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Discussion Starter #1
My son has an 05 Predator 90 that has started running poorly. It starts and idles fine but cuts out and backfires under load.

I have cleaned the carb thoroughly, drained the tank and installed fresh gas, installed a new fuel filter, new air filter and replaced the spark plug. But, it still runs poorly, maybe even worse. The plug color (very light tan) is indicating that it is running lean, but still seems to produces a fair amount of white smoke. Lately, the oil light started coming on and off with the oil tank full which it has never done before, is this a concern?

Lastly, I checked the compression and it's only 70 PSI! So, I am assuming it's time for a new top end? Before I get into that I wanted to get some advice here to see if I'm missing anything in my diagnosis. I would also like your advice on removing the oil injection system and stock airbox. We run 32:1 Amsoil Interceptor mix in our dirtbikes, so it would be easier for us to just run that in the atv as well.

Any words of wisdom will be greatly appreciated! Thank you!
 

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Welcome to the board!:med:
 

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70 psi is definitely gonna require new rings, at minimum. If I remember right, compression should be at least 120. Even though you cleaned the carb, it sounds like it is still clogged. It doesn't take much to screw them up. It might almost be worth it to buy a new one. They're cheap, and not worth the aggravation.

I'm all for removing the oil injection. The oil injection went out on our 50 and cooked the top end. I replaced the top end, disconnected the oil lines, removed the tank, and capped off the oil port on the carb. We run 40:1 mix and still have the light tan plug. I certainly don't think you can hurt it with 32:1.

What plans you got for the airbox ? I've been trying to figure out something for the 50.
 

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Discussion Starter #4 (Edited)
If figured that was the case, the service manual calls for 90-110 psi. Nonetheless, 70 psi is not going to cut it. I'm not so sure I wasn't having a problem with the oil injection system anyway, it would smoke like normal until you revved it up, then no smoke. I don't know if that means anything or not, but I just got finished taking the injection system off, IMO it's just something else that can go wrong and toast your top end. Just for kicks I am going to check the compression again with the pre-mix, just in case it wasn't get enough oil and I was wrong about the mix we use, we do run 40:1.

I cleaned the crap out of that carb twice. Carb cleaner followed by air. All the passage ways and jets seem to be clear and I checked the float level, but I have no idea if the float needle and/or seat are actually good. I took your advice and ordered a new carb tonight.

As far as the air filter goes, I'm going to try a clamp on Unifilter with Outerwear and I'm going to try relocate it under the seat where the oil tank was. I have a friend with a pipe bender, I'm going to see if he can bend me a piece of 1 1/2" aluminum tubing that will look slick! I'll connect the tube to the carb with a rubber Fenco clamp and I'll fab a bracket using the old lower oil tank bracket to secure the end with the filter. Looks like it will work to me and it will fill up the open space under the seat where the oil tank was.
 

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As far as the air filter goes, I'm going to try a clamp on Unifilter with Outerwear and I'm going to try relocate it under the seat where the oil tank was. I have a friend with a pipe bender, I'm going to see if he can bend me a piece of 1 1/2" aluminum tubing that will look slick! I'll connect the tube to the carb with a rubber Fenco clamp and I'll fab a bracket using the old lower oil tank bracket to secure the end with the filter. Looks like it will work to me and it will fill up the open space under the seat where the oil tank was.
That does sound pretty slick. Make sure you take some pics. I'd like to see how it turns out. Heck, I might even get your buddy to make me one for the 50, lol.

BTW: What part of Va. you in?
 

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Did you do the compression test at WOT with 4-5 good pulls of the recoil or at least as many revolutions with the electric start? At 70 there's no way it would even start. I am thinking you still have a carb problem. Where is your mixture screw set? Sounds like it's lean. Also hard to check the plug unless you can do it at the proper throttle position held there for aprox 20 seconds to get the plug to start to change color. Also agree on getting rid of the oil injection on them.
 

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Discussion Starter #8 (Edited)
Thank you, that's why I asked before tearing into the top end. Yes I checked the compression at WOT and turned it over for about 5-10 seconds each time. The air screw was at 1 turn out, I tried adjusting it and went as far as 2 turns out but lost throttle response and it started backfiring when I revved it up. Now it's back to 1 turn out.

I started to think it wasn't getting enough oil at higher rpm's, so I removed the injection system last night and ordered a new carb. The previous owner had gooped silicone all over the air box to try and seal it I guess and when I took the air box apart I saw that he had gooped more silicone on the inside around the rubber boot going to the carb. The silicone was flaking off so who knows how much was sucked into the carb. I removed the air box because it was such a mess and fabbed some pipe from the carb to a location under the seat where the injection pump was. I will attach a clamp-on uni-filter onto the pipe. Once I get the new carb on, the new air filter on and fill it up with 40:1 Amsoil premix I'm going to check the compression again. If all else fails I"ll pull the top end and see what it looks like.

Lastly, and this may be a dumb question, but is it ok to run premixed fuel through an inline fuel filter?
 

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first thing check and clean the exhaust pipe. if it's plugged with carbon the bike won't run right.
 

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It's fine to run mixed gas through a filter. No problems there. Turning the screw out is leaner, in is richer on that machine. Removing the air box and going to a pod filter is going to require you jet up dramatically from stock in main and probably pilot as well. Going to be a learning curve there. You can do a compression test without the carb on and get nearly identical results as with it. I'd suggest you try another gauge as 70psi is not enough to run at all. I've seen some machines start at 90 with a lot of work to get them going, but need 100 min.
 

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Discussion Starter #13
I'll have a few jets on hand when the filter gets here. I've been racing dirtbikes for a long time, so I would say that I'm some what competent at figuring out the jetting but I'm not an expert. I'll do some searching and see if I can find someone with a similar set-up to get me close on the carb settings... Thank you!
 

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If it were me, I wouldn't bother messing with anything else until you verify the compression is in fact only 70 psi.
 

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Discussion Starter #16 (Edited)
OK I guess I didn't know how to properly use a compression gauge. Just checked the compression again using the kick starter, after several kicks and with fresh pre-mix in the system the compression now reads just shy of 95 psi?
 

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Discussion Starter #18 (Edited)
I found this reply on another atv site. The original question was about compression readings relating to a poor running Predator 90 with 95 psi compression.

This guy works on hundreds of stock and racing mini-quads a year and here is his reply.

"Your clutch rollers are flat spotted...guarantee it.....95 psi is as good as it gets on a stock predator, even new the bike didn't have any more compression than that"
 

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Does it bog if you remove the belt off the clutches?
 

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Discussion Starter #20
Have not tried that. This whole CVT thing is new to me, unfortunately I don't know much about how it works...
 
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