Force 125 think its an 1986

Here is what i am being told by the mechanic, he is not sure what the problem is. My motor will not crank while the spark plus are in, or under compresion, if the plugs are removed it cranks great. We recently put a new water pump on but after that it ran good for a few hours. Then according to the mechanic the head gasket blew, he changed that and now one cylinder has 20 pounds less compression and nocrank from the starter while the plugs are in, I am done with the mechanic and want to see if i can figure this thing out any ideas?

Re: Force 125 think its an 86

Difference in compression from cylinder to cylinder,will stop it from cranking right.
Remove the plugs then throttle full forward.Then do a comp test.
Post your results.
A weak starter will also,along with low comp, do the same.Take your starter apart.Clean all the contacts,turn the brushes upsidedown.Then do a comp test. Then post your results.J

Re: Force 125 think its an 86

Mechanic said 2 cylinders are at 125, 1 at 110 and 1 at 105. But according to the book you should test compression while the motor is warm. And I am not sure if i can trust this mechanic, so i will go buy a comp tool and test it myself and add it here.

Thanks.

Re: Force 125 think its an 86

Warm or cold,it can be tested.I prefer cold.That way you can look at the plugs for rust. A good guage that screws into the plug hole.Then open the throttle all the way.Then test.Like I said check the starter.They are easy to put back together.
If your results match his, then a de-carb is needed.Sea foam or Mercurys power tune works good.J

Re: Force 125 think its an 86

I tend to test cold as well - your reading will be a little lower than warm, but it should not make a difference to the range of the readings - a toasted piston/cylinder or stuck rings will still test low compared to the others.

And agree - a bad starter or weak battery may not be getting the flywheel turning fast enough for the ignition to produce enough power to get spark.

Start with a "known" good battery and see it that makes a difference..