With the car running in Park, I pulled the vacuum line that runs from the vacuum advance on the dist to the carb. There was no vacuum on the line. Should there be? The P.O. had put in electronic ignition but didn’t do the matching coil. I went ahead and done that this week. The carb (Edelbrock 1407) is being rebuilt this week but I’m wanting to make sure I don’t have a vacuum issue too?
Should I consider a whole new distributor and vacuum advance ?
The car is a 70, 351C 4V with FMX tranny.
I think your vacuum line may be attached to the wrong port on the carburetor. I don’t know edelbrock carbs very well, but I remember seeing several vacuum ports on them.
Badcatt, my question is should it be pulling vacuum FROM the distributor? I unhooked it from the carb when it idling in park and there is no vacuum. I thought there would be.
Yes it should have vacuum at idle.
The distributor dose not supply vacuum. Vacuum is applied to distributor from the carb/ intake manifold to activate the diafram with in the disrtributor. Also the vacuum that is applied to the distributor is usually routed through a distributor vacuum valve that is screw in to the thermostat housing. See atached diagram’s
The carb has a ported vacuum port where there is no vac. felt at idle, but upon acceleration it supplies vac. to dist.
Backing up Catlover here - over 40+ years of doing it, I’ve never sensed vacuum from a ported vacuum connection off a carburetor. It comes from above the manifold and above the throttle plates for that matter, so the signal at idle is essentially atmospheric pressure. It doesn’t mean you’re not getting advance as airflow goes up. However, it is a weaker vacuum advance action than a manifold connection would give!
Ok now I’m super confused. My carb is an Edelbrock 1407. I’ve only had the car about a month. It has a vacuum line from the distributor to the left side of the carb which the online instructions says is timed vacuum. Right side is manifold vacuum and it’s capped. So do I have it hooked up right? I think I was checking it backwards. I was checking for vacuum on the line FROM the dist and should have been checking it FROM the carb.
Ok. The vacuum comes FROM the carb TO the distributor.
If you unhook the line at the carb your tube will have no vacuum but you will hear a sucking sound at the carb.
If you unhook the tube at the distributor you will vacuum at the end of the tube.
Personally I would use a vacuum source at the manifold tee and block off the one at the carb…
If you are timing your engine pull the tube end off the distributor and block it off using a pencil or golf tee. Then set your timing.
Could you give me the reason to use manifold vac. instead of timed vac. from carb.
If you use manifold vac. the dist will be advanced all the time, if using timed port, the dist. will only adv when accelerate.
Sure. Might be a mental thing but I’ve been chasing the best timing for a 69 Cougar I bought a month ago with a modified 351w. I know nothing about this engine except the previous owner told me there was 7K invested in it. I get better throttle response and no pinging and smoother idle using vacuum off the manifold tee. I YouTubed it and I think it is explained good by a guy named Thunderhead 289. My engine is running a 650 double pumper but again I know nothing about its internals. My previous two Cougars M code and H code that I lost in a fire were stock but I’ve always run the vacuum from the intake.
I also would like to hear more people chime in on which way they think is better???
Premium gas when these cars were new was 100 octane or more. If the distributor is set per the factory settings then with 92 octane unleaded you get lots of pinging. All of us who drove these cars every day in 1977 faced the same problem. At that time leaded premium was no longer available and cruddy 92 unleaded was the best that you could get.
In order to crutch the situation we accepted the fact that in order to drive the cars we had to change the ignition timing. Simply retarding the distributor makes the car run warm all the time. A better result is obtained by limiting either the vacuum advance or the centrifugal advance, or both. The vacuum advance can be shimmed (original vacuum advance) to limit advance. There are aftermarket vacuum advance cans that are adjustable via a hex key through the port where the hose connects.
Centrifugal advance can also be limited. Many of us used the cheap method of installing two of the heavy springs instead of one light and one heavy.
Better solutions involve reducing compression using either bigger combustion chambers or negative piston compression volume. Then timing can be near the factory settings. You can get back the lost performance with headers and a roller cam.
This reduces pinging but performance of course suffers compared to using more compression and good fuel. My solution nowdays is to keep a 55 gallon drum of race gas 110 octane in the back yard. The 427 and the 428CJ don’t like bad gas.
Royce, we should all have your problems. LOL (I’m just jealous)
That is the correct port on the carb. There should be no addtional advance from the vacuum port at idle. As you cruise the timed port (as Edelbrock calls it) raises the vacuum and increases the timing and burn on the cylinders.
Doing it with manifold vacuum applies maximum timing at idle and nothing at top-end. Completely contrary to how a 4-cycle engine works.
During cruise both provide vacuum, the biggest differences are at idle.
No advance at idle (ported):
-Engine will burn cleaner (emissions), and also run hotter
-Less efficient, you will lose idle RPM
Advanced at idle (manifold):
-Exhaust will not be as clean (emissions), but motor will run cooler
-More efficient, you will gain idle RPM
The distributor vacuum control valve that screws into the thermostat housing switches between ported and manifold vacuum based on coolant temp. For cleaner emissions it uses ported vacuum. When the coolant temp gets to around 225 degrees it switches to manifold vacuum to prevent overheating.
I run manifold vacuum on my car.
I’m thinking that as rpm increases gradually as in cruisein’ the manifold vacuum also increases. So it could be a coincidence that the two vacuums are close and you are getting the proper advance. However pedal to the metal would cause a drastic drop in manifold vacuum and when you really need the advance its gone which would cut back on performance. Also letting off the pedal at higher rpms would cause a big increase in vacuum meaning a big increase in advance, possibly backfire???
I to was curious, so I did some research, this is kinda lengthy but at least read the last three paragraphs. Please excuse the car website.
http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm
In the early years, there was only mechanical advance. Pretty much any gas engine needs ignition advance as the RPM increases to run efficiently. The vacuum advance came along as a gas mileage device. Engineers figured out that under part throttle or cruise conditions, even a greater amount of ignition advance would make the engine even more efficient. In later years,it got more complicated with all the vacuum devices.
Each engine has it’s own characteristics, so I usually try different vacuum and centrifugal combinations to find the best drivability results.
If you are working on a Chevy then the advice being given in the article is correct. However it is not correct for Ford products built from '67 to '73.
these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC).
This does not apply. Most Ford products have a 6 degree BTDC base timing. Typically it can be advanced to some where between 10 and 15 degrees.
instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees “total timing” at high rpm wide-open throttle to get some of the performance back.
This is almost right. The stock distributor has two limit slots on the internal advance arms. One is probably labeled 15L for 15 degrees of cam shaft advance (30 degrees at the crank) and the other is probably 10L that will give you 20 degrees at the crank. See how this works? 6 degrees initial advance plus 30 mechanical gives you a total of 36. With modern gas, you have to run a richer mix at idle, so you can easily run more initial advance. The engine will run a bit cooler, show a stronger vacuum signal at the carb and transition better when you tip into the throttle. The only problem is that 36 degrees can be slightly too much advance depending on the compression of your engine. The alternative is to run 12 or 15 degrees initial advance and swap over to the 10L slot to get 20 degrees of mechanical advance, for a total of 32 to 35 degrees advance. The arms in your distributor can have different values, 13L and 18L are also common.
