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Discussion Starter · #1 ·
Hello to all! Bought my son a slightly used bike. When got it would start, idle great but would stall after giving more Than 1/4 throttle. Choke would help alittle. Reads the posts, pulled and cleaned tank, carb and fuel valve. Replaced main fuel hose, filter, pilot jet, top cap of main jet. Cleaned carb thoroughly and adjusted fuel air screw under small cap to 2 1/2 turns out. Bike does the same exact thing. Fuel is new! Help!
 

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My bike is new and has the same issues.
When starting the bike let it idle high with the choke.
Let the bike warm up properly about 10mins.
The little nineties for some reason play up when cold I only have to rev it a few times when it's cold and it turns to crap.
Just let it run till its really warm and it should be right.
 

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Did you replace the pilot jet with the oem one?
I've read on here that the oem pilot jet is too small.
Alot of people are replacing the .32 jet with a .37 or a .39 when they upgrade the exhaust.
If increasing the jet size doesnt help you can also try putting a washer under the needle.

Hope this helps
 

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My bike is new and has the same issues.
When starting the bike let it idle high with the choke.
Let the bike warm up properly about 10mins.
The little nineties for some reason play up when cold I only have to rev it a few times when it's cold and it turns to crap.
Just let it run till its really warm and it should be right.

This is very true...

Even my 700 runs best after being warmed up for 10 mins or so.
 

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I have had the same issues with my sons. Like said above I changed the jet to a .37 and let warn up for 10 min. I have found no way around it.They are cold blooded little thing.lol
 

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There are a lot of posts on this. I put a full Big Gun exhaust on ours and then had the pilot drilled to a .37mm. Don't have any issues now. Starts fast and runs very well. In fact, it hits the rev limiter with my 215 lbs on it. Plug looks good now. It was very white with stock .32 pilot jet. Will be changing to a better air filter and may add the extra vents in the air filter lid. Probably will shim the needle after this to get the main jet involved quicker.
It will be very cold blooded with the stock pilot. Also, if the pilot gets any gunk in it, and .32mm is very tiny, it will idle but dies with the slightest throttle input. I had to use a small piece of wire and carb cleaner to get it cleaned the first time.

If you take the pilot jet out and hold a light at one end and you don't see a tiny bit of light through it, it's blocked. You have to be looking directly down the hole, any slight angle and it will appear blocked.
 

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Discussion Starter · #7 ·
Thank you all! I ordered a .37 and it should be here in a few days. Ill try that when it gets here. Is there anyway someone can elaborate on the process of adding the small washer, im not really sure what to do!
 

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I have also found that if you leave gas in the carb over an extended period of time the jets will clog again, even the larger ones. So when your done for a while shut off the gas and open the bowl screw and drain the gas out of the carb. I have doing this for a year now and the little bike starts a run great every time we pull it out.
 

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One more thing.
I only run premium unleaded in all my bikes.
Premium has cleansing agents for fuel injectors and this really does help with the jet blocking thing.

I have had a 90 for over 12months and have never had any issues.
 

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I have also found that if you leave gas in the carb over an extended period of time the jets will clog again, even the larger ones. So when your done for a while shut off the gas and open the bowl screw and drain the gas out of the carb. I have doing this for a year now and the little bike starts a run great every time we pull it out.
Been doing this also! I close the shutoff, run the engine and while running, open the bowl drain. No more varnish in the carb! I have used Marvel Mystery Oil and Stabil for years in snowmobiles. Lately, I have been using Seafoam added to the tank. The little 90 starts right up and idles great. Also, accelerates without the original bog until HOT, warm didn't even help when stock.
 

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Are you getting the jets from the yamaha dealer? or aftermarket place. Thanks
 

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I have a friend that is a machinist drill them to whatever I need. The drills are extremely tiny and he uses a jewelers lathe to hold the jet and bore it. Many order them from an Eton dealer. You will find the link on one of the other threads.
 

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Discussion Starter · #13 ·
I got the 37 pilot jet, do i lightly screw this all the way in or do i leave it out a little, still bogging after a good ten minutes, although has helped slightly.
 

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Discussion Starter · #14 ·
Id say the bike is doing about 10-12mph unchoked, 15-18 choked! Anything past quarter or so throttle, bike becomes sluggish and wants to stall still! Ya gotta help me guys were supposed to be going to durhamtown, and my boy is bouncin off the walls!
 

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The pilot jet seats snug. No adjustment. The .37 mm jet helped ours a huge amount. If you can get it running well when hot, do some easy runs and check the plug. If it is really white, it is still lean. If dark, then do some full throttle passes and check again. Usually, you do the full passes with a new plug to get a good read. You may need more fuel. Check the filter. Many have said their filter was getting blocked. It's a start
 

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Discussion Starter · #16 ·
New plug before tested, as per my previous message made passes as fast as i could (1/4 throttle max without bogging) for 3-5 minutes. Plug is very black and sooty on 3/4 of plug and clean on remaining 1/4.
 

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Did you try and adjust the air fuel mixture? As the weather got colder up here i had to adjust it or it would bog down and have no power, I still have the stock pilot in it as of now and runs good just by that adjustment.
 

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It sounds like you might have a clog air passage. I would fully clean the carb and force carb cleaner through all the openings.

I know how frustrating this can be. I fought with this quad for almost a year. If you have questions post them here, there a number of of folks here that are willing to help.
 

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I normally start at 1 1/2 turns, and then follow the procedure below.


1) Warm up the engine to full operating temp.

2) Turn up your idle a few hundred RPM using the throttle stop screw (basically you want a fast idle). This will make it easier to hear small changes in RPM. Watch for overheating--pointing a big shop fan at your engine will help it from getting too hot during the fast idling. The whole procedure shouldn't take too long though.

Each time you change the screw setting 1/4 or 1/2 turn or so, wait about 5 seconds to let the idle speed normalize. It usually take the carb and engine a moment to react to the change.

3) Turn the fuel screw IN until the idle starts to drop and miss. The engine should die if you bottom out the screw. Your pilot jet is too big if it doesn't die when the screw is bottomed out--it should die before it gets that far in.

4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.

5) The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to drop at 1 turn out and starts to drop at 2 1/2 turns out then 1 3/4 of a turn out should be the correct setting.

If the peak/smoothest RPM is reached somewhere between 1-3 turns then your pilot jet is correct (the 1-3 turns applies to most carb types). If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one. If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up (bigger) on your pilot jet.

To re-emphisize: If the idle never drops when you're turning the fuel screw in, you need a smaller pilot jet. If the idle never drops when you're turning the fuel screw out, you need a bigger pilot jet.

Typical fuel screw settings are in the 1 1/2 to 2 1/2 range.

6) Once you've got the fuel screw set, re-adjust your throttle stop screw (idle screw) to an appropriate idle speed.

And that's it! Your pilot circuit is now VERY close to ideal. From here you can experiment with how small adjustments affect low-end (i.e. small throttle openings) response and make adjustments for weather. The hardest part is usually gaining access to the screw while the engine is running. Also, you may really have to listen carefully to detect the rpm changes in the idle. 100 or 200 rpm differences can be tough to detect when the change happens over several seconds.
 
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