Here's the deal. I've got a '70 w/350 base engine TH400. I've added headers, 2.5 in. dual exhaust (w/x pipe) Performer RPM w/ 670 street avenger w/67 jets instead of stock 65 jets, HEI and MSD wires. As far as I'm aware, the long block is stock. I just installed the Innovate LM1 A/F meter and I'm not sure where to go from here.

Once the LM1 was up and running I found a very rich (12.5) idle ratio. I was able to correct this using only 1/2 turn out from bottom on the idle screws. Then, while the car was in park, I revved the motor up to about 3000 rpm, 500 rpm increments, and found that the ratio progressively went lean as follows (1000 14.7, 1500 15.4, 2000 15.9, 2500 16 and 3000 16.3).

Question 1: Would raising the float level affect idle A/F?
Question 2: What would be the largest jets I could go to with the 670?
Question 3: What other things should I be looking at as a cause to this lean condition?

Any insight/help would be greatly appreciated.

DC118us

 

Sorry I don't have answers to your question, but wondering about the unit your using. I do most the work on my 70 Corvette, but one of the areas I always fumble with is carb tuning if fact I usually stumble on getting it tune and not sure how I got there. Would you or any others here recomend this unit for someone like me.

Sorry I hijacked this, interested in seeing the answers to your question.

 

That's the same spot I'm in, I can't tune by ear (whatever that means), so I wanted to know for sure what my A/F was, so that I could at least get in the correct ballpark without the guesswork. I've read a lot of good things about the LM1, but I'm still a novice to its capabilities. So far it's been easy to set up and start using and I like the ability to record a session then analyze it on the included software.

DC118us

 

ttt

 

1) Yes, raising the float level will affect the A/F ratio (makes it richer by "pulling" more fuel from the higher level in the float chamber with the same "signal") but it will affect all circuits (except the accelerator pump circuit)...i.e. idle, main, power, choke, off-idle. So unless you are lean/rich the same amount everywhere, the float trick is not advisable.
2) Sorry, don't know that one.
3) Nothing, you don't have anything wrong with your readings. As no load RPM goes up, the A/F ratio will go down. Since the engine was under no load, it didn't take much HP for it to be at 3000 RPM meaning it didn't take much throttle opening to achieve as opposed to when the engine is under a load. With small throttle openings/air flow, you may still be on the idle and off idle circuits and it had simply reached the limit as far as fuel flow through that circuit. Or, if the main circuit has started, air flow through the venturi will not be sufficient to "pull" enough fuel to maintain the 14.7 A/F ratio you're looking for while it's making enough HP at 16.3 A/F to overcome engine friction to run at 3000 RPM. It's like the inverse of when load increases at a constant RPM and the A/F ratio goes rich. An engine will actually make more power with lean A/F ratios (leaner than 14.7, there is a limit to how lean) than rich A/F ratios...but not for long as combustion chamber temps increase significantly and you start burning valves and things. But that's under full throttle/load conditions. Under no load in your circumstance, you're not making much heat/HP to even begin to worry about excessive heat. Hope this helps.

 

The Holley Street Avengers run very lean. Bump your primary jets up 2 to 4 sizes from stock, and install secondary jetting about 8 sizes larger than the resulting primary size. Set both float levels to the bottom of the sight hole. Then, set idle mixture at about 1/2 to 3/4 turn out. This will get you in the right range, and will produce greatly improved throttle response and performance.

 

To Glass Slipper and Lars, thank you for responding and I'm sorry I didn't respond earlier. I didn't get on the computer for a couple of days.

I realized after posting that I had also forgot to increase my plug gap to .045 from .035 after installing the HEI. So based on your comments, I think what I'll try is to increase the gap, take it for a test drive (recording the data) and see where I am at, then, if necessary, I'll increase both the primary and secondary jet sizes according to Lars' recommendations.

I think I kinda freaked out by the increasingly lean A/F above idle and didn't want to hurt the engine.... ....

Thanks again for the comments, I'll update this post when I've had a chance to test.

DC118us

 

with Lars. I took his advise on my 670 SA after installing Hedmans. My primaries are now 70's and my secondaries are 78's. However my idle screws are 1 3/4 turns but since I didn't have a vacuum guage and only watched my tach for increasing rpm's I'm not sure my idle is at optimum. Anyways changing the jets made a noticeable difference.

 

No highjacking but I have to chime in on this one. Could be the answer to my madness!!!! Earlier this month I installed a 650 Holley Street Avenger out of the box and now have a nasty stumble under partial load. Stand on it and it pulls hard once you get past a slight stumble, fair idle quality. New fuel filter, plugs, wires, cap and rotor. Fuel pump last year. Timing is set at 14* initial, 30*dwell holds strong. Idle mixture screws make a huge difference when adjusting so I have them turned out to acheive the best idle quality and vacuum readings (about 14-15hg?) Also replaced the vac advance canister. All to no relief.

PLEASE KEEP THIS THREAD GOING!!!!

 

tripple check your timimg first. I had many of the same issues and messed with the carb for weeks, pump cams, power valves, acc pump, jets etc. I got to the point that I was confussed with what I had fouled the plugs replaced them and did the timimg, which was off approx 3 deg. problem solved because of 3 deg.

My advice would be to buy a Holley book (mine was $9 used from e-bay)and read it cover to cover....these carbs are soooo simple you will be able to fix anything once you have the book "Super Tunning and Modifying Holley Carburetors" by Dave Emanuel.

Here is some basic info to get you going

you can run a holley carb with 90/100 jets and it will still run, you need to run the leanest size jets to achieve the best power in a mechanicAL CARB.
Read this article, then come back to us after you did the steps....Good Luck.

ACCELERATOR PUMP SYSTEM

The accelerator pump system consists of three main components: the pump diaphragm, the pump cam and the pump nozzle. This is the carburetor system that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By accomplishing this purpose it acts to smooth the transition between the idle and main circuits so that no stumble, hesitation or sluggishness will be evident during this transition phase.

The first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm, at wide open throttle. This clearance should be around .015". The purpose for this clearance is to assure that the pump diaphragm is never stretched to its maximum limit at wide open throttle. This will cause premature pump failure. Once this clearance has been set take a good look at the pump linkage and work the throttle. Make sure that the accelerator pump arm is being activated the moment that the throttle begins to move. This will assure that pump response will be instantaneous to the movement of the throttle. These adjustments can be made by turning the accelerator pump adjusting screw that is located on the accelerator pump arm together with the pump override spring and lock nut.

The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for this pre-determined amount of fuel to be delivered is affected by the pump nozzle size.

A larger pump nozzle will allow this fuel to be delivered much sooner than a smaller pump nozzle. If you need more pump shot sooner, then
a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it's a sure bet
that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller
size is required.

Holley accelerator pump nozzles are stamped with a number which indicates the drilled pump hole size. For example, a pump nozzle
stamped "35" is drilled .035". Pump nozzle sizes are available from .025" to .052". Please note that whenever a .040" or larger accelerator
pump nozzle is installed the "hollow" pump nozzle screw, P/N 26-12, should also be used. This screw will allow more fuel to flow to the
pump nozzle, assuring that the pump nozzle itself will be the limiting restriction in the accelerator pump fuel supply system.

NOTE: When changing the pump nozzle it's best to jump three sizes. For example if there's currently a off-line hesitation with #28 (.028")
pump nozzle, try a #31(.031") pump nozzle. If you must use a #37 (.037") or larger pump nozzle, then also use a 50cc pump.

The same applies to the accelerator pump cams. Once a pump nozzle size selection has been made the accelerator pump system can be further tailored with the pump cam. Holley offers an assortment of different pump cams, each with uniquely different lift and duration profiles, that are available under Holley P/N 20-12. Switching cams will directly affect the movement of the accelerator pump lever and, subsequently, the amount of fuel available at the pump nozzle. Lay out the pump cams side by side and note the profile differences. This little exercise may help to better explain the differences between the cams and their effect on pump action.

Installing a pump cam is straighfforward. It's a simple matter of loosening one screw, placing the new pump cam next to the throttle
lever and tightening it up. There are two and sometimes three holes in each pump cam, numbered 1, 2 and 3. Placing the screw in position
#1 activates the accelerator pump a little early, allowing full use of the pump's capacity. Generally, vehicles which normally run at
lower idle speeds (600 or 700 RPM) find this position more useful because they can have a good pump shot available coming right off
this relatively low idle. Positions #2 and #3 delay the pump action, relatively speaking. These two cam positions are good for engines
that idle around 1000 RPM and above. Repositioning the cam in this way makes allowance for the extra throttle rotation required to
maintain the relatively higher idle setting. Pump arm adjustment and clearance should be checked and verified each and every time the
pump cam and/or pump cam position is changed:


Lastly, a 50cc accelerator pump conversion kit is available under Holley P/N 20-11 when maximum pump capacity is desired.

POWER ENRICHMENT SYSTEM

The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Holley carburetors utilize a vacuum operated power enrichment system and a selection of power valves is available to "time" this system's operation to your specific needs. Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number "65" indicates that the power valve will open when the engine vacuum drops to 6.5" or below. An accurate vacuum gauge, such as Holley P/N 26-501, should be used when determining the correct power valve to use.

A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should have a vacuum opening point about 2" Hg below the intake manifold vacuum reading taken.

A stock engine, or one that is only mildly built for street use, will have high manifold vacuum at idle speeds. To determine the correct power valve the vehicle should be driven at various steady speeds and acuum readings taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed.

POWER VALVES

The power valve is a key component of the power enrichment system of Holley performance carburetors. The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Holley utilizes a vacuum operated power enrichment system and a selection of power valves is available to "time" this system's operation to your specific requirements.

Each Holley power valve is stamped with a number to indicate its vacuum opening point. For example, the number "65" indicates that the power valve will open when the engine vacuum drops to 6.5" Hg, or below.

An accurate vacuum gauge, such as Holley P/N 26-501, should be used when determining the correct power valve to use.

good luck!

 

sorry I just realized that you folks have the avenger and not all the info will apply, but much of it still will.

Still buy the book you will be amazed what you can do with your carb once you have read it.

 

DC, I have been playing with the lm1 meter for about 9 months in my 71 ls6 that i am tunning for pure stock racing. I first test the car fully warmed up and look at idle mixture first. I will record a session at a stop light for about 10-15 seconds. I look for 13.50, I then drive the car under crusie conditions and record for another 15-20 seconds at 2,000, 2,500 AND THEN 3,000 RPM At 2k and 2.5k you are still on the idle circuit and should be around 14-15, by 3,000 rpm you should be off of the idle circuit and onto the main metering, your reading should get richer. I then find alot of open highway and nail it from 3,000rpm in 4th gear and run it up to 110mph(Trap speed) and try and get it at the ideal number of 12.8. Then i go home and run AND SAVE EACH SESSION, Idle , 2k, 2.5k 3k and full throttle. You must remember what the car was doing to to learn anything. I would think the rpm addition would be helpfull. I then apply the smoothing option to each graph to get a better picture of the true a/f ratio.

OTHER NOTES:
I adjust float levels once and forget them
Idle mixture, I used to use the highest vacume reading, now use the lm1 meter, found that the high vacume reading was a hair rich. I believe that anthing past one turn is to much. With all of the playing around on my car, I end up at about 3/4 out. This is on a 3310 780 vacume carb.

QUESTIONS FOR LARS OR ANYBODY
I have placed a .020 wire in the idle feed restrictors to get the carb lean enough. From what i can tell by testing the A/F ratio from a drag race start up through first and second gear is that it goes dead rich to about 11.0, no hesitation pulls clean. I was running a white spring. I bought a new secondary spring kit and notice that the new white spring was taller. I put it in and a small hesitation occured and then pulled clean, I put in the short yellow and no hesitation, but a sustantial loss of seat of the pants power> I am going to the track tomorrow with the old short white spring and see what happens. What about rear power valves? I pulled mine out and went up, jetting is now 72/84, front nozzle is stock .025, any ideas???????????

 

At the top of the page there is a bar that reads "other" click on it...to "tech tips" then "C3"...There are articles on distributor advance, timing methods and tuning Holley's by Lars...I had the "stumble" too....I methodicly followed the procedures outlined and now my car runs better than I could have imagined...I now have instant response from idle to 6000 RPM's and almost pefect air to fuel ratio. Check it out.

 

Glad to see the papers helped you out - good job with the tuning - keep up the good work!
Lars