I have the 2005 truck of the year Losi LST1. I replaced the original Mach .26 engine with a new tealheaded Mach 427 engine. When I installed the mach 427's carburator, I had to rotate it a bit so I could access the Low Speed Needle. That's because on the LST there's a HI & LO speed selector knob that was in the way.

The problem with this carb installation is, that the throttle return spring doesn't have enough power to fully close the carb's gap (because the servo doesn't have as good straight pull as before). I tried fixing this by replacing the spring with a nylon band (it's pretty flexable and a lot stronger, so I gave it a shot). Now the carb completely closes with no problem, but now my stock servo (JR Z590M, which has 85oz/in of torque at 6.0 volts) doesn't have enough power to go to full throttle anymore.
It goes to about 70% open, and I can see the servo struggling at holding the gap open at 70%. (With the spring installed instead of the band, it can go to 100% open with no problem, but then the carb doesn't fully close if the car loses battery power) What can I do? Is the only solution to get a more powerful servo? I would not have had this problem if I could have skipped rotating the carb, but I want to be able to tune the low speed needle aswell =). Take a look at the attachment picture, it will give you an extra idea of the carb's installation.

Any help highly appreciated!

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You think I can replace my throttle return spring with one of these and it will be good enough to close the carb's gap if the truck loses power? And is lose enough for the servo to go to full throttle?

If in doubt, use two of them, yes - it will close you're carb

If you are right, big thanks!

Imo using a Really tight throttle return will only lead to a bound up carb...i would use a band with enough force to close the Carb ONLY no linkages attached.....you arent going to pull the servo's back to idle if they fail at WOT or go to WOT during the run away.

have you checked to make shure the collet is letting it close?? Easy thing to over look! That's where imqould start. It shouldn't take that much to close it!!

+1 That is the only reason to run a throttle return spring. If the throttle ball cup comes off you want to close the carb.

the only reason i do is for races where they threaten to remove placings if a band or spring isn't found......had that happen at this year's Canadian Nationals....and to say the least I WASN'T a Happy Camper.

BUT 2nd in Pro buggy about an hour later made up for some of the frustration.

I use a band and a spring. To me it's like your visa card "never leave home with
out it". Once during my nitro touring car dayz my power cable unravelled and
started dragging on the ground until it completely wore through the wire. That happened as I was going down the back straightaway at about 65 mph in second gear WOT. I lost steering,throttle and brakes luckily my throttle return spring kicked in and closed my throttle making an ugly situation not so
ugly.

Download Videos here: http://www.filefront.com/14245099/Carb_problem.zip/

Video 1: With no linkage attached, the throttle return spring fully closes the gap.

Video 2: Linkage has been attached. Servo pulls the carb so it fully opens, then car is shut off. You can see that this would happen if the batteries in the car would go fully dead. (That is, the gap doesn't close)

Is this an issue?

P.s. sorry about the video quality, I didn't have my camera with me so I had to use my mobile phone

No worries...a throttle return spring is there in case the linkages themselves fail or the ball pop's off the carb slide.

Ohh ok, great! I have a failsafe, but I was told that if in some case where the power in the car would go dead, the failsafe would not work, therefore I should use a throttle return spring to force the carb close. The day before yesterday, my failsafe safed me from a runaway, I checked the batteries in my truck but they were still 40% charged.

Is my failsafe maybe that good that it engages when the battery is low, so I won't have to worry about runaways?

This doesn't put too much pressure on the servo does it? I'm worried about burning up a $100 servo, but if I get a runaway, it could be more expensive! Anyone got a pic of how this looks?

In looking at the pic you might want to turn your carb a bit. Part of your problem maybe the mis allinment.