'89 40 Horse Low Compression but Starts and Runs Okay, What to look at

[cheaperts]

1989 VRO 40 Horse, VRO has been removed and runs straight 50:1. This engine is on a 20' pontoon I bought a year ago from a small dealer/salvage. Starts easy and idles/runs good, but it seems to have less power than expected. We run a 12.5x13 prop and only get 11-12 mph GPS @ 4500 RPM.

I checked compression a couple of weeks ago and found them even but low, 80 on both cylinders, with my cheap gauge from harbor freight, low but even. Today we sea foamed the engine hoping to clean things up and raise compression. Did not recheck after but we seemed to pick up a little power, getting 14 mph @4500 rpm.

When I serviced the motor and did the compression check I noticed one piston looked like clean aluminum, one looked more black carbon covered, making me think one piston had been changed. Plugs are identical color and dry.

If I pull the side cover will I be able to see the cylinder conditions or just the pistons and rings. Does any of the warrant pulling the head and inspecting it and the head gasket, could a leak between cylinders give these numbers? And can the low compression be limiting the RPM's to the 4500?

 

[Daviet]

Re: '89 40 Horse Low Compression but Starts and Runs Okay, What to look at

I don't have a manual for your engine, but a 2-cycle has to turn the proper rpm at wide open throttle to devlope the rated horsepower.
I would think you should be turning somewhere around 5500 rpm or better at wide open throttle.

 

[cheaperts]

Re: '89 40 Horse Low Compression but Starts and Runs Okay, What to look at

I don't have a manual for your engine, but a 2-cycle has to turn the proper rpm at wide open throttle to develop the rated horsepower.
I would think you should be turning somewhere around 5500 rpm or better at wide open throttle.

Agreed, this motor should get around 5500 RPM, but Im wondering if the very low compression is keeping us from getting above 4500. I've done a link and sync, the advance assembly has a little sloop in it so it may not be getting full advance, but I had a mechanic warn me not to mess with the advance saying that too much and we would smoke motor real fast.

 

[phillnjack2]

Re: '89 40 Horse Low Compression but Starts and Runs Okay, What to look at

before you go jumping the gun, try another compression tester or another known good compression engine with your guage.

how did you test for compression ? did you have both plugs out and have throttle set to maximum ?
did you run the engine first to get it warm, and also switch it off and then do test straight away ?
if not then you have not done proper test and will possibly see low numbers.

if it really is only 80psi then your lucky it even starts at all, so do check that gauge ...

also if you are getting 14mph @ 4500rpm after seafoam then the speed will be the same as the 4500 before the seafoam !!!!!.
you cannot gain speed if the revs are the same !!!!!
its more like you have the wrong prop or dirty carburettors.
seafoam will keep them clean to a point, but it wont do a proper carb clean job if the carbs have never been cleaned thoroughly.

20foot pontoon is far too big for that engine too, so I would suggest a smaller prop as well.
for a 20ft pontoon your should be looking for at least a 60hp and probably a 70 plus would suit it better.
40hp is a tiny little engine for that boat honestly, unless you want to keep around 8mph with it of course then it will do the job.

see if you can borrow a 11 x 11 prop as the 12.5 diameter is killing that little engine.
12.5 x 13 is a ski prop for a 13 to 14ft speedboat with a 40hp Johnson.


just my opinion of course.


phill

 

[cheaperts]

Re: '89 40 Horse Low Compression but Starts and Runs Okay, What to look at

We actually put a 10.5x11 4 blade on it last weekend, had no tach then (installed it this weekend) but it seemed to be rev limiting, it was surging at WOT, when I posted here everyone told me how that prop was too small.....The engine came with a 13p and this is what most everyone has recommended.

Our original 12 mph @4500 rpm is a guess as to the RPM sine we just put the tach on, but the engine sounded about the same, we may have picked up a few RPM after we decarbed it with the sea foam.

Compression test was on a warmed up engine WOT both plugs out

And yes I want to find a larger motor, boat is tagged for 75 max, but till then we will keep putting along, just trying to make the most of it till the right one comes along