I just my car running with the rebuilt 390 4 speed. The engine rebuilder broke the vacuum advance, and said I do not need it.
The motor is putting out 350 hp. He had also rebuilt the holly 600( I had ran this rebuilt carb on my 289 without any issues)
It idles ok but tries to die when I start out and does not accelerate smoothly. I am running premium gas , and getting under 9 mpg. That is combined highway city, I have 2.75 gears with wide ratio toploader.
Would a vacuum advance distributor help.
I also have a Pertronix unit installed.
No, it’s not the lack of vacuum advance. Do you have a timing light? If so, see if the shop has it remotely right. Not knowing the guy, it may be a clue that he “rebuilt” that carburetor. Can you look straight into the carburetor with the engine off and see accelerator pump shot at the slightest movement (by hand) of the throttle? That’s one check to begin with.
I will check this with the timing light. I had a friend look down the carb while it was running and commented that he could see gas at the top of the carb and was not sure the secondaries were kicking in when I accelerated.
( he also drove the car and noticed the lack of acceleration at the start and the hesitation ( fluttering)). It Ideals around 950rpm
The engine guy keeps insisting the car is running right, he has worked on these motors for 40 years. I also have an oil leak that I am trying to find. One of the leaks was from the oil filter mount to the block. This is very frustrating when I have put this much money and time.
Thank you for your help.
If he added weights and converted it to mechanical advance then you’re ok. That would have to be done on a distributor machine. Otherwise, you need the vacuum advance or you’ll have poor performance and bad fuel economy.
I need to check with him, I was trying to see if I was having a carb issue
I was suggesting a test that one man can do. With the car shut off, look down into the primary throttle bores. At the top will be the accelerator pump squirter - there will be two stubby, small discharge nozzles pointing across the throttle bores at a roughly 60 degree angle. With the car warm (but shut off), while you’re looking, slowly move the throttle bellcrank on the carburetor. If you do not see liquid gasoline come out of the squirters at every slightest increase in throttle setting, that’s a clue. The discharge will only occur AS you move the throttle. It adds fuel in instantly to compensate for the extra airflow. Nozzles are the stepped, pointy items below the bright anodized screw:
The accelerator pump system operates via a lever that rides on a plastic cam, and connects to a diaphragm pump through a buffer spring. See this Holley video, juicy part about the arm and spring starts at 1:22.
https://www.youtube.com/watch?v=k-Nx5HEzvlY
This is all so that you can verify that the accelerator pump arm hasn’t slipped off totally, which would cause a BAD stumble.
Perhaps this engine builder is planning to give a $50 gas card every month in the season you drive this.
A distributor PROPERLY set up for the street is going to have both mechanical and vacuum advance. With many variables, your stop and go mileage is tough to estimate. But your cruising mileage should be 18 or so possibly 20. My 68 Monterey with a 2.75 gear and a C-6 trans got 18 mpgs on the highway. 350hp is a mild 390. It shouldn’t be throwing fuel out the tail pipe. Vacuum advance is a vital contraption to increase part throttle efficiency. Race cars, boats and stationary engines do not need vacuum advance-----because they operate in CONSTANT load conditions. Driving on the street presents VARIABLE load conditions. The vacuum advance is there to adjust for the load variations. Here is why. When the throttle is wide open optimal ignition timing is at X amount. Part throttle acceleration can tolerate more timing and light throttle cruise can tolerate much more timing due to less dynamic compression. In fact, it not only tolerates it, that extra timing is what it needs for efficiency. When I say efficiency, I mean you are making the most out of the fuel that is going in the engine. With retarded timing you are wasting it.
Find a car with a distributor that has vacuum advance that is properly tuned. Drive it for a bit and get a feel for it. Take note of how much throttle is required to perform a normal rate of acceleration driving in normal traffic conditions. Now, go unplug the vacuum advance and plug the hose. What you will find is sluggish performance-----lower hp for the amount of fuel being used at that part throttle opening. To compensate for sluggish performance you step on the throttle more, in turn using even more fuel.
Can you fix that by adding more to your mechanical advance? Not really. Mechanical advance is tuned in for a Wide Open Throttle condition. If you change mechanical advance to help a non functional vacuum advance, then you are detuning the mechanical advance away from optimal tune at WOT.
If your throttle is set up like and on/off switch then don’t worry about vacuum advance.
At Carlisle All Ford show I bring my tuning equipment and a couple vacuum advance cans. One example is they got 12 mpgs on the way to the show and 15 on the way home.
Find someone that can tune a street car. No matter how much hp this engine guy gets on his builds, or how much the local circle track guys like him. He gave you advice that is NOT appropriate for YOUR application.
Thank you I will take a look. It just wants do "die for a second when pulling out(especially when I just start out and when I am on a grade)
Thank you for the explanation,
I did check the front jets and they are working
I drove the car today 90 degrees in pittsburgh, temperature was in the normal, but the car was sluggish
and was pinging, I need to check the accelerator pump.
At this point, if accelerator pump is ok, than I will need to
change out the distributor with vacuum advance,
I need to check the timing but I think the engine guy said he set the timing at 20 degrees
You can buy just a vacuum advance and probably fix your distributor.
If you want to enjoy the car, finding someone else to tune your engine should be high on your list.
Bring the car to Carlisle all Ford show in a couple weeks. I’d be happy to tune it up.
Pittsburgh…eh? I might know someone in your area that can tune it.
First off, I’m no expert. My 390 GT 4 speed seems to run well at 10 degrees initial, 36 degrees mechanical advance all in. 20 sounds real high for initial.
This started with a complaint of an off-idle stumble, so I thought uneven fuel delivery first. Without the car in front of us, this is like the first 2/3 of an episode of House.
If it were really like House I’d have thought someone would have mentioned Sarcoidosis or Lupus by now. But then again, there is still another 1/3rd to go!
RB, I think you were right on track with the accelerator pump as cause for the off idle stumble. Without seeing the car, we can only make interpretation of his interpretation to begin to help. There are multiple issues going on. Some proper adjustments in fuel and ignition will make a night and day difference.
I would look close at the carb first, although I do think long-term some thought likely needs to go in the ignition, not to fix this problem, but to get it better dialed in.
As R.B. pointed out, the shooters should start immediately and be clean fuel, no sputter when you look down the carb, engine off moving the throttle. Lots can go wrong with a hamfisted mechanic in a carb in that circuit. If the accel pump uses a check ball, they can stick, or guys can break the housing trying to clean them. If it uses an umbrella seal (later carbs) they can break them during install, and more people incorrectly adjust an accelerator pump than correctly, all can do what you say
Additionally, checking to make sure float level is correct and the carb is adjusted properly is another one. If the car is idling too deep in the transition slots (front barrels too far open) you lose the benefit of transition fuel when you accelerate) The fix in that case is to add some secondary idle and close the primaries.
Finally, if this is a toy you plan to keep a long time, I would likely send the distributor out to be recurved anyway, you will be happy with the performance benefit and know exactly where to set your timing. Stock fords like about 36 total, and there is some drivability benefit starting at about 15-16 initial, so having your distributor set up for those numbers, then having the rate of mechanical advance set to come in at about 2800 will make it feel like it gained 2 cylinders on the street. Replacing the vacuum advance and limiting it to about 8 degrees additional, fed by ported vacuum will be icing on the cake with better mileage.
Hope this helps, 1 - Accel pump, 2 - carb setup, 3 - distributor improvements
You guys are great - and funny!
Rassan68, we await your next update.
I do have one questions, with the car pining under load or when it warmed up and going up hill, is this function of the type of gas or ethanol gas. I am using 91 octane premium gas.
I have attached my Build sheet and Dyno results
Obviously all the symptoms point to a disconnected vacuum advance. You have to use tons of initial advance to get the total advance good enough to run. Gas mileage is terrible because the distributor doesn’t know anything about engine load.
Simple to fix.
Other than needing a full tune up, compression is a bit on the high side for cast iron heads with 91 octane. Though it might not make much difference try to using 93 octane. The compression versus octane will probably be o-k. I’d suspect everyone here will agree your timing is the cause of your pinging. With 20 degrees initial I’m surprised we haven’t heard about a starting problem when it’s hot.
Carlisle is in 2 weeks. If you were to go with the tune in it’s current status you’d need to trailer it. Or, find someone that can get it close to right before taking more than a trip around the block. Like I mentioned earlier, I’d be happy to give it a quick tune. I always bring my tools to the Carlisle.
Thank you for the offer, I need to see if I can bring the car, if not I might try to go.
So the bottom line is I need a new distributor with a vacuum advance. I do have a pertorix unit in the current distributor. And suggestion on replacement distributor.