Topic: hesitation and lack of power from 5500rpm on in 4th and 5th gear

[cavi]

Bike history, 1978 cb750f, 10000 miles, bike was stopped for a long time, I bought it a couple of months ago, the last owner had it for a very short time and changed the fluids and the bike seemed to be running fine.  I rode it twice, and had the cam shaft die, the oil passages clogged and also the retaining bolts for the whole cam retaining cassete came loose.  The local shop put a used cam cassette  and a used cam, that I got from a member here.  The cam was in theory from a F model.  Anyway I picked it up yesterday and took it for a ride today, and two issues, when hot and you are on it and come to a stop and let off the gas, it dies.  If you restart and just rev lightly for a second or two it idles fine.  The bigger issue is in 4th and 5th gear I get a hesitation, in 4th it comes at 6000rpm and in 5th gear I feel it at 5500rpm.  In fifth it actually does not even develop any more power past this point, in 4th it will still develop power till 7000rpm but not past there. 
I took it back to the shop and they are going to tear the carbs apart which makes sense as who knows how long the bike sat for so the mains might be partially clogged.  But at the same time it did not do this before the cam died.

[scottly]

What about 1st, 2nd, and 3rd gears? Does it pull to redline in all three gears, or does the power seem to drop off at a lower RPM as you shift to higher gears?

[cavi]

1st and 2nd pulls fantastic, 3rd, you can get a slight hickup at 5000rpm, but you have to accelerate in a specific way, and it is just a slight hichup, then it continues to pull like it should

[scottly]

*Might* be a fuel starvation issue, maybe on only one carb. In higher gears, it takes more fuel to make power at the same RPM than in lower gears. More load requires more fuel. First thing to look at would be the spark plugs.

[cavi]

That is kind of what i was thinking

[scottly]

Might also be a plugged gas cap vent, if it gets worse the longer the bike is run at speed.

[N30R3L0AD3D]

Sounds like it's lean.

[lucky]

We need more facts. It is not a camshaft issue.

The facts:

1978 CB750 F
What kind if intake system do you have (air filters)
What kind of exhaust do you have?


When you get the carbs back from that shop. Make sure the accelerator pump system works. With the engine OFF and the float bowls full of gas take a bright LED pocket flashlight and look into the intake (air filters off) and turn the throttle and make sure that the  brass tube is squirting fuel when you turn the throttle.
Check all 4 carbs. If they do not work take it back to the shop and ASK that they do the job right.

[Mingo]

*Might* be a fuel starvation issue, maybe on only one carb. In higher gears, it takes more fuel to make power at the same RPM than in lower gears. More load requires more fuel. First thing to look at would be the spark plugs.

What do you mean look at them, Scottly? Do you mean see if they are good or look for signs of certain engine behavior on them?

I've got new plugs in the bike, but this is happening to me too. Though with a different bike: 78 cb750K.

In first and second, it bogs at about 5,500.
In third, it bogs at around 5000.
In fourth and fifth, it bogs at 4500.

Anyone know why?

[rklystron]

Had the an issue similar on my 1977 CB750 F2. I was tuning my carbs and all of a sudden my bike would bog down around 7.5-8K. You could feel it in the lower gears but when the engine was under heavy load in 4th and 5th gears the bike would really bog down, like not getting enough fuel...so I thought carbs must be the problem. I decided to take a close look at throttle position by taping markers on my twist grip showing Idle, 1/4, 1/2, 3/4 and WOT. What I found was interesting, throttle position seemed to have no real effect on the engine stumble problem. This made me look at the ignition because it was seemed more RPM based than throttle based.  So I replaced my ignition module with the same type, no effect. Replaced my 3 ohm coils but not the wires, no effect. I replaced my wires and WOW runs like a champ. Problem fixed. My problem was the plug cap on #1 read 10K ohms and all others read 4K. Just goes to show you just because you are working on tuning carb's doesn't mean some other problem will not show it's ugly face and make you humble.

[scottly]

*Might* be a fuel starvation issue, maybe on only one carb. In higher gears, it takes more fuel to make power at the same RPM than in lower gears. More load requires more fuel. First thing to look at would be the spark plugs.

What do you mean look at them, Scottly? Do you mean see if they are good or look for signs of certain engine behavior on them?

The plugs may give an indication of how the engine is operating: clean and white may mean a lean mixture, black and sooty means a rich mixture, and clean and wet with gas can indicate an ignition problem. Your symptoms are a bit different than the OP's, since your bike never revs past 5500.