Topic: 1975 cb750F engine underpowered...thoughts?

[cheftuskey121]

Hey guys, this is a continuation of a problem I have been having with my 75F motor. it is a replacement motor for my K1 engine that got a little trashed in my accident back in october. now when I am riding in 5th gear, I have to bring it up to 5500 rpm just to go 60mph. the speed IS verified by a gps unit, so it's not the gauge. the tach is not off either because I can not only hear the engine wind up (don't need a gauge for that haha) and I can't go any faster than 80-90mph. not that I speed that much, but something is wrong for sure. I have replaced the clutch plates with the cycle x kit and have no defined slippage, but this "underpowered" feels as though its permanent clutch slipping. am I crazy?



this mileage on this engine is unknown but is higher mileage, between 35k and 50k. it does NOT smoke. I am using recommended denso plugs and my plugs look light tan. ethanol in fuel plays a role in that I know. the carbs are from a K1, the earlier round top style. airbox is K&N, which is the only difference other than the engine itself. running pamco system with the "ultimate" coils. the exhaust is 341s, stock jetting for those carbs/exhaust on the K1...main is 120/slow is 40. needle clip is in 2nd from the top. with the old engine, everything ran smoothly. things were tuned perfectly. my brother is bringing home a compression tester so we can check. maybe the engine is just old and worn out? what would/could cause this issue?

I'll add, all electric connections in the harness are new/good, transplanted from before the accident where things were working well. my charging system is working fine and have no issues there with the battery or such. I have tried new plugs with no change. the ONLY other thing I can think to check is one of my coils where I soldered the connection instead of just using blade connectors. I had to replace the other coil because when I soldered the connection it DEsoldered on the inside making it useless.....maybe this other coil is desoldered but making enough connection to make it run...but not strong? its bothering me anyway so I will try that.

so in summary after this novel...I will (try another "ultimate coil" in there that is known to work great (off my 78F), and will check the compression to see where it's at. those are my only two things to think of. any insight would be great. I want this engine to run smooth and stock, and it is weak sauce right now. thanks in advance guys!

[flybox1]

damn....i hate weak sauce!     
what do your plugs look like after a run like that?
IIWM, id drop the needle clip one slot (thinking you're lean here..) and go at it again.
verify clean carb passages and float height while you're in there..

[goldarrow]

I think you may running too lean at mid range.  Don't recall any of my 750s both k and f having needle clips at 2nd notch from top.

[bjbuchanan]

Definitely lean on that needle. Every bike is different but that is still probably gonna be lean.

Drop the needle to the middle and see whats up. I had my needle set like yours and it was still a 750 but I felt the same way about it, it didn't have that freight train pull like it should when healthy. Dropped one notch and wowzers

[Stev-o]

Need an accurate compression check. Automotive testers dont work well on SOHC when trying to diagnose an issue such as this.

[madmtnmotors]

I concur with the checking the basics first, starting with compression test. Mine got to be hard starting (ether required) even after swapping in a freshly rebuilt set of carbs. Turns out compression was near zero in two cyls and very low in the other two. Currently prepping the replacement mill...

[goldarrow]

Makes sense to check compression.  I've never done it on any of my bikes.  His carbs settings are off to begin with, so I would check that

[cheftuskey121]

Makes sense to check compression.  I've never done it on any of my bikes.  His carbs settings are off to begin with, so I would check that

The carb settings are spot on for the previous k1 engine that was in there. Hondaman recommends lowering the needle one position on the earlier bikes because they were jetted too rich. Hopefully this is just a matter of raising the needle back to get that midrange fuel flow. In principle I'm not sure hot much of an effect the engine plays on jetting when the intake, carbs, and exhaust remain the same. Shouldn't have too much effect?

Side bar: what is odd though is my oil temp reads LOWER than on the old engine. This points me away from a lean condition. Lean engines run hotter. This is odd. Will raise needles soon and report though. Thanks so far guys.

[Jerry Rxman Griffin aka MuthaF'er]

Chef,

I see that the guys are talking about putting the clip in the middle position. That would be correct for the 75 F. Has anyone mentioned that the 75 F uses a 105 main? Just can't say how much difference there would be between the 75 4 into 1 and the 341's as far as jetting may go but the 4 into 1 made better power with a smaller main!!??

[cheftuskey121]

well...my needles ARE in the middle position. this makes sense though because my plugs have been looking fine. I spend most of my time in the idle to 1/2 throttle range. I almost never go full throttle for any reason. for now I am going to put my carbs back on and put my stock air box on instead of the K&N. see if that makes a difference. if it does not I will raise my needles one more time. eliminating one thing at a time.

[classic bike tuner]

go with your motor to a dynojet and test it for output power olso the can messure the mixture

dont be supprised  if you wil not get 50hp on the wheel

[cheftuskey121]

Well riding into work today with coils from the over bike (no issues there) and the original air filter. It was a tiny bit better. 60mph at 4800 in fifth. Still sucks. Clips in the middle position. Have not checked my plugs yet but they were looking good before. Hard to believe the plugs would lie.

I even added some choke while riding to try and simulate a richer mixture and there was no performance increase. When full choke it started bogging down and wouldn't go above 5k (just like on my 78F) so at least there is consistency....still baffled though

[andrewk]

You mean that you have a K&N drop in filter in the stock box, right?

I think your mains are too big.  Go back to a 110, raise the needle at least one notch, and see what it does.  Stock engine, especially with stock exhaust won't require main jets that much bigger than stock.

[andrewk]

Plugs can "lie" because they give you an average.  Idle to half is where you say you run most of the time, so a high speed run on those plugs that ends up rich and bogging out won't blacken the plugs, especially if you ride back home at idle to half throttle.

I would start with a stock tune for the engine you have and make changes from there.  After a compression test verifies you aren't wasting your time, of course.  If you can get a dynotune session lined up like tuner mentioned, it will make your life a lot easier.

[seanbarney41]

Well riding into work today with coils from the over bike (no issues there) and the original air filter. It was a tiny bit better. 60mph at 4800 in fifth. Still sucks. Clips in the middle position. Have not checked my plugs yet but they were looking good before. Hard to believe the plugs would lie.

I even added some choke while riding to try and simulate a richer mixture and there was no performance increase. When full choke it started bogging down and wouldn't go above 5k (just like on my 78F) so at least there is consistency....still baffled though

why do you keep giving us specifics of how fast at what rpm?  If your clutch is not slipping and you have correct gearing and rear wheel size, this really doesn't have anything to do with being underpowered.  How does it accelerate?  If you rarely to never go 3/4 to full throttle, how do you know it's underpowered?
Just seems like everybody is having a communication breakdown here.

[Stev-o]

Well riding into work today with coils from the over bike (no issues there) and the original air filter. It was a tiny bit better. 60mph at 4800 in fifth. Still sucks. Clips in the middle position. Have not checked my plugs yet but they were looking good before. Hard to believe the plugs would lie.

I even added some choke while riding to try and simulate a richer mixture and there was no performance increase. When full choke it started bogging down and wouldn't go above 5k (just like on my 78F) so at least there is consistency....still baffled though

why do you keep giving us specifics of how fast at what rpm?  If your clutch is not slipping and you have correct gearing and rear wheel size, this really doesn't have anything to do with being underpowered.  How does it accelerate?  If you rarely to never go 3/4 to full throttle, how do you know it's underpowered?
Just seems like everybody is having a communication breakdown here.

+1.  And we need compression numbers.


PS. I cannot envision riding at half throttle or less. If thats how you ride, what makes you think its under powered? Try opening it up...

[cheftuskey121]

I think some of you are not reading all of my posts thoroughly. I give specifics for basis of comparison. I have two other 750 engines for comparison and this is the slowest of them. All three engines have been subjected to a 17/48 sprocket gearing as well. All three have been on 19" and 18" wheels. This way I have something to compare to. One engine is in my 78F and the other was the k1 engine in this bike before my accident. The k1 engine was subject to the same jetting, airbox, and exhaust. I never "went" to 120 mains. They were always 120 because that's STOCK for those carbs and exhaust on a K1. I don't own 110s currently to drop in. That will be my next recourse.

Continuing on. Of COURSE I've gone full throttle to test out the engine. I'm not dumb. I don't putt around on the bike. I read something like going 60mph is about halfway on the throttle. I commute around that speed everyday. I keep the engine between 4k and 7k regularly. This engine tops out at about 90mph though. I run out of redline way too soon. Sounds like the engine is working harder than it should. I can't get it to go any faster. The other engines both get up to 110-120mph with ease. So that's how I know I have a problem. It does not bog. It does not stutter. It does not pop on decel. It's just weak, almost as if a cylinder isn't firing. All cylinders ARE most certainly firing though. The acceleration feels ok but without torque, and like I said I'm out rpm range before I know it. It feels like riding on the highway in 4th gear. I keep trying to shift up but I am in 5th.

Now a new question. Is it possible for an engine to be worn to the point of lower power output without smells or smoke? From the outside this seems like it runs well. This is a theoretical question. Can an engine be worn like that if it's a high mileage engine. It seeps at the head gasket but no more than the k1 motor that was previously in it. Maybe I don't know what to look for. I am working on getting a compression tester to see what I've got. I'm trying to be as specific as possible that's why my posts are so long. Please read through and then ask any questions if I'm leaving something out. I am trying to be methodical and I like theory so I enjoying know how/if.

This question was also never answered. If the intake, carbs/jetting, and exhaust remain the same, how much of an impact does the engine solely play on combustion/fuel to air ratio? Remember this is a different engine but it's the only thing that has changed from when Yolanda was kicking ass!

[seanbarney41]

might want to read this thread
http://forums.sohc4.net/index.php?topic=31626.0
keep in mind I have ridden virtually every iteration of cb750 except for a ko, and the differences are not tremendously noticeable.  Judging by my butt dyno, the fastest 750 I have ridden was a butt ugly k2 with pods and an RC 4 into 1, the slowest was a k4 running on three cylinders due to a broken rocker arm.  Even then the difference was not as much as you would imagine.  4800 at 60 mph is not unheard of at all but I don't think it adds up with your your gearing and wheel set up.  Current main rider is a k5 with k7/8 totally thrashed/worn out motor and 18/46 sprocket combo (ugg, what a terrible combo).  I have gotten a few rides in on my new f3 with stock sprockets and it feels quicker, but not by that much.  Basically, what I am saying is you either have a serious hidden problem, or you have some kind of perception issue, or maybe a little of both.  Without some kind of real data, we cannot help you.

[Stev-o]

Chef - you need a compression check before we can help diagnose more.

Yes, I suppose an engine could be worn and not show signs of smoke.

I once had a K5 with 63K miles. Started up and ran pretty good, but "felt" low on power as you describe, was worn and had low compression.

You also need a Sean's Butt Dyno!

[cheftuskey121]

Haha butt dyno. Come check it out. The only real data I have right now is I can't get it over 90mph without going over my redline. That's SOMETHING there. I will work on getting compression numbers hopefully tonight. 

[Jerry Rxman Griffin aka MuthaF'er]

Some of us are talking compression. Yes, very important. What I'll ask you is how many miles have YOU put on this engine and is it using any oil? If so how much in how many miles?

I'm sticking with the 120 main jets being too large for a stock 750 F0 engine in lieu of answers to compression and oil useage. 110's may be sweet for it but keep in mind it was used to 105's as that what they came from Honda with, not 120's.

Buy a set of 105's and 110's. That's easier to test than buying and using a compression tester.

But then I can't seem to understand "I can't get in over 90mph without going over my redline". My F0 will do 100mph in 3rd at the top of redline. Are you talking redline in 5th at 90mph and what exact rpm here? If this IS the case then you ALSO have a clutch problem.
 
Without reading through this again, have you done correct method plug chops at mid-range AND top end?

[lucky]

This is why I do not like engine "swaps".
I like engine rebuilds much more.
Why swap a 40 year old engine anyway.

[cheftuskey121]

This is why I do not like engine "swaps".
I like engine rebuilds much more.
Why swap a 40 year old engine anyway.

I understand that and it was not my first choice but it was the only whole engine available to me and I wanted to get back on the road. I don't have experience rebuilding an engine or anyone around me that does without driving 4 hours away. I would rather drop an engine in it to get riding, and work on an engine on the bench. It was also $400, rebuilding an engine is MUCH more than that, and not everyone has the money to do so, myself included. That's why I swapped a 40 year old engine. This isn't the end of the world as far as riding goes, just slightly annoying, haha.

[cheftuskey121]

Some of us are talking compression. Yes, very important. What I'll ask you is how many miles have YOU put on this engine and is it using any oil? If so how much in how many miles?

I'm sticking with the 120 main jets being too large for a stock 750 F0 engine in lieu of answers to compression and oil useage. 110's may be sweet for it but keep in mind it was used to 105's as that what they came from Honda with, not 120's.

Buy a set of 105's and 110's. That's easier to test than buying and using a compression tester.

But then I can't seem to understand "I can't get in over 90mph without going over my redline". My F0 will do 100mph in 3rd at the top of redline. Are you talking redline in 5th at 90mph and what exact rpm here? If this IS the case then you ALSO have a clutch problem.
 
Without reading through this again, have you done correct method plug chops at mid-range AND top end?

Thanks Jerry, I have put about 300 miles on this engine so far. it does not use much oil to my knowledge. I have not needed to refill the oil. the level has dropped maybe 5mm on the dipstick. not much more than the other engine used. I will work on getting compression. my brother has an auto tester and I know its not ideal but its something haha.

I will get some 110s and 105s to try out. they are easy to change!

yes 90mph is 8k in 5th gear. I try not to exceed redline. this is WEAK. my original thought was a clutch issue, I had a separate thread about it. I mentioned the clutch does not slip traditionally like when you give it too much beans and then you lose power and your rpms shoot up...it does not do that. that regular clutch slip has happened to me on the other engine one time before I adjusted that clutch. never happened again. this feels....consistently loose? I replaced the clutch plates with cycle x plates and the stack seems to line up just fine. nothings broken and I can shift fine. this is just a new issue so it's hard to describe, please bear with me.

this F0 engine did not come with clutch fiber plates. just some metal plates and the basket. it did not contain the double plate that F1s and later seem to have. it had the same metal plates as my K1 motor did so I put my K1 clutch basket and plates in. shifted fine but had this rpm issue. so I tried cycle x clutch plates and the issue is still present. that should bring everyone up to speed

[cheftuskey121]

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!

hey thanks Cal! good insight. that was another thought of mine, the advancer! I originally tried the K1 advancer I had and this issue was present so I switched to the F0 advancer and the issue is still there. the K1 advancer was definitely looser than the F0 though. I am kinda scared to cut the springs but that is an option as well. I will double check my timing advance tonight as well and see what it's showing.

thanks for the info on the cylinders, that's the kind of stuff I like knowing. there is definitely NO smoke, and minimal if any oil usage. I haven't really used it enough to know. I have a leak on the stator cover but its because the grommet there on the inside is funky. I can peek in the crack and see the dread red rtv. that issue will be taken care of soon when I get my chrome stator cover and new gaskets. kinda waiting to take care of that at once. sorry for the side bar there. just wanted to account for the minimal oil loss I've had, I am pretty sure its from that. it drips once every 1-2 minutes while riding and a few times after I shut down but no more when the engine is off.