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YFM 250 Moto 4 Quad: backfire around 1500-2000 rpm


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Hi All

Great site, lots of good reading, and helpful members!

I have a 1990 YFM 2050 Moto 4 Quad with a new somewhat annoying backfire issue.

Right off of idle, somewhere in the 900-1000 RPM range it seems to backfire through the carb into the air box. This continues until it hits around 2000 RPM then it seems to subside, but still runs rough.

I've been playing around with different things, (see the list below). I went out to a local dealer showed them the same list and they said "Jeeze, your doing the right tests, and based on # 8 we really can't put a finger on your problem, but try the other things you have outlined and let us know what the result is".

Sigh....

So while I was at the Yamaha dealer I talked them into selling me a Yamaha service manual for the YFM250A, which I prefer over the aftermarket manuals. The upside here is they sold it to me for a smoking deal, not much more than the aftermarket manual price!

Here is what I've done (#1 to 13) and have left to try (#14 to 21):

1. Spark Plug: clean and gap, replaced with new and gap
2. air cleaner: clean and replace, remove and test
3. Carb: Clean twice, all jets removed, carb cleaner, air blast and .005 tungsten wire clean.
4. Carb: fuel level check,
5. Carb: mixture screw, 1.5 1.75 2.0 2.5 turns out
6. Throttle cable: free up adjusters, adjust and lubricate
7. Adjust valves: .003" intake .005" exhaust
8. Swap carburetor for one from working YFM250 moto 4 identical to mine... same backfire problem ([email protected]#...argh!!)
9. Check Valve timing: Cam sprocket "^" mark and crankshaft "T" mark aligned... perfectly
10. Check timing chain tension: Good
11. Remove gas cap
12. Check gas shutoff valve flows properly
13. Check Compression: 135 to 142 psi.
14. check spark timing (loose source/pickup coil)

Timing right on, in between the "F" and "T" marks on the mag hub.

Timing light does not cut out during the backfire condition.

15. Intake manifold leak

Took manifold off, inspected for cracks, deterioration, none found.

---to be checked: ---
16. CDI Malfunction: ?
17. Source Coil: ?
18. Pickup Coil: ?
19. Ignition Coil: ?
20. Exhaust: restricted / plugged?
21. Bad gas?

ANY help or ideas would be appreciated.

Thanks!

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Burnt inlet valve , not seating , causing back fire into airbox,,, try a compression test

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Thanks guys,

I had done a compression test (see #13 above) and the 142 psi was slighty higher than spec in the workshop manual.

You got me wondering though, maybe an intermittent sticking valve or a weak intake spring.

Is there any way to check a intermittent sticking valve without tearing the head off?

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do a leak down test with 3-5 psi of compressed air and listen to the air box throttle wfo motor a tdc or bdc,you can make one out of a old spark plug by knocking out the center and welding a fitting on or get a compression tester hose from snapon or mack that has the fitting allready on,even better is to get a leak down tester.

Ok paul dont mess with this LOL

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do a leak down test with 3-5 psi of compressed air and listen to the air box throttle wfo motor a tdc or bdc,you can make one out of a old spark plug by knocking out the center and welding a fitting on or get a compression tester hose from snapon or mack that has the fitting allready on,even better is to get a leak down tester.

Ok paul dont mess with this LOL

Good idea! Thanks!

question(s)

  1. only 3-5 psi?
  2. what do u mean by "throttle wfo"? (I guessed WideF'ingOpen)
  3. who is Paul, and why would he mess with a couple of Cannucks?

I have a compression tester hose, gonna try that.

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Update on the list from first post:

checked #14 and 15:

14. check spark timing (loose source/pickup coil

Timing right on, in between the "F" and "T" marks on the mag hub.

Timing light does not cut out during the backfire condition.

15. Intake manifold leak

Took manifold off, inspected for cracks, deterioration, none found.

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Only 3-5psi so you dont seal the valve with air pressure if its a week/broken spring. And your right about wfo.

Airhead is Paul we had some post up on here and got to be a mess so he cleaned them up and in the process accidentaly deleted my thread about leak down testing.

By the sound of all your testing and still comming up empty sure sounds like a valve isuse,these motors eat valves/springs now you have the factory manual it shows how to check for bent/saged springs and if you look the service interval is only 10 hrs 6 races or 600kms,if you have gone well past this then it's time to open her up anyway.

Rings ,springs, valves + shims and a head gasket gona run you a grand.

The only other thing I can think of is if yours is equipted with the auto decompression cam then it might be sticking ,the pin should retract as soon as the motor starts ,you can check it by having the pin off the valve and move the spring loaded weight arm on the end of the exhaust cam sprocket. But that would not be back firing but firing out the exhaust.

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Well I went out and got a compression leak-down tester and found that the comperssion would not hold.

It was not leaking out the intake or exhaust valves, but was leaking by the piston and rings. Tore it down and found the piston is pretty burnt in one area from the top and down the side between both compresion rings.

So its off to the dealer to get a oversize piston, rings, wrist pin, top end gasket kit, and take the cylinder and new piston to a machine shop for a rebore.

Oh well, I love doing this stuff, ... quality time in the garage/man cave. But since I just did a top end and carb rebuild on my YZ250 and brake job on my buddies ATV and now this.... I think I'll be taking my wife out for a few dinners.

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After measuring the cylinder, I found it was still within spec, 2.795", spec is 2.794->2.796, but the piston was out of spec in one measurement by about .006".
so I didn't need to go the re-bore and oversize piston route... whew!

Now for the bad news...

Yamaha has discontinued the standard piston (3GH-11631-00-Y0), and only has the 1st and 2nd over-sized pistons available.
ARRRGGGHHHH!
The parts guy looked up the Yamaha timberwolf yfm350 and apparently it uses a piston which is the same dimension as the yfm250. So we ordered that.
When it comes in I'll be measuring the heck out of it, comparing it to the original piston for wrist pin size, position from top and bottom, ring land size and position and anything else I can measure... before I install it.

Apparently Wiseco doesn't make a piston for the 1990 yfm250a frown.gif
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