Compression test / top end rebuild difficulty

[AllanCaveNZ]

Hi guys,

Was putting my bike together recently and had a question about carb / timing and received a couple suggestions that were a heap of help so thanks.

I did the PMA / Pamco Ignition / Mikuni VM34s combo and trying to get it dialed in, it starts up best with the ignition set at the middle of it's advance (plate has equal room on left and right of the screw), and the right hand cylinder backfires until I tweak the carb to run a fair bit richer than the left carb, runs okay like that but a little lacking in power at lower revs.

I presumed this is a sign of average compression, did a compression test today and got a tad under 110psi on right cylinder and just a little over 100 on left cylinder, but didn't realise I needed to remove the spark plug from the opposite cylinder while testing, will this have influenced the test or just made it harder to turn over while doing so? Engine was warmed up fairly, though might have cooled down a bit by then. Cam chain / valve clearance adjusted fairly recently but could check that again before retest I guess.

Should I retest without the opposite plug in? Any thoughts on how challenging it is to do a top end rebuild having never done one before on any bike, but being moderately experienced with bikes otherwise?

Any advice appreciated.

 

[Airwolfie]

Have a look at my engine rebuild thread.

http://www.xs650.com/forum/showthread.php?t=30786

To be honest, it is a LOT LESS daunting than it sounds, as long as you follow all instructions in manuals, and on the forum. Then nothing can go wrong(he says not having started up his engine yet!).

Top end is the easy part, the only tricky bit is aligning the cam with TDC,but I managed after a few goes!
If you do a topend rebuild, you may as well go all out and replace the camchain guide and tensioner while you in there, if you haven't done so already.

 

[pamcopete]

Allan...,

1. When doing the compression test you must hold the throttle wide open else you will get low readings.
2. Use a timing light to set the timing for best results.

 

[gggGary]

Compression testers do vary on what they read but in general 100 PSI is on the low/marginal side of things. I don't worry about the the other plug either way. Throttle at least cracked open will get the number up to it's highest reading quicker. I did some back to back testing and can get a bit higher reading with my foot compared to the e-start.
All that said it should run OK with 100 especially with a Pamco ignition. Having to adjust one carb different than the other most likely points to an issue in the carbs.
The 650 is on the easy side of top end overhauls. But the quality (result) of the work is going to depend on how rigorous you are with precision measuring or how much you spend on new parts and quality machining services.
I am thinking about measuring valve guide wear, valve seat machining and piston/ring/bore measure/machine/replacement. while you are there front guide/cam chain replacement. Add all that stuff up and it's real easy to blow right through $400 and keep going, assuming your labor is free.

 

[AllanCaveNZ]

Thanks guys, well there's a guy down the road a bit called Daryl Hutcheon who does xs rephasing and such and while I can't justify spending that much maybe I can do the tear down myself and then pay for his time for some advice and help sorting it out. Could run it for a while with average compression but don't really want to.

On another note shorted a wire today and think I may have fried the Pamco... definitely fried the coil but put an HD one in and it fired once then stopped, might be a bad earth though, will check the wiring better before I decide what's going on.

 

[scabber]

Welcome to the site. The compression is on the low side. Not knowing the history of the bike or year and model barn find a picture. Do the basics all the usual adjustments recheck.Set the timing and get the carbs working not knowing your setup. Some good threads on the VM 34 carbs google search. Daryl is a good guy to have around. I would go see him if you need to do a top end or not. Get it running good first on the compression you have.

 

[AllanCaveNZ]

Thanks scabber. Have already taken those steps and it's running as well as it can I believe, the 10% compression difference is annoying though as one cylinder likes to run richer and hard to get it running really nicely between 2.5k and 3.5k revs.

Will have a chat to Daryl, would like to get him to do the whole rebuild but not sure I can quite justify spending that much, has anyone heard of any other shops in Melbourne, Australia that have worked on XS650s at all?

If I do go through with the rebuild myself what sort of special tools would I be looking at? Just a valve seating tool and cam chain breaker tool and valve spring compressor? Is a normal digital caliper alright for measuring tolerances etc?