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Timing Curve

General engine tech -- Drag Racing to Circle Track

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BobbyB
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Timing Curve

Post by BobbyB » Thu Oct 03, 2019 1:53 pm

Can too much base timing cause carb tuning trouble?
New 347 sbf, 10.4 to 1, 224-229 at .050 110lsa hft cam, 255 cfm aluminum heads, rpm air gap, 650 QF vac secondary carb, 2500 stall.
Current base timing is 25 degrees but plugs are darker than I want. AFR is 13.0 to 13.5 at 950 in park, 12.8 to 13.3 in gear at 700 rpm. It does not like leaner at idle. Cruise AFR is 13.5 to 14.2.
5.5 power valve. Main jets down from 68 to 67, (considering dropping to 66 main jet). I have a vac gauge in the car and power valve is not opening except under load.
I am concerned that rear butterflies are covering the transfer slot and the front butterflies are almost completely closed. I started with .020" rear transfer slot showing and closed the secondary stop screw 1/8 turn at time while adjusting the mixture screws and driving the car and watching AFR gauge. The car drives great with very small hesitation (sometimes) from a stop on level ground. Front(only) mixture screws are only out about 1/2 turn.
13"hg at 950 rpm in park 11" hg at 700 rpm in gear.
Using vac advance and very conservative total timing so far. Street only for now.
Duraspark Ignition with built in 5 degree start retard, starter starts it fine.
(Looking for a vacuum leak, but cant find one so far)
Should I try dropping my base timing some to see if I can open my front butterflies more without increasing idle speed? Will opening the front butterflies help the carb give a better mixture at idle?

Lizardracing
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Re: Timing Curve

Post by Lizardracing » Thu Oct 03, 2019 3:29 pm

4V vacuum secondary carbs don't have four corner idle so the valves should be shut. All idle A/F must go through the primaries. Ideally. Opening the secondary on these carbs is like adding a controlled air leak. A tiny bit prolly won't hurt but thats air not be accounted for and too much of it means you the carb loses control of the idle mixture and the screws aren't very effective.
More base timing typically helps, not makes it worse. Set you timing and leave it until the carb is working good and really close, then if you wanna, try a timing change and play with the carb again. Don't go back and forth or you'll be chasing your tail.
The only QF carb I've had needed some IFR work to be right.

ClassAct
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Re: Timing Curve

Post by ClassAct » Thu Oct 03, 2019 3:45 pm

BobbyB wrote:
Thu Oct 03, 2019 1:53 pm
Can too much base timing cause carb tuning trouble?
New 347 sbf, 10.4 to 1, 224-229 at .050 110lsa hft cam, 255 cfm aluminum heads, rpm air gap, 650 QF vac secondary carb, 2500 stall.
Current base timing is 25 degrees but plugs are darker than I want. AFR is 13.0 to 13.5 at 950 in park, 12.8 to 13.3 in gear at 700 rpm. It does not like leaner at idle. Cruise AFR is 13.5 to 14.2.
5.5 power valve. Main jets down from 68 to 67, (considering dropping to 66 main jet). I have a vac gauge in the car and power valve is not opening except under load.
I am concerned that rear butterflies are covering the transfer slot and the front butterflies are almost completely closed. I started with .020" rear transfer slot showing and closed the secondary stop screw 1/8 turn at time while adjusting the mixture screws and driving the car and watching AFR gauge. The car drives great with very small hesitation (sometimes) from a stop on level ground. Front(only) mixture screws are only out about 1/2 turn.
13"hg at 950 rpm in park 11" hg at 700 rpm in gear.
Using vac advance and very conservative total timing so far. Street only for now.
Duraspark Ignition with built in 5 degree start retard, starter starts it fine.
(Looking for a vacuum leak, but cant find one so far)
Should I try dropping my base timing some to see if I can open my front butterflies more without increasing idle speed? Will opening the front butterflies help the carb give a better mixture at idle?


One thing jumps put. The power valve is opening way too late. Set your cruise A/F ratio by main jet and WOT by power valve channel restriction.

You may have to open the idle air bleed a bit, or close down the idle fuel restriction a bit.

GARY C
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Re: Timing Curve

Post by GARY C » Thu Oct 03, 2019 4:28 pm

Is 25 total?

I would increase initial timing until I get the highest idle vacuum at desired idle speed and then limit total to 34 and start there.
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MadBill
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Re: Timing Curve

Post by MadBill » Thu Oct 03, 2019 5:56 pm

Understand guys, he's looking to use more air, resulting in more TB opening and more/some transfer slot area below the TBs, so retarding the spark to increase the idle air demand is unusual step but for a plausible reason.. :-k
Felix, qui potuit rerum cognscere causas.

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Re: Timing Curve

Post by BigBro74 » Thu Oct 03, 2019 6:08 pm

Is your vacuum advance hooked to manifold vacuum or timed port?

I would try and use the manifold vacuum with that 25 initial and 34 total as a base curve before you plug-in the manifold vacuum. It will idle like a champ with the vacuum advance hooked up which will get more like 40 degrees idling—- and part throttle cruise will be cleaned up too- you will have too check for detonation at part throttle / higher load - like cruise up hills, low rpm lugged down..

steve cowan
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Re: Timing Curve

Post by steve cowan » Thu Oct 03, 2019 6:11 pm

I put a 585cfm QF adjustable vacuum secondary carb on my uncles 350 vortec.
It has 4 corner idle and adjustable air bleeds
I set front transfer slot gap around 20 thou and rear nearly closed, rear idle mixture screws half turn out max, fronts you want one to one and a half turns otherwise idle fuel restrictions is to big, I didn't restrict the IFR as I got it close. In my opinion your afr are to rich at idle, my stuff I run around stoich at idle and cruise, you are about 10% richer.
My WOT is 13.2-13.5.
I figured your vacuum should be higher with small cam but it what it is, as suggested the more base timing the better with no off idle pinging, float level and fuel pressure dictate fuel mixture as well so worth a look.
Once sorted idle look at entire fuel curve, I agree with Classact, a 65-75 PV might work better also tuning PVRC for WOT.
steve c

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Re: Timing Curve

Post by BobbyB » Thu Oct 03, 2019 6:39 pm

Just tried 66 main jets in front. Then double checked timing to be:
25 deg at 950 rpm
25 deg at 1500 rpm
28 deg at 2000 rpm (a little erratic at this rpm)
28 deg maxim and stable above 2000 rpm

Drove great with tiny hesitation from stop on level ground as I eased onto the throttle.
AFR was the same idling but came up as expected at cruise to more like 14 to 14 .7

I disconnected the vac advance this trip.

The car weighs about 3000 pounds with 3.55 gear and 24” tires. I could tell that it was going to be too lean as I tacked it up to about 4500 rpm in 2 nd gear. Not WOT, but it was getting above 15 to 1 AFR before the vacuum dropped into the power valve range.

Mad Bill understands what I am asking about getting more air, but, I have very little carb experience so thanks for all input.

What should I do next to keep my engine safe?

GARY C
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Re: Timing Curve

Post by GARY C » Thu Oct 03, 2019 6:51 pm

BobbyB wrote:
Thu Oct 03, 2019 6:39 pm
Just tried 66 main jets in front. Then double checked timing to be:
25 deg at 950 rpm
25 deg at 1500 rpm
28 deg at 2000 rpm (a little erratic at this rpm)
28 deg maxim and stable above 2000 rpm

Drove great with tiny hesitation from stop on level ground as I eased onto the throttle.
AFR was the same idling but came up as expected at cruise to more like 14 to 14 .7

I disconnected the vac advance this trip.

The car weighs about 3000 pounds with 3.55 gear and 24” tires. I could tell that it was going to be too lean as I tacked it up to about 4500 rpm in 2 nd gear. Not WOT, but it was getting above 15 to 1 AFR before the vacuum dropped into the power valve range.

Mad Bill understands what I am asking about getting more air, but, I have very little carb experience so thanks for all input.

What should I do next to keep my engine safe?
Could be late power valve opening, as others have said, the only way to know is to test it.
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ClassAct
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Re: Timing Curve

Post by ClassAct » Thu Oct 03, 2019 6:59 pm

BobbyB wrote:
Thu Oct 03, 2019 6:39 pm
Just tried 66 main jets in front. Then double checked timing to be:
25 deg at 950 rpm
25 deg at 1500 rpm
28 deg at 2000 rpm (a little erratic at this rpm)
28 deg maxim and stable above 2000 rpm

Drove great with tiny hesitation from stop on level ground as I eased onto the throttle.
AFR was the same idling but came up as expected at cruise to more like 14 to 14 .7

I disconnected the vac advance this trip.

The car weighs about 3000 pounds with 3.55 gear and 24” tires. I could tell that it was going to be too lean as I tacked it up to about 4500 rpm in 2 nd gear. Not WOT, but it was getting above 15 to 1 AFR before the vacuum dropped into the power valve range.

Mad Bill understands what I am asking about getting more air, but, I have very little carb experience so thanks for all input.

What should I do next to keep my engine safe?

If that's what you are wanting, drill a .0625-.080ish hole in each primary butterfly. Sounds like you need more air at the same throttle opening. If so, drill the holes. Start small. You can always go bigger.

And your power valve is opening too late as well.

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MadBill
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Re: Timing Curve

Post by MadBill » Thu Oct 03, 2019 7:25 pm

Again, he's trying to get the butterflies more open to expose some of the transfer slots, without raising the idle speed too high. Drilling holes in the blades will only make it worse.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Timing Curve

Post by steve cowan » Thu Oct 03, 2019 8:06 pm

If you expose more transfer you will be richer at idle, I have 3 x holley carbs and a Barry grant demon, I fitted adjustable transfer slot jets in main body, all mine were pig rich prior.
Set total timing to 34 and retest
You are going lean because main jet is to small, get idle,transfer and main jet sorted first, don't worry about WOT at the moment
If primary idle screws are still half turn out your IFR needs to be reduced,
steve c

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Re: Timing Curve

Post by BobbyB » Thu Oct 03, 2019 9:51 pm

I think the factory IFRs are .030”. I have .020”s, .025s & .028s which one should I fry first?

With smaller IFRs I will be able to open the mixture screws more to get the same AFR at idle. How will the car behave differently with smaller IFRs and the mixture screws further out?

Will the car idle better with a higher AFR with smaller IFRs?

The car does not like to be leaner at idle than my current setting with current parts.

Holley says Power Valve should be half the idle inches of vacuum. What should I do go back to a 6.5 or even higher?

Gary...total is 28... Right now idle speed is too high if anything. That’s what makes me think I might need to reduce timing at idle.

Mad Bill you understand my original question and the basis for it. Am I off base with thinking I might be better off with more open butterflies and more transfer slot?

I agree that WOT does not matter until I get idle and cruise sorted out. I want to do this right. My biggest concern is getting the idle right first. I don’t want to hurt my engine being too rich.

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Re: Timing Curve

Post by Firedome8 » Thu Oct 03, 2019 10:38 pm

Do not look at afr for idle look at vacuum, you should have at least .020 xfer slot showing start at 1 turn out and adj screws 1/4 turn in or out to obtain the highest steady vacuum, take note how far out the mix screws are. Note timing. Now turn the idle speed screw in 1/4 turn at a time until fuel is issues from booster (aprox 2000 rpm) listening all the while for smooth engine response at each setting. If it falters or runs splashy at any point change the ifr (the ifr feeds the slot and base idle) and start process over. Timing should return to same * at same idle speed.
Test drive afr of 14 to 15 at cruise should be good as was said I would try a earlier opening pv and see what happens.
Find a chassis dyno and you usually find someone with more carb experience and you can do wot pulls for mixture and total timing.

What heat range plug ?

An adjustable vacuum adv. Is a good idea.

I have started using the MSD programmable timing box it's great. Learned that on this forum.

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MadBill
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Re: Timing Curve

Post by MadBill » Fri Oct 04, 2019 12:58 am

BobbyB wrote:
Thu Oct 03, 2019 9:51 pm
...Mad Bill you understand my original question and the basis for it. Am I off base with thinking I might be better off with more open butterflies and more transfer slot?
You may be able to bandaid things by juggling slot exposure but when the calibration is right as above, the mixture will remain steady as the revs are slowly raised.
Here's a snippet of the tuning process from David Vizard's Holley book, but there are at least half a dozen other good ones available and you really should take advantage of one or more of them; it just takes too many words to get into all the details in a forum post. (I started to post the preview but realized it was running into a dozen pages...):
https://www.musclecardiy.com/performanc ... ion-guide/
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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