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Discussion Starter #1
Hi All,

I have a 1968 Mustang Convertible, 6 cylinder 200ci with automatic transmission. I did the 5 lug conversion with front disc brakes and the rear-end is from a 1967 mustang with 9" drum brakes.

When I drive the car it's sluggish and the gas mileage is bad, maybe 10mpg. When I hit on the accelerator it takes a while before it gets up to cruising speed.

I checked for vacum leaks and compression loss and everything appears normal. I have an Autolite 1100 carb and it's tuned up correctly. I also have the duraspark electronic ignition.

When I did the 5 lug conversion, I didn't swap out the drive-shaft so I'm using my original driveshaft that was coupled with the original smaller rear-end. Would this affect performance? Should I use a V8 drive-shaft to match up with the beefier rear-end?

Or is an engine rebuild my only option?

I'm open to any and all ideas.

Thank you
 

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Hello tikiron,
Congrats on some great work and a nice looking car.

I would say your issue has nothing to do with the driveshaft.

Keep in mind that it is a 6cyl and may not be as peppy as a small block
however
It sounds as though your car may need a complete tune-up.
PLugs, wires, rotor, cap condensor, perhaps air filter

You should be getting MUCh higher gas mileage.
Once you know the components for a tuneup are good- -then
you need to check timing, point dwell and of course a carb adjustment.
Let us know when the car was last tuned

You will be surprised how peppy your motor can be in stock condition

Let us know about a tuneup and we will go from there - - pay special attention
to the choke and carb adjustments

Print Dad
 

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You don't say if the problem started when you did the swap, but if you have brakes dragging or a drive shaft binding I would imagine you would be able to tell by trying to push the car while it was in neutral. I would also suspect that when you let off the accelerator that you would notice a change in how rapidly it decelerates. If brakes were the culprit I would also suspect you would smell them after a drive.

My 8 inch rear end in my 67 had a gear ratio of 2.8 :1. Depending on what ratio you moved from and to, might be why you're experiencing a difference in acceleration.




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Discussion Starter #4 (Edited)
Hi Print Dad \ fhu667,

Thanks for the advice. I'll do a tuneup this weekend to see if that alleviates the problem, if not then I'll take a closer look at my gear ratio of the rearend.

The car has been in the garage for a few months after I did the 5 lug conversion, so I don't recall if it started around this time. But I do remember at one point it was abit more fun to drive.

The original read-end was the 7.25 with I believe the 2:80 ratio, but now that I have the larger 9" rear-end I don't know or recall what ratio I have. I guess I'll have to jack the car up and mark the drive shaft to see how many turns it makes for a complete wheel turn. Since this car is my weekend car, it will see more city driving than highway driving. So I'm thinking a gear ratio of 3/1 would be "ok", maybe 3.25 tops. Unless I hear other suggestions \ recommendations.

I'll keep you posted and let you know how it turns out.

Thanks!
 

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It could be a number of things as said above.
The only way to solve the issue is a process of elimination.
Was the diff you purchased from a running car? It could need new bearings if it has been sitting for a while.
Start with the tune up.
Jack up rear of car and see what diff ratio you have, while you are at it se if the brakes are dragging and check the handbrake too. It might be an idea to run the car with the rear off the ground to check for noises, Please do this safely, chock the front wheels.
Jack up front end and test front brakes, check for dragging.
You will find it and hopefully it is a simple fix.
 

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Hello tikiron,
You may have provided another clue here.
If the gas has been sitting in the car a few months it
could be going flat (stale).
Todays gas has a short shelf life.
At this point I think I would add about a 1/2 tank of high test
and drive the car - -see if the situation gets better.

Print Dad
 

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Tiki, another thing to check is whether your 1100 is a "Load-O-Matic" carb.

If so, it'll look like this:


Those were designed to work with a very specific distributor that had no centrifugal advance. Instead, the carb had a vacuum circuit that would advance or retard the ignition based on load, in addition to the vacuum advance. If your carburetor has this feature, and you are not running the original dizzy, then you have a vacuum leak. It would definitely mess with your mileage and performance. You might be able to modify it by plugging certain passages and preventing the leak, but that would also ensure that it could never be used as a stock carb with the original dizzy.

This is also something you might not want to hear, but your 9" rear end "upgrade" may also be hurting you significantly. The 7" rear end, while weak and lousy for performance applications, did not have nearly as much friction as the 9" you put in. On a V8, the higher taxable horsepower would not be very noticeable, but on your little I6, you may be losing as much as 10% of your engine's power to the rear wheels, because of that change alone.
 
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