Here is what I have:

Top quality 351 rebuild, Edelbrock carb and manifold, Edelbrock heads, mild cam, Mallory vacuum distributor.
3500 miles, will not tune up satisfactorly.

Symptoms:

Decent idle, white plugs, low speed stumble, pinging on hard acceleration with vacuum advance connected to carb, no pinging with vacuum on manifold, lots of high speed power.

Replaced rods on new carb to go two steps richer to get out of lean condition. No improvement

Findings from shop:

Vacuum leak on #6 cylinder. One bank rich on hydrocarbons.
22* mechanical advance, 6* static advance, 12* vacuum advance.

Obviously, replace leaking gasket. So:

Suppose we go to 10* base timing, leave the vacuum disconnected, get 32 all in timing.

Can the two step richer rods work for the mods described? Should we re-weight the distributor to reduce mechanical advance and hook up vacuum advance?

It is just a driver. To and from work with an occasional punch for adrenalyn

 

Buddy I am having almost the same problems you are , I am currently experimenting with a few timing curves if I am successful I will let you know .

1970 351W block / E7 Heads Decked .010 ( 10 / 1 comp ) Double spring
Carter 625 with RPM Dual plane intake
1.61 Stamped rockers with Comp VR lifters
Pertronix IG 3 billet Dist W flame thrower coil
Cam Old comp .598 lift 110 Lobe separation
Vac gauge reads low at idle unless timing is way up or 1000 rpm idle

1st - 10* 24* M with Vac Adv. - Fail Felt like it was fighting itself and ping
2nd - 8* 24*M No advance - Fail felt like it was lack of power ping
3rd - 12* 24* Vac advance - Fail again fighting for power had some yet ran out fast pinging

4th - 12* 20* M no Vac. - don't know lost spring have to get new one

 

You have a little more cam and compression but the same general engine and malady. So let's stay in touch.

Have you disconnected brake booster and other sources of vacuum leaks? Tightened manifold?

Have you richened up the box 625 Carter with skinnier metering rods? I went two stages richer on the same carb. Helped

Gas. We have crapo California summer mix, 90/10 corn gas, and tuning anything is a bear. You run premium, of course. But maybe we have to buy five gallons of aviation fuel for each tank and mix.

 

With the right cam you should be able to run pump gas. I run 91 octane in my 331 with 10.4 to 1 compression. You're not going to know what rods will work till you try them. If it's pinging on acceleration, you're adding too much timing for the gas you're running.

 

Just tried 13* and 20 Mec. No Vac.. A lot better if I had a car this would be the hit. But my heavy F-250 is still not liking something.. Still had a slight ping not 1/2 as bad so I am going to mess with rods... What's the chance I have low vac lean det...?

 

What is the compression ratio in the 351?

 

Not to intercept the last poster, but you are running 33* total. If you still hear a ping, you will have to do something. Something is coming up too fast in the distributor. Springs too tight.
Shut the base timing back to get rid of the ping and eat the performance. Or recurve the distributor. Better than a hole in a piston.
You guys are from all around the country and have different fuels. I am in CA and am trying to deal with our crap premium with 9.5:1 compression.
Not much luck yet.

 

MJL, we found the problem. It was sucking air on the #5 intake gasket.
I had to send it to a carb/injection shop and eat the cost. They found the problem by spray and tailpipe hydrocarbons. We sprayed everything but the intake because two years ago it was rebuilt by a "top" rebuilder. Never suspected the manifold.

We now are trying to figure out timing, whether or not to run vacuum advance or not.

So much for top buck rebuilders. How can you screw up an intake manifold install when you have new heads, new manifold, engine stand. But it can be done.

Now, runs and idles fine. 12* initial, 24* max on the mechanical advance, vacuum disconnected. We shall play around with combos.

What is your situation, MJL?

 

How is this the fault of the rebuilder ? Two years down the road you have a leak ? That's not the rebuilders fault. This is why you as the owner should have taken the time to go over the torque values with a wrench after running the engine a few times. Gaskets DO settle. Especially with this engine design (wedge shaped intake) You can go through the torque sequence 4-5 times and after running it, those correct values will change, depending on what gaskets were used. Don't lay this on the guy who built it. He didn't run the engine, you did. I had the same thing happen a few years back with my 331. My 19yr old son took it out, ran it up to 7 grand and then let off the gas in second gear, the resulting vacuum pull sucked one port's gasket down into it. Wasn't the gasket's fault, nor the builder who assembled it 4 years before (Me) It was the result of gasket settling and too much vacuum pull. If I had thought to go back over the torque sequence, maybe that wouldn't have happened, or maybe it would. **** happens to the best of us. Don't blame that on someone who had no control over what happened 2 years after the fact.

 

Ouch. I am chastised.
I have built engines for fifty years and installed many intakes on engine stands that ran without problems for decades.
Thank you for the guidance.

Semper Fi

 

Millions of these engines were built and installed into cars and trucks that didn't develop leaks of air for decades, with no retorquing of fasteners.

But it may be harder to seal a rebuilt engine because some of the gasket surfaces may have been machined, upsetting the fit of the parts when assembled.

 

Everything I have read emphasizes everybody should run vacuum advance unless they are a drag racer in which case it would be pointless because the engine lives at wide open throttle. For a street engine it will have better all around driveability, run cooler, and get better fuel economy. It's understandable some folks disconnect it because they don't, but it's worth getting into.

 

Yes, the statement above yours was odd. Taking a 351 to 7k and parking it to let the gasket suck in is... odd.

In my situation a rebuilder put a new Edelbrock manifold atop new Edelbrock heads on a virgin block and managed to screw it up on the engine stand. Hard to do. But the big bucks guy managed to do it.

If people come into the two sites I visit who have symptoms of a vacuum leak, I advise them to revisit the intake manifold torque. But if an intake manifold is properly installed with proper gaskets on these motors, they will live forever.

Semper Fi

 

Sorry for my absence fellas I been so busy I just today got to do some test runs. I did read that ya found a intake leak glad to hear ya made headway.. Some is better then none!.. So re-jett went 4% rich on cruise and 4% power.. Huge difference lot more responds and less ping.. Went with 12° INT. 20°Mec. It's a dog and pings around 5000 with 4bbl open.. Has a feel like it's fighting to run in lower rpm range. With Int set at 15° no sluggish feeling going to recurve at 15° Int 14° mech. And see what I get tomorrow. ( taking the whole day off so I will have results) .. I am open to any ones wisdom!

 

Good, MJL. Good to hear back.

We are at 24 mech (according to the Mallory catalog) and 12 initial. With the vacuum leak fixed, much better performance through all ranges. No pinging.

It is my son's car. He is now playing with adding vacuum advance and retarding initial. . More to come.

We went two rod sizes richer on the Carter primaries and the shop tailpipe sniffer said we are on the money. It is a mildly rodded 351.

Stay in touch.

Semper Fi