Engine racing at idle

Looking for some confirmation on my troubleshooting. I figure this has to either be an air leak at the intake boot or something stuck internally in one of the carbs.

Suited up and started my '79 KZ750 to go for a ride today and the engine just started racing. I pushed the choke lever all the way off and the racing topped out at about 5k.

I checked the throttle cables and everything is moving as it should. The throttle stop seems to be working. It's not like the throttle is stuck open or anything.

I tried spaying some WD-40 around the intake boots but it had no effect.

Carbs were removed and cleaned thoroughly less than a year ago. It's been running great.

The only recent work I've done is on the starter motor and starter clutch. After re-assembly the bike started just fine and I didn't experience this racing. That was about a week ago and I haven't ridden since.

So, do I need to just pull the carbs again and see if something's stuck in there?

How about the idle adjust screw? The big thumb screw between the carbs.... I'm assuming your bike has it.

Well this seems to be fixed. Must have been a leaky boot.

I pulled the carbs and checked everything making sure the butterfly valves and slides were moving freely. Nothing looked wrong with the diaphrams. Everything checked out.

Put it all back together and everything is fine. No engine racing.

I guess it had to be a bad seal at the intake boots or carb holders. I just couldn't find it with the WD-40 trick.

Gonna go for a short ride and make sure everything is good.

if you have a small propane torch. you can check for a leak that way too. it's a lot cleaner way too.

Thanks guys. I've never tried the propane torch method, but have seen some videos on YouTube. That looks pretty slick.

It wasn't the idle adjust screw. I checked that and backed it way out, made sure the throttle linkage was back as far as it would go and the engine just hung at 5k.

I'm really leaning towards a leaking intake boot. When I was putting the carbs back in, the right intake boot popped free of the airbox with very little pressure on it. I think that's where the leak was.

Does your bike have a mechanical ignition advancer? Most are behind small, CD sized engine covers at the end of the crankshaft. Sometimes they stick, affecting idle, and low speed engine performance. On the bigger 4's, removing a bolt permits removal of the advancer for cleaning and lubrication. Light oil or thin grease, is about all it will need now and then.

Topper wrote: ...I'm really leaning towards a leaking intake boot. When I was putting the carbs back in, the right intake boot popped free of the airbox with very little pressure on it. I think that's where the leak was.

Leak at the carb holder area on the engine side of the carb may result from whatever reason such as a crack in the carb holder or just a worn out carb holder, or perhaps an insufficiently tightened clamp that holds the carb inside the carb holder -- typically results in a racing idle due to overly lean air-fuel mixture attributable to extra air being sucked in through the leak.

Am thinking that a slight air leak on the air box side of the carb doesn't generally result in a racing idle.

Good Fortune!

Patton wrote: Am thinking that a slight air leak on the air box side of the carb doesn't generally result in a racing idle.

Hmmm... well that throws a wrench into my theory about what was wrong.

The boots and carb holders are in pretty good shape. There's no visible cracks or wear and they're all still pretty flexible.

The other thing I noticed when removing the carbs was that one of the ends of the balance tube (that connects the carb holders) had slipped a little. It hadn't come off, but it was a little loose. Maybe that would do it?

What was the fuel level? Did you turn the petcock on? Am asking because mine will race when I shut the fuel off, that is until it stalls due to lack of fuel

Hi there

I've been working on my KZ750 basket case for a while and had a similar symptom. On starting it would race up to 5/6000 rpm. It had pods and a 4 into 1. Long story short, swapped pods for stock air box, and after checking for (many) air leaks,finding and fixing 'em, it now idles WAY better. So I'd say keep checking for air leaks. Even the smallest leak made a BIG difference to the idling. Good luck, regards Bos

The right carb on my twin has always run a lot leaner than the left. I have to have the mix screw out nearly three turns on the right, but the left is right around 1.5 turns out. (Out enrichens and in leans on these carbs)

I've cleaned the pilot passages, set fuel float levels with the clear tube method and balanced the carbs with a vacuum gauge. I was about to re-test the carb balance, but this has me thinking maybe I've got an air leak on that side.

Topper wrote: The right carb on my twin has always run a lot leaner than the left. I have to have the mix screw out nearly three turns on the right, but the left is right around 1.5 turns out. (Out enrichens and in leans on these carbs)

I've cleaned the pilot passages, set fuel float levels with the clear tube method and balanced the carbs with a vacuum gauge. I was about to re-test the carb balance, but this has me thinking maybe I've got an air leak on that side.

Good deduction Topper. Maybe between your carb holders surface and the head. Maybe too small of a leak to be detectable using the WD-40 method but enough to have an effect you described on your right carb. Hope you nailed it. Good luck.