No compression after cylinder head resurface (GS650G)

[Guest]

Hi. I came here to find some thoughts on what should I do in order to get back my '83 GS650G.


Mi bike was running a little bit bad cause the carbs y some stripped threads on cylinder head. After rebuild the carbs I went to a machine shop and let them my cylinder head. They fixed some threads, studs and resurface the flat (combustion chambers). After that, mi bike didn't star again, I measured the compression on each cylinder, and all 4 of them is below 30 psi. Suzuki said the minimum is 100 psi (normal should be 142 - 199 psi), so I'm pretty sure I lost compression after my cylinder head was resurface.


But, I came here to get more toughs on what to check or what other things may cause such a low compression. My gaskets are new and I'm thinking to buy new piston rings and a good condition cylinder head... do you have any other recommendations?


thanks to all.

[red1100]

The head would have to have a severe warp to get numbers that low.Did you torque all the head bolts down to spec in order?

[hampshirehog]

Did you check the valve clearances?

[tom203]

Did you check the valve clearances?

Yeah, this is what comes to mind.....or maybe camshaft to crankshaft postioning

[Tom R]

Yep - sounds valve related.

[gfpunto]

I did a complete valve service before I took the head apart. Maybe I'll need to do it again. Clearances are good, but I'm not sure if they're perfectly sealed.

So, this is what I'm going to do this week.
- Take the head apart again.
- Perform a valve job and valve seats.
- I'll order a new set of piston rings.

Any other suggestion to look at?

thanks for all the comments.

[Steve]

I did a complete valve service before I took the head apart. Maybe I'll need to do it again. Clearances are good, but I'm not sure if they're perfectly sealed.

If your machinist also cleaned up (or re-ground) the valve seats, the valves are sitting deeper in their positions. Sitting deeper will reduce the clearance at the other end, where you change shims to adjust clearance. It would be very likely that ALL clearance has been removed, meaning you will have no compression.

First, verify the proper timing of the cam in relation to the crankshaft. When that is assured, re-check your valve clearances.

.

[Nessism]

I did a complete valve service before I took the head apart. Maybe I'll need to do it again. Clearances are good, but I'm not sure if they're perfectly sealed.

So, this is what I'm going to do this week.
- Take the head apart again.
- Perform a valve job and valve seats.
- I'll order a new set of piston rings.

Any other suggestion to look at?

thanks for all the comments.

What is a "complete valve service"? Does this mean you took the valves out and cleaned/lapped them?

Regarding the compression issue, my first guess is that you got the camshafts incorrectly timed. Did you make sure to put cylinder #1 on TDC, 1-4 T mark aligned on the crank, and then install the camshafts such that the notches on the ends face inward, toward each other? It's easy to get the cams timed 180 degrees out of phase.

[gfpunto]

Exactly. I took the valves, cleaned and lapped... valve seal are new.
I did exactly what service manual said. Put the T make align with the arrow on the crack (on the points area). Align the "1 ->! mark horizontally and "2" mark facing up (exhaust cam), count 20 pins (of the chain) and align with "3" mark on intake cam.

[Nessism]

Exactly. I took the valves, cleaned and lapped... valve seal are new.
I did exactly what service manual said. Put the T make align with the arrow on the crack (on the points area). Align the "1 ->! mark horizontally and "2" mark facing up (exhaust cam), count 20 pins (of the chain) and align with "3" mark on intake cam.

You might want to double check the timing. Remover the spark plug for #1 and make sure you can see the piston at the top when the 1-4 T mark is aligned, then make sure the notches in the camshafts are facing each other.

[gfpunto]

Thanks to all. The bike now starts great. I changed gaskets, piston rings, and another full services to the valves. Now I need to focus in other issues haha.

[alke46]

Thanks to all. The bike now starts great. I changed gaskets, piston rings, and another full services to the valves. Now I need to focus in other issues haha.

C'mon man, don't leave us hanging here. What was the cause of the "no compression"?

[gfpunto]

C'mon man, don't leave us hanging here. What was the cause of the "no compression"?

I'm pretty sure it was a piston rings issue and maybe the cylinder head gasket. I bought all that new (rings are aftermarket).