[pontiac 455ho]

OK question 79 ta 301 60 over. Melling spc-9 cam qjet carb set up
72 jet
49k rod
0.41 idle tube
0.52 down channel
13/32 float
2.5 turns idle mixture
3 turns apt
10hg pp spring
OK timing
16 initial
23. At 4000
16 vacuum advance
OK I'm getting 21 highway cruise in 2300 at 75 mph in the city only 10. Car runs great crisp throttle responce any ideas would be great.

[Ben M.]

Seems low for in town. Are you mashing the carb into the secondaries every time you take off?

[pontiac 455ho]

No just light throttle

[Ben M.]

My '78 Bonneville with a 400 would get 21 on the freeway and 14 in town. What rear gears do you have?

Any chance it is running rich at off idle? Might want to try hooking a vacuum gauge up and driving around town to see what the engine is doing.

[Cliff R]

Timing curve too slow. Base setting of 16 degrees is OK, plus 16 from the vacuum advance is fine as well.

Not seeing the rest of the timing till 4000rpm's is not putting enough timing in the engine for best efficiency in the rpm range you spend most of the time in.

In a little engine like a 301, moving big/heavy cars, I'd put in the mechanical timing earlier, so you see full advance by 2800-3000rpm's, even if you have to limit the amount of vacuum advance timing a few less degrees.

A slow timing curve on a small engine will have you depressing the throttle too far moving away from stoplights and changings speeds in traffic. Vacuum falls off, vacuum advance falls out, and power piston moves up, timing falls back to the mechanical curve, causing excessive fuel consumption.

Put a vacuum gauge on the engine and monitor what it is doing in city driving as recommended above, and also test the vacuum unit to see what vacuum it allows the timing to fall out at?

The carb set-up is also pretty lean, as K rods don't offer much adjustment with the APT. Lean settings seldom, if ever delivery best fuel economy for all driving situations, as they require the operator to press the throttle down further in many driving situations so the engine gets the fuel it wants from the carb anyhow. A really lean cruise A/F will get you pretty decent highway fuel economy on flat ground and very light load, but in all the testing I've done over many years the average economy city/highway will be less than being just a tad richer.....FWIW.....Cliff

[pontiac 455ho]

OK used dial back timing light and timing is all in at 3200. I did put in copper springs engine did not like it. No 4bbl power. And being that my idle tubs are .041 would that make it rich during city driving.

[pontiac 455ho]

And my dist is the flamethrower d1200

[pontiac 455ho]

And I do not run a egr valve. And no cat.

[68 Firebird]

Well I would have to ask what rear gears and trans are you running?

I am getting around 15-16 avg on my 68 400 with melling spc-7.
My HEI has factory heavy springs and total is 34 at 3200rpm, initial is 12 with
vac adv adding 10 from a ported source. I believe my curve would also be considered
slow, but have NO pingning or detonation.
But I have a conservative 3:08 rear gear and a T400 with a 13" Continental Conv.
My MPG seems to be better since trans overhaul and new converter(was slipping)


Gerry

[Blued and Painted]

Is this a turbo 301?

[Cliff R]

"OK used dial back timing light and timing is all in at 3200. I did put in copper springs engine did not like it. No 4bbl power. And being that my idle tubs are .041 would that make it rich during city driving."

That makes no sense, as the springs only change when you see full timing, not the amount. Once past where full advance occurs, power would be EXACTLY the same with any spring, even if the timing were "locked-out".

.041 idle tubes could be small, or HUGE, depending on the upper and lower idle airbleed sizes, location of the upper idle bleeds, and down channel restriction size......Cliff

[pontiac 455ho]

Carb body is a stock 301 down channel are stock as are the air bleeds main air bleeds are. .060
And the engine seems like it was being bound up. With the quick timing curve. It doesn't make sense to me

[pontiac 455ho]

And 79 non turbo

[Cliff R]

What is the carburetor part number?

Are you using the 301 airhorn on it, as they have larger MAB's than you listed?

Maybe the engine just doesn't want as much total timing as you are giving it, not how fast you are bringing it in.

Here is what I would do. Use the springs that bring in the total timing around 2800-3000rpm's.

Do some full throttle runs off idle with traction and retard the timing till the engine is happy with it.

Look to see where the initial timing is at?

Next do some light throttle driving with the vacuum advance hooked up and applying 16 degrees. Move the distributor until it absolutely runs the very best. Play with the APT as well during this tuning and get things dialed in for best off idle throttle response, light throttle smoothness, fuel economy, etc.

Now look at where the timing is at for initial setting, and compare it to what you found for the full throttle testing.

Set the timing back to where it was happy during the full throttle and modify the vacuum advance to make up the difference in degrees between the two results.

This is how I tune everything here, heavy/full throttle first, getting the carb and distributor total timing dialed in, and how fast the curve is, then light/part throttle adding to or taking out timing via the vacuum unit for best results.

Kind of interesting is that most V-8 N/A engines will like pretty close settings. This assumes a well chose static compression ratio and good cam choice.

Most engines I've tuned here will want about 8-12 degrees initial timing, 20-24 degrees from the mechanical advance all in around 2800-3200rpm's, and another 10-14 degrees from the vacuum advance. Really "low" compression engines typically want greater initial timing, especially if they have long duration cams in them, and high compression engines with smaller cams will want less timing, as a general rule.......Cliff

[pontiac 455ho]

I will do the best. As I don't have access to a track. And in nv ther is so much traffic. Might use the desert to tune. And I'm using a 79 Pontiac If horn. And carb number is 17059271. I got the carb body from you a bought 2 years ago you did the bowl'plugs and finish.

[pontiac 455ho]

OK update. First I tested the vacuum can it starts at 10hg and full on at 15hg OK out of gear vaccum reads 19hg tested while driving on accel from light drops to 5hg and this is light accel. Once going 10hg 15 if I barely touch the throttle. Now on high way 0hg accel 10 on cruise 15 again if I just touch the throttle at 2000 gauge reads 10 to 13hg deaccel 21hg in gear 15hg OK next step went with one copper and once silver advance spring. Vacuum went up 3hg and rebounded quicker. But under heavy accel engine was missing. Did not do it with the slower curve. I so put in two crane cams silver springs and miss was gone. Has pleanty of tourq nails it sidways from a corner. Now any ideas way the engine is not making vaccum under a load.

[Cliff R]

Vacuum will always drop off under load. The leaner the primary side is set up, the more throttle angle it takes to achieve any given vehicle speed, so the vacuum is even lower.

Probably doesn't help that it's a tiny 301 cid engine pushing around a heavy vehicle.

From your results it doesn't appear that the vacuum unit is able to add much timing during most driving situations.....Cliff

[pontiac 455ho]

Would dirty intake valve cause the low vacuum. And under most driving the pp will be up be cause I'm using the 10hg green spring. And trans is th200 so a small trans to power to. And I would think that crusig at 2200 I would,d have more than 10hg vacuum . and it does not take much throttle to get the car going . any ideas would be great

[pontiac 455ho]

Tested the car again on open high way best is 10hg in city on accel 5 or less. Still runs good . don't know what to do.

[Blued and Painted]

Put together a 9 to1 iron 400 with a fuel mizer cam. You will have more power and mileage than the slug 301. I once had a 455 in a fully loaded shock bumper 75 lemans. 19.5 mpg hwy.