Derbi PR Tuning: What am I missing?

I can't seem to get this engine dialed in and i'm not sure what I should be focusing on or what order to tune in.

Here are the specs on the engine:

- Metrakit (cast iron cylinder but aluminum head) I think 70cc

- Powerdynamo w/ timing at ~1.8mm BTDC to try and keep temps down

- 21mm PHBG, red foam/mesh air filter, main jet=82, not sure of others

- Metrakit blue baffle pipe

- 6 roller vario, right now has 2.5gr polini weights in it

- blue polini clutch springs

- gearing 20/35

I can't get this thing past 47 mph @ 7500 RPM. On the stand I can rev it to ~10,000 RPM but not under load. It doesn't feel like i'm into the pipe at all when i'm riding it. I've tried different weight combos. With heavy weights it doesn't variate fully. Plus the engine temps keep climbing into the 400's at WOT.

So what am I doing wrong here? Too much carb? Weights way off? Powerdynamo not adequate for this setup?

Re: Derbi PR Tuning: What am I missing?

small air leaks or lean carb tuning will give you low power but rev on the stand.

15 grams sound pretty decent for the tjt, but you could try to drop weights and see if you can rev more.

Re: Derbi PR Tuning: What am I missing?

Probably Fred /

To make good power you need perfect jetting, one jet size or needle selection off and your bike will be a dog,

Jet it/test it with no filter then add k&n filter and then see if rich or Lean/do plug chops/whatever it takes to get clean crisp jetting throughout the range

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

What's your squish?

I'm betting you're lean.

Open filter req much larger jetting.

At least 90+.

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

Using this filter w/ an adapter. Not sure what the squish is off the top of my head, i'll have to measure it tonight.

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Yeah, that's basically open.

With case Reed and a big kit you need to give it some juice bro.

Have you pulled the enricher while running to see if it runs better?

You need to get squish around .8 to really feel snappy.

7500 is low as shit. Either your timing is way too retarded,

Or your jetting is way off or both (edited)

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

Ah I hadn't pulled the choke to see if runs better. Derp what a good idea.

Powerdynamo recommends 2mm BTDC in their assembly instructions but I was running to hot so I retarded it more. Kinda tricky getting timing perfect, no woodruff key and stator doesn't move.

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Good squish and fat jetting and moderate timing keeps you cooler.

Overretard timing can heat you up just as bad.

Re: Derbi PR Tuning: What am I missing?

don't forget to replace rear spring. there is a post about it century spring number C-806

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Yes, lugging the engine out of the powerband can create way too much heat.

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

Dan O Wrote:

-------------------------------------------------------

> don't forget to replace rear spring. there is a

> post about it century spring number C-806

This one?

https://www.mopedarmy.com/forums/read.php?7,2062522,page=2

Also, holy shit long thread:

https://www.mopedarmy.com/forums/read.php?7,3027947,page=1

Thanks Jbot

Re: Derbi PR Tuning: What am I missing?

the century spring is bullshit.

Re: Derbi PR Tuning: What am I missing?

meh, I like it. but I've never been able to get it to variate 100%, like 90-95% though and the mid-range slays.

Re: Derbi PR Tuning: What am I missing?

That's the spring and you can machine the rear vario to close tighter. Posts on that as well. I've always had good results with those springs

Re: Derbi PR Tuning: What am I missing?

campeona del mundo /

yeah amish gold is a ripper but limits it easily 20%

Re: Derbi PR Tuning: What am I missing?

I had a similar setup PR metrakit, blue baffle, tjt, stock ignition, with a 15 sha, got me into the mid 50's

I used the weights that came with the pipe, 4g i think.

I never had any luck with the powerdynamo's. Just seems like a basic stock cdi.

Did you check your crankcase seals?

How about the reeds? those reed blocks have some room to open up in there.

I really like those metrakits, Those motors and pipes are almost all gone now.

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

If a spring is stopping you from variating it might be getting coilbound.

Gotta check the century sping fully compressed, vs the space inside the pulley when fully open.

I had problems with this on hobbits with those red and yellow aftermarket springs. Just a bit too long whrn fully compressed.

If it's just a touch too strong, I've had incredible results with shaving a section of the outside of the spring in a bench sander to weaken it. Makes it act more manageable.

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

Reeds are opened up as far as they can go. All seals are good as far as I can tell.

I fixed a small airleak @ the carb and messed with the timing today, I got up to mid 8000 rpm but not any more. I tried upjetting but anything more than 82 it just four strokes. It really feels like it is right on the cusp of hitting the powerband.

@jbot how do I check the spring?

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Are you using the aftermarket spring?

That's the only one you'd need to check.

Put spring in vice, compress, measure coilbound length.

Then open the rear pulley without the spring, measure distance spring has to live with the pulley in the fully open position

I like to give my springs at least 1-2mm space to not get coilbound.

I doubt that's your issue if you're rocking a stock spring.

Here's how you find out your problem.

Pull out your weights.

Try it with no weights and you'll eventually hit the power unless something god awful is wrong with your engine.

Then tune the bike to that power.

I bet your rear spring is weak as fuck

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

I think it's the stock spring but I'm not sure, think I'm the third or fourth owner of this engine since it was kitted.

I'll try running without the weights and see what happens. Thanks!

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Another way to test a weak contra is if you rev the bike just a touch on the stand, if it variates all the way or just too far really fast, your spring is toast.

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

Yeah it variates stupid fast. Think my spring is toast. What's a solid replacement? Looking at Treats there is the red contra, yellow contra, and tjt.

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Welcome to rear pulley tuning.

Experiment bro

Re: Derbi PR Tuning: What am I missing?

If the century spring is limiting what do y'all recommend? red? yellow? I've been running the century spring and I noticed it won't variate fully. Definitely rips but...

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

Mod the spring like I said.

I'm not writing just to exercise my fingers

Check for it getting coilbound,

And if not weaken a 1/3 of it with a belt sander or grinder

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

If it's getting coilbound, you gotta shorten the spring a tad

Re: Derbi PR Tuning: What am I missing?

gotcha thanks

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

JBOT, Bro Wrote:

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> Welcome to rear pulley tuning.

> Experiment bro

Ok will do.

DAMN there are a lot of tuning options on a Derbi.

Re: Derbi PR Tuning: What am I missing?

Totally normal JBOT /

As far as tuning all dual variated bikes,

I prefer to use the weakest spring possible that will not allow you to overvariate, and weak enough to fully variate.

In general, the first 1/3 to half of pulley travel can be weaker and shift faster,

While generally the upper half of variation needs more resistance.

I really try and not use a super strong spring because it does waste power.

especially down low.

Interesting fact.

On my 70cc kitted gy6 4t scooter engine,

The stock rear spring was so over tight it was nuts.

Variated all the way, but it was a slug getting there.

So I found the weakest shortest contra I had off of a stock 50cc 2t bike.

Could compress with one hand.

Holy crap, it made that bike come alive. Such fast acceleration for a shitter.

A kitted 2t is much more peaky so more care must be given to the spring choice as falling out of the band is a real concern.

That's the main reason I run 2 different tension springs in my hobbit rear pulleys.

Soft for the beginning, hard for the end.

Modding the single spring can accomplish the same thing by thinning out a section as I described. Many laugh at my tuning ideas, but he who laughs las las las.

Re: Derbi PR Tuning: What am I missing?

MPB Andyrew /

So is this variating too fast?

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