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Discussion Starter #1
Im sure this has been answered before, but Im new to the forum and cant track it down. I just bought and old 1986 350 fourtrax 4x4 and am trying to restore it. I rebuilt the carb, and it cranks up after just a few turns and runs great. Problem is it smokes like the mosquito truck. The compression on it is a steady 90 psi. I do not have a service maual, so I do not know what is normal. Does anyone have this information? Im thinking it is possibly the valve seals are leaking but not sure. And also it seems to not smoke with initial crank up but after about 1 minute it starts smoking, and it really smokes more when you open up the throttle to rev the engine. I did see the post from someone earlier on this same model, but they never mentioned actual compression readings I should be looking for. Thanks adhead of time TDC
 

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Welcome to the site. I'm not sure what the compression should be but the valve seals are a definite possibility. You could also take the exhaust pipe off the motor and see if there is any oil built up in the head. There should be a way to take the end cap off the silencer and the packing material is most likely soaked in oil. As soon as it starts heating up it is burning the oil.
 

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I would say that much smoke you will need a complete overhaul, imo.
 

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Welcome to the Forums!! PM Pain he had a quad around that year and model and was looking to get rid of it and he had a manual for sale. He may still have the Manual. Good Luck.
 

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Most likely valve seals, if your exhaust valve is not seating properly...

Start with the cheap stuff.
 

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Discussion Starter #7
Quad Racer your most likely right, I was just hoping. I still want to find out what compression reading im looking for...90 psi seems pretty good
 

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If you have a way to put air in the cylinder with the piston at top dead center you can listen to were the air is coming from . If you hear it from the exhaust or intake its the valves or if the air is coming from the engin its the rings.but if the valve seals are worn that will let oil into the combustion chamber.
 

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it's probably piston rings, you might get away with just a new set but if you're going that far into the engine, I'd do a complete oversize piston. A new fourstroke should be about 125-150 psi for that style of engine... under 75 psi usually won't even run so that gives you an idea how tired your engine is. the 86 model had a hotter cam than later years, so if it is an original 86, it's worth fixing IMO. That's an old atv introduction trick, put lots of power in the first year then detune it after everyone is impressed with it. suzuki did that to the 250 4wd in 87 as well FYI....... to find out if it's the rings, add some oil to the spark plug hole and do the compression test again, it'll seal worn rings enough to do a wet test.
 

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Discussion Starter #10
Scooter, I appreciate the info and that pretty much makes up my mind. I saw on Ebay a company offering to take my cylinder, bead blast and bore to next oversize, Provide new 10.24 to 1 piston, rings, wrist pin, cir-clips, top end gaskets including valve seals for $200. Im skeptical of Ebay, but the guy has 99.7% rating for almost 20k reviews. Does that seem like a pretty fair price.

A friend of mine had mentioned the compressed air and oil test, I got to the compressed air test and from what I could tell, the valves were sealing off as they should. I got called away before I could do the oil test, Im going to try that this afternoon.

And yes, even with bad compression the engine feels strong, I have a prairie 650 with performance chip, and I think the Honda in the SL 1st gear and a rebuilt engine it might give it a good pull.

Im probably going to break some Honda Lovers hearts, but I purchased this machine for parts in order to build a 4x4 mini 2 seater buggy for my daughter. I picked it up for $300 off Craigs List, and beside plastics being shot, missing front break Master Cyl. and the tube between air box and carb it is in good shape. I did a carb rebuild, and beside the smoke, it will crank on the second turn. I Plan on using the complete rear end, extending the width to 50". The front diff is in good shape, but I dont want to use the strait axle, so Im going to try and find and use a Honda 300 4x4 front axle, diff, and possibly, a arms. I of course plan on upgrading the shock all the way around.
 

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Discussion Starter #11
Scooter, performed the wet test and it pumped up the compression to almost 120 psi, Ill be doing an overhaul on this one, and yes I think the engine is worth it, especially for what I want to use it for. I need to get my hands on a manual before I tackle this though.
 

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Yeah, just sold that same bike and also sold the manual I had for it.

I do still have a few parts if you find you need something. I have the right front half shaft and brake drum with cylinders. It's a complete right side from a whole front end that I got off ebay, but only needed the left side.

I also have the left brake drum with one good cylinder and one leaking cylinder, so you could salvage one of them in a pinch.

Good luck, that machine is bulletproof! Unfortunately, finding parts is hit or miss and ebay is your best bet for a lot of them.
 
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