Yeah, its another valve gap question...
I have read several pages of posts about valves and compression. I am just ready to get this bike running.
I will give my situation first then ask the question?
I just got the valve gap measured:
Left Intake=.102mm gauge goes .127mm doesnt (2.55 shim)
Right Intake= .038mm goes .051mm doesnt (2.60x shim)
Right Exhaust= shim turms .038mm doesn't fit (2.30 shim)
Left exhaust= .076mm goes .102mm doesnt (2.55 shim)
With these specs, I get 60 psi on my screw-in compression gauge. I get no ignition. I get enough pressure to barely lift my finger off the spark plug hole for that test.
I made sure the rings were staggered according to the manual. I put a little oil in the cylinder and still get 60 psi. Before assembly, I verified valves were seated. I am 100% that the timing is correct. (2,3 and 18 teeth stuff...)
I have changed the shims to this:
Left Exhaust=unchanged
Right Exhaust=2.25
Right Intake=2.55
Left Intake=2.60x
I still get 60psi...
What do you all suggest? Different shims? Specific sizes for each cylinder would be helpful. I have been going to the local dealership to get parts and ask questions on my bike. I have access to the valve tool and shims if I need them. We have traded shims in the past. ( The mechanics didn't want money for their help. So I bought them a 12-pack of Mt. Dew for their advice and generosity...)
Any advice would be great,
04MtBee
Are you getting only 60 psi in both cylinders? How did you do the compression test?
Your shim choices look mostly OK to me (I would have left the Right Intake alone). Since you've just rebuilt the engine, I suggest double checking the timing first.
Why are you rebuilding the engine? Was there a catastrophic failure?
Did you use new gaskets? Are all bolts torqued properly? Were the valves reground and seated properly?
When you say "no ignition" - do you mean it's dead (electrical), or is it trying but just won't start?
Yes, 60 on both cylinders... I did a compression test with a screw-in tester: Throttle wide open, hitting starter button...
I can install a 2.60x in the Right Intake again...
I had to replace the head because a PO bent an exhaust valve. Yes, to catastrophic failure...
New Haskets, and o-rings...
Torqued correctly...
Valves NOT reground, but appeared to be seated. Did not do leak test... (Can look at this again, but wanted to see if valve gap might be culprit before I take it apart again.)
"No ignition" it just won't start.
Thanks for reading this...
04MtBee
Compression is made when the valves are closed (compression stroke). If there is ANY clearance between the cam and the bucket they will be closed (barring bent valves or bad seating or what-have-you). Unless there is no gap at all, changing valve shims will have zero effect on compression. If your compression is low, either the valves arent sealing or the rings arent sealing. A leakdown test will tell. If there are new valves installed, i definitely recommend lapping them in.
A gasoline 4 stroke engine, rule of thumb, needs at least 100 psi to run at all. I'd look at the compression before worrying about anything else.
Who rebuilt the head?
Valves should have been lapped in.
If you rebuilt the head yourself- Did you replace ALL the valves? They don'thave to be bent much to not seal.
Quote from: 04MTBee on June 29, 2009, 05:35:07 AM
Yes, 60 on both cylinders... I did a compression test with a screw-in tester: Throttle wide open, hitting starter button...
I can install a 2.60x in the Right Intake again...
I had to replace the head because a PO bent an exhaust valve. Yes, to catastrophic failure...
New Haskets, and o-rings...
Torqued correctly...
Valves NOT reground, but appeared to be seated. Did not do leak test... (Can look at this again, but wanted to see if valve gap might be culprit before I take it apart again.)
"No ignition" it just won't start.
Thanks for reading this...
04MtBee
Quote from: AccidentalF on June 29, 2009, 07:12:04 AM
<snippage>
A gasoline 4 stroke engine, rule of thumb, needs at least 100 psi to run at all. I'd look at the compression before worrying about anything else.
This is true but in a way its backwards. The compression is actually also a measure of suction. That lack of suction will not let the cylinders fill. Better compression almost always means better suction and hence a 100 compression motor makes good enough suction to pull the mix in through the carb.
Anyway I would get it timed accurately too, you want the piston comming up when the 2 valves are shut obviously on the compression stroke.
Cool.
Buddha.
How many teeth can you be off on the chain before things hit, anyways?
Quote from: The Buddha on June 29, 2009, 08:00:32 AM
Quote from: AccidentalF on June 29, 2009, 07:12:04 AM
<snippage>
A gasoline 4 stroke engine, rule of thumb, needs at least 100 psi to run at all. I'd look at the compression before worrying about anything else.
This is true but in a way its backwards. The compression is actually also a measure of suction. That lack of suction will not let the cylinders fill. Better compression almost always means better suction and hence a 100 compression motor makes good enough suction to pull the mix in through the carb.
Anyway I would get it timed accurately too, you want the piston comming up when the 2 valves are shut obviously on the compression stroke.
Cool.
Buddha.
On a GS I suspect you would never hit - you may have the valves hit each other more than likely, I have to check in one of the dropped valve heads I have.
But for valves to hit you need to be off relative to each other not neccesarily with the crank. Like your cams are 1/2 a rotation or there abouts out.
Cool.
Buddha.
Quote from: intergalactic on June 29, 2009, 10:28:48 AM
How many teeth can you be off on the chain before things hit, anyways?
Quote from: The Buddha on June 29, 2009, 08:00:32 AM
Quote from: AccidentalF on June 29, 2009, 07:12:04 AM
<snippage>
A gasoline 4 stroke engine, rule of thumb, needs at least 100 psi to run at all. I'd look at the compression before worrying about anything else.
This is true but in a way its backwards. The compression is actually also a measure of suction. That lack of suction will not let the cylinders fill. Better compression almost always means better suction and hence a 100 compression motor makes good enough suction to pull the mix in through the carb.
Anyway I would get it timed accurately too, you want the piston comming up when the 2 valves are shut obviously on the compression stroke.
Cool.
Buddha.
">Valves NOT reground, but appeared to be seated. Did not do leak test... (Can look at this again, but wanted to see if valve gap might be culprit before I take it apart again.)"
without lapping the valves, the motor wont seal. I once did a tercel like this, replaced a burnt valve without lapping... and regretted it forever. The cylinder always leaked. expensive lesson.