Classic Mustangs (Tech)Technical discussions about the Mustangs of yester-year.
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So this is a problem that's been going on a couple weeks, I'm posting around a couple different spots for input trying to figure this out.
I have a 1964 1/2 Ford Mustang, according to the VIN the car originally had a 170 CI 6-cyl but previous owner dropped in a 302 engine block with a 289 intake manifold. Local NAPA guy referred to it as a 'Mickey Mouse' in that its got a lot of things cobbled onto it, aftermarket electric fuel pump, prob an aftermarket radiator, 4 barrel carter AFB 9605s carb.
The car starts, runs and idles normally from a cold start, and if you turn it off and start it up while still not fully warmed up continues to behave normally. Driving the car at a warmed up temperature from a cold start seems to be just fine. From all appearances once the car reaches a certain temperature, if you turn it off, then starting it up again it'll run with an low idle (300-500 rpm, semi-surges), giving it gas it runs rough between idle and around 900/1000 rpm, if lightly pressing on the peddle it'd often hover/descend from 900 rpm, with occasional backfire out tail end if holding here. If you give it enough gas to climb over 1000/1100 rpm the revving begins to sound more normal.
A commenter on another board suggested that possibly the carb was overheating and partially vaporizing the gas early in the carb and reducing fuel pressure? He suggested holding a high rev to try and restore fuel pressure, I tried this and my idle climbed to 600 rpm for a minute or so before slowly working back down. This came about because a week or so before this issue got bad I would have to hold the engine at about 1500 rpm for 30 seconds or so to restore its normal idle at a warm start.
Speculation so far has ranged from a low vacuum pulling on the advance diaphragm to something acting up with the fuel line, any help people can give would be greatly appreciated. Thanks!
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its fine until you shut it off... then on restart it dont run so great...okay I think I got it!
Let me ask this.
If its running fine and you shut it off and count to 10 then restart it is it fine?
Haven't timed out the quick restart but if i remember right I started it very briefly(under a minute) after turning it off when running well and it showed symptoms. But I'll want to double check that.
What fuel pump do you have? does it have the two cylinders that hang down below it?
the numbers are a bit worn/faded but I believe I saw either a 50104 or 60104 part number and a label reading 1.5-4psi, I'll include what photos I could manage.
upon restart how hot is the carb..is case temp over 130f?
manual choke right?
Haven't measured carb temp, but it is a manual choke. The guy that mentioned the high revving diagnosis said he resolved his carb temp issue with a spacer gasket between the carb and manifold, but the only hangup is this symptom (seeming) to develop over time for a couple weeks now, or at least being reliably run-able until a week or 2 ago.
1) I have never seen a worse looking electric fuel pump setup
2) even 4 psi is too low maintain any consistency I could imagine this causing lots of problems especially if sometimes its 1.5 psi
3) if it had been okay and is now getting worse maybe that pump is performing worse than it had been.
these carbs are designed to run on 6.25 to 6.5 psi (notice the narrow range) of course this changes some amount with engine load.
this may or may not solve the issue but here is my suggestion.
I know from looking that your fuel pump setup is not all that hot so lets get rid of it.
1) spend big bucks on a good electric fuel pump and a good regulator that will allow you set 6.25psi dead nutz and will maintain that pretty well.
2) that engine should have a block off plate covering the mechanical fuel pump port. If the fuel pump eccentric is still installed (you can take the plate off and see) then you can simply install a nice mechanical fuel pump and be done with all that mess. Hell even for test purposes you can throw a mech pump on there and run some rubber fuel line in like 1 hr be done enough to test it out.
Will it solve the issue I dont know but we are fixing a problem so its not a total waste.
Masters of Industrial Technology intelligence thats why I own guns made to drop elephants.
So I ended up taking the car in for a general inspection, had also wanted to see what other issues might crop up down the line, etc. I guess I found the world's most generous mechanic, one of the guys at the shop went through and fiddled w/the carburetor, did some minor adjustments, corrected the timing on the car for no extra cost. It's running a lot better, symptoms have vanished.
Someone else mentioned it sounded like the vacuum advance was hooked into the wrong vacuum port on the carburetor, and sure enough when I got it back from the shop it was hooked into the other small port (above the throttle) instead of the other(below the throttle). Which leaves me wondering if or when it was hooked into the constant port(I'm pretty sure it was hooked in there when I got it) and why it was running well until the hot start. Oh well, onto the fun that is electrical.
Since the vacuum advance line was hooked to the lower vacuum port, the distributor was at full advance during idle. But, you set the idle timing at the retarded setting for idle. When you open the throttle, with the hose connected to the UPPER port, the ignition timing advances. But, because you were on the lower port, the vacuum actually decreased ( the intake is not pulling against the throttle plates) and the ignition, not seeing full vacuum as it did at idle, retarded further. A retarded ignition causes the engine to run hotter. Opening the throttle a little ( going up to 900 rpm) would drive the timing crazy, as it would initially retard because of loss of vacuum but the centrifugal tries to advance (Mustangs had the Dual advance distributors).
The original carb setting was probably done with the hose connected correctly. After the hose was put on the incorrect port, when the engine was cold, the choke high idle would keep the engine running. But when it came off high idle the distributor retarded the engine ran too slow.
So the main problem was almost assuredly due to the incorrect placement of the advance hose.
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