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Low RPMs on takeoff

Posted: Tue Oct 17, 2017 9:33 am
by ansneds
I've been experiencing low RPMs on takeoff for awhile now and I can't seem to figure out why. I've read multiple posts saying that it's either the main or pilot jets. I am using the stock carb with a 78 main jet. Also have polini variator with unknown weights in it. Just put in 9:8:1 gears in too. Please let me know if there's anything else I should look into.

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 9:56 am
by Meatball
Is your motor stock with a PG Short exhaust?

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 10:06 am
by ansneds
Yeah it is

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 10:49 am
by Meatball
That pipe likes 8k-10k rpm's....not 2k-6k....especially on a stock bore.

Tall gears are a trade-off as well. If your mountain bike was in a tall gear, it would be tough to pedal...until you get the RPM's up. Multiply this with an exhaust that has no power at low RPM's and you have a recipe for sluggish take-offs.

Stock bore + PG Short + Tall gears = slow take-offs.....add a BBK, Intake, Carb, etc and you'll fly.

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 1:31 pm
by ansneds
Alright I'll try the stock exhaust to see if it's any better. Do you think I need to upjet?

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 1:53 pm
by Meatball
Whenever you change exhaust you should re-jet. A higher flowing/revving exhaust would require an up-jet to normalize the additional air provided. Having changed ONLY the exhaust wouldnt require a very drastic jet change since the surrounding components (airbox, carb, intake, bore) are still in restrictive "stock" form, limiting the amount of additional flow your PG Short is capable of providing. Maybe only a few sizes up. Ultimately, you have to be realistic in your expectations of the stock bore. It can only give you what it has...which isnt much.

My suggestion: up-BORE!

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 5:56 pm
by bonesv
Meatball gives good advice on this but you could first try changing the springs in the clutch and maybe a few total grams lighter roller weights to help the revs come up before the drive starts shifting and see if you can get the power of the stock bore to at least get you off the line better. I have a stock bore with 9:1's and clutch spring/roller weight mods and it gets me to the store economically and a little faster! You will need to practice tuning the drive system anyways if you go bbk so it could be good practice. It's cheaper than all the other stuff at least.

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 6:10 pm
by Jonadz
With the pg short I found 5g weights in the vari to be the game changer, yellow contra spring and shoe springs to launch at 4500-5000 rpms, also stock belt, not a malossi for some reason.

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 9:51 pm
by ansneds
Jonadz wrote:With the pg short I found 5g weights in the vari to be the game changer, yellow contra spring and shoe springs to launch at 4500-5000 rpms, also stock belt, not a malossi for some reason.
Is that all on a stock bore or bbk?

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 10:56 pm
by Jonadz
That was on stock! I can dig out my carb tomorrow to check jetting but itÔÇÖs really an individual number, MeatballÔÇÖs ran great and I tried to mimic his, ended up being different by at least 5 sizes but IÔÇÖm pretty sure I had I mean air leak back then. Regardless rode it 69 miles a day in hilly terrain for a month and pulled on everything.

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 11:17 pm
by ansneds
Dang, what did you top it out at?

Re: Low RPMs on takeoff

Posted: Tue Oct 17, 2017 11:55 pm
by Jonadz
My highest gps reading was 67, granted it was on a spree swap but still a stock elite bore.

Re: Low RPMs on takeoff

Posted: Wed Oct 18, 2017 9:33 am
by ansneds
And you didn't have any issues with the yellow contra spring on the stock pulley?

Re: Low RPMs on takeoff

Posted: Wed Oct 18, 2017 10:17 am
by Jonadz
Red contra is the only one IÔÇÖve read about that presented issues with the stock pulley, I believe most with stock pulleys run a yellow and have yet to hear of failure myself.

Re: Low RPMs on takeoff

Posted: Wed Oct 18, 2017 2:12 pm
by bonesv
Jonadz wrote:My highest gps reading was 67, granted it was on a spree swap but still a stock elite bore.
You need to get a new gps :roll: