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Author Topic: Throttle / butterfly shaft vacuum leak  (Read 1768 times)
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bochnak Topic starter
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« on: November 28, 2011, 11:34:04 AM »

I'm working on a 92' nighthawk with CVK carbs.

I found a vacuum leak on 1-2 & 3-4 throttle shaft seals.

I found the felt seals here and replaced them:
http://www.motorcyclecarbs.com/FELT_CV_BUTTERFLY_SHAFT_SEAL__P11503C680.cfm

The 1-2 leak disappeared, however the 3-4 is still present.

The throttle shaft rides directly on the carb body, no bronze bushing. There is no major signs of wear or play in the shafts/bodies.

IMHO, the felt seals are a joke. Anybody have this issue?

I will probably try opening the idle mix on problematic carb another turn. They are at 3 turns out currently. Otherwise I may have to try some o-rings instead.

Thanks,
Matt
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Bumblebee
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« Reply #1 on: November 28, 2011, 12:15:21 PM »

$5 a pop? You gotta be kidding me!  eek7

Felt is not going to create an airtight seal. It is highly porous by design. In reality they are more along the lines of a dust seal more than anything else.

When I had mine apart, the felt was filthy. I already had some extra moderate density felt from the local hobby lobby. I simply cut it to shape and used that. Small sharp pointed scissors and $0.65 will get you about 100 of those seals. They even come in about 20 different colors if you're fanatical about color coordination.  giggle
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bochnak Topic starter
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« Reply #2 on: November 28, 2011, 12:36:57 PM »

$5 a pop? You gotta be kidding me!  eek7

Felt is not going to create an airtight seal. It is highly porous by design. In reality they are more along the lines of a dust seal more than anything else.

When I had mine apart, the felt was filthy. I already had some extra moderate density felt from the local hobby lobby. I simply cut it to shape and used that. Small sharp pointed scissors and $0.65 will get you about 100 of those seals. They even come in about 20 different colors if you're fanatical about color coordination.  giggle

Yeah, not cheap and don't work  banghead

I aksed the vendor if any of the rubber seal would work, apparently these would work:

There are 2 rubber shaft seals available with the same measurement as the felt shaft seals.

Click http://www.motorcyclecarbs.com/RUBBER_CV_BUTTERFLY_SHAFT_SEAL_P9607.cfm

Click http://www.motorcyclecarbs.com/RUBBER_CV_BUTTERFLY_SHAFT_SEAL_P9608.cfm

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Bumblebee
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« Reply #3 on: November 28, 2011, 02:10:35 PM »

Quote
Yeah, not cheap and don't work

They work for what they're designed to do. You can't blame the part for that.

Quote
I aksed the vendor if any of the rubber seal would work

Rule #1 about parts department drones: Just because a person on the phone or at the counter thinks or even knows something will fit doesn't mean they know squat about mechanical design or operation - or even what you're talking about. (yep, nozzle fits in the fuel tank, engine should run with that liquid -- even if it's water or kerosene) They assume you're an expert and you assume they are an expert. No good can come of that.

Don't start altering original designs to make something do what you think it should do. You'll just be shooting the parts cannon at the problem. Those Japanese designer guys are pretty smart and knew exactly what they were doing when they put it together the way it is. If they say felt goes there, put felt there, not something else.

Are you certain the leak is through the throttle shaft gap specifically? There are a lot of other places and parts that are more likely to cause air leaks or similar symptom problems. It takes a lot more than a less than 1/1000th inch gap air leak past a throttle plate shaft to cause vacuum related problems. Think in terms of vacuum pistons, jets, needles, fuel and air passages, sync problems, etc which cause great levels of pressure or fuel flow differential.

What is the book numbers for the pilot screws. If you just keep turning them randomly hoping it will get better, it won't make the problem go away.
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bochnak Topic starter
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« Reply #4 on: November 28, 2011, 02:21:25 PM »


Are you certain the leak is through the throttle shaft gap specifically? There are a lot of other places and parts that are more likely to cause air leaks or similar symptom problems. It takes a lot more than a less than 1/1000th inch gap air leak past a throttle plate shaft to cause vacuum related problems. Think in terms of vacuum pistons, jets, needles, fuel and air passages, sync problems, etc which cause great levels of pressure or fuel flow differential.

What is the book numbers for the pilot screws. If you just keep turning them randomly hoping it will get better, it won't make the problem go away.


I have checked all areas for vacuum leaks. Spraying just a bit of carb clean near shaft seal area causes idle to rise. There is popping in the exhaust, which I believe is a lean pop. Once bike is revved past 1/8th throttle, it runs fine. I have cleaned the idle/pilot circuit.

I've tried to sych the carbs, but is difficult with vacuum leaks. They are benched synched at the moment since carbs were de-ganged.

I have the manual at home for stock pilot setting. They were 2.5 before I messed with it. They also had factory limiters on them. Exhaust baffles have been removed on this bike.
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« Reply #5 on: November 28, 2011, 03:49:37 PM »

FYI - the bike runs lean from the factory.  All of them pop a little through the exhaust at idle.  You can do the carb washer mod to fix that and the "just off idle" flat spot on the bike.  But your fule mileage will suffer a little.  Run a search on the carb washer mod if you are interested.

Maybe you also have a throttle shaft problem  as well - just food for thought.
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bochnak Topic starter
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« Reply #6 on: November 29, 2011, 05:23:37 AM »

FYI - the bike runs lean from the factory.  All of them pop a little through the exhaust at idle.  You can do the carb washer mod to fix that and the "just off idle" flat spot on the bike.  But your fule mileage will suffer a little.  Run a search on the carb washer mod if you are interested.

Maybe you also have a throttle shaft problem  as well - just food for thought.

The washer mod was performed by the bike's owner. I don't see how this would affect my off idle hesitation though. The jet needle comes into play 1/2 - 3/4 throttle.

The manual calls for 2 - 2.75 for pilot screws. They state to start at 2.75 and adjust from there. In my case 3 sounds good since the exhaust has been modded.

It only pops on the 3-4 pipe, where the vacuum leak is.

I did not see anything strange with the throttle shaft/body when apart. I'll take a closer look next time I disassemble.
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« Reply #7 on: November 29, 2011, 05:58:25 AM »

I've never heard of a vacuum leak where you are describing it.  Not saying it can't happen, just wondering if your problem is from somewhere more obvious.  Are you absolutely sure your carb boots on both sides are sealing well?
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bochnak Topic starter
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« Reply #8 on: December 01, 2011, 08:08:48 AM »

I've never heard of a vacuum leak where you are describing it.  Not saying it can't happen, just wondering if your problem is from somewhere more obvious.  Are you absolutely sure your carb boots on both sides are sealing well?

100% certain it is the throttle shafts on 3 & 4. I've sprayed the boots and no signs of leaks.
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bochnak Topic starter
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« Reply #9 on: December 01, 2011, 08:12:07 AM »

Update:

I went ahead and synched the carbs. Runs much better now. At least the hesitation went away along with slow return to idle when revved.

The leak is obviously still there on 3 & 4. I can pull the plug wires on 3 or 4 with virtually no change to idle. It has a lean misfire every so often on #4. Exhaust still pops from time to time.
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