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Revs wont come down??? 03 Oct 2012 13:28 #552224

  • krifartida
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I am working on an 81 750 H2 LTD.

I have finished doing the breaks and electrical, and now have to tackle a problem that was present since i drove the bike home from previous owner.

The bike starts and idles ok.

When i throttle it the revs go up nice and even, and then they stay up. If i rev to 5000 they stay at 5000 and do not come back down. If i put it in gear and let the clutch out, it will come back down, and stay there, untill i rev it again. I can drive it around the block, but it revs up fairly fast between gears, so i cant really drive it.

I just finished cleaning the carbs completely for the second time.
Everything inside looks clean, in place, all rubbers are good.
I cleaned all 4 diaphragms, springs look good - i am at a loss.

Cable is brand new, and has slack as needed, and the action on the cable is good.

Any ideas???

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Revs wont come down??? 03 Oct 2012 14:14 #552228

  • mark1122
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Try starting it , then spray wd40 around the carb boots and vac lines. if the rpm changes then u have a vacumm leak. also make sure the cables r lubed and not binding.
hope this helps
76 KZ, frame gusset work,1200CC.Ported by Larry Cavanaugh, 1.5mm.over intakes, Carron Pipe, ZRX12 rear end, and seat,96zx9 front end.
01 CBR600F4i Track bike.
Cobourg, Ont. Can.

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~ (k) / (z)

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Revs wont come down??? 03 Oct 2012 14:31 #552232

  • Del_Herring
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Symptom is typical of a lean condition. Frequently caused by the leaking boots like Mark described. Do that spray check and see what happens. Are you using the original airbox and filter setup?
1983 KZ750-N2 Spectre

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Revs wont come down??? 03 Oct 2012 15:07 #552234

  • krifartida
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Yes - i am using the original airbox and filter.
The boots between airbox and carbs are in poor shape and i am using hose clamps to keep them in place, and they do sit nicely now (not so when i got the bike).

The boots between the carbs and motor are in much better shape, though it appears that at one point in time someone used a smear type silicone gasket maker all around where the boots meet the engine.

I did a spray test and the revs do go up when i sprayed some carb cleaner around the area.

Is it possible the air filter is clogged? at last inspection it seemed to be fine, i guess the way to find out is to take it out and run the motor.

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Revs wont come down??? 03 Oct 2012 15:42 #552240

  • Del_Herring
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Air filter being clogged would tend to cause a rich condition (i.e. less air getting through, not a lean one).

If spraying around the boots caused your revs to change, that's 99.999% surely your problem, those should be sealed, spraying there and seeing revs change means you have a leak. People always try putting that silicone stuff on those when they start leaking, and it never seems to hold. If air's getting in there, what happens is your engine pulls the same amount of air, but since it isn't all going through the carbs, they lose some vacuum, and don't get enough fuel in, thus causing the lean condition (i.e. too much air, not enough fuel).

I asked about the filter, because a lot of times guys switch to a high flow air filter, or just take the filter out, and have this kind of problem. I'd check out Z1 enterprises for some new carb holders (the boots between the engine and carbs), and maybe some new boots for between the carbs and airbox too if those are old and crusty, they should be nice and soft.
1983 KZ750-N2 Spectre

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Revs wont come down??? 03 Oct 2012 16:38 #552247

  • krifartida
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Z1 have the part for about 60$, and the airbox boots for 40$ so i think i will try to seal it with silicone just to see if it is actually solving anything, before i commit to 100$ on this super fixer upper. I need other stuff from them anyways so i might just get the stuff on the way. In any case i think i will get that stuff, this bike ride amazing when it take it around... :)

I will post as soon as i have more time to play with this. Hopefully this will be soon (as i have no work :( .... )

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Revs wont come down??? 03 Oct 2012 17:35 #552254

  • krifartida
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While doing my new coat of smear, i notices that at least one of the carb holders has a sheet metal\wood type screw holding it in place, i suspect the reason these are so old, along with the previous owner being frugal is that stuff started breaking on this bike and it was easier to patch up then to really fix.... i will find out soon enough.
Compared to my VF700 taking the carbs off and on this bike is a 5 minute breeze compared to the full day pain the carbs on the VF700 are...

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Revs wont come down??? 03 Oct 2012 20:11 #552291

  • DesertKZ
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Do yourself a favor and while replacing those boots, switch over to allen head bolts so you can torque them on better. An easy $5 upgrade.
1979 KZ1000 Shaft
1979 CX500C
1975 GT750 Smoker

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Revs wont come down??? 03 Oct 2012 22:43 #552308

  • hx35w
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I had the same problem with my ELR. The float level was set correctly according to the float gauge. I still had a lean condition causing a uncontrollable high idle. My solution was to raise the fuel level in the bowls slightly higher. The gauge is just a good starting point.

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Revs wont come down??? 04 Oct 2012 09:54 #552387

  • havakaw
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The carb holders are a critical part for the running of this bike.They need to be sealed good against the head,as well as the carb.New units and the correct hardware and clamps will really help out.One other spot to check is the advance weights in the pick-up coil area. A lot of times they get rust build up and will stay in a timing advanced mode.The advancer should be easy to give a twist to the right and then snap back quickly.If if doesnt,take it off and clean/lube it and then recheck the timing with a light.Fairly easy job.
'80 KZ 750 LTD-H
'82 KZ 1100 A-2 Shafty (ex-bagger)

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Revs wont come down??? 04 Oct 2012 11:15 #552411

  • krifartida
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I will see if i can find this in the shop manual - otherwise can you point me in the direction of the advance weights location?

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Revs wont come down??? 04 Oct 2012 12:14 #552416

  • DiamondSkyBlue1000
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The advancer and pickup coils are behind a round cover on the right side of the engine. The part that protrudes out the most.

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