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compression diminishes to nothing from 1 to 4

  • Thread starter Thread starter Paul's Zuki
  • Start date Start date
Are you talking free end gap or compressed in the cylinder? 4mm is actually below the service limit for free end gap (5.2mm min). Service limit for gap in the cylinder is 0.7mm max.

Paul's Zuki said:
bear with me, I'm new at this. The rings had a gap that measured about 4mm, I mean. pretty good size gap though
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I dont understand and I'm so freaking frustrated.

so after figuring out I didn't have sufficient compression and 2 cylinders had no compression a top end rebuild was in order. This is what I did per advice from you guys, good advice, but I don't understand what's happenning.

replaced bad valves (2)
lapped all valves and did the leak test. (no leak)
checked clearances on valve shims and replaced 3 now all within spec
replaced valve guide seals
checked tolerances on valve springs(still in original spec)
flex honed the cylinder to find that they were scored so I purchased another and it immediately looked better.
flex honed those bores for about 40-60 second with a slow cordless drill. cleaned out very good and made sure a white towel came thru clean
replaced all piston rings and made sure the ring end gap was no more than .008"
replaced and used all oem seals & gaskets
Timing is all perfect and aligned per manual instructions

Still only getting about 60 psi per cylinder at least all are even now.

Please help. I'm getting to the end of the line with this build.
 
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chef1366 said:
Did you seat the rings? Did you run it?
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well I've heard that I at least need 100 psi to get bike running. can i get it on with only 60 psi? I know I need to seat rings by running, but I'm confused about getting it on.
 
I have not read the whole thread in depth so apologies if these are stupid questions;
1 Did you make sure you installed the rings the right way up and not upside down?
2 Did you make sure you spaced the ring gaps roughly 120 degrees apart around the piston and not all lined up below each other?
3 Are you sure the battery is full and the engine is cranking at the correct speed when checking, could also be caused by a dragging starter?
4 Are the carbs being held wide open while checking, making sure there is no restriction?
5 have you checked for warpage on the head? may need to have it surfaced to get it straight.

Yes you can start it with 60psi and run it, it will increase with some running as the motor warms up and the clearances settle, but you should be getting more than that, I doubt very much it will double, something is wrong somewhere.

I know you said it was fine, but just for giggles, double/triple check your cam timing, I was frustrated for a week with a motor that would not run right, the timing was out one tooth even after I was convinced it was right, I had checked and double checked it, it happens.
 
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Even is better. 60 psi is disconcerting I'm sure to look at: but you need to get it running to seat the rings with the cylinder. The service limit for compression on early GS motors was actually 90 psi - and that is hot! Once it catches the compression strokes will be much quicker and there will be less opportunity for ring leakage.

Slap those carbs on and try to get her running.
 
Flyboy said:
I have not read the whole thread in depth so apologies if these are stupid questions;
1 Did you make sure you installed the rings the right way up and not upside down?Yes, installed the correct way
2 Did you make sure you spaced the ring gaps roughly 120 degrees apart around the piston and not all lined up below each other?Yes, spaced apart correctly as I was lowering head
3 Are you sure the battery is full and the engine is cranking at the correct speed when checking, could also be caused by a dragging starter? has been sitting for about a 2 weeks. Its a brand new battery, but will start charging to full this morning while installing carbs.
4 Are the carbs being held wide open while checking, making sure there is no restriction? when checking the compression just last night, I did not have carbs installed yet, only the boots, so, yeah, I guess it was fully open
5 have you checked for warpage on the head? may need to have it surfaced to get it straight. Did not check warpage per se, but the two heads I used were off bikes with only 9000 miles, surfaces were impeccable, from the first head when removed the gasket did look completely pressed

Yes you can start it with 60psi and run it, it will increase with some running as the motor warms up and the clearances settle, but you should be getting more than that, I doubt very much it will double, something is wrong somewhere.

I know you said it was fine, but just for giggles, double/triple check your cam timing, I was frustrated for a week with a motor that would not run right, the timing was out one tooth even after I was convinced it was right, I had checked and double checked it, it happens.
Click to expand...
I will check again. I was actually having some trouble last night about 3-4 times and finally got it at the position it was supposed to be with the 20 chain pins between the 3 & 2 marks on the cams.
 
If you did everything right as you say I wouldn't worry about the compression yet. It will be fine. The engine needs to be run before you will get decent numbers.

Focus on making sure the cam timing is perfect, the cam chain tensioner is working properly and released, get the airbox installed and sealed, check spark, and make sure the carbs are clean and full of fuel. Do all this and the engine will start.

Good luck
 
Thanks guys, you sure are giving me some hope. So, should I install the cam cover also?
 
so I got it all assembled. any good tips on first start ups? I've tried starting it up and nothing. any tips please
 
Ok. So let me ask this. my bowls have fuel in them. I did rebuild the carbs. But since vacum pulls fuel up from bowl how do i get vacuum? compression? If my compression is low to begin with how to i get it started up or how can i get fuel to the cylinders?
 
Where do I start? Manual doesn't say anything about dry or wet plugs
 
I know this is not politically correct, but just to get things moving, dribble a little gas in each cylinder and reinstall plugs. It has worked for me with a cantankerous Yamaha turbo at the beginning of each season...
Curt
 
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bellucci said:
I know this is not politically correct, but just to get things moving, dribble a little gas in each cylinder and reinstall plugs. It has worked for me with a cantankerous Yamaha turbo at the beginning of each season...
Curt
Click to expand...

anyone second this motion? I have thought about this and did a google search and couldnt find anything. I will search here.
 
I understand your hesitation. The truth was, I made sure the oil pressure has lubed the cylinder walls and just used ether to speed up the combustion process. I suggested gas as most mechanics don't approve of ether due to it rinsing the oil off the cylinder walls. And since you have a fresh motor, oil is your best friend for those rings...
Curt
 
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