I did some more road testing today. I ended up changing my jets from 62 to 64 hoping this would help with my surging and besides my plugs are reading a bit lean with the 62 jets in. The truck didn't seem to surge as much but it was still surging a bit. I also changed out the Autolite 26 plugs for the stock Motorcraft plugs, the Autolite plugs were one heat range hotter than stock, I will post a picture. I had my vacuum gauge hooked up while driving and I noticed most every time I got on the gas pedal gradually at cruising speed I get a hesitation somewhere between 9 and 7 inches on the gauge, I posted recently about having a 9.5 power valve with 19 inch reading at idle. I definitely had a off idle hesitation with the 6.5 power valve.

 

I also have individual pictures of every plug if anyone wants to see one closer.

 

Let's define our terms so everybody is on the same page. "Surging" aka "Lean surge" is something experienced at steady cruise speeds at part throttle, say from ~ 40 mph to 70 mph on level ground. This surging or bucking is caused by jets that are too small, has nothing to do with power valve.

It is important that the power valve is "sized" correctly but, there is no larger or smaller power valve as such, the amount of fuel delivered by the power curcuit is determined by the size of the power valve channel restrictors (PVCR) underneath the valve itself, these are basically a fixed jet. The PV size # determines when, not how much.

It's useful to plumb the mechanic's type vacuum gauge into the cabin and get a feel for what your engine's steady level cruise vacuum is, and at wide open throttle, climbing hills, decel, etc.

Thing to avoid is a power valve that opens, even a little, at steady cruise, this "idle vacuum divide by 2" business is tailored for high performance engine tuning in my opinion. If it's got a big cam in it particularly, the avg idle manifold vacuum will be very low and probably erratic. If the best a hot cammed engine can pull is 10" hg at idle a 4.5" power valve maybe makes sense.

My old slick idles around 20" hg that doesn't mean I want to install a 10.5" pv. Stock pv in the Shop manual is a 7.5" at Sea level. And it probably cruises around 15" hg on the highway. Them Enjuneers was smart!! That's the next thing. If you live at any kind of altitude, a power valve must be selected that will not be open or dribbling at anything other than upon demand i.e. higher load, high demand situations. For tuning purposes you want to get jetting squared away first, make sure the PV isn't tipping in when it shouldn't be, or it will screw your tune, and you'll need a fuel tanker to follow you around.

 

I did some more reading last night and now I think I need to go from 64 main jets to 60 because the top threads are black and cruddy, that is supposed to be caused by the main jets being too big. The porcelain being white may be a sign of detonation? So turn back my timing some?

 

That last picture was taken with 62 main jets,9.5 power valve, 35 squirter.

 

You went back to the stock heat range Motorcraft plug, right? Have to start there, because timing and fuel mixture changes the plug tip temperature. The ground strap should show a definite color change about halfway when the heat range is correct. Porcelain can show a glazed appearance if heat range too high, or too lean - that's why we need to start with a stock baseline before changing. Btw: Need super sharp close up focused pics for anyone to help you online.

Detonation will show by tiny metallic spherical ball deposits, meaning the piston is burning through. Again, a high heat range plug can cause pre-detonation, keep in mind, this is not the same thing as ping or engine knock. Use a magnifying glass with a strong light.

Also need to look deep inside the plug, where the porcelain transitions to plug shell. There will be a fire ring of color there. This indicates the wide open throttle fuel mixture. Basically want to be nice and lean on the highway cruise and nice and rich under power or load. Porcelain color itself doesn't mean much with today's gasoline. Ground strap, base ring, fire ring.

 

Yes I went to the stock heat range Motorcraft spark plug. I'm at 14 initial timing which puts me at 36 total timing at 3000 rpms. I check my plugs in my garage after a good rigorous drive. If I killed the engine on the side of the road everything would look even leaner I would guess? I looked down into my spark plugs and they are as white down deep as the porcelain is up at the ground strap. I'll add that my truck pulls great at WOT.

 

That doesn't sound good, lean fuel mixtures are perfectly safe under steady cruising but never, ever, want to go lean under any kind of load or acceleration. Think of what a lit acetylene torch does when you first dial in the OX. In kind of the same way a distributor has an advance curve, carburetor setup and jetting &c, is tailoring the best air/fuel mixture for each circuit or condition.

You can really fine tune by jetting to get the steady cruise mixture optomized, and then open the pvcr's slightly with a pin vise drill to raise the wide open throttle fuel mixture back up where it needs to be. This is where a wideband O2 sensor is slick as hell. Some guys really get into modifying idle air bleeds and emulsion tubes and all that stuff. I never bothered with that, though using a wideband sensor I was able to improve highway fuel economy from about 9 mpg (with a good tailwind) to 16. Mistuned carburetors/engines waste an amazing amount of fuel. That's why fuel injection replaced carburetors.

 

The pictures I showed were with the 62 main jets and the hotter than stock spark plugs, so with the stock spark plugs and 64 main jets it should be richer at WOT right? I don't care about gas mileage as this is just a weekend wanna be hot rod. I would just buy a new carburetor but I'm about to dump a bunch of money into a new 9 in.possi 3.70 third member, a new carb will be coming in the future.

 

Yes, larger jets will also richen up the power circuit mixture. See how things look with stock heat range first though.

 

Middle plug is said to be on the edge of too fat at open throttle. Two plugs on the right is kind of what you want to see.

Spark Plug Reading 101

 

I decided to take my carb apart today and spray it and all the passages really good with a couple of cans of carb cleaner when I removed the power valve I found two gaskets instead of just one, I suppose that might of been some of my troubles. I put it back together with 62 main jets, 6.5 power valve, 35 squirter, plain secondary spring, and I moved my orange accelerator arm cam from the first hole to the second hole. When my son and I took the truck for a test drive the back firing on decel was gone and the truck ran much better in the transition circuit where I was having most of my problems. I parked it feeling pretty happy with my work. I will also add, the truck had a very steady 20 vacuum reading once it was up to operating temperatures.

 

That second gasket will do it every time. Always the simple things huh ? With the stock secondary spring in there, the secondaries won't open till way up in the rpm band. I always used the lightest ones in a 302 to get them to open up a second or so after punching the go pedal.

 

My thinking on the plain spring was to try to keep the secondaries out of it until I can get the transition circuit issues worked out than start working in lighter secondary springs?

 

Nothing wrong with that.