Engine stutter at high rpm

[Herd]

Hi all, I just picked up a nice 1978 CB750K this weekend. I've had a few bikes, but this is my first Honda 4, and I think it is waaaaaaaay cool. It's bone stock at the moment, and I promise to get some pics up soon. PO said the carbs were cleaned (I don't think so), the plugs replaced (maybe...), and the battery was replaced (maybe with a used one).There was fuel mixed in with the small amount of "blacker than night oil" that was in the bike.

Anyhow, I changed the oil, cleaned the points (did not check gap yet or timing yet), and took it out for a few miles. Rode for a bit till the old gas was almost gone, and then filled it up with some fresh gas, and then rode her some more. It was running great. I found myself on an open straightway and opened her up a bit (70-80mph, maybe 5-6k rpm). As I was cruising, the engine studdered (2 separate times) like it wasn't getting fuel or spark. It lasted just for a sec, but when it happened, the bike lost power and felt like it was gonna shut down. I laid back off the throttle and everything went back to normal. I waited a bit and then pushed it some more. Sure enough, it happened one more time another couple hundred yards down the road. When I backed off and road 50-60 mph, everything was fine, and she ran as she should.

Any thoughts on how to proceed with diagnosis?
Check point gap? Check timing? Recommended reading?

Ideally, I would like to get her as reliable as possible for the season. I would love to take her on a few long rides over the next couple months.
Any thoughts on how to proceed would be greatly appreciated.

 

[dirtdigger]

If it starts to die and you back off the throttle and it runs again then you are running out of fuel. You are either not getting enough fuel into the float bowls or you have jets plugged or restricted. Check the jets and pull the petcock off the tank and see if the screen is plugged. These bikes are also touchy about the points and timing so you need to get them adjusted and timed...and check them every 2 to 3000 miles. Check and adjust valves, adjust cam chain, sync carbs after you rebuild or clean. This is 70's tech it needs regular maintenance.

 

[Herd]

Dirtdigger, thanks for the reply. I have been taking my time with the bike. Rode it for another week or so with no issues. A couple days ago, it stalled out while driving, like it was running out of fuel. There was fuel in the tank. I checked the plugs, 1 and 4 were wet with gas, 2 and 3 were dry. I put a screw driver in the coil wires, grounded it to the motor and got spark (and then shocked the crap out of myself... twice). The shock did not stop until I dropped the screwdriver. I checked the points and found that they are arcing. I checked and adjusted them, setting the timing mark to 1&4 F (is that correct?). 2&3 were way off, and did not even separate. They also arced quite a bit. I checked the condensers, both seemed to be non-functional. I ordered new points and condensers, put the condensers in, checked the gap and cleaned the old points and fired the bike up. Started up and idled ok (not great). Points were still arcing. Not as bad... 1&4 displayed minimal arcing, while 2&3 continued to display quite significant arcs. I am pretty sure the timing is off. The 1&4 point did not start opening until about a half inch or so past the F mark.

This is my first encounter with points. Why would 2&3 point still be arcing if a new condenser was installed? Failing coil? Bad or dirty points? I have not tested coils yet, due to the "light show" happening at the points.

 

[mkaser81cb750c]

Would love to get an answer to this. Sounds like your problem is similar to mine. 1981 cb750c dohc I am unable to get passed 5k rpm while in gear, it revs fine in neutral... bogs out and no power until I'm get completely off the throttle. Runs fine up to 5k rpm. I have ultrasonically cleaned and synced my carbs, set timing, gapped plugs, tried iridium plugs, noticed I wasn't getting a reading on my regulator/rectifier got a new one, 4 into 1 exhaust, 3 turns out on air/fuel screws, stock airbox, sprayed around carb boots with starting fluid to make sure no air leak, made sure the spark advancer was free and had strong springs, checked battery with a volt meter (12.2 at rest and 14.5 at 6k rpm = good), gas flows from tank to carbs fine... probably more just trying to get this thing fixed.

At this point I have no idea. Going to make sure a slide is not sticking (which to me if it revs high okay in neutral i don't think a slide would be sticking), check the valve clearances (same revs fine in neutral), buy new coils and timing plate. If it is not that I am completely stumped but still seems like weak spark. Does this in every gear at 5k no matter if I slowly roll the throttle on or gas it hard. Learning a ton about this bike but I also want to ghost ride it off a cliff.

 

[tommycarl]

I went through this 2-3 years ago with a CB900C. Everything pointed to the carbs so I cleaned them, but problem didn't go away. t checked everything else... and cleaned the carbs again; and again; and again. In the end, I cleaned the carbs 6 times before I finally got the problem solved.

 

[eihander]

Having the same problem at the moment. Readjusted and cleaned the points. Cleaned my carbs as well. Problem went away for a few hours and then came back again. Will update if I solve my issue.

I took it for a spin and shifted at 7,000 RPM every gear. It ran great! Was almost home and then it started sputtering and backfiring again.

 

[mkaser81cb750c]

So after a new reg/rec and another set of spark advancers and going through my carbs many times I was all but ready to give up and take it to someone. I was reading some random thread that said take off the airbox cover and see if it runs better. I thought well I might as well try it. Took off the airbox cover and the thing ran perfect. Went through the entire tach and didn't die out anywhere. Airfilter must be just old. It is not dirty at all. It's now months later and the bike still runs great. I just have my secondary cosmetic chrome cover covering up the open airbox hole and you can't tell. I'm sure a K&N filter might work better but I'm just leaving it as is. Idles great and everything.