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Discussion Starter #1
My little brother owns a 91 indy 400, pretty nice little quick sled when it's running right. Here's the problem, we put brand new plugs in and it seems to run fine for 5-15 minutes it will then foul a plug out on one side, the same side every time. We put fresh plugs in again and the same thing will happen. You guys have any opinions on what could be wrong? One polaris owner my dad talked to said he had the same problem on an older indy 500, this guy said he grounded the motor to the frame and the problem went away. What do you guys think?
 

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Check the cap at the end of the plug wire. If it is not moulded on it should unscrew. Cut a little piece off and screw back on. It may have a bad connection. Also make sure that when you put the cap on the plug it fits real tight. If not replace.
 

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On the last trip we were on we were trying to eliminate things that could be wrong, it just so happens that that cap on the bad side was cracked so we bought a new one and screwed her back on. I rode her around for a while and it felt like she was back, then gave her over to my brother sure enough 5 min. later same S##t.
 

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If this was my sled the next thing I would try is pull both plugs. Have someone pull the rope and look to see if both plugs have the same color spark. They should be a bright blue color. If they are the same color I would look at carburators. Let me know and I can tell you what I would look at next. I'm bored in W. Mass and don't have much else to do, but share what little I know.
 

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It could be the fuel pump. The diaphragm on the pulse side of the pump can rupture and raw gas can flow directly into the cylinder. If the cylinder that is always fouling is the one that the fuel pump is connected to then I would check the pump. I believe a 400 uses the pto cylinder for the fuel pump but I might be wrong. To check the pump pull the hose off of the crankcase and stick it in your mouth and suck on it. If you get a mouth full of gas it is bad. If you don't like the taste of gas there are other ways to check it for example by using a automotive hand held vacuum pump. Simple and cheap to fix if it is bad.

xltboy
 

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The fuel pump is brand new, we checked the spark of the plugs and it seemed to be fine, both were bright blue. The carbs were taken off,checked and were clean before it was ridden this year. No jets were clogged etc. If your getting a good spark from both sides it wouldn't be the cdi wuold it?
 

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Why don't you try this. Swap the carbs around. If the other cylinder fouls you will know it is in the carburator. You should also do a compression test. They should be within 10% of each other. Good luck.
 

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Thanks, we're going to try a bunch of stuff tonight, It'll be a nice night to work on it being it's 54f outside right now. Seems pointless to work on it because of the weather, no snow, warm. I'll let you guys know how it goes and let ya know if I could use anymore info.

Thanks alot, Steimy ;)
 

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your problem is in the carbs, the main jet in the one that is fouling, is too large, and it is getting to much gas for the amount of air, jet down one size in that carb, and check the plugs alot!!!! they should be gold, or chocalate brown, take it from me, dont lean it out, i make that mistake and didnt check my plugs enough and melted a mag side piston. Doest it do this at top end or midrange or idle. If midrange, try the small main jet, and a one size smaller pilot jet, if its the idle needle jet, but i can almost bet its the main and pilot, and clean the carbs real good while your at it, make sure not to loose any parts, and make sure there spotless!, its realy quit easy. Erik
 

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Did you check to make sure the carbs are synched? Also make sure the choke is set properly on each carb. You can also check to make sure the clip that holds the throttle cable on the carb slide is in place.I had one come off once and had the same problem.
 
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Back in the day I had a 1979 polaris cobra did the same exact things you would put new plugs in it and it would run like a champ but after a half hour it would foul them out and I went through everything on that sled carbs were rebuilt new wires everything....Stilll couldnt find the problem I had never had the sled rebuilt but my guess would be high milage time for a new sled!!!!
 
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