Vintage Mustang Forums banner
  • Hey everyone! Enter your ride HERE to be a part MAY's Ride of the Month Challenge!

1 - 9 of 9 Posts

·
Registered
Joined
·
7,376 Posts
Discussion Starter · #1 · (Edited)
I have had a random pop that comes and goes at idle and low speed. Sometimes when I try and rev the engine up into the 3000 rpm range it feels like it is running a bit rough. Other times its perfect so its kind of unpredictable. I also seem to be having to tweak my idle screws around a lot. It will look way lean so I fatten it up and then at some magical point I can't identify what has happened it goes way rich again by itself. The only questionable thing I have is what is going on with the fuel pump.



The thing I have always wondered and never found out the answer to is the question of fuel pressure. If the fuel pump is not putting out its set fuel pressure at the regulator, or say it is very intermittently shutting off and coming back even though the carb may not be get emptied while its off could this cause these random engine running issues I started have? The idle a/fr is also totally wonky sometimes. It will drop out to 18 as if it is not getting enough idle fuel to even register so I would expect a lean pop if that is an accurate reading and it sounds like a lean out pop. Then if I rev it it will come back to 14 but it may not stay at 14. It never used to act like this so I have been thinking this is connected to the pump going out maybe. My reasoning only works though if fluctuating fuel pressure has an effect on the carb draw even if the carb bowls have not gone completely empty yet. It is possible that the pump is surging too when it tries to recover its brain but I have a decent regulator so I don't think it is over running the regulator. At the track with no alternator I used to pre-prime the carb then shut off the fuel pump as I cranked the engine to give the starter all the amps it could. However, I would instantly flick the pump back on as soon as it cranked. I don't think that would have found an on/off and fluctuating fuel pressure issue like this. I am still inclined to think that the failing fuel pump hasn't been the cause of these other engine quirks though.


I knew my old Holley blue pump was on its way out because instead of its normal cheerful froggy noise it had a rolling grumble in it and would pull way down low once it made some fuel pressure. Yesterday it just shut off completely with me on the street but I couldn't hear it over the engine. Fortunately the carb ran dry just as I pulled into my shop drive way. Pushing the car uphill back into my shop was,,,,,,,,exhilarating, LOL. I managed to not pull a hamstring this time.



When I got it parked finally, I turned the ignition back on and the fuel pump is running again, engine fired up but I don't trust it at all anymore so I ordered a new one. I've been putting this off for a while. The techsperts said I have gotten way more than my expected life span out of this thing which is probably true since it is at least 10 years old which is as far back as my brain tends to retain stuff like this. It may be more like 14 or 15.
 

·
Registered
Joined
·
376 Posts
Low fuel pressure will cause popping back thru the carburetor. I had a similar problem and replaced the fuel pump and put a fuel pressure gauge in line next to the carburetor so I could diagnose the problem if it returned with the new pump.
 

·
Premium Member
Joined
·
7,515 Posts
I'd say, get the new pump up and running then monitor the carb "tweakiness". Am assuming you're running 5-7 lbs? Yeah, you shouldn't be adjusting the air mix screws often. Does the eng. vac appear normal for your engine?
 

·
Registered
Joined
·
7,376 Posts
Discussion Starter · #4 ·
I have it set on 6. I went ahead and ordered a new pump last night. I had Summit bucks too so I got 20 bucks off and free shipping. I have typically ignored my Summit bucks emails so I think I'm going to start paying more attention to this. 20 bucks is 20 bucks after all.


If I catch a time when this problem is not happening, my idles are working ok and I have a solid 14 idle a/fr, it will run up clean and my vacuum looks normal for this engine. When this problem starts up it will mess with everything.



So is this actually being caused by the fuel pressure exerting a certain amount of force on the carbs circuits or is it simply gravity since the fuel levels in the float bowls is dropping lower what it should normally be?
 

·
Registered
Joined
·
7,376 Posts
Discussion Starter · #6 ·
"Junk" in the carb.

Could be the carb but not for my non-lack of trying anyway. If the fuel pump doesn't clear it up the carb will be up next,,,,,,,again. I had to clean it totally out once already from having sat on the engine stand for several months and the carb is essentially new. I'm starting to think that these Quick fuel carbs are prone to getting clugged up if you are running pump gas unless you drain it out after every time you run the car. That is presuming that the car is going to sit for a while which is how my car has been operating for a long time now. I may try putting something in the fuel for a cleaner like B12. I didn't know this until some time back when somebody told me they run it in their motorcycle so I looked on the can. Says right on there that you can use it as a fuel additive to help keep the system clean.


I did all the other stuff this time in the restore with a new gas tank(flushed it out just in case), new lines and a new fuel filter. It shouldn't have picked up anything other than sediment/residue from the fuel but 93 unleaded pump gas now leaves a reddish brown residue behind at least where I am. It looks just like what the old octane boosters used to do.
 

·
Registered
Joined
·
37,851 Posts
Could be the carb but not for my non-lack of trying anyway. If the fuel pump doesn't clear it up the carb will be up next,,,,,,,again. I had to clean it totally out once already from having sat on the engine stand for several months and the carb is essentially new. I'm starting to think that these Quick fuel carbs are prone to getting clugged up if you are running pump gas unless you drain it out after every time you run the car. That is presuming that the car is going to sit for a while which is how my car has been operating for a long time now. I may try putting something in the fuel for a cleaner like B12. I didn't know this until some time back when somebody told me they run it in their motorcycle so I looked on the can. Says right on there that you can use it as a fuel additive to help keep the system clean.


I did all the other stuff this time in the restore with a new gas tank(flushed it out just in case), new lines and a new fuel filter. It shouldn't have picked up anything other than sediment/residue from the fuel but 93 unleaded pump gas now leaves a reddish brown residue behind at least where I am. It looks just like what the old octane boosters used to do.
If you haven't completely disassembled, knocked out the welch plugs and run all the passages with a wire it ain't totally clean....
 

·
Registered
Joined
·
1,332 Posts
I had a similar problem after some old gas carboned up my spark plugs.
 

·
Registered
Joined
·
7,376 Posts
Discussion Starter · #9 ·
I had some time between waiting for welds to cool off so I did some more experimentation with my carb on the mustang. I have more of the primary throttle plates exposed than the secondaries by a small amount. I turned the primaries idle screws out to 1 1/2 and the secondaries to 1 turn. All 4 screws are still very responsive. I can shut the engine down by turning 1 screw all the way in. I think that means I am still good with transfer slot exposure. I can't trim this thing down to the ragged edge of lean red. It isn't very happy set that way.



So after that my idle pop and low speed stumble went away. A/fr sits at 13.5 or so though. Maybe that is what it needs since it cranks up almost instantly and seems to run up smooth now.



I'm still replacing that grumble grumble fuel pump though anyway since it is likely the motor is about shot on it.
 
1 - 9 of 9 Posts
Top