Am I In For A Total Rebuild?

[Guest]

So, I picked this bike up last September (78 GS750). Bike had been well taken care of and odometer shows just under 28 miles. Seemed to run just fine, too. Over the winter, I stripped her down to the frame, tore down, cleaned and rebuilt the carbs, checked and adjusted valve clearances (not perfect, but on the road to improvement), blah, blah, blah........except, during my excitement, I forgot to do one valuable thing. I forgot to run a compression test.
Recently, I got her all back together, and got her prepped for some shakedown runs. She won't idle, alone, below 1500, and off a dead stop, between 1/4 and 1/2 throttle, she sputters and chugs. All factory jetting, airbox and filter, and stock exhaust. I've played with the pilots and needle positions, nothing improves matters, much.
The other day, I realized I didn't comp. test, so I warmed her up and pulled out my tester. All chambers were within 10% of each other, with a horrible average of 35 psi! How was this thing even running?! Pulled the air filter with the airbox sides off, nothing changed. And, yes, I fully opened the throttle for the tests. Adding a little oil jumped the results up by about 15 psi.
Points and timing were checked and in spec. Right now, all valves are either in spec or a tad loose, except for, I think, #4 exhaust, which is on the tight side. Static reading on the coils were 4 ohms, each.
The other day, I test drove it down two blocks, to test some tweaks. It sputtered one block down, died, and refused to start, no matter what I did. The battery died from all the attempts, it wouldn't kick over, nor would it catch in gear. Had to call the wife, she came down, and we towed it behind our SUV up the hill to the garage.
I suspect more than just a compression issue. Right now, I just need to take some time away from her in an effort to regain some sanity.I apologize if my writings seem crazed. I've worn myself ragged on this build, and with these issues, I've become slightly unravelled. Once I have a chance to relax, I'm gonna run a leakdown test. That should prove fun!

[BassCliff]

Hi,

With that many miles I would not think you need a total rebuild. If all of the other maintenance has bee properly done, usually it's a matter of stuck rings which will seal up once you get it running and ride the snot out of it.


Thank you for your indulgence,

BassCliff

[Road Rash]

Cool. Was hoping for something like this. Let's hope this is the case! Fingers crossed.

[Starion007]

Hello Mr. Rash;
I was reading your post and at the beginning you say around 28 miles, not 28k miles?
and it seemed to run good. I'm wondering if you drove it much before taking it apart. I say that as a possible clue, cause if it ran good before and now has low compression. Did you take any of the motor apart? I'm not trying to bust your balls just trying to piece it together. You would have to have a stuck ring in each cylinder, that seems unlikely to me if it ran good before.
Maybe make a list of things you did that might help you remember something.
Mark

[nejeff]

She was running fine last September? Hmmm...A mystery. I'm interested how this turns out for you. Got a vacuum cap off anywhere? Maybe manual petcock and #2 vacuum tap is open? Maybe a bad petcock and not getting flow? I don't think the compression all of a sudden went kaput if it ran ok in September...maybe a bad tester? Just thoughts...

[LarryJ]

I'd start by ck'g your valve clearances again. Do anything to the cam chain tensioner? It's most likely something very fundamental.
So, don't tear into the engine until you have diagnosed the cause.
Otherwise, you could just wind up chasing your tail endlessly.

Just my 2 cents.

[LarryJ]

Ck all the carb boots & O rings, & try another compression gauge too.

[Road Rash]

Hello Mr. Rash;
I was reading your post and at the beginning you say around 28 miles, not 28k miles?
and it seemed to run good. I'm wondering if you drove it much before taking it apart. I say that as a possible clue, cause if it ran good before and now has low compression. Did you take any of the motor apart? I'm not trying to bust your balls just trying to piece it together. You would have to have a stuck ring in each cylinder, that seems unlikely to me if it ran good before.
Maybe make a list of things you did that might help you remember something.
Mark

28k. Sorry. Typo.

[rwcfrank]

Did you do the compression check with the throttle wide open or closed?

[Road Rash]

Yup, guys, did all those things, including swapping compression test gauges. Petcock's good. Just rechecked valves and ignition timing, as well as cam timing and chain. At this point, I think I have two issues: 1) The compression, will very well may be a ring re-seating issue, and 2) I'm wondering if I have an ignition/electrical component issue, like something's heading south. Before and after she died, it seemed like it was sputtering more than it was running right, like there would be zero power, followed by a very brief power surge, like it wanted to run, but would then go right back to crap. If I so much as tried to let out the clutch when in gear, the power was so faint, that it would immediately begin to die at any sign of friction from the clutch.

[Big T]

Okay - ran fine last September
You did maintenance on it and now it runs like crap

So, what's the failure point?

If the cam timing is correct and the valve clearances are good, then there's no way it ran well (if at all) last September with 35 Lbs compression

So, it's got to be cam timing or valve clearance letting the gases out, thats the main things that changed since last fall

[LarryJ]

gotta be something simple. don't give up.

[Road Rash]

Yeah, T, that's what I'm thinking: 35 psi? How the hell could it have run, even recently? Cams are timed properly with the crank, and the only thing I did with the valves was actually loosen the clearances that needed it, bringing them into spec.
Larryl, I've been thinking the same thing. Unfortunately, it hasn't been found, yet. But it will. You know what they say. It's always the last thing you check.

[Road Rash]

Okay, update, and the plot thickens.

Yesterday, went out and tried to start her, cold, as is (haven't touched her in almost a week after stranding me), and she would not fire. So, it was not a heat issue. I started pulling apart electrical connections and ensuring the contacts are brushed and sandpapered nice and shiny. While I was in there, I threw my meter on some things. Battery read 12.62 vdc, so while there was a charge, it needed topped, so I threw her on the Battery Tender. The coil connecting plug read 12.63 vdc, so it appeared to be getting the full monty. Testing the battery at that state while cranking, it dropped to 9 vdc.
Static stator readings, leg to leg, were all about 1.4 ohms, in spec. Each leg to ground read infinite, again, which is good. I then left it untouched while the battery charged.
This morning, the battery was topped, and while I set the float levels with my calipers, I decided while it was still cold, to check and tweak them with the clear tube method. I had read how the bench method, while it is good, could prove inaccurate once buttoned up and fuel flowing. I'm glad I checked cause numbers 1 and 4 were low. Got em back in, first try.
Next came the moment of truth. Time to crank it...........AND nothing. Yeah, it cranked, but would not fire up. Pulling the plugs revealed not only sooty plugs, but two, numbers 1 and 2 had no spark at all. I remembered how a dirty plug could prematurely ground, not allowing it to spark. I also read how if you don't have a sandblaster plug cleaner, which I don't, you could spray it with oven cleaner. Eh, what the hell. Boy, did they get clean. I sprayed, wire brushed, then rinsed and dried each, leaving MUCH better looking plugs.
I reinstalled them, said a little prayer and it cranked and fired right up. Took her out for a test ride, and she ran the best, to date. I'm guessing the plugs got loaded when I got stranded (OR that's what LEFT me stranded), and I continued to try and crank her over.
On the test run, she ran well, at least at first. I noticed once she got warmed up, more, the sputtering returned on the first half of the throttle, especially on load off a dead stop. That made me a tad twitchy that it might strand me again, and not just blocks from home, this time, so my right hand never left the throttle at stops and my attention stayed on it.
So, is it problem solved? Not quite, but it's a step in the right direction. I'll be curious to pull the plugs tomorrow and see how they look. I'm trying to run her so the rings stand a chance of re-seating. I wonder if they could be what sooted up the plugs, too. Could be oil blowby, I dunno. Even though I'm not new to wrenching (More used to tooling on my old 68 Chevelle and 89 Camaro, plus I had a Yamaha Maxim before this), some of these things are uncharted territory for me. Rings and compression issues, especially. But hey, learning is fun!