Low Compression?

[Anonymous]

I have a 82 Gs 750T that has 35,000 miles. According to the factory manual specs, my comp numbers are out of tolorance and vary from one hole to the next. I borrowed a gauge that was the tapered rubber type. Are those gauges reliable? I replaced the head Gasket with no change in the numbers. Should I look at honing and rings next? Thanks Steve

 

[Billy Ricks]

Squirt a little oil down into each cylinder and see if the numbers go up. If so rings are the problem, if not it's valve seating. Also, make sure the slides in the carbs are up when checking compression. With CV carbs it means physically holding the slide up or removing the carbs from the manifolds.

 

[saaz]

If it seems like the valves, check valve settings. make sure you hold that guage in tight, otherwise you will get false readings. It is easier to get someone to help so you can hold it on properly.

 

[Anonymous]

Low Compression Thanks

Low Compression Thanks

Thanks guys for the tips. How much oil is a little? I didn't didn't raise the slides, I just removed the plugs but, I will next time. I did check and recheck the valve settings after the new head gasket install. Steve

 

[massakins]

GSSteve, a more definitive check is a leakdown test. This can identify whether its rings, intake valves or exhaust valves. Similar procedure to compression check but different tooling. Note that if the motor was warm and cooling off as the compression check was performed you may get varying readings. Also if its been sitting for a long time and the rings are stuck in their grooves. Or if the petcock leaks by while the bike was parked on its side stand the left cylinders may read low. Been bit by them all. Have fun.

 

[Don Lobacz]

What are the compression numbers that you are getting? I would see if you can borrow a screw-in compression gauge and compare (Sears sells a good one for $20, I think). Raising the slides means holding the throttle open when checking the compression.

 

[Anonymous]

My Service Manual tells me that Standard Comp. range is 128-170 psi. Max allowed difference between higest and lowest cylinders is 28psi and the low limit is 100. I had only one cylinder barely in the stand.range and two below the low limit. I don't remember the exact numbers. The screw in gauge sounds like the only way to know for sure where I am at. I hope the rings are the problem rather than the valves because I can't find anyone around here that works on old gs heads but I think I can deal with the rings ( I think!!!) Thanks Steve

 

[Don Lobacz]

Most likely it is not the rings, and that is fortunate (they are fairly expensive, require base gasket/big O-ring replacement, require a quality hone and replacement without breaking/lipping is tedious). Carbon buildup on the valve seats often is the cause of low compression. A solvent test should have been done when you had the head off, and it would probably have revealed slight leaking around the valves. The fix is head disassembly, valve/seat/combustion chamber decarbonizing, and valve seal replacement. All motorcycle shops should be equipped to do this, but quality of work may be a question. I have personally done many, and if you can't find anyone that you trust locally, I can do it (as many other people here can). If you recheck the compression with a better gauge and the readings are no higher, then the head has to come off again anyways.

 

[Anonymous]

Don....Thanks for the info. I am going to start by getting a screw in gauge. After reading your post and finding valve servicing info in my manual, I have a much better grasp on whats involved here. The light is starting to come on! Thanks again Steve

 

[Billy Ricks]

Just twisting the throttle won't work on CV's. They need motor vacuum to raise the slides. You'll have to prop the slides up or take the carb bank off to check compression. If the slides are closed you will get a low reading.

 

[moto_dan]

Just twisting the throttle won't work on CV's. They need motor vacuum to raise the slides. You'll have to prop the slides up or take the carb bank off to check compression. If the slides are closed you will get a low reading.

I understand about the slides being raised, but was wondering...could you just remove the bolts in the intakes where vacuum gages are hooked up or do the cylinders require more volume?

 

[Don Lobacz]

There is a gap under the slides that is sufficient for checking compression once the butterflies are open. Think about it - when the bike is idling, the butterflies are almost completely closed, yet enough air passes (through small side channels) to fill the cylinder and achieve combustion. If you want to recheck, pull the sych stops and see if anything changes (or pull the carbs too).

 

[Billy Ricks]

All I know is when I had the stock carbs on my bike you couldn't get a good reading without raising the slides along with cracking the throttle. There isn't much of an opening with the slides down. And that's where they stay until engine vacuum sucks them up. That opening is wholly dependant on having enough rpm to keep it open.

 

[Don Lobacz]

Billy, your engine design is more advanced than the average GS and I must admit that I have never had a set of carbs like yours to look at. How much lower are the numbers if you don't raise the slides (a ballpark figure)? I try not to overgeneralize since it won't help anyone, even me, when we run into exceptions to a rule.

 

[Anonymous]

I have a screw in gauge on the way. Hopefully the numbers will be better. I really couldn't believe the low numbers based on my bike's 1/4 mile times. I was thinking that as much as the numbers were out of tolorance that there would be a noticable horsepower drop. I do however think that with the minor mods I have done that the bike should be a little quicker since it runs almost the exact times the new GS750T I bought in 82 did box stock. 12.50s

 

[Billy Ricks]

Don,
I haven't run the stock CV's in a long time. It's been about 13 years since I did a compression check with those carbs. That was after missing a shift and burying the tach needle. That spun a couple of the threaded valve adjusters right out of the rocker arms. I did a compression check at that time to make sure I hadn't bent any valves or anything. I want to say there was about 20 lbs of difference between the readings with the slides down vs up. It's been so long though that I can't say for sure. I was disappointed in the readings I got with the slides down. After raising the slides the numbers were better.

 

[Don Lobacz]

Thanks, Billy. That is a significant difference.

 

[saaz]

The final compression numbers should ultimately the same, slides up or down. The key is how many revolutions does it take to get to the max reading...one or two, and things are dandy...if it takes a few turns, then things are not as healthy... Slides up gives a better chance for consistent readings and observing how the pressure is reached. If the bike still performs well, hopefully the readings are still within acceptable limits.

 

[Anonymous]

I got my screw in gauge. With a warm motor here are the readings. Cyl. #1 - 140 #2 -114 #3 - 116 #4 - 148. These were w/slides propped up and wide open butterflys. I checked one cyl. w/slide down and the numbers for that cyl. dropped 3 psi. I put a little oil in cyl. # 3 and the numbers dropped by almost 40 psi. I don't know what was going on there!
Maybe too much oil eh?

 

[Billy Ricks]

Most likely too much oil. You didn't happen to notice any up in the hose to the guage did you? If you had bad rings you would expect to see the numbers go up. If they don't go up, or very little, the rings and cylinders probably aren't in too bad of shape and valves are the problem. The only other thing I can think of, and it's not likely, is the extra oil didn't allow a valve to seat right and that led to the 40 psi drop.

A leak down test would tell you exactly where the problem is. Unless you know someone with a kit and a little know-how you may have to either bite the bullet and pull the head or have a leak down test done. I've seen guys make there own valves to fill the cylinder with air out of an old spark plug. You fill the cylinder with a certain air pressure and see how long it takes to drop. You can hear where the air is escaping. If it is valves you can hear it coming out the intake and/or exhaust ports. If it's getting past rings you can pull the oil filler cap and hear the air getting into the crankcase. Naturally some air is going to get past the rings and valves. That's why you need the kit to guage what is normal and what's not.