Technical Carburetor Jetting The Old School Way

WOT, correct? That's right in the ballpark from everything I'm reading, but if the ratio is way lean, or way rich, how is this adjusted? Power valve says "when" not "how much", see where I'm goin' with that? It's a factory stock engine, draws lots of vacuum at idle, I believe it is a stock cam.

 

Jeez Louise, did it run like that?

 

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Absolutely the best argument that I know of for efi.

Gimpy - depending on the specific application:

Both Stromberg and Zenith made single barrel carbs with externally adjustable main metering jets. I have seen some gearheads using these with an adjuster hooked up to a dash-knob, so the carb could actually be tuned dynamically from the driver's seat.

If using most Carter carburetors, one can make/acquire metering/step-up rods for various elevations. Roughly a 3 minute job to change them on a Carter AFB; and while I don't recommend this procedure, I have actually changed them in a running engine simply to prove a point.

Jon.

 

I'll bet he said 'and I tuned it myself!' just as proud as could be. I rebuild lots of carbs and I've even seen them com ein with any jets in them! One dragracer brought me his dominator after grumbling about how it'd go flat from halftrack on and I found he had the rear float spring on it upside down and it was keeping the needle & seat shut - he was running on a 2bbl carb! 'And, I tuned it myself!'

 

I'll tune racecar for WOT but a hotrod etc I drive thru town until I'm satisfied and then the highway. Adjust by jetting, remember the idle circuit operates full time so you can play with that a little for highway driving too. With a strong stock engine & cam you should be able to make a smallish adjustment and see results - I get vague because none of the tuning thing is 'fixed', thats why EFI has the worlds smartest computer making tens of thousands of adjustments timed in milliseconds. Carbs are based on how many pounds of air is going thru them (you can calculate it from CFM and about .00627lbs per cubic foot and my memory might be a little off but you get the idea) an dthose pounds of air are what is available at this moment, this barametric pressure, elevation and temperature. They'll all be different tomorrow.
About the power valve, it isn't an 'on' or 'off' thing where is snaps closed at 6.5" of vacuum. It'll start opening at a vacuum somewhat less than its rating (6.5 for instance) and continue to open further as vacuum falls, as vacuum rises it'll start closing until at the rating it'll be fully seated. It'll let extra fuel into the wells in drops and dribbles as you drive normal or dump as much as the restrictors allow when you floor it.

 

OK, but let's say for example idle and cruise jetting is dialed in slightly rich, however there is way lean WOT air/fuel mixture condition. This is bad. How is that remedied on a carburetor like this? Or more likely how to lean out a pig-rich condition?

Idle and cruise jetting seems pretty straightforward, the rest not so much.

 

If idle & cruise is satisfied - we are talking theoretical here - and you are lean at WOT then the carb is sized wrong. I doubt if your 'what if' could occur but the opposite can happen. When I build a 350cfm carb for NASCAR restrictor cl***es the carb cannot flow enough air to satisfy the engine but the fuel circuits will supply ever greater amounts of fuel - the opposite of your 'what if' where the engine is satisfied at part throttle but fat at WOT. The easy fix it to get a bigger carb where it is fat at WOT, if lean then your carb is overly large. Since we cannot - legally - go bigger than 350cfm then we have to modify the fuel curve with what is known as the 'emulsion bleeds' and that isn't something you want to mess with.

 

The current carburetor is the bone-stock 1.02" venturi C3TF 2100 carburetor. OEM for this engine. Flows approx. 245 CFM, it is a great carb in terms of low end torque and idle quality. One torture test of ignition components and carb tuning is running about 10 miles per hour in high gear without bucking, this thing really flows nice at low RPM.

One of the many webpages on Wideband tuning described an initial routine, I may have misunderstood this, of running the carb through its paces neutral for example running it up to around 2500 to 3000 RPM steady and confirming jetting.

I'd have thought that the only valid A/F numbers would be under load, on the road. Guess it would be a ballpark at least.

 

Today warmed up some to around 40°F and rainy, snows gone mostly and so took it for a spin to see what level highway cruise was at. I'd previously set the idle mixture at 14.7

Also hooked a vacuum gauge up in the cabin so I could read that too. Was so impressed with myself I forgot to rig up a tach. Oh well that can come later. Idle vacuum ended up around 20", about 15" at 30 mph, and maybe 12" or so at 60 mph, it's tough to find absolute level road sometimes, monitor gauges, and not run off the road. Engine vacuum seems real good.

Cruise AFR isn't bad - 14.2 to 14.8 at anything steady from around 35 mph up to 65. This is in fairly cold, rainy/foggy weather though. It would likely be pig rich on a hot summer day, right? Stock 48 jets right now. I have a set of 46 on hand, they seemed a little anemic last fall, will order a set of 47 size, I bet they will be close to ideal. 46 might be good for summertime.

WOT couldn't get below 13 or so, and the accelerator pump rod is set as rich as it will go. 12.5 is a target to shoot for. Running a stock 7.5" economizer (power valve). It looks like steady cruise vacuum never drops below 10" or so under most conditions, so I think around a 8.5" might work and would bring enrichment in earlier. But maybe not the amount?

The idle AFR was too rich on the test drive, had to readjust that on return, that might lean the cruise circuit out a smidge.
Real nice to see what's what, and not guess.

 

WTF are you on about ***? You're no help.

 

Individual cylinder tuning going on !!!

 

I post early in this thread when it began ,It takes a lot of time, and Notes ,
You need to play with air bleeds , Inertia tube hole sizes along with jets & PV

 

You're right, but forums like this at least make it so I don't have to reinvent the wheel. Getting the cruise jetting close was the idea today. It occurred to me that running so much ignition advance compared to the stock spec should mean bumping up the PV a notch, on average manifold vacuum is higher than it would be.

 

Truck64 ,in a way We are reinventing the wheel , we are now using modern technology to see exactly what the engine is doing through all RPM ranges , so now you have to do more adjusting & modifications to carburetor to correct air fuel mixture's from idle ,cruise to WOT,
In older days you just tuned to the plug color, BUT that does not mean the AFR was corect threw entire RPM rang , Now you can see what the AFR with Wide band O2 , SO more work to do , Earlier in this thread I stated the the new modern gas formula always look Rich , look around new cars that have EFI ,tail pipes look black , But we know that the PCM is always cycling Rich Lean ,, But tail pipe by color Look Rich ,
I also stated that Just on my Flatty over 40 hr in tuning carbs ,
Also I stated I was interested going to EFI on my 1930 A with carbs making North of 550hp almost daily driver ,time & time in tuning those carb also.
I contacted holly MSD and Fast, I told them what I was wanting to do & I discuss my tuneup and air fuel mixture's now through all rpm range .I was told by all three that I would not increase in fuel mileage ,probably Same or less MPG But would increase in performance , for me not worth the $3,500-$4,00 cost , to get more MPG it would be a EFI System over $6,500 !!
if I wanted just the performance edge I would install a
50-70 shot of N.O.S
I was looking for more MPG ,

 

Mileage isn't so much the factor, I like range, though they go hand in hand. But a proper tune also means that excess fuel isn't diluting the oil and washing the rings out, glazing cylinders and wiping out the baby seal populations.

It ran super rich when I first got it some years ago, for a long time. I didn't know anything about tuning. This was before the internet (mostly) when today, so much more info is at our fingertips. Installed a hotter plug was about the extent of it, of course that wasn't the problem and ain't the fix. Now have everything dialed in pretty good, but I wonder if the rings are washed out or something like that.

It pulls 20" of vacuum at idle - but only at lots of advance, way beyond factory. I think it's a factory cam. The manual sez it should pull 18" at idle, but that spec should be at the factory spec of 4° ignition timing..

 

Do a leak down test , "gas wash rings "

 

The leakdown test is definitive, right? Would a compression test be any benefit? I have a compression tester. I should have known better, but haven't done that yet. Probably today.

Washed rings would normally mean an oil burner and/or smoker, and it's neither. Valve lash is perfect. Maybe the cam was never installed correctly? So I am correct in ***uming that all vacuum gauge diagnostics must be performed at factory timing spec of 4°? I'll do that and report back. Thanks for your advice.

I can see where THIS is going. A leak down tester isn't too expensive.

The leakdown test is definitive, right? Would a compression test be any benefit? I have a compression tester. I should have known better, but haven't done that yet. Probably today.

Washed rings would normally mean an oil burner and/or smoker, and it's neither. Valve lash is perfect. Maybe the cam was never installed correctly?

Am I correct in ***uming that vacuum gauge diagnostics must only be performed at factory timing spec of 6°? Manual says timing can be further advanced another 5° over "normal" for al***ude, performance, or fuel mileage reasons. I'll do a comp. test and report back. This will get the engine warmed up a bit that way anyway and then pull the plugs for compression test. Thanks for your advice.

 

Leak down is more accurate , to many Variables when doing a compression test ,
Pull all plugs . All so if you are not experiencing blow buy , smoke , Oil coming out of the breather or dipstick , no gas in oil ? You should be ok !
Bad guid's /seals will cause smoke .

 

Nope, no blowby, no oil consumption. Tailpipe looks grey-brown.

Decided to try a stone cold compression test, just to see what's what at this point. Plugs removed, battery charged, removed primary wire from coil, carb throttle stuck open. 5 to 8 cycles, record first puff and highest reading for each cylinder is the plan.

 

1. 135
2. 145
3. 135
4. 130
5. 140
6. 140
7. 135
8. 140

I'm no expert but I think this is acceptable for a cold test. 150-170 is spec for engine hot.

 

I mention this before on using spark plug electro ground strap & porcelain for tuning ,
You also need to look DEEP down inside porcelain with magnifying gl***
Shining plugs are more Accurate them black plugs ,
Also None ethanol blends fuels

 

I suppose it's regional, but finding fuel that's ethanol free ain't getting any easier. Around here you have to go to a marina that's geared for fueling boats. I'm about 20 mins from the nearest marina, once you get the coastline of Lake Erie there's a marina every few miles.

And I don't give a **** what the sticker on the pump at the corner gas stations says, the ethanol content out of the nozzle from one station to another is different even if they claim to be the same.

There was only one station here that sold 100 out of the pump. They changed that over to E85. Now there's nothing but 110 in several places.

 

Right where the porcelein transitions to the plug shell. I tried to get some of that in the pics. There is a definite brown or tan ring. WOT was 13 AFR, in the ballpark anyway.

If the wideband can believed the cruise jetting can lean out some.

The plug base ring and first couple threads shows rich, but in order to check this accurately it's important to chop the throttle at highway speed and coast to a stop before observing color.

We can get fuel that ain't laced with corn squeezins, but it costs more, there's even one station with high test 92 w/o alcohol. I don't use ethanol fuels mainly because they don't store well. Can go 6 months without driving the old beast sometimes because of snow and ice. I keep it garaged and a full tank and it seems to store OK.

 

Are you running leaded fuel? If so that explains some of the color on the plugs but more importantly lead will contaminate the O2 sensors. Bad juju that. If you want to use the Innovate then you need to have ethanol laced fuel.

 

Sorry for the confusion - I'm burning 87 unleaded pump gasoline, just w/o any ethanol. It costs a little more than the 10% stuff but it stores better. Here's how to find who carries it in your area.

http://www.pure-gas.org

 

Not sure if this will bring Ohio
http://www.pure-gas.org/index.jsp?stateprov=OH

Marinas (boats) and Amish country
Imagine that ! Not one in Cleveland

 

There is no pure gas within 350-miles of me, according to that site.