Piston Port Derbi tuning help please

I did not do the build on this bike, and it has been sitting for a couple of years. It went from Indiania to Seattle to Orlando and now I am trying to get it roadworthy. Derbi Variant TT, It has a Metra 65cc piston port kit, what I think is a Zen pipe, and a 19 Dellorto PHBG. Carb started with a 260 atomizer, W9 needle, 45 pilot, and 86 main jet. It starts and idles fine, but really doesn't like anything but WOT. Even then, it seems low on power and doesn't fully variate. It does variate fully on the stand, just not going down the road. I have gone one up and one down on the main and the 86 does seem correct. I dropped the pilot from 45 to 40 and it still seems to be 4 stroking at part throttle. I get what feels like a bog from takeoff, but once I get up to speed and back off to part throttle it 4 strokes, so my guess is the pilot is still too rich. I don't have anything smaller than a 40, yet, and just wanted to see if this seems normal and I am heading in the right direction. I tried to search, but not a lot of piston port Derbi info, much less with a PHBG. My only comparisons are to piston port Puchs with PHBG carbs, and most seem to run pilot jets in the 40's. Does this seem normal?

Re: Piston Port Derbi tuning help please

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Re: Piston Port Derbi tuning help please

Sounds like you need to check your timing

Re: Piston Port Derbi tuning help please

coupla years is a while, fresh gas?

Re: Piston Port Derbi tuning help please

Fresh gas. I will check timing tomorrow, it has the HPI ignition on it, I don’t know why I didn’t think to check that. Tunnel vision with the jetting I guess. Phil put this thing together in Indiana, but that was 4-5 years ago I think, then it sat in a “collection” in Seattle but really never got ridden. I got it, then shortly thereafter I fractured my knee on my last Monday night ride with the Fleet before moving to Florida a little over a year ago, so now I am finally getting back to riding and wrenching. I am still trying to find tools and parts in my boxes of stuff.

Ryan, i just read your hpi review on Treats, that gives me a good idea where to start, thanks.

Re: Piston Port Derbi tuning help please

Mikey Antonakakis /

Maybe it's an illusion, but the tuned length of that pipe looks really short! Like it's set up for crazy high rpm. Does it rev out a lot higher on the stand than when you're riding it? I'm guessing if you aren't fully variating, it could be that you're shifting at too low RPM and aren't able to get into the pipe...

Like your pipe looks about the same length as this bike's, which seems like it is set up for pretty wild high rpm race stuff:

Re: Piston Port Derbi tuning help please

OK, timing is definitely an issue, it was as close to TDC as the slots would allow. I adjusted it a bit by eye and definitely better, now I need to find my micrometer and temp gauge, and go buy a timing light for good measure. I took one brief ride, got closer to full variation, better take off, but still 4 stroking when I back the throttle back down to half and just try to putt along.

Re: Piston Port Derbi tuning help please

yeah is it HPI mini or the inner rotor?

Re: Piston Port Derbi tuning help please

> Ryan N. Wrote:

> -------------------------------------------------------

> yeah is it HPI mini or the inner rotor?

Mini.

Re: Piston Port Derbi tuning help please

SABAT! andDestroy /

lighter weights , your falling off the pipe and falling into limbo

Re: Piston Port Derbi tuning help please

Adjust one thing at a time - as you know the timing is not spot on start there

Re: Piston Port Derbi tuning help please

Just a quick update. Timing was definitely off, I have it set at 2mm, maybe a touch more, but that is as far as I can get it without opening the slots more in the base plate. It was definitely better, but still shitty on the low end. I got some more idle jets from Treats, down to a 37 from a 40, 2nd clip from the top on the needle; better, but still a little rich. Put the needle on the top notch, better, might even still be a little rich, but more rideable and can cruise part throttle. I thought I had ordered a CHT gauge when I ordered the idle jets, but apparently ordered the wrong thing and got a Tach&Hour meter....cool, but not what I really needed. On my short trip around the neighborhood I think I hit about 45, the speedo seems fairly accurate compared to GPS; it's got more in it but I can't go much more than a 1/3 mile before I have to slow down for traffic, kids, or stop signs. Once I actually get a CHT I'll try a 35 idle jet, then maybe I can play with variator weights. But maybe I'll fix the brake light and get it tagged first. :)

Re: Piston Port Derbi tuning help please

> Mikey Antonakakis Wrote:

> -------------------------------------------------------

> Maybe it's an illusion, but the tuned length of that pipe looks really

> short! Like it's set up for crazy high rpm. Does it rev out a lot higher

> on the stand than when you're riding it? I'm guessing if you aren't

> fully variating, it could be that you're shifting at too low RPM and

> aren't able to get into the pipe...

>

I am not sure what the tuned length is, and I can't find a damn tape measure, but I am fairly certain it is this exact pipe https://www.treatland.tv/derbi-ZEN-performance-exhaust-pipe-65cc-p/derbi-zen-pipe-td-1000c.htm

Re: Piston Port Derbi tuning help please

Dear diary,

Update, got the 35 pilot jet installed, 2nd clip from the top on the needle, best yet. More responsive, cruises good at part throttle. My phone GPS says 47 mph top speed, pretty good but I still think it should be faster. The HPI Mini is set at right about 2mm, and that is far as the slots will allow it to go so I'll leave it alone for now. Now it is time to check out the variator, I have no idea what weights are in there, and change the clutch springs since it engages very early.

Re: Piston Port Derbi tuning help please

You need hella lighter rollers

Re: Piston Port Derbi tuning help please

U want it to 4 stroke a lil at part throttle cruise

Re: Piston Port Derbi tuning help please

What size needle were you running?

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