Bandits '77 GS750 Revival

[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/a/suz/50d3fefff8700230d8b4dd65/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.

 

[Grimly]

If the battery's down enough to just about crank the engine but not supply suffienct oomph to the points or ignitors, it won't start.
This problem was common enough back before a 77 bike or car.

 

[93Bandit]

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.

Gotcha, thanks for the clarification.

 

[93Bandit]

If the battery's down enough to just about crank the engine but not supply suffienct oomph to the points or ignitors, it won't start.
This problem was common enough back before a 77 bike or car.

Maybe that was the issue then.

 

[93Bandit]

Just a little update for you guys, or rather compiling the information into the build thread.

I got a shim stuck in the bucket when adjusting valves and had to wait on some engine parts so decided to work on something else.

The brakes are locked up so I disassembled the front and rear calipers. Carefully unstuck, cleaned and polished the piston and bore with 0000 steel wool, but when I got ready to order seals and pads, I discovered they were outrageously priced and difficult to find for the front caliper. So looked around for newer calipers in better shape that I could swap on. Purchased a set from Burque73 and they arrived weeks ago but haven't had a chance to dig into them yet so I'm not even sure they fit, but the GS Hive mind seems to think there's a good chance they will.

I've got a few other projects in the garage I hope to wrap up in the next few weeks and then I plan to get back on the 750.

The short list I plan to attack once I get back on the 750 is to sort out the brakes by:
-Master rebuild/cleaning
-New brake lines
-"new" calipers installed (hopefully they fit and don't need rebuilt but we'll see)
-new wheel bearings

I think once I get brakes and wheel bearings done I'll get back to engine work. My plan for now is just to get it back on the road for this season. I think with brakes and getting the engine sorted, I can ride it this spring. We'll see how it goes.

Oh, definitely going to need some new tires too. They're hard and dry rotted so I'm not going to take a chance on them.

 

[93Bandit]

Had a chance to work on the 750 last night.

I finally got that stuck shim out, and it was a pain. Tried a blast of air, two magnets, sharpened tiny screw driver, razor blade and heat but it would not budge. So I pulled the cam chain idle sprocket and the exhaust cam bearing caps and carefully moved the cam enough to get the bucket out. I kept the cam sprocket engaged with the chain as I moved it, and kinda rolled the cam in the chain to walk it over far enough out of the way in an attempt to keep it in time. With the bucket out, I tried all the same methods again and it still would not budge. So then I put the whole thing in the freezer for a while, then quickly heated the bucket and tried to tap the shim out. STILL STUCK... So at that point I'd been messing with it for hours and was getting frustrated, so I stuck a small screwdriver in the window and started tapping harder than I probably should have with a small hammer. Finally started to move and after some prying and generous tapping it came out.

I put in a 2.55 shim and now clearance is 0.125mm. It's too much per spec, but I don't have another 2.6 shim on hand. I think it'll be OK? What do you guys think?

I put the new cam cover back on with a new gasket so engine is all back together after triple I checked the cam timing per the manual. I think engine work is done and ready for the carbs to go back on.

I also checked the 82 650G front L brake caliper on the bike and it fits like a glove! So next I plan to continue working on the brakes.

 

[93Bandit]

Brakes are done for now. I "rebuilt" the master cylinders and calipers buy disassembling and thoroughly cleaning. Was able to reuse all the seals and boots. Decided to replace brake lines at a later date when I do a proper rebuild. For now the bike will just be for around town so I'm not concerned about the brake lines that are on it. Flushed them well until fluid came out clear, and installed the calipers and bled them.

Also installed the carbs and throttle cables.

All I need to do now before a test ride is front wheel bearings and carb to airbox snorkels. Hopefully will be ordering those this evening or tomorrow.

 

[93Bandit]

Lose the crash bar!

I'm sure you'll be happy to hear the crash bar has been removed. Had to take it off to get a vice grip on the head to hold the cam down when reinstalling. As I was taking the bar off, I thought of you and how proud you would be.

 

[93Bandit]

Was able to get more work done on the 750 today. I installed new front wheel bearings and put the bike back together for a test ride. Still waiting on several parts (petcock rebuild kit, new airbox snorkels etc.).

Here's a few pictures from today.

Front wheel off for bearings.


Back together and ready for a test ride. Notice the "new" front caliper courtesy of Burque73.


Aaaaaand, out side warming up for a quick ride.

 

[93Bandit]

I was able to ride it down to the end of my street and back. Maybe 1/2 a mile. It was really cold so I didn't want to go far, just far enough to get through all the gears.

It ran through all gears, cruised well at 50mph, but it's still not running right. Some backfiring through the carbs, falls on it's face if you whack the throttle to WOT, and it has an erratic idle that sometimes hangs at 3k rpms. It acts like it's getting air somewhere, but I have no clue where. It's got new OEM intakes with o rings and I ran it with the airbox on. Is it possible my pilot jet and air screws are causing this? Seems unlikely to me, they're all at the base 1 turn out setting.

 

[93Bandit]

Worked on the bike some more today. Installed the new airbox snorkels and air filter which were the last original components on the air/fuel side. I thought that might fix the hanging idle but it didn't seen to, although it is better than before. Adjusted the idle air screws on all 4 carbs but they didn't seem to do much. The idle is still unsteady/erratic. There some popping from the exhaust at idle as well. Took it for another ride around the block and it ran better than last time, cruised really well and accelerated pretty hard too. But every time I came to a stop the idle was hanging around 3-4k. Adjusted the idle screw and it settled at 1100 but after more riding it would start to hang again.

Not sure what's going on. I checked compression with it hot. 150, 140, 140, and 145. That seems pretty healthy to me.

Any ideas guys? I'm kinda at a loss at this point on the idling issue.