Topic: Where'd my power go?

[Laminar]

I have a '77 CB750K7 that feels as if the power is a bit lacking. The acceleration is "okay" at best, and it tops out at a hair over 100. Based on quarter mile times, it should be roughly on par with my Nighthawk, and my butt dyno says that's definitely not the case, and I see people here posting top speeds of at least 120 regularly.

Since getting it in August, here's what I've done:
- New air filter
- Upped jets from stock 115s to 120s to account for the 4-into-2 exhaust (plugs were whitish)
- New points & condensers, timing is spot on
- New battery
- New fuse box
- Thorough carb cleaning, accelerator pump appears to work great
- New sprockets and chain

It has a muffled 4-into-2 exhaust, compression is 150+psi on all cylinders, spark is strong, valve clearances are set properly I've run some 91 ocatane and Seafoam through the bike, but it still feels...lacking. I'm not so sure that it could beat my 550 cafe.

One more symptom - when coming off of full throttle at higher speeds, as I let off the throttle, it stutters then surges. It did this with both 115 and 120 jets.

Am I missing out on something or is this par for the course?

[mlinder]

What do your plugs look like now?

Are your carbs synched?

Letting off throttle causing surging can sometimes be a lean condition, but I'd need to know more about the surge and sputtering.

Lean carbs actually make a weird noise (well, the engine does) at WOT, then as you let off the throttle, you will get a slightly richer mix, which causes the engine to surge a bit since it is getting a better AF mixture.

This kinda thing can happen too, with unsynched carbs.

[Laminar]

Thanks for the reminder - I forgot to mention that I synced the carbs and adjusted the cam chain tension, and I'm using 10w40 oil.

The carb sync was before I installed the new jets, though, so it might be due for another. I'm also considering raising the needles a tad.

The center electrode of the plug I checked is tan and the ground is whitish. I might throw some new plugs on there, too, as these were on when I got the bike.

Does it sound like I'm missing some performance? At 100mph it's doing a little over 7000 and it just won't go any higher.

[pampadori]

7000 rpm in 5th gear when you are at 100? 
you sure its running on all 4 cyl?

[mlinder]

7k in 5th would be somewhere in the vicinity of 125mph with stock gearing.

If the tach and speedo are correct, and your gearing is stock, sounds like your clutch is slipping.

[Laminar]

Sorry - gearing is NOT stock, and that's in fourth.

I installed a 520 chain/sprocket conversion a few weeks ago, and adjusted the gear ratios for lower revs on the highway. Stock '77 630 sprockets are 15/41 (2.73), and I went to 18/45 (2.50). Before doing the conversion (also before the new air filter, jets, etc.) it wouldn't top 105 on a slight downhill section.

[mlinder]

Unless my math is wrong, 18/45 should put you around 115 (assuming an 18" rear wheel), so, if that's the case, could still be slipping clutch, but, since it's just sticking at 7 grand, maybe not.

Full advance happening?

[cycleman]

Sounds like it's not getting enough gas or the valves have started floating at that RPM level.  Have you re-checked the float levels.

Also in theory changing the gearing should make it capable of going faster but that's not always the case. Some cars etc. just won't pull at highway speed or under load when the gearing gets lower numerically. It seems you where faster on the top end before the gear change.

[Don R]

Just how big an ole boy are you? sorry, I couldn't resist.

[GammaFlat]

Unless my math is wrong, 18/45 should put you around 115 (assuming an 18" rear wheel)

K7s and K8s have 17" rear wheels but I don't know how that impacts the overall height...

[Laminar]

Just how big an ole boy are you? sorry, I couldn't resist.

6'5", 193lb.

[mlinder]

Unless my math is wrong, 18/45 should put you around 115 (assuming an 18" rear wheel)

K7s and K8s have 17" rear wheels but I don't know how that impacts the overall height...

Alright, 110mph or so, then, with a 120/90 tire.

So whatever, it's pretty close, I guess.

Again, is ignition fully advancing?

Also, as I've said many times, weight really has no effect on top speed, only how quickly you reach it. I'm pretty sure the k7's and k8's are capable of better than 110mph, especially in 4th, although I hav eno idea what the power curve looks liek on them.

[Laminar]

Alright, 110mph or so, then, with a 120/90 tire.

So whatever, it's pretty close, I guess.

Again, is ignition fully advancing?

I need to check this.

Also, as I've said many times, weight really has no effect on top speed, only how quickly you reach it. I'm pretty sure the k7's and k8's are capable of better than 110mph, especially in 4th, although I hav eno idea what the power curve looks liek on them.

Yes, weight can only add rolling resistance due to tire deformation, which is pretty much negligible. But the 6'5" part is hard to overcome, there's only so far I can tuck and maintain a safe grip on the bars.

[Laminar]

Alright, here's what I did tonight:


- Adjust valve clearances (all were spot on or very close)
- Verify plug gap (Honda manual says 0.024 to 0.028, they were 0.026)
- Verify timing and advance using timing light (perfect)
- Verify accelerator pump (worked great, pump plunger gap looked good)
- Reset idle pilot screws to 1 1/2 turns out (hadn't touched these before)
- Adjusted clutch cable (full engagement was at the very end of the lever travel)
- Sync carbs (only #3 needed the tiniest adjustment to get them all in sync)

There's still a slight hesitation off-idle, as I've read is common with these PD carbs since they tend to run lean. Above 2000 RPMs, revving is great, but below 1500, there's a slight stumble before it revs up correctly. I'm wondering if some new slightly richer idle jets would help, since, from what I've read, my idle pilot screws control both air and fuel at the same time, instead of one or the other.

I didn't up the jet needles any as (I believe?) this requires pulling the carbs and I didn't quite want to go that far.

All of this was in anticipation of a trip to Kansas City on Friday. We'll see how it goes.

[750Caferider]

If you get in a jam in KC let me know you can use my garage... I've got plenty of tools and access to more with a phone call.
Oh and a buddy of mine back in the day did the same thing with his sprockets to save gas on a cross country trip and just about destroyed
the damn motor.  Said old man Honda had it right the first time.  PM me if ya want my #

Cheers,

Anders

[Laminar]

If you get in a jam in KC let me know you can use my garage... I've got plenty of tools and access to more with a phone call.
Oh and a buddy of mine back in the day did the same thing with his sprockets to save gas on a cross country trip and just about destroyed
the damn motor.  Said old man Honda had it right the first time.  PM me if ya want my #

Cheers,

Anders

Out of curiosity, how was it detrimental to the motor? At 70mph it's still over 4000 rpm and if revs ever get below that it's easy to kick it down a gear.

[Duke McDukiedook]

Yeah, I don't see how upping your gearing for highway running could ruin your motor.

Someone please elaborate on this if I am wrong.

[Jerry Rxman Griffin aka MuthaF'er]

Perhaps gearing, weight, and jetting or a combination. I don't think 193 is the factor though. Now the 6'5" can create quite the wind resistance  but I'm sure you're tucking down. Even though you have good compression you may not be able to pull the 45 rear in top gear with your current jetting. I would expect more speed in 4th unless it's perhaps lean. If lean I don't think it would pull the r's in 4th or 5th. Try 1 step larger on the mains and see what you get and go from there. Easiest potential solution. You could actually end up with worse mileage if it's not jetted properly. 

[750Caferider]

He drove it cross country from eastern Mo to Cali and back  My guess is he had the problems in the mountains Maybe the exhaust valves were tight? dunno.  He didn't elaborate on what he did to fix it, I just remember him saying to keep the gears stock.  As he was running next to a stocker the whole way that didn't have a lick of trouble.

Cheers,

Anders

[Laminar]

I made it there and back with no problems! It was the darndest thing though...the trip down to KC was awful. It was cold (especially for the first 100 miles) and the bike seemed to be having trouble keeping up to speed. I had to give what I felt like was an awful lot of throttle just to maintain 75mph. I figured it was just geared too low. Gas milage reflected this - my first stop was after 100 miles and it took 3.6 gallons to refill - 27.8mpg. The second stop for gas was 104 miles later at my destination, and it took 3.5 gallons - 29.7mpg.

The way back was a completely different story though. I don't know if I had a nice tailwind, if something in the bike just "clicked" into place, if the Shell 89 octane had something magical in it, if there was a significant change in elevation, or what, but it took considerable effort to keep the bike BELOW 80. I typically found myself doing 85 without realizing it. Cracking the throttle open at 80 in fifth would push my head back. At one point I opened the throttle to pass a couple cars and didn't let off. It climbed right past 110 when I remembered that I really like having a driver's license so I let off.

I'm not sure what did it, but it seem to be running great now. If the plugs still look whitish, I'll try some 125 mains or maybe raising the needles a bit to see what that does. But that might have to wait until next season.

[Laminar]

Oh, and I forgot the best part about the trip back home. For the first leg, I stopped at the same halfway point that I did on the way down for gas - 105 miles, 2.3 gallons - 45.6 mpg. That was mostly at 80-85mph and with plenty of cracking the throttle open for fun.

[Jerry Rxman Griffin aka MuthaF'er]

have you run any fuel cleaner through it? Shell gas is said to contain a high amount of detergents with the premium containing the most.

[Laminar]

I put 1/3 can of Seafoam in the tank I used for the first leg of the trip.

[Laminar]

Well last night I tried again. I had upgraded to 125 jets a couple of months back to see if that would reduce the surging when coming off throttle, but it didn't.

So last night, I went back to 120 jets, and I disassembled the carbs to the point that I could get at the needles. Turns out the clip was on the highest setting, making it as lean as possible. I moved them back to the center slot. Then I set the valves, cam chain tension, lubed all of the cables, and performed a good sync. The bike is very very different now. It is very obviously much more powerful now, which is great.

[mlinder]

Holy thread resurrection Batman!

Nice to hear you got it running properly, though.

[Laminar]

Holy thread resurrection Batman!

Nice to hear you got it running properly, though.

Yeah, it hibernated from November to April and I'm not home very much so projects tend to take a lot longer to complete, especially with four project bikes.

[Really?]

You said you are running 10w40 oil.  I hope that is not the energy conserving type.  It will make a clutch slip.

[Laminar]

Hey, I'm new here. What type of oil is best? And I want my 750 to compete with modern literbikes, I have $200 to spend. What should I do?

[mlinder]

Virgin Olive Oil, NOS.

[Laminar]

Virgin Olive Oil, NOS.

Hmm, installing the whole system looks complicated. If I just slap on the stickers, will that work?

[mlinder]

Virgin Olive Oil, NOS.

Hmm, installing the whole system looks complicated. If I just slap on the stickers, will that work?

What do you mean, 'system'? The installation is the stickers.

[Really?]

Hey, I'm new here. What type of oil is best? And I want my 750 to compete with modern literbikes, I have $200 to spend. What should I do?

I guess you got that the wet spot between your legs you were looking for there.