I've had it with the spark knock. My initial advance is set to 6 degrees, when it should be higher. My vacuum advance has been adjusted back. I have a performer 400 intake and a holley 670 carb, and it still pings when I let out the clutch, but not at cruise for some reason. I suppose it could be 30 years of carbon buildup, but I'm slightly reserved in seafoaming the intake system for fear of what the carbon is holding together or sealing after all these years. I suppose building up an EGR system is in order to lower the cylinder temps. The intake is the EGR version, but the port is blocked off right now. Easy fix, but what parts do I need? EGR valve, vacuum line, obviously, but what about those vacuum control valves and amplifiers? I have a 1978 F150 with a 400, manual transmission, and it's not a California truck. Any help would be appreciated. I guess I'll tinker with the centrifugal advance for now. Thanks

 

I apologize for not answering your question directly in advance. I live in MS where there are no emissions inspections, at all, so EGR systems are not my specialty.

Your EGR system should not cause this. If you are sure your timing is in good order, you may want have a look at your distributor, excessively worn dizzy's can cause erratic timing.

Also if you have as much carbon as you say you do, I wouldn't doubt that your chambers have excessive carbon deposits thereby increasing cylinder pressure and causing the pinging.

A little higher octane might help your problem.

 

Haha, yeah, I'm in Louisiana, so I also have no knowledge of emissions systems other than PCV. My distributor is fairly new, only maybe 2 years old. But it's always had a slight knock from a stop, and it gets worse when it's hotter outside, or if I take a nice freeway drive. I guess I may just bite the bullet and try to seafoam it. I've been filling with midgrade to eliminate it, but it's getting expensive.

 

Your Distributor might need to be re-curved. If the springs are too light* I think, it can cause pinging.

 

Don't know if this will help any because it's for a '77 and I don't know how clear it'll be.

 

Actually, a missing (or non-functioning) EGR system is probably one of the most common causes of spark knock on smog motors.

The reason is because engines designed to run with EGR (a '78 400 is definitely one of these) come from the factory with a steeper timing curve and leaner jets to compensate for the altered combustion chemistry. If the EGR system is removed, the resulting fuel mixture is too lean, and the timing is too far advanced. Even with an upgraded carburetor, the distributor curve may be enough to cause pinging in the absence of the EGR system the motor was set up for.

That being said, if it does NOT make the noise at cruise and DOES make the noise with the clutch in, then I am hesitant to believe that this particular case is not that of pinging, but something else. Can you describe the noise?

To answer your direction question - the most basic EGR system is a ported vacuum switch (PVS) in a coolant passage (often off the thermostat housing), one side of which is connected to ported vacuum from the carburetor. The other side of the PVS runs to the EGR valve.

Upping the octane will only waste money. There is no way a stock '78 400 requires anything but regular. If a motor with that low of compression pings, there is a pre-detonation issue caused by heightened combustion chamber temperatures because of a lean mixture or overly-advanced timing.

 

Well, I think it's pinging. It sounds like rocks in the cylinders, slightly like a diesel. Right now it's limited to taking off from a stop, and going up a hill in fourth. If I drive for like 20 minutes on the freeway, it gets very aggravated, and will happen from the beginning of every shift until about half way to the next, if that makes sense. It also gets way worse if I bring initial timing up. I have also installed a new timing set on the cam that I believe has removed the factory retard.

 

Is yours a single or duel vac canister on the dist.? Is the diaphragm holding vacuum?

.

 

Mine is a automatic. It doesnt ping or knock and my egr is plugged off. I know this dont help but i have also been thinking of putting a holley economaster on my 351m and trying to get my egr up and running to.

 

All of my vehicle run smoother and get better fuel economy on higher octane. I know this is a fact , i have tested and writtne down on every car, truck and motorcycle i own. I also do alot better without ethanol fuel. Not trying to start a fuel debate but this does benefit in my vehicles and i actually dont save money but really it ends up being about the same buy the time u figure it burns more on regular and on fridays my local food city sales there premium for the price of mid grade. Ive just always believe in good fuel , and tested my opinions too. Better fuel does help with spark knock!

 

When i say i get better fuel mileage with premium , i do do my testing on the inerstae. It doesnt make any diff in town. It really sucks in town! i hoping the holley economaster would help this. Sorry i am so off topic. Im done here. Great thread!

 

The only reason I can see higher octane fuel resulting in better mileage would be because it contains more heptane by volume, if that's even a realistic reason (and I am not a chemical engineer, so I actually have no idea).

No one can make 100% accurate claims about getting better mileage with higher octane fuel because there is no way to make it a controlled experiment on the street. It's difficult to replicate exact driving conditions on a day to day basis, and it's not like you drain the tank completely each time you switch (there will always be a mix in the tank). As far as "running smoother," that's a qualitative statement.

However, whatever the physical explanation behind getting better mileage may be, the gain has to be marginal and very small compared to the high price of higher octane fuel. At the end of the day, the goal behind tracking your mileage is to save money at the pump. There's a point of diminishing returns - if you're getting a marginal increase in mileage but spending 20 cents more per gallon, at some point you're losing money.

Higher octane fuel will only solve spark knock on a high-compression engine which is not the case for a '78 400 (that's about as low compression as they get). That's the whole reason behind octane ratings - how much it can be compressed. If that engine knocks, something is wrong.

 

Without starting this debate back, what cars/bikes have you tested this on?

 

OldStyle, it's a single, and I believe it is functional because adjustments will help or aggravate it.

I've been letting this sink in for a while, and have a theory. Let's say I have a broken or weak advance spring, this would cause knock at low speed because the engine would not be at a high enough rpm and the mechanical advance is coming in way too fast. When the engine revs higher, it catches up to the total advance, and the knock goes away. I'll have to find time to pull the distributor apart and look at the springs. I have a couple sets of MSD springs laying around here, do you think they could work?

 

As a basic test, pull your distributor cap off and twist the rotor against the advance springs, then release. If it hasn't rotated back to where it was, your springs are bad. You also shouldn't be hearing any strange popping or ticking noises when you twist it.