Horrible mileage

[DoubleL]

Hey guys! First off I'm not looking for 25mpg here. I'd be happy to hit the teens!
I think I am getting about 7 to 8 mpg (my speedometer is broken so I'm not positive). Here is my set up:

1966 C20 stock 292
Recently installed 700r4 (professionaly installed TV cable)
4 barrell Holley 390 cfm
Clifford intake
Long tube headers
Electric choke (works fine)
Howling stock rear end
HEI distributor (Full 12V power and 8mm wires, vacuum advance NOT hooked up)
32" tires
Regular gas

The truck sputters and hesitates when cold and is generally gutless on the freeway.
I have a new 3.73 posi rearend i am cleaning up for it.
Any ideas? Thanks as always

[Rich 5150 69]

First of all get your vacuum advance hooked up and dialed in, and your total timing, thats one reason it hesitates, plus it will help with your MPG....emensly...!

[brokenspoke]

Originally Posted by Rich 5150 69

First of all get your vacuum advance hooked up and dialed in, and your total timing, thats one reason it hesitates, plus it will help with your MPG....emensly...!

Ditto

[chevyrestoguy]

^^^Agreed. That's a great place to start, and probably the biggest contributor to the gas mileage issue.

[DoubleL]

I hooked up the vacuum advance tonight. Won't have time to drive it until tomorrow. I don't know how when I installed the new carb/manifold and the HEI I just didn't hook it up! Dang old-timers disease! I'll get it timed this weekend once I figure out what the timing should be set at.

[awbrown]

I have never had any luck with gasmilage and holley carbs ... A Q-jet will out mpg a holley any day ....

[brokenspoke]

Originally Posted by awbrown

I have never had any luck with gasmilage and holley carbs ... A Q-jet will out mpg a holley any day ....

Its a 6 cyl.....

[siggyfreud]

Theastronaut has provided excellent advise. I'd be surprised if you didn't double your MPG following it.
Posted via Mobile Device

[theastronaut]

Originally Posted by siggyfreud

Theastronaut has provided excellent advise. I'd be surprised if you didn't double your MPG following it.
Posted via Mobile Device

If the engine internals are in good shape, there's no reason why the mileage shouldn't double.

As said, I picked up 12 hwy mpg going from the stock carb to dual two barrels. This was a stock 1600 engine, only had ratio rockers and exhaust otherwise. VW advertised "An honest 25 mpg" so I picked up 11 mpg over a factory tuned single barrel with carbs that flow around 205 cfm per barrel- it's not always about how big they are as to how well they run on the street. My bug doesn't even have vacuum advance, and guys commonly pick up around 3 mpg just adding vac advance. This was also with a 4.37 ring and pinion- the car needs to be geared to run where the engine is most efficient. Too much overdrive can drop the engine below it's powerband and make it work harder (getting less mpg's) than if the rpm was up a few hundred rpm. That's why many new cars have 6,7, and 8 speed or transmissions- they keep the engine in an rpm range where it's most efficient.

The combustion chamber of a engine doesn't know whether there's a carb or injectors metering air/fuel in; it's all about how they're tuned. EFI has to be tuned just like a carb does, and with the right carb/manifold and a tunable centrifugal and vac advance distributor, you can get similar mileage as an EFI engine.

[HEI451]

Stop chasing your tail, you DO NOT need a fancy gauge to tell you it doesn't help.

First, the vacuum advance needs to be restricted down on degrees to function correctly for that engine. You need to make a stop that limits the pull pin for the pickup plate to allow only 10 crankshaft degrees if ignition timing. To get that 10 degrees, the pull pin needs to have only .112 inch travel, NO MORE. Use the e/mail address I supply below for pictures of how to do it right.

Once you get the vacuum advance stopped down correctly, it goes to FULL MANIFOLD VACUUM ONLY, NOT A PORTED OR "TIMED" PORT, PERIOD. Anyone that says different, hasn't got a clue as to doing it right. And, if you do not do tghe mod to the pull pin, make the stop, it never will work correctly, so, stop here, you are stilol lost without the stop.

Next, I am a former factory Holley Pro-Stock carb tech, and, the 390 is entirely THE WRONG CARB FOR THAT ENGINE, PERIOD. You need to find a Holley LIST-1850 (any suffix will do, from -1, -2, -3, etc.). FAR, FAR better for that application than any 390 carb. Trust me, you will NEVER get the 390 Holley's to work well on that engine.

For the 1850, they mostly come with 66 primary man jets, you will need either 64's or 65's. The secs plate should work OK, but, possibly, a different secs spring might be needed. A spring kit and quick change top for the canister are available, not expensive. Start with the 1850's 6.5 power valver, should be right in the ball park for one of those engines, if it doesn't have a radical cam in it. If it does have a radical cam, might need lower numerical P/V. Squirter should be OK, as the pump cam too.

If you must use a Q-Jet, use either an OEM carb for the OHC 230 Pontiac engine, or, one of the early ones for the 231 Buick V6's, they should both be very close right off the bat.

Stay away from Edelbrock Q-Jet and AFB cones, they are more trouble than they are worth, right on par with the 390 Holley's.

Request stop plate pictures for the vacuum advance here: info@davessmallbodyheis.com

[theastronaut]

There's no right or wrong when using ported vs manifold, you just have to set the distributor's initial and mechanical advance accordingly.

I prefer to leave the vacuum advance unhooked and set the initial advance, then adjust the total centrifugal advance. This way the vac advance (hooked to ported source) is only used at light throttle/low loads when added vac is actually needed. The initial and full throttle centrifugal are dead steady this way (mechanically controlled only) and the vacuum canister has zero effect except when cruising- the only time it's needed. The centrifugal advance rate (how quickly it starts advancing) can be set with lighter/stiffer springs and the advance stops are set for total advance; this way you have dead-repeatable full throttle centrifugal advance from idle to redline every time. If any part of the vacuum advance system fails, you still have a fully functioning and unaltered initial and total advance timing that works plenty well enough to run the engine; mileage and throttle response will be the only things that are affected, and it's only a minor issue. VW set up millions of dual advance (vac and centrifugal) distributors with ported vacuum, so it can't be wrong.

Manifold vacuum pulls all the time, so setting the timing is somewhat backwards from ported vacuum. You have to take into account both vacuum and initial timing at to set timing at idle, and if any vacuum component fails, you're stuck with the wrong timing under any condition except full throttle. The engine idles with vac advance already added in, and throttle application retards timing since there's less manifold vacuum for less vac advance. Just seems backwards to me. Any variance in manifold vacuum will cause a variance in timing. Obviously it works though, and either way can be effective.

Like I said, there's no right or wrong way so I'm not arguing; just stating that it makes more sense to use the vacuum advance only when it's needed (part throttle) and not tune around the vacuum advance at all other points when it's not needed.

[HEI451]

"There's no right or wrong when using ported vs manifold, you just have to set the distributor's initial and mechanical advance accordingly."

Really? I guess the years I spent working for GM engine development and Duntov directly in Skunk Works were all wasted, because we didn't find that statement correct at all. The ONLY reason for ported (timed) vacuum souring for a vacuum advance is EMISSIONS. If the engine has a functional EGR valve, ported is your disaster, and loss. If not, full manifold vacuum is the right way to do it, as long as the advance is restricted down to give the correct amount of degrees for the engine application.

On a non-emissions engine, ported vacuum advance is a second acceleration advance curve, and it just does not work, case in point, the ZZ4 crate engines. Years ago, I did a revision of one of those distributors for Limeworks Speed Shop, Whittier, Ca. They use a lot of crate ZZ engines n all their specialty built cars, and always had over heating, fuel economy and performance issues with the engine set up as GM demanded it be. Simply restricting the degrees of vacuum advance down, and using full manifold vacuum over ported, along with speeding up the mechanical curve speed, FIXED ALL THE ISSUES. Many have used that same fix on their distributors, for all sorts of small blocks over the years since I did that revision, and, IT WORKS.

Non-emissions engines do not need, nor like two separate advance curves for acceleration, they DO need a load compensating curve (full manifold vacuum sourced vacuum advance curve), and a separate mechanical advance for acceleration only.

"Manifold vacuum pulls all the time, so setting the timing is somewhat backwards from ported vacuum." Well, this is so far from the truth, it isn't funny at all. Full manifold vacuum is strong at idle, and at low load engine operation, and virtually goes away when large throttle loads are used.

And, then it is stated that vacuum advance impairs correct initial timing. Well, "timing" is finite, not variable in specification, so, both the initial AND vacuum advance degrees need to be set to give a correct IDLE timing, such as for the ZZ4 4ngines, 11 initial degrees, full manifold vacuum advance degrees of 8, for a total idle degree setting of
19.

It always just amazes me that everyone can come up with all sorts of incorrect info for all to revere and awe at, usually from sites off the net, when all that stuff just doesn't work at all. The worst is the "dyno hero". Dyno's LOVE two things that a real world engine doesn't tolerate well, extremely excessive timing, and to the fault rich over jetting. we have all heard that a person had their engine "dyno'd by the best guy racer at his shop" and then, when that engine hit the street in the dyno trim, wouldn't get out of its own way. Then, we hear all sorts of excuses and "fixes" for it, when in truth, it is the curves and usually, a mis-adjusted and/or disconnected/plugged off vacuum advance, incorrect mechanical advance and way too rich jetting, drom the billion dollar "dyno tune".

Sorry, I don't buy the argument that there is no positive truth about vacuum source differences, I have fixed so many mis-applied setups in the 40 plus years I have been in factory engine development, racing and design to know quite differently to know different.

Iritinal poster, fix the vacuum advance degrees, use full manifold vacuum, go with the LIST-1850 carb, and that 292 should work great, mine did. Use my e/mail, I will help if you wish.

[theastronaut]

Originally Posted by HEI451

On a non-emissions engine, ported vacuum advance is a second acceleration advance curve, and it just does not work

Non-emissions engines do not need, nor like two separate advance curves for acceleration

Ported vac advance does not act as an "acceleration curve" since the ported vac source sees zero vacuum at WOT, so there's no added vacuum advance. Vac advance drops back to zero, and the centrifugal advance is the only advance there is.

Originally Posted by HEI451

"Manifold vacuum pulls all the time, so setting the timing is somewhat backwards from ported vacuum." Well, this is so far from the truth, it isn't funny at all. Full manifold vacuum is strong at idle, and at low load engine operation, and virtually goes away when large throttle loads are used.

I did forget to say that manifold vacuum goes away at WOT, this is generally understood though. My point was that with manifold vacuum you are starting at idle with added vac advance, and it lessens as more throttle is applied. That is backwards from ported vac advance- zero at idle, added in at low loads (the only time it's needed), and drops back to zero at WOT.

Originally Posted by HEI451

Sorry, I don't buy the argument that there is no positive truth about vacuum source differences, I have fixed so many mis-applied setups in the 40 plus years I have been in factory engine development, racing and design to know quite differently to know different.

It worked for VW for millions of cars, and is simpler to time since both advance systems are completely separate, and the total advance under any condition doesn't have to be dependent on the vac advance. If you lose manifold vacuum, you lose your initial and part throttle timing. If you lose ported vacuum, your initial advance stays the same so your idle isn't affected. Either system can be "misapplied" if the distributor's initial and centrifugal advance isn't optimized for the vacuum source you're using.