Good report, but where to begin...

88SS

Active Member
I recently purchased a 1980 cb750f super sport for a very good price. It started and ran ok for a few days, then as I started tinkering it started running lean. The baffles were rotted out so pulled them, then the exhaust, and then replaced it with 4 into 1 exhaust. I tuned up the carbs and it ran ok, but still a touch lean. Condition became worse. Then I changed the plugs, coils, and wires. Replaced the carb boots and did a rejet to 68 and 105. Still lean. Changed the inline filter. Lean. Then my battery starts going dead on me. Changed all the charging parts. The r/r, stator, and rotor.... and the battery cables. I did an oil change, and replaced the k&n filter (in the stock air box) with a stock paper one. Fixed the shift shaft oil seal leak by replacement, and switched to iridium plugs, and replaced the petcock. This is where it gets interesting.

Immediate changes with the new petcock. I'm assuming it was causing most if not all the lean condition. The carbs start pissing gas immediately out of 2 and 4, so I pull them. When I get the bowls off, I notice the ones leaking are adjustable, and the ones not are non adjustable and stock. Long story short, I adjusted them, put them on the bike, and no leakage. Took the bike for a spin and it was like new money. Rode like a dream. No issues for the first time in months.

I talked to the p/o about the carbs, and apparently they are off an 83 model bike and he replaced a couple floats with adjustable. Hopefully my issues with be resolved for a bit, but what should I look into next? I've got a faint tick, so I'm going to tighten the cam chain in the morning. I'm thinking a valve shim wouldn't hurt, but I'm really just looking forward to riding. Anyone ever taken one on a long ride? Any info is good info, and thanks in advance.


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- Check and adjust valve clearances to .005".
- Fresh oil/filter can't hurt.
- May require new valve stem seals.
- Adjust your timing chain. There's a guy on the cb110f forum called 605brent; his company in New Zealand has developed the perfect replacement chain/guides.
- Clean and tighten all the connectors - No sense wasting electrical power through corrosion.
- Consider doing the ignition coil relay mod. It's on here somewhere.
- Ride the hell out of it!

Welcome to DTT, 88SS
 
I did go through all the connections and cleaned the shit out of them. I replaced the oil filter when I changed the oil. Timing is dead on, so I'm not sure if there's any issues with the timing chain. Pretty sure there's a tiny bit of slack in the cam chains, per the faint clicking at idle. I'll tighten it tomorrow. My electrical connectors are dry rotted in a lot of places however.

The plan is to ride as much as possible this summer, and take it down to the frame over the winter. I need to paint the frame, motor, and tank. Really looking to restore the bike, so I'm hesitant to mess with the paint too much. I do have rust however. When I get it to the frame, I'll replace the wire harness.

It seems like valves are pretty important on this bike, so I'll probably give them some love sooner before laterally


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These were taken when I first got the bike. I'll get some fresh ones next weekend. I would love to get a Corbin seat for it


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Well..... while tightening the cam chain, I totally broke the bolt off. Will this still be ok to ride with until I can replace it?


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I would choose to error on the side of caution and park it until your replacement tensioner bolt comes in. You can't know for sure if it's doing what you want on the inside, or if it'll stay in.
 
Sounds like solid advice to me. Luckily I'm out of town this week. The bolt should be in by the time I return home. I honestly didn't think I would ever find one. It took me all day. Also, it's going to be tough getting the broke bolt out


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It may require a drill and a timesert. Not the osteoporosis fun.
 
Tried tapping the bolt out with no luck. So, I went and bought a drill. The local motorcycle shops just laughed and told me I would have to pull the motor. F@&$ that! I'm going to drill a hole and easy out that bitch, and I don't care what they say. The moto community out here sucks


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Still dealing with carb issues. I'm really almost fed up. Thinking of a carb upgrade. Any suggestions?


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Nice set of CR's would wake her up, but you'll still have to dial them in. What's the current carb issue?

Orrrr... http://www.speedmotoco.com/cb750-DOHC-vm34-dual-carb-Mikuni-Speedmotoco-p/km-vm34-dohc.htm

Believe speedmotoco is a site supporter as well
 
coyote13 said:
Nice set of CR's would wake her up, but you'll still have to dial them in. What's the current carb issue?

Orrrr... http://www.speedmotoco.com/cb750-DOHC-vm34-dual-carb-Mikuni-Speedmotoco-p/km-vm34-dohc.htm

Believe speedmotoco is a site supporter as well




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88SS said:
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At first I was running too lean. So I pulled the carbs and everything looked great. The bike had been running fine before, so I'm thinking it's because I went to the 4 in 1 exhaust. I bumped the jetting up from 68/100 to 68/105. Not a huge jump. Bench synced, put the carbs on, vacuum synced, and it still ran lean. On top of that, the idle was all crazy. Next I adjusted the idle mix screws on the bottom to richen up, and the fuel starts flowing out of 2 and 4, so I pull the carbs again. 2 and 4 have adjustable floats while 1 and 3 are stock (I believe). Adjusted the floats, and resynced. Bike ran awesome for a few days, and then I started having problems cold starting. I'm thinking I've melted a ring, but the compression was good. Upon further inspection I notice the choke isn't moving properly. Pull the carbs again. The spring had came off that links the choke b/w 2 and three. Jackpot, I think, it should be perfect now! Slam the carbs back in and it fires right up, but is running terrible so I sync the carbs again. All seems fine until I start pissing fuel out of carb 1. Readjusted the idle mixture screws, and take it down the road, and it still seams lean.

I'm pretty frustrated, and just want to ride at this point. The idle is back to being all crazy. Setting at a light it will just rev up to about 2500 and then drop down until it dies, so it's like I'm back to square one.

I'm running stock air box with paper filter. 83 carbs on an 80 bike. Should I try 75/100 or go back to 68/100? I also put a temp gauge on the pipes. 2 pipes would stay about half the temp of the other two. It really seems random though. Sometimes it will run ok, and other times it runs lean, then other times seem like it's flooding. I changed the petcock and accelerator pump. Any ideas? What am I missing?


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Carb 1 was pissing gas today, so I pulled the bowl and a tiny grain of sand was causing it. I guess I'll pull the carbs again, but I'm really tired of dealing with it. Some days you just want to ride.

Still acts like it's running lean. Popping and running like crap. Idles super high then drops down until it dies


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Thought I would write an update. After pulling the carbs and syncing them again there was still no improvement. I couldn't figure it out on my own. So, I started reading up on carb theory and came to a conclusion. I did a rejet when I lived in Birmingham. I went from 68/100 to 68/105, and it ran good there. It's 700 feet above sea level. Not that big of a change, but the jets still may have not been dialed in perfectly. Then I moved to Fredericksburg and started having problems, at 60 feet above sea level. I believe (and I may be wrong) the bike is running lean on the idle jets, causing poor idle. Then when it revs up and the needles rise, it's too rich causing it to die. Anyway, I changed the floats where all four were the same (non-adjustable), and I also changed the jets to 70/102. Fired the bike up and it seems to idle well. Smells good/no smoke. The only problem is, the sun went down so I didn't have time to vacuum sync but I'll do that tomorrow after work. Fingers crossed


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