Hi, I am almost done rebuilding the 636SA carb for my 1948 CJ2a. It sat for a few years and would not idle - only fast idle. The rebuild has gone fairly well and all that is left to do is set the metering rod height. However, I noticed that there is a lot of slop in the throttle linkage. I took that assembly apart and noticed that the boss on the cast throttle body is worn where the stamped bracket rides on it, creating a lot of slop. Is there a fix for this? Will performance suffer if I leave it as is? I suspect it wont be as responsive, but then again, this isn't a sports car. How hard would it be to come by a replacement carb base?
Will performance suffer if I leave it as is? I suspect it wont be as responsive, but then again, this isn't a sports car.
Your biggest problem will be idle recovery. The linkage may have enough slop to prevent full return to the idle setting. A significant amount of wear will be found where the throttle shaft lever is connected, by a pin/screw, to the main linkage lever. This slop leaves you with a high idle because the pin is not fully engaged. The typical required response from the driver is to lift the pedal with the right foot.
How hard would it be to come by a replacement carb base?
NOS and “only slightly used” 505 throttle flanges are very, very few and very, very far between. I have none available. I’m sorry, but I sincerely doubt you will find one any time soon. If so, it will be on a NOS carburetor.
There are a couple of ways to try and tighten the action. A combination of the following may work for you.
The easiest thing to do, provided the wear is not too bad, is to lengthen the adjustable linkage rod. This puts more tension on the return spring.
You can change the long civilian spring to the shorter military spring. Again, more tension.
In slightly less-than worst case I use Flat Teflon bushings, cut to fit where they are needed.
I did. Having a hard time remembering now but I think it was around .314-.315 and the shaft was around .310. I got the engine running this weekend but haven't been able to get it dialed in yet. Idles around 700 ok with a little choke, when I push the choke in it wants to quit after a 10 seconds. And I have to give the throttle a pretty good goose to prevent it from stalling when starting off or shifting. I put a vacuum gauge on it and the needle was really bouncing around (like a blur) between 17-20". I'm going to check for vacuum leaks next. I can hear a sucking sound but not sure if that is just the normal sound through the air horn of the carb (air cleaner was detached for the testing).
Having a hard time remembering now but I think it was around .314-.315 (.313”ID is max wear here.) and the shaft was around .310. (.310”OD is max wear here.)
You have a Total Air Gap (TAG) of .005”. That’s a vacuum leak which will only get worse.
Idles around 700 ok with a little choke, when I push the choke in it wants to quit after a 10 seconds. (This is because of the vacuum leak.) And I have to give the throttle a pretty good goose to prevent it from stalling when starting off or shifting. I put a vacuum gauge on it and the needle was really bouncing around (like a blur) between 17-20”. (You have valve issues.) I’m going to check for vacuum leaks next. (We’ve already found it.) I can hear a sucking sound but not sure if that is just the normal sound through the air horn of the carb (air cleaner was detached for the testing). (This is normal.) A little light reading for you. How to Interpret a Vacuum Gauge
One more vacuum gauge tidbit..... Mine is a Autozone special, I finally figured out to hold it vertical in my hand to get a accurate reading. I was laying it horizontal on the fender, the needle always bounced. I spent a lot of time trying to troubleshoot valve issues that didn't exist... Gonna make a hook for it one of these days....
We have miles to jeep before we sleep! “Keep the Lord in your heart, and keep your powder dry.” from the song “In The Mean Time” by Chris Knight.
Thanks for the replies. I had done some googling and also found information about worn valve guides. But now that I think about it, the vacuum gauge was lying horizontal on the fender which was vibrating like crazy while I was trying to smooth out the idle. Perhaps that was causing the needle to bounce. Let me repeat that test holding the gauge vertical in my hand.
Is the .005" cleareance on the throttle shaft a big enough vacuum leak to cause me idle issues? I suspect that l might also have another leak so I'm going to try to sort that out before I go the route of throttle shaft bushings and/or a new shaft.
Yes. .005” is the point at which a noticeable vacuum leak presents. Other issues could be loose screws, faulty gaskets and a diffuser gasket with copper rings that have not been flattened with the proper torque. As Gary Indicated, the vacuum gauge should not be in contact with a vibrating vehicle. Free hanging vertical is the best position for accurate readings.
There is one more very rare possibility and that is an internal crack in the main body/bowl casting.
Last Edit: Nov 6, 2017 6:50:04 GMT -5 by Scoutpilot
I haven't had time to do a leak test with the propane yet but I did do a little investigating. Turns out all 4 nuts holding on the intake manifold were loose. One wasn't even finger tight. So I tightened them up. I also cranked down on the nuts holding the carb to make sure the copper rivets get crushed. And finally I noticed that the outside of the carb was wet with gas. Tightened all of the plugs and will check it again tomorrow. So hopefully I was able to address any potential vacuum leaks. Will do a propane test on Friday and let you know what I find. Thanks for listening.
I played around with it a little more today. Hard to get it to idle at anything less than 900 RPMs. I tried doing a leak test with a propane torch and honestly I couldn't really tell if there was a noticeable increase in RPMs because it already moves around without introducing propane. I'll give it another try tomorrow. I did notice that the throttle body (base of the carb) is wet with gas on top of the flange that connects to the manifold. Not sure where it is coming from. Could be from he throttle shaft or could be from the copper plug or the square plug that is threaded into the throttle body.
On a positive note I took it for a spin around the neighborhood and the engine seems to have good power. But the vacuum gauge does flutter a bit so my valve guides may be worn. If so it's not something I really want to deal with now.
Post by Scoutpilot on Nov 17, 2017 17:22:58 GMT -5
Re-check the float setting (3/8”) and inspect the float pin for grooves. If the pin is grooved it will cause the float setting to change from too high to too low and back again. I suspect flooding. There should be no liquid fuel in the throttle.
It's definitely flooding when parked. I took it apart and focused on the float. Sanded the tang flat, replaced the slightly grooved pin and reset with a 3/8" gap without gasket. Checked to see if needle was getting cocked to one side and it looks like it is not. Reassembled and problem is not fixed. Is it possible that the orifice that needle goes into is leaking? I did replace it during the rebuild but I did not take it out during the most recent work I did on the float. Could it be leaking from somewhere else? Starting at the double seal and spaced everything on the outside is wet with fuel after sitting for a few hours after a short drive. I'll post a picture if I can figure out how to.