Throttle Shaft Vacuum Leaks


Information and questions on GL1100 Goldwings (1980-1983)
  • Sponsored Links
Post Reply
User avatar
DWade1990
Posts: 60
Joined: Thu Apr 27, 2017 7:27 pm
Location: Akron, OH
Motorcycle: 1976 GL1000
1983 GL1100 Aspencade
1981 CB750K

Throttle Shaft Vacuum Leaks

Post by DWade1990 » Mon Jul 02, 2018 11:29 pm



Hello all,

I'd like to discuss something that I can't find a clear answer to. I've noticed a vacuum leak on the left rear carburetor at the throttle shaft. I'm certain this is the leak location, and not the air cutoff. I can audibly hear the leak with the bike running. I shot some WD40 onto the carb body, and the air hissing sound changes when I spray the throttle shaft. I don't think it's a noticabley large leak, as the rpm doesn't change when I spray the area, but it's still enough that I can hear it. Other than an annoying hissing sound, the bike runs great. No hanging or creeping idle, or any other typical signs of a vacuum leak.

The confusing part is that I'm getting some conflicting information. I've read articles, some of them quoting Randakk, stating that the carbs are designed to allow air to enter the carbs there, even showing photos of tiny air passages behind the felts. Then in other articles, I see people making new felt seals for the throttle shafts, and stating that the bike runs much better.

So what do you guys think? Are these carbs designed to have a leak there? I can't see how letting in unmetered air could possibly be beneficial for performance.



User avatar
Maz
Posts: 496
Joined: Tue May 19, 2015 4:43 am
Location: Kent, UK
Motorcycle: 1975 GL1000K1
1976 CB500T
1979 CB750L
1990 Yamaha FJ1200
1993 Suzuki GS500E

Re: Throttle Shaft Vacuum Leaks

Post by Maz » Tue Jul 03, 2018 1:48 am

DWade1990 wrote:
Mon Jul 02, 2018 11:29 pm
Hello all,

I'd like to discuss something that I can't find a clear answer to. I've noticed a vacuum leak on the left rear carburetor at the throttle shaft. I'm certain this is the leak location, and not the air cutoff. I can audibly hear the leak with the bike running. I shot some WD40 onto the carb body, and the air hissing sound changes when I spray the throttle shaft. I don't think it's a noticabley large leak, as the rpm doesn't change when I spray the area, but it's still enough that I can hear it. Other than an annoying hissing sound, the bike runs great. No hanging or creeping idle, or any other typical signs of a vacuum leak.

The confusing part is that I'm getting some conflicting information. I've read articles, some of them quoting Randakk, stating that the carbs are designed to allow air to enter the carbs there, even showing photos of tiny air passages behind the felts. Then in other articles, I see people making new felt seals for the throttle shafts, and stating that the bike runs much better.

So what do you guys think? Are these carbs designed to have a leak there? I can't see how letting in unmetered air could possibly be beneficial for performance.
Having totally stripped my carbs, I can confirm that there is a tiny hole right through the casting behind the felt "seal". The felt is there to keep out dust/grit. However, if you can actually hear a hissing sound, but the rpm doesn't change when you hit that felt seal with WD40, I doubt that the fluid is being drawn into the engine there, so it's unlikely to be a vacuum leak there. A hissing sound would indicate a quite substantial leak. Just thinking out loud.

Have a look at www.wingovations.com. Go to 'Technical Tips' then 'Carbs and Fuelling', then look at 'Carb Throttle Shaft Leaks'.
Maz
Ironically, Common Sense is the LEAST common of all senses!

User avatar
DWade1990
Posts: 60
Joined: Thu Apr 27, 2017 7:27 pm
Location: Akron, OH
Motorcycle: 1976 GL1000
1983 GL1100 Aspencade
1981 CB750K

Re: Throttle Shaft Vacuum Leaks

Post by DWade1990 » Tue Jul 03, 2018 12:53 pm

Maybe I should rephrase this. What I'm hearing isn't a constant hissing sound. It's more of a hiss that corresponds with that cylinder's intake stroke. More of a hiss-hiss-hiss-hiss-hiss.

User avatar
Maz
Posts: 496
Joined: Tue May 19, 2015 4:43 am
Location: Kent, UK
Motorcycle: 1975 GL1000K1
1976 CB500T
1979 CB750L
1990 Yamaha FJ1200
1993 Suzuki GS500E

Re: Throttle Shaft Vacuum Leaks

Post by Maz » Tue Jul 03, 2018 1:43 pm

Silly question, but you don't have a slightly loose spark plug, do you?
Maz
Ironically, Common Sense is the LEAST common of all senses!

User avatar
DWade1990
Posts: 60
Joined: Thu Apr 27, 2017 7:27 pm
Location: Akron, OH
Motorcycle: 1976 GL1000
1983 GL1100 Aspencade
1981 CB750K

Re: Throttle Shaft Vacuum Leaks

Post by DWade1990 » Tue Jul 03, 2018 1:55 pm

I can't see that I do, but it's worth checking. The only thing that convinces me that it's the throttle shaft is when I spray WD40 directly onto the throttle shaft, the hiss-hiss-hiss sound changes like it's sucking it into the carb, almost like the sound a straw makes when the cup is almost empty. But I'll check the plug, you never know.

indianakid
Posts: 186
Joined: Sun Sep 26, 2010 5:28 pm
Location: long beach, ca
Motorcycle: 83 GL1100 I

Re: Throttle Shaft Vacuum Leaks

Post by indianakid » Tue Jul 03, 2018 6:00 pm

OK...... I will throw my opinion into the hat. For a hiss to be that loud it is not a small leak around a throttle shaft. And since you are hearing it in the throttle shaft area I think that the small o-ring for the air cut valve cover is missing on that carb.

User avatar
DWade1990
Posts: 60
Joined: Thu Apr 27, 2017 7:27 pm
Location: Akron, OH
Motorcycle: 1976 GL1000
1983 GL1100 Aspencade
1981 CB750K

Re: Throttle Shaft Vacuum Leaks

Post by DWade1990 » Tue Jul 03, 2018 6:13 pm

indianakid wrote:
Tue Jul 03, 2018 6:00 pm
OK...... I will throw my opinion into the hat. For a hiss to be that loud it is not a small leak around a throttle shaft. And since you are hearing it in the throttle shaft area I think that the small o-ring for the air cut valve cover is missing on that carb.
I thought the same thing too, but I'm positive it's not the air cutoff. I rebuilt these carbs about 2 years ago and put a complete Randakk kit on it. I even took the air cutoff cover off that carb last year because I thought the same thing, and maybe I missed it during installation, but it was in there correctly with the flat side against the carb body. I'm 100% certain the sound I'm hearing is the throttle shaft. I can pinpoint it with the straw on my can of WD40. No sound change when I spray anywhere else besides the throttle shaft.

It isn't a very loud sound, but I also dont hear it on the other side of the motor.



Post Reply