unwanted revving issues

[MarkD]

Hi -- I'm new here, trying to solve a vexing issue....
I'm having revving issues with my newly purchased '79 F. After rebuilding carbs with all new standard jets ( I have the stock airbox with new air filter on ), it started right up. Problem is, it races up to 2,000-3,000 rpms after I blip the throttle. I bought a Motion Pro (Ebay) sync tool, put it
on, and the levels were very close,( I had previously done a bench
sync). I did a little adjustment to bring them equal. I have sort of the straight pipe mufflers on it, not really the type that has much of a baffle in them, so maybe that's a contributing factor. I bought new
carb boots, and made sure the throttle cable was a little loose. I have checked for cable 'hang-up, and it's smooth and clear. I
have screwed out the mix needles about 2 or so turns out, but noticed
the plugs all had sort of a dry black carbony look to them. Maybe I
need to turn them out another 1/2 or so to richen the mix? Previously
I sprayed wd40 all over, and there was no increase/decrease in the
idle speed, which is indicating no vacuum leaks at the boots or elsewhere on the carb bodies.
One weird thing, I held the rpms at 3,000 for a bit, and it increased
on its own to 6,000, every time. I have read a lot of the threads, and I'm stumped.
Thanks-- MarkD

[Old Red]

Fuel mixture too lean will cause idle speed to go up and ignition timing advancing too much.
What size jets? Try unplugging the vacuum hose to the vacuum advance and plug the hose.

The Other Earl

[Gallaecus]

I would be interested to know if you have tight valve clearance

[donebysunday]

Hi -- I'm new here, trying to solve a vexing issue....
I'm having revving issues with my newly purchased '79 F. After rebuilding carbs with all new standard jets ( I have the stock airbox with new air filter on ), it started right up. Problem is, it races up to 2,000-3,000 rpms after I blip the throttle. I bought a Motion Pro (Ebay) sync tool, put it
on, and the levels were very close,( I had previously done a bench
sync). I did a little adjustment to bring them equal. I have sort of the straight pipe mufflers on it, not really the type that has much of a baffle in them, so maybe that's a contributing factor. I bought new
carb boots, and made sure the throttle cable was a little loose. I have checked for cable 'hang-up, and it's smooth and clear. I
have screwed out the mix needles about 2 or so turns out, but noticed
the plugs all had sort of a dry black carbony look to them. Maybe I
need to turn them out another 1/2 or so to richen the mix? Previously
I sprayed wd40 all over, and there was no increase/decrease in the
idle speed, which is indicating no vacuum leaks at the boots or elsewhere on the carb bodies.
One weird thing, I held the rpms at 3,000 for a bit, and it increased
on its own to 6,000, every time. I have read a lot of the threads, and I'm stumped.
Thanks-- MarkD

To me it sounds like your chokes/enrichment valves are hanging up, make sure they all go back to seat.

[DGXSER]

I have sort of the straight pipe mufflers on it, not really the type that has much of a baffle in them, so maybe that's a contributing factor.

This will make it a lot more of a challenge to tune. These bikes like to have some back pressure.

I bought new carb boots.

OK, but how did you seal them to the head? Use some RTV? Paper gasket? Paper gasket does work well in this application.

I have screwed out the mix needles about 2 or so turns out, but noticed the plugs all had sort of a dry black carbony look to them. Maybe I need to turn them out another 1/2 or so to richen the mix?

Opening the screws up richens the mix, you describe a dark sooty plug which would indicate overly rich condition. I would turn them in a 1/4 turn.

Previously I sprayed wd40 all over, and there was no increase/decrease in the idle speed, which is indicating no vacuum leaks at the boots or elsewhere on the carb bodies.

I do not belive WD is a good test for this, try carb cleaner or starting fluid. "Poor mans method", an unlit open propane torch if you have one around.

One weird thing, I held the rpms at 3,000 for a bit, and it increased on its own to 6,000, every time. I have read a lot of the threads, and I'm stumped.
Thanks-- MarkD

This increase in RPM on its own indicates either a lean condition, which your plugs would say otherwise, or out of synch. Although you day you already synched the carbs.

[bikerphil]

Make sure your vacuum advance is hooked to the #2 carb body and not the carb boot. An overly advanced timing condition may cause a high RPM.

Another thought, the clamp on the #3 front carb boot may be hanging the cable up, all of the forward carb clamps should have the screw on the bottom side of the boot so they don't interfere with the throttle linkage,.

[jjz28]

I also had this happen on my xs750 and it was my butterfly seals. These other guys know a lot more than me, just throwing that out there. My bike would slowly idle up to about 3,000 and sometimes go down if I blip the throttle but go right back up.

[motoman]

Make sure your vacuum advance is hooked to the #2 carb body and not the carb boot. An overly advanced timing condition may cause a high RPM.

Another thought, the clamp on the #3 front carb boot may be hanging the cable up, all of the forward carb clamps should have the screw on the bottom side of the boot so they don't interfere with the throttle linkage,.

.......and #1 and #4 vacuum is going to petcocks, with #3capped......

[MarkD]

unwanted revving issues

Hi guys-- thanks for the suggestions.My jets are:main-137.5 / pilot-42.5, so this should be correct for the airbox.
I sucked on the vac adv. hose, and the mechanism moved, then returned to its original position. I have it connected to #2 carb body, not the manifold, and all other lines are going to the right place. I will try the propane or starter fluid or unlit lighter test next, to see if indeed there is a leak. I checked the (new) carb boot gasket, and they are the ones that came with the boots, and they look good. I will replace the spark plugs, too, to get a better read on any changes.
Again-- thanks for the help== this is getting me on the right track.
MarkD

[DGXSER]

Ooops..should have rad paper gasket does NOT work well for this application. I strongly advise using some RTV or similar on the carb boots myself.

[Diskman01]

My 78 did the SAME thing. Drove me NUTS trying to fix it. It's a vacuum leak somewhere. I replaced my sync-port caps, RTV'ed my carb boots and replaced the intake gaskets.

The sync port caps made all the difference. Made it drivable last summer at least..

What REALLY helped was new carb boots this past winter. Got the $70 off ebay. They came with new bolts, gaskets, clamps and of course nice squishy boots. WELL worth it.

On my 78, I had to really open up the pilot screws. Not sure if your bike has em on the carbs. Make sure that none are broken off. With todays crap gasoline, I think the engines like it a bit richer.

Too lean will give it a "hanging idle" too.

Don't be afraid to turn those screws out a bit more. Ride, and see how it behaves at lights.

Float level makes a HUGE difference also. Pick up a 12 dollar digital caliper and set the float heights as per the book.

[MarkD]

unwanted revving issues

Hi all. Diskman, sounds like you are on to something. I did have 2 mix needles broken off and stuck in the bodies before I rebuilt the carbs, but removed them w/ the push pin method. Tomorrow night I'm working on it, and will look into all possibilities you kind folks have put out there. Thanks to all so far!
MarkD

[MarkD]

unwanted revving issues

Hi helpers, i appreciate all the ideas and help in getting my issue solved. Today, ( after setting the float heights to 25 mm, they were at 23 upon remeasuring). I also went through the whole process of installing the carb boots, to make sure they were sealed to the engine. I started the XS11 up. The revs were a bit better, hung at 2000 rpms for a bit, but settled quicker. Held at 3000 rpms, it did race to 5000, but not 6000 as it did before, and settled upon release of throttle. I still have the K&L pilots, but may change them out to Mikuni as was advised might be the real issue. I backed out the mix screws 3/4 of a turn also. The bike runs better, but the plugs blackened up a lot, and were carbony still, but not wet. Progress is being made, but more suggestions are appreciated.
On another note, a PO bypassed the main 30 amp fuse, and connected the red wires together. He also tin-foiled one of the 10 amp fuses. I installed a 30 amp inline fuse, and will redo the fuse panel tomorrow, as one of the fuseholder clips broke as I checked it out.
Another note, only one turn signal works. The bulbs look good, and I cleaned the sockets with WD40. So, as you can see, a few issues still plague this scoot.Again, keep posting, and I appreciate the experience you guys bring to the table.===MarkD

[MarkD]

unwanted revving issues

Sorry -- forgot to mention that at idle, I sprayed carb cleaner on all boots, and pretty much on each individual carb all over. There was no difference in rpms upon doing this, so I'd say --no vacuum leaks anywhere near the carbs.

[CaptonZap]

WD-40 is not the best substance to use to find air leaks. It evaporate to slow, and it is the volatiles that need to be sucked into the leaks, and richen the mixture, thereby raising the RPM's. A propane torch, unlit, naturally, set to produce a small flame, then blown out, (which will give you an idea of how much propane should be coming out of the nozzle), and played around any part of the induction tract that has any chance of having a leak to the inside will give you the best chance of finding a leak.
Use common sense, and do not use this method if your timing is not correct, if you are not sure that there are no leaks in the ignition wires, and if you have any raw gasoline dripping around the engine.
YOU have to be just a little bit smarter than the stuff you are playing with.

CZ


ps Do you have a fire extinguisher, and do you know how to use it?



OOp's ignore the preceding, I see you used carb cleaner, (I assume the flamable kind), and that is a good alternative to propane.