First Scooter-bad start!
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Member
GT125
Joined: 09 Nov 2020
Posts: 14
Location: Ireland
Fri Nov 20, 2020 1:44 pm quote
Need help. Bought a GT125 ('03) in September. Really enjoying my new hobby but after less than 100 miles, it has started to give trouble. It won't take full throttle. Will do 40mph with throttle slightly open but any further movement causes it to die. When it slows, the engine comes back and repeats. Been off the road since it happened cos it's dangerous. Here's where the fun starts. Convinced that it is a fuel issue, I checked the vacuum lines. Head in engine compartment, sucking hose from manifold. (Think scene from Airport re. auto pilot). Air getting in at fuel pump. New pump was the same. Opened old one and there is a vent hole. Removed carb for first time. Made a meal of this. Then watched "Robot" on youtube make it look easy. He recommended cleaning carb before stripping. Cleaned with toothbrush and paraffin. Washed down with carb cleaner but hit candle that i lit to drive away flies. Set carb on fire and also the bench and my gloves too. Lucky I didn't spill my beer. Opened carb to clean but someone had been there before me. Someone who is still using the screwdriver he got with his Meccano set 40yrs ago! Couldn't get top off to inspect diaphragm but cleaned jets and refitted. Still the same. Removed a 2nd time. No candle or beer this time. Heated carb body and got diaphragm out. No tears. Cleaned up slide and refitted. No better. Removed carb again. Checked float level and cleaned inlet valve. No better. Been off 6 times now! Replaced fuel lines, fuel tap, plug and coil. Thinking of getting the candle out again!! Anyone help me here? I'm on the ropes:(
Hooked
S150, Beo 500ie
Joined: 14 Aug 2019
Posts: 403
Location: Bermuda
Fri Nov 20, 2020 3:02 pm quote
The fact that the engine dies when fuel is in high demand, and recovers when fuel is in lower demand, suggests to me that the needle valve is not letting enough fuel into the carb float bowl.

Assuming the needle valve is clean and moving smoothly, there are actually two adjustments to it. Both are subtle but can be very significant and, confusingly, they control the same thing: how much fuel gets into the bowl.

The needle valve is held in place with an L-shaped bracket touching the float. The bracket is held in place and pivots around a pin that can be removed for these adjustments. I think there's also a tiny spring on that pin. Take a picture of all of it before you disassemble it to make sure everything gets reassembled correctly.

One of the legs of the L touches the float. If the L is set so the float is too high, it lets too much fuel into the bowl, flooding the engine. If the float is too low, which might be your problem, not enough fuel gets into the bowl and the engine runs out of gas.

The way you're supposed to adjust this is to measure the height of the float above the carb bowl flange when the L is just touching the needle valve, and bend that leg of the L so the float sits at the right height. I don't know this number for your carb but most carbs have manuals that specify this number, which is sometimes called "droop."

The other leg of the L rests against the needle valve. If it's too high, the needle valve will be pressed into its chamber too far (or for too long as the fuel in the bowl is used up), and it won't let enough fuel into the carb so the engine runs out of gas. This could also be your problem. If it's too low, the needle valve will let too much fuel into the bowl and the engine will flood (ie, never start, or start with billows of smoke until it stalls).

The challenge is that these two adjustments affect each other. And although it sounds odd, they don't COMPLETELY affect each other. You can bend one a little bit without affecting the other, somehow.

I think the leg of your L touching the float is too low, or the leg of your L touching the needle valve is too high. Try tweaking them both a tiny bit, just a few degrees, and see what happens.
Member
GT125
Joined: 09 Nov 2020
Posts: 14
Location: Ireland
Fri Nov 20, 2020 3:15 pm quote
I agree with your theory Juan. That's why I pulled the carb off for a 6th time tonight. The main jet lifts fuel from the bottom of the chamber and when I drained it tonight, there was a complete fill in it. I did try to check the float level as best I could and it seemed perfect to me. From standstill, the fault happens before i reach the house next door. Thank you for your very detailed reply. I think we are in the right area.
JR
Hooked
S150, Beo 500ie
Joined: 14 Aug 2019
Posts: 403
Location: Bermuda
Fri Nov 20, 2020 3:26 pm quote
I suppose draining the carb and measuring the amount removed could ascertain that it contained the appropriate amount of fuel, ie, the float is at the right height, without disassembling the carb. But I don't think that's the standard metric. Might be an interesting data point if you had a known, working carb to compare it to.

If you're happy with the amount in the bowl, and the scoot is running out of gas, then the problem is that not enough is getting into the bowl. So...the other side of the L touching the needle valve.

Again, assuming the needle valve is clean, free to move, and the chamber above it is free and clear of gunk.
Member
GT125
Joined: 09 Nov 2020
Posts: 14
Location: Ireland
Fri Nov 20, 2020 3:36 pm quote
I think so. I feel a 7th removal coming on. However I'm wondering if the main jet is opening. There is wear on the slide so before I take the carb apart again, I'm gonna try for a replacement diaphragm/slide. When I blow into the side vent, the slide lifts but I dont know if the engine vac is enough to do this. If I'm feeling brave tomorrow, (and it's not raining!) I might connect a tube to the vent and blow into it when the engine starves of fuel.
Member
Vespa Supertech
Joined: 17 Nov 2020
Posts: 16
Location: Australia
Fri Nov 20, 2020 6:36 pm quote
Re: First Scooter-bad start!
Rustybike wrote:
still using the screwdriver he got with his Meccano set 40yrs ago!
same (and yes, approx 40yrs ago also)
Member
GT125
Joined: 09 Nov 2020
Posts: 14
Location: Ireland
Tue Dec 01, 2020 11:32 am quote
All sorted now. I was correct with my thoughts that my cut-out problem was related to main jet opening, however I was completely wrong with my thoughts that fuel was not being supplied. It fact too much was being supplied and engine was choking. Although I checked the level of the float, and found it to be perfectly level, when I found that the engine had flooded, I adjusted the level by a small amount. I have spent hours understanding how too much fuel in the bowl could give an over-rich mixture, but it can. Think of sucking liquid using a long straw. More difficult when liquid is far away,easier when it's close. Same with the lift tube.
By the way, the new fuel tap I fitted failed after less than 2 miles. Began dripping fuel over the exhaust from a vent hole in it's side. Had to refit original. Plan to refit all the covers tonight and give "wee pet" a good polish. Thanks for your help everyone. JR
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