76 kz750 have idle questions

so i just bought a 76 kz750 for a coupe hundred bucks outta the bargain corner, everything is fine except for some idle issues,

if i set the idle screw so that the bike idles nicely and either crack the throttle or ride it when i come up to a stop sign the idle will not drop it will sit at about 4k. and IF it comes down it is very sloooow

and if i set the screw so that the idle drops properly it will not stay running, all cables are lubed up thats not the problem carbs have been off and cleaned, no debris all looks good.

this bike came with what appears to be an aftermarket 2 into 1 exhaust that has been pretty well boared out there isn't anything left inside the muffler.

would this type of exhaust modification cause my idle dilema?

any helpful hints welcome
thanks
ratrider ben

ratridekz750 wrote:

...if i set the idle screw so that the bike idles nicely and either crack the throttle or ride it when i come up to a stop sign the idle will not drop it will sit at about 4k. and IF it comes down it is very sloooow

and if i set the screw so that the idle drops properly it will not stay running, all cables are lubed up thats not the problem carbs have been off and cleaned, no debris all looks good...

First, welcome!

The exhaust might cause a performance problem, but I don't think an idle problem.
When you say, "idles nicely" - do you mean about 1000-1200 rpm or so?
You also say the carbs have been cleaned, but have they been balanced? Has timing been set? Have the float levels been validated?

Perhaps a leak of outside air being drawn into the intake manifold. Could be a crack in the manifold. Or old hard deteriorated rubber vacuum plug caps. Or loose clamp holding carb onto manifold.

May test with engine running (transmission in neutral) by spraying propane gas (unlit) or WD40 or starting fluid all around the suspect areas and observing any change in idle speed.

Memo -- this applies to the engine side of the carbs. Any leaks in the intake boots from the air box are not critical for this test. Should avoid getting spray into the carb intakes.

the clamps and whatnot are definatly not loose but i will try the starter fluid leak test and see how that goes, thanks

I had the same problem with a 79 KZ750 twin, setting the fuel levels in the float bowls cured it.

You may have greased the cables but make sure there is some slack in them. Also, the orings on the mixture screws may need to be changed.