1984 XR350 starts and runs fine for 5-10 minutes then dies

[xr75kid]

Any help appreciated. I have a 84 xr350 I bought new. It has always ran fine even after sitting for a year or 2. I recently took it to a OHV park and it started right up , ran and rode fine for 5-10 minutes then would die. It would lose power on and off , sometimes backfire and then die. Starts back up after a few kicks then runs for a few minutes and dies again. I barely made it back to the truck. Brought it home and performed the below repairs on it and it does the same exact thing after riding it today.

 

Things I have done since I brought the bike back home from the OHV park:

 

Taken off the carbs (dual carb setup) and cleaned the inside of the bowls,  the main and pilot jets and blew compressed air through most of the carbs orifices that I could get to-did not separate and totally dismantle the carbs.Also cleaned the outside of the carbs as much as I could without separating them.

 

Taken off the intake manifold and re-installed. Did notice the 2 O-rings that go between the head and intake manifold were nearly flat but did not replace them-but thinking of that as an option now-will need to order those if I go that route. 

 

Adjusted the valves.

 

Cleaned and re-oiled the K&N air filter with K&N air filter spray.

 

Drained the fuel in the tank and replaced with fresh fuel.

 

Petcock was also replaced with a good used one-original one started leaking.

 

Gas flows freely out of the overflow tube on the gas cap.

 

Stock header pipe was removed re-installed. New exhaust gaskets were not installed-originals were still in head and reused. Does not appear to leak.

 

Bike has a aftermarket DG RCM alloy slip on muffler on it connected to the stock header pipe and a K&N air filter with stock jetting-Have run this same exhaust/air filter for years with no problem so I never felt the need to change the jetting.

 

Have not removed the spark plug yet to examine it. Will take a special thin wall socket to do this. Have not located one yet.

 

I set the air fuel mixture screw first at 1 1/2 turns out after I cleaned the carbs then again today at 2 turns out- I just got a factory Honda manual and manual says 1 3/4 turns. Thought I would go a little more and see if it helped-but didn't.

 

I research on line what I could find on the problem and still haven't found the fix yet. I did read that the plate on the side of one of the carbs held on by 2 screws sometimes gets filled with gunk , checked that today and its clean inside.

 

Again thanks for any advice/help. Hoping there is a expert on these 83-84 dual carb xr's out there that has seen this problem before.

[patuca]

Xr, this sounds like a typical gas tank vent problem....Gas will flow freely until a vacuum builds up in the tank and slows the flow,  With a partially blocked gas cap vent it would flow okay at at first when you test it....but after a few minutes running it would stall.   When the bike is stopped the tank slowly looses the vacuum and is normal again......Try it with the cap loose before you tear it apart.

 

patuca

Edited March 4, 2014 by patuca

[ohio rusty spokes]

I agree with the above. Also check that the float bowl vent lines are clear.

[ickfinger]

I seem to recall mid 80's xr's having heat related ignition isuues/failures. Before pulling apart the fussy dual carb setup, I would check the spark when the bike acts up.

[knudsen]

Get that wrench and get a new plug in it. So cheap and easy, its usually my first troubleshooting step even when it doesn't seem likely.

Maybe a bad coil.

[xr75kid]

Thanks for the ideas. I pulled the overflow tubes and 1 was clogged. Unclogged it and thought that was it...but it wasn't. I also loosened the gas cap and blew through the gas cap overflow tube. After it died it would start right back up. When it died I started to loosen the drain screw on the bottom of one of the carbs to see if there was gas in the fuel bowl which there was and gas came out of one of the overflow tubes and ran onto the ground. This was a first. Think I will be pulling the carbs (again) to disassemble and inspect .

Edited March 4, 2014 by xr75kid

[cynicryder]

just went through this last fall.. I ended up replacing CDI with used Ebay one, new coil/wire, new plug, new needle valves, cleaned seats, new floats and it now runs again. it ended up being a combination of problems, but I think mostly ignition (intermittent weak spark). it was difficult to diagnose, since it wasn't one specific thing. it ran fine the day before an enduro, and when I started it next morning, it would run for a few minutes and then die, like you describe.

[xr75kid]

Well, today I turned the gas on and it started coming out of the primary left side carb overflow tube again. This is the same overflow tube that was blocked previously. I spent a couple hours removing both carbs, cleaning everything, checked the float level and reassembled. When I removed the carbs I noticed the intake manifold on the primary carb side had excessive gas in it as did the air box on the primary side. Just threw the tank back on and hooked up the gas line , turned on the gas and same thing...overflowing through the primary overflow tube. What in the carb is causing this ? Anybody know ?

[cynicryder]

Well, today I turned the gas on and it started coming out of the primary left side carb overflow tube again. This is the same overflow tube that was blocked previously. I spent a couple hours removing both carbs, cleaning everything, checked the float level and reassembled. When I removed the carbs I noticed the intake manifold on the primary carb side had excessive gas in it as did the air box on the primary side. Just threw the tank back on and hooked up the gas line , turned on the gas and same thing...overflowing through the primary overflow tube. What in the carb is causing this ? Anybody know ?

it could be the float needle valve and/or seat or the float, assuming it was adjusted right at one time...

 

Edit:

 

the needle valve has some rubbery coating on it that gets old. also the float sometimes degrade ans start absorbing gas. also, the seat could have some of the old gas gunk in it.

Edited March 5, 2014 by cynicryder

[xr75kid]

Thanks again, I will pull them back off except this time im thinking there is no sense in mounting them on the bike, may as well just set up the tank and carbs somewhere and try it like that. I will inspect the needle valve , pin, seat and float and see if I can figure something out.

[cynicryder]

Thanks again, I will pull them back off except this time im thinking there is no sense in mounting them on the bike, may as well just set up the tank and carbs somewhere and try it like that. I will inspect the needle valve , pin, seat and float and see if I can figure something out.

yes, I usually test them on the bench to make sure the float level is good. it's a pain to get the bowls off on the bike.

Edited March 5, 2014 by cynicryder

[xr75kid]

Got the gas to stop coming from the overflow tube. I ended up taking the carbs back off again and removing the float bowls on both carbs and comparing their condition. They both looked very clean and in great shape. Couldn't see anything wrong with any of the parts inside so I figured I would switch the float , needle and pin from one carb to the other. It quit leaking. Started it and it ran great for a good while but died again. Found my original Honda spark plug wrench and removed the plug and it looked great , nice tan color. Guess I will be looking at replacing the sparkplug,coil and cdi now.

[717 MOTORSPORTS]

KISS. Keep it simple stupid. Remember that. Start with a plug change. Then the cap (resistor could be going bad) Check wiring for breaks, cracks, dirty or loose connections. Check grounds. Test coils, and so on down the line. In my experience CDI boxes don't have intermittent problems. The back firing tells me you are momentarily losing spark, then when the plug re-fires, it ignites the excess fuel in the pipe causing a backfire. I have seen many spark plugs do that

[xr75kid]

I may have fixed it. I started checking connections and I pulled apart the coil and the cdi connectors and they looked like new so I figured I was at another dead end. Then I decided to check the ground which is under the tank and has not been off since new. Found the ground to be heavily rusted. Cleaned it up and started it and it ran longer than before and I finally killed it manually . Will ride it tomorrow and see what happens. See before and after photos.

[cynicryder]

I may have fixed it. I started checking connections and I pulled apart the coil and the cdi connectors and they looked like new so I figured I was at another dead end. Then I decided to check the ground which is under the tank and has not been off since new. Found the ground to be heavily rusted. Cleaned it up and started it and it ran longer than before and I finally killed it manually . Will ride it tomorrow and see what happens. See before and after photos.

good find.

[knudsen]

Yup, that will do it. Some old coils had no ground and relied on the coil core contacting the frame. Those fail until you add a wire.

[xr75kid]

Rode it yesterday. Started right up. Ran great. Was able to hold it wide twice. Ran better than ever it seemed like. The third time I held it wide as soon as I let off the throttle it backfired and died then started right back up.

 

I noticed when I took the carbs apart , the one with the diaphragm that hadn't seen the day of light since new had some tiny imperfections in the diaphragm but they didn't look bad enough to warrant replacing and I didn't have another one. What exactly does that diaphragm do ? This could be another dead end Im just reaching here.

 

Spark plug is next.

Maybe replacing the resistor in the cap with a piece of wire ? Is that wise ? I read those go out sometimes.

Trying to do the inexpensive things now as Im currently unemployed.

 

Wish I knew someone with a similar bike, I could send them my coil and cdi and they could try it on their bike to see what happens.

[cynicryder]

Rode it yesterday. Started right up. Ran great. Was able to hold it wide twice. Ran better than ever it seemed like. The third time I held it wide as soon as I let off the throttle it backfired and died then started right back up.

 

I noticed when I took the carbs apart , the one with the diaphragm that hadn't seen the day of light since new had some tiny imperfections in the diaphragm but they didn't look bad enough to warrant replacing and I didn't have another one. What exactly does that diaphragm do ? This could be another dead end Im just reaching here.

 

Spark plug is next.

Maybe replacing the resistor in the cap with a piece of wire ? Is that wise ? I read those go out sometimes.

Trying to do the inexpensive things now as Im currently unemployed.

 

Wish I knew someone with a similar bike, I could send them my coil and cdi and they could try it on their bike to see what happens.

I think the diaphragm you're referring to, is part of the anti backfiring valve. when you shut off the throttle, the carburetor will become really rich for a short while, because of the difference in mass between fuel and air, causing unburned fuel to enter the exhaust, which will then ignite and cause the backfire. the anti-backfire-circuit, senses this situation (high vaccuum) and lean outs the carburetor, to prevent it. if the diaphragm is OK, it could just be the little valve seat behind it, or a missing spring, or not sealing correctly on the cover, are the vacuum port being clogged.

Edited March 7, 2014 by cynicryder

[xr75kid]

Thanks for the reply. The spring is there. There were 3 or 4 spots on that diaphragm that looked almost like a tear but I couldn't confirm that because they were so tiny. The carbs have been blown out with compressed air. I think Im getting closer but not there just yet. Its riding weather now here in Nor Cal and the bike is calling me to fix it so I can ride.

[cynicryder]

Thanks for the reply. The spring is there. There were 3 or 4 spots on that diaphragm that looked almost like a tear but I couldn't confirm that because they were so tiny. The carbs have been blown out with compressed air. I think Im getting closer but not there just yet. Its riding weather now here in Nor Cal and the bike is calling me to fix it so I can ride.

where are you located ? I'm in sacramento.