LCGB Forums

The ability to post messages is restricted to LCGB members. Any questions contact us at lcgbadmin@googlemail.com

Surging when backing off throttle

Want to know which carb to choose? Having problems setting up the jetting details on your scooter? Wheter you are using a standard or tuned carb, ask your question here.

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.
User avatar
Hughieboy
 
Posts: 162
Joined: Tue Jun 17, 2014 7:15 am

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
User avatar
coaster
 
Posts: 2647
Joined: Fri May 30, 2014 1:00 pm
Location: Norfolk, Flying 8 Balls

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.
St George
 
Posts: 825
Joined: Fri May 30, 2014 3:48 pm

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.
User avatar
Hughieboy
 
Posts: 162
Joined: Tue Jun 17, 2014 7:15 am

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...
User avatar
Hughieboy
 
Posts: 162
Joined: Tue Jun 17, 2014 7:15 am

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.
User avatar
Fast n Furious
 
Posts: 1502
Joined: Sat Nov 19, 2016 3:56 am
Location: York

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....
User avatar
Hughieboy
 
Posts: 162
Joined: Tue Jun 17, 2014 7:15 am

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 .
Mel K
 
Posts: 924
Joined: Sat May 31, 2014 7:18 pm

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
Hat
 
Posts: 71
Joined: Sun Jan 27, 2019 9:16 pm

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.
MickYork
 
Posts: 1476
Joined: Sun Jun 01, 2014 8:37 am

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...
User avatar
Hughieboy
 
Posts: 162
Joined: Tue Jun 17, 2014 7:15 am

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.
roli150lam
 
Posts: 1354
Joined: Fri May 30, 2014 8:16 pm
Location: Lincoln


Return to Carburettors

Who is online

Users browsing this forum: No registered users and 4 guests