Where's the beef?

[gunslinger45]

I've got an 88 rm250 that just doesn't seem to be running as strong as it should at the bottom and mid range. Doesn't seem to have any pull unless you work the throttle up and down and then it will really take off. I've got a 58 pilot jet in and I'm not sure of the others. The jet that the thottle needle slides into said 380, but I haven't seen any references to that size jet. (Is this the needle jet?). Air screw is 1.5 turns out. Anyway, I'm just not sure where to look anymore. I've thoroughly been through the carb and things are all clean and clear. I have no idea how old the reeds are, but I could not see any gaps when I last looked at them. Any ideas? Keep in mind I'm no pro and I'm not talking about tweaking to get another 1 hp out of this thing, something is definitely not right.

Thanks guys,

Trev

 

[reelrazor]

What's the plug look like?


What does the bike do when you get to that throttle opening in 2nd or 3rd...and hold it right at that spot? Does it buck or surge, or just decrease revs(try to die)?

Can you reach the enrichener(choke) while doing the above? If so, does the enrichment make it worse or better?

 

[gunslinger45]

Plug is a slightly blackish, but not overly gunked or anything. I have been thinking that the bike was running rich which was hindering its response, but then I read that if the bike bogs down it is a sign of running too lean. If I have the bike in 2nd and just hold it at the unresponsive throttle opening, it will drag its butt and then sometimes it will pick up, other times, it won't. Seems like another sign of richness to me. I haven't tried the choke thing in gear when riding, but I put it on for a second or two earlier today when I had the bike idling and it definitely did not like it (tried to stall). Also, I have a Keihin PWK carb and it seems that my main jet should be in the 170 to 185 range. I pulled what I now realize is the main jet earlier today and it said 380, which is not a Keihin jet size, but it is a middle of the road main jet for a Mikuni. Are these interchangable? I don't know, maybe I'm just not familiar with what an 88 250 size bike should do. If I am in first gear and nail the throttle it drags and drags, then takes off.....from what little I know, if I do that with a normal 250, even from that time frame, then I should damn well flip over right?

Thanks for your thoughts so far.

 

[reelrazor]

Have you removed the emulsion tube(needle jet-the brass tube above the main jet-needs the main jet out and comes out the top) when you have checked the carb for cleanliness? It has a series of holes up its' sides which need to be clear-if not, the fuel doesn't get emulsified (frothed with air) before it exits into the venturi and leads to a false rich condition as the large droplets are too large to burn clean. Also check the air passage/air jet leading to the well that the emulsion tube is enclosed in (air enters in upstream carb throat).

Get a proper Keihin jet. Refer to the carb specs for THAT bike.

Sounds to me like someone who had no clue started playing with that thing. #58 pilot seems excessive to me, the mikuni main is scary. What's the needle clip position?

I would try to find the carb specs and start fresh.

Yep, that bike should put you on your ass with a hard, first gear, clutch out from a roll, handful of throttle.

Ohh, and here's a often overlooked point on older bikes-check the carbon level in the exhaust. Both at the headpipe/manifold/ex port area and the silencer baffle. especially if the bike seems quieter than it seems like it should.

The exhaust should feel like crisp slaps when you put your hand behind the silencer and blip the throttle.

Pull the reed cage and give them a good look from all angles.

 

[gunslinger45]

Thanks Razor, I think I'll take your advice and get the carb back to square 1 and see if anything in there helps befor moving down the line.

trev