VF500F idle doubles after an hour

Hi!

I'm riding an '85 VF500F. Overall, it's running pretty well. Starts on first try, idles without choke after a few minutes. I've got the idle set at 1500. My issue is that, after riding for about an hour, the idle will go to 3000. No amount of playing with the throttle will get it to go down. I first noticed it about a month ago, and it is entirely repeatable. It will idle at 1500 until I ride it for about an hour, and then it will be 3000. I presume this is something going on with the throttle cable(s), but I thought I would ask ....

- John

 

The temp gauge moves around a bit, but mostly stays to the left of the middle (just above the hump that starts operating temp). It is a bit slow to warm up, but driving down the road with wind through the radiator, it will sit right at the lower end of what I view as operating temperature. If I let it sit at idle, the needle will lean over towards the high end, but the fan comes on and all seems OK.

When I notice it idling at 3000, it is usually after I've been driving down the road for a while, so the gauge is definitely not leaning towards overheating.

 

Don't know anything about 500's or their carbs, but have you tried resetting the idle back to 1500 rpm when its hot?

 

Yeah, a couple weeks ago I idled it down to 1500 from 3000. But then the next day when I rode it wouldn't stay running at intersections, so I idled it back up to 1500.

You have to understand that I ride primarily on weekends, usually about 40-50 miles. so the high idle that I see is manifesting itself at the end of my ride, pretty much every time.

 

It's normal for all bikes to idle faster after the engine and oil warm up, so nothing unusual there.

Like Tinki suggested, a carb synch might help your issue, but the idle speed adjustment screw is there for a reason !!! Use it to boost cold, choke-off idle and again later to lower idle when engine reaches operating temp. Use it !

It your bike won't idle (using the adjustment screw) without the choke after about one minute, your carbs need cleaning and/or adjustment.

 

OK. I just finished syncing the carbs. I've been meaning to do that for a while, anyway. They really weren't that far off; but it seems like it idles smoother now. I'm pretty sure that's not my imagination. They now read as close as my IEMIGO 4-cyllinder gauge can show. I set the idle at around 1500, with the bike (very) hot (from sitting an idling). I really won't know what the end result is until Saturday when I can start it from cold and run it through the normal routine.

Thanks for the ideas. I'll report back within the week.

 

Honestly, if the sync doesn't get the job done, then the next step will be to pull the carbs, clean them, and then do all the setting from the ground up. I'd really rather just ride it until winter gets here, though.

 

It could also be a vacuum leak that isn't presenting itself until the bike is fully warmed up.
While the bike is running spray starter fluid around the boots and carbs and see if the idle spikes.

 

Sorry for getting back late. And no, I haven't tested the bike since syncing the carbs - I'm just slow.

Just to be clear, should I be looking for a vacuum leak when hot or cold? To put another way, is the theory that 3000 is the "healthy" idle, but a leak when cold is dropping it to 1500? Or would the leak be cause the idle to race? I can look for leaks around the carb boots; where else might I be looking?

 

False idle is a classic sign of vacuum leak. Every changed the boots on the carbs?

 

My XJ750 Seca would be sync'd and idle fine until about 20-30 minutes of riding them the idle would shoot up.
The spray test led me to a vacuum leak at the intake boot gaskets.

 

Well, I'm back. I can report that the carb sync had no effect whatsoever on the idle issue. I still haven't checked for a vacuum leak, because i got distracted....

After deciding that the sync didn't do the trick. I decided to set the mixture screws. The process is certainly easy enough, and I felt that there was some chance that the bike was running lean and overheating the carbs, in spite of what the temp gauge was telling me.

So, I dialed in the first screw, at it was set at two turns out. I dialed in the second screw, and it was about a turn and a half out. I forgot what the third screw was set at, because the fourth screw was/is *stuck*. Won't budge. Someone was at it before, as the screw head is totally *stripped*. I have no idea what it's set at.

I set the other three screws to significantly richer mixtures, and it idles quite a bit smoother. I'm not going to bother doing a road test to see if it still races when hot, because right now I am obsessing about that fourth mixture screw. I don't see any way to resolve that without pulling the carbs. I've spayed some WD-40 on it, and tried to tap the screwdriver into a bite with a hammer. But it won't budge, and it looks like I'm going to have to drill it. Can't do that with the carbs on the bike (radiator hose in the way). And I need to get another screw. Or maybe just a replacement carb. I dunno.

 

Northern Virginia.