Why is my top speed 70 mph?

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ansonservice

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HI everyone. I am new here, and I am eager to get into this new project with my son. We bought a 1975 Dart Sport with a newer 318 and newer automatic tranny with a shift kit. The problem is that it is SO SLOW! In 3rd gear it is topping out at 70 mph. 2nd gear tops out at 40-45 mph. It is not even fast off the line. The engine runs great and purrs like a lion, but dang if it is not going to be passed by a moped. I took it in and my mechanic (whom I have lost confidence in) did not see anything wrong. Any ideas where I should start?
 
First thing to check and it costs nothing. With the car off of course, get someone to hold the gas pedal on the floor. Open the hood and confirm the carburetor is open all the way. I bet it is not and the linkage needs adjusting.
 
that's not a lot of info to go on...........it could be alot of different things performance-wise, timing, carburetion, compression could be bad on the motor, timing chain could be worn...................u say the car has a newer motor, is this what the guy told U? did you have this verified by a mechanic?

i would probably start with a compression test............

explain "newer"
higgs
 
73AbodEE said:
Maybe it has 5.30 gears lol
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that's the first thing we need to know- rearend gears. My duster has 4.10 gears. Now tops out at about 70 lol (stock beatup 1979 pickup 360). With the 340 I had before it would only do about 120, but it hit it in a quarter mile.
 
RustyRatRod said:
First thing to check and it costs nothing. With the car off of course, get someone to hold the gas pedal on the floor. Open the hood and confirm the carburetor is open all the way. I bet it is not and the linkage needs adjusting.
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This ^^^ is an excellent starting point.
 
Could be as simple as a nearly clogged fuel filter....
 
Too bad it doesn't have a cat, you could start there. If it has dual exhaust you can put a little back pressure to each pipe, I just use my hand for a second. Both sides should have enough to blow your hand off. Any restriction in the exhaust could cause your problem.
 
RustyRatRod said:
First thing to check and it costs nothing. With the car off of course, get someone to hold the gas pedal on the floor. Open the hood and confirm the carburetor is open all the way. I bet it is not and the linkage needs adjusting.
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x16.
 
You all are great! Good questions. So, I was told the engine was less than a year old. It looks like it too. Very clean. The problem is that the wiring is all original and is terrible shape, so I will be replacing wires as we go. Right now I am having intermittent problems getting spark due to a wiring issue somewhere. Anyway, the engine revs well, and the carb is open. It sounds like we are doing 120 mph, but yet, we struggle to hit 70. I am thinking that we do have a gear issue going on. Maybe they are 4:10 gears in it? I don't know. Any way to know without taking the cover off the rear end? Even then, I am not sure what I am looking for or what I am needing to replace it with. Basically, I just want it to not be revving high at 40 mph in 2nd gear before it goes into 3rd, and I want us to be able to hit 80 with plenty of pedal left. This is a hodge podge of a car from several people before us who did some things well, and others not so well. I wish I knew the history of the car and could talk to whomever did the work over the years, but I cannot. So I will be learning as I go on this one. BTW - - my mechanic I took it to to give it a one over said it had a shift kit on it because I was concerned about the hard shifting.
 
It could be a trans/converter and maybe a timing issue. It looks like you have a long list of possibilities but it will pay off when you find the problem and you will know your car inside and out. If it has a rear end cover it's most likely an 8 1/4 if it was originally a v8 car and a 7 1/4 id not. I doubt you would have high gears if you have either.
 
My stock 318 with 2.76 gears would smoke the tires all through first gear! Check the throttle travel as mentioned. Also see if you are getting enough fuel, it may just be getting enough to run. Timing is also a good thing to check, timing chain jumped a tooth?
 
Did you actually DO the test I outlined?
 
RustyRatRod said:
Did you actually DO the test I outlined?
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R.R,R,. gonna go out on a limb, here. The experienced people , have a built in flow chart.( You , myself, KrazyKuda, 66 Sedan/Keith..., know this, by experience). This internet guy, may not have that guile , or that frame thought.
 
Check the trans fluid. Maybe its burnt to a crisp. No reason a high revving engine should be running 70 in high gear. Even my big block with 4.56 gears would run a hundred without exploding. I had a pickup with a 'vette engine in it that only ran eighty. But it had the wrong speedo gear in it.
 
I doubt gearing is a problem or it wouldn't be racing in the moped class. I bet it's a 318 in a /6 car witha stock diff.
After following RRR advice, and checking the kick down, I would check the fuel delivery and put a fresh filter on.
If your mechanic didn't see anything wrong then he must be a Ford mechanic.
I would start by doing a thorough inventory of what you have- engine #, tranny, rear end and gearing, carb, distributor etc. get to know what you have so that you can build on your mopar education.

Lots of help on here and if you don't know- just post a picture and someone will tell you what it is
 
H20loo said:
I doubt gearing is a problem or it wouldn't be racing in the moped class. I bet it's a 318 in a /6 car witha stock diff.
After following RRR advice, and checking the kick down, I would check the fuel delivery and put a fresh filter on.
If your mechanic didn't see anything wrong then he must be a Ford mechanic.
I would start by doing a thorough inventory of what you have- engine #, tranny, rear end and gearing, carb, distributor etc. get to know what you have so that you can build on your mopar education.
Click to expand...
think I`d pull the carb loose and look under it, and in the intake just for grins.:wack:
 
Is the kickdown linkage properly hooked up from the carb to the trans? If it's not then the trans could be slipping and burning up.
 
I only read one post that said it dosen't have a Catalytic converter.It was not posted from ansonservice.That will be something we need to know,,,What type of exhaust Original? Aftermarket Duals? A cloged exhaust MAY cause this.Next would be fuel supply from the tank forward.We can make many suggestions.You need to injest all and get a starting point and elminate from there. Hard to give a solution by computer.Take your time,,,,, you will find it.When you do PLEASE tell us as it may help others in the future.
 
Most common cause is a slant 6 throttle cable on a V8 swap. In this case you'll get a lot more throttle travel by hand under the hood than you can get from the pedal in the cabin.
 
Being slow does not necessarily mean not quick. Does it have enough "power"
to spin tires? You said it revs..? Do you have a tachometer in car?
 
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