Any Holley Carb Experts ?

[Learpilot]

Anybody running a 80670 Holley Street Avenger 670 CFM Carb ? I had a 4777 650 Cfm Double Pumper that was on my '40 when I bought it. It ran great but the gas mileage was real bad. I bought a 670 Street Avenger  for my SBC 350/350 and a 2.79 gear.  It runs just as good as the double pumper ,but a lot better gas milage.
I was just wandering if anybody has leaned out this carb for better gas mileage ? I have done all the set up that Holley recommends.
I think the main jets are 65 and secondarys are 67.
Thank you !!!
Rick

[crdnblu]

I believe that a bit more information would be helpful for us to offer assistance to you; for example:

1. What cam is currently installed? (if no specs available, is it close to stock or "hotter", with a "lumpy idle?)

2. What is your engine vacuum reading @ idle?

3. What ignition initial advance, and total timing are you running?

4. What intake manifold is installed on the engine?

5. Is there currently a carburetor spacer installed?  If yes,  how thick, & what composition?

6. Dual, or a single exhaust?  Also, if duals, is an "H" pipe installed in the exhaust system?

These answers are important as a carburetor is part of the total engine package that all works together; any changes in any ONE of these areas can have an effect on recommended carburetor tuning tips.

[wayne petty]

there is more to getting fuel economy...

with the gear set you have.. and the size of the carb...

you are just barely going to be off the idle transition slots...


do you have any friends with an unmodified 390 holley laying around you might try.. ?      08007...  there are several versions...

these allow the carb to operate at the lower RPMs at a wider throttle opening..

wait.. its not that confusing.. its the same amount of air..  but the fuel is being metered by the main jets via the booster venturis not the idle transition slots..

just curious.. does your exhaust have any 18MM bungs installed for oxygen sensors and a place under the dash to temp hang an air fuel ratio gauge..

i wonder if the individual exhaust sensors with the single gauge mentioned in this almost 8 year old article on piston engine aircraft fuel injection systems are becoming available for cars... or dyno tuning???

http://www.airspacemag.com/flight-today/church.html

[Mikej]

The new carb is a vac secondary? Single pumper?  Both would be more street friendly. Make sure idle adj and throttle plate position is perfect. Than you can lean out the primary jets. If it starts surging at cruise it is to lean. Go up 2 jet sizes. Watch your plugs and the tail pipes. The leaner it is the lighter colored the tail pipe will be. If your foot works like mine, its to blame for most of my bad mileage. Would be worse if I had a double pumper. I'm getting 13 MPG average on my BBC.

[Learpilot]

I will try to answer these questions.
1. The cam has a little lope at low idle of 650 rpm but is smooth at about 750 to 800 rpm. I don't know the spec. I can run regular gas so I think it is a 9 to 1 compression engine. I know the 350 engine came out of a 1977 C-10 Chevy truck.
2. The vacuum at idle is 18 inches in nutrual and about 12 inches in drive holding the brake.
3. The ingition initial advance is 8 degrees and the total is 38.
4. Edelbrock Peformer RPM dual plane non air gap manifold.
5. 1" phenolic 4 hole spacer.
6. Dual exhaust, block hugger headers with no H pipe. 2 1/4" exhaust pipes.
I think the '40 weights about 3200 to 3300 lbs.
Thanks for your Help !!!

Thanks for the link to the pilot seminar. When I flew Piston Aircraft we would only lean out the planes that were equipped with a EGT gauge, because you can not hear the engine detonating.
Rick

[wayne petty]

hmm... tire diameter?  just for more info to the pile...

http://www.omix-ada.com/gear-ratio-vs-tire-size.asp

vacuum at cruise speed... on level ground..

the above chart will give us rpm's at cruise speed..


glad you enjoyed the air craft engine tuning article.. it opened my eyes.

this is sort of off thread.. but i wonder how many people know there were different injector flow rates on the L83 Cross fire corvette motors.. with the twin individual throttle bodies..   gm engineers must have really been on their toes in the dyno room on that one..

its a shame there is no easy way i know .. to monitor throttle opening angles on a holley...  so you know how much or how little you are opening the throttle while driving..

as if you are just cracking the throttle..  you are running on the idle transition ports.. not the mains...

and most holley's don't have venturi vacuum passages to keep track of when the flow is enough..

its a shame that demon closed its doors... i would have ask them to create a square bore carb... with the front half a 390 holley and the rear half a 650 or so...   not a spread bore.. that requires a special intake...  one that nobody knows its a tiny primary  carb unless they look...  and then they notice that you pass a LOT of gas stations..

in the search for mileage...  one could also count on some of the other brands of carbs... creating an adaptor plate... to mount a 2300 holley. or a motorcraft 2100, 2150...  even a 2GC from a similar sized motor application to get the benefit of the factory tuning..

the whole idea again is to go smaller to get the 2:79 gears and your probably 27 inch tall tires to spin the motor at a RPM that the carb size can work at ... other than at WOT...  since we are sipping fuel instead of spraying it in..  we need to get the suction on the straw flow rate to match the needs of the movie viewer.. or you will be out of the soda before the movie is over..  i think i need a long nights sleep... i have more 300ZX fuel injection ground problems to sort out tomorrow morning.   where they grounded the various sensors to individual grounds on different points on the firewall and the intake..  but did not hook them all together somehow..

[crdnblu]

Learpilot........Your responses to the numbered questions all look to be in range for your best mileage, so far.  Additionally, I would suggest considering a bit more initial advance, in the range of 10-14 degrees.  Small block Chevy engines typically respond well to this setting.  NOTE:  A larger advance-stop bushing should also be installed in your distributor to limit total advance to ward off WOT detonation.

You haven't mentioned, but does your existing distributor have vacuum advance?  (Always recommended for the best mileage results.)

Regarding your exhaust system, do you have an "H" pipe in your system?  If yes, good; if not, it's a modification worth considering.  Flowmaster says:

Flowmaster strongly recommends using a crossover tube, otherwise known as an "H" pipe or balance pipe on all true dual exhaust systems. The crossover pipe equalizes the exhaust pulses and allows the sounds waves to communicate between both banks of the engine. Not only does this usually improve torque in the low to mid rpm range, it also creates a deeper mellower tone both inside and outside of the vehicle and helps eliminate "back-rap" on deceleration.

The above recommendations will potentially help for improved mileage........
However, with your existing gearing, you will still have the potential for only cruising on your Holley idle circuits, as previously mentioned by Wayne.

[Learpilot]

Thanks , I will bump the timing to 10 degrees now and try 12 later and will also check the total advance. I do have a vacuum advance and it is hooked up to full manifold vacuum not ported. I have an adjustable vacuum advance. It is not installed but if I have to limit the timing I will install it.
I don't have a H pipe, but will.
Rick

[crdnblu]

Also, don't forget that after changing, & re-setting the initial advance, [with the distributor vacuum disconnected] you'll need to re-adjust your idle RPM, & mixture screws, slightly.  I'd suggest doing it with a hand-held vacuum gauge connected.  
As you know, you're shooting for the highest vacuum reading @ the idle speed that you choose, with both mixture screws turned out EQUALLY on your Holley.  This method is more full-proof than just relying on a steady tachometer reading.

[Mikej]

I  do the lean drop method.  Start rich on the idle screws ( 4-5 turns out) and go to lean(1/4 turn at a time). Watch your RPM you go. Pay attension to the RPM on each screw. As you lean it out the RPM will rise. When you get get to your highest RPM lean it out so each screw drops 20 more RPM. The idle adjustment screws are not made the same so they don't have to be turned out an equal amount per side but should be treated as individuals. This takes alittle time but if you do this and adjust the throttle speed as you go you will have both set when your done.  That is the way I was taught. For mileage and no tip-in hesitation.  I'm not sure but airplanes adj from rich to lean as they go up but stay rich so you don't burn a piston.

[ACEMAN419]

has the engine back fired any if so check the power valve, if the vacuum is all over the place you could change the power valve to a diffent size if its moving to much could kill mileage  al