1991 Yamaha Moto 4 250

[Arizona]

So my latest find is a Moto 4 250. Looked in good shape, started right up but would backfire and die upon acceleration but you 

could get it to go if you were good with the throttle. I figured it was the carb. was a brand new Chinese carb, he didn't have original.

I took it apart and cleaned it, only thing I found was some debris in main jet and the main adjustment on bottom was at 41 turns. Cleaned 

it out and set at 2 turns, re installed. Starts up, runs better actually drove it but seems really sluggish, once I hit 3rd gear could barely pull

it. Rechecked everything, even ordered a new carb as I had this problem with another atv but still same problem. Now I pull it apart and 

decided to check compression, I show only 85 psi. I checked valve adjustment (after aligning up cam gear to mark on case). to make sure not too tight still right at 85 psi. 

I assume I need to pull the head and look at the valves, Any ideas I am missing something?  motor does not smoke but is super sluggish. 

The specs I find are off the downloaded manual of a 94 but figure should be the same. It states compression standard is 128  and Minimum is 114 and Max is 142

Any other Ideas before I tear it down? Thanks in advance Az

[Frank Angerano]

Could be a head gasket ? Spray the head down with soapy water and crank it up. See if any bubbles show up.  If not then it’s the usual suspects, valve seals or piston/rings. 

[Arizona]

Well was going to tear it down today but double checked a few things

rechecked compression, right at 85psi then rechecked compression with

throttle wide open and I get 125 psi. Double checked and confirmed with throttle

open 125 psi. so I assume the motor is not the issue.  Still looking at the carb as it 

feels like a carb issue. Similar to problems with the Kaw bayou 300 I had. So gonna 

set the valves before I put it together, Since I don't have original carb and have tried 

2 Chinese carbs, should I try a jet kit and install in this carb? If so what ones do you have

the most luck as far as accuracy with?    Might also see if ign. coil is putting out enough

volts, I am assuming it should at least put out 20kv Any other ideas? Thanks Az

[Frank Angerano]

Hey Az, if I’m not mistaken and maybe a few others can correct me if I’m wrong but the  compression reading should not change wether the throttle is open or not? The compression reading is contained at the top of the piston stroke so the only place to loose it would be valve seals, head gasket  or ring wear and not opening the throttle. 

 If you have the oem carb then tear it down and go with a kit, oem size jets obviously. 
I’ve used a few different  types. I like the Hi-Caliber brand, $35 bucks on Amazon. So far so good with them. 
 

 

[Arizona]

Yes the compression changes every time I either open or close the throttle. 

 Never heard of 

the readings changing with throttle angle. It will read 125 as long as throttle is opened.

If not it reads about 85psi. The motor runs super smooth at idle and doesn't smoke. 

Any ideas what I should do with it or just leave it alone?  Now second thing. I do not have the

oem carb. Should I try the jet kit on the new Chinese carb? or wasting my money? Az

Edited October 10, 2019 by Arizona
add pics

[Frank Angerano]

Wasting money on the jet kit with a Chinese carb. I would rather try to locate a used oem carb on eBay and try my luck and at worst it will need a cleaning maybe new jets. The only reason I can think of that the compression reading keeps changing is the cam lobes are worn down and there is not enough intake based on valve lift ? Throttle movement should not change the psi on the gauge as far as I know.  
Strange but maybe @davefrombc or any other members can expand on this a little.

 

 

[Arizona]

I agree weird readings. But they are repeatable. Just depends if throttle is open or not. 

Would think cam problems would be constant. I cant think of anything that would make

them change due to throttle angle. Im stumped. Any Yamaha motor builders out there that

have ideas?  

 

On a second note, after screwing with the second carb I got it to go quite a bit faster but still not right.

I can floor it in neutral and it wont redline. Just revs about half of what it should be. Pretty odd as it 

idles pretty smooth and starts pretty easy. Hoping for some knowledge on the motor Az

[Arizona]

talked to an atv engine builder and he said you will get different compression readings

between no throttle and full throttle. Should be checked at full throttle. I checked the online 

manual and it says check at wot

[Frank Angerano]

Me too. I looked it up as well but what I want to know is why? 

[Frank Angerano]

I would assume that as the piston goes down and takes in air through the intake with an open throttle that allows more air intake thus more volume and more to compression on the way back up .  Makes sense now never thought about it really.  

[Arizona]

Well im looking into a used oem carb. The online engine builder says he has a few and should call me back. 

also found 1 on ebay but looks pretty rough. Ign. coil was putting out about 28kv

I sure find the weird ones! Az

[Frank Angerano]

Lol yes you do. Me too though, my guess is that people do crazy sh** to there bikes OR it’s a broken bike  and everyone and there mother’s are “mechanics” and try to fix them and tear into wiring etc.  

Although this is a hobby it  also takes some brains to work on this type of sh**. 
 

[mnmsmith7304]

Just got a 1991 Yamaha Moto 4 YFM250 for my kids and have some basic maintenance that I know needs to be done and am somewhat new to ATVs, so any advice is much appreciated.  Thanks for all the detailed info about yours.