Accelerator pump rod travel

[Kool_Biker]

Hi all.
After having done everything by the book in my newly rebuild 79 carbs, when opening the throttle, the total travel of the accelerator pump plunger is 2 to 3 mm tops (I have not made precise measurements).

Is this about right, or am I missing anything??

The pump has been tested working on the bench. Also both specified adjustments, i.e. rod clearance and arm stopper have been checked and adjusted accordingly. Also the spring is standard.

Thanks in advance, Aris

[daves79x]

Aris:
Your rod travel is about right. I'd adjust the tab/rod clearance to zero. The other trick is to remove the spring and stretch it about 1/2 again it's length. This will give more push to the linkage. It is hard to do more to make the system better than that. The problem is that, with many of these carbs, the accelerator pump system's effeciency deteriorates rather quickly. All the crud from the engine funnels it's way back to the center of the carb linkage where the accelerator pump resides. This gums up the linkage and results in poorer pump performance. Also, the pump daphragm gets hard over time and is hard to push. I have never rebuilt a set of CBX carbs that had a working accelerator pump when I got them. And I believe some never worked from the factory. You seem to have a good grasp of the function and you'll be fine with those couple of tweaks.

Dave

[Kool_Biker]

Dave, thanks, this is very helpful, and could not agree more about the state the pump was. But now it tests good.

Not sure though what you mean though by " ... remove the spring and stretch it about 1/2 again it's length ... ".
Do you mean, pull out - stretch - the spring beyond its elastic limit, to make its length when it returns to rest 1.5 times what it was before?

Cheers, Aris

[daves79x]

Aris:
Stretching the spring is not a precise science by any means. I usually stretch them out about 1/2 more than their uninstalled length. You will have to really push to get it back on the rod, but it will be fine. It will likely eventually return to it's normal length at some point, but it gives a good healthy squirt while it works.

Dave

[Mike Nixon]

Aris:
I have never rebuilt a set of CBX carbs that had a working accelerator pump when I got them. And I believe some never worked from the factory.

Dave

Agreed. The accelerator pump is the weak link for sure, and unfortunately, unlike other bikes that have accelerator pumps, the CBX really needs it, relies on it for the carbs to do their job. Which means incidentally that the carbs are too big. I would disagree however that 2-3mm is adequate accelerator pump travel.

[Jeff Bennetts]

Another solution to stretching the accel pump spring is to instead shim it with a small diameter washer.

[EMS]

Another solution to stretching the accel pump spring is to instead shim it with a small diameter washer.

...or with a small nylon bushing that fits over the rod. Available at any hardware store and you can cut it to the desired length.

[Kool_Biker]

I would disagree however that 2-3mm is adequate accelerator pump travel.

So Mike, how many mm should the accelerator pump rod travel be?

Thanks
Aris

[Mike Nixon]

Aris -- I like to see at least 6mm.

[Kool_Biker]

Hi Mike!
Assuming the accelerator pump rod with the throttle valve closed, is 0 mm (just touching).
Then the ONLY way to go from my existing 2 - 3mm pump rod travel to ~ 6mm, is to change the clearance between the adjusting arm and stopper to much more than the Honda specified 3.1 - 3.3 mm. More like 5 or 6 mm. Is this right, or am I missing anything.

P.S. I do understand the need for a 'stiffer' spring but this will not change total travel, it should make things more instantaneous.

Thanks in advance
Aris

[daves79x]

Aris:
Doing some quick math, I see that 6mm is about 1/4 inch, which I can more easily visualize. I'm sure I've assembled carbs to nearly that spec also. Yes, you do have to bend the limit tab to get that much travel. I suppose there is such a thing as too much 'squirt' from the pump, but most times that does not seem to be a problem. Just as long as you are getting a good healthy shot with each 'blip' of the throttle.

Dave

[Mike Nixon]

Ditto. Thanks Dave. Aris, you got it.

[Kool_Biker]

Gentlemen, this is quality and to the point information.
I am already at it, thanks a lot!
Aris

[Kool_Biker]

Guys Hi

To vent my frustration for NOT being able to get anywhere near the 6mm pump rod travel suggested, no matter what I tried and how I bent the accelerator pump travel limiters, I decided to dismantle bowl no 3, to test the free travel of the pump rod on the bench.
Imagine my surprise when this measured just shy of 5mm, closer to 4.5mm.

P1050754 (1).jpg

Now the pump spring is original, and so is the pump diaphragm.
So, at least in my case and for my 79 carbs, there is no way I will get the 6mm travel suggested and will have to settle for what travel is physically available - unless I am missing something??

Thanks in advance.
Aris

[Mike Nixon]

Wrong diaphragm, wrong spring, misassembled, something. I wouldn't recommend it, but it is possible to have accelerator pump all the way to about half throttle.