Dialing in my Holley Carb

[sstock]

OK today, we had a better weather day in the Pacific Northwest, not exactly sunny but it was cloudy dry and about 55 degrees.

I took this opportunity to weld in a bung in the exhaust collector for my wideband and started the tuning of my engine. Just to give an update, I'm running a Holley Street Avenger 570 CFM with a vacuum secondary, electric choke 4150 series carb. It sits on a Weiand single quad tunnel ram. Engine is a bored 283 to 301 cu in., basically a .125 over bore. Have a .450 lift cam with 224 degrees of intake duration at .050 and 230 degrees of exhaust duration at .050. Stick is setup 4 degrees advance on 110 degree lobe separation. Engine starts cold on one crank and comes up on fast idle. After it was warmed up, I set the idle mixture on all four corners of the carb for best manifold vacuum which is about 11 inches. The carb is rich on warm idle with A/Fs of about 12.5 to 1. Went for a easy cruise and main metering is also rich with A/Fs in the mid 12's holding 35 mph on level ground, I would of liked to of seen it more towards mid 14s there so I'll need to jet the main jets down (it came with 64s), then I can work on the pump shot which I changed to a 35 squirter, but it still isn't enough. The engine was popping back through the carb wit the 31's that came stock with the carb. The pop is gone but it isn't still rich enough with A/Fs going into the 15s and 16s with throttle tip in. Secondary side of the carb is totally out to lunch , extremely lean. As soon as I started to press down I could see we aren't even in the ballpark. I'll get the main jets accelerator pump done first then proceed with power mode after that. I'll need another power valve too it is a 8.5 stock, probably go with a 5.5. I don't know how anyone can tune a carb by just reading plugs, I've had this wideband for about 5 years now and it takes all the guess work out of it. Now I've seen it first hand: cold startup on the money, main metering pretty rich, pump shot very lean, power mode lean, secondaries extremely lean. I'm pretty confident though I'll get the single quad tunnel going down the road without too many issues. If anyone has any ideas please post up, I always value input and help in any way.
Thanks in advance,
Steve

 

[railroad]

Your manifold vacuum at idle seems low to me, for that cam. This should be interesting.

 

[sstock]

I thought so too, but it does have fewer cubic inches than the norm. I ran a similar cam on a 350cuin some years ago and it had about 14" of vacuum. If I bump the idle up a hundred rpm it does much better on the vacuum.

 

[2old2fast]

If you go down to a 5.5 PV , the enrichment will come in later rather than sooner, if you're on the lean side already, may be the wrong way to go...
Hook up your vacuum gauge & watch the relationship between vac/a/f at the point of acceleration, might be helpful info..
dace

 

[sstock]

Dave, you have a good point, maybe I'll wait on the PV until I get the main jets squared away. The more I analyse this I might just keep things on the rich side of things during cruise and WOT because of the big plenum of the tunnel ram, it will probably won't suffer nearly the drivability problems of a leaner tune. A buddy of mine, whos opinion I really value said I would of been better to of bought a 650 double pumper based on my stall speed of my convertor and shorter gears that I'm running, even with the turbo 350 trans.

 

[2old2fast]

...
dave

 

[Wild Mango]

Another way to look at it might be that with the lamda sensor in the collector you are reading a sort of an average of the four cylinders feeding it. Tunnel rams can be tricky to achieve even mixture distribution at idle, let alone with the hammer down, and with a little engine and one carb you can rightly be proud of your efforts when she runs sweet.
Thats a lot of advance on a designed ICL, IMHO.
P.S. Is your sensor a two or four wire type (heated or not) ?

 

[sstock]

Wild, it is a heated O2. It is a LM1 from Innovate, pretty neat little tool that I have been using at the race track the last few years, really takes any kind of guess work out of the equation. The cam is a comcams grind, it surely would of idled better if it was ground on 112 LCA

 

[Mike]

Sounds like you're getting pretty close. Have you tried moving the accelerator pump cam, to see how that works out for you? With the big plenum, you're going to want things to be wetter than you would with a dual-plane intake. You've a lot more manifold area to hide with pump shot. A 50 CC pump conversion would likely make life a bit easier for you.

Out of curiosity, how about a taste more ignition timing? You'll likely clean up some of the idle A/F numbers and be able to tune yourself into a higher vacuum number, as well. You might stick 1° in it and see if it wants to play nice.

Some plenum stuffing is something to consider. A little hard to do with that particular plenum design, but killing some of that volume would really start bringing things back to your side of the table. If you're careful with how you think it through, you can really wake things up.

Rolling the cam forward is helping you build cylinder pressure, but it's not always a win-win fix, because you're also opening the exhaust valve earlier. Doggone the trade-offs we have to endure, right? I say to the devil with people like Newton and Einstein.

And you're right. It's impossible to read plugs in a street-driver. A few seconds of idle time can completely wipe out what WOT will clearly indicate.

 

[2old2fast]

Something you're probably aware of is how much heat affects especially idle settings. I've found that I need to Drive at highway speeds at least 10-15 minutes in order to be able to adjust the idle & have it be stable, also, you want to be slightly on the rich side.

dave

 

[Wild Mango]

Wild, it is a heated O2. It is a LM1 from Innovate, pretty neat little tool that I have been using at the race track the last few years, really takes any kind of guess work out of the equation. The cam is a comcams grind, it surely would of idled better if it was ground on 112 LCA

Same sensor I use. A real time saver. Mikes comments about the cam timing same as mine would be - thats where the vacuum went. Newton should have stuck to being an orchardist

 

[sstock]

Tuning this carb has been a chore. Right now I have the car starting up nicely cold, with high idle and kicking off when reasonable warm. You can actually start it throw it in drive and pull out of the driveway onto the main road and accelerate up to speed without a hiccup. I played with the timing curve and idle speed to get 13 inches of vacuum and I stayed with the primary jet size of 64 and they put the main metering cruise in at about 13to 1 AFR. Been trying to taylor the right size pump shot nozzle and accelerator pump cam to get rid of the lean bog when the vacuum secondaries begin to open. I can get by the lean bog if I have enough rpm in the motor, seems at lower rpm the carb is loosing the signal. Seconday jets I went all the way from the stock 68 to 80 to get a full throttle A/F of 12.5to 1. Didn't matter if I used a 8.5 or 6.5 power valve. As it stands now, I'm back playing with the squirters. Stock was 31, right now it has a 35. I have a 37 and 40 as well as a 50cc accelerator pump kit and plastic cams to go with. I am cautiously optimistic that I will close in on it.
BTW, the tech at Holley thinks I am p!ssing in the wind, he said there is no way to get a tunnelram to live on the street with a single vac seconday carb, he wants to sell me a 600 double pumper, he might be right, time will tell, I don't give up easy, and have a few more things to try.

 

[Screaming Metal]

OK....I've been sitting on the sideline watching and listening....I believe essentially what you have is a leanout there between the transitions. Its very common on street driven TRs, gotta ignore most folks that tell you you can't drive with one on the street. If it were a 350, that stumble probably would be a cough.
Anyway, yes, play with the accelerator cams, you should have a set of holes on the bellcrank, put it on the first set of holes, where the cam will activate the pump arm quicker. What you want to do IS Hold that pump lever down longer.
Get it to shoot sooner (1st set of cam holes), then put on a cam that will shoot longer. That 50CC kit it gonna come in handy because I think its gonna come in handy.
On a tunnelram, it weakens your vacuum signal. The powervalves are marked to open at certain preset vacuum levels. WHAT WE WANNA DO HERE IS KINDA FOOL THAT POWERVALVE till we get our rpms up where things are a little better.

Idle it about 800 rpm, back out your screws on your front metering plate till you richen it up till the rpm starts to fall, your gonna be fat at this point. Do both sides, readjust your idle rpms. Now, using the 35 Squirter, back off on that adjuster with the spring that actuates your front accelerator pump, to where your almost holding the pump open at idle.

What you need to do now, is have your secondary adjusted with the a middle of the road spring. I'd say when you have the thing to where you crack the throttle hard, the rear 2 tilt open at about 4000.
With it richened up, it will be easier to tune. Then just take the fat outta it where its needed.

Don't give up on that timing either....my old 283 that I ran for years on the street, she liked for me to take some of the exhaust timing out on the exhaust valves by opening up the lash on the exhaust side.

 

[Screaming Metal]

It does sound like your really close....

 

[2old2fast]

I'm really surprised that the sec. jets were that far off,especially on an alleged "performance" carb & your small C.I. engine! AFA the P.V. is concerned , if your floorboarding it , it really doesn't make alot of difference if it's a 2.5 or a 10.5, as soon as the throttle blades are wide open the P.V. "sees" near 0 so it's open. The power valve was intended to add fuel during LOW vac. [pulling a heavier than a "T" vehicle] so it's function is somewhat diminished, it still adds needed fuel, but only under WOT operation.. I don't know what the biggest shooter you can get is , but don't forget that the shooter screw can also be a restriction..

dave

 

[sstock]

I'm really surprised that the sec. jets were that far off,especially on an alleged "performance" carb & your small C.I. engine! AFA the P.V. is concerned , if your floorboarding it , it really doesn't make alot of difference if it's a 2.5 or a 10.5, as soon as the throttle blades are wide open the P.V. "sees" near 0 so it's open. The power valve was intended to add fuel during LOW vac. [pulling a heavier than a "T" vehicle] so it's function is somewhat diminished, it still adds needed fuel, but only under WOT operation.. I don't know what the biggest shooter you can get is , but don't forget that the shooter screw can also be a restriction..

dave

Thanks Dave, I believe I need the hollow screw at about .040. You info on the power valve seems logical. My thoughts too on the secondary jets but seem to remember a long time ago when I played with tunnelram and holley it was the same thing too. Thinking it has something to do with plenum volume. Holley even alludes to huge jets size jumps with TRs.

 

[sstock]

OK....I've been sitting on the sideline watching and listening....I believe essentially what you have is a leanout there between the transitions. Its very common on street driven TRs, gotta ignore most folks that tell you you can't drive with one on the street. If it were a 350, that stumble probably would be a cough.
Anyway, yes, play with the accelerator cams, you should have a set of holes on the bellcrank, put it on the first set of holes, where the cam will activate the pump arm quicker. What you want to do IS Hold that pump lever down longer.
Get it to shoot sooner (1st set of cam holes), then put on a cam that will shoot longer. That 50CC kit it gonna come in handy because I think its gonna come in handy.
On a tunnelram, it weakens your vacuum signal. The powervalves are marked to open at certain preset vacuum levels. WHAT WE WANNA DO HERE IS KINDA FOOL THAT POWERVALVE till we get our rpms up where things are a little better.

Idle it about 800 rpm, back out your screws on your front metering plate till you richen it up till the rpm starts to fall, your gonna be fat at this point. Do both sides, readjust your idle rpms. Now, using the 35 Squirter, back off on that adjuster with the spring that actuates your front accelerator pump, to where your almost holding the pump open at idle.

What you need to do now, is have your secondary adjusted with the a middle of the road spring. I'd say when you have the thing to where you crack the throttle hard, the rear 2 tilt open at about 4000.
With it richened up, it will be easier to tune. Then just take the fat outta it where its needed.

Don't give up on that timing either....my old 283 that I ran for years on the street, she liked for me to take some of the exhaust timing out on the exhaust valves by opening up the lash on the exhaust side.

Thanks for the .02 Screaming and spending the time for the lengthy reply. I have read it twice now, to not miss a detail. In fact your idea to reset the idle mixture screws to the rich side as opposed to the lean side (like I have it) jogged my memory, that the carb function on the main jets does change with this change, enough so it might have a desireable effect. Great idea there. I'll tighten up the spring on the AP like you say too. It has been a chore but I really enjoy challenges like this. Hope to have a favorable report soon.
Steve

 

[Flatsman]

Good luck with your Carbs Steve! I will be right behind you, and it sounds like you are getting really good advice. I haven't played with Carbs for for a long time but I really enjoy it too. It is pretty Cave Man compared to what we are used to messing with. My carbs should arrive in the next few days but I am still a little ways from tuning them. I do plan to put rebuild kits in both of them and check them out pretty good before they go on the car. Take Care and best of luck!

 

[Screaming Metal]

Thanks for the .02 Screaming and spending the time for the lengthy reply. I have read it twice now, to not miss a detail. In fact your idea to reset the idle mixture screws to the rich side as opposed to the lean side (like I have it) jogged my memory, that the carb function on the main jets does change with this change, enough so it might have a desireable effect. Great idea there. I'll tighten up the spring on the AP like you say too. It has been a chore but I really enjoy challenges like this. Hope to have a favorable report soon.
Steve

Steve, got faith in ya dude! Your close.....really close, hang in there....

 

[2old2fast]

I'm curious, what RPM did you run to & were the A/F readings consistent under WOT ?

dave