Topic: 1977 CB750 K7 Resto-mod, Knoxville

[Desert-SOHC]

That's great! It's amazing how much better they run when the carbs are sync'd

[enwri]

That's great! It's amazing how much better they run when the carbs are sync'd

And how it should sound,  good to have a baseline on how it should run.
It might not have been quite right since you've owned it, and unless you have ridden a new one, you would never know otherwise.

P.S.....Always wear a helmet....

[AintNoEasyWay]

That's great! It's amazing how much better they run when the carbs are sync'd

And how it should sound,  good to have a baseline on how it should run.
It might not have been quite right since you've owned it, and unless you have ridden a new one, you would never know otherwise.

P.S.....Always wear a helmet....

Enwri, absolutely right. I had nothing to compare it to before & didn't realize that it was off. Makes a big difference. I've always got my helmet, gloves, & jacket with me. Thanks!

[Stev-o]

When properly tuned, it should start right up as long as you follow the correct procedure. 
My bike likes full choke when doing a cold start, if you have PD carbs, give it two twists of the wrist to shoot some gas in the carb. 

[AintNoEasyWay]

When properly tuned, it should start right up as long as you follow the correct procedure. 
My bike likes full choke when doing a cold start, if you have PD carbs, give it two twists of the wrist to shoot some gas in the carb.

Thanks, Stev-o. I assume it doesn't matter that the choke is out when I twist the throttle since the valves have those small holes in them for the accel pump nozzles to shoot through? Just checking.

[AintNoEasyWay]

What's up guys, haven't updated in a while! I've been busy riding as much as possible before this Tennessee winter hits.

Everything with the bike is good except I seem to be experiencing some loss in power in the past couple weeks. I'm not sure if this has to do with the weather or is carb related. I have to be up in 5th gear by the time I hit 50 mph, which seems like a lower speed than when i first got the carbs tuned up & had it screaming down the interstate.

Also had a mishap with the bike early last week. I parked my bike & went into a coffee shop to get some work done. When I came back out to the bike I had a note on my bike from a girl saying that she had accidentally backed into it & knocked it over. It fell over on its right side & my right mirror scuffed & bent back. Other than that there was no major damage. Fortunately I guess a guy saw it happen & helped her pick it back up. At least she left a note letting me know...

I just ordered some Hagon Type A shocks from Dave Quinn (very helpful guy!), so I'll be adding those to the bike when I rebuild the front forks over winter. Hope you guys are doing well!

[enwri]

 I have to be up in 5th gear by the time I hit 50 mph, which seems like a lower speed than when i first got the carbs tuned up & had it screaming down the interstate.

Does that mean the motor is revving out but you haven't gained speed, or that the engine won't rev fast enough to go faster than 50?

If it's over revving but not accelerating it will be the clutch slipping, either needs adjusting, or it's worn, or friction modifiers in oil.
You can get away with putting spacers under the clutch springs to try to get a bit more "squeeze" into it, for a little while, until you run out of adjustment, but replacing the plates is the way to go.

Mine got knocked over three days after I re-registered it, broke the hi beam filament in the headlight bulb and pushed the handlebars about 2 inches sideways through the clamps, nothing else. It's a tough old thing. But it does wear case savers, I put my feet up on them when I'm going past the nice new Harley dealer on the main street.
Lots of new harleys in Cairns, not too many CBs.

[Desert-SOHC]

They do have some weird quirks from time to time. I have noticed that when I'm getting really close to having to go to reserve I cant get Mayble to accelerate past 75mph. If I flip it on reserve there are no issues and I can leave it in "on" as long as I just run around surface streets. It may be time to replace or rebuild the petcock.

[AintNoEasyWay]

They do have some weird quirks from time to time. I have noticed that when I'm getting really close to having to go to reserve I cant get Mayble to accelerate past 75mph. If I flip it on reserve there are no issues and I can leave it in "on" as long as I just run around surface streets. It may be time to replace or rebuild the petcock.

Hey Desert, I was searching around the forums for power loss earlier this morning & faulty petcocks definitely seemed to be culprit in some situations.

 I have to be up in 5th gear by the time I hit 50 mph, which seems like a lower speed than when i first got the carbs tuned up & had it screaming down the interstate.

Does that mean the motor is revving out but you haven't gained speed, or that the engine won't rev fast enough to go faster than 50?

I can get it up to about 70-75, but that's it. It's as if the bike can't do more than that. Last night on the interstate I tried to get to about 75 & the tach gauge was wobbling back & forth between about 4.5-5k rpms. Could the possibly be a clutch issue?

[madmtnmotors]

Sounds like it's either starving for fuel or low compression. You know these later models have a filter in the tank, above the petcock. Have you checked this filter?


I see in your first post you "de-rusted" the tank. Was there a filter in the tank before you started? Did you remove it? After they have been in there a while they almost start to become "one with the tank" and require some deliberate effort to remove. You can shine a flashlight through the filler opening at the petcock and see if the in-tank filter is in place. Don't use a lighter, or any other type of flame!

[AintNoEasyWay]

Sounds like it's either starving for fuel or low compression. You know these later models have a filter in the tank, above the petcock. Have you checked this filter?


I see in your first post you "de-rusted" the tank. Was there a filter in the tank before you started? Did you remove it? After they have been in there a while they almost start to become "one with the tank" and require some deliberate effort to remove. You can shine a flashlight through the filler opening at the petcock and see if the in-tank filter is in place. Don't use a lighter, or any other type of flame!

I replaced the old filter inside the tank when I de-rusted it. Used the pen filter from Honda. Maybe it is compression then. I don't have a compression tester but maybe someone I know does. I'll have to ask around.

[madmtnmotors]

I replaced the old filter inside the tank when I de-rusted it. Used the pen filter from Honda.

I remember references to an in-line fuel filter in previous posts. So you're using the stock filter in the tank as well as an in-line filter? You can check the flow rate by routing a longer fuel line to a container. I suspected fuel flow issues at one time and measured the flow on two separate K8's side by side. The poorly running bike would only flow half the fuel compared to the properly running bike, but I cannot remember what those volume rates were. I'm thinking one quart in 3-1/2 minutes vs one quart in 7 minutes. I had removed the in-tank filter which in turn had allowed debris from the tank to partially obstruct the petcock. Since you are using the in-tank filter this would not be your issue, but since you are using two filters I'm wondering if the second filter is obstructing flow. This is easy enough to test with a clean container and a watch. Check the flow under three conditions:

>Flow rate through both filters
>Flow rate using just the stock filter in the tank
>If these two values are similar, then loosen the fuel cap and check again with just the stock filter in the tank

This will show you whether the in-line filter is too restrictive, or if your fuel cap vent is obstructed...

and it's free!

[Desert-SOHC]

I remember in the gas crunch in 79, with every station pumping there tanks dry. I had to remove my petcock 3 or 4 times to clean the filter and all the crap out of the petcock itself. Might be time for me to check mine out, I never really removed the petcock....just made sure the tank was clean.

[enwri]

I can get it up to about 70-75, but that's it. It's as if the bike can't do more than that. Last night on the interstate I tried to get to about 75 & the tach gauge was wobbling back & forth between about 4.5-5k rpms. Could the possibly be a clutch issue?

No, doesn't sound like it.

[madmtnmotors]

I can get it up to about 70-75, but that's it. It's as if the bike can't do more than that. Last night on the interstate I tried to get to about 75 & the tach gauge was wobbling back & forth between about 4.5-5k rpms. Could the possibly be a clutch issue?

No, doesn't sound like it.

I concur. A slipping clutch would let the engine rev much higher without producing any more forward speed. I can only come up with three potential culprits:

>Fuel restriction
>Something in the ignition system failing under load (points, condensers, coils, plugs)
>Low compression. This can be worn rings, worn valves, or valves adjusted too tight (not closing)


I recommend a compression test before buying any parts.

[70CB750]

Do you get to WOT at 75, or is therea sudden lose of power at that level?

I had the later and it was during tuning jets/clip position, when the pilot jet no longer supplies enough fuel.

[RAFster122s]

Could you pull the spark plug caps and check their ohm reading out of the circuit?  They could cause some issues at higher revs.
The fuel bowls running out of sufficient flow could do it as well.  A cheap Chinese petcock could just not pass enough fuel due to bad design/quality control.
Time the tank to fill a quart container from the petcock after checking the caps.

Double check the points and advancer is fully advancing.  Also that you are not getting some arcing at high rpms on paths that could sort the power to thr coils at around the points.
Condensor might be weak. What brand and how new?
David

[Restoration Fan]

IIRC, he's running a Pamco electronic ignition.

[Desert-SOHC]

When below 2/3 of a tank my petcock doesn't flow enough to let me rev over 4500rpm

[AintNoEasyWay]

Could you pull the spark plug caps and check their ohm reading out of the circuit?  They could cause some issues at higher revs.
The fuel bowls running out of sufficient flow could do it as well.  A cheap Chinese petcock could just not pass enough fuel due to bad design/quality control.
Time the tank to fill a quart container from the petcock after checking the caps.

Double check the points and advancer is fully advancing.  Also that you are not getting some arcing at high rpms on paths that could sort the power to thr coils at around the points.
Condensor might be weak. What brand and how new?
David

Didn't get time to look at the bike yet, but the petcock is original. I haven't rebuilt it or anything. I'll see if I can check the plug caps tomorrow evening, along with checking fuel flow. I was also able to borrow a compression tester, so I'll check that too. Ron is right; I've got a PAMCO ignition, so it's not points related.

I should clarify that I'm not having a "loss of power" in the sense that the bike is dying on me. Rather, there seems to be a decrease in power from how the bike ran a couple weeks back when I first had the carbs tuned.

When below 2/3 of a tank my petcock doesn't flow enough to let me rev over 4500rpm

Desert, that's interesting. Still, I try to keep my tank topped off so it rarely has less than 1/2 tank in it.

Do you get to WOT at 75, or is therea sudden lose of power at that level?

I had the latter and it was during tuning jets/clip position, when the pilot jet no longer supplies enough fuel.

As mentioned above, it's not like the bike gives out, it's just as if it's saying "Hey, I can't do much more than this." Whereas a few weeks back, I barely had to twist the throttle in 5th & I was good to go around 75 mph.

I concur. A slipping clutch would let the engine rev much higher without producing any more forward speed. I can only come up with three potential culprits:

>Fuel restriction
>Something in the ignition system failing under load (points, condensers, coils, plugs)
>Low compression. This can be worn rings, worn valves, or valves adjusted too tight (not closing)

I recommend a compression test before buying any parts.

Would be amazed if the rings wore already. Although I didn't buy original Honda rings (maybe that was a mistake). I have new exhaust valves, but the bike shop I took my head to said the intake valves were fine. Other than that, it's possible I've over-adjusted valves; that's another thing I'll the time to check out.

Thanks everybody. I'll report back soon.

[AintNoEasyWay]

I'm back. The bike has been winterized in the garage for the past couple months. Safe & sound.

Yesterday I began the process of rebuilding my front forks, installing new front wheel all balls bearings & steering stem bearings. I'm also sending my front disk to Godffery in exchange for a drilled one. Once all this is done, I'll basically have the bike as functionally complete as I'd like it. From there on, I'll just do minor aesthetic touch ups -- repaint emblems, maybe change tank paint, etc.

[AintNoEasyWay]

Anyone know how to remove the bottom forks from the fork tubes? I've unscrewed the bottom allen bolt that goes into the bottom of the fork & also removed the clips that go in the top over the fork seals. Still can't get them to come apart.

[RAFster122s]

Sometimes sediment settles if they were not serviced frequently/to recommended intervals and any wear of internals can cause an aluminum paste to form that gums up things and make it hard to come apart. Sometimes after draining you have to use them like a slide hammer to break it free with one end clamped in a padded bench vice. Some kerosene or good penetrant can help break this bond if you let them sit in that for a few days, putting bolt back in the bottom or a rubber plug so it holds thefluid.

[AintNoEasyWay]

Sometimes sediment settles if they were not serviced frequently/to recommended intervals and any wear of internals can cause an aluminum paste to form that gums up things and make it hard to come apart. Sometimes after draining you have to use them like a slide hammer to break it free with one end clamped in a padded bench vice. Some kerosene or good penetrant can help break this bond if you let them sit in that for a few days, putting bolt back in the bottom or a rubber plug so it holds thefluid.

I could be wrong, but the forks have probably never been serviced, RAF. There's some sediment over the seals that I'll try to remove. Could you explain in more detail what you mean by using "them" as a slide hammer. Not sure I understand what you mean. Thanks!

[Desert-SOHC]

Clamp the tube in a vice and work the lowers like a slide hammer. compress and yank, compress and yank...it will eventually come apart.  Maybe shoot some penetrant in the bolt hole on the bottom in between workouts.