TOPIC:

Hi, I new to bikes(sort of). I have 1981 kz750 4cyl, it has the normal knock at low idle due to the primary chain. so I have increased the ilde to about 1400rpm. my problem is when you rev it, it is slow to return to 1400rpm(it does return but not instantly). but if you set idle lower(noizey primary chain louder) and rev it the idle returns quicker.
I have slack in cable, cables free and lubed. butterflys all returning evenly. is there heavier diapharm springs i can use?
Any help would be great.
thanks
sonny.

sorry, it has keihin cv carbs

From what I understand, those carbs work the same as the CV carb I put on my bike. If you put a stronger spring on the diaphragm, I just makes the engine run richer because the piston doesn't lift at much and it sucks more gas than normal, especially at idle. Are you sure it's the primary chain rattling and not the timing chain?

dead sure.

My KZ1000 has another cable going to the carbs. That one pulls the carbs closed. Is there a spot to hook up another cable on your carbs?

Paul Sr.

The slow return to idle is most generally associated with not having enough slack in the throttle cable. The KZ750 uses a single throttle cable and the springs on the carbs return the throttle.

If you are happy that you do have slack in the cable, next thing to check on the CVK40s is the intake manifold seal and caps on the vacuum ports. If some of these ports are still in use (hoses connected), be sure none of the hoses are cracked and that the caps are not cracked. Even a small crack will result in an air leak and the idle will not return as it should.

If you are happy with the state of your intake manifolds and vacuum lines/fittings and are sure they are not leaking, about the ONLY other place that can leak (except for a leak in the carb bodies itself where the pivot arms go through) are the pilot mixture screws. Under these there are small metal washers and under the metal washers are small orings. If the rubber orings are older and one doesn't seal, your idle will be slow to return and may be a bit erratic. Unfortunately, the only source for the little orings and metal orings is in a rebuild kit(s). The rebuild kit from a KZ440 will work if you can't find any for a 750 Keihin carb assembly. These kits include the oring, metal ring, the mix screw, a bowl gasket and float needle (and not much else).

thank you all for your input, I will check the vac caps, when i looked at the manifold boots when carbs were off they all seemed to be soft and free from cracks. will write back with any outcomes.
thanks again.