Keihin carb vb 42a accelerator pump

V10Pilot

Been Around the Block
Quick question. I bought a 79 CB750L and it all appears stock however I think the previous owner may have replaced a few soft parts on the carbs (keihin vb42a)

I can’t find any really good pictures of the accelerator pump rod in an idle position on line to compare. When the throttle is completely closed, the actuating plate that is part of the throttle linkage assembly is about 4mm from the end of the pump rod. This is my first time messing with these style carbs but I’d imagine they are supposed to always be touching each other so any time the throttle is applied, the rod is immediately pressed down actuating the pump.

The way is sits now no matter how far I open the throttle, it never touches the rod and I don’t see any adjustment to bring the actuator plate closer. I’m guessing that maybe the previous owner bought the wrong pump and the rod is too short.
 

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At this point does anyone have experience with this style pump? It doesn’t have to be the exact carb, I’m just curious if the actuator plate is always resting on the pump rod.
 
typically with the carbs i have used, the accelerator pump rod only engages well into the opening. say at 50% or better throttle. not seen them actuated form idle.

JMO
 
Have you tested the throw on the rod? When you pull the throttle, can you see if and when it's squirting? You definitely need to be able to time the squirt, so there are certainly adjustments. It looks to me like the rod is in it's fully pushed position, as though the spring is missing inside the pump, or you have the wrong rod.
 
Haynes Manuel says for my carbs keihin pd50 that the setting is between 0.40 and 0.60
I've set mine to 0.50 but not running bike for a while so cannot comment if it's working correctly. Except seeing it opperate on the bench

Sent from my SM-G955F using Tapatalk
 
Thanks TheCoffeeGuy, I’ll keep that in mind.

irk Miller, the rod never touches the actuating plate above it. I’ve pulled it apart and made sure the spring is not only in there, but it’s on the correct side of the diaphragm pushing the rod toward the plate that is part of the throttle linkage. I’ve even made sure the diaphragm is flexible and I can operate the rod with a flathead. I’m trying to figure if there is an adjustment to the actuating plate that I’m not seeing or if the previous owner may have installed a pump with too short of a rod.

Bruders8146, is that the gap between the rod and the plate? What unit of measure are you using? 0.40-0.60mm?
 
Well I finally found a pdf of a factory manual that says the rod and the plate that actuates it should have 0 - 0.04mm tolerance. So it sounds like any time the throttle is cracked open from idle, it actuates that pump to correct for the momentary lean condition.

Now I just have to figure out if the rod is too short or the plate that actuates it is out of position. It’s a solid 4.0mm gap now. The total length of the diaphragm rod is 55mm(2-1/8”) from the outer most points. It has small letters on it NOK and 78.
 

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Well it’s glaringly obvious now that I’ve figured it out. The previous owner replaced the pump with a new one and he installed the dust protecting bellows on the shaft inside the housing. It was so compressed that I just figured it was a bumper. It finally hit me looking at kits online that all the pictures had a bare metal disc with no such bumper. Duh!!!
 

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