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YFM350 Moto 4 - Engine Rebuild Question

This is a discussion on YFM350 Moto 4 - Engine Rebuild Question within the Other Models/Brands forums, part of the Performance /Technical category; I have a 1987 YFM 350 Moto 4. Its old but in extremely good condition. Been running decently for years. but about a month ago, ...


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Old 09-15-2009, 12:37 PM   #1
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Default YFM350 Moto 4 - Engine Rebuild Question

I have a 1987 YFM 350 Moto 4. Its old but in extremely good condition. Been running decently for years. but about a month ago, it has some running issues and I found the Cam Chain Tensioner had cracked, so I got a new one, then I decided to get a new chain and while I was at do some more upgrades. Which leads to my inquiry.

My question is. the 4-Wheeler is 22 years old give or take. It's been burning oil lately and I am sure the power is lacking so what I need to know is. How should I go about honing the cylinder.
It has a 83.0 mm Bore x 64.5 mm Stroke according to spec.

I am not a large scale small engine person, but I do my share when necessary. This is just a new step for me. So I am trying to learn, but from reading multiple sites and forums, there are too many options...

resurface with new rings
resurface with rings and a piston
bore it out and go oversized rings
go with a big bore kit.
Just run some kind of miracle bore cleaner...

Its confusing. And from reading online, this forum is THE place to go for my questions so here I am...

If you need anything from me, be it pics, specs, more details. Just say and I will supply!

Thanks!
-TEK

PS: If any of you pro's have any suggestions for getting more horsepower, better response or increased performance by ALL means, please share!

Last edited by TEKNiK; 09-15-2009 at 01:09 PM.
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Old 09-15-2009, 08:28 PM   #2
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I'm not positive, but I believe your engine is almost if not the same as a warrior's. If you have the cash, bore it and install a higher compression piston. There are plenty to choose from. Lap the valves and replace the valve stem seals. Try going up a couple sizes on your main jet. Then drill a couple 1" holes in the airbox lid. If exhaust is stock, try to remove the baffle. If it's too loud, just drill some holes in it for more flow.

Last edited by 366warrior; 09-15-2009 at 08:33 PM.
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Old 09-16-2009, 11:21 AM   #3
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Quote: Originally Posted by 366warrior View Post
I'm not positive, but I believe your engine is almost if not the same as a warrior's. If you have the cash, bore it and install a higher compression piston. There are plenty to choose from. Lap the valves and replace the valve stem seals. Try going up a couple sizes on your main jet. Then drill a couple 1" holes in the airbox lid. If exhaust is stock, try to remove the baffle. If it's too loud, just drill some holes in it for more flow.
Well, the Engine is stock, so I will try the muffler mod when I get everything put back together. Curious, where does one drill the holes? anywhere or somewhere specific.
And I also believe it is an exact to the Warrior's. So that makes it easier.

Now, on the other things, could you treat me like an idiot...
Who would you recommend for a higher compression Piston? And when you say I should bore it, are you recommending boring it larger or just resurfacing?
What should I use to bore it out with. Or should I have someone else bore? I have been researching this, but I am not sure what to use to bore.

And on Laping valves, Stem seals, Main Jets and airbox lids.
Please excuse the fact that I am sounding like an absolute moron, but I am a n00b who's trying to learn and I have no clue how to do those things.

I know where the Valves are, and I believe I know how to lap. But Probably not... I don't want to come off as a jerk or unappreciative of your reply, but could I request a few pictures of the things you are referring to and maybe some short, curt, simple instructions on how to do some of the things you are recommending.

Heck, a list of parts to buy would be ultra cool. Because I could then try to figure it out by studying what you recommended. If I am too much a pain in the neck, I will attempt to figure it out myself.Google images helps alot.

And to be honest, if it ends up costing too much, I will just skip most of it. But I do atm have some spare cash so I will do as much as I can.

Thank you for the reply and I hope to hear back from you!
-Michael
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Old 09-16-2009, 12:53 PM   #4
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There is a bolt or rivet in silencer(muffler)at end on bottom. remove and then remove the baffle. it probably won't come out easily. spray w/PB blaster or heat w/torch or both but it should come out. drill holes anywhere you want(in baffle) to increase flow. might even want to trim with saw to open it up or leave it out(very loud and mean sounding) also take airfilter cover off and drill or cut a few 1" holes. This allows more air in, but then you need more fuel in(larger main jet in carb)

boring isn't usually needed unless cyl is out of round. a bore scope is needed to do this. boring and a larger piston will give more power. any machine shop can bore usually $50-$100. J&E makes killer piston kits. 10.5:1 will boost power. check out e bay. type in "350 warrior piston kit" and many choices should appear. if going the boring route, order piston and give to machine shop w/cyl(jug)

more later........

Last edited by 366warrior; 09-16-2009 at 05:19 PM. Reason: missing info
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Old 09-16-2009, 05:13 PM   #5
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parts list:

piston kit(j&e 10.5:1 compression) choose bore size($150)

top end gasket kit w/seals($30)

valve lapping cmpd($10)

main jets ($5 each) find stock size and go up 5, then 2.5 from there

repair manual($30)or download from sticky at top of warrior page for free

the book can walk you through everything step by step

check out this carb tuning site: Mikuni carburetor operation and tuning
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Old 09-17-2009, 09:41 AM   #6
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First order of business would be measuring that cylinder. The oil burning is most likely from the top end not worn rings. Unless the atv has obviously been flogged. Don't buy any parts until you measure the bore, just like with large scale stuff, if the cylinder is within specs there is no benefits to a .030 difference in bore size, just more cost. If the jug is in specs, get the head rebuilt, or put some valve seals in yourself.

If you want a little more spunk from the engine put a higher compression piston and Stage1 cam in it. I don't usually recommend a stage one, but its a utility that is geared different and has more transmission mass than the warrior.

If you don't worry about water in the airbox, cut the airbox lid up to allow more air in. Be careful the lid holds the filter in place after you are done cutting. As stated above grab some jets and get your jetting where it runs good.
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Old 09-17-2009, 02:12 PM   #7
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http://motors.shop.ebay.com/i.html?_...+Jet&_osacat=0

Would this constitute as the jet I would want?

And I think since I got the head off and I am taking the arm and piston off, I will take them to the local ATV guy to mic it. Then from there I will see about having him rebuild it or recommending something to me.
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Old 09-17-2009, 10:11 PM   #8
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not sure what carb is on your machine, but i do know the stock warrior main jet is a 145. remove carb. remove bowl(on bottom.)careful not to strip screw heads. you should see a brass piece with a small hole in it. it's about the diameter of a pencil. look closely for a number on it. that's the size. look for a main jet 5 #'s higher and 7.5 #'s higher. jets only cost a few dollars at a Yamaha dealer.
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