I've noted that some members have removed the secondary throttle stop screw from beneath the base plate and reseated it above the plate to make access to it easier. Any info or comments would be appreciated.

JamesC

 

Depends on the carb... not all base plates are drilled (& tapped) all the way through.

Not hard to do that though. 6-32 if memory serves but I could be wrong, just pull the screw out and match it up. Then you can turn the screw upside-down and adjust it from above.

I'd suggest, if yours is an old one with a slot screw, go to the hardware store and get an Allen one to replace it. MUCH easier to deal with.

 

Adjusting the screw in or out changes the angle of the blades, of course, but how do the mechanics of it work?

JamesC

 

The idea is, rather than using the primary throttle location to set the idle speed, instead, locate the primary throttle blade at its ideal location (with respect to the transfer slot); then use the sec throttle opening to control the idle speed, without disturbing that other critical relationship.

 

I understand. I'm curious, however, about how the screw physically opens the blades. EDIT: I think I've found the answer to my curiosity. It appears that the screw adjusts a lever on the secondary shaft.

Also, how sensitive are the blades to changes in screw adjustment? A big or little change, say, at an 1/8 turn?

JamesC

 

Yup, that little tab thing comes to rest against the screw.

Couldn't say about the sensitivity in terms of turns, since it varies depending on the particular carb's linkage; but, the carb being more or less "square", the effect of any given angular motion of the throttles, is about the same. If you wanted to figure it out the other way, you could look at the distance from the center of each throttle shaft to the place where it rests on the tab, and the pitch of the screws; and from that, figure out pretty close what the ratio of the 2 adjustments is.

Usually I just set the idle right w the pri, then flip it over and re-adjust; and however far the pri moves from where it was to get to "ideal", I just adjust the sec the same amount. Usually only takes acoupla shots to get it right. With the screw accessible from above though, you can take it off once and set the pri to the right spot and approximate the sec once, then start it up and see where the idle is and shut it off and re-tweek the sec, repeatedly if necessary, until you get the idle speed where you want it. It might even change so much that you have to re-adjust the mixture screws, which you can do with vacuum if you have a gauge; find max vac, then lean the idle 1/8 to 1/4 turn.

 

Thanks, Sofa. I've got the jets and power valve right, next the above tweaks.

JamesC