Keihin CR Flat Carburetor issues

[Wicked Al]

OK Geniuses, I need some help on this one.

While racing up at Snowflake and doing oh so well, my carb started giving me issues.

I thought it might be fuel delivery issues as my quad is set up as gravity feed and the fuel line went up and over the motor and I didn't have any issues until 40 miles into the race at less than half a tank. The reason I don't think it's fuel issues prior to the carb is after the engine died, I was able to start it by pulling my choke out and the bike ran just fine, but the choke doesn't stay out on its own so I was continually pulling it out and then that didn't even work. After pulling over for an extended period of time I rerouted the fuel line around the engine so I could be sure that wasn't an issue. The quad started right up and I took off. As I came into the pit area the quad died again, I think that may have just been out of fuel...not sure. When a generous racer came over with a fuel jug, they splashed about a gallon or so into the bike, even though it started right up, it still sputtered and ran rough until I got to my pit. At my pit I topped it off and ran fine for the next 25 miles then I started getting sputtering again, usually going uphill, on the face of jumps (waterbars) or at top speed.

I had similar issues when I pre-ran the course a couple weeks earlier, before the race I had the carb on the table with all the jets out, cleaned and put back. I also installed a new inline fuel filter.

Ideas?

Thanks!

Alex

[jdwrigh]

That's a tough one since it appears to run fine for a while. I'm no genius but maybe a float issue.

You said you cleaned it, does that include all the vent hoses? One time I "cleaned" my carb by just spraying everything and it still didn't get all the gummed up gas out until it was physically scrubbed out but it ran poor all the time. It's also weird that your choke doesn't stay out.

Could it be heat related? I've never personally heard of an ATV vapor locking, but it might be possible.

Stock gas tank right?

I'm just grasping at straws but I'll ponder it some more and see what I come up with. If you have any more pertinent information it might help narrow it down. I'm assuming the engine and ignition system is in good shape as well.

[Wicked Al]

Thanks for the ideas Jeff. Here is a page with tons of good and confusing info on how to tune a carb. http://www.factorypro.com/tech/carbkei.html

I went ahead and took the carb apart again, all of the jets were clear. The aforementioned page however recommends blasting everything (all passageways) with carb cleaner and then an air gun to ensure all is good prior to trying to tune the carb. I did not do that so i guess that'll be on my list for tomorrow. I was careful in opening up the carb, I wanted to see if there was any debris in the bowl, looks like there was a little bit.

On my YFZ carb (different year of an FCR) it has a jet in the bowl for the throttle pump, the one in my carb is a slug, that's right, a jet size of "0". I went ahead and swapped jets so now it has a 50 jet in the bowl.

I did notice the vent tube from the crankcase had a lot of residue on the end that attached to the carb, there was no clamp on that end, I think I'll go ahead and put a clamp on it.

I thought that maybe a vacuum was being created in the tank as there is no vent tube on the gas cap, then I remembered there is a vent tube on top of the tank, I'll verify it isn't being pinched.

I measured the gas still in the tank after doing my last lap, more than half a tank, so that shouldn't be an issue. I also looked carefully at the petcock level and the bottom of the tank, should be no problem feeding the carb.

The inline filter is for 3/16 lines and I have 1/4 lines. I have small hose clamps, so it shouldn't be sucking air, but I'll get the right filter tomorrow just to make sure there is no air getting sucked into it.

Other than a thorough cleaning, I'm not really sure what else I can do. I spoke to Pro-ATV today and they said it was gravity feed when Jeff ran it, so I should be good to go!

Yup, stock gas tank. After I moved the fuel line to the side of the engine, it shouldn't be getting very warm so I don't think it's vapor lock.

My choke has a pretty good spring return though it also "clicks" into place when I pull it out. Maybe the spring in conjunction with the vibration is enough to get it to go back to normal.

I'll tackle it again to tomorrow and see what I can do.

Thanks!

Alex

[RyGuyRacing]

I would check and make sure the fuel line isn't being pinched by the gas tank or something. I would make sure the vacuum line on the gas cap isn't pinched also. Sounds like for some reason if it only ran with the choke on that it wasn't getting enough fuel maybe for that elevation??? Maybe you need to lower the clip on the needle to richen it up? I guess a good question would be what gear were you in and what speed were you going and was it WOT or mid range throttle and you can figure out what circuit needs to be adjusted

[EPDP99]

We had an issue with ours kinda doing similar things and tried tons of jetting changes with no consistancy in changes from same jets to same jets after we tried other jets. We found it to be the coil wire was not making a good enough connection and would sputter at times. We were told to put a set screw in the carb where you talk about your acc pump jet. We are going to put the actual jet back in because it gets really rich and is hard to start after putting the set screw in. Basically all the fuel went to the acc pump and none got spewed back into the bowl. Might be your issue but not sure.

[Wicked Al]

Hmmmmmm.....thanks for the input and your experience, it's great to get different ideas to investigate. Quite frankly the coil makes more sense as there were times when the sputtering got so bad it killed the engine. But then why would it start up with the choke on and not without it? I pulled the plug and it was dark, but not black and moist so I'm guessing the mixture wasn't too rich, and if it was too rich the choke would have made it worse???? I went ahead and blasted every orifice with carb cleaner and followed that up with the air gun. We'll see where that gets me. I guess the next step is to go to the track and run it with half a tank of fuel or less in it and see what happens, hopefully nothing. I ordered an oring and washer for the slow jet screw, but I haven't had any problems at low RPM's as far as response is concerned and I have been running it without the oring and washer all this time and for the most part it has been running great.

I guess there's nothing to it now except to go ahead and run it and pay attention to the conditions if it acts up again

Alex

[EPDP99]

Id also looks for air leaks between the carb and headmaybe you had/have a leak and it needs the lack of air through the carb to start. Get it running and spray carb cleaner at all the connections and if it revs up or bogs when you spray it you have a leak(if it is steadily idling). Then try getting the leak secured and see how it is. I'd try a new plug as it sounds like it might be fouling out even though it isnt wet. Plug should be a light chocolate brown, not dark and def. not yellow/white. Also the place to check it is deep on the ceramic inside the end of the plug not nec. right on the tip. Then you will have to try it at low, med, full throttle with new plugs to tell where the jetting needs to be adjusted otherwise if you ride it and see where it is while in mutiple throttle ranges you dont know exactly where to adjust. Run it at whatever throttle you need then shut it off while at that throttle position and coast so you can check it. Hope you get it! Good luck.