Hi all. It's been awhile since I've been around here (formerly LTLCRZY427). Needless to say I've not been playing with the car for quite some time. Anyway...

Completed an engine swap this week (with the immense help of my brother). New combo is a 468, 10.25:1 compression, AFR 315 heads, XR288 hydraulic roller cam (236, 242 dur), Performer RPM with a 950 HP Holley on top. Engine starts and runs but is very rich at idle. I have a LM-1 wideband O2 and it shows 11-12:1 at idle. I adjusted the mixture screws but with 3/8 turn out it still is rich. I can go further, but I was under the impression that that is too far in. The idle transition slots (in the primary bores) are barely exposed, just a small square is visible. The rear throttle blades are completely closed (had to do that to get the transition slots to even appear). Float level is set as well. 10* timing at idle and it pulls 12" of vac at 750 RPM. 79 jets all the way around and the air bleeds are 32s with 74s in the high speed ones (basically carb is untouched from Holley at this point).

In driving the car around the AFR at cruise is low as well, tho it leans out slightly when the pedal is pushed halfway to the floor (no secondaries). I'm not concerned with this part as much as the idle (first thing is first).

I tried putting some small pieces of wire in the idle feed restrictions today with little success. (Before anyone lambasts me, I made sure there was no way they'd get sucked into the engine or metering block. :cool: ) After that there really was no change in the idle AFR. I'm still at 12:1. I had no scientific method of picking the wire I stuck in there, and I'm unsure what size IFR it had from Holley. Going to pick up some really small drill bits tomorrow and see what's in there.

I may be jumping the gun a little here, but I have a copy of a magazine article where they tuned the carb using a LM-1 wideband. Funny enough it was very similar to what a carb shop did with my previous engine/carb...so I'm trying to do it myself this time. Buying the LM-1 instead of paying someone to tune my car.

Anyhow. Anyone done this sort of thing before? Any pitfalls to look out for, or any other suggestions for where I'm headed? I'm certainly no expert but willing to give this one a try. Thanks in advance.

You might want to read up on Holley's recommendation about putting holes in the throttle blades, I bought the Holley book years ago and it's been a great help.

If your idle transition slots are barely showing, that's good, but turning the mixture screws in should kill it. Based on this info, I'd look at the power valve. It's either opening at idle, or blown. I had the exact symptoms on my buddy's 440 Dodge pickup yesterday, power valve was blown. He's running an 850 Demon, but it was doing exactly what you described. Swapped in a new primary pv, truck idled down smooth, ran great again.

maybe some more advance. 10 deg seems very light for that cam at idle. I would think 20 is closer to what you need with a total at 38 ish.
See if Scott W will jump in here and offer some advice.

I would agree with Levon, more timing. I'm at 22*/36*, and I have less cam. Your getting pretty good vacuum though as I only get 11"-11.5" at 800rpms w/less duration.

You can try dropping the primary jetting till you get a stumble/cruise, then go back up a notch or 2 till it dissapears. This would lean everything out a bit since the mains feed the other circuits, I'm positive though that carb comes with an agressive calibrations (richer) for power. All the 750/800/850 DP I've messed with had 12.5 AFR across the board, the vac sec were leaner. Probably why the DP's always seemed to run a little crispier to me.

Doesn't that carb have adjustable IFR's? Get some more timing in there first though.

some might not agree, but the carb is prob. too big, untill high rpm's, although some people use them-also, if what you are saying about the airbleeds, their reversed-the high speed ones should be .032, not .074-the idle should be the larger-
your jetting is small for a 950, was that the factory jets? a 236 cam is prob. gonna be a little dirty at idle anyway, but the vac. seems pretty good, prob. because of the dual plane-you could try cracking the sec. a tiny bit, might let some air in and help-make sure the n/s's are good and clean, not letting fuel slip by-if you had some spare idle air bleeds, you could try opening them up a little, or just remove one and watch the lm-1 and see what happens
dont drop in down the carb though

Read this page linked below and take note of the information about accuracy. People put way to much stock in these meters. They have been tuning carbureted engines for a 100yrs without them and you can too.
Mike

http://www.autospeed.com/cms/A_1595/article.html

Your intial timing @ 10' is very low. If you have a MSD distributor install your black bushing total timing. Your main jet doesn't come into the picture at idle by the way. The stock jetting is 78 square.

Dave

I agree 10* is too low, good spot I missed that one.

Will check the power valve. Brand new carb but I did have one backfire through the carb. I thought all new Holley's had PV blowout protection. I could be wrong. Easy enough to check since I have to pull it apart anyway (I'll be a pro at dissassembling this once this is all over with. ;))

Air bleeds...correct. The larger ones were the air bleeds and the smaller were the HSABs. Got typing a little too fast.

Will up the timing at idle, tho I see that raising the idle speed, meaning I have to back off the idle speed screw. That would most likely completely cover the transition slots. Maybe that would help.

Throttle blades came drilled from Holley...all four of them have a good sized hole pre drilled in them. I thought that mod was for trying to get the transition slots covered properly, which it appears I have correct.

IFRs. I think they are adjustable...tho you have to pop them out and get new blanks and drill them. Just little brass plugs it appears. Anyone ever messed with them?

Thanks...keep 'em coming and I'll keep you guys updated.

What is your fuel pressure at idle and what is your idle speed?
Mike

Unsure of the fuel pressure. Don't have a gauge for that one. Pump is new Carter Strip pump. Rated at 7.5 psi I believe.

Idle is at 750 RPM.

Timing is a very good point. Larger cams like more timing...I'm running 28 degrees at idle although that is with vacuum advance connected.

Holley does seem to always provide a richer than expected carburetor...

Good Luck.

Didn't get a chance to play with the timing this evening.

At the recommendation of a local shop I plugged the holes (factory Holley drilled) in the throttle blades. With the holes plugged, and the secondary blades closed I was able to get a better idle out of it...in the neighborhood of 14:1. The vac has dropped to 10" at idle (750 rpm). I'm still a 1/4 turn out from seated on the mixture screws. (No wires in the idle restriction.)

Now the car has a lean stumble to it off idle. A definite lean stumble actually. Just after the stumble it goes back to rich (like 11:1) and stays there until about 2200 rpm where it goes real rich (like 10:1) and another stumble. I'm guessing that's where the main jets are coming in and just dumping more fuel on my problem.

Local shop feels that much of this can be cleaned up with the air bleeds, and they are probably right. Too far out of whack and maybe the idle restriction is the answer.

Another weird situation I have a question on. While driving around I noticed that if I put the car in neutral (while rolling along at say 25-30) the engine would go way lean. (16:1+) It's not just a lean spot when the throttle blades snap closed...as long as the car is rolling along (and after it comes to a stop) it's lean. Any idea where that is coming from?

Thanks guys.

Your getting way too deep into the carb IMO too fast. You need to add to that timing, it makes a HUGE difference in evrey respect.

There's guys here that are running 10 sec 1/4 miles here with that carb out of the box.

So you say the vacuum dropped as you got a better idle???

I would think 10 inches of vacuum is way low for that cam with the static compression you have. The valve adjustment may indeed help this...knowing that if they are out of adjustment, it can make an engine run very odd! .25 turn from fully seated is not the sweet spot for the idle mixture screws, but you know that. Something is out of whack.

Leanness when coasting...don't coast! You should always be at WOT or hard on the brakes...only way to drive...ok that is on the track...

Is the exhaust popping during this time? What negative is there? If none, then I don't think it is a worry-carbs way of saving fuel!

Maybe if you unscrew the 950 Holley and insert a 407 SBC beneath it...guaranteed to run better! Ha.

:-)

Time for another update.

Have gotten the mixture to a 13.5:1 at idle using a turn out on the mixture screws. Have a solid 750 RPM idle. I think a recent valve adjustment along with upping the initial timing to 14* helped...Also upped the idle air bleeds to 85. I hesitate to give it more initial timing, for the fact that I would have to limit the amount of centrifical advance then. I'll be looking into that here next week.

Car pulls 9.5" of vac. at idle. Seems low, but it drops sharply when setting the idle at 750...a little higher and the vac is at 12".


Engine runs much better. Very little hiccuping while driving around. Only thing is it is still rich at cruise RPM. Like 1700-2500 it's between 11:1 and 12:1. Would you lean toward a jetting issue or power valve being too large?

Can you hook up a vacuum gauge that you can read while your driving? That might be helpful. Does the car still smoke at WOT?
Mike

Andy,you issues are very similar to mine. I've been using the LM1 from day 1 this year on this new engine. The only good thing for me is WOT is dead on,but idle and cruz speeds are pretty bad like yours. I've been through it all except air bleeds.

Glad to hear your making progress. Jetting might be too large in primaries for cruz speeds and checking fuel pressure would be ideal too.

69 CHVL e gave you good advice get that dist recurved to give you the 18-20* you need at idle plus VA .....then reajust mix screws for best vacuum if you get A?F you want but vacuum fell off then increase idle rpm A/F will pbly get better too and AF........then do air bleeds ...........I'll guess those 85"s too lean pbly 80 or 78 will be close, 750 idle seems alittle low for a HP950 to properly function ...FWIW

Curious what is the 950's power valve number?

950 has a 6.5 PV front and rear.

Car runs pretty well now. Still richer than I would like to see cruising around. Took a few logs tonight...Have to see what they say tomorrow night and let you know what I'm getting for AFR at WOT.

Thanks guys!

try putting lower pv in say 4.5,which is about 1/2 your vac reading@idle. the lower the number iab the richer the mixture, (smaller hole less air) if you get an irratic idle when you let off the gas,the idle circuit is too lean. after you get this mess straight then you need to work on the jets and squirters

tm69408,

Holley PV are all the same size, the number on them refers only to the vacuum at which they open. the "size" part of the deal is determined by the drilled hole in the metering block behind the valve.


>>the lower the number iab the richer the mixture, (smaller hole less air)

PV's don't meter air at all, just fuel.

There's several good books on Holleys available, you might try reading one or two before posting about how to tune a carb.

What is your total timing goal? Is it around like 36'? If it was me I would install the 18' bushing its the black one. So your intial timing would be 18' have it all come in around 2500 to 3000 rpm. A lot of guys with these erratic cams are locking out their advance which I don't agree on with a street car. But try running as much intial as possible, I am sure it will help. Put your jetting back to stock which is 78 square or 78 primary 86 secondary if you already ditched the rear power valve. Stock Holley jetting is always really close.

Dave

Were is your sensor mounted?

What is your total timing goal? Is it around like 36'? If it was me I would install the 18' bushing its the black one. So your intial timing would be 18' have it all come in around 2500 to 3000 rpm. A lot of guys with these erratic cams are locking out their advance which I don't agree on with a street car. But try running as much intial as possible, I am sure it will help. Put your jetting back to stock which is 78 square or 78 primary 86 secondary if you already ditched the rear power valve. Stock Holley jetting is always really close.

Dave

Bingo ........exactly what you should do ....with your cam and vacuum reading the 6.5 PV is fine I have alot more cam and run a 6.5 .........stock jetting and PV and even the Air bleeds should be very close for your set up your issue is timing .focus on the total timimg at your idle speed sb 18-20 depending on idle speed plus vacuum advance ........ I have a Pro Systems 950 HP and my carb set up with bigger cam is very close to daves suggestion in fact 76 primary may be right with proper air bleeds and definitely 86 no rear PV ....all you should be doing to get your A/F at idle sorted out is timing first .....then idle mixture / idle speed then air bleed size

>>the lower the number iab the richer the mixture, (smaller hole less air)

PV's don't meter air at all, just fuel.

There's several good books on Holleys available, you might try reading one or two before posting about how to tune a carb.

Hey Tom you might consider following your own advice.

The idle system supplies fuel at idle and low speeds. The idle system requires a richer mixture than at cruise speed. Unless the
idle mixture is richer, a slow and irregular combustion will occur, known as a rough idle. Decreasing the IDLE AIR BLEED size
richens the idle mixture by increasing the pressure drop in the system. Increasing idle air bleed size leans the idle mixture by
reducing the pressure drop across the idle air bleeds. The same conditions can be created by backing out the idle mixture
screws, which will increase the pressure across the idle air bleeds, pushing more fuel from the idle well creating a richer fuel/air
ratio. The idle mixture screw is the only adjustment recommended for controlling the idle fuel/air mixture richness or leanness.

950 has a 6.5 PV front and rear.

Car runs pretty well now. Still richer than I would like to see cruising around. Took a few logs tonight...Have to see what they say tomorrow night and let you know what I'm getting for AFR at WOT.

Thanks guys!Have you checked out the LM-1 forum? There's some extensive tuning going on over there, air bleeds and all.