Bandits '77 GS750 Revival

[93Bandit]

So I went out to the garage on my lunch break. Tried to start the bike but it would not start, wouldn't even cough. Just endless cranking. Checked fuel, checked spark, all good. Checked petcock, all good. Hmmm...

After cranking for a very long time, it started barely running on full choke but only if I continued engaging the started. It would run with starter on, but not once I let off the starter. Then I cranked it again, heard a loud metallic POP and it roared to life and idled at around 1500. Then another pop and idle surged to 3k. Then the top end really started clattering (and I'm not talking about cam walk), sounded like cards in your bicycle spokes. So I shut it off and went inside.

I haven't adjusted the valves yet but it fired with zero effort yesterday and ran well. What happened over night? I'll be pulling the cam cover today or tomorrow to start investigating.

Maybe the cam timing somehow got off? The popping noises I heard scare me...

Any ideas are appreciated.

[93Bandit]

I re-watched the first start video I posted. If you go to the 30 sec mark, you can hear the metallic pop/tick I mentioned in my previous post. I jump a little as it happens. I didn't think much of it at the time, but that's the same exact sound it made today when the idle surged.

[93Bandit]

Pulled the cam cover and inspected the cams and timing chain. Didn't see any signs of damage, the chain was tight.

Measured valve shim clearances and below were my results. Service Manual says I: 0.02-0.05mm, E: 0.03-0.06mm

Cylinder 1:
I: 0mm clearance, 2.7mm shim
E: 0.076mm+ clearance, 2.7mm shim

Cylinder 2:
I: 0.038mm clearance, did not measure shim
E: 0mm clearance, 2.7mm shim

Cylinder 3:
I: 0.051mm, did not measure shim
E: 0mm clearance, 2.7mm shim

Cylinder 4:
I: 0mm clearance, 2.8mm shim
E: 0mm clearance, 2.7mm shim

So 5 valves are tight, C1E has substantial clearance, more than spec, and C2 & C3 intakes are in spec. Could this really have caused my starting issue today? Since I didn't see anything else going on I'm going to order some shims and see if that fixes it...

Also peaked inside each cylinder with my bore scope and didn't see any issues. No PTV contact marks that I could see. Can still see good crosshatching on the cylinder walls.

[Grimly]

Those numbers would normally lead to hard starting, yes.
The reason it started yesterday was probably the presence of sufficient fuel vapour through some splashing around over the previous while and sittiing in the inlet tracts.

[93Bandit]

Those numbers would normally lead to hard starting, yes.
The reason it started yesterday was probably the presence of sufficient fuel vapour through some splashing around over the previous while and sittiing in the inlet tracts.

Appreciate the input.

I've got 8 thinner shims leftover from the last time I shimmed my 850 so I might just have what I need to re-shim without having to order any. I do need a new cam cover gasket though. I'll have to order that but that's not a big deal. I'll shim the valves and see if it starts/runs any better. Probably won't happen until after the 1st of the year. Stand by...

[93Bandit]

Shims have been ordered along with new cam cover gaskets. Assembly ETA, first week of January.

[93Bandit]

Welp...

Adjusted the valves with new shims and tried to start the bike. It absolutely will not start. Won't even try to fire on one cylinder, not even a cough.

I have spark, I have fuel (at the petcock) and I should have compression. Haven't put my compression gauge on it yet but I seriously doubt it lost compression on all cylinders over night.

I'm at a loss, so I'm going to take the carbs off and see if they got clogged somehow? Maybe I didn't get the tank cleaned out enough?

[Nessism]

Did you try putting the petcock on Prime in order to fill the carbs?

[93Bandit]

Did you try putting the petcock on Prime in order to fill the carbs?

Prime and reserve.

Also pulled #1 fuel bowl and it had fuel coming out.

[eil]

One way to rule out lack of spark or compression is to dump a teaspoon or so of gas into the cylinders through the spark plug holes, quickly thread in the spark plugs, connect the wires and try to start it with half-open throttle. If it runs for a second or two and then dies, then you have spark and compression but you're losing the fuel somewhere between the tank and the intake.

[93Bandit]

One way to rule out lack of spark or compression is to dump a teaspoon or so of gas into the cylinders through the spark plug holes, quickly thread in the spark plugs, connect the wires and try to start it with half-open throttle. If it runs for a second or two and then dies, then you have spark and compression but you're losing the fuel somewhere between the tank and the intake.

I was planning on doing this as a quick cause elimination test. It probably wouldn't hurt to clean all the coil circuits and grounds as well. The bike sat in a dirt floor barn for years so it has plenty of surface oxidation. I wouldn't be surprised if some connections do as well. It has spark but maybe it's too weak?

[93Bandit]

OK, so I went out to the garage today to check compression of the 750, but just for kicks and giggles I tried to start it. It fired and ran...

Backstory: Yesterday I installed heated grips on my 850 so I pulled the battery from the 750 and put it in the 850, but the 850 wouldn't even crank. OK, probably drained the battery trying to get the 750 to start (battery is new, purchased it July of 2021). Charged the battery and then went for a ride on the 850. Got home, took the battery out and put it on the shelf. Today I went out to do a compression test, so I put the freshly charged battery in the 750 and hit the starter button just to see what would happen. Cranked for a second, then started stumbling, then fired and ran. Ok...

Is it possible the 750 ignition system is that sensitive to low voltage? The weird thing is it cranks the engine just fine, for a long time but wouldn't fire. So I don't know if it's a coincidence or if the charged battery "fixed" it?

Regardless, I let it get to operating temp and noticed a few things. I heard some backfiring through the air box and when going WOT I noticed black smoke. Idle is still somewhat erratic, hangs after revving, and surges sometimes. However, when revving it, it rev's smoothly and doesn't backfire or hesitate. Seems most issues are just while idling. I did noticed the petcock is leaking, so maybe it's sending too much fuel to the carbs contributing to my issues?

So, I think I have a list of things to do first and see if that corrects the starting issue and erratic idle/backfiring.
1: New OEM snorkels(?) between carbs and airbox
2: Rebuild petcock***
3: Pull carbs and double check idle circuits
4: ???

***I've searched for an OEM replacement petcock but cannot find one available, it's been discontinued. I've searched the part number online trying to find a seller with no luck. I've read countless threads about people being unsuccessful rebuilding them, but I don't see any other options. Instead of ordering a kit on ebay, I planned on ordering items 2, 3, 4 & 6 from here https://www.partsoutlaw.com/oemparts...d65/fuel-cock# to attempt a rebuild. What do you guys think?

[rphillips]

Good to hear it running, ain't it? What a way to start off a new yr., Congrats.. Personally I couldn't imagine a batt. cranking the eng. but not firing the ign. Yes, maybe on the newer electronic computerized stuff but nothing as old as "77". The Petcock can't send too much fuel to the carbs, that's controlled by the needle valves inside the carbs. I've been lucky, rebuilt sever GS petcocks, over the yrs. with no problems. Have read here, a lot, about the rebuilds not working properly & have also read the aftermarket petcocks don't do well. I don't know what to recommend about that, hoping someone else has a good idea. Main thing is just keep up the good work.

[93Bandit]

Good to hear it running, ain't it? What a way to start off a new yr., Congrats.. Personally I couldn't imagine a batt. cranking the eng. but not firing the ign. Yes, maybe on the newer electronic computerized stuff but nothing as old as "77". The Petcock can't send too much fuel to the carbs, that's controlled by the needle valves inside the carbs. I've been lucky, rebuilt sever GS petcocks, over the yrs. with no problems. Have read here, a lot, about the rebuilds not working properly & have also read the aftermarket petcocks don't do well. I don't know what to recommend about that, hoping someone else has a good idea. Main thing is just keep up the good work.

That's what I was thinking which is why I asked if anyone thought it might be possible? I seriously doubt it is, but weird things can happen with low voltage...

As for carbs, I know the the needle valve shuts fuel flow off but I've heard of people flooding their engines when leaving the petcock on reserve or prime. Not sure how that happens, but apparently it has...

Thanks, it's definitely nice to hear it run, but I wish I knew why it's running now when it wouldn't before even though nothing has changed (other than charging the battery).

[allojohn]

It's the CV carbs that can flood the engine - VM carbs flood out the back of the carb into the airbox or pod. You're safe.