idle help

[73ss]

I need some help getting the idle dialed in.

Recipe: 17084228,  single main air bleed.
Main air: .088
Upper idle air bleed: .070,  Lower: .077
idle tube: .038,  idle down: .058
Idle port: .090,  Bypass: .125
75 jet, 50m rod, 1/4 float.

Engine: 489 BBC, Brodix race rite oval port heads. 10:1 squeeze. Domed pistons.
pistons 10 in the hole, block has been decked.  .040 cometic head gaskets.
cam: Howards roller,  intake: 232,  ex: 246 @ .050.  .601, .612 lift.  114 lobe sep, 110 intake center line.

Timing: 20 degrees lead, stable in and out of gear. 850 rpm.

I can only get 10in of vacuum with 20 degrees lead. Engine idles fairly well, doesn't stall. Slight hesitation right of of idle. If I disable the accelerator pump, The off idle hesitation is all but gone. I assume this is because too much transfer slot is exposed.

If I close off the choke at idle, I get 11 inch of vacuum and about 50rpm increase. This tells me I'm lean on idle. Mixture screws are unresponsive past 4.5 turns out. I did open the idle down from .054 to .058. It did make a difference but it acts like it wants more. I currently have 2/3 of transfer slot exposed. Boosters are not dripping at idle. Engine will stall with mixture screws seated. Plugs are white.

Carb feels good otherwise, No stumble going to WOT from stop, May need to tweak ATP or primary rod size yet.

Any suggestions on what to do next, Increase idle tube, idle down channel? Decrease lower idle air bleed?
Thanks for any suggestions or help.

[Kenth]

I have measured mid-70´s Chevy BB and 350 HD Truck Q-jets with up to .044" idle tubes from factory.
I would say your race-engine needs larger than .038" idle tubes.
Also, most if not all BB Chevy in the high performance era uses .138" idle air bypass.
And i would open the idle needle ports to .095".

HTH

[Cliff Ruggles]

You would be better off reducing the LIAB than putting in larger idle tubes.

Before doing anything remove a small manifold vacuum hose and see if it wants or needs any additional idle bypass air?

A few days ago I set up one of those later truck carbs for a 690hp Big Block Chevy engine and used .038" idle tubes.  I reduced the LIAB's to .063", .070" uppers, and .055" DCR's with .136" bypass air.  It was well on the idle system and excellent response with the mixtures screws with only .089" holes under them.

The larger LIAB's vent enough to require much larger DCR's and idle tubes with heavy cammed engines.  Even when you open them up to compensate the response at the mixture screws isn't nearly as good vs closing down the LIAB's.   

I'd also add here that you aren't making nearly the vacuum I'd expect from that cam in a 489CID build at 10 to 1 compression with 20 degrees initial timing in it.  Not quite sure what's up with that deal but it shouldn't need much past 10 or 12 degrees timing to make 12-14" vacuum at 750rpm's with that cam.

What are the actual seat timing numbers (advertised) duration and where are they rating it at, .006" etc?.........Cliff

[73ss]

Advertised duration is : 284.6 in  298.6 ex. It's not listed as to what the rating is.

I am puzzled as well with the low vacuum and 20 degrees of lead. I did verify TDC and the timing pointer when the heads were off, spot on. I would like to think I got the quench good. The shop said pistons are 10 in the hole, I measured around .008, It was hard for me to get a precise measurement due to the domes and several thousands worth of piston rock at TDC. I went with the cometic gaskets , they have a smaller crevice volume than the fel-pro equivalents. I was expecting at least 12"s of vacuum with around 14~16 degrees of lead.

Pulling a small vacuum hose has no effect, I did block off one of the idle bypass slots in the air horn with a piece of tape. RPM's dropped but idle did smooth out somewhat, so I think I am close on that.

The throttle is open far enough that the ported vacuum advance port is just on the verge of working, But not far enough to cause nozzle drip.

I verified the ignition system is working correctly by swapping out the MSD setup with "old reliable", a known good working points distributor that I have for troubleshooting. No difference, Pops right off at the flick of the key.

 
The original '73  7043202 ? BB carb does in fact have the .138" bypass ports. Idle discharge ports measured out at .082.

Thank you for the replies!!!

[Cliff Ruggles]

That is NOT a big cam in that build.  I suspect late intake closing or the compression ratio is less than calculated, or a little of both.  The 114LSA is the right way to go with the big CID engines, they thrive on it.  I like the ICL around 109-110 as well.  I typically use cams around 244/254 @ .050" in them if they are around 10 to 10.5 to 1 and pump gas street builds.  They will act no bigger than using a 234/244 @ .050"ish cam in a 454-468 build.

My smaller 455 CID engine at 11.2 to 1 compression idles with 13.5" vacuum with only 10 degrees initial timing.  My cam is close in duration at 289/308 @ .006", 236/245 @ .050", 112LSA with the ICL at 109.5 degrees.

I've built 496-505cid engines with bigger cams than you are using around 10 to 1 compression and they idled with at least 12" with only 10 degrees initial timing at 750-800rpm's and nothing really special required from the carb in terms of bypass air, idle tubes or DCR's......FWIW......Cliff

[73ss]

Specs are @ .050 : Intake closes at 46.3 ABDC  opens at 6.3 BTDC.

                             Exhaust closes at 5.3 ATDC  opens at 61.3 BBDC.

Your thoughts? (Tried to post pic of card but I can't shrink them small enough to post)

[77cruiser]

My 421 sbc idle about 12" cam is 227/239/107/103. 18 deg initial 10 vac.
Carb has LIAB .063 move the UIAB to the cover .043 tube is .031 I think DCR is .05 I think.
This is the best one I've done.

[Cliff Ruggles]

My thoughts. 

Anytime I've found myself having to run more than about 10-14 degrees initial timing to get good vacuum at idle the cam has been just a tad big for the CID and static compression ratio.

I ran into this about 15 years ago after assembling a 455 Pontiac engine.  I did NOT choose any of the parts, just hired to assemble and dyno it.

It used the same cam I've put in other 455 builds and the same CNC ported aftermarket aluminum heads.  The only difference was this engine had lower compression, around 9.8 to 1 instead of 10.5 or so.

One wouldn't think that would be a big deal but that engine didn't like 10-12 degrees timing at idle, and it needed more idle fuel and bypass air to be happy.  I'd also add this it wasn't quite as "peppy" right off idle, at low RPM's, and it made less power on the dyno as well.

Not a deal breaker for sure, but the experience taught me that it would have been a little happier with about 5-8 degrees less duration, or a half point or more compression......

[73ss]

I'm slowly making some progress. I shrunk the LIAB's down to .063 and it did not like it. Lost control of mixture screws and had a noticeably stinkier exhaust, lost a tad bit of vacuum. Opened them back up to .070, seems better. I'm able to get a decent idle, in and out of gear, 11~12"vacuum with 18 Degree initial and another 10 from the vacuum advance for a total of 28. vacuum wanders from 11~12".

We tried a friends QFT 750 holley. With that carb, I'm able to get a steady 12" vacuum without the added timing from the vacuum advance, for a total of 18. We cracked open the secondary adjustment screw to stay of the front transfer slots to get a nice idle. Exhaust had a better note to it as well. The QFT has the 4 corner idle and only needed 7/8 turn on each screw to be happy. Better throttle response when slowly cracking the throttle open also. I probably could have taken a few more degrees of timing out at idle and still had a good idle. Nice and steady on the vacuum gauge.

This tells me that the engine is capable of producing at least 12" without the need for added vacuum advance at idle that the Q-jet wants. Not sure what to try next. It didn't like the LIAB's taken down to .063. A little better with them opened up to .070. No difference when pulling off a small vacuum line. It picks up about 30 rpm and a tad more vacuum if I partially close of the choke.
Since it responded to opening up the idle down to .058 I'm thinking opening them up to .060. Take the idle tubes up to .039~.040?
Any thoughts or tips would be appreciated. Thanks

[Cliff Ruggles]

The results make no sense to me.

IF you close down the LIAB this puts more fuel to the mixture screws.

IF you tip in the choke flap and get an improvement in vacuum richening it up, then it wants more idle fuel at the mixture screws.

In some cases when making the LIAB smaller I've also had to use smaller idle tubes and/or smaller DCR's at the same time to get control of things. 

One thing for sure is that on engines with low vacuum at idle speed smaller LIAB's produce more sensitivity at the mixture screws and clean up the idle some. 

I've also found that with some engines smaller IAB's just work better.  This simply happens when the vacuum is low and signal to the idle system is a little "weak".

Holley carbs shine in that area because they were NOT produced to have to pass emissions so very generous in the idle and off idle system, always have been.  This is good for idle with big cams but heavy fuel consumption is typically the result.

The Q-jet actually has an EXCELLENT idle system and the bypass air being a controlled vacuum leak is superior to drilling holes in plates or opening up the plates to add air.  If we raise the throttle plates in a Q-jet to add idle air they very quickly pull from the boosters (nozzle drip) as the system is considerably more efficient than a Holley style carburetor.

In any case you are still baffling me with the engine wanting all that timing and "low" vacuum even when you throw timing and fuel at it.  I've build hundreds of engines and the only time I've ran into that sort of thing was full-race stuff with super tight LSA camshafts, locked out timing, and "flooding" them with fuel to keep things happy.  In a "race" engine nobody cares anyhow.......Cliff

[73ss]

I started out with the above specs. .077 LIAB size. I closed up the LIAB's to .063. This resulted in a very rich idle and no control of the mixture screws, Very little difference with them fully seated. engine would not stall. By closing up the bleeds this provided more than enough fuel at the transfer slots to keep things running, correct? Engine would load up if I closed up the choke flap.

I then opened them back up to .070. This resulted in regaining control of the mixture screws. Engine will stall fully seated. If I close up the choke it will stabilize the idle somewhat and pick up about 30 rpm.

Any idle circuit mods also effect the amount of fuel at the transfer slots correct?

I think I need more bypass to get it off of the transfer slots and also slightly more idle fuel. I have quite a bit of transfer slot exposed, but not opened so far as to have any nozzle drip.

With the holley, I was able to ditch additional timing from the vacuum advance at an idle and have a nice steady 12" of vacuum. Tryin to get the Q-jet to do the same.

[Cliff Ruggles]

You were on the right track with the smaller LIAB's, just too much transfer slot exposed and probably DCR's a little to generous.

That engine should EASILY be pulling 2-3" more vacuum than it is without a butt-ton of timing at idle, at least by looking at the specs you provided.  I'd be real tempted to advance the cam 2 degrees, I'll bet that will make the process a lot easier.

In any and all cases we still have to give the engine what it wants to make it happy.  Believe me I've tuned engines effectively that had 5-6" vacuum at idle and struggled below 1000rpm's!  You will get there, it's just going to take exactly the right combination of timing, bypass air and idle fuel

[73ss]

That engine should EASILY be pulling 2-3" more vacuum than it is without a butt-ton of timing at idle, at least by looking at the specs you provided.  I'd be real tempted to advance the cam 2 degrees, I'll bet that will make the process a lot easier.

I did call Howard's last week for their feedback on this cam. Said it should be pulling 13", especially in a near 500 inch engine. Asked what my cranking compression was, 160 psi. (forgot to mention that in my original post). The tech said that's good but could be a tad higher. He asked if I degreed the cam. No. He suggested advancing it 2 degrees......
Also suggested getting some miles on it before getting to carried away.

I've got to give Howard's some credit. Answered the phone right away, didn't get put on hold, didn't treat me like a idiot, and didn't even bash on the Q-Jet like so many of them like to do. I went with Howard's because they had the largest selection of Gen 6 cams. All of comp's stuff for this generation was the extreme thump your mama stuff and we all know how that works out.

I do appreciate help on this. Thanks

[Cliff Ruggles]

Last time I looked it was 2021, but I still hear folks "bash" Quadrajets and regurgitate old, inaccurate and outdated information about them.

I woln't touch one made in 1965-66, and try to avoid most of them made before 1969.  By the time Rochester corrected the main issues with them it was too late and they had a BAD reputation for bogging and leaking bottom plugs.  To this day folks still think all of them are leaking and dab a big glob of monkey chit over the plugs when the "rebuild" them.

The same idiots that tell you to throw it away will tell you to put a Holley Street Avenger on it in the same breath.  What a complete and utter POS those are.  I've had locals bring vehicles up here using them and despite my best efforts I can't get them to work for chit. 

Judging by what I saw at the Car Show at First Friday in town couple of weeks ago Edelbrock Performer carbs are the "go-to" for most.  There were at least 150 cars there and at least 80 percent of them had one of those AFB clones sitting on an adapter or aftermarket aluminum intake.  Didn't really see too many Holley carbs, and not surprised and the newer varieties aren't all that great.

If I mess with a Holley carb it's going to be from the 70's or early 80's, downleg boosters and rear metering block.  My favorite is the 4781-2 850 DP and early 750/780 cfm vacuum secondary models. 

I avoid the Edelbrock AFB clones like the plague.  It's cool to see what folks do to install them.  Cut the factory fuel line with a hacksaw blade, run a long piece of rubber hose around to the left rear fuel supply and secure it with a couple of hose clamps.  Talks about a fire waiting to happen....YIKES!.......

[old cars]

Single plane or dual plane intake . Idle Rpm ? Vacuum readings in and out of gear.

Optimal idle advance is typically about 32 degrees. So you could have 12 initial and 20 vacuum advance.
You shouldn't need those huge idle tubes and down channel then.
I run a similar cam with 17080213 Quadrajet in my BBC that idles 900 rpm with 14" vacuum in neutral and 750 rpm 12" vacuum in gear. relatively smooth idle.
Next I converted it to DMAB. Idle fuel ratio 13:1 , cruise fuel ratio 15.5:1 with .031" idle tube and .046" down channel.
If you want throttle feel/response like that 750 holley you will need to work on the linkage of that quadrajet to get the secondary butterflies opening much quicker. Then you'll never look back.