The Stovebolt Site Forums The Shop Area Engine Shop where things go

Hi Guys, I have not been around for a while I had to sell my fifty to help my kid. But I have a new truck now so all is good. This one is a 55 first series 1/2 ton short bed its an Arizona truck and so far rust free.I went to pull the carb and noticed that there is an Automatic Choke.I looked at the dash and dont see a knob for the choke lever but there are a couple of open holes in there too. The carb has three vacuum ports. One coming from beneath the carb hard plumbed to the automatic choke and from the choke there is another piece of tubing that points down toward the intake but I dont know where it goes. and another tube that comes from the other side beneath the carb and I believe It runs to the vacuum advance of the distributor.So my questions are. Did this truck come with an Automatic Choke? And if so where does the free line coming out of the choke go. And does the vacuum advance attach to the other port below the carb facing toward the motor.Thanks in advance for any advice.Its great to be back and I can hardly wait to drive this old truck.

Tony

Hi Tony, new project, got pictures?
I can't help you with the carb, I barely understand my '26 engine, I just wanted to bump your post on top of the pile again. I'm sure someone will come along and give you the answers or you drop a note to one of the first series truck owners in the gallery.
Good luck
Richard

Assuming the '55.1 carbs were the same as the '54s, and I think they were, the trucks originally used a manual choke. The cars had auto chokes.

The tube coming from the choke should go to a choke "stove" on the exhaust manifold. The stove is a little shield around the manifold to collect hot air that is sucked through the choke spring housing to heat the spring and open the choke when the engine is warm/hot.

The rule of thumb (although not agreed upon by all) is that the vacuum advance hose should go to a "ported" vacuum sorce. That is, one that only has vacuum when the carb is in a "non-idle" state. In other words, there should be no vacuum at idle and vacuum at part throttle.

Some motors/distributors are picky and run better with a full-time vacuum, so it doesn't hurt to try both ported and non-ported sources.

I really don't know what the factory routing should be for your setup, but these are some general suggestions.

Along the same lines... I just put a H/W on my 52 and do not know where to hook up the vacuum advance. From what I understand the vacuum port on the exhaust manifold is for the vacuum wipers. Mine have been converted to electric so I plugged that port. I am picking up no sign of vacuum on any of the other ports on the carbuerator at idle and when giving it throttle. What gives?

Tony maybe this is the answer for you... grab a vacuum gauge (under $20) and see which ports gives you vacuum on the carb. Looks like the choke line question has been answered.

I think that my 216 53 pickup vac advance has vac on when at idle.???????????Will have to let someone watch when I crank it up.

If I hook the vacuum advance to the vacuum port on the manifold I have vacuum at idle... 20 pounds of it. The vacuum advance retards when the engine rpms increase rapidly. This seems backwards to me. Shouldn't the vacuum advance increase timing when the motor accelerates?

Thanks for the info. I have sent an email to one of the guys in the gallery and will wait to see what he says about placement of the choke lever. I'll take a trip to the junkyard this weekend and see if I can come up with a carb and linkage and see if I can get the location of the vacuum advance and the pre-bent plumbing (thats wishful thinking in and around LA there's no parts left). I'll get some pics and get them posted ASAP. Look forward to talking with you guys.

Tony

Looking at other posts on this site it looks like the advance needs to be hooked up to a ported vacuum port.

There should be no vacuum at idle but as acceleration begins the carb opens and vacuum accumulates advancing the timing. As the motor rpms increase the vacuum decreases and the timing retards. At WOT there should be no vacuum timing advance.

I still don't know where to hook up the vacuum advance on the H/W. I have checked every port on the carbuerator and there is no vacuum at idle or when accelerating. Any thoughts?

This is incorrect. The "no vacuum at idle" is ok, but vacuum should only be present at part-throttle. It has nothing to do with engine speed. Advance for engine speed should be performed by the centrifugal advance weights inside the distributor. In most cases, vacuum will drop to near zero at WOT, something people familiar with vacuum-operated windshield wipers will be painfully aware of.

The only reason for vacuum advance is to increase mileage during part throttle situations. It doesn't have to even be used. Many hot rodders don't even run a vacuum advance can.

Here's a good write-up on vacuum advance:
http://www.classicchevytrucks.20m.com/vacuum.htm

My mistake... should have been as the throttle opens not as the rpms increase and almost no vacuum at WOT. Thanks for the corrections. Still trying to figure this stuff out.

By the way... love the rat rod!

there should be vucuum at idle beneath the carb next to the adjustment screw there is a port.I'm sure this is where I will pick it up at. I will let you know what I find out at the junkyard. The engine is a big pump and when it draws in air thru the intake that is your vacuum if the throttle is almost closed (at idle)the vacuum is at its highest as if you had your hand over the opening. at wide open throttle the vacuum is lowest as though you moved your hand away and the pump can suck air freely. that little port is into the throat of of the carb and if you dont get manifold vacuum there the port is probably plugged

Just saw this post, albeit a little late. The correct vac port on the holley /weber 32/36 carb for the vac advance on the 235 chevy is the vac port just above the air/fuel adjustment screw.