CRF70 Upgrades/Mods

My son & I are wanting to improve the performance of his CRF70. We are not ready to do any bore kits etc... at this time. Based on what we have read, it seems that we need to change the sprockets in the front and possibly the back. Also, we need to look at the intake/air filter. Lastly, possibly the suspension. I would like to hear from some of you who have done this. What we need to know is the following:

1) What sprocket sized do you recommend on the front and back? 13/42?

2) On the intake, we are looking to keep the same carb., so do we just get a Uni filter?

3) On the suspension, need something tighter on the front, and would like to know if there is a swing arm which fits the back with no mods needed?

Any help will be appreciated.

1 and 2 are right. I think BBR makes a swing arm.

do aftermarket exhaust help in any type of proformance? if so tell me a good brand name. if not thats fine.

I would keep the stock exhaust if your going to leave the motor the same. for the front, you can get Damping rods from BBR that will give you better travel, and get a BBR swingarm in the back to even it out...

where should i get my sprocket from

Sprocket Specialties or any one of a number of 50 suppliers carry 13t. sprockets.

The foam filters offered by for the 20mm carbs will fit the stock 70 carb!!! :D Supposedly there's a UNI available but so far I've not found it. Currently I have a K + N and a foam filter I got from an Amsoil dealer, although it's not specifically designed for the carb mouth size on the 70--it came with rubber adapters that seem to work although you have to make sure they're oil free and not to overtighten the hose clamp.

I preach the gospel of using 50 primary gears on the 70 for even more acceleration. The part numbers you need:

23110-GK4-010 big gear

23120-086-020 small gear

23112-GBO-910 clutch outer drive gear

The big gear is narrower than the 70's but if the 50 gears hold up under 88cc+ motors I figure they'll last on a 70.

Main jets are available from Sudco ( or 323-728-5407). They are part number 99101-116-(jet size). They're available in sizes from #50 to #120.

Also the intake manifold isn't matched well to the intake port of the head. The head hole is larger and off-center from the manifold hole. I copied the head hole location on a piece of paper, cut out the hole, and then used the the paper template to match the manifold to the head. I had to remove a lot of aluminum to get it to match. Won't give you 10 HP but every little bit helps and all it costs is your time!!

i just dropped 2 teeth on front and it came from rocky mountain, price was 6.95


Consistantly hear of dropping to 13T on front. Anyone change the back? Is so, to what size? I was thinking of going to a 42, but don't want it to kill the effects of going to 13 on the front.

i was thinking the same thing

bigger in back and smaller in front, slower top speed/lower gearing

Well, I got the exhaust tip today. It's very nicely made and fit up well on the bike. It's about 1/4 the weight of the stock insert. The bike is considerably louder and revs out quicker.

I have not changed the jetting yet. I still need to modify the air filter or put a pod filter on the bike. I'm at 5,000'+ so I'm probably a little rich as it is anyway.

I am racing the 70 in a motard class right now i am 5ft10 165

fast 50's stiff fork springs/10 weight fork oil

fast 50's riser

skid plate

fmf pipe

1/4 turn throttle

extended throttle cable

12 inch front and rear with street tires

i like bmx bars but its all personal pref

werkes rear shock a must

cdi ripper box

kn air filter

bigger pegs and folding shifter helps

13/43 gearing but as sprokets are cheap i would play around with that

some guys are running a 19mm carb

hot cam

some guys are running the 4 speed manual tranny i prefer the stock but you may want to play around with the clutch

i need to come up with some sort of lower fork brace as the lower tripple bends easy

and most likely you will constantly be working on the jetting

it worked great.

Where are you guys getting the pod filter from. I orderd a 4112st uni filter and it does not fit even though the catalog says it will. Anyone know the correct size or where I can get one? Thanks 04 crf70F


i have the crf70 2008...sweet bike stock but if your looking to throw on a little more power for little cash and without bore-ing your engine put a bbr round exhaust pipe. they are only 170$ and you can install your stock end cap so that its a simmal quieter sound. Go to and check it out for yourself! its a pretty decent mod, as well try to get a aftermarket rev box its another cheap mod that helps with the power.

Just keep the back the same if you just want a little more pull bc you dont want to ruin the top speed on it bc then 1st gear and 2nd will be almost usless...

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