Weird throttle issue

[DHXrider]

So I've got my bike running good as new again but there is still a small issue that is annoying me. I had this issue at the end of last year also, with a perfectly good running engine. YX160, runs fine but about 1hr into riding, the throttle get's a little stiff and it takes a while to come off of revs at times, basically sounds like a lean issue but it's not, almost like the slider get's stuck and doesn't go completely down to idle, like it get's stuck a bit. If I blip the throttle all is good and it will come back down to idle, almost like the blip unsticks it. Because it only happens about an hour into the ride, I have a theory that the carb becomes hot as the engine warms up and the slider bore expands slightly therefore making the slider stick.....My cables are good and the twist grip seems ok as far as I know. Again, everything works fine and the engine is tuned in terms of jetting etc. until about an hour or so into a ride. If I stop the bike, the twist grip rotates without any sticking and I can hear the slider closing fully shut. Another mystery issue. *argh.

I havn't troublshot anything yet, just want to hear a few theories first....what does everyone think?

 

[my67xr]

Have you checked for any kink's/bend's in the throttle cable that might be making it a little tight ?
You might be able to re route the cable to solve that ?
How much play do you have a the grip's on the throttle cable?
You need around 2mm-3mm movement before the throttle start's lifting the slide.
Is your throttle clamped to the bar's tight ?
How much room is there between the throttle tube and the bar's at the end of the bar's ?

Do you have an insulator between the manifold and carby mounting face ?
You can get on that fit's between the manifold and head too if you search around.
Engine temp shouldn't get that warm (depending on what you are doing)
Is it still easy to start when hot ?
Do you have a decent oil cooler on it ?

What carby and jetting does it have again ?

 

[DHXrider]

Well my bikes are definitely different.
So yes, the throttle cable does have a very tight bend towards the end before it goes into the carb because the gas tank is right on top. So for sure, I think I'll try rerouting the cable or adding a little length to the line from where it's clamped to the frame.

I've pretty much thought of all the troubleshooting tips you mentioned and I WILL try them and report back but I just find it weird how it only sets in later into a ride, the only thing I can think of that would cause a delayed issue like that would be heat creep.

2. Yep, there's a little bit of play in the throttle twist grip as there should be.
3. Yes, throttle is clamped tight, maybe too tight? yes maybe not enough slack on the end of the bar. Will foucs on that.
4. No insulator on carb manifold to MOTOR as it would lift it too high and hit the gas tank. The manifold to carb has the boot for the VM26.
5. Very easy to start cold, needs choke, warm it up for a minute, choke off then I'm good to go, sometimes some adjustment on the fuel screw is needed to get it purring just right, depending on the temp and humidity.
6. Easy to start when warm, no choke required, will usually start as is but is guaranteed to start with throttle held at mid point to full open.
7. Yes, YX160 with the good coolers with the small fins like a standard liquid rad.
8. Carb is VM26, 180 main, 20 pilot, needle middle clip. Fuel screws hovers anywhere from 2-2.75 turns out, depending on temp and humidity. Engine runs great with this setup. The issue certainly could be confused with a lean issue but because it goes away when I blip the throttle and due to the onset of "stiffness" in the throttle it's almost certainly a slide/cable/twist grip issue.

thanks for all the input, i'll try a few of these out and report back.

 

[DHXrider]

So a quick update:

Did a bunch of troubleshooting with the focus being the twist grip and throttle cable but sure enough after about an hour of riding it started doing it again. It really is the weirdest thing. I can shut the bike off and twist the throttle with zero drag or resistance, works perfectly, then when I start it back up, there's resistance the first 1/8th throttle and then clears up from there to full. It almost feels like it has drag or get's stuck from 0 to 1/8th throttle. If I turn it only to 1/8th, it will stay revving at that amount. Either the slide is getting stuck or there's some issue with the pilot, which for some reason affects the throttle slide???? In anycase it's very annoying. It's funny how much you can take for granted just a simple working throttle. Just a quarter turn of your hand but it really is the essence of the ride.

A few things to note, for some reason when I went to start it this time around, I lost compression, I'm assuming a valve got stuck or something, then I managed to start it and it ran from there so not quite sure what was up with that but anyway. Another curious thing, to get it running properly, I had to turn the fuel screw in (more fuel), but I ended up turning the fuel screw in fully seated just to get it running properly. Hmmmm... that's not right. Funny because I ran a 20 pilot all last season with no problems, and I was able to run it at 1.5-2.5 turns out on previous rides this year with that 20 pilot. Maybe this is a pilot issue after all? I'm going to try a few larger pilot jets and report back.

 

[my67xr]

Pilot jet or pilot circuit blockage i think.
Try taking the air filter off and hold the palm of your hand across the intake side of the carby to seal it off.
Now kick the bike over a couple of time's, your hand should get wet with fuel.
Now refit the air filter, turn the mixture screw back out to 1.5 full turn's out and try starting the engine again
See if it start's and run's then.

Sometime's if there's a blockage in the pilot circuit that method ^ can help to suck it through.

If it seem's good after doing it then i'd pull the cabry down and clean it out well., including removing the mixture screw, spring, washer and rubber o'ring and blow through all the port's on the air filter side of the carby while checking that air is coming out where the jet's screw in



Have you tried removing the cable from the throttle and lubing it, run some decent teflon type lube through it like Inox MX5.

 

[DHXrider]

Well, because I had to adjust the fuel screw almost to closed, yes, definitely something got into the pilot.....

So to recap I have the notchy 1/8th turn, throttle issue after about 1hr of riding or once the motor is really heated up. And issue two now is that it seems like the pilot is becoming blocked.

So took the carb apart and noticed a varnish build up on on the interior, mostly in the engine intake side and some in the throttle slide, this would probably explain the stickiness of the throttle slide. The thing is, I've literally had two-three rides on this carb since it was meticulously cleaned.....weird.




So I cleaned everything and put in a new #20 pilot, I didn't try putting in a larger pilot as I remembered telling myself that #20 is the right pilot, if it doesn't work with #20, something else is wrong. So I put everything back together and she fires up easily, this time I have the fuel screw set to 1.5 turns and that seems about right, I can back it off a little more to 2-2.5 turns and all is good, if I turn it in all the way (as I had to at the end of the last ride), it get's too rich and starts to fumble. So the pilot system is working ok.

Again, first 1hr riding, bike rides great, smooth, nothing a miss. Then I shut it off, wait five minutes, fire it up again and then bam, notchy 1/8th throttle, also feels every so slightly more rough when putting on the gas but that might just be me. But the sticky 1/8 throttle is very annoying, it just ruins the ride, you have no modulation or control at low revs, and it constantly over revs and I have to flick the throttle to, I imagine, unstick the slider.........

So basically did that ride last night, going to take it apart tomorrow and see if that varnish is there....I'm really stumped. It's fresh gas, my tank is sealed with a sealer but it's good stuff, I used it before without any problems. I suspected it was my air filter, which I spray with a thick oil but it should have been all sucked out by now, it doesn't feel overly oily to the touch. Just stumped.

One thing I noticed is I'm blowing white smoke now and then but I've chalked that up to an overfilled tranny. I overfill on purpose slightly due to the way the motor is mounted in my frame. I've overfilled before and never had smoke issues so not sure if this is a sign of something. Also, the notchy throttle was doing it before I did the oil change, I was on tranny oil from last season so it should have been around the standard level.

hmmmmm..... I appreciate all the advice, really helps to have multiple minds on the issue. This is a tough one though...

 

[DHXrider]

I think my next step will be to try a fresh air filter, sans oil. Just to rule that out. Obviously I'll probably have to clean the carb out again.....

 

[my67xr]

I would be taking the throttle housing and throttle tube off the handlebar's and check it out carefully looking for burr's of plastic where the cable travel's.
Also check for any burr's on the large rounded section that takes the main weight from the cable, if it is a plastic tube then there is more than likely bit's of plastic from the casting hanging off it that can be trimmed with a box cutter

Remember it'll be something silly/simple that's causing the issue.
Do you have another throttle you can try out on it to rule that out of the equation?

 

[DHXrider]

Yeah I have a few lying around...on my list to do....bleck...just such a pain running through all the stpes. Another thing I want to try is to stretch the slider spring a bit, it would be nice if they made slightly heavier return springs for the throttle slider....

 

[my67xr]

Take your old one to the local hardware store or Walmart and match up the length but find a slightly stronger one

 

[DHXrider]

Did a complete work through of the twist grip and throttle cable, the pulley wheel in the twist grip had a small notch wore into it, so replaced that, lubed the throttle cable etc. cleaned the slide, Again, about an hour into the ride, starts to stick. I can feel it, it's the slider, not the cable or the twist grip. Everytime I take the slider out, there is the slightest bit of gum here and there. Something is up with the slider, it's just not sliding in the bore properly. I'm stumped. Bike runs great aside from that. I might try to polish things up a bit, if that doesn't help.

 

[pitsterproAUS]

if its a std throttle assembly , most feel as u describe the first part kinda tight sometimes clicks ,

 

[DHXrider]

lol..no, this is an issue. trust me. It's notchy at 1/8th turn, so much so it affects the ride and throttle modulation. It's not just "the way it is", it's the slide somehow getting minute bits of gum on it when it's ran for a little while from crap knows where, Clueless as to why this is happening...think I might try to order a new carb see what happens. Def a weird issue.

 

[DHXrider]

Hey guys, so a little update on this issue, in point form:

Ended up buying a new carburetor VM26.

- Swapped for new carb
- New cables, new twist grip
- Fixed a broken cam chain tensioner, it was broke and didn't notice, thought that might be the cause of the problem in itself.

Results:

About an hour into riding, still doing the same thing, this time, even worse. The twist grip was really sticking and this time I knew it was the slider sticking, I could here it snapping shut at odd times. I took it out and noticed black residue gum just on the front side. This tells me there is some crud or oil coming up through the intake port and splashing on the slider, I've also read that this can happen if your valve seals are leaking.
I also wanted to elimate the potential of my gas tank causing the problem, as it is made with fiberglass (the gas resistant kind), but you never know. So I replaced it with a temporary metal tank that I know is non reactive, clean etc.

Knowing it was probably oil coming up and either combusting and leaving residue or possibly even oil vapour getting into the intake, I added some Lucas injection/valve cleaner to my gasoline.

- Long story short, I rode for two hours without a problem, my bike smokes now and then (which I know is not good and means the valve is leaking) AND I can here the slider snap shut now and then, but so far, with all the changes (metal tank, injection cleaner add to gas), it seems to work ok. Bike runs super strong, feels great, and throttle doesn't stick. I'm going to go back out and give it another rip and will report back. So my thinking is that the injection cleaner is just enough to clean off the oil from gumming up the slide. Obviously I should try to fix the valve issue but it's seems to be a temporary fix.

So my question is, am I right on the issue of it being a leaky valve seal? I just replaced the head, valves and seals last season. Maybe it's rings also? I never replaced them. My bike definitely smokes from time to time, but at some times can run very clean, it's almost like the valve or seal spins and looses it's fit/seat/seal or something. It's pretty random. What do you guys think? Do I need to pull the head? I'm dreading it, my frame is all enclosed, I have to pull the whole motor out to rip the head off, I'm trying to avoid it as much as possible.

 

[my67xr]

When was the last time you adjusted the valve/rocker clearance's ?
And what are they set to ?

 

[DHXrider]

They should be ok.. set to .003, .004. If they were off, bike would be hard to start and would run rough but it runs great, and rockers won't cause smoke, just bad timing. The smoke is coming from oil getting into the head somehow, probably leaky valve seals would be my guess.

 

[DHXrider]

actually just got back from another little session and all was good as far as throttle sticking goes so it looks like the additive is helping to break the goo from the oily combustion. But again, she smokes like a chimney now and then. Never did that before, black oil coating on the tail pipe. Definitely not right. Oil is getting in somehow. So it's either leaky valve seals or crank seals or something.

 

[my67xr]

Blocked breather hose from top rear of engine ?

 

[DHXrider]

???????????????????? lol.

 

[DHXrider]

all signs point to leaky valve seals, just not sure. Bike runs A1. I've never had to deal with this before so I'm wondering if leaky valve seals would make for a crappy running machine?

How would a blocked breather tube cause smoky combustion...I don't see the connect.