82 gs 750 engine rebuild help!

[Cashmoney89]

thanks for taking the time to help me out guys, just had a new CDI installed in my bike, at a suzuki dealer, ( im usually a DIY but not a fan of electrical problems) so as i go to pick her up, hes telling me the bike needs a lot of work, being the compression is way under spec @ 70-100 cfm. Telling me my rings are shot ect... So im home now, with the motor disassembled...and the freggin cylinder walls and pistons look great! I cant see any hot spots, carbon build up, score lines, nothing....

Problem- bike was running, but suffering at higher rpms, backfiring...compression test showed low compression...what can i do to solve this....engine is already torn down to crankcase.

Recent work- new battery, charging system was redone, electrical tested at suzuki, and CDI from same year/model 750.....i took off the carbs, and then the bowls, cleaned the floats and a few valves, nothing was clogged, or gummed but a lil water was in the bottom of the bowls

Any ideas! much appreciated, there are attached pics

 

[harley10]

Could have just needed a valve adjustment , others will chime in soon, Brian

 

[Flyboy]

I agree with Brian, it could and most probably is a small problem/collection of small problems.
Low compression is going to have you down on power, and worn rings will have burning oil, which would see a lot of carbon in the cylinders, as it is a by product of burning oil.
The only person to work on your bike is you, shops don't know, nor do they care about older bikes, if they can't plug a laptop into it to tell them what is wrong, they aye not interested.

Had you posted before tearing down, everyone here would have called BS and saved you a bunch of work, Go and read my rebuild thread "Jennifer's Makeover" link in my signature, and you will see what I mean.
I have just refreshed her top end with new rings and a valve job, mileage is 375 000Km, yes, count the zeros, and the rings and bores were still in factory specs.

But what is done is done, and you just need to get on with it now, there ar many things that could cause low compression readings, when taking the readings, the engine must be at operating temperature, throttle wide open, fresh, strong battery, this all affects the numbers, a cold engine, a throttle that is not held open or a weak battery will all give you low numbers.

If the numbers are low, it could be valves that are not set correctly and not closing properly, or burnt, or both, that will give you the backfire you describe.
Incorrect ignition timing or valve timing will give you a backfire, carbs that are not cleaned and set up correctly will give you a backfire, and

"i took off the carbs, and then the bowls, cleaned the floats and a few valves,"

is not going to cut it, you need to clean them properly, grab the carb cleaning tutorial and clean them properly by dipping them and then cleaning each and every orifice, set the float heights as per the manual, otherwise you are going to land up chasing your tail and frustrating yourself.

Now that it is open, give it a hone, throw in a set of rings, clean the carbs properly, CHECK THE VALVE CLEARANCES and lets take it from there.
You might want to refresh the valves while you have the head off by lapping them in to make sure they seal properly.
Turn the had upside down, fill the combustion chambers with gas and you will know soon enough if any are leaking, after letting it stand for 30min, a wet port indicates a leaking valve.