Low compression 120psi/8bar problem or not?

[thelonious]

I'm doing up a 1996 Monster 750 that is only showing 13k miles (I'm still not sure if this is genuine).

I did a compression check with the engine warmed up I got 120psi / 8 bar from both cylinders. I put in a few squirts of oil into the cylinders and it did not change the readings at all.

Normally I would think 8 bar is pretty low and probably shows that something is wrong, but the service manual quotes "standard compression" 9-11 bar and "minimum" 8 bar so actually it is in spec, just about.

Plus the compression ratio is only 9:1 on this engine anyway. It's good that the cylinders match, and that the test with oil added was the same as that suggests the rings etc are fine.

So then I checked all the valve clearances. The horizontal cylinder was very close to original spec, the closing shims turn freely with the arm held down but with no significant clearance and the opening ones are close to target. But on the vertical cylinder it's all a bit loose with one out of limits (see pic). I didn't want to get too deep into removing the shims and belts etc yet so I only measured the one that was out and will just replace that for now.

Given the compressions readings are the same for the two cylinders, I don't think the loose valve clearances on the vertical cylinder are a problem.

What do you guys think? Is the motor good to go or not?

[Howie]

Adjust that loose valve.  Also, if the timing belts are over two years old, change them before running the engine.

[ducpainter]

Was your test performed with WOT?

[thelonious]

Yep.

[thelonious]

The belts were changed last year, or so I was told.

Any thoughts on the compression readings anyone?

[ducpainter]

While 120 psi isn't ideal, the two cylinders being identical is a big plus.

How does it run?

[thelonious]

So far I have only done the few miles from the sellers house to mine, very carefully as the fork oil had leaked all over the brake disk, the exhausts were very loud and I had not yet MOTed or taxed it so I did NOT want to attract any attention!

However the engine seemed to start, idle and run fine. The gear box and clutch felt good, it didn't seem the fastest but as I say I have not really tested it yet. Also my first V-twin so not really sure how it should sound/feel.

Currently it's in quite a few bits undergoing a light restoration. I have done most of the work now, just waiting for the parts to come back from the powder coaters and it should be back on the road in a couple of weeks.

[ducpainter]

I'm going to say run it and see how it goes.
.

My dad used to say, "Liars figure, and figures lie."

If it runs good and works how you want it to...who cares what the numbers are?

[thelonious]

Well, it seems to be going very nicely.

New belts, loose valve adjusted, carbs balanced, fuel system cleaned, full service and a ton of other stuff and it's riding very sweetly and looking a lot better than when I bought it. It's almost back to standard now, I am just waiting for a rear brake disk and some indicators.

I am also waiting for new tyres before I can really push it (the ones on there are Michelin 2CT and look like new but are dated 2006!), but for a bike that only had 60bhp out of the factory in 1996 it feels to me like it is all still there.

I will stop worrying about compression figures and just enjoy it now! I am going on a run with the local club on Thursday, and will see if I can keep up...

[ducpainter]

Well, it seems to be going very nicely.

New belts, loose valve adjusted, carbs balanced, fuel system cleaned, full service and a ton of other stuff and it's riding very sweetly and looking a lot better than when I bought it. It's almost back to standard now, I am just waiting for a rear brake disk and some indicators.

I am also waiting for new tyres before I can really push it (the ones on there are Michelin 2CT and look like new but are dated 2006!), but for a bike that only had 60bhp out of the factory in 1996 it feels to me like it is all still there.

I will stop worrying about compression figures and just enjoy it now! I am going on a run with the local club on Thursday, and will see if I can keep up...

Have fun, but I'm going to suggest that you ride your own ride, and don't worry about keeping up.

[koko64]

Expect around 160 psi on a hot motor. WOT and don't stop cranking until that gauge needle stops and no longer makes progress. People quit too soon due to being worried about stressing the starter motor.  Have both plugs out to halve the strain on the starter. Make sure they're connected to the ignition cables and the plugs are grounded to the motor to protect the ignition components.

If you did this ignore me


Whats good is the vaues are the same.

[thelonious]

I am still showing 125psi on both cylinders with a hot engine, WOT, both plugs removed and grounded.

But either 125 is enough or my gauge is crap, because the bike is running great. Now running Pirelli Angel STs.

I am going to stop worrying about the compression figures and just enjoy the bike, which is a little cracker.

Do they have rev limiters built in to the ignition units or is that up left to the rider? The user manual suggests 9000rpm max for the 900, but of course the 750 has no rev counter.

[koko64]

It is a little cracker . I never assumed a rev limiter on the stock ignition.

[Orange16]

I would not expect to see much more than 125 psi on a 750, no matter how long you crank it.  They are a comparatively low compression engine and the camshafts were retarded from their original setting in the F1 models.  Even with the cam timing advanced I do not believe you'd see more than 140 psi.

The valve clearance adjustment will make some difference to the idle quality and low speed running.  Have the closer clearance as close to zero as possible.  0.01mm is obtainable.  While the Ducati specification shows up to 0.20mm, you will find at 0.20mm that the idle will be poor.

The Bikeboy graphs show the power peaking at 7,500 rpm, so you should notice the power falling well before you reach 9,000 rpm.  There are no limiters in the Kokusan ignition units.

[thelonious]

Thanks Orange16. Good to know.

I am thinking of stripping it down over the winter to get the frame powdercoated and to repaint/vapour blast the engine. In this case I might as well strip the heads and decarbonise, lap valves and reset clearances.

Meanwhile I am just having some fun with it.