Compression Issue

I have an 82 VF750 magna. I'm having a huge issue. I did a compression check recently because the bike smokes pretty bad. I had good compression in all the cylinders but 1. both back cylinders are reading right at 150 cold and the front (we'll say drivers side as I don't know the exact number of the cylinder) is right at 150. however the front passenger side cylinder won't even register on the gauge. what could cause this?

 

stuck valve, blown rings, badly burned piston, cracked cylinder wall, warped or cracked head

now that ive scared you, try pouring a couple teaspoons of oil down the plug hole and crank her over a couple tiume then try again to test compression, if that doesnt work your pulling her apart

 

I doubt the oil trick will show anything. Normally you would at least get some type of a compression reading first. Then you try to up it with oil. If it goes up then bad rings.
If a valve is stuck open it would have to be at least half open for no compression. A hole in the piston will read zero. Broken rings 50/50 on reading zero. A crack in the wall should show some compression. Take the valve cover off and look at the valves for #4 piston (pass side front). If all looks good, then drain oil and look for metal particles. Or first poke around in the cylinder with a stiff wire to see if any holes in piston or ring pieces. Make sure piston is at the bottom. No matter what you find, I believe a tear down will be in order.

 

Another way to go is to pull the exhaust and carbs(your going to no matter what) and look in the ports. I will put the pink on any of my bikes it is a valve issue and you will be able to see the damage throught the ports. Smart money is on tipped valve and damaged piston. Sorry but thats my bet.

 

I would like to offer one more possibility.

If you get nothing on that cylinder it may be because no air can go in or out of the cylinder.

This can be tested using a vacuum guage. It can be from a clogged exhaust or clogged intake on that cylinder. Or one other possibility. The tappet on the intake rocker arm has backed out because the locknut came loose, so the valve does not open at all.
It has happened.

Just removing the rocker arm cover would let you know.

 

Thanks for all the help guys. I've been shopping for motor's on ebay and craigslist and have a guy that was gonna sell one for cheap but said if I'd come get it I could have it. IS there any way I can check the compression on it without having to put it in my bike. Being that it's free and not guaranteed to run, I really don't want to spend a whole day putting this thing in my bike only to find out i'm no better off than I was when I started.

 

I totally do not understand this kind of thinking!
I would think you have some loyalty to the bike.

You find out your bike has a problem and the first thing you guys think of is just to go get another engine!!

WHY don't you think to just make a repair to the engine?

It must be a generational thing. If it was me I would want to keep my original engine and that way the serial numbers match the frame too.

You have to remove the engine to repair it - the same as if you got another engine you would have to take the old engine out.

I just do not understand the thinking.

Just take the carbs off and look into the ports . Look at your spark plug on that cylinder for damage.
If you see damage just put in a new valve and piston if needed.

But PLEASE, PLEASE do not tell me you took your engine out and threw it away!!!! That would REALLY be stupid. The parts are valuable to others.

 

the reasoning is it is a 28 year old bike, the parts alone to rebuild my engine would cost way more than an entire replacement engine. on top of that I'm unable to do that kind of work so I would have to pay someone to do it. and we're talking 10 hours at $60 an hour minimum (most places $80) so by the time its all said and done I've paid $1000 for parts and labor on a bike that I paid $700 for. plus its summer so I've found its next to impossible to get the bike into the shop for anything other than a tune up. so if we're talking a full rebuild, I won't get my bike back til next march. Also If I choose to remove the engine I won't throw it away I will definitely try and sell it or part it out.

 

Yes..I understand, but when you replace your engine with another you are never sure what you really have. If you rebuild YOUR engine you have a NEW engine!

And think about this. If you pay $1000 dollars and the engine is completely rebuilt or a top end job. It is like getting a NEW bike for $1000 dollars and another motorcycle would cost more than that.

Guys are still rebuilding HONDA 750 4's from 1976 and you can get all kinds of after market parts too.

Motorcycles are not like cars in that motorcycle technology is further ahead and a 10 year old motorcycle may still be faster or have better features than the new ones.
All the parts are more valuable than any car.

Wait until all of the aluminum framed motorcycle frames start breaking from getting brittle. Then you will wish you had a CB9000 or something with a steel frame that lasts almost forever.

When all the throw away plastic and aluminum computerized motorcycles are gone and a new one costs $20,000 dollars those CB 900's will get snapped up in a heart beat.

 

A $50.00 dollar engine? What was good. Maybe the cases.
I guess it is possible but how many miles were on it.?

 

Yes.... that is also a good plan.

 

I dunno tink...TOE has had his 86 since......86? but he has probably put enough money into it to buy it four times....buffalo...go get the engine check the compression. if its good then voila you have a good bike if its bad your not out anything but the gas money to go and get it.. get a bill of sale with it nad when you register it take it in and show that it was swapped out and have the serial numbers on the bill of sale......

 

I think that's what I'm gonna do. If the engine that I'm getting will get me through the rest of this season then it will give me time to get my old engine rebuilt this winter and nothing else I can put it back in at the beginning of next year or when the engine i'm putting in it fails. What it all comes down to is it's already the end of june, I have yet to get to ride this year because of this problem. if I don't find at least a temporary fix to get me through the year I'm going to go insane haha

 

Please let us know what the reason for the '0' compression was in the engine, when you finally discover it. Curious minds want to know. Sheared in half cam would do it too. (not likely though)

 

ok so...gas is getting into my oil (my guess is thru the bad cylinder). I need to get the bike to the shop, but have no way of hauling it there and the gas is making the bike smoke like a freight train. now my question is, can I safely remove the gas flow from the carb to that cylinder and make a small like 20 mile trip? I was just gonna get the gas out of that cylinder and cut off the flow then do a quick oil change so i don't smoke all the way across town. I don't want bashing opinions, just to know if it would hurt anything?