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Old 01-12-2013, 04:17 AM   #1
Jdeks OP
Joined: Aug 2009
Oddometer: 1,022
Total loss of WOT/top end power.

Okay, so I've just done my valves and installed pyndons prefilter, and configured my carbs as per below:

42 pilots
158 front main
162 rear main
IMS 1.5 turns front, a.625 turns rear
stock needles, thrid from top
floats at 3mm
all else stock

My SAS and EPC are gone, and I have a 2-1 exhaust with a 'straight-thru' style pipe (Barrett). Carbs are synced, TPS calibrated.

Since putting it all back together, I no longer have enough top end power to even get to 100 kph. The very first little jump when you hit the throttle low in the revs is there, but it dies instantly, and even on full throttle, acceleration is like that of a 250cc.

I'm following the carb tuning advice on the HOW, which seems to say my symptoms indicate my mains are too small. Pulled the brand new rear plug, very end of the tip is white, which I take means lean. So I;ve stepped the mains up to 162 front 167 rear, then 167 front 172 rear. Slight improvement each time, but not much, reluctant to go any larger. Also, she snaps, crackles and pops like a bitch on deceleration.

Where should I look for what I've done wrong? Could this be the result of botched valve clearances, or is it going to be carbs? If so, which part?

Thanks heaps for any help.

PS I found the original working carb settings: 155 rear main, 150 front main, factorypro needles on 3rd clip, front and rear IMS at 1.5 turns, floats at 2mm

Jdeks screwed with this post 01-12-2013 at 04:43 AM
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Old 01-12-2013, 06:30 AM   #2
They call me......Ronski
Joined: Feb 2007
Location: Wishing I was back in Grand Junction, now in MN
Oddometer: 717
Look for a fuel delivery problem- plugged filter, pinched fuel or vent hoses, failing fuel pump.
Enduro Racer & Big Bike Adventurer
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Old 01-12-2013, 09:52 AM   #3
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Joined: Aug 2010
Location: Austria
Oddometer: 127
Maybe just the fuel line is squeezed when you mounted the tank, happend to me, same symtons.
We blew it
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Old 01-12-2013, 05:33 PM   #4
Jdeks OP
Joined: Aug 2009
Oddometer: 1,022
Hey guys, thanks for the suggestions. Checked the fuel lines, all good - but by pure luck it put me onto the actual problem.

I hadn't seated the seals under the black plastic end caps properly - the little copper tube was crooked on both of them. So the slides were moving back and forth WAY too easyilty (no suction noise/resistance when i played with them). Re-seated both seals, running fine.

My knowledge of carbs is limited - if someone can chime in and tell me what effect that would actually have when running the engine, that would be great. My theory is that when I hit the gas, the slides weren't opening (no suction in the chamber behind the black end caps), and so I had bugger all flow into the cylinders.
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Old 01-12-2013, 06:10 PM   #5
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Joined: Mar 2007
Oddometer: 2,651
The carburetors are called Constant Velocity Carbs. I just found this simple YouTube video on how they work..

Sorry, I don't know how to embed that.

They are marvelous at providing good smooth throttle response and compensate for altitude mixture adjustments automatically..

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Old 01-13-2013, 02:58 PM   #6
Jdeks OP
Joined: Aug 2009
Oddometer: 1,022
Hey, thanks for the video, most educative.

Still having minor problems with the carbs, though, and my knowledge isn't quite enough to know where to go

Pulls hard as heck from 7500 rpm + ,literally gives a yank spot on 7500. So I think I have the mains right. But the low end doesn't have the kick it should. 155/158 mains, stock needle on third clip, little over 3mm floats., IMS 1.5/1.675,42 pilots. Still running the 2-1 pipe, and pyndons pre filter.
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