non bmw question

[kmac]

Evening guys,

I have a question about a honda vf700c magna. I got it for free and it was supposedly running when parked a few years ago. I know the carbs need a MAJOR cleaning. But while i pulled the carbs off i thought i would also check the compression. The bike has virtually NO compression. No difference between plugs in or plugs out on the pressure turning the crank over with a ratchet on the crank nut.

 

Is this possible on a 4 cylinder motor to have 0 comp in all 4 cylinders equally? If all are bad I would assume it threw a cam chain and is an interference motor and bent valves in all 4 jugs. Is it possible that there is some kind of auto decompressor that is just making it feel like there is no compression but once it fires up it will be fine?

 

Should I just clean the carbs and see? or is it likely that the motor is toast?

 

Thanks guys

[cris nitro]

I would seriously doubt that none of the cylinders have no compression at all. If the engine spins free, why not spin the motor with the battery and starter and see what kind of compression you get. I've never heard of checking compression by turning the engine over by hand.

[ka5ysy]

I seem to remember that particular engine has a compression release mechanism because it is a high compression engine. This is to allow the engine to spin up and start without burning up the starter motor. I would clean the carbs and try a start. Bet that is what is going on.

 

FWIW, you could give the local dealership a call and ask the parts guy to look it up and verify the compression release setup.

 

 

[dhanson]

Service manual download:

 

VF700C service manual

[kmac]

I've never heard of checking compression by turning the engine over by hand.

 

I didnt have the right spark plug hole adapter for this motor to use my compression tester, so i dont have a compression number. When I turned it over it just had no compression POP to it. Every motor i have owned has a POP right at TDC of the comp stroke for each cylinder, this motor does not have that. it just turns round and round with no POP. Dang my KTM 530 had a serious pop even with the plug out of the motor it was hard to turn over and still had a POP at TDC.

[Skywagon]

Before you just start spinning it after it has been sitting a long time, I highly recommend you pull the plugs and put something like a capful of ART, Marvel, or something and let it sit a bit. Yep it will smoke and foul, but dry walls suddenly being cranked can do a lot of damage from scoring, to ring brekage, to valve breakage.

 

Ask me how I know?

[kmac]

I did pull the plugs and put some marvel in the cylinders as soon as i got the carbs off....i always do that on a bike that has sat for any period of time also.

 

There doesnt seem to be any bad noises on slow rotation, no knocks or clanks or grinding sounds, just no real resistance or POP feeling as you usually feel with motors.

 

?????

[mrduck]

On our KTM motors, you have one BIG piston, on a long stroke so lots of "pop".

What ya have there is four little pistons on a short stroke, so not near as much volume for compression.

Throw a good battery in and then spin the motor over with the starter and I'll be willing to bet there is a big difference

My cheap advice for today

 

 

Don J

[CoarsegoldKid]

I owned a V45 and having all at zero compression would be very odd indeed. I'm with Mr. Duck on this one. With all plugs out, a good battery, the starter motor should turn that motor freely without a popping.

[cris nitro]

Pop, pop.

[kmac]

By POP I mean that thump of compression right before the stroke starts to head back down.

[4wheeldog]

The most likely cause of no compression in an engine that has been sitting for a considerable length of time is valves hung up in their guides. If you remove the cam/valve covers, you will be able to verify this, (There will be excessive clearance when the valves should be shut). You should also be able to soak the valve stems with penetrating oil (PB blaster would one choice.......Mouse milk would be another), give 'em time to soak, then turn the motor over again. If they move at all, they should loosen up. Good Luck!.

[cris nitro]

The most likely cause of no compression in an engine that has been sitting for a considerable length of time is valves hung up in their guides. If you remove the cam/valve covers, you will be able to verify this, (There will be excessive clearance when the valves should be shut). You should also be able to soak the valve stems with penetrating oil (PB blaster would one choice.......Mouse milk would be another), give 'em time to soak, then turn the motor over again. If they move at all, they should loosen up. Good Luck!.

 

Or, you could just push the starter button!

[kmac]

my issue with just cranking the motor is I dont know if it is an interference motor and i am a bit worried about bending a valve. although I have turned it over slowly so i guess the damage would be done