84 250r Carb Swap

[SiiCHo]

ok, so i had a thread in the new members area about my carburetor. the little nylon tang inside that keeps the slide from rotating had snapped off, so i was able to make a temporary repair, but now i want to replace it. it was recommended that i upgrade from my stock 30mm to a 38mm airstriker. i posed the question of where i can find a new air boot to go from the carb to the reed that will accept the 38mm carb, but have not gotten a response. any help would be greatly appreciated. also, i kinda think that the 38mm would be overkill, and that a 32 or 34 would be better, what do you guys think? im at about 5000 ft above sea level, i have a k&n filter, DG silencer, and stock pipe. i would prefer to buy a new carb over a 30 year old used one, so if you guys have any recommendations, that would be awesome.

original thread: http://www.3wheelerworld.com/showthread.php/158295-Carb-stuck-open...

[yamaha225dr]

I don't have a clue what to tell you about the boot, since I don't have any knowledge of the air cooled R's. I do however have two 34mm 85 takeoff Carburetors if you are interested in one.

[barnett468]

Hello siCHo


CARB - That’s a subjective opinion to a point. Some people like as much rpm as they can get and don’t care about bottom end at all. Others don’t want to sacrifice much bottom but want a little more mid and top. No matter what, after a certain size you will reach a point of diminishing return where the carb you install for a particular application is simply too large and actually takes away from overall performance. In my experience since your bike sounds basically stock the point of diminishing return in your case would be 34-36 mm. I suggest that unless you want to give up a lot of bottom end you go no larger than a 34. You could put a 40 on it and it would not have any more peak rpm than a 36 but it wouldn’t start to run hard until maybe 5k rpm.


CARB STYLE – Due to your elevation you might be slightly better off with an oval bore than a round one if you are concerned about losing bottom end throttle response. Keihins are available in oval bore and although a Mikuni is an xlnt carb I have not seen one with an oval bore.


COMPRESSION – If your bike is stock one big improvement you can make due to your elevation is to increase your compression. For every 1000 ft increase in elevation you lose approximately 5 psi in cranking compression. 5 psi x 5000 ft = a loss of 25 psi. This is substantial. An increase in compression can be achieved by either a cylinder head mod done by a professional or the installation of a thinner head gasket if available.

[SiiCHo]

thanks barnett.

basically its like i thought, a 32 is all i need for a little more go... in that case, im probably just going to stick with the stock size, just to alleviate trying to find a boot that will work. thanks for the the advice on the compression. i have 2 and a half basket case motors on the shelf in my garage, when i build them up, ill keep this in mind.

[JasonB]

Thats solid advice Barnett! I dont know what carb to recomend in that size range, but if you do go with a larger jug (ie 300cc big bore for the air coolers) than a 38/39mm carb would make it happy. For that, an FL350 intake boot from the honda odyssey is what bolts on to the air coolers for a larger carb. I ran one on my fooler

[SiiCHo]

So after some research, i think im going to go with the mikuni vm 32-33. Now i just need to figure out if i should order jets too, or just run it stock and see if i need to jet it. Most likely i will have to go down a jet size or 2 due to my altitude. The question is, can i get away with just jetting the main and pilot, or should i change the needle jet too? And if i change the needle jet, do i have to change the needle too? I figure i will have to down in size because at least with cars, the rule of thumb is -1 size for every 2000 ft above sea level. Im at 5000, so -2 sizes should get me where i need to be. Thoughts?

Eta... Ok so after looking through the factory service manual, it looks like im only going to have to go down 1 size. Anyone have any input?

[toki]

air foolers.com has this topic covered a million times. check em out.

a popular upgrade is a 34mm keihin PJ from an 85-86 250r or TRX. it will slide right into your existing boot and if you run an air box that bolts right up to that too. i have an 83 (basically the exact same as your 84 except the rear suspension) and i swapped out to an oval slide keihin 34mm. holly hot dog. huge improvement. also with jetting you may be in the dark with little help running a mikuni on your air fooler. not many people run mikunis on them.

and also also with jetting. on my stock carb the stock jetting was 50 pilot 130 main. moving up to the 34mm im now at a 45 pilot and 145 main. some people say bigger carb flows more fuel so drop the jetting. others say they flow more air so raise all jetting. truth is. different carb, different slide, different everything. might as well buy a bunch of jets and get it dialed in perfect rather then burn it up. I started with a 152 main and worked my way down. 130 on my stock carb was good, i tried it in the 34mm once for 5 seconds. it was soooooo ungodly lean.

whatever you do. buy some jets

[SiiCHo]

so i just ordered a keihin PJ34 and 3 jet sizes smaller and one bigger for both the pilot and main. ill let you guys know how tuning goes when i get it

[yamaha225dr]

How much did you pay for the 34mm carburetor? I had mine listed on Ebay for $50 and free shipping for four weeks and no one wanted them. lol I even started another auction with a $20 starting bid and no reserve.. Still nothing! Guess I will just hold on to them before giving them away.

[SiiCHo]

i paid $150 for it, brand new in the box

[nicker71]

good call not to go with the mikuni. The 34mm PJ works great for the 250. I have a 38 PWK AS for my 300kit, but the 38 maybe a tab bit too large for the 250. THey also have a 35mm PWK AS that would prob be the best carb for the 81-84 250r engine.