71 mustang, increase vacuum? - Tilted Forum Project Discussion Community

ltdaywalker · 07-23-2003, 04:32 PM

I have a 1971 mustang fast back with a 302 and a c4 tranny that just got rebuilt and a shift kit installed about a month ago. The guy who rebuilt the trans test drove it after I put it back in and we both noticed a hesitation between 2nd and 3rd gear. He ran a vacuum test and told me that the vacuum was low at idle so he adjusted the modulator on the trans and sent me off. It shift much better but still has a slight hesitation again between 2nd and 3rd, it also does not like respond like I know it should with i slam on the gas from a light. I think that by increasing the vacuum it might solve these. If you have any idea how to increase my vacuum (there are no leaks) or solve this another way it would be a great help!
thanks!

WarWagon · 07-23-2003, 07:07 PM

I honestly don't think that the vacuum is the problem. What was it pulling at idle? Do you have power brakes, and if so, is there problems with braking?

There are a few options, though I think it may be related more to ignition timing/carb adjustment. If you need to get the vacuum up, you can raise your idle, use a vacuum canister, or an electric vacuum pump.

Adjusting the modulator on the trans shouldn't have anything to do with your overall vacuum unless there is a leak. If the c4 modulator is like that on a th350, adjusting the modulator would only effect your part throttle shift points.

How do you know for sure that there are no leaks? What you can do is take a can of lighter fluid, carb cleaner, or something of the sort and spray around the engine, paying attention to the base of the intake, carb, etc. If your idle jumps suddenly, you found a leak.

The when revving to the point where you're shifting from 2nd to 3rd, you should be pulling ample vacuum to shift properly. When flooring it off the line, vacuum should not cause a bog or poor performance unless its affecting your advance, or if your secondaries are coming in too early.

In order to solve that off the line performance problem, the first step is to put a timing light on it, check your timing with the advance plugged, then check it again with the advance connected.

mighty mouse · 07-24-2003, 02:05 PM

First question: How big is your cam?
Second question: Dual plane or single plane intake?
Third question: What heads are you running?
Fourth question: What is your compression ratio?
Fifth question: What ratio rearend gears are you running and how tall are your tires?

07-23-2003, 04:32 PM	#1 (permalink)
ltdaywalker Upright Location: Chicago, IL	71 mustang, increase vacuum? I have a 1971 mustang fast back with a 302 and a c4 tranny that just got rebuilt and a shift kit installed about a month ago. The guy who rebuilt the trans test drove it after I put it back in and we both noticed a hesitation between 2nd and 3rd gear. He ran a vacuum test and told me that the vacuum was low at idle so he adjusted the modulator on the trans and sent me off. It shift much better but still has a slight hesitation again between 2nd and 3rd, it also does not like respond like I know it should with i slam on the gas from a light. I think that by increasing the vacuum it might solve these. If you have any idea how to increase my vacuum (there are no leaks) or solve this another way it would be a great help! thanks!

07-23-2003, 07:07 PM	#2 (permalink)
WarWagon Loser	I honestly don't think that the vacuum is the problem. What was it pulling at idle? Do you have power brakes, and if so, is there problems with braking? There are a few options, though I think it may be related more to ignition timing/carb adjustment. If you need to get the vacuum up, you can raise your idle, use a vacuum canister, or an electric vacuum pump. Adjusting the modulator on the trans shouldn't have anything to do with your overall vacuum unless there is a leak. If the c4 modulator is like that on a th350, adjusting the modulator would only effect your part throttle shift points. How do you know for sure that there are no leaks? What you can do is take a can of lighter fluid, carb cleaner, or something of the sort and spray around the engine, paying attention to the base of the intake, carb, etc. If your idle jumps suddenly, you found a leak. The when revving to the point where you're shifting from 2nd to 3rd, you should be pulling ample vacuum to shift properly. When flooring it off the line, vacuum should not cause a bog or poor performance unless its affecting your advance, or if your secondaries are coming in too early. In order to solve that off the line performance problem, the first step is to put a timing light on it, check your timing with the advance plugged, then check it again with the advance connected.

07-24-2003, 02:05 PM	#3 (permalink)
mighty mouse Crazy	First question: How big is your cam? Second question: Dual plane or single plane intake? Third question: What heads are you running? Fourth question: What is your compression ratio? Fifth question: What ratio rearend gears are you running and how tall are your tires?