Topic: Bogging or missing at 5k RPMs...which is it?

[coffee_brake]

I've started a new thread because the old one was so convoluted.

I'm putting a 1982 650 Nighthawk back on the road. Bone stock. The carbs were cleaned by the book with the ultrasonic, but I did not replace the fuel rail o-rings with stock size parts. Looks like I'm going to have to do that, I have a significant fuel leak but not from the float bowls. I've rebuilt carbs before so I think I did it right...but goodness knows I may have missed something obvious. This is my first SOHC, I'm learning a LOT from this machine.

The bike is running an auxiliary fuel tank while I trouble-shoot.

The bike pulls well up to 5k, and then it bogs down, or begins to miss, I can't tell which. If I try to slowly throttle past 5k, the engine dies.

I already know I'm going to have to pull the carbs again, I'm just trying to see what normally causes this kind of problem so I can tackle it before pulling the bike off-line. Seems it's back to the old riddle of "fuel vs. spark", and I'm asking if you more experienced folks here have seen this before to send me to the right path of trouble-shooting.

You've already helped a WHOLE lot, thank you...

[Vinhead1957]

I think you know your answer already

Those Orings on the fuel Tees could very possibly be your problem.   If it makes you feel better I may be next.  Doesn't leak all the time just occasionally.

[coffee_brake]

A fuel leak causing a miss at 5k? I don't see how...

[cheftuskey121]

sounds like fuel starvation. when I had some fuel delivery issues on my 750F, it would not go above 5k without bogging down to almost nothing and dying. in MY case it ended up being an aftermarket petcock I bought that the output hole was too small. it literally was not getting enough fuel to the carbs.

I am not sure where in the chain it is, but you aren't giving your carbs enough fuel.

[LesterPiglet]

Are these bikes CDI? Do they have an advance mechanism?

[cheftuskey121]

after reading your whole post. get those correct o-ring sizes and STOP your carb bowl leaks. also check your petcock (whether its on the aux tank or the real one) and make sure its large enough. also if there is a fuel filter make sure its not clogged and reducing output. you want fuel to flow greatly into the carbs, AND stay there. good luck

[coffee_brake]

I did have to adjust the ignition pickups, somebody before me certainly did it wrong and boogered up the screws in the process. 

I also did the valves, they weren't too bad.

I'm pretty sure there's nothing catastrophically wrong with the bottom end now that I've ridden it (heehee it was fun, too!  )

I was hoping for fuel starvation too, since this electrical system is all new to me. Fuel I can understand; the electrical stuff is a lot harder for me to wrap my brain around.

Where do ya'll source the fuel rain o-rings?

[coffee_brake]

Also during the carb go-through, the 3-way joing was disintigrating:

http://www.partzilla.com/parts/search/Honda/Motorcycle/1982/CB650SC+A/CARBURETOR+ASSY./parts.html

It's part number 10 and 11 on the carb assembly fiche:  16036-460-771 and 16047-460-771.

Aren't these just vents to the atmosphere? I wasn't too concerned with making them solid, I patched them with RTV. The price to replace them is cost prohibitive. What do you folks do about these when they fall apart?

[Tdinova]

Did you check the ohms of the spark wire caps. They should be ~5000 ohms

[coffee_brake]

There's going to be lots to check, including the plug wires and caps, and the coils themselves. But I KNOW I have to fix the fuel rail o-rings.

Chasing this kind of trouble can get really confusing and overwhelming, so even if I know it's going to take longer, I have to tackle one unknown at a time.

I found fuel rail o rings for like $5 so I'll get those on the way.

But I do wonder what ya'll do for those three way connectors that seem to only be open to the atmosphere. It seems the connector and its hose only exist to keep dirt out...so it seems my RTV fix should be OK, but I wanted to ask the big group of folks here who may have been here before.

As for the

[LesterPiglet]

"But I do wonder what ya'll do for those three way connectors that seem to only be open to the atmosphere."

If it's the same idea as my 550, mine haven't had anything at all connected to them for the last 30 odd years.

[John Eberly]

It seems most likely that you're having a fuel delivery problem. The symptoms point to something like plugged jets, collapsed fuel line, plugged petcock, or something wrong in your jury-rigged tank.

Engines that"run ok unloaded" but die when you try to make them work are usually going lean because of fuel.

[coffee_brake]

Very good to know.

Sean who is famous for his affordable o-ring kits on the DOHC forum is sending me the correct kind and size for the Nighthawk 650, since its carbs are so similar to this SOHC model.

[coffee_brake]

After going through the carbs again, this time with new fuel rail o-rings, I was able to ride the 650 today and get it up to speed.

It did very well all through the RPM range, but after a few minutes, it began to pour fuel from the carbs again. It didn't leak the whole time I was warming the bike up, nor for the first mile or so. I'm running fresh gas through a fuel filter.
It's not coming from the vents, indicating a stuck float. Instead it's coming from a bit higher up but I can't figure out where. The bike was bogging down while underway from the flooding.

I'm not familiar with the function of the accelerator pump on these carbs. Could that be my culprit?

[coffee_brake]

Oh, and the bike was riding great with the air box cover on normally, no more miss or bog at 5k.