1982 CB750 DOHC Cafe racer + some issues

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1982 CB750 DOHC Cafe racer + some issues

nuwonder
Hey peeps! Gonna be longish story but hope you find time to read :) Just found this place when googlin' some data for DOHC Hondas and found this place very useful! Thought to share a recent project and ask some questions as I'm having some issues with the bike.

So the bike is (supposedly 1982) Honda CB750K and it was brought to me and my buddies' garage by friend who wanted some cosmetic work done so the bike would be more black and aggressive, cafe racer style and it had some running problems too, plugs were getting wet etc. Carbs had just been in a shop and electronics looked like shit by previous owner: wiring was done with crappy connectors, everything was dirty and corroded, fusebox was without cover and just wrapped in electrical tape and stuffed in where nobody would see it. So we checked carbs quickly, they looked clean (the shop must've washed them) and in working order and thought it must be ignition problem as the electrics were so shot.

We upgraded main electrical components: reg/rectifier, battery, coils, plug wires, plug caps, plugs, i tested ignition modules and they passed. I rewired the bike, replacing fuse box with flat blade fusebox but otherwise staying in original wiring pattern. Bypassed clutch switch by just connecting the two wires which should hook into that lever.  Everything back together electrics work nicely but there's one thing, and this must be me done something stupidly wrong when wiring but what could it be: Everything works normal when bike is in neutral but when i push gear in, i lose front light. Blinkers, rear light etc work ok, just front light goes off. What the hell? Can't find anything in the wiring that would conflict with neutral switch wire.

And then the other problem: it still has the plug wetting problem. Actually it's not firing properly on cylinders 1 and 4. Pipes get little warm, as pipes of 2,3 get very hot. Also plugs in 1,4 are like new (but wet from fuel) and 2,3 get sooty. Getting nice fat constant spark on all cylinders though and swapped ig. modules and problem persisted. So the problem might not be electronic then. I went thru carbs again, teared them apart, used that nice manual by MacGregor fella. Cleaned slow jets very thoroughly and made sure they worked, then noticed air cut on #1 carb was shot, so i bypassed them all and removed also the vacuum valve system sitting on top of the carb bank and ran fuel straight to the carbs. Now, we did install K&N pods on the carbs and I hear these can be a bit of a problem but I don't think they should present this big of a problem. After all the work I did on the carbs, I slammed then back on and... nothing, still firing properly only on 2,3. Now I've ran out of ideas. Then I thought well I guess the lesson is not to ASSUME whats wrong but to do all the tests you can and remembered we never even checked valves. So I just hooked a pressure gauge and got 120-150 PSI on cylinders 1,2,3 but 4 was 75. I know thats low as hell, but if it's valve, why cylinder 1 doesn't fire either?

Any advice where to look next on these issues? And sorry if I didn't provide enough info, ask away and I'll be happy to tell more :)

Oh, here's a shot of the bike after all the work. Can't wait to test ride her properly, it's gonna be sweet!



-Sami
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Re: 1982 CB750 DOHC Cafe racer + some issues

Re-run
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If plugs are dripping, then there is way too much fuel. First off, you should be hooking clear fuel tubing to the drain nipples. Crack the screws open and make sure the bowls are filling up but not over filling.

Second, I hate to say this, but the pods are better off ditched. These carbs are cv carbs, they rely on vacuum from the airbox to work right and no amount of fiddling with the carbs will change that. Some people, in a bid to do anything they can to keep pods, end up taping most of the filter surface of the pods to help create this vacuum. So, now they have pods with tape on them. Maybe that's cool? Still, pods will never offer better performance, and will actually lead to other issues from crosswinds and rain. May as well stick on velocity stacks and custom make covers to close the stack down for vacuum purposes. It would look better.

When there is that much fuel, things just are not going to burn right.

For #4, put a teaspoon of oil in the cylinder and check compression again. If it stays the same, it is valve related. If it goes up, then it is rings.

Still the bike looks pretty nice. I would probably try making a breadbox style filter for it. Keeps the more open look, while still providing a sort of airbox.
The ride IS the adventure. The destination is just to get gas!
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Re: 1982 CB750 DOHC Cafe racer + some issues

nuwonder
Thanks for the Input Re-Run.

Plugs are wet, but not dripping wet. It's certainly getting fuel and it doesn't seem like excessive. It also fires up with choke nicely and settles to idle.

Man, I know the pods are gonna be bitch but airbox probaby won't fit back after new electronics box under the seat (might not be visible well in the pic). Still, if pods were making things go haywire i'd expect problems in all carbs, not only in carbs 1+4. Also had talk with the owner and he said he had indeed same problems before pods, when original airbox was installed, and it also ran like it ran on my test ride (like crap, that is. doesn't pick up rpm, sluggish etc).

Gonna test cylinder pressure tomorrow with oil. Can valve clearance in this bike be such an issue that it has this kind of symptoms? Sorry for stupid questions, first time with DOHC bike for me so it's a school day almost in every way :) Also, any advice on the wiring issue?

-Sami
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Re: 1982 CB750 DOHC Cafe racer + some issues

TOOLS1
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Since the running problem is cylinders 1-4, and they are fired by the same coil, I would look at that ignition circuit for the problem. As for the headlight going out when the bike is put into gear, the headlight must be grounding through the neutral indicator wire/switch. Try grounding the headlight straight into the ground circuit (green wire in the factory harness.)
TOOLS
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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Re: 1982 CB750 DOHC Cafe racer + some issues

nuwonder
Thanks TOOLS1 for the wiring tip! Gotta check the grounds. At the moment i've grounded headlight to the green wire on the harness but maybe it's still conflicting somehow with the neutral switch wire.

When I ground spark plug to the cyl. head from either cylinder 1 or 4, i get nice constant fat spark when bike is running so I don't know, feels weird to think something was funky there. However, if I switch plug wires from coil 2.3 to go to cylinderes 1.4 and vice versa, I can't get the bike started, it just bangs from the pipe very violently.

Maz
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Re: 1982 CB750 DOHC Cafe racer + some issues

Maz
Swapping the plug wires to test the coils is a good idea  BUT make sure you swap the low tension feeds to the coils too, otherwise all plugs will be firing at the wrong time, hence the backfiring in exhaust.

Maz
'76 CB500T
'75 GL1000 'Wing
'79 CB750L
'90 FJ1200

In a little place called Bexley, Kent, UK (Just south of London!)
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Re: 1982 CB750 DOHC Cafe racer + some issues

nuwonder
'whoopsie, that explains it! :D i don't think coils are problem tho 'cos they're brand spanking new. Only thing I can think is that if I fumbled the wiring but it was so straight-forward and as I said, is giving spark , that the coils are probably fine. But we're now on double- and triple-checking territory anyways :)
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Re: 1982 CB750 DOHC Cafe racer + some issues

nuwonder
Huh, seems like it has something to do with ignition circuit like you suspected. Tested coils, they're fine, but noticed that although i had spark when bike was running, on startup i can't get anything to cylinders 1 and 4. So gotta check the circuit and pulser for any problems there. Thanks for the tips guys!

-Sami