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Missing at constant throttle! !

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My xr650l has had this problem since I purchased it. It has a miss at 1/4 to wide open constant throttle. If I wack the throttle from any position and it runs great! I tested all the wiring and it checks out ok. I tested the coil and can't get a reading on resistance through either side of it to the spark plug boot end. I purchased a new coil and boot from EBay not oem. Same problem with it and runs even worse with the new coil. I also cleaned the ground connections to the frame.

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I have installed a FCR MX carb. I went through the entire carb several times. With new jets installed. I am going install the old coil tomorrow and see how it runs. It missed a little with the stock cv carb too.

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I agree with Backwoods Bomber. When you bought the bike did the guy say anything about how long it had been since the last time he rode it? look in the gas tank and see if there is any particles. Check the petcock for screen damage (fuel filter at on/off switch). Usually on bikes that have been sitting for a while, the gas cap rubber can erode as well as the petcock screen. It can be a bear locating the little piece of rubber clogging a hole in the carb and yes even the smallest piece of rubber can cause the problem you are describing. Cleaning the carb takes more than just opening it up and wiping things down. If you choose to take the carb out and apart, take LOTS of pics along the way it will help. Compressed are is practically a must. Maybe keyboard cleaner if you don't have a compressor. Lots of carb cleaner but watch that you don't soak the rubber pieces in carb cleaner. Lastly, I have had success in boiling the carb in lemon juice and water, but don't boil too long because it can eat at the metal the carb is made out of.  If you find that the petcock screen is damaged, don't order a new one, you can buy a universal filter from the auto parts store, just cut your hose from the tank to the carb and insert it there. I can almost guarantee you if the bike starts, there is nothing wrong with the electrical. It is in the carb and there is a tiny little whole that is clogged, you just need to find it and clean it. Hope this helps and good luck. 

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I have a new tank installed. Acerbis 5.8 gallon and a fuel filter. I will try the old coil since it ran better with it. If that doesn't help. I will take the carburetor apart again and boil it with lemon juice. Thank you for the tip.

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Well you may not need to take apart the carb again but I have taken carbs apart way too many times in my learning curve. I was thinking about it though and have you checked for air leaks? Mainly after the carb but also before. check for cracks in the intake manifold and that the gasket at the engine is in good condition. You can do this visually and also spray some starting fluid all around there with the bike running. if there is air leaks, the bike will stutter. 

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it's funny that you posted this problem since mine started doing this yesterday. Stopped and put some 93 octane in, but this didn't make a difference. Seemed to miss less with the choke full-on for whatever reason, especially since I have changed nothing in the carb for at least 20,000 miles. Pulled the #9 iridium sparkplug, which looked good color and wear wise,  but replaced it with a new one anyway. Also, opened the screw at the bottom of the carb with a paper towel underneath to see if there was any signs of dirt. Gas was clean. Don't expect that these measures will stop the miss, but will test later today and report back.

 

what i am wondering........... if this type of engine miss could be related to a failing CDI(maybe some cracked solder contacts) since I have 25,000 miles on an '02 and have never opened mine up to see for sure?

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i dont have the stock cdi any more but the cdi could be a suspect. i would suggest a possible vacuum leak between the carb and the boot or boot and head. using the choke made it run better. there is an o-ring in the boot to head connection that had gone flat, left it in there and put a bit of right stuff on the o-ring, reassembled, and the problem was gone. that missing was annoying!

ps i had a backfire once that coughed the carb slightly out of the adapter. it was hard to see, but would not start. epoxied the adapter on so it wouldnt happen again and made sure this connection was sealed up from leaks.

Edited by garthr

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Usually whacking the throttle doesn't smooth out a misfire that is caused by an ignition issue. I'm not saying it isn't possible, just not something I have seen.

The fact that you are running a pumper carb combined with the fact that gassing it smoothes things out leads me back to insufficient fuel supply. Considering you have a New tank, New fuel line and filter, that puts the issue in the carb.

I'm sure the acidity in lemon juice would do the trick but I just don't see myself boiling motorcycle parts in fruit juice. I know lots of guys swear by soaking them in pine-sol. I have always just taken them apart and gone to work with a tooth brush, carb cleaner and torch tip cleaners.

My recommendation for what its worth is to just leave the carb in the bike for now, pop out the jets and make sure nothing is plugged. That is assuming you can get at them with the carb on the bike.

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I installed the original coil and it ran better less of a miss. I still get a open reading on both coils thru the cap. I just ordered a oem coil and boot to see if I that is the problem. I've had the same miss with the stock CV carb and the FCR MX carb. I will keep everyone informed what I find!! Thank you all for the input.

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I just put the old coil back on with the new cap. I couldn't get a reading from coil to the end of the spark plug cap. But it runs a lot better. What is the rev limiter at? I seem to be able to hit it pretty easy with the FCR installed. I also have a wiesco 102.4 10.5 compression piston. Hot cam. Installed.

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glad you're getting things sorted out..........got mine running better also. I guess the plug was shot even though it looked healthy. Runs strong once again.

 

Rev limiter, i don't know of one...... could be wrong though. Maybe you're just floating the valves.

 

Oh, did you remove the snorkel from the top of the airbox under the seat? Definitely runs stronger, although, if I remove the left sidecover, it won't even run. Strange shit!

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As Natey mentioned, have you tried the choke or richener circuit to see what difference it makes?  Wierd fuel/spark issues often present similar symptoms.......

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What is the rev limiter at?

 

Higher than you would ever need to spin an RFVC that didn't have very major work. all the reports I have seen are 7200-7500rpm. I've had my 600 up into that range and it never hit a rev limiter, but the engine was far beyond the power curve at that point.

 

Have you checked the entire ignition system? How much resistance is the pulse generator showing at operating temp? The fact that your misfire changes with different coils tells me that the carburetor, if it is in fact causing issues, is only a secondary problem right now.

 

if I remove the left sidecover, it won't even run. Strange shit!

 

That sounds like some pretty serious restriction in the intake tract when the cover is installed.

 

Restriction upstream of the carb reduces air density and increases pull on fuel through the carb which ultimately richens the mixture. That's why airbox and filter mods necessitate fatter jets; to get the extra fuel with less vacuum pulling on it.

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That sounds like some pretty serious restriction in the intake tract when the cover is installed.

 

Restriction upstream of the carb reduces air density and increases pull on fuel through the carb which ultimately richens the mixture. That's why airbox and filter mods necessitate fatter jets; to get the extra fuel with less vacuum pulling on it.

don't want to hijack this thread, but i'm now in over my head on this one. Now the bike won't pull out until it's run for 5 min. Choke doesn't work like before when I could just start it up and go with choke on. So now I got two problems, the choke and air intake. 

 

I'll see if i can do some reading to figure these  out.

 

thanks for the input.

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Restriction upstream of the carb reduces air density and increases pull on fuel through the carb which ultimately richens the mixture. That's why airbox and filter mods necessitate fatter jets; to get the extra fuel with less vacuum pulling on it.

 

In a roundabout way I was saying that if the bike will run with the airbox cover installed, but not when it's completely open, then the mixture could be/is probably very lean.

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My air intake is new w/o any leaks. It runs a lot better after warmed up. The choke and hot start knobs work . After I get the new oem coil and cap next week. I'll let you guys know what I find. Since my bike was a basket case from the PO. I now get to rebuild the rear brakes!

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I sacrificed the old spark plug boot to see what and how the connection was made from the plug wire and boot. There is what looks like a copper wood screw that the plug wire screws onto the cap.

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