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 15) and have left to try (#16 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 (!@#...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?)... done timing check, right between T and F marks
15. Intake manifold leak ... checked for cracks ... none found
----left to do ----
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!

 

I would check the alternator is charging first (13 to 15 volts), then go for changing the fuel, then check voltages and resistances at the CDI plug are right before swapping the CDI from the "identical" bike, then swap coil, then pickup coil. Another possibility is the cam "gone soft" and not opening the valves fully. A turned baffle in the exhaust is a possibility too.

 

Thanks merryman
All great ideas. I will check and get back.
btw, I had replaced the battery about a month ago, the old one was over 7 years old and just would hold a charge. Thank God for the pull starter!

The soft cam idea got me thinking about the compression test I did.
When I start cranking the engine the compression immediately jumped to 120 psi then over the next second or two after 5-7 more revolutions of the crank, jumped to 130 and then 142.
I usually like to see it jump to 80% then to 100% within a couple revolutions of the crank.
What do you think?

 

Don't know, compression testing does depend on the throttle setting anyway.

Most new quads don't have a pull start, another consequence of fuel injection.

 

I did the compression test with the electric starter (new battery) and the throttle set at full wide open.

I'm trying to buy a Cylinder Leak-Down Tester to check for slow compression leak and also buy a borescope/endoscope to check for a burnt or sticking intake valve without pulling the head.

New technology is great but the pull starter is old school, but nice to have when you're stuck in the bush, something to be said for carbureted & mag systems.

 

Cool. Please keep us posted.

 

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.

 

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

 

It is extremely unlikely that the piston has caused your problem. Check the cam lobes while you have the head off, and go through my previous list if it still doesn't run right.

 

measured the cam:
Intake : A=1.441" B=1.89" ........... Spec is A=(1.438 ~ 1.442 in) Limit=1.435 B=(1.186 ~ 1.190 in) Limit=1.182
Exhaust : A=1.442" B=1.90" ........... Spec is A=(1.440 ~ 1.444 in) Limit=1.436 B=(1.191 ~ 1.195 in) Limit=1.187