The Stovebolt Site Forums The Shop Area Engine Shop Air/Fuel Mixture Question plus Vacuum adv.

Ok. here is my problem. The carb. does not seem to be getting enough air. It idles ok but when I press the throttle at the carb. it bogs down. Turning the mixture screw doesn't help. After doing alot of experimenting, I discovered that if I press the throttle down and at the same time turn the distributor towards the front of the truck, it revs up like a clock. Could this be a bad vacuum adv?(When revving up, the distributor does not rotate on its own) Also the fuel is running way too rich. I believe this because the exhaust is really rich smelling. Any Help?

The vacuum advance should be turning the distributor when you rev up. Either the vacuum advance or the connection-to-carb line is bad I'm thinking.

The vacuum advance has a rubber diaphram, so these do fail after a period of time like fuel pumps, etc.

This is a 235, right? Suck on the advance line, see if distributor moves. The vac advance won't really affect performance in neutral. Either your mechanical timing advance system is gummed up, or your carb is screwed.

Mixture screw only affects idle. The main jets/fuel system inside carb controls above-idle mixture. Could have a vacuum leak somewhere. Check sparkplugs for carbon fouling.

Check timing with a light. Shoot for about 10-12 degrees at idle, rising to about 35 degrees total by 3,000 RPM. Look for no or erratic spark advance.

How does truck run going down road? I've seen 235s do weird things at idle, but run fine going down road.

chip

Truck bogs going down the road. I have put on two different rebuilt carbs so I'm pretty sure its not that. It is a 1954 235. Oh yeah, I am unsure what the slang is for Mechanical timing advance system. Is this the vacuum advance?

Hi 53,

The mechanical advance is a device in many distributors under the breaker plate that allows the rotor to advance timing through centrifigal weights and draw back springs. The tension on the springs control how fast you get mechanical advance. On GM V8 engines the mechanical advance is on top of the distributor, the two things that pivit out and springs pull back and holes drilled for the rotor screws. Its not slang as per say, but what the device actually does.

Truck Tinker

Newold53 there are 2 advances on those distributors. one is external and is the Vacuum advance. The other is Internal (in the dist under the Point or Breaker plate) mechanical advance. The mechanical advance is 2 weights that advance the disty by use of centrifigal force. The faster the dist turns the farther out the weights go advancing the timing. If you have a shop manual there should be a break down of the distributor and you can see the weights that I am talking about. These weights can get gummed up and not work correctly.

Just to clarify a little bit: there are generally 2 types of ignition timing advance. First is centrifugal or mechanical. Its purpose is to increase the amount of timing advance as engine speed increases. Then there is vacuum advance. Its only purpose is to increase timing advance slightly under part-throttle conditions in order to increase gas mileage.

In most cases, you can disconnect the vacuum advance completely and still have a good-running motor...as long as you plug the vacuum line and don't induce a vacuum leak.

In my experience, a throttle bog comes from running too lean...whether it's because of a carb mixture problem, a malfunctioning accelerator pump or a vacuum leak.

Vacuum advance is normally not the cause of a throttle bog.

Your distributor advances are not working well because the engine runs poor. Check engine condition with a compression test and go from there. Could be anything from a warn engine to fried points. It's best to start from the beginning,
Scott