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Timing Curve

General engine tech -- Drag Racing to Circle Track

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Geoff2
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Re: Timing Curve

Post by Geoff2 » Mon Oct 07, 2019 1:46 am

In case it has not been mentioned. Are you running a PCV? If vac is low, or the PCV is faulty, it will admit too much air at idle & require closing the t/blades.

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Re: Timing Curve

Post by BobbyB » Mon Oct 07, 2019 5:53 am

Geoff2 wrote:
Mon Oct 07, 2019 1:46 am
In case it has not been mentioned. Are you running a PCV? If vac is low, or the PCV is faulty, it will admit too much air at idle & require closing the t/blades.
I am running a PCV valve. It is from a 74 ford Torino 302. It seems to be in good shape. Can you recommend a PCV valve from a particular car/ engine combo that might work better?

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Re: Timing Curve

Post by Tuner » Mon Oct 07, 2019 3:06 pm

BobbyB wrote:
Sun Oct 06, 2019 9:27 pm
GARY C wrote:
Sun Oct 06, 2019 7:53 pm
BobbyB wrote:
Sat Oct 05, 2019 10:49 am


Gary, Are you saying I am starting in left field? If so, how?
Just trying to figure out if you are wanting to tune it (power/economy?) or trying to do something that no one else has done or does?
Trying to get best of both with clean plugs and a long lasting strong engine.

Today I got about an hour to test and make changes.
Quick Fuel 650 Brawler Carb was changed from 5.5 PV to 6.5 PV. IFRs down from .030 to .025.
Opened front (only) mixtures screws to 1/2 turn ( from 3/8 with 030 IRFs) and engine started right up.
While it warmed up, I sprayed carb cleaner looking for a vacuum leak but found none.

After warm up I turned mixture screws in a little at a time but saw almost no change from 12” hg & about 13 AFR. Idle speed was about 950 rpm. Finally with front mixture screws fully closed idle speed dropped to about 900 rpm. Front butterflies were fully closed. Engine was idling at 900 rpm on secondaries only with no rear transfer slot showing.
To verify this, I sprayed carb cleaner in to the primary venturies with no change in idle. Spraying carb cleaner into the secondary venturies stalled the engine. I repeated a few times to be sure.

Based on this, I closed the secondary stop screw another half turn. This brought the idle speed down to 600 rpm in park. I then opened the mixture screws 1/2 turn and opened the idle speed screw to get 850 rpm in park. Adjusting the mixture screws I got 13”hg maximum in park.

Driving the car 20 miles on country roads it ran great. The tiny hesitation from stop signs on flat ground was gone. Afr at idle was about 13 in park, 13.5 in gear at 650 rpm. Cruise afr was 14.5 to 15.5 . Up hill, with PV open, AFR was about 13.

I approached WOT in 2nd enough to open secondaries about 25%. (Based on how much my zip tie slider moved on the secondary rod) the engine pulled well and showed about 12.5 AFR.

Pulled plugs after cool down and they looked a little better, but the 2 rear plugs were a little wet shiny... I think it was fuel based on their smell. (When I pulled plugs a couple days ago, plugs 2 & 6 were wet.)

Overall, I think I am making progress. Car drives fantastic with really nice throttle response. Trying to get it perfect.

Thanks for all your input.
This sounds like you got it in the ballpark now. :)

Generally with a vacuum secondary carb and most street driven carbs, DP or VS, the secondary should be closed all they way and then just cracked open enough the throttles won't stick in the bore.

T-slot exposed to manifold vacuum on the secondary side results in erratic A/F when the vacuum varies above and below about 14-15" Hg or increases a lot from idle to cruise.

Depending on the local barometric pressure, sonic velocity and "sonic choke" occurs at near 14" Hg and the result is above that value the A/F remains relatively constant but below 14" the idle system fuel flow varies with the amount of vacuum and fuel flow varies disproportionally with air flow.
Last edited by Tuner on Mon Oct 07, 2019 3:28 pm, edited 1 time in total.

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Re: Timing Curve

Post by BobbyB » Mon Oct 07, 2019 3:26 pm

Thanks Tuner.

With the carb on the car, how do you know when the secondary butterflies are open the correct amount?

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Re: Timing Curve

Post by MadBill » Mon Oct 07, 2019 3:37 pm

A change I make the first time I have a Holley off the vehicle is to remove the secondary idle stop and reinstall it* with the screwdriver slot up, so that any further adjustments do not require carb removal. If you're looking to achieve Tuner's suggested setting, you'd back off the screw until it cleared the throttle lever, maybe flip the linkage a couple of times to be sure the butterflies are seated and then tighten it down until it just starts to move the lever. (*My preference actually is to replace it with an Allen socket set screw with a dab of weatherstrip adhesive on the thread to keep it from vibrating loose.)
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Re: Timing Curve

Post by BobbyB » Mon Oct 07, 2019 3:59 pm

Thanks Tuner & Bill.

My Quick Fuel Brawler came with an allen set screw that I can access from the bottom with out taking the carb off. It is a pain, but it can be done.

How the heck can Quick Fuel /Holley techs say to give the secondaries .020" of transfer slot??? I am way past having the transfer slot covered up and based on Tuners advice I need to close the secondaries another full turn of the screw.

With the .025 IFRs instead of the factory .030s I still have the metering screws out less that 1/2 turn. Should I try going down to .020 on the IFRs?

At what point do I change the idle air bleeds? Holley/Quick Fuel tech says NEVER TOUCH the air bleeds!

I drove the car to work today after making a couple of adjustments. It sure is fun to drive. :D

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Re: Timing Curve

Post by Tuner » Mon Oct 07, 2019 6:52 pm

How much primary T-slot is exposed now? If you want the idle/T-slot circuit to be leaner I would first try a larger air bleed. I have no idea why anyone who actually has practical carb tuning experience would advise against changing air bleeds, particularly idle air bleeds, they have the most significant effect on part-throttle, the frosting on the cake for driveability tuning.

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Re: Timing Curve

Post by BobbyB » Mon Oct 07, 2019 8:39 pm

Tuner wrote:
Mon Oct 07, 2019 6:52 pm
How much primary T-slot is exposed now? If you want the idle/T-slot circuit to be leaner I would first try a larger air bleed. I have no idea why anyone who actually has practical carb tuning experience would advise against changing air bleeds, particularly idle air bleeds, they have the most significant effect on part-throttle, the frosting on the cake for driveability tuning.
Not sure how much primary tslot is exposed without pulling carb, but idle speed screw is turned in 1/4 turn if that correlates to tslot exposure.
How much larger IAB should I try, in area percentage, to change idle AFR half a point?

Should I change IAB before going smaller on IFR? (Already dropped IFR from 030 to 025)

I closed the secondary screw another half turn before the drive home and the car drove great with idle afr at 13 in park. The engine “likes” the afr at 13 at idle to get highest vacuum of 13 in HG in park, 11 in HG in gear. If I change IAB will the engine “like” to idle leaner?

Thanks again for everyone’s help.

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Re: Timing Curve

Post by Tuner » Tue Oct 08, 2019 8:12 am

BobbyB wrote:
Mon Oct 07, 2019 8:39 pm
Tuner wrote:
Mon Oct 07, 2019 6:52 pm
How much primary T-slot is exposed now? If you want the idle/T-slot circuit to be leaner I would first try a larger air bleed. I have no idea why anyone who actually has practical carb tuning experience would advise against changing air bleeds, particularly idle air bleeds, they have the most significant effect on part-throttle, the frosting on the cake for driveability tuning.
Not sure how much primary tslot is exposed without pulling carb, but idle speed screw is turned in 1/4 turn if that correlates to tslot exposure.
How much larger IAB should I try, in area percentage, to change idle AFR half a point?

Should I change IAB before going smaller on IFR? (Already dropped IFR from 030 to 025)

I closed the secondary screw another half turn before the drive home and the car drove great with idle afr at 13 in park. The engine “likes” the afr at 13 at idle to get highest vacuum of 13 in HG in park, 11 in HG in gear. If I change IAB will the engine “like” to idle leaner?

Thanks again for everyone’s help.
Should I change IAB before going smaller on IFR?
Change the IAB first and last, easiest tuning thing to or un-do. Drop by Wm. Baldwin's and get some bleeds and advice.

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Re: Timing Curve

Post by BobbyB » Tue Oct 08, 2019 10:05 am

I hope to see William tonight. I will see he says about IABs. Thanks.

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Re: Timing Curve

Post by BobbyB » Fri Oct 11, 2019 8:00 pm

William’s advice was to try an old 750 cfm 3310-2. I hope to drive it tomorrow.

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Re: Timing Curve

Post by Tuner » Sat Oct 12, 2019 6:57 am

Probably because the T-slots in the QF base are too wide and too long and make tuning difficult or near impossible on some engines. It's much easier to fine-tune an old-school traditional carb that has the right T-slot to begin with.

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Re: Timing Curve

Post by RevTheory » Sat Oct 12, 2019 9:35 am

Tuner wrote:
Sat Oct 12, 2019 6:57 am
Probably because the T-slots in the QF base are too wide and too long and make tuning difficult or near impossible on some engines. It's much easier to fine-tune an old-school traditional carb that has the right T-slot to begin with.
Wow. When looking at the myriad of Holley-style carb options out there, how does a guy know whose version has a funky this or a funky that?

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Re: Timing Curve

Post by Knight rider » Sat Oct 12, 2019 4:30 pm

This might have been asked already, but how was the cam timing set and what heads are you running. Cheers

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Re: Timing Curve

Post by BobbyB » Sat Oct 12, 2019 5:41 pm

Knight rider wrote:
Sat Oct 12, 2019 4:30 pm
This might have been asked already, but how was the cam timing set and what heads are you running. Cheers
Cam timing was set per the Cam card, 106 icl for 224/228 at .050, 110 lobe centers .528” hft. Heads are older afr 165 s with 1.94 intake valves that flow about 255 cfm max.

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