compression test

you might have to download some manuals and check bore specs and stroke. Like you said earlier, the wildcard is wrist pin location. I have been told there is a difference in wrist pin location from the 750 and 700's.

Did you discover anything definitive causing the low compression in two cylinders?

 

Donald, did you read the thread where he spec'd out what the cylinders are reading?

It is a double overhead cam. One cylinder on the same set of cams has great compression, the cylinder next to it is bad. Real bad.

 

post #12, he checked for bent valves by using fluid in the manifolds and checking for sepage.

 

I'm pretty sure the 700 and 750 are a straight across swap. It's been discussed before and I guarantee there's another thread on the topic of you do some searching. As long as the motor has a starter still bolted to it, you should be able to take a battery and jumper cables with you to crank it.

The proceedure for verifying cam timing and installation is in the manual and would be lengthy to type out here. I'm assuming you have a manual if you tore it down and reassembled it?

You haven't answered the question about what you found. Did you do the leakdown test as recommended earlier? You were going to perform a leakdown test and your next post was about needing pistons? What's going on?

 

No it doesn't.

Post #9, you were going to apply compressed air into the cylinder and see where the air was leaking to.

Post #12, you explain that you used carb cleaner in the manifolds to verify that the valves were seating properly.

You do not explain if you did a leak down test by applying compressed air to the spark plug hole and how you came to the determination that you need pistons.

If you aren't going to provide more information and answer questions appropriately then I'm done offering my help. If that's ok with you, then we're both done wasting eachother's time.

 

There is a significant point to performing a proper leak down test.

There are probably a dozen reasons the motor can have bad compression, all listed earlier. The OP believes he put the rings in right, he believes he checked the valves for proper seal, he believes he put the cams in correctly too.

The best thing anyone can do here is assume the OP doesn't know shit about what he's doing. Sending him back to check cam timing which is a cumbersome process he believed he did properly is not wise IMO. If the guy did it wrong once, he'll look at it and still think it's right. That's why in quality control they have the guy who makes a part inspect it and a second set of eyes inspects it as well.

So the best thing to do here is to take a methodical approach and start with easy shit. We know the motor runs like shit and we know the comrpession test results are bad.

Doing a proper leak down test on each cylinder will easily prove the OP's assumption that there's a ring/piston problem, prove a valve was damaged, or prove the valve timing is off without disassembling anything.

The test is very simple. Remove all spark plugs. place a screw driver in the hole and turn the motor over BY HAND until the piston is on the top. Presurize the cylinder and listen for air leaking to the exhaust, carbs, the radiator, and the crankcase. There will be air leaking out the crankcase but the question will be how it compares between the good cylinders and the bad. Start each test with your compressor at the same PSI and turned OFF. You can use a clock to time how long it takes for all the air to bleed off your compressor thru the cylinder.

If the cylinder is blowing air past the valves - turn the crankshaft over again BY HAND 360 degrees. The piston will go down and back up. Try the test again.

If it still blows air past a valve then we know the valve timing is off and we can tell the OP he F'd up, go look for cam timing, bent valve, or valve/tappet clearance.

Here is the other important thing; There's a chance that this test will have all four cylinders holding pressure and leaking down in the same ammount of time.

How can that be when the compression test is bad??!!

Again, the cams can be in wrong and the valves are opening either late or early but ARE completely closed at the top of the piston stroke.

What's the point?

We will have proven that there are no bent valves, the pistons/rings, and the headgasket are all just fine.

And the whole test is easier than pulling valve covers.

I'd rather not chase rainbows until I've proven there's a pot of gold there.

 

You say the lobes (intake and exhaust) are both facing upward while trying to pressurize the cylinder?

If both lobes are up and away on that particular cylinder and you are getting noticable air blowing past the valves then either the valve/tappet clearance is too tight and isn't allowing the valve to seat or the valve is tweaked. The key is making verifying that the lobes are not near the valves.

Remember; you have 4 cycles Intake, compression, power, and exhaust. When the piston is at the very top at the beginning of this process, the intake valve is opening while the exhaust valve is closing. So the lobes will be contacting the valves and they may not be seating. Then you would turn the crank 360degrees to get the piston at the top of the stroke again and between the compression and power stroke. At this point the cam lobes should be a long ways away from the valve rocker and air shouldn't be blowing past the valves.

regarding the lack of cleanup in the honing process; Even if you didn't get 100% cleanup in the honing process, you should still have better compression than what you reported.

 

You have nothing to loose. If you're real dam lucky then maybe there's a chance it'll keep coming up. Give it another couple hundred miles and see what happens.

Understand two things;

1 keep a close eye on your oil level. That much blowby will cause you to loose oil out the crankcase vent so check it before every ride.
2 the engine will be out of balance so don't expect it to run well or to have carb vacuum balancing do you much good.