No vacuum at idle

[hellcat287]

Hi,

I recently bought a 64 falcon post with a 200 in it. The car has the original one barrel carb (an autolite 1100 i think). The car idles OK but hesitates on acceleration from a stop / slow cruise.

I checked the static timing at 12 degrees which I think is OK. I put a vacuum gauge on the carb and found it's generating no vacuum at all. Just to make sure my gauge was OK, I tried it on another car successfully.

Would the lack of vacuum at idle mean that the carb needs to be rebuilt? Or could there be a vacuum leak elsewhere?

thanks in advance!

Brian

 

[JackFish]

Depends on the port you checked. I know that on my 1946 The two ports on either side higher up on the barrel do not generate vacuum at idle. This is why they are used to control the distributor, because they generate vacuum in response to accelerator, throttle plate movement and airflow. Thereby controlling advance under varying loads.

 

[ludwig]

You should be able to pull the vacuum off the side of the manifold log. My 200 had a brass block with a couple different sized nipples. As for leaks, there are many places. Check all the rubber hoses connected to the carb and anything that pulls vacuum from it for cracks or leaks or split ends: automatic transmission modulator, power brakes, spark advance mechanism and any plug or bung that could be used as a vacuum source.

Spray WD40 around the base of the carb to see if there is a hole in the seal there. It will pull vacuum when the WD40 hits and then the vac will drop as the spray is pulled into the carb throat. Also spray it around the thing you are hooking on to for vacuum testing. There could be a hole or crack there too. Like a solder joint or something.

With the engine off and the distributor cap off, test the advance can on the distributor by hooking a new rubber tube to the advance can on the side of the dist and suck with your mouth. The rotor should move if you do.

If you close the choke at idle and there is no appreciable degradation of the motor function, you have air getting in somewhere else. And that would be a leak too.

 

[Frankenstang]

You should be able to pull the vacuum off the side of the manifold log.

Yep, vacuum should be measured at the manifold port. You'll have to enlarge this thumb to see an arrow pointing to it at the bottom of the pic.


As Jackfish mentioned the distributor port on the side of the carb will not show any vacuum at idle, this is true on a scv 1100. The scv valve is at the tail of the arrow in the enlarged pic (octagon headed valve with six holes), and the distributor vacuum port is the capped one (for my app.) with plug & teflon tape.

EDIT: Brian, yes vacuum should be measured at the manifold port Ludwig mentioned (and the arrow in my pic points to)...this is where you should check vacuum relative to timing. You want to set timing so as to give the highest vacuum reading at the manifold port (about 18+hg IIRC). I hope my pic does not confuse you, but on my scv 1100 the port where you normally connect the distributor line is plugged as mentioned (you can see white teflon tape on the plug in pic). This is the correct port for hooking up the advance line to distributor on stock/load-o-matic setups (but not the correct port for measuring vacuum at idle), in my 'unique' application it is plugged...because my aftermarket dizzy has no vacuum advance.

 

[hellcat287]

thanks for all of the information. On my car the vacuum line is coming from the side of the carburetor into the distributor. I didn't have a chance to check the port on the log. I guess i should be testing that port for vacuum correct?

 

[wallaka]

It sounds like typical SCV (Spark Control Valve) problems. The distributor and carb on your car are a matched pair. The vacuum port on your carb sends a modified vacuum advance signal to the distributor depending on throttle position and engine load, as there is no centrifugal advance on the dizzy. It was a decent system when new, but not as good as dual-advance technology. If you continue to have problems, 1968 or later have the dual-advance system and can be hooked to manifold vacuum to work. 1973 and later are DuraSpark electronic ignition, and are an easy upgrade.

Look at the Stickies at the top of the forum for posts detailing the SCV and upgrades.

 

[Frankenstang]

Just to clarify my earlier comments regarding vacuum measured at the dizzy (scv controlled) port on the side of the carburetor (where I think you were checking), again it is normal to show no vacuum at that port if the car is running at a normal curb idle (ie less than 800rpm). At that same curb idle the manifold port however will should show 18hg or better depending on the age/wear on the engine.

Spark Control Valve is a potential culprit, for incorrect advance under acceleration. There is great info that should help you learn all about the SCV in the "Small Six Tech 'Stickys'" thread at the top of this section. Also, while it is normal to see no vacuum from the dizzy (scv) port on the side of the 1100, you should be able to rev the engine and witness vacuum at that port increase up to ~10hg (I don't recall the max vacuum delivered by the dizzy/scv port on the carb).

Another candidate for your hesitation is the accelerator pump on the driver side of the carb (small square cover with four screws, and linkage atttaching to throttle) that houses the accelerator pump diaphram. A faulty accelerator pump will not deliver the required extra squirt of fuel upon initial throttle response, and the car will stumble. This extra squirt is required to offset the lag/drop in normal venturi vacuum delivery of fuel when you first push the gas. This could be your problem if the hesitation is only intially when you punch the gas, and does not continue to hesitate after that.

 

[hellcat287]

Frankenstang - actually there is no vacuum at all on the carb port either at idle or revving the engine. I put the light on my timing mark to set the static 12 deg and did see it increase to 25 deg at about 2000 rpm. I'm guessing this is the mechanical advance kicking in but without any vacuum from the carb i'm not sure if this is enough.

I also checked the accelerator pump and did see a decent "squirt" of fuel.

I think the next thing to do will be to check the vacuum on the log and if it's OK there then maybe i need to rebuild the carb.

I'll check into the SCV as well.

 

[Frankenstang]

Hey hellcat287, sounds like you got a handle on it, and wallaka had you pointed in the right direction, scv problem (defective valve would be my guess or could be internal passage obstruction) since the scv controls the strength of the vacuum signal(hg) the dizzy receives from the 1100 (sticky or shop manual give a good description of function)...at 2000rpm you should definitely be reading some vacuum at the carb port.

I think you'll be best off with a rebuild kit as you mentioned. Last I knew the GP Sorenson kits still came with a new scv in the kit, needle&seat, main jet, gaskets, diaphragms, etc...no float though And if you haven't taken an 1100 apart before, I'd be sure to preview the exploded diagram. There are some check balls that you'll want to get back in the right spots on re-assembly.

Probably not a bad idea to check vacuum strength at manifold and other possible sources of leaks mentioned. Good luck, will be interested to hear how you make out.

EDIT: Another source on SCV & load-o operation http://www.classicinlines.com/Loadomatic.asp

 

[ludwig]

Disassemble it over unprinted, light colored paper on a table or similar surface (not the garage floor) so that you can find all the balls and springs that fall out. Newspaper is okay but you can easily lose the little bits among the letters when they roll away form you.