Weak Accelerator Pump & Adjustments

[pinholenz]

I've been trawling through the postings about accelerator pump setups. Mine is a bent accelerator pump rod, vacuum flapper model with a stop screw. I want to cure the "bog" that I mentioned in a previous post, when I open up under load

Jimustanguitar mentioned he used a shortened syringe with a wire around it to hold under the accelerator nozzle to check the volume ejected. A 1ml diabetic syringe works great for this. (Cut the end off at the 3 unit mark by rolling it under craft knife, Then cut the black plunger end off the syringe an use it a plug into the end at the 3 unit mark so that the plug comes up to the 4 unit mark)

My first test on the bike revealed that the gas volume through the accelerator nozzle was just 0.1 ml. According to Jim the spec volume should be 0.25ml down each carb throat. How could I adjust the volume?

Off with the carbs again (Getting better at this). The stopper screw was not reaching its stop mark and although the rod activated the pump diaphragm, I figured the rod spring mechanism was so weak that it wasn't very effective. When I activated the pump directly (by bypassing the spring) I found that well over 1ml of gas reached the nozzles. Here is what I am thinking:

First ,the screw stop adjuster on the accelerator pump rod actually controls the volume of gas delivered to the nozzles by limiting the travel of the pump diaphragm. A little adjusting and testing and I was able to get 0.5ml delivered to the rear carbie. When I reconnected the front carb the volume through the nozzle on the front was 0.2 ml and on the back, 0.3ml Close enough?

Next, it seems to me that the purpose of the accelerator rod adjuster is to put just enough tension on the rod spring so that when it is compressed, it activates the pump diaphragm without delay and with sufficient pressure under agressive acceleration, to fully press home the diaphragm lever to the stop. Additional travel enables the rest of the throttle to open without more raw gas from  the accelerator pump. All of this should be able to be adjusted on the bench.

Does that make sense?

I'd say my rod spring is so weak that it needs to be replaced or doubled up. I'll see how it goes and see if my theory works. Cheers

[Tiger]

To start...make sure your carbs are squeaky clean and synced properly. This basic procedure cures MOST Visionaries running problems, including the one you have. If not, then and only then look elsewere, (like the acc' pump).

            8) ....... TIGER ....... 8)

[QBS]

Your operational analysis seems valid to me.  Thanks for putting it out there.

[Rikugun]

Tiger makes a very good point. Assuming they are clean and tuned it sounds like you are on the right track with your understanding of the accel pump mechanism. Keep us posted.  :)

[pinholenz]

Thanks for the feedback.

Found replacement compression springs for the accelerator pump connector rod at my local hardware store in their "assorted springs" box. It hadn't seen the light of day for a while. When I removed the existing (rusted) spring, my replacement spring was short. I chopped one of the new springs in half, found a thin nylon washer to put between the springs and threaded   1 1/2 springs on to the rod. Put it all back together.

Next, on the bench I filled the rear carbie bowl with gas, primed the accelerator pump with 4 or 5 quick flicks of the accelerator sync rod until I got gas at the jets. Then I adjusted the accelerator rod length until the merest flick of the accelerator produced gas at the jet in the carb. Put some thread lock on the accelerator rod, tested the delivery volume again to see if the stop screw needed adjusting and assembled the carbs back on the bike.

My starting routine is now two flicks on the accelerator to prime the carbs, full choke and she fires into life without hesitating. Once warm there is no bog when I open up. So far so good. BUT haven't yet synced the carbs and done a road test. (Waiting for a new front tyre).