I am four months into the purchase of a 2008 Big Bear 250. When I got the quad I was told that it only needed new rings. Yeah, I fell for that line. Fortunately I got it for what seemed like a great deal.

It registered 60 psi of compression when I got it. I got the cylinder honed from a machine shop and put in a new piston and rings. That increased the compression to 110. After the install it would barely start and run. I then rebuilt the carburetor with little effect. The service manual specs call for a minimum 114 with a max of 142 compression.

After doing a leakdown test I confirmed I had a considerable amount of air leaking past the intake and exhaust valves so I replaced the valves, valve springs and seals. When replacing I realized my intake valve was bent. After installation I set the intake and exhaust valves to service manual specs. I checked my compression and it went up to 140 and no more air was leaking past the valves. I then tried to start it and still had trouble starting it and had to use starter fluid to get it started. Once started it could barely idle, would only run with the choke on and would die with any kind of throttle.

I wasn't convinced that my carb was not the culprit so I bought a cheap ebay carburetor to confirm that this was not the problem. I also cleaned out the gas tank and put in new ethanol free gas. After the carb install I tried to start it and I could only fire it up with starter fluid. It would idle better than previously but it has no power. When I push the throttle to full power it barely reacts. At this point I don't know what to do. I appreciate any advice...

 

Sound like you've addressed any mechanical or fuel related problem,but if leak down still shows good,compression is still good,but only starts with ether which is lighter and ignites easier than gas,problem may be electrical. Might be worth while to check the specs on the electrical system. Good if you have a manual,if not here's one.https://www.tradebit.com/filedetail....ice-manual-pdf

 

Thanks OPT. I forgot to mention that it keeps fouling plugs. Would that be a symptom that would lead to electrical - spark problem or could this be contributed to the starter fluid? On the below two pictures the first spark plug was brand new. This is from about 3 minutes idling and trying to give it some throttle in neutral. The second picture was from a backup plug that was not previously fouled but was after about another three minute run in neutral.

 

Start with a new plug..Could be as simple as that. Plus did you check the new carb before you installed it,comparing the jet sizes,float level,etc to the old carb? I'd eliminate the carb as a problem,then check electrical specs if need be.

 

I set the float height and made sure everything was tightend on the new carb. I will compare the Jets to my old carb.

I have included a video of the spark when trying to turn the engine over. Does this look like a healthy spark? I know this might not tell much but I would assume if I have compression, fuel, and a good spark it should at least start. I can't get it started without starter fluid...

I will try to use my multimeter on the electrical this week to check the resistance per the manual.

 

Looks good through the tester.

 

From the video it looks like it is jumping a good gap, is the spark blue or orange? Orange means weak, blue means strong at least it did in the old days.. Take a clean and dry spark plug and turn it over a number of times without adding any starting fluid. If it doesn't start, remove and look at the plug....is it wet with fuel or is it still dry? If wet then you have to much fuel and is fouling the plug, or the spark is very weak, if still dry then fuel is not getting into the cylinder so carb work is needed. You might try running a compression check as well, if compression is low fuel will not ignite well.

 

It appears that not enough gas is is going into the engine. If I crank it for about 10 seconds and then pull the plug there is no sign of gas on the plug. If I drop a teaspoon of gas directly into the spark plug hole it will start right up. I can also get it to start if I take off the air filter off and block the air intake. Once running it will only idle with the choke on and idles best with the fuel mixture screw completely backed out. If I try to push the throttle it has very little power, maybe 20% throttle. If I try to put it in gear it acts like it wants to go forward but limps forward a few inches with very little power, it won't even idle in gear. When I pull the spark plug it appears fouled. I don't understand why I seem to have a lean idle condition but am fouling plugs? I'm lost on this one. I appreciate any insight.

 

Ok now back up a minute. when cranking you had a dry plug, you added fuel and it started but fouled when accelerating the throttle. IF your spark is hot (blue) spark, and your not getting fuel when cranking, but you are fouling when accelerating then you have a carb issue. Now if the spark is yellow then you have a spark issue. Just a guess here but my guess the carb is way out of wack with wrong jets ets or you have a serious blockage inside.
If you have a blue spark then a complete carb overhaul with a proper kit would be in order. That is what I see from what you have posted.

 

Thanks. I definitely have what appears to be a blue spark. I can't verify the Jets on my new carb but I did rebuild the old carb with a new rebuild kit and it ran similar to this new carb. I'm not sure what else to try on the carb.