Topic: DOHC 750 help needed-Update

[Jay B]

Thought I'd run this by you guys before I bothered the 900f board. I got my brothers '79 750ltd going last winter after 20 years of sitting in a shed. Here's what I did-

Motor was lightly stuck, soaked the jugs a few days with PB Blaster and it came free easy. Doesn't smoke, uses no oil.

Carbs were crappy and corroded. eBayed a set of carbs, went through them thouroughly. Carbs don't leak, all jets and emulsion tubes perfectly clean. Tank also cleaned.

Trimmed ends of plug wires. New plugs.

Rebuilt petcock.

New Honda air filter

Bike is set up 100% stock.

Now for the issues. The bike starts and runs well at low speeds. Ripping up through the gears though, it starts misfiring at about 6k, will run up to 9k but not well. Very little top end power. When it is good and hot, the idle becomes very erratic. It will sit and idle perfectly well at 1500rpm then shoot up to 3000 or so, then eventually come back down.

I have checked for intake leaks 3 times with starting fluid, none found. I have checked the operation of the choke/ high idle mechanism and things seem to work fine, ie. high idle is not screwing with regular idle. When I went through the carbs I verified that the CV slides were free by lifting the slides manually through the bore of the carbs. Accelorator pump is working fine. Could the slides be sticking and starving it out at high rpm? I know nothing about the ignitions in these things. Not even sure which side of the motor it's on. Does it have a mechanical advancer like a Dyna S setup? If so maybe it's sticky and not advancing properly.

Any help given would sure be appreciated. I'm a SOHC guy, I really don't know much about these things.

[mystic_1]

Check your vacuum hoses for leaks, seems these things are pretty finicky about vacuum pressures.  Also check the vacuum-driven fuel shutoff mounted on the top right side of the carbs.

The ignition is a CDI system, pickup is under the left-side front engine cover.  Ignitors are under the seat, I think, or the right-side side cover.  No mechanical advancer if I remember correctly.

mystic_1

[bill440cars]

        Even thought I have a 79' CB750K(that wasn't running when I got it) and an 80' CB750K, I haven't had much to do with those particular things yet and won't be of much help. Sorry bout that. Maybe I'll pick up on something I can use later, on mine.

                                       Take care, Bill 

[Jay B]

Thanks for the replies guys. I've been searching the 900f site, and found a couple things. These do have a mechanical advancer, but dang it, it wasn't stuck. I was hoping for something so simple. I did find one vacuum leak in the shutoff system, which I just eliminated. Helped a bit. I found out these carbs are overly complicated nightmares that have to have everything perfect to work right. (their words, not mine) I found I've got bad anti-backfire diaphrams so the carbs will have to come off again to fix. The good news is the guys over there say you can just eliminate them by plugging the vacuum inlet to the diaphram, no parts required.

[crazypj]

Honda calls them air cut valves and if they leak it means you'll have after fire on deceleration. ( its lean misfire and in exhaust, backfire is through intake)
 Its more likely they are stuck.
They pass air to pilot jets, so, as long as the outer seal is good it wont matter. I don't remember if they have a tiny 'O' ring as well as the outer seal.
There should also be blanking plugs on the pilot jets, they are fed from either the primary or secondary mains (I forget which)
PJ

[Jay B]

They are on the left side of the carb body, so you can get to the left one easy. The valve wasn't stuck, but the diaphram was in bad shape, split in a couple places. Looks to me like that would be a continuous vacuum leak. Sounds like lots of guys who do performance mods especially ditch these carbs in favor of something else like cr's.

[mystic_1]

I have a buddy with an 81 DOHC who's wanting to cafe it. He said he wants to run pods but I advised against it due to the finicky CV carbs, but I have no idea which carbs to suggest to him.  So, if you find a good option, keep us posted please.

Cheers
mystic_1

[crazypj]

I have a buddy with an 81 DOHC who's wanting to cafe it. He said he wants to run pods but I advised against it due to the finicky CV carbs, but I have no idea which carbs to suggest to him.  So, if you find a good option, keep us posted please.

Cheers
mystic_1

 They are 'jettable' but you need to know how they work.
 You only change the main jet thats NOT connected to the pilot system and usually have to DROP the needles instead of the more normal raising them.
 PJ

[Jay B]

Sounds like CR's are the hot ticket over there. A couple guys mentioned Mikuni VM33's. From my limited research, pods are really tough to do, although exhaust changes are no problem. Every part of the intake needs to be right, including the seal between the airbox and the inlet rubbers. PJ's right, there don't seem to be anything intuitive in these things, for example I read a plugged low speed jet will cause a very RICH mixture down around idle because they will suck fuel from somewhere else. I think if I ever owned one of these I'd budget in money for some CRs if there's nothing else to make work, these carbs seem exasperating to me.

[crazypj]

personally I don't think its worth putting the money a good set of CR carbs will cost on a DOHC, probably going to cost more than the entire bike is worth, (even off e-bay.)
 Mikuni round slides are probably a viable alternative though or just get some Mikuni CV carbs off a Suzuki/Kawasaki/Yamaha and modify them to fit.
PJ

[tsflstb]

I've owned two bikes with the CV carbs - both did/do what you describe.  I feel your pain. 

I went through the first bike's (Kawasaki Zephyr) carbs top to bottom and never found a leak.  It was a monumental task to get the carbs off and on - probably did that 20 times.  I ended up selling it just to be rid of the damn thing.  The guy I sold it to ended up selling it after a couple of months without ever fixing it either.

I now have a BMW R80 that is doing the same thing.  I've replaced the carb diaphragms, o-rings on the air screws and have sprayed WD-40 all over the intakes to check for leaks and haven't found anything.  Last night I slathered high-temp silicone all over the intake tract junctions just to see if that helps.  It's curing now, so maybe I'll find something.

I've also heard it could be the valve clearances, which I really doubt but that will be the next check.  I don't think out of synch carbs would behave that badly, but if the valves check out and there are no air leaks I can't think of anything else to do.

[crazypj]

I've also heard it could be the valve clearances, which I really doubt but that will be the next check.  I don't think out of synch carbs would behave that badly, but if the valves check out and there are no air leaks I can't think of anything else to do.

 good call, no one else thought of that.
Its common as the shim motors get quiet when valves are too tight so it doesn't seem a problem until it get hard to start when cold but seems fine when warmed up (because the alloy head expands more tthan the steel valveand gives you some clearance)
 If they don't have a tick they are probably too tight
PJ

[Jay B]

I've heard that too, but this damned thing starts in about 1/2 crank every time. I'll be checking the clearances though as soon as I get the time.

[Jay B]

Checked the valves, some were way tight. A couple less than .002. Got tools on the way to adjust. BTW, sounds like early GSXR750 carbs are another good option for these things.

[kghost]

Binding between the CV piston and cap causes just what you have described.

I hope you put the slides back in the holes they came out of.

Make absolutely sure none of them are sticking and make sure they are sync'ed

[Jay B]

Made some progress. I pulled the carbs again and went through them, everything looked fine. I went ahead and blocked off the vacuum port for the air cut valves. Sounds cheesey but the guys on the dohc board do it often. All idle issues are gone, and high rpm issues are much improved. As for disabling the air valves, now there is just a light popping on overrun, which they are supposed to eliminate. Barely noticable, no big deal. Got a tool coming to change shims, once the valves are adjusted I'm hoping everything will be resolved.

[mystic_1]

Interesting.  To me, this is a sign that something else is wrong.  If you have to disable a system in order to get the bike to run properly, something's awry.  Obviously these things worked out of the factory, as designed.

So, this is good data, should help narrow down the real problem, but I wouldn't necessarily consider it a final solution.  Workarounds can often come back to haunt you as the original issue is still there.



So, if you had to block the air intake for the air cutoff valves, to me that implies a leak past the diaphragms which is throwing off the idle speed mixture and vacuum pressures, OR sticking cutoff valves which are not retracting properly.

mystic_1

[Jay B]

So, if you had to block the air intake for the air cutoff valves, to me that implies a leak past the diaphragms which is throwing off the idle speed mixture and vacuum pressures, OR sticking cutoff valves which are not retracting properly.

mystic_1

Exactly. But the whole circuit really isn't neccessary for good carburation. It's just there to eliminate the light popping on overrun. I guess it refines the bike more. By eliminating the vacuum source, problems with diaphrams and valve seal just go away. I usually don't do things like this, but the whole thing is extranious anyway.

[tbpmusic]

Exactly. But the whole circuit really isn't neccessary for good carburation. It's just there to eliminate the light popping on overrun. I guess it refines the bike more. By eliminating the vacuum source, problems with diaphrams and valve seal just go away. I usually don't do things like this, but the whole thing is extranious anyway.

I agree Jay - they took the old 450 carb, a blindingly simple and efficient design, and totally garbaged it up with springs and diaphragms and other crap.

Simple is good.

bill

[mystic_1]

Me and my 750K1 series carbs couldn't agree more

mystic_1

[detdrbuzzard]

i'll take my '75 carbs over the '79s anyday

[Jay B]

Thought I'd dig this back up. I talked to Jeff at Z-1 a while back, and ended up ordering a Dyna 2000 ignition kit for it. Solved the problem completely. Starts instantly, revs to the moon.