Carb slide sticking???

So, I've got my colortune, and though I don't have the YICS yet, I've done a tune and balance. Everything is dialed in pretty well and I can even get an smooth idle out of the engine. Things are looking up.

But I'm still having the problem of the high idle sometimes - especially when out for a ride. She rips nicely off the line, but as I run through the gears, I can tell the idle is hanging around 3-4k when the clutch is in.

Sitting at a light, it'll hang around 3k if I don't engine brake all the way to a stop. If it's idling high like that at a stop, letting the clutch out will bring things back down to normal. The idle does not climb back up at that point, but if I rev it, it'll usually hang up there.

I've done the WD-40 test around the boots and the shaft seals. I don't think it's a vacuum problem because I didn't notice a change with the WD-40, and it's such an intermittant problem whereas a vacuum leak would likely be constant (in my understanding, correct me if I'm wrong).

So I'm starting to think one of my carb slides migh be hanging open. Before I put the carb rack back on the bike, I tested them with compressed air, and they each seemed to come down on their own at the same rate. The is the only thing I can think of, though. Does this logic/troubleshooting seem sound? Or should I wait for my YICS tool before I leap to this conclusion? Is there any way to isolate the slides as the culprit? Too bad there's no window to show what they're up to at any given point.

I'll probably pull the carb tops off this weekend to check freedom of movement again and maybe give them a dip in cleaner (keeping the diaphragms clear, of course). I've also found a site that says the emulsion tube can wear which can then hang the slides - so I'll check those closely as well. The springs all looked OK last time I had them apart. Any other suggestions?

Thanks.

 

Have you checked your float levels, maybe to high.

What do the plugs look like?

Cable adjustments.

Sticking fuel enrichment valve, see if there seating all the way.

My 2 cents.

Good luck

 

Richard is right about checking the float levels. Although I suspect one or two may be low rather than high. I would check all he mentioned while waiting for the YICS. The rising idle can be intermittent. If the throttle shaft seals are leaking they may leak more as the carb body warms from engine heat. The rubber seal becoming more pliable and more easily suck in by the vacuum. Your check of the slides is sound. You are only checking movement so as long as hey go up and down at about the same pace you should be fine. It is when one hangs up or falls down ahead of the others that more attention is needed. IN the case of one falling down ahead it is a sign of a cracked or leaking diaphraghm.

 

When the carbs are off the bike try using a vacuum cleaner on the engine side of the carbs, making sure there is NO GAS in the float bowls.
You can see how the carb slides move with throttle operation using this method and not worry about overpressurizing anything by compressed air method.

I would tend to think of throttle shaft seals causing your problem as BlueMaxim said. One thing to try is a quick visual inspection of the outer two carb seals by removing the E-clips and washers to expose them. Do they look soft or hard, any cracks?

 

Sounds weird, but it was a free fix for my creeping RPM. I just turned idle down to about 900 rpm, if it is set at 1050-1100 it creeps.

 

I would tend to think your fuel mixture is to rich. Pull your plugs like Rich suggested after a high speed run and look at them.

 

Checked float levels before I put the carbs back - all within spec.

Plugs are clean/tan - if anything a little whitish on the top of the electrode, but as I said, the colortune plug shows the mixture is good.

Cable adjustment and lube is fine. There's a little slack with the throttle closed. The enrichment cable as well.

For the fuel enrichment valve seating - is this something I visually inspect from the outside? They all look in synch. Would/could the enrichment valves move on their own - causing the problem later in the ride after things were OK to start with? FWIW, I have verified that the enrichment lever itself is all the way 'off' when this is going on.

As for the shaft seals, I visually inspected the outer ones when all this started and they look fine. As for the others, as I said, I tested those with the WD-40 method - should I still suspect them? If I get a chance to do a warm-up ride today, I'll blast 'em again.

If I set my idle anywhere below 1000 RPM, the engine slowly dies out, so that's not really a long-term option, but in my testing, I DID have it set lower, and still had the hanging idle problem.

One thing to make clear - this is NOT a creep in idle. I used to have that. Mixture and synch seem to have fixed that. This is an engine not RETURNING to idle after a rev. I just want that clear in case it indicated a different cause.

Thanks for all of your suggestions. I had checked the majority of this stuff already, so I should have said that in my original post. I might have time to pull the carb tops today and just make sure the diaphragms and springs are seated correctly and that the slides are moving freely. I like the idea of the vacuum test, but honest to god, I'll live with this idle and pull it down with engine braking before I pull the carbs off for a third time in a few weeks. When I had them off last, I tested them with a medium pressure jet of air from about a foot in front of the carbs. I did this not only to verify slide movement, but check that the seafoam/gas mix was getting though the circuits in all four carbs (yes, I did this in a WELL ventilated area away from any source of spark - atomized gasoline is a very dangerous thing). As I said, the slides looked fine then. But things change, so who knows. If I have to pull the carbs again, so be it< but I'm waiting to get my YICS tool before I do that.

Thanks again, guys. Any more ideas, let me know.

 

WE HAVE A WINNER!!!!!!!!!!!!!!!!!!!!!!

After another frustrating evening with the bike on Saturday and holding myself back from dropping a match into the gas tank, I managed to get things straightened out on Sunday to the point of getting the bike to sing, and plastering an ear-to-ear grin on my face!

First of all, I decided that since the leak tests revealed nothing, I should re-evaulate the mixture of the engine. I THOUGHT the clortune was the right color blue. I THOUGHT the plugs were the right tinge of light brown - but something still bothered me about them being a little ashy. So I was going to take her for a ride and check the plugs again - I had to be missing something.

Well, in trying to get down the street, I discovered a new problem - I was now only hitting on 3 cylinders when I HAD recently been firinf on 4. What was this? After checking the exhaust headers, I foud that #1 was cool to the touch. I pulled the plug and didn't see any spark, so started to suspect the coil. But the other side of the pair was OK. So I limped the bike back into the garage and took a deep breath. And thought. #1 wasn't firing, but it also wasn't fouled - it was dry and still a little white. "We're missing fuel!" I thought.

Ladies and gentlemen, boys and girls - just because the float levels WERE fine doesn't mean they still will be in a hour, day, week, etc. Yes, the #1 fuel level was down about halfway. A few love-taps with a screwdriver handle, and the level came upto where it should be.

Problem #1 identified and resolved (at least temporarily).

Now, on to the mixture. See, when I put the carbs back on the bike, I did what I was told and screwed the mixture in to the stops, then backed out 2.5 turns and started my colortuning from there. Looking at the spark, it was a little white, so I backed off the mixture screw and there - that's blue -right?

So, this time I decided to actually folow the colortune directions and find the full-rich end of the spectrum. Get this - to properly tune the mixtures, I needed to back them out an ADDITIONAL 3 COMPLETE TURNS!!!

Already the bike was running better. I had to bring the idle setting down a few times, did a quick carb balance (still don't have that YICS tool), and decided to take her for a spin. And she spun!

I stopped a few times to put the finishing adjustments on the idle setting, throttle cable, etc, but by the time I got home, there was no floating of the revs (it had still been doing that a LITTLE though not nearly as much), and it wasn't dying out at an 1,050 idle. I even pulled the plugs one more time when I got home, and they were the nice EVEN tan I had been expecting all along, not that 'just barely, mostly white, tan'.

She's alive! I don't think this thing has sounded this good in the 11 years I've had it. I keep waiting for something else to break now, like the exhaust blowing out or something. You know what they say - riding high in April, shot down in May

I already know the winter holds a new valve cover gasket and a clutch kit,
but for now - I ride!!!

So here's my summary of this latest hurdle:
1) Persistence pays off!
2) The help here is great, but only if you're smart enough to follow it.
3) Related to that - just because something was working/nominal yesterday doesn't mean it still is today - CHECK IT AGAIN!
4) If you have a hunch (the spark plugs weren't reading QUITE right), follow it up.
5) Guidelines, ballparks, and rules of thumb are great, but PAY ATTENTION TO THE HIGH-PRICE TOOLS YOU BOTHERED SPENDING MONEY ON!!! You might START at 2.5 turns rich, but don't stop until the colortune TELLS you too. "Good enough" isn't good enough.
6) Again, persistence can even overcome ignorance. Sometimes.

Thanks for playing/following along. If you're interested, I took some new pics of the ride. She's not fully polished up, but still - not looking too bad for being 25.

http://flickr.com/photos/oblivion/tags/xj750