Compression Check with Motor Removed

Motor is removed from the frame, and all connectors are unplugged. I'd like to check the compression in the motor, so I reinstalled the starter in the motor and applied 12v, to which nothing happened but minor sparking. Motor is a 750, from 1982 maxim. Bike has run before (last year, started on the starter fine). My question is how am I able to run the starter with the engine removed.

Thanks again to the forum for all the help so far.

 

You grounded the engine right?

 

Was it in gear perhaps?
Did you apply +12 to the stud on the starter and - to the case?

 

are you trying to start it with jump leads?
if so make sure you have a good earth on the -ve to engine ground .
if still no joy try the starter motor on its own , removed from engine
if it spins ok, then can you turn the motor with a spanner on the end of the crankshaft, l/h front cover?
stu

 

Engine turns over fine. Just changed valve shims to within spec and engine is full of oil. I grounded the motor strap directly to the battery and applied voltage to starter. Have battery on charger now and if I can't get it I will remove to check starter. I mostly wanted to know if there were any other voltages expected by the motor in regards to safety cutoffs.

 

Not to spin.

Ground the case, 12V to the starter and it should spin. You're bypassing everything else that way, which is good. If it won't spin it has to be the starter.

Make sure you have an assistant to help steady the motor, it may want to jump around a bit from torque reaction.

 

You have 4 connections - - each one must be clean enough to handle 200 amps - - you have one bad, corroded connection, I think.
Minor sparking?? maybe lots of crud from the starter to the case??

If not, it's the starter.

Remove all 4 plugs, also.

 

He's doing a compression test, ya kinda gotta... you know, that gauge thing and all?

If by some chance the carbs are still on the motor be sure the throttles are blocked wide open.

 

Well - no kidding, but I ran across a HELICOPTER MECHANIC that was doing a compression test, with an assistant, on a 1981 GoldWing, with carbs on and plugs in. Throttles closed. (it turned 120's)

I made the suggestion of repeating the test with throttles open, just to satisfy my curiosity, and got hit by such a barrage of insults that I just left them alone.

Some people just don't know.

 

Okay thanks for the info as far as the safety bypasses. I had figured that was the case but I recall the starter working fine when last I checked. I'll give it a check in the morning and post an update. Thanks again for the help. Much appreciated.

 

put a few nuts on the starter stud and touch to them, if you touch to the stud directly you'll ruin the threads

 

Okay, as I understand it, the starter is fed voltage through what appears to be the only stud, and grounds through its contact with the motor casing. The motor itself has two grounds. Correct me if Im wrong but I was about to jump and the wire and rubber protectors are black. I don't understand why yahama engineers would have chosen this but for aesthetics. Anyway, I don't want to proceed unless I know for sure, but this the correct location to apply voltage?

 

It appears it was answered further up in the thread, and I confirmed with the manuals. I believe it may be in gear, but since Im bypassing everything, shouldnt it still work?

 

The starter motor is failry small. If it's in gear, and on a compression stroke, the engine won't want to turn. This is particularly true if the starter motor needs new brushes and the commutator cleaned.

Take it out of gear and see if she'll spin. If not then have a look inside of the starter motor.

 

Alright, I'll go and check it now. The motor is removed, so I didn't think the added resistance from just turning the gears would really affect it, but Ill post an update later.

 

You should also take the ignition cover off and try to turn the engine by hand. You'll need a 19mm open ended wrench. Be careful to not damage the ingnition pickup coils.

 

Right, I had already done that for valves. For reference, I went through and cleaned up all the contacts again and put some dielectric grease and it worked straight away. Dumb case of not checking the easy stuff first I fear. compression numbers are cyl 1-4: 140, 140, 145, 145. Not bad for 41k on the odo I reckon.