Rough Idle 1979 KZ750B4

Still having issues with weak idle at 1k RPM.

Went up to 50 on the pilots, stock airbox, stock carbs, 2into1 exhaust with muffler.

Replaced the points and condenser and that made a big difference on strength of spark.

Mix screws are at about 2 turns out.

Bike starts up great on the chock and warms up nice. Idles ok when set to about 1.5K RPM but idle get's weak and rough going down to 1K RPM.

Do some older bikes just need to idle a bit higher than what the book says? Does this cause overheating issues?

Checked the valves last night and everything was in spec.

Thinking that you just need to have the idle screw higher until the thing really warms up and then back it down over the course of riding.

Looking for other ideas.

It should idle fine at 1000 rpm. Carb holder leaks can cause very rough idle at low idle rpm. Here's how to test the carb holders:

Make sure the vacuum caps are tight on the carb holder nipples.

Warm the engine up to normal operating temperature.

Temporarily set the idle as low as possible without stalling the engine. It should be down below 1000 rpm. The lower the better because when it is set low the carb slides are closed (or nearly closed) and this raises the vacuum level in the carb holders. The combination of the increased carb holder vacuum level and the very low idle amplifies the effect of the leak test.

Once the idle is set low, spray carb cleaner around each of the carb holders where they bolt to the cylinder head and where the carbs attach to them. If there are leaks, the vacuum in the carb holders will suck the carb cleaner in and this will affect the idle. It may cause the engine to stall. If the leaks are at the carb holder / cylinder head mating surface you need to replace the holders. If the leak is where the carbs attach to the holders you may be able to tighten the clamps enough to seal the leak. Some folks use starter fluid or other substances for the test, but carb cleaner works best as it doesn’t evaporate too quickly like ether and leaves no residue on the engine like WD40.

If you do need new holders just bite the bullet and buy them. Trying to seal things up with some kind of sealant or goop will only lead to frustration as it will at best only last a short time.

Ed

Oldercrow wrote: Still having issues with weak idle at 1k RPM. . . .

Perhaps needing to sync the carbs.

Good Fortune!

Spark advance springs weak?
Steve

I checked the timing and F is lined up with mark at idle, but the || won't quite reach the mark. Is that also indicative of weak advance springs?

Had the bike running, sprayed starter fluid around from the airbox to the holders, no change in idle so probably no leak.

I do hear a high pitch sound when really running the bike. I may reseat the headers to make sure there's no exhaust leak.

Timing advancer cam is also a little pitted and a little rusty. Would it be ok to hit it with a wire brush or buff it out with a dremel?

Clean it up and lube it. The spring loops can be closed a little to get it to go back to F.
If not you need new springs. I wound up doing this.
Steve
kzrider.com/forum/2-engine/600671-ignition-timing#703561

Ended up getting Dynatek DS2-2 electronic ignition for the 1979 KZ750B4.

I had to clean up the advancer quite a bit and bent the springs in a little.

Since the B4 is a twin, the Dynatek sensors were in the wrong place for the timing. I removed one of the sensors and moved the other to the space across where they sat down around 8-5 o'clock (see picture).

Used the crimper box to add in the 12v current wire and it fired right up, first kick (since my starter clutch needs replaced too).

Here is a video of the carb syncing:

goo.gl/photos/11sVY9e8QSa5zLnt5

Valves adjusted?
Steve

Yes, all in spec, left exhaust was .6mm but others were about .9mm. Going to check again in about 5k miles?