Throttle Shaft Vacuum Leaks

[DWade1990]

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.

[Maz]

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

[DWade1990]

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.

[Maz]

Silly question, but you don't have a slightly loose spark plug, do you?
Maz

[DWade1990]

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]

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.

[DWade1990]

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.

[DWade1990]

Here's something I noticed the other day while riding. The hiss-hiss-hiss is pretty pronounced when the motor is cold, but once the bike is up to temp, I don't think you would even notice it if you weren't listening for it. It's barely audible.

Just so we're clear, the bike runs beautifully. It starts almost instantly from dead cold, idles at 900rpm, and will rip all the way up to redline when I ask it to.

Does anybody else experience this kind of hissing from the throttle shafts when the motor is cold? This is honestly just to satisfy my curiosity at this point.

[DWade1990]

Well don't I feel stupid. Last Saturday I rode down to Harding's Park Cycle to pick up a cherry used FJR1300, and before I left, I took another look at the carbs. After playing around with it for a while, wouldn't you know it, I was wrong and the sound was in fact coming from the air cutoff cover. I pulled the cover off, and the small o-ring had deteriorated and turned to mush. This is the second air cutoff o-ring I've had go bad from my Randakk's kit.

Luckily my 81 CB750 uses the same o-rings, and I had a few good stock ones that I saved when I rebuilt those carbs. I put one in the Oldwing, and now the carbs are silent again, so I guess my throttle shafts aren't leaking after all.

Now the question is, are the remaining two air cutoff o-rings going to hold up, or am I going to use up my stock of used CB750 o-rings. I already contacted Randakk's to see if they sell the part by itself, and they told me my best bet is to get the entire air cutoff kit from Z1Enterprises. Not a bad price, and much better than purchasing the $200+ kit again, but still kind of a bummer that they aren't holding up.

[indianakid]

Glad to hear that you were able to find the problem. Its got to be disappointing to have parts fail after having spent large money for Randakk's kit. I wonder if anyone else has experienced that problem with Randakk's parts ?