I have a 87 trail boss 250 , It will start but does not stay running , then when I go to give it gas it dies , no bogging down or anything it just dies someone please help

 

If you mean it starts, with the choke and the idles a bit, but dies when you goose the throttle then first thing is to adjust the carb for the low end of the range. If it works better fully choked when you do that, then almost certainly it is too much air in the mix.

low-end carb adjustment on a Mikuni VM is done via a screw (philips) at the carb (on the right side). That is the "air screw".

Turn it clockwise to close (careful, do NOT overtighten, when it snugs up, call that "closed"), then back off 1 and a half turns counter clockwise. (another caution: *3* turns back is the max, after that the screw could come out)

That is the theoretical "correct" position for starting with the carb adjustment procedure.

From there, with the ATV idling, you can do slight adjustments either way to bump the idle speed up. Once it is warmed up, you can test the low range of the throttle, adjusting the screw further as necessary (again, *slight* adjustments)

A good test is if you have slight slope to run the ATV up; if it handles that without bogging down and you seem to have good power output with the throttle in the low range (i.e. ATV is moving slow), that is a rough way to determine it has been adjusted "good enough"

 

Now Ive been havin trouble startin it , but Im goin to try what u said thank u I will let u now if it worked

 

If it isn't simply carb tuning, and engine is getting fuel, spark, etc -- in other words, the straightforward stuff isn't the problem -- then, you could be having issues with the sealing of the crankcase. It needs to be tightly sealed as the internal vacuum is key for engine health. Per any number of other posts on the subject, the most common culprits are the crankshaft seals.

I have replaced mine as part of an overhaul of the whole engine including taking it off the machine and splitting the crankcase, but I believe you can replace the crankshaft seals without taking the engine out.

Search this site for posts on the subject.

 

Great info carl-in-nh good to see someone that really knows what he talking about.
As for replacing the crank seals there easy. once you remove everything in the way I use 2 small self-taping screws and a cordless drill. Run 1 screw into the metal housing of the seal on each side being careful not to scrape the engine case once you have the 2 screws in I use 2 sets of pliers and gently (But with force) pull in the screws and the seal will pop out.
Hope this helps Randy.

 

Hey thanks Randy ! I am a newbie, to ATVs and engines, but I got a free 1988 Trail Boss a year or so ago, and I am having GREAT fun working on it and learning all about it.

My sources of info for learning are this site (of course!), Haynes, and talking to knowledgeable friends and family