Where to Start-1979 750f Super Sport? Carb/Compression or both?

[Motogrady]

Sounds normal to me.
It’s not gonna have the low end torque that that v twin does.
I know mine, it does seem to run higher rpm then I’d like at cruising speed.
That’s just one of the differences between a
bigger twin vs an inline 4.

Add a tooth to the countershaft sprocket,
or drop 2 or 3 teeth on the rear sprocket will give taller gearing. This will help drop the rpm of the motor at cruise, and give more absolute top end speed, but will sacrifice even more low end torque.
That’s basic gearing 101.

For low end torque this helped my 79f.
I’ve drilled I think 6- 1/2 inch holes in the bottom of my air box. To help the thing breathe better. I still run a paper air filter.
I also increased jetting 2 sizes, figuring more air I could add more fuel to the mixture. With the holes in the box, 75/110 jets, and stock gearing it seems to run stronger throughout.
Still no where near the low end a Dyna has,
but it is better. Better to the point I feel 2 teeth off the rear just might be next to help with motor rpm when cruising at say, 65 or 70 mph.

I mean, nowadays, around here anyway,
you don’t run that yer in the way.

Hope this helps guy.

 

[brettp]

My CB750k revs at 5k (I think) at 60mph. And I think torque is normally higher in a v-twin design. Maybe my understanding is wrong, but that's what it is today.

 

[Motogrady]

My CB750k revs at 5k (I think) at 60mph. And I think torque is normally higher in a v-twin design. Maybe my understanding is wrong, but that's what it is today.

Yes Brett. The torque on a v twin is stronger at lower rpm. Even more so with a big single.

But neither will rev like an inline 4. Or 6 for that matter.

Not for long anyway.

 

[Hastings]

Great info Motogrady and Brettp, Thanks!

 

[brettp]

By the way, that cb750 power range is above 5k, I think. You really feel it pull through 5-8k.

 

[kirky]

the revs and speed are same as mine i keep going for 6th gear , i'm used to brit iron except for a kwack triple 2 stoke a long time ago

 

[mrtwowheel]

I don't putt putt around, at the same time I'm not a speed demon either. I like to take the main roads at the speed limits. I'm currently running the stock front size sprocket, 18 teeth, and a smaller than stock rear sprocket, 39 teeth, with a 104 link 530 chain on two of my '81 CB750C's. 55mph and 70mph on the freeways is very pleasant, even two up. I think that running a larger than 18 tooth on the front is a terrible idea with these fat O-ring chains, asking for busted cases if a chain fails.

 

[plantguy]

I have a 1980 CB750F and a 1982 CB900F that I have been messing with. They both had carb and ingnition issues that were driving me crazy. I fixed the carb issues with a set of Keihin CR carbs, expensive yes but they are better than the crazy complicated stock carbs. I was going to try the Murray set up but just not sure about having 1-2 and 3-4 on the same manifold. Seems some like them and some hate them.

On to ignition. I pulled the triger on a Dana 2000 and coils after having issues with stock style after market junk.
I pulled the tank off of the 900 getting ready to install the Dyna on that bike and I thought I would check the wires and coils with a meter.

1-4 23 ohms, 2-3 open!!!! Those coils and wires have less than a couple hundred miles on them. Ends up it was the cap that has a resistor in it and it’s the open component. Screw it I have a known good set on the 750 and since that’s the bike I am for sure going to keep it’s gona get the Dyna ignition.

Swap the coils and wires, drop the mains on the 900 to 110 and ride it. Runs fantastic.

After I installed the Dyna on the 750 I did more reading about these older stock ignition systems and found out a few more problems associated to these waste spark systems. Seems the wiring is kind of goofy in that, power comes from the battery through a starter relay and main fuse, into the headlight bucket, through the ignition switch, through the kill switch, into the ignition modules, to the coils and a lot of 42 year old connections.


I checked the battery voltage 12.5 then the voltage at the coils 11.4. That’s over a 1 volt drop! I know these ignition systems need good, clean voltage to opperate correct because the stator quit charging on the 900 a few months ago and it started to run real ratty. Rather than do voltage drops at each component I decided to install a relay that would switch straight battery voltage directly to the coils. Yep the 750 with the Dyna and 900 with stock ignition are both running great.

With all that said the valve adjustment that you show will not give you the running issues you are seeing. Your compression looks good, but reading your plugs tells me it is most likely a fuel issue you are having. I recomend you check your charge rate, coil positve voltage and make sure your restiance for coil 1-4 and 2-3 is around 25 ohms just to check that box and then move to focus on a fuel-carb issue.
And I highly recoment a set of NGK DR8EIX Iridium spark plugs for these older CB's ignition systems.

I am sorry to say it but your going to have to spend some time and money espescaly if you have someone else do the more advanced diagnostic and repair work.

 

[Hastings]

Well it has been running great until yesterday, it was a cooler day and it was fine up until 6K rpm then it was if you closed the throttle
Anything below 6K rpm was fine. I assume its fuel related since I just replaced the coils less than 100 miles ago.
It still has the stock jets even though the previous owner change to a Kerker exhaust.
Will be interesting to see how it does on a warmer day as I dont recall it doing this when the temp is up.

 

[plantguy]

Well it has been running great until yesterday, it was a cooler day and it was fine up until 6K rpm then it was if you closed the throttle
Anything below 6K rpm was fine. I assume its fuel related since I just replaced the coils less than 100 miles ago.
It still has the stock jets even though the previous owner change to a Kerker exhaust.
Will be interesting to see how it does on a warmer day as I dont recall it doing this when the temp is up.

Make sure it's charging,14.5 volts above idle. My 900 did that exact thing, Turned out to be the stator.

 

[brettp]

Coils can fail at high RPMs. But I had the same thing happen to me, above 6k it acted like it had zero power left but it wanted to go. Absolutely felt fuel related. But it wasn't fuel, it was a valve problem. My valves were out of spec. A valve job later and I was revving to 9k without issue.

 

[Hastings]

Rode again yesterday and experienced the it again at 6K rpm no sputtering just as if you hit the kill switch.
I let off the throttle and tried again and reved fine to 7K-8K.

 

[brettp]

Sounds like valves out of spec to me

 

[Hastings]

Did do an adjustment on them about 150 miles ago, they were not at .05 across the board but varied .06-.07 think I need to get them closer?

 

[brettp]

Your 6k limit is exactly what mine did. The fact it revs slowly past 6k but not quickly tells me valves. But it could also be piston valves holding the carb needle having friction on the way up. Might try polishing the piston valves and caps.