My 71mustang w/ 1970 351c 4bbl recently started running badly and pinging. I noticed that fuel was squirting out of the carb when I would snap the throttle. After some discussion with a local mech. we figured that an intake valve on the driver's side was probable cracked or not closing. I did a compression test and the passenger bank all read about 150psi however the drivers side read differently:
cyl # 5 - 155
# 6 - 18
# 7 - 26
# 8 - 130

I couldn't visibly see anything wrong with the springs or the keepers. Do I have 2 bad valves? What is the possibility that it could be a head gasket? There wasn't any white smoke when it was running, but it was ticking/ knocking when you gave it hard gas or going up an incline.

Any help or thoughts would be greatly appreciated. Thanks.

Caddyman
aka Tom

 

If the compression is low on two adjacent cylinders which is what your case is, it would indicate a.) There is a "blown" or weak sealing surface at the head to block mounting area, which basically means a bad head gasket.
b. Another possibility is that the camshaft is broken in an area that operates valves for two adjacent cylinders.

 

Have you overheated it recently? You can warp a head and cause problems like that. Ask me how I know.

 

Yep check the head gaskets, while you have it apart have the heads redone. Put some new valves and springs in them, they arent that expensive and its cheap insurance. Do you really want 30+ year old valves in there?

Also that is a high compression engine, most likely pump gas wont work in it well. You will have detonation issues under load, even if you cant hear it. Its called inaudible detonation and it tears things up. If you havent been running a mix of race gas in it, maybe you should or rebuild it and lower the ratio with dished pistons.

Detonation can wipe out a head gasket, smash rod bearings flat, collapse a ring land, and hole a piston. Checking the bottom end is probably a good idea as well.

 

are you still using the 1971 multi groove valves and locks?

 

I'll have to check the head gasket. The motor was recently rebuilt b4 I aquired it so I believe everything is sound. I will have a look see when it is apart. I've only used high octane pump gas 91+ octane ratings. From what I've read this should be fine even in older stock high compression engines. I'm a bit rusty with tearing down an engine so I'm afraid to get too deep into it myself. I need to go slow and try to keep things simple. Thanks for all the help so far. I'll be looking for more updates. Thanks.

tom (caddyman)

 

Wait I was thinking you had a 70.. My bad. A 71 will run pretty good on 91 or even 89 as long as it isnt a BOSS or HO. They dropped the compression a bit in 71, more in 72.

91 is said to work in the older engines that had over 10:1, in factory condition that probably isnt going to work. You end up lowering the compression when you do valve jobs, because the seats get ground down and the valves move farther into the heads creating more space. Another way its lowered ona rebuild is the pistons. Some are shorter and will end up with a negative deck, or down the hole a bit. Sometimes as much as .100" that is a huge amount to an engine. headgasket thickness also plays a part, a thicker gasket will reduce compression, a thinner one will increase it Make the space larger and compressionwill drop.

So if its been rebuilt and any of those items is in effect the compression will be lower, sometimes by as much as a full point. Other things that affect when an engine pings is altitude, ambient air temperature, barometric pressure, humidity, chamber design(head and piston), oil in the cylinders, weight of the vehicle, gearing, engine temp, oil temp, head material (aluminum vs iron), and timing. Weather makes a huge impact on what an engine can stand. For some reason people in Seattle WA can run more compression on pump gas than people in South carolina or even Nebraska can and not have a problem with preignition/detonation.

So it depends where you live as to how much compression you can run.

As for tearing it down, Clevelands are straight forward and simple. All old Fords are really. You should be able to handle it no problem

 

Altitude also has an impact. I am in Denver so my 11:1 compression is fine here. It would be rough on the motor without race gas at Sea level.

 

first thing i mentioned, it has the largest and easiest to see impact weather wise. good call man..