1993 CB750 NH dying on longer rides

[bickel777]

Hello everyone,

This is my first post here, but I'm hoping someone can share their advice/wisdom with me on this problem I'm having...

I got this 1993 CB750 Nighthawk 2 months ago...the previous owner had it for ~2 years, and before that it'd been sitting for ~8 yrs. The bike was running rough when I bought it and I figured it needed some carb cleaning/tuning up etc...

I had my (1st) mechanic rebuild the carbs, which he said were "all gunked up". He really is just a car mechanic but has bikes and works on his own. Anyway he got the carbs rebuilt/cleaned but couldn't get the thing to start right...He said it was having some fuel supply issue and ended up disengaging some spring inside the petcock so that it always flows. Then it started but wouldn't idle <4K rpm. He gave it up and I took it to an actual shop that went back into the carbs and fixed the idling issue.

However, neither of them addressed the original issue I had with the bike since I bought it. The bike dies mid-ride after ~15 miles or so and sustained highway speeds (>45 mph or so). It's fine, then suddenly the throttle doesn't respond, the bike sounds terrible (low rumble) and smells very rich, and I pull the clutch in and the bike dies...the bike starts back up if I let it sit for 5-10 minutes, but will die again a few miles later.

I took it back to the shop, and told them it was still dying. They rode it around some more and deemed that the issue is a bad gas cap, and may not be venting correctly causing a vacuum and therefore fuel starvation (which I've not heard of before). They claimed they rode it for ~50 miles straight (@65 mph or higher) with the cap popped open, and has no issues.

Since, I've gone on 3 rides myself with the cap closed, waiting for it to die so i can pop open the gas cap and see it that fixes it. However, the bike hasn't died on any of the 3 (~30-50 mi each) rides. Today, after I got home, I let the bike sit for awhile then decided to open the gas cap...When I opened it I heard a loud hissing then a pop.

I'm just not sure what to do with this bike, it used to die all the time, but now it isn't but I don't feel the true issue was ever solved.

Other useful facts about the bike:
- Brand new battery
- New gas tank added by previous owner (OEM), but gas cap is original

Hopefully someone has some suggestions or can make sense of all this (Sorry for kinda long story).

- Scott

 

[Travis]

Sounds like a bad coil or coils. They can stop working when they get hot. No real way to test them for that problem other than check for spark immediately if it happens again. By the way, welcome to the site.

 

[K2750tracy]

Travis i agree bad coils have seen this many times with lawn equipment (what i worked on for many years) they can be very eratic in failure too. Hissing noise was most likely pressure from engine heating up fuel in tank, gas cap will vent air in but hold fuel and vapors from comeing out. Fyi a bad cap vent will cause vacum and stop flow of fuel on gravity fed systems.

 

[bickel777]

Thank for the reply guys,

So far everyone has told me it's a bad gas cap venting issue...however you both suggest bad coils.

Is there anyway I can check for bad coils myself?

 

[Travis]

Well you can't really check them at will becuase it's only a problem when they get hot. Like I said before, the only way to check them is if your bike stalls again after about 15 miles and won't start, you have to check for spark right away. If you have to spark or weak spark, but then have good spark after they cool down, then they're bad. You could just replace them to eliminate that variable, but it might be throwing money away if that's not the problem.

 

[K2750tracy]

Does all 4 cylinders shut down or just 2? Its unlikely you have 2 bad coils failing at the same time. Im not fimilar with the 93 ignition system but id bet it has a spark controler or computer of some sort. If all 4 shut down i would look there.

 

[Travis]

K2750tracy, good point... You definitely should be able to tell if it's running on 2 cylinders, but maybe the idle would be too low and cause it to die anyway.

Anyway I just re-read the original post and since the problem hasn't happened again, and you haven't been able to test the venting gas cap, you don't know whether you have a fuel problem or spark problem. I would keep riding it and bring a spare plug in your pocket. If it dies again, open the gas cap and if the bike has a vacuum petcock set it to prime to let the bowls fill up, then try to start the bike. If it doesn't fire up right away, use the spare plug to check for spark on all 4 cylinders. If no spark on 2 cylinders, then it's probably a coil, and if you have no spark on any cylinder, then it's probably the ignition.

 

[CB750-Bobber]

I completely agree with Travis..nice response Sir!

 

[Travis]

Well thank you... now I just hope what I said was correct!

 

[Mydlyfkryzis]

if the bike has a vacuum petcock set it to prime to let the bowls fill up,

No prime setting on this Bike. I have a 91 NH750.

Another possibility is the little vacuum tube that runs from the left carb boot area to the petcock. My hose became brittle and was leaking vacuum (making for hard starting). If it is really loose, the petcock vacuum valve would close, starving the engine for fuel.

Worth looking at.