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Help me fix this 2001 YZF 600R #5443901 06/01/20 12:29 PM
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Kurtatron Offline OP
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Hello Everyone,

I hope you all are staying safe. Fixing this bike keeps me sane. It's an old carburated Yamaha Thundercat. It sat for 4 years under a shed. I'm trying to get it running again. I'm hoping you guys may be able to help me get it running again.

It's got 19k miles on it.

Before the owner stopped riding it, the bike wouldn't idle. It would stall. The RPMs would dip. He gave up on it and let it sit for 4 years until I came along and told him we should fix it. Our first idea was to clean the carburator. We opened it up and found the jet's covered in green varnish. We cleaned both the pilot and main jet (soak in carburator cleaner fluid for a week). We even used a thin wire to go through the tiny holes. We attached everything, tried to fire it up, and it did! and then, died. And it then it kept failing to fire up. It'll fire like one cylinder while the starter keeps it running. It'll putter. We gave it starting fluid, and it runs on starter fluid just fine. We got it to rev to 7k rpms on starter fluid no problem. I was thinking maybe the fuel pump might be a suspect.

When flip the engine switch, I hear very subtle clicking, I am guessing the fuel pump. But the clicking gets slower and slower and then just stops completely. Is there a way to test the fuel pump for it working correctly? We didn't mess with the carburator diaphragms, NOR did we spend tons of time cleaning the inside passageways behind the jets (Have no idea how). What do you suggest we do next? [Linked Image]
Any input is appreciated. Thank you.

Last edited by Kurtatron; 06/01/20 12:30 PM.
Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5443915 06/01/20 12:47 PM
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02SE Offline
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Take it to someone who can clean the carbs properly, and charge the battery.

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5443929 06/01/20 01:14 PM
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Kurtatron Offline OP
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I think I know where we may have screwed up. I think we failed to turn the pilot jets the correct amount from bottoming out. We just tightened the pilot jets until they bottomed out, and did not turn them 2.5 times out. We will try that, and see if that works.

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5444003 06/01/20 02:48 PM
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GSCJR Offline
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Let me know how how you make out with those jets, l'm having the exact same issue with a Yamaha generator I'm trying to get going.


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Re: Help me fix this 2001 YZF 600R [Re: GSCJR] #5444018 06/01/20 03:04 PM
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Bonz Offline
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Yes, if you turn those in till they were lightly seated, and leave them you won't have an idle LOL. I think you found part of your issue.

As advice, never bottom them out hard or you'll break the tips or deform them. Remember, lightly seated.

Disconnect the fuel line at the carbs and turn the key on, see if you get fuel flow. The fuel pump in my cars whir when I turn the ignition on and then stops until I start the engine. It just gets up to whatever pressure it's supposed to and then is at the ready. But I may be wrong on that last part.

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5444053 06/01/20 03:41 PM
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JohnG Offline
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You answered your own question. If it runs on starting fluid, it's not getting gas. A lot of bikes have auto shut off valves under the tank that can get plugged or gummed up. Same with the in tank screen. You need to go over the entire fuel system.

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5444143 06/01/20 06:27 PM
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Blkstanger Offline
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Start by cleaning the entire fuel system.


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Re: Help me fix this 2001 YZF 600R [Re: Blkstanger] #5444228 06/01/20 08:47 PM
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Bonz Offline
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See if it's getting fuel from the tank by disconnecting the fuel line and turn on the ignition. If you're getting fuel then do the carbs, you basically described a biology project going on in the carbs with that green varnish.

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5465084 06/28/20 06:42 PM
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Kurtatron Offline OP
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Hey Guys, Update:

We removed the carbs and backed out the pilot jets 2 rotations. It ran much better, but still not road worthy. Once it was fully warmed up, it actually did idle for a while, but would still stall out every few minutes. I am going to attempt another cleaning, as well as do a better job sealing stuff. It was leaking gasoline everywhere. My friend has let me take possession of the carburator. This time I have removed the diaphragms and the big needle that goes through the throttlebody. Cleaning guides I have researched included removing the diaphragms and needle, to spray cleaner down there.

But I am still messing up the pilot jets somehow. This is the second time this has happened. I open the carburator bowl, and the pilot is just sitting there separated. Am I missing something? Are the jets supposed to get threadlocker? It DOES seem weird to leave the jet in there loosened 2 turns, like leaving a bolt unfastened on a car suspension component. BUT it did run better leaving it loosened 2 turns. What am I doing wrong for the pilot jet to come separated? Please help me guys, I plan to go back this upcoming weekend with a working carburator. Thanks.

[Linked Image]
[Linked Image]

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5465259 06/28/20 10:14 PM
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ZeeOSix Offline
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Originally Posted by Kurtatron
Hey Guys, Update:

We removed the carbs and backed out the pilot jets 2 rotations. It ran much better, but still not road worthy. Once it was fully warmed up, it actually did idle for a while, but would still stall out every few minutes. I am going to attempt another cleaning, as well as do a better job sealing stuff. It was leaking gasoline everywhere. My friend has let me take possession of the carburator. This time I have removed the diaphragms and the big needle that goes through the throttlebody. Cleaning guides I have researched included removing the diaphragms and needle, to spray cleaner down there.

But I am still messing up the pilot jets somehow. This is the second time this has happened. I open the carburator bowl, and the pilot is just sitting there separated. Am I missing something? Are the jets supposed to get threadlocker? It DOES seem weird to leave the jet in there loosened 2 turns, like leaving a bolt unfastened on a car suspension component. BUT it did run better leaving it loosened 2 turns. What am I doing wrong for the pilot jet to come separated? Please help me guys, I plan to go back this upcoming weekend with a working carburator. Thanks.


The pilot jets inside the bowl are supposed to be fully tightened, and with no tread lock, not turned out a specific amount of turns. There may be screws (either for air or fuel metering a idle) on the outside the carburetor that require so many turns out from the full in lightly seated position. Do you have a manual for this bike that shows the details on the carburetor adjustments?

Also, carefully hold the rubber diaphragms on the slides up to a strong light and look for any tiny holes or tears. If they have some, the vacuum that operates the slides will be lacking and the slides will not operate correctly.

Re: Help me fix this 2001 YZF 600R [Re: ZeeOSix] #5465309 06/29/20 12:48 AM
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Bonz Offline
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Do not spray ANY carburetor cleaner inside of carburetors when they are assembled with rubber parts inside. It is meant to clean the outside of the carbs and metal surfaces. It will ruin the rubber tips on your float needles and mess up the o-rings on your needle seats. You're going to have to take everything apart and replace those items if it's dumping gas, because the damage is done to the rubber parts.

What do you mean by better job "sealing stuff"? What didn't you seal up of the first time? Thread locker in a carburetor is never used. Think about it, any excess would easily clog passages. The brass jets with a light snugging in the aluminum carb body WILL NOT come loose, so get those put in correctly.

As has been said, you adjust the pilot screws (not the jets!).
However, the screws are usually under pressed in caps to seal them at the factory and you may not be able to see the screws. I would suspect that is the case since you tried to adjust the jets themselves inside of the carburetor...

The pilot screws once you see them or take the caps off to access them on your bike will control fuel as they are on the front of the carburetors on the engine side. Any modern 4-stroke uses fuel screws, air screws are on the backside of a carburetor on two stroke engines. Do an internet search for removing pilot screw caps and you will find the information to do it.

I'm not sure what videos you're watching, there has to be a Yamaha forum for the YZF you can go to. So much is being done out of good effort but lack of knowledge of what it will do.

In my opinion and experience so many things have been done incorrectly a fresh start is the best and only thing you can do. As well, the carburetors are almost 20 years old regardless, you cannot expect o-rings or float needle tips to be reliable at this point after spraying cleaner inside and the age of the carb alone dictates a full rebuild.

DO NOT get any type of spray cleaner on the diaphragms, it will cause the rubber to swell or you will be replacing those as well. Pin holes in the diaphragms would not cause any of the problems you're dealing with.

Unless you can unrack the carburetors which means separating them from each other and taking them down to their individual pieces, fully cleaning every passage and replacing the rubber parts that I would bet a dollar are now damaged, I don't see this problem being rectified.












Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5465364 06/29/20 05:57 AM
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ccs368 Offline
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The pilot jets are suppose to be seated tight and snug to the carb body. What you are researching about the screw that adjusts xxx turns out is the low idle mixture screw and typically sealed from the factory. To get access to it as Bonz mentioned, quite often you have to center drill the cap just to break the face and then thread a sheet metal screw and pop out the pressed on cap. DO NOT drill too deep as you may damage the actual adjustment screw underneath. If you are lucky the screws will be slotted so you can adjust with flat headed screwdriver. If you are unlucky, the adjuster screw will have a half moon face and need a special tool to turn unless you carefully take it out and Dremel a slot for future adjustment. No need to replace the cap after removing.

Based on your photo, that screw is like behind that raised nipple I see on the bottom of the picture just outside the bowl. It looks like they are still on there which is a good thing and you move forward tuning the carb.

Here’s the thing though. It is likely you need to mechanically synchronize your carb once put back and tightened to the intake manifold. This synch using mercury sticks or vacuum gauges will guarantee you have a proper functioning carb. What this means that the 4 carb bodies are matched to one another to within specs and not seat of the pants adjustment. This will alter idle when done properly and just a better performing bike. If you want that done, those adjustment screws are located in between the carb bodies and there are 3 of them. Screw 1 to adjust carb 1 to 2. Screw 2 to adjust carb 2 to 3. Screw 3 to adjust carb 3 to 4.

The taps or cap head bolts should be located on the casted intake manifold where the rubber boots for the carb is mounted for attaching the synching tubes if you venture to try this out.

I have done this dozens of times and really missed tweeking carbs.


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Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5465416 06/29/20 07:29 AM
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Kurtatron Offline OP
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We followed some youtube guide online, and he sprayed carb cleaner into the main jet bore and pilot jet bore. I don't know if there are any rubber seals in there. When we ran the motorcycle, the carburator was leaking fuel. We suspect it was either these cheap chinese knockoff bowl gaskets we bought or 20 year old o-rings we had reused on the lower hose shown on the second picture. There were two of them. They did not look damaged in any way. The daiphragms look fine to my untrained eye.

I am going to soak these again in carb cleaner, one of the Jets is still coated in green (corrosion?) gunk. and we had forgotton to separate the main jet from the needle jet. We thought they were one assembly. This time I will soak them as detached parts. I will not spray carb cleaner into the passageways shown on the second picture, even though that's where I've seen every youtuber spray into. What would you recommend for cleaning those passageways?

I will look into synchornization after we reinstall the carburator. I have seen the 3 adjustment screws.

Thank you for your help all.

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5465479 06/29/20 09:08 AM
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Bonz Offline
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When you spray carb cleaner into the main jets it goes backwards through everything that gas comes up through. I may have misunderstood, if you did this with the bowls off and the floats removed and the float needle seats pulled out that is a different scenario. That would indicate you are spraying it through the bare metal car bodies and jets which wouldn't pose a problem.

You have the right idea, keep working on it!

Re: Help me fix this 2001 YZF 600R [Re: Kurtatron] #5465480 06/29/20 09:19 AM
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ccs368 Offline
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Compressed air is ideal to blow out any spray solvent or cleaner you use to clean all the gunk and grime in there. Once I remove the diaphragm and the bowls, I do spray with cleaner until I see if come out another end and blow out with a nozzle.

Reading back your post here is what I can make out. You basically can separate a carb into three segments just for discussion sake. Idle, mid range, and wide open throttle. Originally you said you were able to get the bike to rev to 7K with starter fluid. That's strongly suggesting the mains and its passages are clear. You can see daylight through the mains when looking into the sun, right? Now for midrange or something like ~3K-7K, that's the needle jets that has multiple tiny orifices in the body. You mentioned you wire brush cleaned them but did you strip back some electrical wire and take a single strand of wire and actually ream every hole? Doesn't take much to clean them but don't go hog wild and ruin these tiny orifices. You said you soaked for a week so that suggests these jets are clean. Now idle to off idle response which is idle screw and low idle pilot mixture screw(hidden and probably 1.5 times turned out). This is where I think based on what I read you need to focus on. Once you have access to them, unscrew them and clean out those passages and then get them to xxx turns out that you are trying to tweek. Before turning all of them out, turn them back in until just touches BUT count the number of turns. This way you have a data point on what the factory set and now you can adjust from there.

Also, I am also guessing there's no slip-on of full exhaust as that would suggest the factory screws are probably too lean and need to be turned out to compensate for the free flowing exhaust.

Because you removed and installed the carb many times, make sure there're no air leaks in the rubber boots from the carb to the manifold. Once you can get it to reliably idle, spray water in the boot area and hear if the engine idles differently digesting water to tell you if you have a leak or loose band that can be tightened more.


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