'85 250R fuel consumption

[pismorat]

My '85 250R has been bored twice (2mm over) with the bigger mm flat slide carb. on it and a full FMF exhaust. The thing rips when up in the rpms, but bogs a bit down low and tends to smoke more in general. I compare this to my dad's '85 250R. His is stock bore with stock carb. and a FatBoy silencer. Both mixed at 32:1. This past weekend we rode out in the desert and had a blast...thing is his used 2.5 gallons of premix the whole weekend and mine used 5!! I cannot believe that a mine uses double the fuel on the same, or even less, riding. Should I really chase my jetting? When I first rebuilt it I did a plug chop and it looked good to me at the time. Suggestions to get more 2-stroke bang for my buck?

[pismorat]

We went to Superstition this past weekend, and we have dirt bikes we use on longer (60+ mile) rides. The 250R's are more for duning, wash rides, night rides, etc. I'm just worried cause in a month we're headed to Gordon's Well and I'm thinking I need to bring more fuel! The carb. is a 39mm Keihin with a 150 main jet (I believe). I will be double checking all this later in the week. This is mine:
http://www.supermotors.net/getfile/1071754/fullsize/dec_12-doug-and-bikerz-beware.jpg

[barnett468]

My '85 250R has been bored twice (2mm over) with the bigger mm flat slide carb. on it and a full FMF exhaust. The thing rips when up in the rpms, but bogs a bit down low and tends to smoke more in general.

I would do a compression test with a good gauge . . If it is below around 150, you will get a bit more power everywhere by increasing it to around 170 for premium gas.

[barnett468]

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Your description is quite short we do not charge by the word !


Think of typed words as something you're going to run out of and choose them wisely.

I suggest you go with less is more.

. . . http://www.mustang.org.au/forum/Smileys/smilies/thumbsup.gif

[El Camexican]

bogs a bit down low and tends to smoke more in general.

Might try a smaller pilot (slow jet). How many turns out was your air screw that day?

[threewheelin-feelin]

if your running a 39mm carb and his is stock wouldnt that answer the question right there? 34mm is stock if my memory serves me right.

[Rustytinhorn]

My '85 250R has been bored twice (2mm over) with the bigger mm flat slide carb. on it and a full FMF exhaust. The thing rips when up in the rpms, but bogs a bit down low and tends to smoke more in general. I compare this to my dad's '85 250R. His is stock bore with stock carb. and a FatBoy silencer. Both mixed at 32:1. This past weekend we rode out in the desert and had a blast...thing is his used 2.5 gallons of premix the whole weekend and mine used 5!! I cannot believe that a mine uses double the fuel on the same, or even less, riding. Should I really chase my jetting? When I first rebuilt it I did a plug chop and it looked good to me at the time. Suggestions to get more 2-stroke bang for my buck?

I would not chase jetting based on fuel efficiency. It doesn't seem likely that jetting alone would be the sole contributor to the fuel consumption difference. There are too many other variables effecting fuel consumption. Riding style and RPM range would make more difference to fuel consumption than a couple jet size changes. However, with that being said, the flat slide carb will run more rich on the bottom with stock size jets.
5 gallons of fuel sounds typical for a couple days of dune riding. 2.5 gallons sounds pretty fuel-efficient.

[RIDE-RED 250r]

Agree with pretty much everything said thus far....

Can't stress enough that that larger carb will be a big contributor in the increase in fuel consumption, especially when riding conditions and habits are less than friendly to higher fuel economy. I can pretty easily blow through 5 gallons a day riding my 350r on Tug Hill in NY. And alot of that type of riding is in higher gears sort of cruising with occasional hard pulls at high speed. And the funny thing is, I didn't go through that much less fuel riding there before I did the big bore. I guess I don't need to wack the throttle as hard on the big bore to get the same effect I did with the OEM top end. ;) Sure is fun to let 'er eat though!

It does sound like you may be a little rich down low though... but unless you ride around all day at low rpm it's probably not going to make a huge difference in fuel economy.

Do a plug chop while riding at a steady speed at low RPM without going more than about 1/4 throttle or less. This will give you an idea as to your fuel mixture at low RPM.

[barnett468]

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Post a photo of the end of your plug from a slight angle and around 1 foot away just as it is now if you want.

[pismorat]

I think you're all correct. Riding style and the bigger carb. probably have the most to do with it. My dad keeps his in a higher gear and only revs it high occasionally. I like to let mine scream :beer Personal preferences I guess. When we race up the dunes too, mine is in a different league. He probably needs a new set of rings, but it still wouldn't be a fair comparison. I do run premium gas though, but its crappy CA 91...

[barnett468]

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A motorcycle engine is no different than an automotive one . . if you run around with the throttle wide open all the time, the mileage will be reduced by around 50%.

[pismorat]

OK, so I know it's been a long time since this topic was up, but I wanted to give an update. I "think" the problem is fixed. I will not know for certain until after our next trip in a month, but I finally took the carburetor apart and found that the pilot jet had backed itself out and was only being held up by the plastic shield. The thing was running, basically, with no restrictions to the pilot circuit! I couldn't believe it. When running it out of gas, it took about 4 times longer to drain the bowl than before. Hopefully...problem solved!