I havea 68 Mustang w/ a 351. The enging pings under high load. The engine itout of a 70 cougar and the vacuum advance hose was disconnected. Which i didnt think anything of it, which i should have, untilI started getting a ping at high load.With it disconnected the car idles and runs great. When i hook it to vacuum it runs rough and wants to stall. if anyone has any suggestions it would be appreciated.

Thanks

 

Timing could be off. Also if your compression is over 10:1 it'll get a ping on regular gas.

 

+1 Adrenolin
Many of the '69 and '70 351s will ping if the timing is too high or you are running lower octane gas. With the high compression that they ran in those years, you just have problems with unleaded. Try 93 octane gas with lead substitute. If that does not work, check your timing. I ran 93 with additive and about 9.5* inital and 38 total timing in my Cleveland with no problems, but if I did not add the sub or if I ran 89 it would rattle and ping.

 

run the highest octane you can get.

here in the bay area its 87, 89, or 91. i go with 91 in all our cars.

 

Yeah but the question is why does it run rough and stall with vacuum line conected.
Are you talking about at idle?
Possibly a vacuum leak on the vacuum advance unit. Is it connected to ported vacuum?
Ported vacuum should provide, virtually, no vacuum at idle, so really shouldn't change anything.
Sounds like the initial timing is too advanced and/or the distributor is advancing way to much or sumtin.

 

if it's running fine idling without the vaccuum hose (to ignition advance canister) and horrible with it's connected, then you have connected the vacuum hose plugged to the wrong port on the carb (you should hookup to ported vacuum, not manifold vacuum on carb).
that would at least be my first guess. what carb do you have? model or pictures needed.
If that's not it, then your car is running very rich (vacuum off gives more airt to mixture causing it to run normal mixture). once you connect it that extra air is gone and you're running rich. But that shouldn't cause such problems. So check on the carb port first.
At the same time, as all the above say: when it's pinging then please please with loads of sugar on top retard the timing until you found your problem.
Pinging is lethal to an engine

 

Okay so i dont want to hook it to main Vacuum. That explains why it would stall out when hooked up. I have a Holley 650 4v carb. I believe it might be a 4150. It has avacuum secondary. So i should look for the port on the carb with minimal vacuum at idle it sounds like. I will try that.

 

that's correct. near or nearly no vacuum at idle. block all unused vacuum ports on carb.

if I picked the right carb here's a schematic from holley manual




[IMG]local://upfiles/52145/55F5084A423C4AE6B067692E8FA90D5F.jpg[/IMG]

 

Alrighty I have It working. I tried 93 octane and that didnt work so i turned the distributor about 1/8 of an inch CCW. That took care of the ping. However it seemsto be running a little rougher at idle and lower load situations then it was before. I checked the Dwell and it read around 22 degrees. Now by the book it should read 26-32. Is this the time to pony up for a new distibutor. I havent check to see it the shaft has any play yet, I plan to do that when i get out of work. If I do replace it i definently think the way to go would be electronic. So Anybody with some good suggestions?

Thanks again

 

Before you pony up for a dist., pony up for a timing light and mustang manua l