Low Fuel Economy - Running rich, not sure why

[ds1506]

Hello everybody,

So I bought myself a 1999 GZ250 a few weeks ago as my first bike. I got it for a steal since the previous owner left the prime valve open the night before I picked it up. After oil change and a new spark plug, I got the bike running again. While it seems to run fine, I'm getting absolutely terrible MPGs for this thing (I hit reserve about every 70 miles). Now since I've been working on it, I've been noticing that the bike is not very stock, so I've come to see if you guys could maybe point me in the right direction to maybe boost the economy of the bike.

As far as things that I've noticed are different goes: The bike has an aftermarket exhaust and the air box was taken out and replaced with a pod filter (which is kinda dirty, but still ok). I attempted to adjust the fuel mixture screw (the cap was already removed) and it seemed to be already screwed in all the way (that's supposed to lean out the mixture, right?). The bike is definitely running rich though, the plug is blackened. There's also a noticeable smell of gas when I'm sitting in traffic. I'm not entirely sure what my options are on how to get the bike to stop running so rich and I'm on a bit of a tight budget so I'm hoping it's not too big of an issue.

It also seems to be consuming oil, it smokes occasionally from the front, I'm thinking about where the exhaust meets the engine. I figure I can keep on top of the oil thing until I get the fuel issue resolved first.

I put a couple caps worth of seafoam in a couple days ago, but I'm not sure if that'll really fix my problem or not. The bike operates normally, runs fine, and gets me back and forth to school, but I'd like to do more than a weeks worth of commuting before I have to fill up again. Any help is greatly appreciated and I'd like to thank you guys for supplying the service manuals here, they've been very helpful so far.

Here's a couple pictures of the bike while I'm at it:


That's the condition I bought it in, and it's the same condition it's in now more or less (I moved the tail light and licence plate and got it running, that's about it).

[JohnC]

If it idles with the pilot screw all the way in there's something really wrong with the carburetor. (Leaving the petcock on prime shouldn't cause an issue unless the float valve in the carb is leaking).

A dirty air filter can also cause high fuel consumption.

If gas is getting in the oil your oil consumption will be high, too. Keep an eye on the oil level until you get a handle on things!

[jonathan180iq]

Looks like this guy put an aftermarket exhaust and intake on the bike and then WAY over compensated with his jetting.

If you're a newbie at this, you might feel a little intimidated by doing this, but you need to set the carb back to the stock setting and then rejet from there.

I agree with JohnC about something being really FUBARed if the bike runs with the pilot screw closed.

You need to at least pull the float bowl and see what size main jet he has gone with. It should be stamped somewhere on the outer rim of the jet, like this:

This would be a 103

For reference, no one on this forum has ever used anything larger than a 140 without eventually coming back down to earth. 145 is too rich for any setup that I can remember.

After that, check the needle setting and then we can get back down to the pilot setting.

[ds1506]

Thanks for the responses guys. I guess I'm going to take the carb off tomorrow and try to check on the float and the jets. I guess if at this point I managed to get everything back to stock settings, everything needs to be readjusted from there too. As far as the jets go, any idea of what kind of size I should be looking for and should I be returning it to the stock size if it isn't? Also, the pod filter/exhaust combination shouldn't be bring the economy down too much right?

[jonathan180iq]

You won't be getting 65MPG, but it won't be too bad. Guys with a full setup were still getting in the mid 50s.

You don't need to settle on the stock carb configuration; it will be too lean for your exhaust/filter system. Returning it to stock will just be your mental starting point when you start to rejet and adjust your setup. So you are right in assuming that you'll be making adjustments from there.

The stock main jet size is listed in the service manual, which you can download here:
viewtopic.php?f=3&t=27

IIRC, it's like a 115-117.5 or something like that. You will probably be in the 130 range. Since you have both an exhaust and a pod filter, then maybe a 135. ( But everyone's jetting is going to be slightly different. If you are going to buy some jets and try and sort this out and get this right. I would buy a 125, 130, 135, & 140.

You need to find the right main jet first, as it slightly affects all other jets, and then go from there. The needle and pilot jet you can simply adjust if you need to, along with setting your idle with the bike whit knob that is sticking out from under the carb.

One last thing, for guidance, do a search for Jaime's Rejet on his Marauder 250. It's very detailed and has photos. I also did one a long time ago called "Jetting: Part 1". There was also a part 2. It should at least have lots of good discussion and stuff about what you're getting into.

Sometimes talking about carbs may sound complicated, but they are really simple once you understand what you're looking at.

Keep us posted.

[ds1506]

Ok, update time:

I didn't accomplish as much as I would've liked to this weekend. Some work stuff came up and I spent way too much time adjusting and readjusting the chain (looks like I'm going to need to replace it soon) and I didn't really have the chance to give the carb much attention. I did get the chance to pull the float bowl briefly though, It looks like the main jet is a 125. I pulled the plug too, and it didn't look too dirty, a little blackened but not nearly as bad as it was before. Just as reference: the pilot screw is about 1/2 - 3/4 of a turn out from tall the way in. I'm assuming since the main jet is one of the smaller ones, that's probably not the cause to my economy issues anymore, right?

I also gave the filter a good cleaning. It was a lot dirtier than I thought. With that all in mind, I've made it to 83 miles since last fill up and I haven't hit reserve yet, but that'll probably happen later tonight.

Hopefully either Tuesday or later in the week I'll have time to pull it apart more and do some more inspection, but this thing's my main mode of transportation currently, so I can't really put it out of commission during the week.

[jonathan180iq]

125 isn't stock, so at least the PO tried to adjust the jetting. But it's a little lean for the set up that you're running of pipes plus intake... I'm sorta surprised the plug wasn't white. Oh, well. The results speak for themselves.

I understood your first post to mean that the pilot screw was all the way in. Having it out like it was an adjustment that the PO did, and again, if it worked, then let it ride.

A horribly dirty air filter would cause poor fuel economy. And it would suffer if your chain was too tight, so you're on a good path there. Find the tightest setting of your chain as you rotate the back tire and adjust it to that point. It won't all be uniform. But it's better to be slightly loose than to be slightly tight.

All the usual stuff? Air in tires? Condition of your oil? Clean fuel with some cleaning additive?

[ds1506]

In my first post I did mean that the pilot screw was all the way in. Although at this point I'm not entirely sure that was the case, I had a lot going on when I inspected it, so I could be remembering wrong. As far as the chain is concerned, I'm pretty sure I got the adjust right, the main issue ended up being that it needed some kind of lube action going on, but it was still out of whack. I'm thinking a new chain will be needed in the near future.

Otherwise I'm going to keep messing with the little things, as far as adjustments go. Oil was changed not too long ago, and it's slowly leaking, so I have to top it off every so often, so I'm thinking it's pretty clean right now. I put a little bit of seafoam in the gas a couple weeks ago, I may put a little more in tonight and see if that helps anything. I figure it can't really hurt.

I'll check the plug in a couple days too just to be sure of everything. I just filled the tank last night, so this is the first full tank since I started messing with things. I'll keep you guys posted as far as what happens.

[raul10141964]

I runing
filter mod
2.5 turns pilot
140 main
needle mod
get 62MPG

[ds1506]

So, I pulled the carb off the bike today and I think I made some headway.


Here's the pilot screw being almost all the way in. Adjusted it out to 2.5 turns, it doesn't smell any richer when I went on a quick ride, but I'll check the plug later after I get some more riding in.


Here's the float needle, I pulled it out to make sure it wasn't worn or damaged. Didn't look too bad.



Here's the jets inside. PO really did a number on the float pin screw. Go figure. Anyway, I pulled all the jets and cleaned them. I was talking with my father and he suggested that I check and make sure that the float actually worked properly. I dropped it in float bowl full of gas and it floated fine at first and then one side sank a little bit. So I'm thinking that the float might be at least partially responsible for all the gas consumption issue. Although, I did forget to check to see if it was just a float adjustment issue, so I'm going to check that a little bit later as well.