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Old 04-06-2021, 11:08 PM   #1
Gofastcart2020
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Default Low power after 1/2 throttle

I have a yamaha g16, rebuilt the motor. Bored .025 over new gaskets head gone through new intake valve and seal. Ran good for about 2 weeks just driving around the lot at work. Top speed of 25 baby-ing it. A buddy and I took it out at my house on the trails, nothing super crazy. Ran great for the first 25 min then I went to throttle it and it just seemed dead, thought it was getting low on gas, it wasn't. could get about 18mph. Seemed like past half throttle it was slowing instead of picking up speed. Wouldn't climb the hill it had no problem climbing the day before. Like it wouldn't downshift to get the rpms up. Got home and set the valves, and checked the plug, it was black.swapped it and tried again, no difference. Compression was 143ish. Cleaned the carb, tried different gas, different belt, flushed the fuel pump out, Ran a ground from the motor to the battery neg. Nothing seemed to help except tuning the pilot mixture screw from about 2 turns out to 1, 1.25. Will climb better but when I get past about 1/2ish throttle it's doggy and kinda jerks lightly driving down the road.. Almost like the Governor was kicking in.. but it's been removed.. Like I said it was running great than all of a sudden it lost power. 1 thing I noticed is that while driving with the seat off I can't get the belt to the top of the primary, bottom of the secondary while driving. Its fine when sitting in neutral and throttling it up and down. Takes off fine from a stop and gains speed til I get around 1/2 throttle. Helix buttons are there, ramps are a lil grooved on the back of the secondary where the buttons ride. Primary is new off ebay, 130$. Secondary is what was on it when I bought it. Gonna look into swapping the primary and the secondary if needed.. Otherwise any suggestions would be greatly appreciated. Camper opens up in 3 weeks, on a time crunch now. Thanks everyone.
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Old 04-07-2021, 03:43 AM   #2
CustomCurt
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Default Re: Low power after 1/2 throttle

ignition coils go bad, look for oem or high performance replacement
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Old 04-07-2021, 09:18 AM   #3
matthd24
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Default Re: Low power after 1/2 throttle

Primary is new off ebay, 130$. Secondary is what was on it when I bought it.

I don't like the cheap after market clutches I would invest in a good primary clutch. put it in neutral rev it up and see how you clutches are working
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Old 04-07-2021, 11:08 AM   #4
Gofastcart2020
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Default Re: Low power after 1/2 throttle

Quote:
Originally Posted by matthd24 View Post
Primary is new off ebay, 130$. Secondary is what was on it when I bought it.

I don't like the cheap after market clutches I would invest in a good primary clutch. put it in neutral rev it up and see how you clutches are working
I have a stock spare one I'm going to try this weekend. In neutral the belt rides at the top of the primary and bottom of the secondary as it should.. as far as the coil, it's a pickup coil on the flywheel.. It crossed my mind that's the prob. Again the crazy part that it ran great for awhile than all of a sudden didn't have crap for power kinda pointed me to the coil, but I wouldn't think heat would matter being it's right in the airflow from the fan.. This weekends plan is to swap Tha primary. 🤞
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Old 04-10-2021, 08:21 AM   #5
slonomo
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Default Re: Low power after 1/2 throttle

I know you said you cleaned the carb, but it sounds like a fuel issue. If the plug is black it's running rich. Trust the plug readings. Is the carb the stock Mikuni? Check all your carb boot connections and air filter connections etc. Is the air filter the stock setup? If using an aftermarket filter, they sometimes make the stock engines run funny. Put the stock filter setup back on with a new clean filter.
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Old 04-10-2021, 01:24 PM   #6
Gofastcart2020
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Default Re: Low power after 1/2 throttle

Carb is mikuni stock carb, sprayed all the possible areas of airleak , including around the airbox lid, which is all stock, with carb cleaner while idling, no change in rpm. I put a spare primary on it and lubed up the secondary shaft, and it seems better 22mph from 18. And the hill climbing is all there again. I'm guessing I am having secondary problems. Ramps are worn on the sheave and buttons are close to gone on the Helix. When I pull up on the belt the secondary opens and stays there..? I feel it's sticky. Or is this normal.?
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Old 04-11-2021, 10:51 PM   #7
sho305
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Default Re: Low power after 1/2 throttle

IT should close unless the belt is holding it open. Also those springs can break if you don't look close you wont see it broken in there...in the secondary. Make sure the helix is not hitting the trans case some have snap ring that can fail. You can put a new bushing and seals in the secondary if still available, yamaha cut some parts off now. I've done a few its easy. Make sure you preload the stock spring properly or it can be doggy with no preload.
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Old 04-12-2021, 07:06 AM   #8
slonomo
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Default Re: Low power after 1/2 throttle

If your engine is running smoothly and you're getting full revs, then yes the clutches will cause issues like this. Sho is right, check what he suggested. I've seen those snap rings broken before. Also any irregular wear on the sheaves and buttons will cause issues with torque/power and speed etc. I don't really like stock Yamaha carts with stock springs, they just don't have enough pulling power for me. I've always upgraded to a Plowman's type spring and/or a Plowman's secondary clutch kit, which doesn't have tangs on the spring and have "built in" preload so to speak, they are stiffer than stock.

On an older worn in cart I always recommend investing in a good quality new secondary clutch and belt. The secondary clutch controls your power delivery and shift etc and really needs to be in tip top shape to perform correctly. These clutches do wear over time and a worn out secondary will not perform well no matter what you do to it. Plowman's Carts has the best product for that IMO. Check the sponsors page.
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Old 07-20-2021, 11:52 AM   #9
Gofastcart2020
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Default Re: Low power after 1/2 throttle

So the final straw that brought this thing back to life, and this was a couple months ago, was pulling the foam pre-filter out of the airbox... 🙄🙄
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Old 07-23-2021, 04:34 AM   #10
Cartmaster
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Default Re: Low power after 1/2 throttle

Wish i had seen your original post back in April. I would have been able to diagnose it for you lol. Black plug plus low power = clogged air filter ( Rich mixture ). If it has a PCV hose going to the filter box, i am surprised it was not pulling oil up from the engine into the airbox.

In Fact, Slo was almost on the money at one point.
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