XR400 Top End

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#1
I intend to freshen up the top end on my 2000 XR400 in July. My question is should I simply bore it to the first oversize (87mm I think) or take this opportunity to resleeve and go to a 440 kit? It seems that theres only about $125 difference in the cost to resleeve to a 440. I know the 440 kit will give me more overall power but is there a major tradeoff in operating temperature or reliability? From what I've seen either the first oversize or the 440 kit will up my compression ratio to 10.5:1. I will also be having the head worked.

Any input on this matter would be appreciated.
 

penguin

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#2
If you go to a 440 and ever need to rebuild the topend, you will need a new cyclinder. I never have been a big fan of going to the limit on a overbore. And yes the bike will run a little hotter than a stocker, you will need to change the oil very often. I would just go with the first oversize with more compression. If you want really big hp, get a bike that is bigger and more powerful to begin with:cool:
 

mtngoat

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#3
I'm certainly no expert, but I can offer my observations and input from my local shop and White Bros., who've done a lot of 440 kits. I have two XR4s, one a 96 with a 440 kit, which I bought used a year ago.

The 440 kit is is considered by the shops to be a very reliable upgrade. Performance-wise, I notice a significant improvement in low-end power, which I like for my kind of mountain trail-riding. I don't notice a lot of difference in top end, which you may desire for desert riding.

I've compared performance with a 01 XR4 and a 98 with the HRC cam. The cam'd bike has similar performance mid-top end, but not as good on the bottom. All comparision bikes are set up similarly with (pipe, airbox, etc.), although age/engine wear may also be a factor, since the 01 XR4 is not far behind.

Also, I noticed a few months ago riding in the desert, my head gasket started "seeping". After checking with the shops, this apparently is not unusual since they claim the aftermarket head gaskets aren't as good as stock. I also suspect I was running lean and hot, because it stopped with cooler mountain riding.

BTW, The shops recommend a cam upgrade with the kit. I still have the stock cam. One shop doesn't like the White Bros. cam, except for racing, because they claim those cams are made from modified stock "cores" and have rapid cam lobe wear, compared to the HRC cam.

My XR4 riding group's amateur conclusion is that the 440 kit seems preferable for trial riding (better low-end) and the HRC cam seems preferable for desert. Or you can go both. Also, if you have a lot of hours and remove the head, valve seal replacement might be a good idea, since we notice these fail frequently due to high head temps.

I'm not sure the HRC cam is still available and it's pricey. But, if I rode mostly hot flat-out desert, had a newer XR and wanted the best reliability, I'd go with the HRC cam.
 
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#4
Originally posted by penguin
If you go to a 440 and ever need to rebuild the topend, you will need a new cyclinder. I never have been a big fan of going to the limit on a overbore. And yes the bike will run a little hotter than a stocker, you will need to change the oil very often. I would just go with the first oversize with more compression. If you want really big hp, get a bike that is bigger and more powerful to begin with:cool:
No, you would not have to replace the cylinder if you need a rebuild on a 440. You just replace the sleeve. The 440 kit is not an "overbore" it's a re-sleeve.

The bike WILL NOT run hotter. The sleeve is larger, and creates the same heat displacement properties as the stock liner/stock bore.

Higher compression WILL cause the engine to run hotter. Any compression high enough to make a performance difference will require a race fuel mix on the 400.

Oil should always be changed often on a four stroke. It's what keeps the engine running well. A larger bore doesn't make this requirement change any. But if you are lax about maintenance, a performance engine will die faster than a stocker. The stocker will still die though.

I'm sorry Penguin, but I think you're head is in the ice on this one :think

TexXR - there are other options also. The stock bore is 85mm on the 400. Your next overbore is + .010" (85.25mm). The 87mm piston is an option. It's .080" over the stock bore, and gives you 415cc, still leaving room for larger pistons in the stock liner (if they're ever needed).

I really like the power the 415cc kit gives. It's snappy and revs quicker than the 440. The 440 shines in tight technical riding situations, or if you're a big fella.

Both the 415 and the 440 are great kits. They do not hurt reliability on the XR, and they definitely add something to the fun factor of this bike!
 

ochster

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#5
TexXR, I believe the single best mod for these bikes, is a pumper carb. You certainly have to have some patience to dial it in, but it makes a huge difference. Aside from that the valve trains on these bikes are inhearently weak. A nice set of stainless valves, and a good valve job will only help. These items on a fresh bore, with no more than 10.5:1 comp. will make a huge difference. I have had problems with temp, and fuel when running higher compression ratios. The last couple kits I have been a help with, we used the 88mm 10.5:1 J&E piston kit (430). XR's Only sells the kit complete, pretty reasonable. My .02
 

penguin

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#6
if you pump up the engine, you increase the maintenance requirements and decrease the reliability and put more stress on other parts of the engine that is a fact of mecanical law, more power means more heat, and resleeving the motor the second time on the 440 is not as simple as you think. But I'm not paying for it, so someone can do what they want ;)
 
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#8
Thanks everyone for the advice. My bike runs fine now although is down on power from when it was new. My concern is if I don't freshen up the top end I'm asking for trouble. I've got 11 Enduros and 5 Cross Country races on it along with countless number of trail rides on it. It might be prudent to just go to the first oversize and concentrate on the head.

I agree that the pumper carb makes a big difference. I'm just leary of it. I've seen too many blue bikes on the side of the trail with their rider turning blue in the face from kicking.

BTW, me and my red mule XR400 are leading the Open C class in the Texas State Championship Enduro Circuit (TSCEC, championship division). Most of my competitors are riding two-smokes of the 300 variety along with some big blue thumpers (giggle giggle). ;)