Compression Problems

[Pickup_man]

Hi everyone, I'm looking for a little help here. I recently rebuilt the top end of a bike for a buddy of mine because it was burning oil. The bike is an 83 Honda Shadow 750. I've had it apart several times with different issues now so here's the work I've done on it. New valve stem seals all around, new standard size piston rings, standard bore honed by a professional machinist, seated the valves. The lifters on this bike are hydraulic and are supposed to automatically adjust but there are shims underneath the tappets that I'm not %100 sure went back in where they came out, but I think they're right. So here's my problem, I tried to start it yesterday and it wasn't having any of that, it would pop occasionally but wouldn't fire, so I broke out the compression tester and I have 150 psi in the rear cylinder, and maybe 20 psi in the front, both cold measurements. I'm going to do a leak down within the next few days but I was looking for some input on what you think it may be? I should also note that I had the bike running after the hone and new rings, but it was running really bad, that's when I pulled the heads off again, ground and seated the valves and moved one tappet shim, but I don't remember which cylinder that was on.

Any input is appreciated. Thanks!

[beets442]

With the compression that low on the front cylinder,sounds like front ring/piston didn't seat right. A ring in upside down?Ring damaged/wrong,piston right direction?
Need to measure piston,cylinder and make sure gap is right too.
If its popping,its taking air in when it shouldn't,not timed right?,check intake manifold for air leak also.
Hope this helps

[Six Stroke]

Got the cam timing set right?

[Pickup_man]

Thanks, been a while since I've checked this, still haven't gotten around to diagnosing yet. It's possible that the cam timing is off, but it was checked, then rechecked, and rechecked once again because it can only be done with the engine out of the frame, so I think it's unlikely that it's off. I likely have to pull the engine out anyway, so it will be checked again.

The rings were direct replacements, I doubt they are wrong, it's possible that they were damaged though, although it's really tough to get them in if they aren't perfect (I wrecked one ring already and replaced it) Piston is the right direction, it was never taken off and the cylinder only goes on one way. I think the popping is just excess gas that gets built up then fires, because it's not a popping while running, it's a crank for five minutes, pop once, crank until the battery is dead (not constantly cranking) without another one.

I'm leaning towards the rings not being seated right and the cylinder being too big. I had the stock cylinder honed to break the glaze for the new set of rings and my guess is that it got honed just a bit too much and is now out of spec. Once I get some time again I'll do a full leakdown and determine where it's losing compression. I'm guessing I'll have to buy another stock cylinder hope it's in spec (check it when I get it) and have it honed again, (because that's much cheaper than new pistons, rings, and a bore for both cylinders).

Thanks for the advice!

[El Camexican]

The leak-down will tell you more than any of us can, but with a comp reading of 20psi I would speculate in this order:

- Cam timing
- Incorrectly set valves
- Bent valve (if you were turning the engine over without the cam timing being correct at any point may be the problem)
- Head was not torqued down, or the gasket was incorrectly installed
- Cracked head
- Broke a ring when installing the cylinder and put a very deep grove in the cylinder

A slightly over honed cylinder, or a loose piston, large ring end gaps, upside down rings etc. are not going to make all the compression go away. You’d still get a reading of 80+ psi if it were any of those things.

[Six Stroke]

+1 for valve trouble. I'm curious about what the leakdown will tell you.

[Pickup_man]

Thanks for the help! Hopefully it is in the valves, I may have to just buy the dial indicator and do a full check on the valve shims, as much of a pain as that may be :rolleyes: but... do the job right the first time right? If I did I probably wouldn't have to pull the engine out for a third time. I'm guessing that re-seating the valves and possibly having a shim in the wrong place would put a valve out of adjustment. Hopefully I can get to the leak down this weekend, the fiance's car broke so that's taking priority, I was hoping to get the leak down done this week but oh well, it'll get done eventually. Thanks again for all the help, I'll post some results once I can get to it.