Topic: CB750K4. Light frame mod and performance upgrades.

[Johnie]

Here is a pic showing the slide down and the threads on top. Might give you a little idea. You said you had a lot of threads showing which tells me the slides are to high and causing your issue.

[D-Ral]

Here is a pic showing the slide down and the threads on top. Might give you a little idea. You said you had a lot of threads showing which tells me the slides are to high and causing your issue.

Perfect. Thanks Johnie, I'll double check when I get home from work.

[Davez134]

I dont think the slides can go in backwards on these carbs. You shouldn't have more than a couple threads showing above that adjustment screw. If you use a drill bit for bench sync make sure it is a very small one. The few things I can think of that would cause idle to do that are: a leak around carb boots, idle adjustment screw turned in to far( a very small turn can cause idle to shoot up) idle air mixture screws not set equally/ properly, not drawing same vacuum pressure (seems pretty likely after reading your post. Start over trying to get closest possible bench synch possible. Also make sure ignition timing is correct after getting idle close. I'm far from an expert, but had all these problems while learning these carbs in and out.  Bike is looking good already too!

[iron_worker]

I have a K4 and the slides are keyed to the carb bodies. You couldn't put them in backwards ... you could somehow put them in upside down but I don't think you could assemble the top of the carb like that though.

IW

[D-Ral]

I dont think the slides can go in backwards on these carbs. You shouldn't have more than a couple threads showing above that adjustment screw. If you use a drill bit for bench sync make sure it is a very small one. The few things I can think of that would cause idle to do that are: a leak around carb boots, idle adjustment screw turned in to far( a very small turn can cause idle to shoot up) idle air mixture screws not set equally/ properly, not drawing same vacuum pressure (seems pretty likely after reading your post. Start over trying to get closest possible bench synch possible. Also make sure ignition timing is correct after getting idle close. I'm far from an expert, but had all these problems while learning these carbs in and out.  Bike is looking good already too!

Do you suppose it'd be worth my while to change the oil, do valves and timing, points, etc before I rip the carbs/box off again? I can get the bike to idle steadily a little under 2000 with the choke fully closed.

[D-Ral]

Its Father's Day, so I don have much time to dick with the bike today, but I did manage to start it again and verify that it is still doing the same thing. And, there are probably too many threads showing on the slides:

[Johnie]

Yah, it does not take much to get the idle high with the slide adjusted wrong. Look at the difference in threads from my pic to yours. And you are correct...the sync is usually the last thing you should do...very last. You should change the points, condensors, plugs, and check the timing and valves before you do the sync. However, since you know the slides are to high I would at least get it close so she does not rev on you. Then once you get that idle back down you can work on the rest, but sync again when done with the points, timing, valves, etc. Who knows, maybe once you get the idle back down you will be happy with how she runs. I hope you have a friend with a vac gauge for the sync. They are real handy...good luck with it.

[Davez134]

I was going to reply to ya but johnie pretty much said it all!

[D-Ral]

I have a vacuum sync! So what you are saying, is re bench sync so that it will idle without choke, then everything else, and then vacuum sync, correct?

[D-Ral]

Yah, it does not take much to get the idle high with the slide adjusted wrong. Look at the difference in threads from my pic to yours. And you are correct...the sync is usually the last thing you should do...very last. You should change the points, condensors, plugs, and check the timing and valves before you do the sync. However, since you know the slides are to high I would at least get it close so she does not rev on you. Then once you get that idle back down you can work on the rest, but sync again when done with the points, timing, valves, etc. Who knows, maybe once you get the idle back down you will be happy with how she runs. I hope you have a friend with a vac gauge for the sync. They are real handy...good luck with it.

And are you saying I should replace my points?

[Davez134]

I have a vacuum sync! So what you are saying, is re bench sync so that it will idle without choke, then everything else, and then vacuum sync, correct?

Correct. And you don't have to replace points/ condensers if yours are ok. You want to check the contact point surface, make sure it is smooth and gap is correct. The shop manual you can download from this site explains how to test very well. On flip side replacing that whole points plate as a unit isn't too expensive, but if surfaces are smooth and everything checks out just adjust and go.

[D-Ral]

I have a vacuum sync! So what you are saying, is re bench sync so that it will idle without choke, then everything else, and then vacuum sync, correct?

Correct. And you don't have to replace points/ condensers if yours are ok. You want to check the contact point surface, make sure it is smooth and gap is correct. The shop manual you can download from this site explains how to test very well. On flip side replacing that whole points plate as a unit isn't too expensive, but if surfaces are smooth and everything checks out just adjust and go.

Thanks! Yeah, I've got every manual under the sun, I just haven't reached the points, valves, etc., parts yet.

[D-Ral]

I pulled my airbox and carbs. I'm having a difficult time bench syncing my carbs so that my slides look like johnie's. When I do, I am unable to show any threads on top of the locknut. I'm wondering if I have my carb rack set up properly.









As you can see in the photos, I have #3 a paper clips width from the bottom of the throat, the lock nut isn't revealing any threads on the rod, and my idle screw is not even touching its mate. I have no way to lessen my idle. If I put one full turn on the idle adjuster, I'll have even less room for the lock nuts.

[Johnie]

You should have some threads showing, but keep in mind you are bench syncing mainly to get all slides even and not so high she will rev. You will be using that idle screw mainly to get the idle RPM you want. So set the slides lower than they were and go from there. I have used a small paper clip before too. The rack looks to be set up correct from what I can see by the pics...

[disco]

Hi D-ral,

I'm pretty sure that you have got the slides for the 1&2 carbs installed in the 3&4 carb bodies and vice versa. The cutaway on the slide should be on the inlet side of the carb.

[D-Ral]

Ok, I think I have it better now! I think I have it figured out. Thanks guys. I have some rod sticking up out of the locknut, some idle screw is engaged so that I can adjust less or more, and my slides are barely open. Not really sure how I did it, just kept screwing with them. Of course, I had enough time to get everything back on the bike but it is too late to start it. We've got some young ones next door who've already gone to sleep, so my wife warned me not to wake them. I better obey, for while I was messing with my bike, she was painting.

Ordered new condensers and points today. And a new points cover. And a timing light. It never ends. Still need to bleed the front brake and then the rest of the tune up stuff. Checked some of my wiring tonight, as well, and I still have some work to do. Gauge backlighting and rear brake switch work, but that's about it. No headlight.

Excited to try starting it tomorrow.

[D-Ral]

Started the bike and it ran much better. Was able to idle around 1500. Played with the throttle a little bit and it seemed to stick or something. It wasn't returning very well. I sprayed more wd40 around the boots and got no reaction from the engine, so I assume there is no vacuum leak.

  I let it idle for a few minutes and then changed the oil. Wow, what a difference. No smoke from the exhaust anymore. Plus the throttle felt very positive. Woohoo! Was idling nicely around 1,100 - 1,200. There was a small popping noise every once in while, but that's it. Not loud at all.

I'm pretty busy this week, but hopefully I'll be able to get the valves, points and timing adjusted before the weekend!

[Johnie]

Good on you...glad you got it sorted. Now enjoy...it only gets better.

[Davez134]

Very nice! I know that feeling, still fresh in my mind! Sounds like it's going good.

[D-Ral]

Ok, moving forward!

The PO had three 14A fuses installed. I cleaned up the contacts and installed proper fuses and the now the headlight and tail light both work! I bled the front brake this morning, too. Managed to get fluid everywhere, but it worked and feels solid.

Now my issue is the horn. I unplugged the horn and and tested the black and light green leads with a multimeter. With the horn button pressed, it fluctuates between .5 and .7 volts. The battery reads 12 V. All of the light green wires are good, and i've polished up all of the contacts in the switch and my soldering job inside the handlebar switch is good. The green frame ground and negative ground at the battery have both been cleaned up as well. Anyone else have any ideas?

I don't understand how pressing the horn button would complete a circuit, anyway. How does the ground current get to the little metal contact within the button? I'm soooooo close to being done, it seems a shame to be held back by my inability to wire a horn!

[D-Ral]

Aaaaand my oil pressure and neutral switches aren't working either. And I seem to have no clutch switch. The previous owner had the green ground wire and green/red clutch switch wire bound together in the headlight bucket. Is this proper?

[Johnie]

Take the horn off and touch the leads to the battery to be sure it works?
Not sure what you mean by clutch switch?
Neutral switch is located under the motor on left side. Has one lead to it. Check to be sure it has not been cut or is loose. Did you check the bulb?
Oil pressure switch will occasionally get oil in the small hole which will plug it up.

[D-Ral]

My wiring harness shows a clutch switch having two leads attached: green with red strip and green ground. The switch is tripped when the clutch is pulled and tells the safety module it's ok to start the bike.

I have a new speedo with dummy lights, so I don't think the bulb for the neutral or oil light would be dead.

Either way, it's the horn that is keeping me from having the bike pass a safety inspection. I've tested the leads that connect to the horn, and its only .5 volts. I don't think that's enough to sound a working horn.

[D-Ral]

When I said wiring harness, I meant diagram

[D-Ral]

Ill try jumping the horn to the battery when I get home from work.