laying flat at full throttle.

I too once wasted some effort on a 70's 9.9 that was out of timing with the ignition. Those engines are seriously finicky about link/sync timing and funny how you never seem to forget the 'I gotcha's'.

 

[islander9]

Sorry, should have also added that the behaviour of this 7.5 is just like the throttle has not been advanced in time with the ignition.

Appreciate every suggestion to hopefully stumble upon something I have missed to save me replacing rings that might not make the difference.