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Discussion Starter #1
Sorry for the quick nondetailed details, but I'm writing this before my break is over

Bike: 1984 Yamaha FJ600
Problem: Runs only on choke, given throttle makes it drop rpm and or kill the bike.
What I've done: Rebuilt the carbs, replaces all hoses with new ones.
My thoughts: I believe it could be the intake carb boots, there seems to be surface crack on it.
Your thoughts: (you reply)

I'll add photos and a video when I get home. Thanks.
 

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If the intake boots aren't sealing air tight, a vacuum leak could be making the mixture lean, causing the symptoms you are describing.

You can test this by spraying some carb cleaner around the boot area while the motorcycle is running. If the engine RPM changes while spraying, the cleaner is getting pulled in through the boots. That means you have a leak.
 

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My particular 1998 bike has a Manual Choke. In fact, my bike LIKES to start (cold) with the Manual Choke.

If, after I've started it, I don't give it a chance to warm up a tad, before making throttle demands, my bike TOO, will "gag" (for a lack of a better way of saying it).

Once however, the bike has warmed up, (doesn't take that long) I can gradually reduce the Manual Choke, until finally there is no Choke on at ALL.........and the bike runs fine.

In fact, I gauge the warm-up status of the engine BY the way the throttle responds. It "talks to me" (if you will).

-Soupy
 

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Discussion Starter #4
I sprayed around the boots and it seems to spike up in revs on boot #3. I'll have to wait til tomorrow to try to seal it and see if that helps.
 

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Have you allowed the bike to warm up sufficiently before shutting off the choke?

Assuming you have it sounds like it's not pulling enough fuel for one reason or another. Are your float levels set OK?

Definitely fix that carb boot you found!
 

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Discussion Starter #9
Hmmmm before I replace the boot and or seal the boots, could a fuel filter be the proboem? I remember it had a fuel filter in between the gas line.
 

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Choke me.

More choke causes more fuel in relation to the air, to be sucked in.
So there is either too much air, or too little fuel. Too little fuel at idle or just above is usually a plugged low speed jet, which is also the smallest and easiest to plug. Too little fuel can also be a plugged air filter, or a plugged vent in the gas cap. Open the gas cap. Replace the fuel filter. Look at the small jets with a magnifying glass.
The above is assuming the jets were the correct size before the problem started.

Unkle Crusty*
 

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Discussion Starter #11
So I sealed up all the intake boots with some gasket sealant, it seemed to help, I managed to be able to pull throttle with it dying most of the time, but I had a feeling something wasnt right in the carburetors so i reopen it and found this. Gonna buy another diaphragm and ill get back to you guys.
Diaphragm in C#1
h t t p ://i.imgur.com/oU30EyM.jpg

Heres some pics of the boots and the bike
h t t p ://imgur.com/a/Z4lTQ
 

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So I sealed up all the intake boots with some gasket sealant, it seemed to help, I managed to be able to pull throttle with it dying most of the time, but I had a feeling something wasnt right in the carburetors so i reopen it and found this. Gonna buy another diaphragm and ill get back to you guys.
Diaphragm in C#1
h t t p ://i.imgur.com/oU30EyM.jpg

Heres some pics of the boots and the bike
h t t p ://imgur.com/a/Z4lTQ
I was guessing carb related...took me 3 times before it was cleaned right on my bike project.
 

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Discussion Starter #14
So I sealed up all the intake boots with some gasket sealant, it seemed to help, I managed to be able to pull throttle with it dying most of the time, but I had a feeling something wasnt right in the carburetors so i reopen it and found this. Gonna buy another diaphragm and ill get back to you guys.
Diaphragm in C#1
h t t p ://i.imgur.com/oU30EyM.jpg

Heres some pics of the boots and the bike
h t t p ://imgur.com/a/Z4lTQ
I was guessing carb related...took me 3 times before it was cleaned right on my bike project.
4th times the charm for me :l
 

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My particular 1998 bike has a Manual Choke. In fact, my bike LIKES to start (cold) with the Manual Choke.

If, after I've started it, I don't give it a chance to warm up a tad, before making throttle demands, my bike TOO, will "gag" (for a lack of a better way of saying it).

Once however, the bike has warmed up, (doesn't take that long) I can gradually reduce the Manual Choke, until finally there is no Choke on at ALL.........and the bike runs fine.

In fact, I gauge the warm-up status of the engine BY the way the throttle responds. It "talks to me" (if you will).

-Soupy
I'm learning this to be the case on my '81 SR250 Soupy...decided to take it for a little spin around the block last night, it was a bit chilly...I could have fought it for 10 min but I actually remembered I have a choke on the carb this time! It actually started before I pressed the button it was so happy to see me do it right. ;)

Of course then I rushed it, dropped the choke too soon and cough cough cough! I'm learning she needs a little foreplay before I romp her! :thumbsup:

SirBlue I think I'll crack that carb open one more time just to keep up! I don't have a fuel filter inline yet, and I can imagine between putting some gas through my new used tank and the old cleaned by me carb it wouldn't hurt to go through it again.
 

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Discussion Starter #16
I'm being patient, I'm not really rushing things. However, now I'm pooped.

Diaphragm is replaced, carb cleaned, replaced gaskets, replaced hoses, petcock cleaned, tank cleaned of rust and debris, benched sync the carbs, boots are replaced and sealed.

Everything seems to be in tune carb wise, but still the same ol' problem. It would bounce from 3k-4k rpm(with choke) and when choke is off it drops and the bike cuts out, even after 5+min of choke, the bike is running warm/hot.

I'm pooped. If anyone has any idea what the problem might be and what i should do, leave a reply. In the mean time ill be taking another look at the carb, specifically the floats and thinking of rebuilding the petcock.
 
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