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Discussion Starter #1
at 92 mph:(

I'm finally getting the chance to take the gray 66 out to the strip to make some passes. It seems to run strong through the 1/8th mile but flat out feels like it runs out of gas on the top end so I let off.
I only made a few passes because of the top end problem.
60 foot was 1.67, 1/8th was 7.30 @ 95mph.

I had one other run that was almost identical to this run but I put my foot back into the gas one time after I let out on the top end and was rewarded with an 11.83 at a nice 102mph:mad:

So, my plan is to install some rear jet extensions in my 750 Demon carb and go try again in a few weeks. The car was hooking very nicely but I had a timid launch trying not to spin the tires. If the jet extensions do their job and I don't launch it like a wussy, I expect it to pick up a lot even in warmer weather.

Anyone else cure a huge top end problem with some jet extensions?
 

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Jet extensions help on launch to keep the jets from being uncovered. I have always thought that jet extensions actually restrict flow to the jets a little bit but I have never isolated this 1 specific thing to test. I don't think this is what is hurting your top end but you need them anyway for the 60 foots you are getting.

I've also been having a problem falling over where I used to shift. I've lost maybe 2 or 300 rpms. The engine starts to go flat on the run out at the same rpm. It will wing up fine sitting still. It seems like if this was valve springs that it would be a problem whether the engine was sitting still or going down the track. The prevailing thought is still that I need new valve springs which I have but not installed yet.

I suspect that its in the carb and something regarding under load or not under load. I need a wide band really bad to tune my stuff but money has been a bit tight the past few months.
 

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fuel pump, or possibly sucking air instead of fuel because fuel is all behind the sump.

What fuel system are you running?

Is the fuel sump at the back?
 

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The 1/8 looks very promising for the first time out. I would be surprised if jet extensions helped on the back half of the track. 60 ft might get better though.
 

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Very nice 1/8 mile.

When a car lays over hard on the top end, its normally either fuel related or valve springs. It will feel like it hit a brick wall and just won't go anymore.
 

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I'm thinking fuel related. What does your fuel system consist of? Have you checked your float levels lately? Have you checked your fuel filters? Often, carbs have a screen where the fuel line screws in... check those as well. If electric, check the voltage at the pump... while it's running.

Good Luck!
 

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Discussion Starter #8
The carb is a Mighty Demon 750. My fuel pump is a mechanical Carter 170 gph. Fuel line is 3/8". I did have front and rear jet extensions in it when I ran one time last November. It ran 11.47 @ 117+. But it was running really rich and burning my eyes at idle. That's when I found out that the factory floats and standard jet extensions wouldn't work together. So I took them out.
That is the only thing I can think of that I changed over the winter. (I started tinkering with dirt bikes;) I had forgotten about the extensions when I went to the strip the other night. And then this happens??

A few years ago my car was doing the exact same thing on the top end with a lesser 110gph mechanical pump. Once I switched to the big Carter it cured it.

So...I just recieved 2 flattened jet extensions from UPS yesterday that will work with the factory floats. Not sure that is the whole problem but they can't hurt.
 

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How much fuel was in the tank when you ran 11.5 vs. 11.9?

My car had fuel delivery issues starting in the 12's when I was using a stock tank. If the tank was not > 1/2 full, the fuel would simply move to the back half of the tank uncovering the sump.

What size fuel line are you using?

Is it a stock tank with the sump in the center bottom?

Monitoring A/F makes problems like this obvious, even before you can feel or see them.
 

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Discussion Starter #10
The tank is not sumped but I was racing with 3/4 tank.

My fuel level in the bowls is about 1/3 the way up the clear sight glass.

Springs may be another area I can find gains because my AFR heads are old and I'm still using the factory .550" lift springs that came with them that everyone laughs at:)
It does rev to 6500 quickly and I'm sure it would like to go over that but I am limiting it to that because of the springs.
 

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The carb is a Mighty Demon 750. My fuel pump is a mechanical Carter 170 gph. Fuel line is 3/8". I did have front and rear jet extensions
Do NOT run jet extensions in the front.

Jet extensions are used the rear because inertia causes the fuel to move toward the back of the car... uncovering the jets.
In the front, it's not a problem, as the inertia of the car taking off brings the fuel in direct contact with the jets. Installing jet extensions is a step backwards.

If installing the new pump helped things in the past, it could be a clue that you have a fuel starvation issue. Since you're running a mechanical pump, you have possible issues.
First, is... How much rubber fuel line are you running on the suction side? Rubber line tends to collapse in on itself with pump suction instead of drawing fuel.
Next... The stock pickup in the tank is only 5/16" diameter. I removed the pickup assembly, drilled another hole, and stuck 1/2" aluminum tubing through it. I then used a hose clamp to attach it to the stock line and JB-Welded where it went through the drilled hole in order to seal it. I don't use that outlet any more, as I've sumped the tank, but it's still in the tank over 20 years later, and doesn't leak.

The 'sock' filter on the end of the the 5/16" line in the tank pick-up can also get clogged.

You can also use air pressure (or your lungs) to blow backwards through the fuel line to see if there is an obstruction. It should blow freely with no restriction. You might even blow out a partial clog or find a kink in the line. If you use your lungs, don't inhale. LOL

Install a fuel pressure gauge (even temporary) as close to the carb as possible. This will let you know how much fuel pressure you have AT THE CARB. Keep fixing things until you have adequate pressure.

Good Luck!
 

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Discussion Starter #12
Thanks for the great advice. Yeah, I had already realized jet extensions in the front bowl made no sense unless I was racing backward. I did change the stock in tank pickup to a 3/8" one. It also has no sock on it. Mike, I like the idea of what you did with your pickup. I'm using just a few inches of rubber line in the rear at the tank and maybe 8" in the front before the pump.
I've got the flattened rear extensions now that I'm gonna install this weekend. The bad thing is I cant test it on the street because it happens at over 100mph.
 

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Did you blow your line yet? Take it loose at the fuel pump and blow it back towards the tank. There should be no restriction once you purge the fuel... just bubbles in the tank. Check for kinks in the line, loose clamps that can suck air, etc. It can even be the pump. Sometimes the diaphragms can go bad. Make sure the needle valves are not restricted. Basically, work your way from one end of the fuel system to the other.

If it still doesn't work, you'll need to install a fuel gauge at the carb to make sure it's getting plenty of pressure on the top end.

Good Luck!
 

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It just occurs to me that I've been running lately with only 2 or 3 gallons of gas in my car. I wonder if this is what is causing my fall over too. Mine does it when I come up on my shift points as well as the run out though.
 

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It just occurs to me that I've been running lately with only 2 or 3 gallons of gas in my car. I wonder if this is what is causing my fall over too. Mine does it when I come up on my shift points as well as the run out though.
Very likely if your using a stock tank and pickup, fuel pressure gauge or wide band will tell you.
 

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If stuff would quit happening to me I could get a wide band. Another tree just came down on my shop tonight. This one didn't punch any holes in the roof though so maybe I lucked out.
 

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Discussion Starter #17
Quick update: After 4 months I finally got back up to the dragstrip. The only and I mean only change I did was to add jet extensions to the rear bowl. Same timing, same 20 psi air pressure in the drag radials, and once again had 3/4 of a tank of gas. I only made one run because the car got hot on the way to the strip and I didn't want to overheat in the staging lanes. Anyway the run looked like this:

R/T ... .065
60' ... 1.648
330 ... 4.707
1/8 ... 7.292
MPH ... 94.80
1000... 9.556
1/4 ... 11.483
MPH ... 116.83

It hooked nicely and ran strong the whole way. On the top end the front end felt squirrely and I had to let off the gas for a moment because I was going toward the center line. So I need to do a "drag race" front end alignment on it I'm guessing.

Anyway, it looks like the jet extensions did their job. To hear the timeslip guy tell me I need a 6 pt rollbar was nice. One day I'll get around to that.
 

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I was wondering how this ever came out. I don't see how the jet extensions helped on the top end like that. Below 11.50, 6 point cage yep. My track is kind of picky about that and will ban the car if you make more than 1 pass under until you get the cage.

I don't want to put a cage in my car although the old Shelby 4 point cage might be a practical safety thing. I would be perfectly happy if it would run 11.51 every time which it has not ever done so far. My best short times look a lot like yours(1.62 best 60 on slicks, low 7's at 90+ 8th) but I run out of legs or something and my car has always fallen over on the top end. My best quarter has been an 11.72 at 112. It looks to me like you are really starting to go on the back half of the track. I'm thinking from looking at your numbers that you should expect some better short times on slicks considering what you are running now.

I have my heads apart for new springs and decided maybe I'm running out of head so I've been grinding for the past couple months as I have the time. They should have been done by now but the universe or Murphy or somebody has been messing with my life lately.

I have wondered about fuel pressure in a carb'd engine. If there is fuel in the bowls do you need any fuel pressure at all to keep running?

These old mustangs have always had a tendency to want to try and fly above 110. A skinny front tire for the track will help your car's tendency to drift around. I run VW beetle front tires on skinny drag lites. I assume that you already have all the basic bars to keep the front suspension from flexing. Good luck finding any shop that can do you a drag strip alignment. I found one place around me that could do it. The owner was a dirt track racer. He went out of business though or sold his shop.

Something else I did was lower my whole car to try and keep wind from getting under it so much and put some front to rear angle on it. Wind will get under your hood and try to make the car fly above 110 or it did mine. I lowered the front some 3 inches and the back 2 and also run a taller rear tire as you can see in my sig pic. It gives it some down force I guess. It still lifts ok on launch because I put drag shocks on it and I don't run those donkey rough front coil springs.

I have 6 degrees caster too which helps and my steering box is good and tight which also helps prevent wander on the track.
 

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My best quarter has been an 11.72 at 112. It looks to me like you are really starting to go on the back half of the track.
I think that you are getting it done with the HP that you have (as evident by your mph.)

1320/MPH = optimum ET

1320/112 = 11.78

If you want to go faster, you need more HP.
 
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