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300 Fourtrax - Spark Plug Heat Range?

12263 Views 23 Replies 6 Participants Last post by  rottenscoundrel
Hey guys. I bought a 94' 300 4x4 last April. I just got to see it for the first time a week ago. I moved from Alberta to New Brunswick.

So my bike has a Highlifter 355cc big bore kit, web cam, clutch kit, white brothers stainless steel exhaust, 2" lift, a warn 2x4 kit, 124 main jet ( I believe ).

So I have a few questions, I want to change the spark plug and I'm going to change all the fluids. The original owner ran full synthetic oil with lucas additive. He said when it's cold out you had to let the bike warm up or else the clutch would slip. NO good for me. I'm going to run conventenal oil. BUT my question is... should I change the heat range of the plug considering it has the big bore kit?? He has the stock plug in the bike and says it runs hotter with the kit?!?!

I need to replace the front shocks and the front wheel bearings too. I bought all ball wheel bearings.

Now that I'm in my new house and hopefully get to get out on the bike I'll be on here a lot more.

I know for sure I want the detroit gearless locker and 12" rims so I can install disk brakes. And yes I'll research everything before posting as most stuff is already covered. I couldn't seem to find much on heat range of plugs though.

I'll get some pics of the bike later this week and post em up.

Thanks for any advice.
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I agree with you on that! If you run it wide open in 3rd or 4th then shut er down, pull the plug it will tell you how its running.

Now with the ss exhaust I have...... I ran the bike wide open down a main road tonight, and it got cherry red!!!!!!!! I don't think this is normal.... I do have exhaust heat wrap I can install....

tomorrow I'll run the bike in 3rd wide open then shut er down and see what the plug is like... But I is it normal for a ss exhaust to get cherry red??
ahh yeah......... it was red past the first bend right back to where it tucks under the fender!!!!!!! He told me it has the largest jet it can take. I 124 I think he said.
Ok, I'll look into the size of jet he has in it. I'll call him and ask him again. As far as it being red that far back I think it's to hot myself. I'll do some more research and see what I can find out.

Hopefully someone on here may have the 355 big bore kit and can chime in.........
well I just fired the bike up to put it in the garage. It's dark out seeing how it's late at night and just idling it gets red up to the first bend. bike runs great and fires up good. hopefully all is good.
Yup, sure enough the when I removed the plug today, it was white/grey. The plug that was in it was the factory DPR8. I'm going to change the oil & filter, and change the plug out to a DPR9. I'll double check all the settings on the carb as well.

I know for higher speed riding you want a cooler plug. I may buy a heat range 10 and 11 as well. I will be doing a lot of trail riding but logging roads as well ( where we fly )
Ok, I've been doing a lot of reading on how carbs work and how to set/tune them.

I understand now that idle to 1/8 is controled by the pilot jet ( slow jet ) and the screw should be backed out 2 1/4 turns for a 94. IF it backs out more than 3 I need to up size the jet.

The needle jet controls the mid range. I understand that you can richen or lean it out buy adjusting the c-clip. You can also change the needles for size.

The main jet controls the wot and top end. From my understanding it will also play a part with the overall function of the carb as well.

My pilot screw is backed out 3 turns. The needle c-clip is on the lowest setting, and the main jet is a 132. ( I found the paper work for the stage 1 kit he installed. )

So I have a DPR9 plug. The bike runs great, idles great, pulls great through out the whole throttle range. BUT the plug is still white in colour. So I do believe I need to increase the pilot jet, and the main jet, and get a smaller needle.

Does this make good sense??

I understand the only way to know for sure is to pull it apart and change the jets then go from there.......

any info would be greatly appreciated.
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Ok, I ended up changing the main jet from a 132 to a 142.

I ordered a new vacuum cup because I found a crack in the old one.

I still am running the heat range 9 plug. I have the pilot screw out about 2 3/4 turns. I set the needle second from the bottom. I fired it up and it drove good. I then took it out on the road and gaver.... I could tell it just wasn't 100% kinda like it had more power but it wasn't there.

I pulled the plug and it was black around the first thread, and the tip was hard to tell.... looked a light white/grey. So I raised the c-clip one higher. What a difference. Bike ran amazing...... I took it out for a really good rip out on the ***** and main road. I then came back to my place through the woods on a small narrow trail. You have to ride in second gear. This is when the hicup started. The bike in second gear was like bogging kinda...... I had to open the throttle up more and then the bike just roared, but way to fast for the trail. The throttle was prolly in around the 1/4 position.

This would tell me the pilot screw should be turned in correct? Maybe too much fuel on the first circut?! As I pulled into my back yard I hit it hard and killed it. I'm waiting for it to cool down to check the plug. I cleaned it all up before I took it out.
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I was thinking the same thing.

I just pulled the plug. It was black all around and the very top of the electrode was more a brown colour. So I now know I've made an improve ment and richened it up! So I'll try now to turn the screw in, and I may have to raise the c-clip again as well. It worked excellent in mid range though. I'll start with the turning the screw in and go from there.

Thanks for all the good info on this site. I've done a lot of reading on carbs, jetting and how they work. Great info.
Well I raised the c-clip again and turned the screw in from 2 3/4 to 2. I took the bike out for a drive and wow worked amazing, no spitting or sputtering smooth acceleration. I hit it hard and killed it...... Let it cool, checked the plug.... and it was close to a brown with a small hint of black. I'll have to take the bike out for a good run and check the plug. I'm pretty sure I'm close now.
Yeah, sounds like you're real close.

I do believe your right. I haven't been out on the bike for a good run yet. I'm heading out on the trails today with my brother in law.

I had the bike out last night around the yard and up the road. Worked great. At idle and low rpm the exhaust got a little red coming off the head. No big deal. As soon as I start driving and getting more air on the motor it it cools right down and red is gone!

So because I do a fair bit of wide open running I'm going to try out this heat range DPR9 plug. I'll pull the plug at the end of the day and see how it looks. I'll even post a pic.

So I wanted to thank everyone on here! This site is Awesome and has tonnes of great info!

I'll be sure to post up some pics of the bike when I get home!
too much air not enough fuel.

You are correct!
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