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  #1  
Old 01-17-2011, 04:34 PM
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Default Bike idling with choke on, dies when applying throttle

My 94 F2 has developed a really strange problem that I'm having problems diagnosing. I know that there are a few "bike not starting/bike not idling" threads in this forum now, but none seem to fit exactly the problem that I'm experiencing...

To provide a little bit of background: So about a month ago, I brought my bike back to life after 3 years of sitting in a garage. I ended up changing out the gas, cleaning out the carbs (got new boots, new fuel bowl o-rings, and cleaned out the jets), and changed the fuel hose/fuel filter. I also did some other maintenance work like change brake fluid and flushed the coolant. After all this work, the bike worked perfectly and I took it out for a few test runs. Everything seemed to be fine.

I then had to get the tires replaced, so the bike stood on stands for about a month. After I got the tires back on, it ended up raining for a bit, so I didn't get to go out for another ride until now.

So last weekend, I start my F2 back up using my normal starting procedures: turn on choke, hit the start button, and it fires up. I let the bike warm up with the choke on, I gear up, and hop on the bike. It takes me usually about a minute to get fully geared and hop on the bike, so the motor's somewhat warm by now, but not warm enough to stay idling without the choke. So I disengaged the choke a little bit, drop it into gear, and hit the throttle.

As soon as I touched the throttle, the bike bogs down and dies/stalls. I then try a couple more times- choke on, start up fine, hit throttle and dies. I was finally able to get the bike going, but only if I hit the throttle pretty hard (bike dies if it drops below 5k rpm). It continues acting like this, so I only take it for a quick spin around the block in fears that I'll get stranded.

So I'm not exactly sure what is going on here... so I'm just trying to get some advice on what the culprit might be. There seems to be fuel in the carbs, the vaccuum line/fuel hose lines are attached, and I can start the bike up everytime with the choke on. The bike just does not want to respond to any throttle input... I just pulled out the spark plugs yesterday, and they seemed to be pretty fouled up (black, some oil, and the #4 spark plug seemed to be a bit wet), could this be the problem? I can post pictures of the spark plugs online later tonight.

What else can it be??? It's got me really baffled!!!

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  #2  
Old 01-17-2011, 05:18 PM
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Quote:
Originally Posted by switched View Post
My 94 F2 has developed a really strange problem that I'm having problems diagnosing. I know that there are a few "bike not starting/bike not idling" threads in this forum now, but none seem to fit exactly the problem that I'm experiencing...

To provide a little bit of background: So about a month ago, I brought my bike back to life after 3 years of sitting in a garage. I ended up changing out the gas, cleaning out the carbs (got new boots, new fuel bowl o-rings, and cleaned out the jets), and changed the fuel hose/fuel filter. I also did some other maintenance work like change brake fluid and flushed the coolant. After all this work, the bike worked perfectly and I took it out for a few test runs. Everything seemed to be fine.

I then had to get the tires replaced, so the bike stood on stands for about a month. After I got the tires back on, it ended up raining for a bit, so I didn't get to go out for another ride until now.

So last weekend, I start my F2 back up using my normal starting procedures: turn on choke, hit the start button, and it fires up. I let the bike warm up with the choke on, I gear up, and hop on the bike. It takes me usually about a minute to get fully geared and hop on the bike, so the motor's somewhat warm by now, but not warm enough to stay idling without the choke. So I disengaged the choke a little bit, drop it into gear, and hit the throttle.

As soon as I touched the throttle, the bike bogs down and dies/stalls. I then try a couple more times- choke on, start up fine, hit throttle and dies. I was finally able to get the bike going, but only if I hit the throttle pretty hard (bike dies if it drops below 5k rpm). It continues acting like this, so I only take it for a quick spin around the block in fears that I'll get stranded.

So I'm not exactly sure what is going on here... so I'm just trying to get some advice on what the culprit might be. There seems to be fuel in the carbs, the vaccuum line/fuel hose lines are attached, and I can start the bike up everytime with the choke on. The bike just does not want to respond to any throttle input... I just pulled out the spark plugs yesterday, and they seemed to be pretty fouled up (black, some oil, and the #4 spark plug seemed to be a bit wet), could this be the problem? I can post pictures of the spark plugs online later tonight.

What else can it be??? It's got me really baffled!!!

The fouled plugs may be just front running with the choke on and never getting her fully warmed up.

Try to let it idle with the choke on until the point where the revs start climbing because of the choke being on. Let it get fully warmed up... heat gauge to at least 1/4-1/2. Then tell me how it runs.

Go for a good ride if it does.. at least 5-10 miles. Come back and pull the plugs. Still wet?
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Old 01-17-2011, 06:11 PM
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I've actually let it warm up fully during my test spin around the block. I ended up leaving the choke on until the bike got up to around 5k rpm and the heat gauge was at about or over 1/2.

During that quick spin (maybe 3 miles, not quite 5), the bike was still dying as soon as I eased off the choke. Only way that I got home was to ride it around with choke fully on, idling at around 5k rpm when i came to stops. The engine wanted to cut off everytime when i applied the throttle, so I had to go pretty hard on the throttle just to keep the bike from dying.

Should I maybe clean the old spark plugs or install the new spark plugs and repeat what you suggested and report back?
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Old 01-17-2011, 06:28 PM
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What happens if you advance your idle screw until it doesn't drop any RPM when you take it off choke?


Almost sounds like a pilot jet issue - or petcock isn't hooked up or functioning properly? Until you get RPM's up the engine isn't pulling enough vacuum to make the fuel flow right? Did you get all the vacuum port plugs back in?
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Old 01-17-2011, 06:52 PM
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Originally Posted by adrenalnjunky View Post
What happens if you advance your idle screw until it doesn't drop any RPM when you take it off choke?


Almost sounds like a pilot jet issue - or petcock isn't hooked up or functioning properly? Until you get RPM's up the engine isn't pulling enough vacuum to make the fuel flow right? Did you get all the vacuum port plugs back in?
I doubt its the petcock because as he increased rpms it got better and this is usually the opposite for petcock issues.

Vaccum ports may be an issue but again, I think you would see issues across the tach. Still, no harm in double checking these

--- To Switched

Didn't know you let her heat up, skip that part.

Sounds like you are running very lean on your pilot circuit like Junkie said, does this problem start to go away while you are riding above 5k RPMS and go away completely as the RPMS rise? If that is the case it surely is a Pilot issue.

Try to idle the bike just at the fringe of shutting off for a minute or 2, and pull your plugs. If there is a problem, some plugs should be soaked cuz of choking but the bad carbs would be dryer.

That just might give you some direction as to which carb and pilot is your culprit. You are probably going to have to tear the carbs off and double check your jets and while you are at it, double check the idle screw adjustment.

One time I had a random piece of rubbery glue stuck in my main jet and the bike would die above 8k rpms. Kinda sounds like your problem only with a pilot
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Old 01-17-2011, 11:14 PM
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Originally Posted by Dissevered View Post
I doubt its the petcock because as he increased rpms it got better and this is usually the opposite for petcock issues.

Vaccum ports may be an issue but again, I think you would see issues across the tach. Still, no harm in double checking these

--- To Switched

Didn't know you let her heat up, skip that part.

Sounds like you are running very lean on your pilot circuit like Junkie said, does this problem start to go away while you are riding above 5k RPMS and go away completely as the RPMS rise? If that is the case it surely is a Pilot issue.

Try to idle the bike just at the fringe of shutting off for a minute or 2, and pull your plugs. If there is a problem, some plugs should be soaked cuz of choking but the bad carbs would be dryer.

That just might give you some direction as to which carb and pilot is your culprit. You are probably going to have to tear the carbs off and double check your jets and while you are at it, double check the idle screw adjustment.

One time I had a random piece of rubbery glue stuck in my main jet and the bike would die above 8k rpms. Kinda sounds like your problem only with a pilot
I did try playing with the idle screw, but I didn't get very far with that diagnosis.

I agree that it does sound like the pilot jet could be an issue so I will also check that out this weekend. I'm still at a loss as to why the bike stalls when i apply the throttle, even with the choke completely on and the bike idling at 5k rpm???

I also took some pictures of the plugs I pulled. They look pretty disgusting huh?

Cylinders 1 through 4 from left to right:





*edited for better pictures*

Last edited by switched; 01-19-2011 at 02:07 PM..
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Old 01-17-2011, 11:55 PM
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Is that oil on the plugs? Some is normal, could just be the camera making the amount look more than I'm accustomed to. Did you find any traces of oil inside the plug boots when you pulled them out? The wife had a Mazda 626 that would leak oil into the spark plug wells from a valve cover gasket issue. After enough time, it would start running like crap, I'd pull the plug boots, see the a plug or 2 sitting there in oil. I'd just pull em out, let the oil drain into the combustion chamber, fire her up and fog out the driveway for a few minutes.

#1 looks like it's fouled to me. Cant tell from oil or fuel. Has the bike been smoking? Hard to tell if it has even been firing. 2 & 3 look like they're running lean - see the light tan on the ceramic and the scale/buildup on the ground electrode. #4 looks the most normal to me - but there's some deposits around the center electrode that I can't determine if it's fuel or what.

I think she wants a new set of plugs.
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Last edited by adrenalnjunky; 01-17-2011 at 11:58 PM..
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Old 01-18-2011, 12:12 AM
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With the choke fully on - shouldn't there be some apparatus that is keeping the normal amount of air from flowing into the carbs, hence making the mixture richer to prime the combustion chamber and ease starting. With that in place - I wouldn't expect you to be able to go to full throttle, maybe not even past the equilibrium that the engine finds with the choke engaged. I've honestly never tried it before. I'll test this on my bike tomorrow - it's past midnight and the neighbors wouldn't appreciate it right now.

Do our carbs even pull air through the butterflies primarily with the choke on, or is there a sort of bypass that is opened/closed. Is activating the choke also advancing the throttle position? (forgive me, I haven't taken my carbs apart yet, or I'd know this - I'm just comparing it to how the choke works on a gasoline R/C airplane engine (weedeater/chainsaw - anything with a Walbro-style carb setup)

Also - don't hate me - but others on the forum have had it wrong, and lets just make sure - in the off position - the choke lever is pushed fully forward, toward the front of the bike, and you're pulling it toward you to turn the choke on - right?
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Old 01-18-2011, 09:15 AM
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Originally Posted by adrenalnjunky View Post
Do our carbs even pull air through the butterflies primarily with the choke on, or is there a sort of bypass that is opened/closed. Is activating the choke also advancing the throttle position? (forgive me, I haven't taken my carbs apart yet, or I'd know this - I'm just comparing it to how the choke works on a gasoline R/C airplane engine (weedeater/chainsaw - anything with a Walbro-style carb setup)
Our bikes have an enrichening circuit. As you turn your choke on (toward you) it opens up another jet to squirt in more fuel.

---- to switched

Your plugs look bad, hard to tell if it is due to running with the choke a lot, or oil fouled. You will have to make that determination. Oil fouled is more gunky and slimy... thicker. Fuel fouled would be wet or if dried, a dustier or dryer black. Not slick from oil.

All of them are fouled, here is as close to perfect as I could get on a spark plug.

Click the image to open in full size.

I should have told you to clean your plugs first. They are fouled, but usually you can get them to run ok with a solid cleaning and sanding. Sand the plugs and clean them up a bit first. Make sure the gap is correct after, and put them back in the bike. Get the bike again on the edge of shutting off for a minute or 2. If this means holding your throttle slightly open to get it to bog then do it. Pull the plugs again.

All this is doing is giving you a direction to go. If getting the engine to bog for a while makes the plugs look dry then you are lean, if they still look soaked, you could be extremely rich, though doubtful.

On top of that, check for oil like said above. It is hard to tell from your pic, but you could be having problems due to an oil leak as well.

Also, did you try starting it without choke?

Oh, and I don't think you answered the question of how the bike runs beyond 5k rpms and 8k rpms? (maybe i missed your answer)

Last edited by Dissevered; 01-19-2011 at 12:12 PM..
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Old 01-19-2011, 11:31 AM
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Thanks Dissevered and junky for providing the helpful advice. Unfortunately I still haven't had a chance to work on the bike. I get home pretty late from work and like junky, I can't really fire up the bike during night hours (neighbors hate that lol).

Dissevered, to answer your question, when I was able to get the bike in gear (meaning cruising around in 1,2,3rd gear with choke completely on - meaning pulled the choke completely towards me), it seemed to operate okay in the 5-8k rpm range as long as I gave it enough gas. But I caveat this statement with the fact that I have limited knowledge of what "okay" feels like.

The bike just really wants to die between idling @ 5k rpm with choke fully on, and when I start to rolling on the throttle. And on top of that, the bike dies when i turn the choke off (choke level completely away from me), even when the bike is fully warmed up.

Last edited by switched; 01-19-2011 at 12:01 PM..
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Old 01-19-2011, 11:31 AM
 
 
 
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apply, bike, bog, bogs, cbr, choke, dies, full, gas, giving, idled, motorcycle, ride, stalls, throttle


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