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If you have sheet metal screws holding this on, I would replace them with nuts and bolts. Sanding the paint under the tab or running a separate wire from a known good ground would work.

Rock Auto has Motorcraft for $100 and Standard for $20. I would buy Standard. Suggest you make sure the started relay is grounded securely to clean metal. Seen this cause similar problems before.

40+ years ago I put a C4 from a 71 full size into a 69 Mustang with a 5.0. Only problem was the bell housing hit the tunnel. My recollection was the converter was a larger diameter. Took a few hits from a 3# sludge hammer to add more clearance. Tire would chirp when manually shifting up through the gears. Not sure if this was just due to the difference in the converter diameter or a good rebuild.

Had a similar problem once- If the play is within the control valve, pull the cap off the left end and readjust the lock nut. Find the instruction online how to adjust the lock nut which will influence the preload within the valve.

I found a few companies which make new FMX converters for racing applications but could not find any who make new ones with the OEM/stock stall speed. You may have to settle for a rebuilt. These are available through Rock Auto, Advance Auto etc. Ask JW Performance if they know who makes new FMX to OEM stall speed spec.

Here is a couple of links. I will look through the OEM service manual later to see if it is stated.Looks like 1500-1800 RPM for stock. Fords are known to have problems with teeth on the flywheel/flex plate. I suggest you inspect carefully for broken or rounded off teeth on the side the starter engages from. ttps://www.allfordmustangs.com/threads/1969-mustang-torque-converter-stall-speed.156394/ https://www.carid.com/1969-ford-mustang-performance-transmission-parts/jw-performance-mileage-torque-converter-2419441604.html?parentsubmodel[]=TRANSMISSION|Automatic https://www.vintage-mustang.com/threads/torque-converter-stall-speed.532293/ https://www.tciauto.com/stall-speed-chart?SID=n5krg8j2k92moj0le9f2r1b5d0

If anyone has changed the flywheel (flex plate) or damper make sure they have used the proper one. Ford changed the way they balanced small block Fords. See link below. The 302 engine is what changed and some of the 302 parts will fit the 351. If someone has changed parts make sure they only applied 351 parts or if 302 parts were used, they use the parts from the proper year (parts designed for the 28-ounce offset balance ). The picture of the flex plate you do NOT want is shown below. The 50 ounce version shown has a large cut out opposite the weight. Since it has bee 2 years since you had the work done, you may want to run a compression check on the engine and verify something else has not gone wrong to make sure the current problem is not related to the engine. Vibration problems are not easy to solve. Hope you succeed. https://www.motortrend.com/how-to/1601-avoid-this-mistake-creating-a-balance-issue-with-the-wrong-flexplate-and-dampener/

Once bought a new flex plate from Ford which came through with the weights missing. Had the same problem you described- vibration while in neutral when engine speed was increased. Depending on the application sometimes the weights are on their to counterbalance the crankshaft, not balance the flex plate. Could be the flex plate is your problem. When I brought it back to Ford they said they sell it with and without the weights. The weights will be obvious if they are there as you can see below. I noticed on the Rock Auto website they also sell the flex plates without the holes drilled for the torque converter. If a shop does not drill the holes properly, the torque converter may not be properly centered with the flex plate and cause a vibration. If it vibrates in neutral with the transmission mounter and did not do it when a bell housing was attached, it is directly related to the transmission. You may want to verify they did not put on a rigid properly balanced flywheel when they ran it with the bell housing. I am not sure you can unbolt the torque converter on a Ford and spin the flex plate with out it. On a GM you can because they use bolts. Ford uses studs on the torque converter which protrude through the flex plate. If you cannot push the torque converter back far enough the studs may not disengage the flex plate so it can spin free. There are many companies who sell rebuilt converters for stock applications. I suggest you do the flex plate at the same time in case it is the issue.

In 1971 Ford went to the an integral power steering box similar to the Borgeson in the Mustang. Has anyone ever tried to adapt this system to a 69? I see differences in size such as the length of the shaft going to the pitman arm but I not sure if mating parts such as the pitman arm compensate for it.

Sometimes the connector goes causing the switch to overheat and fail. Replacing the switch when this happens is a temporary solution unless you replace the mating connector. If you can get it to work, leave it on for a while then touch the wires coming out of the connector with your hand. If you feel heat, replace the connector as well. Unplug the connector and look for signs of burning on the electrical sockets or plastic inside the connector. This is a sign the connector is bad. If the contact is bad enough, you may not be ale to get the lights to work. As mentioned above many people buy a relay kit and use the switch to only pull in the relay. The advantage of this is you do not run the high current the headlights requires through the switch. This current is what causes the connector and switch to fail.

Having been hit by a Ford Flex fan blade after the blade cracked I would caution everyone from using a flex fan by any manufacture. .

Is there an advantage on setting rollers like this vs while the engine is running? How about non-roller? Reason I ask is it is easy to set it wrong using the above method (I have done it). Some cars such as those with rocker shafts leave you no choice but set to a gap, not turns of the nut. . If you have comp can lifters I suggest you call them, explain what you have and ask for the correct preload to use. My son has them in his 390. We set to a preload equivalent (measured the gap) of 3/4 turn which was on one set of instructions from their website but he had ticking. We then found another set of instructions of a later date for the same lifter series which said to set for 1 turn. This eliminated the ticking. These were not roller lifters. They came with the Comp Cam camshaft kit. Not enough preload causes ticking but should run decent. To much preload causes valves not to close which results in poor idle etc. When I initially started the 351w after rebuild it idled poor. This was because a valve was not fully closing even when set to 1/2 turn (not Comp Cam). I knew this was the problem because I measured the compression. I backed the rocker nut off more and the compression went up. I am not sure if this was because of a lifter problem or I put moly grease on the face of the lifter and may have accidentally put some in the bore which ended up inside the lifter hydraulic system.

If you have conventional hydraulic lifters and adjustable pedestals with self locking nuts- heat up the engine, remove the valve cover, with the engine running back the nut off until it just starts clicking, then tighten 1/2 a turn. Some folks say tighten 3/4 a turn or more but I typically set it at 1/2. If you have Comp Cam brand lifters, refer to their documentation for how many turns to tighten- this may be between 3/4 and 1.5 turns.. You may want to make a baffle the length of the head from aluminum flashing to keep the oil inside the head if it splashes over the headers and fenders. If you have non adjustable rockers like they use on later model engines, let me know and I will explain how to shim them.

Spray WD40 or carb cleaner in areas you think you have a leak. If engine changes speed, you have a leak. You will have a problem with your power brakes at 10" vacuum. You may have to install an electric vacuum pump kit as we did to provide the assist for the brakes. We have about 12" due to cam design. If your carburetor has a vacuum operated power valve, you need to find out what vacuum it opens up at otherwise you may flood your engine. The rating of the valve must be for vacuum lower than the vacuum your engine runs at. If you valve is rated 10.5" vacuum then it will be open and pouring in gas when it should not be. Holley carbs typically have power valves. We had to change ours. You probably want one rated for 3-4" vacuum. They are less than $10. Not sure if the low vacuum will affect the transmission shift quality or how to fix it. https://www.summitracing.com/parts/hly-125-35?rrec=true

In past I have checked for leaks of water into the combustion chamber by removing rockers, bringing cylinder to top dead center and screw an adapter into the spark plug hole to pressurize with 100 PSI air. This is how we used to change valve stem oil seals without removing the heads. With your radiator filled and cap off, look for air bubbles. Who built the engine? Were the surfaces inspected closely for signs of gasket material remaining on them? Are the block, heads and intake the original ones? If you end up stripping it down and do not find a problem you may want to have the head checked for cracks before you reassemble it. Make sure the intake gasket matches up with both the head and intake ports. Maybe a good idea to put a thin coating of RTV made for coolant system around the water ports going through the gasket front and rear. Same with the timing cover gasket water ports. Check the timing cover surfaces closely. They corrode around the coolant passages. I had to replace mine. You may want to start there before you pull the intake. Not sure how to detect water going directly into the oil such as from head gasket or intake unless you take it apart and look for a damaged gasket. Only way I can think which might work is mix a fluorescent dye used for leak detection with the coolant before you take it apart and run it. Drain the engine good before you disassemble. When you open it up look for traces of the dye coming past the gaskets. In the dark a UV light will cause the fluorescent dye to glow. Not sure where you can buy the dye. We use this at work to look for water leakage on products we build. A similar dye is used in AC systems to look for leakage.

Check the connector on the headlight switch. They are known to go bad. Look for signs of heat damage to the plastic or damage to the metal sockets in the connectors. Verify the metal socket in the connector engages the spade on the switch when you connect it. The fuses for the courtesy light and dash lights are in the fuse box, not the headlight switch.

I ended up rebuilding mine twice. It would not idle right after first rebuild. Second time I used pieces of piano wire to clean all of the small ports which solved the problems. When possible I used a brazing torch tip cleaner which is designed to clean the brazing tips for oxygen/acetylene torches. The kit has many different size wires which act as files. Using the carb cleaner itself was not enough for a 50 year old carb. Just because you have flow when you spray the carb cleaner through a port does not mean it is open enough to run properly. If you have an old carb, before you put it back together you may want to do the same.

Check the ground connection between the bracket on top of the solenoid and the fender. This connects the coil which energizes the solenoid to the ground.

If you have to return the system you borrowed, CJ Pony has kits for $350+ Most other places want you to send your power booster in to have it rebuilt then they send back to you. https://www.cjponyparts.com/mustang-power-brake-boosters-power-brake-conversion-kits/c/60030140/

Did you verify the hose is not plugged? Here is a new booster for $85 in USA. Not sure if they will ship to Italy. Comes with rod and check valve. They do no have the dual diaphragm unless you buy complete kit, only 9" single model. On this website the only one with the dual diaphragm is the manual transmision version, says single 9" is for automatics. https://www.cjponyparts.com/cj-classics-brake-booster-bendix-style-9-black-1967-1969/p/BOOSTER6/ If you are doing a conversion from manual to power brakes, the brake pedal is supposed to be changed as well. Possibly because the linkage is different. See the above website.

Bearing looks to be good condition. Let us know how you make out when you run the test with and without vacuum.

Vacuum you have is good. Increase the rod clearance to .125 and see if it helps. Below is a link to a good overview how a power booster works. It also says a way to determine if your power booster is bad on slide #39. Failing this test could also be caused by not enough rod clearance so if you fail, increase clearance.

All master cylinders which should be used with power boosters made for a 1969 mustang and a Ford Granada have a 15/16" bore which is the hole the piston goes in. If yours measures 1", it is not designed for use with a power brake booster. There could be other differences internally to the cylinder or possibly externally which is why I suggested you measure the depth of the hole in the piston.and compare them. If you put a power booster from a Granada in a Mustang and did not confirm the length of the rod with the eye in it which is attached to the brake pedal is the same as the Mustang one, it could also be your problem. Power brake boosters from a 69 Mustang have dual diaphragms. A power booster from a 1975 Ford Granada has a single diaphragm. A single diaphragm can not develop as much power as a dual diaphragm but this does not sound like you problem. Concentrate on checking the clearance noted below. If there is not enough clearance when your foot is off the brake petal, between the rod you made and the bottom of the hole in the piston which it contacts, the brake system will not work properly. This is why I suggested trying a shorter length rod. I read several articles on line and it says the clearance between the end of the rod and the bottom of the hole should be between 1/16 and 1/4 of an inch. It also suggested using clay to determine the clearance. Assemble then remove and measure the thickness of the clay. I did this before and it fixed the no power assist problem. If you pull the vacuum hose off including the check valve which is pressed into the rubber grommet on it, it will release all the vacuum. The pedal should travel much less and come to a firm stop than when you have full vacuum. The same will happen if you stop the engine and repeatably step on the break pedal because this will drain all the vacuum. If you do not notice a difference, either your power booster is bad, you have the wrong master cylinder or you have the wrong length rod. You need a minimum of 17" of vacuum to have full power as stated in the link below. Please read it since it also explains why a MC for power brakes has a smaller bore than one for manual brakes and states a single diaphragm has less power than a dual.. The rod you used does not look like a hardened steel bolt. If it was not, please look for a grade 8 or higher steel bolt which typically will have 8 dash lines on the hex head. If you use a soft bolt, it may break just like your original one and cause you to loose all your brakes. I would buy a hardened hex head bolt and grind the head into the shape of the head on the original rod. To small a contact area with the piston in the cylinder may cause excessive wear or damage. Looking at the 2'nd picture from the top I see damage already in the bottom of the hole. https://www.speedwaymotors.com/the-toolbox/braking-components-selection-and-design/28678

Do not think hoses will help. When I previously experienced short and hard travel while driving it was due to either a failing power booster or an adjustment problem with the push rod you replaced. When you pump the brakes without the engine running you will get short hard travel as the vacuum is used up. If this happens with the engine running, you may have installed a master cylinder designed for manual brakes on a power brake booster so it does not interface to the push rod the same. If you have both master cylinders measure the distance from the power booster mating surface to the bottom on the piston in the master cylinder where the push rod hits. If one is longer than the other it could be part of your problem. If this is the case, try making a push rob about 1/8" shorter and see if it helps. If not, try another 1/8" shorter. Based upon what I wrote earlier, the 1" diameter piston is for manual brakes, not power so using a 1" with a power booster maybe your problem. Do you here air leaking inside the car when the brakes are applied? This is a sign of a bad power booster.

Not sure if you are past this problem but have some thoughts. Is the bore you are looking for the bore of the master or or the disc caliper cylinder? If the master, you can measure the hole with a tape measure to find out. If you click on the "info" tab at the RockAuto site next to the "Cardone" brand master cylinders called for under a 1969 Mustang, it tells you the bore of the cylinder. The bore of a manual brake cylinder is 1" while a power brake cylinder is .937" (15/16"). Looking up the same for a 1975 Granada I see both manual and power master cylinders use 15/16" bore and the cylinders are different.. They also state the size of the threads on the ports (brake lines) going into the cylinder. The Granada threads are smaller than the Mustang. This may help you figure out which one you have. Pump the brake pedal with the engine off to bleed off all the vacuum. Then apply pressure lightly to the pedal and see if it quickly comes to a hard stop or slowly sinks. The pedal should come to a firm stop. If it slowly sinks, you may have a leak between the drum and disc sections inside the cylinder which will cause loss of brake power but not an external fluid leak. The reason I say under light force is because sometimes if you apply a lot of force, it causes the bad seal to work properly. If the pedal does not sink but is spongy when you run the above test, you likely have air in the system. You will send more pressure to the wheels using the smaller 15/16" master cylinder. Not sure if it is enough to make a difference. How much vacuum is there going to the vacuum booster? If the vacuum level is low, it may not produce enough power to lock the brakes up. https://www.speedwaymotors.com/the-toolbox/braking-components-selection-and-design/28678