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Surging when backing off throttle

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Surging when backing off throttle

Postby Hughieboy » Fri Mar 27, 2020 8:57 pm

Hi All

Quick question (hopefully!)

GT186 with 30mm PHBH, 60/110 crank and Avanti Ex Box pipe. Varitronic ignition with Readspeed adjustable CDI.

Was running the following jetting;

135 main
X13 needle, 1 clip from top
AV268
40 slide
55 pilot
70 choke
300 float needle

As recommended by Rich T.

Originally had a TSR Evo on there so I have now put a slightly smaller 130 main jet in.

Runs great, but when you back off the throttle from wide open it momentarily seems to pick up speed. It also did this with the expansion and larger main jet. It just seems like it runs a little freer and prefers to be slightly backed off from full throttle.

Does anyone know what this means?

Thanks.
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Re: Surging when backing off throttle

Postby coaster » Fri Mar 27, 2020 10:10 pm

What's the plug colour like at WOT? could be a little but rich matbe?

Also wondering about your ignition system, I thought the Varitronic had to run with its own cdi
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Re: Surging when backing off throttle

Postby St George » Fri Mar 27, 2020 10:50 pm

Classic rich symptom, some bikes rev higher when they are about to run out of fuel as it weakens the mixture.
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Re: Surging when backing off throttle

Postby Hughieboy » Fri Mar 27, 2020 11:13 pm

coaster wrote:What's the plug colour like at WOT? could be a little but rich matbe?

Also wondering about your ignition system, I thought the Varitronic had to run with its own cdi


It does, but Rich T said he recommends static timing for the GT kits. So I bought the Readspeed CDi that has two variable and two static settings. Does it make any difference? Not sure really. I was struggling to get it to rev past 6000 revs at the time so I tried it. Set it at 17 btdc and it seems fine at that but I think I may have had the timing set a bit wrong previously.
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Re: Surging when backing off throttle

Postby Hughieboy » Fri Mar 27, 2020 11:14 pm

St George wrote:Classic rich symptom, some bikes rev higher when they are about to run out of fuel as it weakens the mixture.


So should I maybe lean off the needle a clip and try that? Carb settings are a bit of a mystery to me...
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Re: Surging when backing off throttle

Postby Fast n Furious » Sat Mar 28, 2020 2:00 am

I'd recommend a weaker main jet first. Try a 130 and see what difference that makes. Do a plug chop to confirm a good burn.
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Re: Surging when backing off throttle

Postby Hughieboy » Sat Mar 28, 2020 7:15 am

Fast n Furious wrote:I'd recommend a weaker main jet first. Try a 130 and see what difference that makes. Do a plug chop to confirm a good burn.


I’m running a 130 now. I could try going smaller....
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Re: Surging when backing off throttle

Postby Mel K » Sat Mar 28, 2020 7:57 am

I always thought this meant it was lean ,a bigger pilot jet might be needed or maybe you have an air leak .
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Re: Surging when backing off throttle

Postby Hat » Sat Mar 28, 2020 8:33 am

I’d plug chop from WOT, it does sound like it could be over rich there. My setup isn’t a lot different a d I’m on a 120 main. If the plug is black I’d drop down again to 128 or 125 and see how it goes. You could try a x7 as well
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Re: Surging when backing off throttle

Postby MickYork » Sat Mar 28, 2020 8:52 am

I wouldn't alter any jets till you've plugged chopped, then you have your datum point. As said, an air leak is a possibility, so maybe a leak down test as well.
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Re: Surging when backing off throttle

Postby Hughieboy » Sat Mar 28, 2020 9:58 am

Plug chop it is then..... not sure if that’s permitted activity at present!

I’ve recently rebuilt the engine/changed the crank and bearings and did a thorough air leak test so I’m pretty sure it’s not that...
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Re: Surging when backing off throttle

Postby roli150lam » Sat Mar 28, 2020 10:33 am

Hughieboy wrote:
coaster wrote:What's the plug colour like at WOT? could be a little but rich matbe?

Also wondering about your ignition system, I thought the Varitronic had to run with its own cdi


It does, but Rich T said he recommends static timing for the GT kits. So I bought the Readspeed CDi that has two variable and two static settings. Does it make any difference? Not sure really. I was struggling to get it to rev past 6000 revs at the time so I tried it. Set it at 17 btdc and it seems fine at that but I think I may have had the timing set a bit wrong previously.



I have a gt240 with varitronic ignition with varitronic cdi.all supplied and put together by Eden Bakewell.
It runs fine.
Might be worth trying the readspeed on a scooter with a 'normal stator' if you can.,borrow a varitronic cdi to try.
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