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GS850 No Compression

  • Thread starter Thread starter wdavie
  • Start date Start date
W

wdavie

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Hi guys. I had two 850 engines that both ran quite well, however one had a knackered gearbox and one had a rattly top end. I swapped the cylinder head from one engine onto the other. I now have no compression on the engine, good spark etc, what could I have done wrong?:-k I know that both engines had run strongly, I never touched the barrels or pistons just swapped top ends over, used OE gasket, I did note however that one engine is the early type and the other engine is the later type with different style clutch cover. Help please as I'm pulling out my hair. Thanks Wayne
 
Cams installed 180 degrees out?

Pull #1 spark plug.

Turn engine over and align the 1-4 T mark alignment on the crank.

Look for piston at TDC

Check cam indexing and look at notches in cams - they should face inward toward each other.

Good luck and hope this helps.
 
Sorry to show my ignorance but would it be possible to have the cams 180 degrees out if I set up the timing etc as stated in the manual:confused:, ie I got the engine to TDC set the numbers on exhaust cam and counted twenty links to the numebr 3 mark on inlet cam? Cheers Wayne
 
My suggestion to you Wayne is to double check your cam timing. If you are confident the cams are correctly timed than disregard my suggestion and look for other reasons.
 
Wayne,
To have no compression in a cylinder, the valves must be open when they are supposed to be closed. Either that or you don't have piston rings. Since you didn't disturb the pistons, your troubleshooting can concentrate on the valves. Did you by chance reverse the exhaust and intake cams?
 
Ok took rocker cover off again today set the valve timing again made sure everything was spot on, tried starting it and again no compression:mad:. What would casue the engine to lose compression like this? Should I put the original head back on again? Any help most appreciated
 
Wayne, have you checked to see if you have any clearances on your valves? If you've changed your heads over it might be that the shims you're using are too thick and the valves can't close.
 
Cheers Wally that hadn't crossed my mind, would be particularly true if I mixed up the cam shafts as well, couldn't remember which shafts went with which engine. Must be more organised in the future.
 
wdavie said:
Ok took rocker cover off again today set the valve timing again made sure everything was spot on, tried starting it and again no compression:mad:. What would casue the engine to lose compression like this? Should I put the original head back on again? Any help most appreciated
Click to expand...

Well, compression takes a sealed piston and closed valves (with the spark plug in place, of course)

so

1. The pistons provided compression before, so that's not the problem
2. So it must be the valves

Did you check the "new" head to confirm the valves weren't leaking before you installed it?

Burned valves are quite common on GS motors

Other than that, as suggested, cam timing would be the cause. It's doubtful there's enough difference between the earlier and later cams to lose compression.

If you've mixed up the cams, are you sure the intake and exhaust aren't mixed up?
 
Both engines ran fine before I dismantled them, used the head from one to go on the other, can only think somehow that the change has caused the problem. The cams aren't mixed up as I checked the timing again earlier today and cams are definately in the right order.
 
Hi all I've been checking the Alpha Sports website and noticed that the part numbers for the cams are different from a 79 engine to the 83 engine. Could the cams have a slightly different profile that could be causing my problem?
 
wdavie said:
Hi all I've been checking the Alpha Sports website and noticed that the part numbers for the cams are different from a 79 engine to the 83 engine. Could the cams have a slightly different profile that could be causing my problem?
Click to expand...

No. The cam timing is slightly different but that won't give you no compression.
 
If the cam have a higher profile and prevent the valves from closing and creating a seal would this not cause compression loss?
 
I'll try to get a picture of it this week, bike is at my lock up garage so only go down when I'm doing work on it. I'm thinking of swapping the cams over anyway set the timing again and see if that fixes problem. If not at least it rules out that theory:-k
 
wdavie said:
I'll try to get a picture of it this week, bike is at my lock up garage so only go down when I'm doing work on it. I'm thinking of swapping the cams over anyway set the timing again and see if that fixes problem. If not at least it rules out that theory:-k
Click to expand...

Basscliff has the factory service manual you can download for free. The manual will show that the valve lift is the same for the different years, the cam timing changed in 1980 but it's no big deal and won't cause the problem as noted unless maybe you changed the cam sprockets (and even if you did I'm not sure that would cause a loss of compression).

Did you check the valve clearances?
 
Last edited:
Did you open the throttle when you checked it ?? Other wise to have No compression it would almost have to be cams out of time or valve adjustment
 
I read the replies but I might have missed this suggestion
pull the plugs remove cam cover on the right side put a 19mm socket on the crank end.
rotate motor using the ratchet and put your thumb or finger over the plug hole and watch the cam lobes you should during one rotation of course feel some compression or air trying to escape the cylinder also at some point during the rotation both valves should be closed or in other words both lobes should be on a flat spot.
The other thing you can do is rotate the motor to the "T" mark and on the left side plug removed piston should be at the top of its stroke and both cam lobes should be flat against the shim buckets if not rotate one more time till you see the "T" and recheck
 
OK Guys pulled the rocker cover off agian today. Set the timing correctly, no difference. Checked all the valve clearances and on number 1,2, and 4 cylinders had massive clearance on the exhaust cams acceptable clearance on number 3 exhaust cam. Inlet cams were ok. I had some compression on number 3 cylinder but nothing on the others. Could the fact that the gaps were big cause no compression? I thought it would have to be the other way round and too small a clearance to cause problems?:confused: I'm more confused than normal now any help greatly appreciated
 
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