Carb Tuning

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BobbyB
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Carb Tuning

Post by BobbyB » Tue Sep 25, 2018 10:24 am

When tuning the idle circuit of a vacuum secondary holley/quick fuel, how far open should the vacuum secondary butterflies be relative to the front butterflies?

How much should you play with the relative opening of them?

What happens if the front is open too much?

What happens if the rear is open too much?

I have read a couple carb books, but have heard conflicting information from people much more experienced than me.

I am working on a 306 ford with hydraulic flat tappet. 13 degrees overlap at .050 cam. 10.4 to 1 compression, performer rpm airgap. Auto trans with stock converter. Trying a 600 holley & 650 quick fuel, both vacuum secondary carbs. I am using a AFR gauge and vacuum gauge to tune.

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Re: Carb Tuning

Post by mag2555 » Tue Sep 25, 2018 12:18 pm

If you or someone else has not messed the linkage adjustment up then leave them as is and don't F with it!!!!
And leave the stock secondary spring in there also!

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Re: Carb Tuning

Post by BobbyB » Tue Sep 25, 2018 1:14 pm

What linkage? There is no linkage on a vacuum secondary carb. The front butterflies have a screw (the idle adjustment screw) and the rear have a separate screw on the bottom side of the carb.

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Re: Carb Tuning

Post by Tuner » Tue Sep 25, 2018 1:31 pm

The link that pushes the secondary closed should be adjusted so at 1/2 to 3/4 throttle both P and S are open a similar amount, so P and S flow is balanced at part throttle. This usually leaves some slack at the closed position. The vacuum servo pulls and holds the secondary against that link at high RPM part-throttle and the P and S move together in a 1-to-1 relationship. Because some folks think the link should push the secondary fully closed they bend the link and that upsets the mid-pedal range the link should be adjusted to synchronize.

99% of the time it is best to have minimum opening of the secondary throttle, just crack it open enough to prevent it sticking.

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Re: Carb Tuning

Post by MadBill » Tue Sep 25, 2018 3:59 pm

Re the idle tuning aspect of vac. sec. carbs, Ideally, the secondary stop should be adjusted as per above. However, I've often found that the (non-adjustable in most/all vac. carbs) secondary idle mixture is incorrect, usually too lean, in which case, for best results the blades need to be closed absolutely as much as possible, allowing more overall mixture control from the primary idle screws.
*If any amount of tweaking is anticipated, it's best to replace the upside down slotted screw with an Allen head set screw, installed from above for easy access. A dab of non-setting adhesive will keep it from backing off.
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Re: Carb Tuning

Post by BobbyB » Wed Sep 26, 2018 12:59 pm

Thanks for the replies.

My problem is that I need to open the front butterflies so much that I am showing too much of the front transfer slot. Why can't I open the rear more and close up the front?

Why not open both front and rear the same amount at idle?

Mad Bill, I removed the slotted rear screw from the 600 Holley and found it to be oversized in diameter compared to a hex 10-32 set screw. There was enough difference that I did not even try the hex screw. What type of non setting adhesive do you recommend if I use the hex set screw?

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Re: Carb Tuning

Post by rq375 » Wed Sep 26, 2018 2:09 pm

that is how I set them up, get the transfer slot exposure where you want it then open the secondaries as needed. On the screw, try teflon plumbing tape.

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Re: Carb Tuning

Post by 427dart » Wed Sep 26, 2018 2:15 pm

What kind of timing are you running? Do you have a working PCV system? Both of these will affect your idle speed and help with getting a good idle with less throttle opening.

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Re: Carb Tuning

Post by treyrags » Wed Sep 26, 2018 2:49 pm

BobbyB wrote:
Wed Sep 26, 2018 12:59 pm
Thanks for the replies.

My problem is that I need to open the front butterflies so much that I am showing too much of the front transfer slot. Why can't I open the rear more and close up the front?

Why not open both front and rear the same amount at idle?
Because during high vacuum conditions like you have during medium to high speed cruising you will have excessive fuel drawn through the rear slots if they are exposed too much

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Re: Carb Tuning

Post by BobbyB » Wed Sep 26, 2018 4:49 pm

Thanks 427Dart

I have tried between 15 to 18 degrees initial, 29 to 32 degrees total at about 3000 rpm. Right now it is set at 17 & 31. (As I recall, at work, notes at home.)

I have tried both ported vacuum & full manifold vacuum on the vacuum advance. Ported vacuum seems to work best for me. With full manifold vacuum and all the vacuum advance it will take before rattling on a small hill I see vacuum readings of 11 to 12" hg in park but it drops to about 6" hg in gear and vacuum advance goes away ( I think they call this the auto trans full manifold vacuum advance death spiral).

If I use ported vacuum, I see only 10" hg in park but it only drops to 7" in gear and is stable. Using ported vacuum seems to use the vacuum advance to help you, where as using full manifold vacuum the vacuum advance seems to go away just when you need it most... off idle or pulling away from a stop sign causing a "flat spot".

PCV system is functional. I asked someone with much more experience if I should consider the PCV system and they said no, so I left it alone. Do you have any suggestions for the PCV system?

rq375, Thanks for your reply. Playing with the opening of the secondaries seems to be helping me. I am trying to understand if I should keep playing with them or leave them alone.

The engine has about 700 miles on it. I used a 600 holley to break the cam in and switched to the 650 quick fuel after maybe 100 street miles. The quick fuel would not restart when hot, but ran OK otherwise. I found that fuel was leaking into the rear secondaries through the transfer slots when the engine was shut off. The fuel would get about 1/4" deep on top of the rear butterflies and could be "sopped up" with rolled up paper towels stuck down beside the rear boosters. I sent the carb back to quick fuel. Quick Fuels conclusion was that there were debris in the secondary side causing fuel to siphon through the rear transfer slot(s). When they returned the carb, they had the rear butterflies much more open than when the carb was new. (I had never touched them.) They were open so far that the car would idle with the front butterflies fully closed and both idle mixture screws fully closed (so, it would idle on the secondaries alone). I closed the rear secondaries up some, but wonder how best to set them.

I made the following changes to the quick fuel carb: Idles jets, .030 from factory, now .025. Main Jet 68 from factory, now 66. Power valve 6.5 from factory now 3.5 (based on quick fuel recommendation.)

Right now I have it idling 1000 rpm in park with AFR about 12.5 (notes at home). In gear, 650 rpm AFR 13.5 to 14. Pulling away from a stop sign Afr drops to 15 or 16, but no real flat spot. Cruise is 14.7 to 16. With power valve open AFR drops to about 13. With secondaries open it is too rich at 11 to 12.

It is using some oil and I don't know if the fuel wash may have hurt it. I figure to drive it while the weather permits and see if it seals up. I need to do a compression test.

Cam person says I should see 15" hg at idle, but I can't get there (yet). Engine has about 700 miles on it & a lot of gas leaked into it from the secondaries of the quick fuel carb before I realized what was going on and sent it back.

What do you think I should change next on the quick fuel? Overall the car drives fine and has good power. I just want to tune it as well as I can myself.

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Re: Carb Tuning

Post by 427dart » Wed Sep 26, 2018 5:25 pm

Looks like you have the carb dialed in well for idle and cruise and part throttle accel. I would lean the secondaries down till you see 12.8-13.0 at WOT up thru 5000 RPM in high gear. I like to do my carb A/F tuning on days with 75 degree temps and 35-45% humidity here in the Mid-Atlantic area.

I have my carbs set where a slight Idle Air Bleed jet change gets me where I want during Spring,Summer and Fall seasons for cruising A/F ratios.

You want the secondary plates just cracked open so you can maintain a good idle 800-1000 RPM in Neutral with the primary transfer slot looking like a square hole from the bottom of carb.

With my dual Holley 650 setup on my 427W I really had to juggle things to keep the idle down to 950-1000 RPM.

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Re: Carb Tuning

Post by BobbyB » Wed Sep 26, 2018 6:25 pm

Dart, do you use full manifold vacuum,or ported vacuum for vac advance?

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Re: Carb Tuning

Post by MadBill » Wed Sep 26, 2018 7:25 pm

BobbyB wrote:
Wed Sep 26, 2018 12:59 pm
Thanks for the replies.
My problem is that I need to open the front butterflies so much that I am showing too much of the front transfer slot. Why can't I open the rear more and close up the front?
The common fix for this is to drill a small hole in each primary butterfly, which will provide the needed air without opening the TBs too far. Start small, say 0.0625" and go up by small increments; it's easier to make the holes bigger than smaller.

Most guys seem to drill the "up" side of the TB, but I don't see why it matters.
Felix, qui potuit rerum cognscere causas.

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Re: Carb Tuning

Post by 427dart » Wed Sep 26, 2018 7:59 pm

BobbyB wrote:
Wed Sep 26, 2018 6:25 pm
Dart, do you use full manifold vacuum,or ported vacuum for vac advance?
Well I don't run a vac. advance. The distrib I use is an old Accel Billetech Race and I run 18-20 at the crank with a total of 32-34 degrees all in by 2000 RPM.

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Re: Carb Tuning

Post by BobbyB » Wed Sep 26, 2018 8:27 pm

Mad Bill, the man that specked my cam says I should have 15 in hg at Idle, I get 12 at the most. He says absolutely no way do I need to drill holes in the blades. I think the only way to get enough air into the engine at idle is to open either the front blades too much or the rear blades way more than “just cracked”.

Where should I be looking for my missing 3 inches of idle vacuum? I am satisfied that there is no manafold or carb vacuum leak. Could my new rings be so bad from the fuel that siphoned through the secondaries? Could the afr165 heads that I reworked have such terrible reversion that they kill the vacuum at idle?

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