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CRF70 Jetting Question


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Hello all.

Correct Jets (?) for a 2004 CRF70 with intake /exhaust mods only:

I just picked up a 2004 CRF70F for my 9 year old son. The only modifications to it are intake and exhaust. It has a FMF POWERCORE Exhaust system and a K&N Pod filter only.

The previous owner never changed the jetting so I'm sure it's lean (probably was even lean in stock form from the factory?)

Any body have this same setup that has successfully jetted there bike? Or a setup that's close to what we have here?

Thanks to all in advance!

-Damon in Santa Cruz, CA.

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  • 2 weeks later...

first just because it has a pipe etc doesnt mean its lean , my sons 70 ran fat stock after the pipe, pod and 88 kit etc, i still didnt need to rejet , just moved the needle .

does it hesitate off idle? or something? do a long wide open run , kill it , and look at the plug.

don]t just assume its lean.

just my 2 cents

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  • 9 months later...
Hello all.

Correct Jets (?) for a 2004 CRF70 with intake /exhaust mods only:

I just picked up a 2004 CRF70F for my 9 year old son. The only modifications to it are intake and exhaust. It has a FMF POWERCORE Exhaust system and a K&N Pod filter only.

The previous owner never changed the jetting so I'm sure it's lean (probably was even lean in stock form from the factory?)

Any body have this same setup that has successfully jetted there bike? Or a setup that's close to what we have here?

Thanks to all in advance!

-Damon in Santa Cruz, CA.

I have the exact same setup, but for a 2008. Please let me know what you did, if it worked, etc...

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Hello, The bike has been put away for a time and so I still haven't changed anything on the little motor. Early in spring i plan to tune it properly.

When i originally posted this i was looking for simply what others have done and what there general set up was (pipe, filter). The jetting and needle adjustments are not always the same for any pipe or filter filter box modification or combination. As mentioned earlier by another member a properly done "Plug Test" is always necessary. I do this with all my bikes (RD350, CB400F, XR200R, XR100R, XR80 as well as my air -cooled VW).

My son has an FMF Pipe and the manufacturer advertises with most of there pipes that no jetting change is necessary. I DO NOT AGREE with that statement! True that it is a possibility that no jet change "might" not be needed. However a plug check tells all ...not to mention a loss of performance change. I believe that FMF states this only to not scare away people from purchasing there pipe with the possibility of having to remove and open a carb (sometimes several times) to change the jet or needle position. Some people are not what so ever mechanically inclined (Moms etc).

Again ...with just there FMF pipe only mod on your bike it is possible that the fuel mixture doesn't change enough for them to state that a jet change is needed. To me how could it be a performance pipe if the jetting doesn't change?

Any performance change to the intake and exhaust flow could change the mixture ratio to a leaner condition. If you are going to add a performance pipe you probably would want the airfilter and or airbox opened up a bit for better breathing, other wise whats the point ...more noise and different appearance? A filter change will change the air/fuel mixture as well. So plug test's and jetting adjustment/changes should be made at the time you change both. Other wise you will have re-jetted/adjusted for nothing.

To be honest with you this little Thumper lacks power at top end with the K&N Pod filter and FMF Pipe on it (Main Jets are a full throttle carb circuit). I would not be at all surprised to find the "Plug" to be a little on the light colored side from a lean condition when i pull it in the future after a Plug test at higher rpm. I plan to order the 2 next main jets up from stock main jet size and go from there with re testing.

Remember, Lean mixtures cause HEAT and the problems associated with heat. Whereas an overly Rich mixtures can strip away cylinder lubrication leading to other problems ;o}

...not to mention power and fuel efficiency loss from either incorrect jetting/adjustment condition.

I will re-post after i finish my my boys bike.

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well im screwing around with my nephews crf70 and it's acting weird. It bogs badly from about 1/4 throttle on up, which I expect when it's cold. It really takes awhile to warm up and the bog to go away. After warm up, I checked the plug and it's black and sooty. So..... that means it's running rich. Is it possible a too rich condition can cause a bog? and the bog would go away once the engine heats up enough to burn the extra fuel???

Ive pulled the carb apart too many times to count, the jets are clear.

Edited by spokexx
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Were there any changes made (modifications or tuning) that possibly lead to this condition? Sounds like your nephew needs to give you the low down on the bike? But maybe he's a young kid? So hit mom or dad up for some details.

Black is a rich condition. But a rich colored plug can also happen when there are other problems such as poor ignition spark. Maybe the air cleaner is filthy also. But it could also be the carb. You say it only happens when its cold and goes away when warmed up. Sounds like the carb. The idle air screw adjustment might have been altered?

I

f the bike has been modified a jetting change is in order:

air screw & Pilot Jet = 0 to 1/4 throttle. Needle = 1/4 to 3/4 throttle

In any case here are some great jetting recommendations from Powroll:

NOTE THAT THEY SAY A PLUG CHECK IS NOT THE BEST WAY TO CHECK JETTING ...its just part of the process in my opinion

.......................................................................................................

POWROLL on JETTING:

1. Find a gentle slope that you can ride in 2nd or 3rd gear. Look for something that will put a decent load on the engine. This will be your tuning test area.

2. Make the recommended jetting changes. Always start rich and work leaner.

3. A basic outline of which jet is active at a particular throttle setting:

Pilot Jet = 0 to 1/4 throttle. Needle = 1/4 to 3/4 throttle. Main Jet = 3/4 to Full Throttle.

● Changing the Main Jet size won’t affect how your engine idles or runs at 1/4 throttle.

● Engine RPM isn’t what determines which jet is active - only throttle position does this.

4. Start and warm up the engine, then ride your ‘test hill’. Any point where you feel the engine stumble or hesitate indicates a rich condition. Note the throttle position and modify the corresponding jet (1/4 to 1/2 throttle = leaner needle. 3/4 throttle or more = leaner main jet).

5. Only change jetting by 1 step at a time, and 1 circuit at a time (don’t change needle and main at the same time). Re-check after every change.

6. Once the engine runs smoothly throughout, you’re jetted!

If you ever notice an abrupt change loss of power, or engine sounds changes, shut it down.

......................................................................................................

To best "Note the throttle position", you can Index the Throttles grip to the Throttle case on the bars. This way you can see 1/4 & 1/2 throttle positions at a glance. Maybe use some masking tape (white) that you can write on wrapped around the grip at the left inside portion of the grip (throttle case side). Then tape a small piece on to the Throttle case with one fixed line drawn on it to "index" from/against your rotating 1/4 & 1/2 throttle marks on the grip.

If you get to the bottom of this re-post your findings.

Edited by TWINFIN
more info
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ya know, i dont remember changing anything on the bike as long as he's had it. I noticed this problem starting last winter when i would ride it around the snow (shameless fun:smirk:)

In the summer when the weather was hotter, the bogging was'nt so bad once the bike warmed up good. But of course the bike warmed up faster.

Im gonna adjust the needle up or down and see what happens.

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