Engine Performance "Flat" ? - Vintage Mustang Forums
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post #1 of 28 (permalink) Old 07-21-2018, 08:56 AM Thread Starter
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Engine Performance "Flat" ?

Engine specs in sig below. Replaced mechanical fuel pump with a new one a few weeks ago (I don't think this is problem). Ever since then engine performance seems to have gone down. I can't really tell at higher rpm but at lower I can. Generally when I'd pull from a stop, without dumping the clutch I could put the pedal to the mat and it would spin. The last couple times I drove it, it wouldn't do this. Otherwise the car seems to run fine. Starts easy, idles, cruises, no misfires or stumbles, just doesn't seem to have that lower rpm "umph". Accelerator pump is working and all plug wires are snug. Any thoughts? I wouldn't think the new fuel pump would have anything to do with it but that's when I noticed this starting. If I had a cylinder not firing for whatever reason, don't you think that would be pretty noticeable? Carb adj? Check wire spark? When I started looking around there was a loose plug wire but when I snugged it up that didn't change anything. Bad plug now?
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post #2 of 28 (permalink) Old 07-21-2018, 09:01 AM
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"Engine specs in sig below" Don't see them
I would think a bad fuel pump would be more of an issue at higher RPM, Wouldn't hurt to check/change the fuel filter though


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post #3 of 28 (permalink) Old 07-21-2018, 09:07 AM
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I’d start by confirming true top dead center with a piston stop (in case the mark on the balancer is wrong), and then set timing. I’d also double and triple check plug wire routing. In my experience these engines run surprisingly well with 2 plug wires crossed.

If you have an auto trans, especially the ‘65-‘66 green dot version, any chance you are starting in second?

HTH
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post #4 of 28 (permalink) Old 07-21-2018, 09:39 AM Thread Starter
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Well.........I used to have a very detailed sig. Must have gotten deleted somehow? Maybe when the site had that issue a few weeks back.

It has a mildly worked 351W with 4 speed toploader. Holley Street Avenger 670. MSD Box.

Fuel filter only has a few hundred miles on it. I don't think lack of fuel to the carb is the issue.

MrFreeze.......Could the tdc and timing change? I really haven't done anything to this car lately other than the fuel pump. Also, wires haven't changed since I put them on new 10 months ago. Car has 4 speed toploader.

1965 Coupe
351W - ~400 Engine hp
Edelbrock Performer RPM Top End Kit
Holley 670 Street Avenger
Bored .30 over
Hooker Headers
MSD Box/Distributor
4 Speed Toploader
3.55 Limited Slip Detroit TrueTrac
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post #5 of 28 (permalink) Old 07-21-2018, 10:20 AM
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I assume you haven't replaced the tires ?

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post #6 of 28 (permalink) Old 07-21-2018, 11:03 AM Thread Starter
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Quote:
Originally Posted by stephen_wilson View Post
I assume you haven't replaced the tires ?
nope

1965 Coupe
351W - ~400 Engine hp
Edelbrock Performer RPM Top End Kit
Holley 670 Street Avenger
Bored .30 over
Hooker Headers
MSD Box/Distributor
4 Speed Toploader
3.55 Limited Slip Detroit TrueTrac
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post #7 of 28 (permalink) Old 07-21-2018, 11:16 AM
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Originally Posted by nrowles View Post
Well.........I used to have a very detailed sig. Must have gotten deleted somehow? Maybe when the site had that issue a few weeks back.

It has a mildly worked 351W with 4 speed toploader. Holley Street Avenger 670. MSD Box.

Fuel filter only has a few hundred miles on it. I don't think lack of fuel to the carb is the issue.

MrFreeze.......Could the tdc and timing change? I really haven't done anything to this car lately other than the fuel pump. Also, wires haven't changed since I put them on new 10 months ago. Car has 4 speed toploader.
Sig showed in this post but not the 1st one ?
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post #8 of 28 (permalink) Old 07-21-2018, 11:20 AM
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Originally Posted by nrowles View Post
Fuel filter only has a few hundred miles on it. I don't think lack of fuel to the carb is the issue.
When you install a new fuel pump, it takes a BIG gulp of fuel from the tank. This is a prime opportunity for crud and gunk to dislodge from the tank and fuel lines. Thus, even if you had a new fuel filter installed, it could easily be clogged with that crud and gunk. A fuel filter is only a few bucks and easy to replace. So, I would recommend that as the first step.

Next check the carburetor. Crud and gunk can and does get past the fuel filter. If your carburetor has crud and gunk in it, it's not going to work as well as it should. Pop the top off the carburetor and take a look at the fuel bowl. Likely, you'll see crud and gunk. If that's the case, you should completely disassemble and clean the carburetor.

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post #9 of 28 (permalink) Old 07-21-2018, 02:42 PM
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If a 400hp 351W with 3.55s and a toploader won't light the tires from a dead stop - you have a problem.

Start from ground zero - verify timing. Check the float level in the carb (fuel pump pressure variations can cause this), new fuel filter, etc.

Something's up.
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post #10 of 28 (permalink) Old 07-21-2018, 03:42 PM
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Clutch slipping? Although it would be more apparent in higher gears.

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post #11 of 28 (permalink) Old 07-21-2018, 08:12 PM Thread Starter
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I have made note of all your suggestions and will work on the easiest ones first. It is supposed to rain almost every day for the next week here so not sure how much road testing I will be able to do.

Would you guys agree if I had a cylinder not firing it would be more noticeable?

1965 Coupe
351W - ~400 Engine hp
Edelbrock Performer RPM Top End Kit
Holley 670 Street Avenger
Bored .30 over
Hooker Headers
MSD Box/Distributor
4 Speed Toploader
3.55 Limited Slip Detroit TrueTrac
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post #12 of 28 (permalink) Old 07-21-2018, 10:10 PM
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Bart's FIRST RULE of diagnosing performance issues:

#1 - Perform cranking compression test followed by manifold vacuum gauge readings.

These two tests are necessary to establish the engine's general state of health.

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post #13 of 28 (permalink) Old 07-21-2018, 10:31 PM
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Originally Posted by nrowles View Post
I have made note of all your suggestions and will work on the easiest ones first. It is supposed to rain almost every day for the next week here so not sure how much road testing I will be able to do.

Would you guys agree if I had a cylinder not firing it would be more noticeable?


Seems like you'd hear/feel it


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post #14 of 28 (permalink) Old 07-21-2018, 10:43 PM
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I had a case of bad gas, and just for clarification no it's not the kind after eating Taco Bell at 3am, in my car. I put some gas in my car right in the transition of winter to spring/summer and it ran like crap until I went through that tank of fuel and then it ran fine again. With the crappy gas that we have today, if a car sits too long it isn't good a good thing, especially in a carbureted car. I'd start by checking both of your fuel filters, you should have one right by your gas tank and one between your fuel pump and carburetor. Check to see if there's a bunch of crap in there and if you don't have a fuel filter between your fuel pump and carburetor, now would be a good time to install one. I also only use premium fuel now. IMO the extra $.10-.20 per gallon seems like a small price to pay compared to having costly engine issues because of getting cheap gas.
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post #15 of 28 (permalink) Old 07-22-2018, 05:30 AM Thread Starter
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Originally Posted by MUSTANG65FBK View Post
I had a case of bad gas, and just for clarification no it's not the kind after eating Taco Bell at 3am, in my car. I put some gas in my car right in the transition of winter to spring/summer and it ran like crap until I went through that tank of fuel and then it ran fine again. With the crappy gas that we have today, if a car sits too long it isn't good a good thing, especially in a carbureted car. I'd start by checking both of your fuel filters, you should have one right by your gas tank and one between your fuel pump and carburetor. Check to see if there's a bunch of crap in there and if you don't have a fuel filter between your fuel pump and carburetor, now would be a good time to install one. I also only use premium fuel now. IMO the extra $.10-.20 per gallon seems like a small price to pay compared to having costly engine issues because of getting cheap gas.
The first thing I did after the fuel pump change was put fuel in. So it is fresh but maybe low quality. I will have to run this tank down further and try new fuel.

As for the fuel filters, I do not have one at the tank unless there is one inside the tank.

1965 Coupe
351W - ~400 Engine hp
Edelbrock Performer RPM Top End Kit
Holley 670 Street Avenger
Bored .30 over
Hooker Headers
MSD Box/Distributor
4 Speed Toploader
3.55 Limited Slip Detroit TrueTrac
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