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Thumper Racing's XR300 kit review ?

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Would those have used this big bore kit like to provide a review ?

Did you replace the carby ? Was the kit worthwhile ?

Does anyone know how much the 300cc piston weighs ?

Sincerely,

Tom

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Had thumper do the machining work on my '88 XR250, now its a 300. I still have to buy a new head, so I havent put it back together. Thumper racing can put it back together. Everything I have read on teh 300 kit is very positive, that why I did mine. Ask Dwight Rudder or Flynall - they both have good experience with the XR300 kit. I think you'll be happy. I expect to be when I get it back together. You'll have to beef up your clutch, and probably go with a 45-46 tooth rear sprocket. I am going to put on an Edellbrock pumper when I get the cabbage.

Mike

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Do a search and you'll probably turn up our previous posts on the subject.

Our XR300 is a really nice bike. 😢

RH

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Would those have used this big bore kit like to provide a review ?

Did you replace the carby ? Was the kit worthwhile ?

Does anyone know how much the 300cc piston weighs ?

Sincerely,

Tom

The 300cc piston might be even lighter than the stock piston. I know it is way lighter than the early 280 pistons from XRs Only. Thumper Racing has done their homework on this mod.

It is honestly FUN FUN FUN. I even have had to debate myself whether to ride a XR300 or a XR440 on which would be best to ride. I like the power of the XR440 but I honestly feel I might be faster on the XR300 ( or 315 to 320cc ) . I say go with the 300 kit and a Edelbrock 30X32 oval bore with pumper. Or a straight 30mm Edelbrock. Be sure to beef up the clutch while you are at it to handle all that torque.

Cher'o,

Dwight

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I don't know who built mine, but I have an '88XR250 (300 bore) and it is awesome. Although it is older it has enough torque to do just about anything other than stuff only a cr500 could do. I can get the front end up and hold it through most puddles up to 2 feet deep. I did almost 3 once and it shot up a wall of water so that I couldn't see. Keep in mind I was riding a big wheelie while standing up and I am 6'4"!! I ran out ouf power so the front end dropped and near the end of the puddle (about 40ft long) and the water swallowed my front end. The fender even dipped a bit into the puddle. Needless to say I made it out and had no problems. All I have to say is it was sick. Anyway, my bike burns oil sometimes, but I dont know why. Its a spaz. If you are thinking about doing the big bore, just realize that the bike will overheat quicker. In 90+ degree weather in the mountains I have to watch it sometimes.

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Shinyboat,

Do you have any pics of your '88? I'm doing a 300 kit rebuild on my '88 also.

Mike

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Has anyone done the 300 by combining the 280 bore and the 3.1mm powerroll stroker? I've got the 280 now. What did the power gains feel like compared to the 280?

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I have the 300 kit on my 1997 XR250R 🙂

It was already on the bike when I bought it but it has the Mikuni 34mm flatslide, stock header and and FMF PerforMax silencer. I"ve since added White Diamond stainless steel valves. I have never had a problem with overheating in the Arizona summers when temps in the early morning are usually around 105 in the desert. The 300 kit gives lots of low to midrange power and it will tractor over anything like there is no tomorrow. It has no problems pulling the steep grades in the Bradshaw mountains northwest of Phoenix where elevation ranges between 3500-6100 feet, or the back side of the Santa Catalina mountains north of Tucson, where the elevation ranges from to 2500-8100 feet.

Even though I picked up an 05 CRF450R, I'm keeping the XR since it's such an awesome trail bike.

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I have built two XR300s using thumper racing's kit. As dwight said, the piston

is very well done and as light as it could be for the size of forged piston that it is. The stock piston is cast and is lighter than this piston.

I have a friend who has a thumper 300cc kit installed in his '86 xr. He is not happy with wear and quality of the sleeve - this was put together 6 or 7 years ago, so maybe thumper has changed their supplier on this item since then. Symptoms are smoking alot on startup and lots of very black oil changes after very little time (iron from sleeve).

I have run two different carbs with the 300cc kit. I think you want to run something larger than stock or the edelbrock's that dwight mentions above. The 35mm keihin FCR from Sudco (kit for xr250r) works very, very well. I doubt that an edelbrock comes anywhere close to it. If you are worried about jetting the keihin FCR - I can get the jetting specs out of my old bike - it was "spot-on".

The thumper 300cc kit uses lower compression to get reliability out of the motor and to let it rev. The other 280cc kits will run slightly higher compression and will gain bottom end torque at the expense of rev-ability. The 300cc kit should yield 28-29 hp with an increase in bottom end hp and torque. This will yield greater HP and torque than a CRF250-X, particularly at lower RPMs. Like they say "there is no replacement for displacement..."

- jeff

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Sounds great Oregon Rider. I can't wait to get mine back together!

Do you still have your XR300? What year bike did you build?

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I got two of the very first kits that they produced. Friend added one to his '86 around same time ('94-'95?). The kits I bought went into a '94 and a '95 - so at that time you had to split the cases to machine enough room to accept the larger sleeve. I had WP 4054 upside down forks on both - one had WP rear shock, the other had an ohlins... I wasted money - heads ported, cylinder head fins welded on (sucks to hear all that valve-train noise), custom header pipes fab'ed, yosh pipes, etc. etc... This time, I am going to keep it simple and get biggest bang for the buck.

FYI - I didn't have to "beef up the clutch" at all and I raced it very hard (but I know I can't fan the clutch like she's a two stroke).

Just went and picked up a shiny new 2004 xr250r today at lunch! Not sure if I'll do the 77mm J & E or the 80mm thumper piston yet. I'll probably do the 300. I have a 35mm keihin FCR "left over" from my old bikes. I'll stick with the stock pipe with a baffle insert. A new set of marzocchi shiver forks (45mm upside downs) go on the front. BRP triple clamps. Maybe an ohlins rear shock - but the stocker in the rear is okay.

After I get her all together, she's going to go out and have some CRF-X's for lunch.

-jeff

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All,

I up'ed my XR250 to a TR 300 (and a bunch more mods). I don't have any significant seat time on the bike yet (as I just moved back to Canada). But as soon as the snows melt, I'll let you know. 🙂

I'm initailly disappointed in the performance, but I suspect that I just don't have the Edelbrock carb tuned properly. To their credit, Edelbrock (and Barnums Pro) are provding great help (via e-mail). 🙂 If you search the forum for "Initial Disappointment with XR300" (I think), you'll find my original post.

I did have to split my cases for the TR XR300 kit; it does require that a sleeve be installed. The piston was Weisco (but is only available from TR).

I'm really looking forward to the bike once it's dialed in.

However, if I were to do it all again, I would have bought something better off the factor floor (orange, perhaps). Buying a bike for $x and then putting $y into it does not get you an equivalent to a $(x+y) bike. And your new bike isn't worth the $(x+Y) you've got in it. Still, if you like to wrench, it's the only way to go (as customizing a bike is fun). But have a thick wallet as y can be greater than x. 😛

Dale

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I saw your previous post about being disappointed with the performance.

I think you "did too much" and went backwards. My best friend builds my motors for me - he says "you can go backwards real fast if you try to do too much with this motor"

My advice would be to remove the cam, reinstall the stock cdi unit, and pick up a keihin FCR 35mm carb - me personally, I think the edelbrock is pretty crude when compared to an FCR (and I know most here will disagree).

Try it again after these changes and I'll think you will more than mildly impressed with your bike...

- jeff

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I remember Dwight Rudder saying to stick with the STOCK CAM when doing the 300 kit. I want bottom end, and mid range, which is what this stock cam gives. If you went with a cam thats mostly top end, I can see being disappointed possibly, as it is a smaller displacement engine (relatively speaking), and then you'd have to rev it to build any useable power, where there may be none below a certain rpm.

Just a thought.

But, Dwight has "been there and done that". He has an impressive list of accomplishments with his XR300's. I am doing everything he has recommended, when I build my bike (except go with the 320 kit, which has nearly 13:1 compression - I'm staying with the 300 kit).

Mike

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Good Point.

When you go to a rev cam like that, you also typically go with a 11:1 or greater compression piston. Stock compression is 10.5:1 on the '96 and newer XRs. The 300cc kit uses a 10:1 piston so that it will be reliable and rev with the stock cam. The combination of a rev cam and a 10:1 piston will not be good.

- jeff

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Thanks for the advice on going with the stock cam. Sure enough, I still do have it.

To the best of my recollection:

1) The Hot Cam's Stage 1 cam is not that radically different than the OEM cam. That cam was recommended to me by Thumper Racing for use with their 300cc kit.

Opps ... my mistake ... a read of the Hot Cams web site indicates that a Stage 1 cam does favour the mid and top end of the RPM range. So perhaps this is a contributing factor to my disappointment (in low end power). Doh! 😛

2) The aftermarket CDI unit does not change the ignition profile when compared to the profile of the OEM CDI. What it does is increase the maximum RPM. I do not know what the new profile is like beyond the OEM limit.

And, for sure, I did go "over the top" when it came to building that bike (especially for such a tame recreational rider as myself). It was a true midlife crisis kind of thing, and it was my medication while living in Dallas, Tx. :D

But I'm home in the mountains of BC now, and can literally ride from my garage. 🙂 And getting paid Dallas-sized checks in small-town Canada is awesome! 🙂

Dale

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Great thread. Of all the xr300 threads Ive seen , this is the first that happens to mention the 300 kit as having 10:1 comp.. 🙂 Now I think Iunderstand why a lot of people sing the praises of the 280 kits that use 11:1 and 12 :1 comp. With pumper carb I bet the perofrmance is close to each other of these two kits. A little extra cc Vs. less cc and higher comp . If this is the case, why sleeve the cyl. and split/mill the cases for the 300 ?.........Why not a xr320 at 10:1 🙂 ? the 320 kits uses a 13:1 comp piston that drives the head temps through the roof and endanger reliability. This may not be possible but I think its worth looking into. It would give even more cc for torque and still keep revability..

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Good point Vic. I wonder what the HP / Tq curves look like between 300 kit with lower compression, versus 280 kit with higher compression.

Anyone (Dwight?) want to comment on what the expected differences in HP / Tq would be, as well as power characteristics between the two scenarios?

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Great thread. Of all the xr300 threads Ive seen , this is the first that happens to mention the 300 kit as having 10:1 comp.. 🙂 Now I think Iunderstand why a lot of people sing the praises of the 280 kits that use 11:1 and 12 :1 comp. With pumper carb I bet the perofrmance is close to each other of these two kits. A little extra cc Vs. less cc and higher comp . If this is the case, why sleeve the cyl. and split/mill the cases for the 300 ?.........Why not a xr320 at 10:1 🙂 ? the 320 kits uses a 13:1 comp piston that drives the head temps through the roof and endanger reliability. This may not be possible but I think its worth looking into. It would give even more cc for torque and still keep revability..

The 300cc kit delivers everywhere and keeping the compression relatively low lets the engine rev freely - this yields accelleration. The increased cc's do more to increase HP and torque than the smaller piston (280 kit) with a slightly higher compression.

The 300 kit is the best, no doubt about it. But, bang for buck a 77mm J & E piston doesn't look bad either. For what it's worth, I wouldn't go greater than 10.5 compression. I have my best friend install my 300 kits for me (honda trained tech with 25 years experience) - he is busy right now - so I will probably go with 77mm J & E piston initially. He has a 300 kit in his XR. We'll dyno both and let you know what it looks like...

Seat of the pants is that the 280cc kits with slightly higher compression boost torque and hp down low but sign off a tad quicker (relative to the 300cc kit).

- jeff

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