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Exhaust and Carb?


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I have a couple questions. Interested in improving the performance of my bike so that it can do some pretty decent speeds on the road. Was considering a supertrap exhaust because a lot of people on here use them and like them, but I don't know a lot about it. What does it matter if it has more or less discs?

Also could someone explain what a pumper carb is and how it works compared to the stocker?

I've already done the gordon mods to my bike and was thinking of a new cam and higher compression piston with possibly a big bore as well. Let me know what else I ought to think about.

Thanks

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The best value for money would be the big bore 277cc Hi-comp. any bigger requires a resleeve. You get more power right across the board. According to Geoff Ballard (from Ballards XR's Only here in aus) the 280 kt is the best value for money you can buy, and will make more power than any carb swap, cam change or port job, yet to make even more power you can do these thing too. Id reccomend starting with the big bore as the standard exhaust and carb are very well up to the task (with the right jetting and baffels removed lol!)

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The pumper injects fuel into the intake. It only helps at the bottom by squirting gas at the initial throttle twist (It eliminates the bog), instead of the vacuum pressure created by the air/fuel screw. That was a very rough explanation and my understanding, somebody else here knows a lot more detail though. You can mod the pump on the stock pumper carbs to get a stronger/longer squirt, a big plus to a snappier throttle. When I did mine I felt I got more power but it really was just the throttle hitting faster compared to stock.

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What all should be done when you do a big bore with a high comp piston? I've heard that with a high comp piston you need a better cam as well. So if I were to do all this would it be best to do something else to make sure it was still reliable, like porting or replace valve with stronger ones, or something like that? Also how large is the piston in a 277cc bike?

What type of mods can I do to my stock carb to make it a little bit better? Right now the bike is fine for what I use it for, but on road I don't think it'll have enough power.

Thanks for the explanation on the carb and thanks for the big bore thought.

Would a stock carb and exhaust (with the welds ground down and baffle removed) handle a slightly higher compression piston and a stage 1 cam? If not what should I look at? And with the higher compression piston and higher performance cam what kind of power am I looking at gaining?

Edited by jostith
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The stock cam works but you can upgrade it to stage 1 or 2 hot cams (whichever suits you). The valves are well up to the job, but you can put some heavier springs if you need to (not neccesary). For the 277cc it is a 77mm piston (4mm oversize).

You can keep the stock carb and get it jetted to suit, or you can replace it with an aftermarket option. The stock carb is well up to the job (if jetted correctly) aswell as the stock silencer if it is opened up (removed end baffle). I have read that the mods will give you something like 20% more power, and if you go the whole lot (big bore, carb, cam, exhaust) it may even be more, its just what your prepared to spend though,

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Thanks for the info. I went to their page but they don't make a model that will fit my 86' XR250R. Does anyone know if the xr200 86' and up or the xr250 and xr400 for 96' and up models will fit?

Here is the site. http://www.supertrapp.com/product_sections/cat.asp?CatID=21

I need to rebuild my engine anyway. It is an older bike that I don't think has ever been taken apart so it needs some new stuff I would say. It runs fine but you know just in case. I will probably just start buying the stuff and setting it away to do it during the winter. I will get a stage one cam, new valve springs, high compression piston, new timing chain, and probably some other smaller things. I'll start shopping around now.

Is there anything else that I ought to know?

Do I need to get my cylinder replated or could I just sleeve it? What are the pros and cons to doing either?

Edited by jostith
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I have had 3 XR280s (actually 277cc), an '87, a '94 and a '96. I am not a fan of aftermarket cams, but that is a personal choice. Removing the exhaust baffle and jetting the carb properly in conjunction with the overbore provides a very noticeable and usable improvement in the already excellent torque characteristics of the 250 engine. Adding an aftermarket cam and exhaust will provide a modest gain over the basic uncorking, but sometimes can be at the expense of a bit of low end torque. Personally, the aftermarket exhaust and cam are not worth the expense for a marginal gain in mid-range and top end power. A mod that is worth it for me is a pumper carb, makes a huge improvement in throttle response. The 250 engine tolerates the overbore very well, no bottom end mods necessary. It is not a good idea to go overboard on compression ratio, too high a ratio will contribute to detonation and excessive top end temps. If there is a competent machine shop in your neighborhood that does automotive work, they can bore your cylinder. They will need the piston to bore to the proper clearances, follow the specs of the piston manufacturer, pay particular attention to the ring end gaps.

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So there is no need for replating? That is my main question at this point. I want to know if you have to replate and if you do would it just be a better idea to resleeve and avoid the hassle of sending it off? For my old CR we needed have the cylinder replated after boring it out or it wouldn't last, or we could use a steel sleeve (I think it was steel). We do have a good automotive shop that does boring, I talked to them. I know they can't do a 125 because it was just barely too small. Pretty sure they could do a 150 so a 250 should be fine.

As for the pumper carb. What is the best carb to use? I know the stock carb can handle it but just want to know what can give me the most power for my money since I want the bike to be street legal and able to keep up with traffic. Stock it'd be ok but at higher rpm's I don't want it struggling and losing the good ol xr reliability because the valves work too hard. So my main concern is reducing rpms. So at 50 if it's at 5k rpms I would like to make it hit 60 at 5k rpms, this is an example not real figures.

Now since you have had an 87' xr250 creeky what kind of exhaust did you have on it? I know that they aren't a good upgrade but figure that I could look for a used system on ebay. I like how the supertrapp system sounds (the thought, not literally the sound) with the replaceable discs and I don't like my old exhaust. But if it is too expensive I won't mess with it.

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The 250 cylinder is not plated, it has a cast iron liner which can be bored up to 78mm. However, that is the last oversize without resleeving, so the 77mm is best leaving one more overbore if necessary.

My personal preference is the Edelbrock, opinions vary on this. The Keihin FCR is also a good carb, but for me, the Edelbrock is the choice due to its' simplicity and ease of tuning.

I ran 14/48 gearing 99% of the time on my 250s, still good in the woods, OK on the road. I ran 14/45 on my '96 in Baja, but it would sometimes not pull 6th gear in the softer terrain.

My '87 had a Skyway (marketed by Cobra as I recall) aluminum muffler. I'm pretty sure that they are no longer manufactured, it was a pretty decent muffler but the only advantage I could see was the lighter weight compared to the stock muffler. I have had a couple of Supertrapps on XR600s, they are OK and tunable for sound.

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Creeky have you, or anyone you know, ran a 35 FCR on a 96-later 250? Sudco lists one but someone I know insists it will not fit.

NW

No, have no experience there. 35mm seems kind of large for a 250, even a 280. I tried a flatslide 34mm Mikuni on my '96 for a while, was a little better than stock performancewise, especially on top end. I didn't keep it on long because the low end performance was better with the stock modified carb. I have heard secondhandedly that the Mikuni TM33 works pretty good on the 250, you might want to research that.

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Great info. I'd love the edlebrock but probably won't go for one. I've heard that the only thing they do is increase throttle response and that doesn't really matter. But I did not know about the cast iron sleeve, that is really good info and great to hear as well. No replating is awesome news. I'm going to look into all that in about a month. Gotta wait to move into my new place with the garage. Going to look into getting a title tomorrow as well. Thanks fella's. I hope everyone else asks questions that I may have missed. Or clarifies other answers and questions.

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Throttle response should not be overlooked it can make the bike, I was a all stocker kind of person until the mod bug bit me. A pumper was stock and a good carb, huge improvement when the mods were done, but man the bike came alive everywhere with the mikuni. I am running the mikuni 34 flatslide on a stock 94 250L and love it. Handles the bottom great (only bogs on the stand when you wack the throttle) and the top is only limited by my motor. 50/200 jetting and it is running a tad lean somewhere. Maybe 250 machine will pipe in he's got the 280 with the same carb and the same jetting or more and really likes it.

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Well, it does fit, quite easily actually. The dimensions are not really larger than the stocker, and there are no clearance issues with the frame at all. One thing I am going to need to check is the cables, I do not see a problem but they are relocated to the opposite side, so I will need to check.

NW

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