Topic: 1975 cb750F engine underpowered...thoughts?

[Schnell]

I can't help much but offer this observation:

Whether the motor is strong or weak, it has no bearing on the redline/top speed condition. In my logic, these are 2 seperate issues.

With the motor swap, perhaps the new motor's tach drive gearing doesn't match with the old tach. That would explain the rpm/speed discrepancy.

The low power from the motor could be caused by one of the ideas presented so far. Personally, I like the spark advancer theory.

[krusty]

If I'm reading this correctly you have an F motor and K carbs, right? Therefore should not the carbs be jetted for the F motor? ie replace the K jets with F spec.

[cheftuskey121]

I can't help much but offer this observation:

Whether the motor is strong or weak, it has no bearing on the redline/top speed condition. In my logic, these are 2 seperate issues.

With the motor swap, perhaps the new motor's tach drive gearing doesn't match with the old tach. That would explain the rpm/speed discrepancy.

The low power from the motor could be caused by one of the ideas presented so far. Personally, I like the spark advancer theory.

I thought about that too with the tach. Maybe the ratio is different between the years?  That doesn't explain the lackluster top speed though.

[Schnell]

Right, you still have an engine power issue, but you also have an unrelated rpm/speed relationship issue, IMO.

[Jerry Rxman Griffin aka MuthaF'er]

I'm thinking your main issue is the clutch. The tach gear is the same in all the 750's. That's a non-issue.

"yes 90mph is 8k in 5th gear" Swap one of your other tachs and see what happens. Let's eliminate the tach itself. For that matter you could swap a tach cable too but I don't think that'd do anything unless the present cable is shredding inside.

If the clutch is the MAIN issue the jets will NOT address the 90mph = 8K. Also the advancver will not address the rpm/speed. And if it's not using any oil or not smoking I'd hold off on the compression test FOR NOW.

My attack would be to replace the tach then look into the clutch. Save the jetting for later. You have new plates but I'm not familiar with Kenny's clutch pack. FYI the double metal did not appear until the F2/K7 or at the VERY end of the F1 run at #58988. The earlier K's also had somewhat different clutches too.

You might want to consider heavy duty springs!

If we haven't asked, are you using MOTORCYCLE non-synthetic oil?

What's the serial number of the K1 engine you took the clutch from? I can do some comparisons with my different HONDA parts manuals.

[Jerry Rxman Griffin aka MuthaF'er]

Right, you still have an engine power issue, but you also have an unrelated rpm/speed relationship issue, IMO.

EXACTLY............

[Jerry Rxman Griffin aka MuthaF'er]

I can't help much but offer this observation:

Whether the motor is strong or weak, it has no bearing on the redline/top speed condition. In my logic, these are 2 seperate issues.

With the motor swap, perhaps the new motor's tach drive gearing doesn't match with the old tach. That would explain the rpm/speed discrepancy.

The low power from the motor could be caused by one of the ideas presented so far. Personally, I like the spark advancer theory.

I thought about that too with the tach. Maybe the ratio is different between the years?  That doesn't explain the lackluster top speed though.

Not an issue, just redlines are different.

[cheftuskey121]

Right, you still have an engine power issue, but you also have an unrelated rpm/speed relationship issue, IMO.

I'm holding out for a "one thing fixes all" solution like the clutch or advancer wishful thinking perhaps

[cheftuskey121]

Jerry about to hop on te bike and head home so I'll respond fully later. I'm using rotella 15w-40 diesel oil. Been using that in my other bike and the k1 motor as well. Haven't had any issues with it, seems to be well used for the sohcs. I'm up for trying something else though. No synthetics here.

[Jerry Rxman Griffin aka MuthaF'er]

Many people swear by Rotella so that should be good.

[cheftuskey121]

Jerry the k1 engine is 1100045. Yeah I've enjoyed the oil. I change it about every 1200 miles or so when I can feel it shifting harder. One thing I've noticed on this F0 engine...it is so easy to get into neutral at a stop when the motor is hot. Impossible on the old engine. It's like butter...probably because my clutch is slipping haha. I also have noticed my mpg decrease from an average of 41mpg on the K1 motor and 78F motor to 32mpg on this motor. Wasting power for sure.

Is it possible I just need to add a fiber? Would the clutch even work properly if I was missing a whole plate? I do have HD springs and plan on popping them in tonight to see if that helps.

[Schnell]

If the tach and speedo function properly and there is no issue with matching the swapped motor, then your clutch has to be slipping to get that rpm and speed combo.

[Jerry Rxman Griffin aka MuthaF'er]

I'd speculate the 32mpg is due to the 120 mains. You'll get 42-45 on the road when it's right.

Can't speculate about the # of fibers. I'd have to do some research and I can't now until later or probably tomorrow. What did the instructions say? and how many did you put in? how many does the fiche show for the K1 clutch for that s/n? 

[Jerry Rxman Griffin aka MuthaF'er]

If the tach and speedo function properly and there is no issue with matching the swapped motor, then your clutch has to be slipping to get that rpm and speed combo.

+1 cut and dry!

[cheftuskey121]

Jerry, the K1 clutch had 6 metal and 7 fibers. thats how many I put in the F0 clutch. there were no instructions. I researched which way the angled grooves go and concluded via a few posts that for the 750 they face \\\ so as the plates rotate clockwise the oil slings out and into the basket. here is a picture showing the edge of my basket. does this stack height look right? it looks like there is enough room to add another fiber...are the fibers supposed to butt up flush with the basket? the bigger edges here are the basket, and the smaller (farther away ones) are the fibers. I am trying to search but I can't find a picture like this to compare to.

[cheftuskey121]

I just put in the HD springs, because that was easiest to try before taking the clutch plates out. not too much of a change. it feels like it accelerates faster in the first few gears and then peters out to the same speed/rpm as before. the clutch lever sure is stiffer....my hand. ouch.

I just ordered an original F0 clutch plate set on ebay. this is a stock setup. when they get here I will throw them in  with the original springs...this should eliminate my guessing on the clutch stack...

the clutch is adjusted great. no slack in the lever, pulls away soon after the clutch lever is released. the adjust screw is between 1/4 turn and 1/2 out from resistance felt. I believe this is all withing spec. I tried loosening the adjuster screw to more than a 1/2 turn out and the clutch did not release until the end of the lever. no good. adjusted it 1/8 out and it let go WAY to soon. both of those settings I took it up to high speeds and all is the same. no adjustment seems to give me the power I am losing. so...waiting on those plates. figured I would update you guys. thanks so far

[Schnell]

We're rooting for you.

[bjbuchanan]

From what you have said I think that the stack is shorter than it should be. You want to measure your stack height vs an F0 stack height. I know that a 76k model is the start of the slightly taller stack, is the F0 taller too?

Either way even before you pop in that original F0 clutch see what the height of it is and compare it to what you have in there now. The differences are very small and very important

[cheftuskey121]

From what you have said I think that the stack is shorter than it should be. You want to measure your stack height vs an F0 stack height. I know that a 76k model is the start of the slightly taller stack, is the F0 taller too?

Either way even before you pop in that original F0 clutch see what the height of it is and compare it to what you have in there now. The differences are very small and very important

thats the plan BJ. the plates were only $20. I am not even expecting them to be great. just a comparison. if they work they hey good money spent. if I want to get new OEM plates its going to be about $160....so I am hesitant to drop cash if the clutch isn't even the issue

news: I found some 110 mains I had sitting around never installed. oem too. put those in and not a damn difference. didn't even feel different riding around. no increase in performance. same speed/rpm crap. gah. will report back after my ride to work and back tomorrow just in case something else has changed.

the only good thing those springs have done is eliminate the clutch stutter I had on takeoff. glad that's gone. it sounded and felt kinda scary. these bikes sure do take a licking and keep on ticking though

[bjbuchanan]

Well from my research on these clutches (I have to replace mine) you don't need to worry as much about the steels. As long as you check them for flatness against a flat surface and a feeler guage and they aren't warped out of spec then your clutch plate swap gets a bit cheaper. That would cut the price in half, a little under.

From the amount of clutches you have laying around I can damn near guarantee you have a good set of steels cumulatively. Stick to the dimpled steels as they grab better. Rounded face of the tooth outward.

Another conclusion that I have come to is that with a good clutch then stock springs will hold plenty good. Plenty of guys on here running 836's and mild workovers with stock springs and clutch. Most of those guys that put in aftermarket clutches seem to give up and put stock springs back in. New ones will be a big improvement

[Jerry Rxman Griffin aka MuthaF'er]

A couple pics





Early vs Late clutch basket - taller one is 77/78 K or F - but let's not go there with your's!! Just for comparison since it was mentioned

[Grinnin]

Wasting power for sure.

If the problem is the clutch you should hear the motor revving with no corresponding increase in speed.  You really can't miss the feeling.

The speedometer can easily be checked with a GPS while on the road and the tach can easily be checked with a dwell/tach in your driveway.  A timing light will show if the advance is working correctly or not and may show if the spark gets weak at high RPM.

If you're not as old as me, ask your retired motor buddies if they have a dwell/tach or timing light in a box somewhere.  These were common diagnostic tools when computers were confined to buildings with extra air-conditioning units.

[lrutt]

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.

[John Eberly]

Chef - lets do the simple analysis: you state the engine does not "smoke" while idling or running. Does it use oil? If no to both of these answers, then I can not imagine a situation where compression would be low.

For compression to be low 1 of 2 situations has to be present: either worn cylinders or worn valve train. In each case, oil is passed into the combustion phase of the stroke and burned. If you aren't using oil, and not blowing, compression is good enough that your high end speed is not compression related. Others may disagree.

Can I offer a bit of a hair brained hypothesis: is it possible your timing advancer is not functioning properly and sparking correctly in the higher power band? I mention this as this appeared to another's situation (believe it was member Maurice) and he found that the spring was weakened. Now, I could be way off, but you seem pretty diligent about bikes settings and comparing apples to apples, you should get apples.

I like your idea that it's coil/spark related,as you posted initially. This is all I have to offer, so I'll shut up now and let others guide you-

Good luck tracking it down!

You can't imagine a burned valve??

[cheftuskey121]

Wasting power for sure.

If the problem is the clutch you should hear the motor revving with no corresponding increase in speed.  You really can't miss the feeling.

The speedometer can easily be checked with a GPS while on the road and the tach can easily be checked with a dwell/tach in your driveway.  A timing light will show if the advance is working correctly or not and may show if the spark gets weak at high RPM.

If you're not as old as me, ask your retired motor buddies if they have a dwell/tach or timing light in a box somewhere.  These were common diagnostic tools when computers were confined to buildings with extra air-conditioning units.

the speedo has been verified with a gps unit. it is working within +/- 5mph of actual speed. I have not checked with my other tach yet. kinda got caught up with the clutch and jets last night. doubt I can do that today because of long work hours but I will see. just takes a moment I suppose.

I have a timing light. I have used it so set the timing initally after I installed the pamco. this is the third time I have installed one and timed it. no issues before. it advances around 2500 and does not stick. both the K1 advancer and F0 advancer have been tried. I did not check this last night but will need to verify. the timing was set correctly after I got the engine fired and it has always had this issue since I dropped it in.

need to look into that tach/dwell meter. thanks!