Topic: 1975 cb750F engine underpowered...thoughts?

[cheftuskey121]

So is the advancer working properly? verified with a timing light?

Also, is your exhaust possible plugged up a bit, too much restriction? If your intake was restricted plugs would look black, but not necessarily so for exhaust.

And you're sure you're running on all 4 cylinders? Dumb question but it sounds like my bike when only running on 3 of 4.

already answered about timing. won't hurt to check, takes me a minute to do. hopefully I can tonight.

Haven't thought to check the exhaust. highly unlikely its clogged as it didnt sit for more than 8 weeks stored inside my bedroom while they were off the old bike. all 4 pipes feel like they have the same air pressure coming out. that is something I regularly check. all four header get hot immediately after starting. I also make a habit of checking this. I have ridden off with a spark plug cable off so I know the feeling. its not THAT bad, and running on 3/4 makes a certain engine/exhaust sound. this isn't it :/

[bikebodger]

one thing no one has mentioned & you dont seem to have checked is the cam timing.if someone has fitted a new chain using a soft link its easy to get it a tooth out.i have seen it on other engines ,if the timings advanced you get more bottom end but it wont rev,retarded & it looses bottom end & needs revs.has someone fitted a performance cam & not timed it correctly .these are pretty simple engines & still pull even when on there last legs it must be something simple.you will have that DOH  moment & wonder how you missed it

[cheftuskey121]

I have not thought about that. How to go about checking this? the history of the engine is fairly unknown, just that its a higher mileage engine. the shifter mechanism was missing as was the clutch plates. didn't look like the came cover was ever taken off as the engine cover bolts were all replaced with hex nut bolts and the cam cover still had the originals. engine also doesn't look to have ever been split either. curiouser and curiouser...

[754]

The bad gas mileage points .......to a worn motor in my mind.
 Clutch?..  Eliminate all doubt..put the K1 complete clutch in,may need old outer cover..

Trouble shooting is just weeding out and eliminating variable.. One at a time..

[bikebodger]

a basic easy check is remove the plugs & turn it over till tdc on either 1 or 4 .when its on tdc two valves will be shut .other two will be rocking ,one just closing other opening.the mid point of the rocking should be virtually tdc.if its a good way off then something is off.this is just a rough & ready check .really you should remove the rockerbox & check the marks line up etc.you can also compare valve lift whilst the valve covers are off ,you never know sometimes a worn cam can be reasonably quiet.

[cheftuskey121]

The bad gas mileage points .......to a worn motor in my mind.
 Clutch?..  Eliminate all doubt..put the K1 complete clutch in,may need old outer cover..

Trouble shooting is just weeding out and eliminating variable.. One at a time..

754 I have done exactly that. K1 complete clutch, and outer plate. that was the first thing that went on there before I fired it up! no dice. if its a worn motor then its a worn motor, I just didn't know if it always smoked when it was old etc. this thing does not smoke at all. I am waiting on my F0 clutch plates to get here and that (along with the F0 clutch springs) will be the last variable to eliminate the clutch as the issue. it will be as stock as stock gets. the 110 mains don't seem to have done much. I know on a F0 the jetting was different, but the exhaust was more restrictive. I am using the 341s which is leaner so thats why the jetting is up. I am ordering more plugs too to do proper chops. this way I can hone in.

I may need to give Bill Benton a call and take my K1 motor and this one up to ATL in his 750 engine collection and come back with a rebuilt motor haha

[cheftuskey121]

a basic easy check is remove the plugs & turn it over till tdc on either 1 or 4 .when its on tdc two valves will be shut .other two will be rocking ,one just closing other opening.the mid point of the rocking should be virtually tdc.if its a good way off then something is off.this is just a rough & ready check .really you should remove the rockerbox & check the marks line up etc.you can also compare valve lift whilst the valve covers are off ,you never know sometimes a worn cam can be reasonably quiet.

thanks sir, I will check that. Bill and I set the valve clearances personally before I left ATL with the engine. it may be time to double check that anyway. all good stuff so far, thanks guys. lots to chew on

[andrewk]

If you only changed the jets and haven't yet tried tuning the needle, you haven't eliminated fueling as an issue.  Once you change the jet, the needle is most likely going to need an adjustment.

[cheftuskey121]

If you only changed the jets and haven't yet tried tuning the needle, you haven't eliminated fueling as an issue.  Once you change the jet, the needle is most likely going to need an adjustment.

I'll be checking that once I start plug chops. Idle seems to be fine. Good color. Next is main jet, then needle? In my head that works. I need to mark my throttle so I can see where I am.

[iron_worker]

8k RPM @ 60mph can ONLY point to a few things:

1) The speedo is off (which you verified)
2) The tach is off ( ... should be pretty easy to tell just from listening though if the motor is revving at 5k vs 8k) My stock K4 with stock gearing and all revs around 4300 @ 60mph. This varies a bit from K to F and through the years but NOT MUCH.
3) The gearing is incorrect. - I don't think this is your issue as I don't think you can even run a large enough rear sprocket to change the gearing that much. Tire size also plays an effect on your overall ratio but the amount of change we're seeing here is way beyond that.
4) The clutch is slipping!!!!

If you've verified that your bike is actually going 60mph and your engine is actually turning 8k and your tires and gearing are reasonably close to stock then surely to goodness something is slipping which is almost positively the clutch.

So,

1) Is the clutch adjusted properly? You mentioned that the lever has no slack ... the manual states it should have something like 3/4" of free play before the handle even starts pulling the cable.

2) Is the clutch assembled properly? I would focus really hard at what is going on here. I think having the K plate stack up inside of the F clutch basket is your main issue. There are differences there that aren't compatible. I believe this is the root cause of your problem.


I think everyone needs to stop talking about fuel problems, timing problems, cam timing, etc etc etc. None of these factors will EVER have an effect on your speed/rpm ratio. Focus in on the things that do!

Perhaps a video of you out on the bike would help?

Good luck Chef.

IW

[cheftuskey121]

8k RPM @ 60mph can ONLY point to a few things:

1) The speedo is off (which you verified)
2) The tach is off ( ... should be pretty easy to tell just from listening though if the motor is revving at 5k vs 8k) My stock K4 with stock gearing and all revs around 4300 @ 60mph. This varies a bit from K to F and through the years but NOT MUCH.
3) The gearing is incorrect. - I don't think this is your issue as I don't think you can even run a large enough rear sprocket to change the gearing that much. Tire size also plays an effect on your overall ratio but the amount of change we're seeing here is way beyond that.
4) The clutch is slipping!!!!

If you've verified that your bike is actually going 60mph and your engine is actually turning 8k and your tires and gearing are reasonably close to stock then surely to goodness something is slipping which is almost positively the clutch.

So,

1) Is the clutch adjusted properly? You mentioned that the lever has no slack ... the manual states it should have something like 3/4" of free play before the handle even starts pulling the cable.

2) Is the clutch assembled properly? I would focus really hard at what is going on here. I think having the K plate stack up inside of the F clutch basket is your main issue. There are differences there that aren't compatible. I believe this is the root cause of your problem.


I think everyone needs to stop talking about fuel problems, timing problems, cam timing, etc etc etc. None of these factors will EVER have an effect on your speed/rpm ratio. Focus in on the things that do!

Perhaps a video of you out on the bike would help?

Good luck Chef.

IW

thanks for the thoughts IW, it is not 60mph at 8k. its between 5k and 7k. I agree it still feel like its slipping slightly. I always thought clutch slip was an all or nothing deal though. I also agree that fuel/air is of little importance when my speedo is accurate both of my tachs show the same thing. engine is making power it's just not getting to the wheel.

I will check that adjustment from honda to the t. It was a little slack before and I tightened it up after I put in the new HD springs. I will readjust it once my 75F plates come in. the parts fiche for the K1 and F0 plates are almost identical. seems to be the exact same number of fiber/metal plates there....and I have already tried my K1 clutch plates in it. I am currently using cycle x plates that do not have a "k" or "F" specification. when the F0 plates get here though it will be as stock as stock gets. the only difference I have seen in the parts is the washer that butts up against the inside of the engine is different for each engine, but I have the correct part in there. will triple check that when I swap plates.

also I should add I have checked timing and it was as close as it can be.

thanks so far!

[iron_worker]

http://www.cmsnl.com/honda-cb750f-750-super-sport-1976-usa_model490/clutch_00000kit0095/#.UvqfnmKrp5E

The picture for the F shows 6 steels and 7 fibers. (Stack up starts and ends with a fiber) ... Also the plate in the center looks different?

http://www.cmsnl.com/honda-cb750k1-four-usa_model14354/clutch_00000kit0097/#.Uvqfn2KrpWo

The picture for the K shows 7 steels and 7 fibers. (Stack up starts with a steel and ends with a fiber.)

IW

[Jerry Rxman Griffin aka MuthaF'er]

No more input at this time, just waiting for you to get your F0 clutch for comparison. Should have 6 friction discs with an outer 7th thats slightly different and 6 steel discs.

What I do NOT know is how the CycleX unit stacks up height wise, etc to the F or K units. Kenny does NOT give any info that I could find on his site as far as comparison or installation. Also the F basket is different than the early K's so can't say how that pack will fit what I believe you've said that you installed, the early K basket.

We'll see. 

[cheftuskey121]

http://www.cmsnl.com/honda-cb750f-750-super-sport-1976-usa_model490/clutch_00000kit0095/#.UvqfnmKrp5E

The picture for the F shows 6 steels and 7 fibers. (Stack up starts and ends with a fiber) ... Also the plate in the center looks different?

http://www.cmsnl.com/honda-cb750k1-four-usa_model14354/clutch_00000kit0097/#.Uvqfn2KrpWo

The picture for the K shows 7 steels and 7 fibers. (Stack up starts with a steel and ends with a fiber.)

IW

mine only has 6 metal and 7 fiber. it definitely worked in the K1 engine....haha. this is odd. I must investigate more. Yes, I am waiting now for the plates to get here so I can measure the height. the K1 plates and the cycle x plates (7 fiber and 6 metal) measure to almost the same stack height...which obviously explains why they both do not work.

crossing fingers

[Bill/BentON Racing]

Chef, do a compression check ASAP. Do it as we discussed. A 4 cylinder CB750 won't run on 0 compression.... Or 35-50 lbs...not enought power made to to effectively do what's needed! I learned on a single cylinder Honda SL 90 a lot about what it takers to run a normal Honda single, a 750 just 4 working as a team!   So my experience, you need 100-110 minimum to run and be able to ride at all without fouling plus etc. the little SL 90 @ 100-110 would still run and pull me. Once it got to 90 psi, it would run, but it wouldn't pull my weight anymore. If you do a compression check you need the engine to get all the air possible. Carbs off the best, but make sure butterfly's/ throttle valves all wide open! Pull all 4 plugs, do one cylinder at a time. Turn engine with charged up battery til needle stops! Record it ,do all four. Let me know. Wilbur, I would imagine you didn't have throttle valves wide open or engine spinning fast enough. Anything down around 100-110 going to be under powered and using more fuel. Wilbur ,chime in, if you do it over, let us know.Good luck all, Bill
Compression low, a leak down will tell us where  .

[bender01]

If the clutch is slipping you really would notice it going over the bridge to work! Is it real noticeable there on your rte.?

[cheftuskey121]

Alright, got my stock plates in, fast shipping form Utah! they are soaking in oil and tomorrow or the next day I will be putting them in. the stack measures only .90mm taller than the K1 stack. unsure of comparison to the cycle x stack currently in the bike...but I doubt .9mm will make a difference. going to check compression tonight as well and will report back.

just a small update. stay with me

Ben, it definitely pulls me over the bridge without slipping. I am thinking it's just worn down. I think it's time to get new cases for my K1 engine and rebuild. I do not plan on getting rid of Yolanda any time soon, so why not put her back the way she was intended

Ok scratch the compression test. The 12mm adapter is missing. Brother is getting on Saturday. No worries. Ain't gonna make the engine run worse or better haha. Everyone carry on.