Low compression after engine reassembly

[MrKaizer]

I had the engine top end of my GS1000G apart because she was leaking from every seam and burning oil like crazy. Accordingly, there were really thick carbon deposits everywhere. I replaced the valve guides and had a shop regrind the valve seats and skim the head. I thoroughly cleaned the pistons and ring grooves, so the rings could move freely again.

Now for the problem: Some time before disassembly I checked the compression and it was 9 - 10 - 9- 9.75 bar. Pretty ok for an old abused engine. Now, after reassembly, all cylinders sit at 6 bar. Really disappointing. WIth some oil in the cylinder it went up to 7 bar. Not a lot so I'm hoping the rings are ok?

All cylinders being so similar makes me think that maybe I botched the assembly. Could the low compression be due to the cam shafts being one tooth off in relation to the crank shaft?

Other possible reasons:

1. Compression was only high before because all the carbon buildup made the volume smaller.
2. I didn't lap the valves after the shop did the valve seats, assuming this wasn't necessary. Was I wrong?
3. Cleaning the cyliner ring grooves freed up the rings a lot --> byebye compression.

Maybe something else I missed? Do you have any hints what else I could check before I tear the head off again?

[Gixer1460]

Reassembly of an obviously worn engine never ends well. The carbon was helping the CR and ring seal. You need to check piston / bore wear and ring wear in bore - likely they will be out of spec!

[Arttu]

So the seats were ground but not the valves? That could be one reason. Now the sealing faces on the valves are worn to match the seat face before machining and don't match properly on the machined seat face. Trying to lap them with grinding paste may not be the best idea. You may end up ruining both the valves and seats in the worst case. 

Wrong cam timing can naturally affect on compression too.

[gs7_11]

Personally I would never re-use old rings on an engine with significant use. I would always at least hone the bores clean, then measure to see if they're still in spec and check a rebore isn't required.

Cam timing should be checked, and it's also possible the compression will improve with some more miles if you haven't done much.

[TonyGee]

when you strip the top end then you always check bores and rings for wear,  best to replace them while its in bits then you know its right. 

[MrKaizer]

Thanks a lot for all your input. Seems like I made a few mistakes. This is the first time I'm doing a big four-stroke and I'm learning as I'm going.

On 7/23/2024 at 11:08 PM, Gixer1460 said:

Reassembly of an obviously worn engine never ends well. The carbon was helping the CR and ring seal. You need to check piston / bore wear and ring wear in bore - likely they will be out of spec!

I checked the pistons when I had the engine apart and they were in spec. I don't remember, if I checked the cylinders but the cross hatching was still visible, if that is any indication. I did not, however, check the rings. No idea why. So this could definitely be the issue.

While researching the low compression issue I found out that you can do a leak-down test. This seems to be a good way to narrow down the cause before I rip the engine apart... So I ordered one from the big A and will have a listen tomorrow.

[Gixer1460]

You say you've checked the pistons - but did you check the ring grooves in the pistons for depth / side groove wear? Its likely that if the carbon build up was a bad as it sounds, the wear there was being 'masked' by the carbon deposits, hence now when clean the rings fit like a 'cock in a sock'.

[gorbys]

might be worth checking the cam timing again, i put mine one tooth off and ended up with like 60psi less than it should have.