Carb tuning update - Vintage Mustang Forums
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post #1 of 23 (permalink) Old 07-07-2019, 04:17 PM Thread Starter
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Carb tuning update

Good afternoon Team VMF

Learning by way of reading and watching Holley Tune Videos on how to really dial in my carb.

Ran some tuning/tests focusing on idle today. Below are end results

Manifold VAC @ IDLE is 12-13 (I have a COMP CAM 31-414-3)
Idle set to 900 (solid idle)
AFR readings: 13.8 - 14.8

I spent a fair amount of time messing with idle screws trying to tighten that AFR reading but 4 corner idle is tough for a rookie to tune. Not sure if I was to expect a steady needle on AFR and VAC gauge - couldn't get either to stick on a single number but I think that is not realistic. Correct me if I'm wrong please.

Carb is a mechanical secondary 750 DP by QF. (Yes - tracking its to big for the car but it is what it is). Primaries are 76's and a PV rated at 4.5. Secondaries are 68's. No PV on secondaries. I believe I'm sized wrong on the PV. I don't think it ever opens hence the higher primary jets....which results in a rich run constantly. Based on that VAC - I should be about a 6.5 for it to open up and dump some fuel so I'm going to replace the PV and jet down the fronts.

Before I buy the PV though - I've left the VAC gauge hooked up and ran into the car. Going to drive around and see how the VAC reacts to cruising at a few different speeds to get a better idea if that 6.5 is correct or maybe something like 7.5. PV's are cheap anyhow so I may just buy a few of them and play around with different jet sizes and PV combo on the primary side.

Anyone care to opine? I'm having a lot of fun learning how to do this (outside of the SC heat)

Thanks
-J

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post #2 of 23 (permalink) Old 07-07-2019, 06:06 PM
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Why are your primaries with the PV jetted richer than the secondaries with no PV? Is that a typo?
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post #3 of 23 (permalink) Old 07-07-2019, 07:46 PM Thread Starter
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Originally Posted by Jsams View Post
Why are your primaries with the PV jetted richer than the secondaries with no PV? Is that a typo?
I see you are in Charlotte. I'm 20 min from Charlotte. Know any CARB jedi knights up there?

Not a typo unlesss I wrote down the secondary #'s wrong when I pulled of the metering plate a week or so ago. I believe we jetted up on the primaries to address a "too lean" at cruise. Not sure what that number was but it was pretty lean. I knew less then than I do now.

Just to update. I let the car cool down.....went to drive it for a cruise but didn't start....empty bowls. Reset the 4 corners back to 1.5 turns out and began to dial it back in. Bottom line - this car won't make 14'ish with the air filter on no matter how hard I try at idle. Best I can get it dialed in at idle now is mid-13's (with filter on) otherwise it will empty the bowls if I try to go leaner. So I accepted this and decided to see what vacuum does at cruise (diff speeds) and WOT. Here are some results

Cruise - vacuum climbs to about 18-19 with AFR in mid high 14's/mid 15's.

Under a load (hills, acceleration) - vacuum drops to 15'ish and AFR richens to 13's

WOT - vacuum drops to less than 4 and further riches to high 11's - mid 12's.

So wondering if this is expected behavior in terms of vacuum. I'm not sure.

Seems somewhat on par for AFR. I'm not sure if I need to go with a diff PV or not yet. I'm planning on dropping in the default jets the CARB came with and seeing how that changes things now that I"m getting more familiar with CARBS.

Or this is just the price we pay for an oversized CARB?

Thanks
J

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Last edited by John_in_SC; 07-07-2019 at 07:49 PM.
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post #4 of 23 (permalink) Old 07-07-2019, 08:37 PM
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AFR numbers look decent. You might consider dropping a couple sizes in the rear to see if you lean it out a little at WOT. Also, what do your plugs look like right after a lengthy WOT pull. Kill the engine at WOT pull over and pull a few plugs.

Not knowing much about the rest of your engine, itís hard to say if your carb is too big. However, I would suspect it is entirely too large. I know guys running well over 400hp 289ís running no larger than a 650DP. Iím only at 350hp and use a custom 650DP.

Regards,
Patrick
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post #5 of 23 (permalink) Old 07-07-2019, 08:44 PM
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Based on your AFR numbers, the carb seems mostly well tuned.

The most important question is, how does it drive?

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post #6 of 23 (permalink) Old 07-07-2019, 08:48 PM
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Do you know how much of your primary and secondary transition slot is exposed (only visible when the carb is upside down)? If too much is, the 4 corner idle screws donít do much. I like to set those correctly- look it up but usually about square looking- and then fine tune my idle speed with initial ignition timing. Also, secondary jets in holleys are typically around 8-10 sizes larger than primary ones, where primary uses a power valve (usually half idle vacuum, so 6.5 for you) and secondaries have A plug in the power valve spot. If both power valves are in, jetting is close to identical front and back.

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post #7 of 23 (permalink) Old 07-07-2019, 08:55 PM
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Quote:
Originally Posted by John_in_SC View Post
(snip) Just to update. I let the car cool down.....went to drive it for a cruise but didn't start....empty bowls. Reset the 4 corners back to 1.5 turns out and began to dial it back in. (bigger snip)

Before anything else, can you describe what you mean by "empty bowls?"

Fuel bowl levels aren't effected by idle mixture screws. Are you sure they (both??) were empty?

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post #8 of 23 (permalink) Old 07-07-2019, 10:04 PM Thread Starter
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Before anything else, can you describe what you mean by "empty bowls?"

Fuel bowl levels aren't effected by idle mixture screws. Are you sure they (both??) were empty?
They both looked empty in the bowl glass. I noticed this after I tried to start the car after it had cooled somewhat. Car turned over fine however AFR was going crazy lean and wouldn't stay on idle. I shut the car down and then looked at the CARB and saw the fuel bowls were empty. I then did the carb tune reset steps (setting back to defaults) - cranked it up and idled like an out of the box carb on first crank.

I just went to have a peak at them now - they seem dry again - or at least not viewable. Dont' want to turn it over - kids are asleep. Suppose I could pull a bolt and see if it drains. No noticeable leaks - didn't mess with floats at all. Interesting.

***EDIT***

Just checked the front bowl - drained it, it was wet.

the first time where they def appeared to be dry and it would start but not idle - start the car, does it not go to the bowls? Its a chokeless carb so when I start it (cold/somewhat cold) - got to bottom out the gas pedal to get it cranked on first go.

Last edited by John_in_SC; 07-07-2019 at 10:15 PM.
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post #9 of 23 (permalink) Old 07-08-2019, 06:10 AM
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Do the floats need adjustment?
Or is the fuel evaporating out of the bowls?
(probably evaporation)

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post #10 of 23 (permalink) Old 07-08-2019, 06:21 AM
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OK, do you have the float bowls adjusted with fuel level in the center of the round window?
Do you have the downleg or annular boosters? I ran a 750 Annular quick fuel for a while. it was a good carb.
There are several local guys who offer chassis dyno tuning of the carburetors.
So you said that you had a lean surge at cruise and you jetted up?
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post #11 of 23 (permalink) Old 07-08-2019, 06:27 AM
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Quote:
Originally Posted by MrFlash View Post
Do the floats need adjustment?
Or is the fuel evaporating out of the bowls?
(probably evaporation)

this was my initial thought. Do you have a phenolic spacer to keep the heat from the motor out of the carb? If however there is a setting of the corners that eliminates this problem it aint the problem...

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post #12 of 23 (permalink) Old 07-08-2019, 08:04 AM
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Some more thoughts.... when you say the float bowl is dry, do you mean you can’t get any fuel to squirt our of the accelerator pump nozzles?
Are you running a PCV?

1969 coupe
372 Cleveland, 4 speed, 9" T.50 Trac Lock axle, Toploader, Global west suspension, Big Lincoln Trans Am Brakes front, Eldorado caliper/ MK VII disk rear.
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post #13 of 23 (permalink) Old 07-08-2019, 10:36 AM Thread Starter
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I'm going to try an answer a few questions in here while I sit on this conference call.

Regarding float bowls adjustment. They were adjusted but I wonder if they got knocked out of whack when I pulled the bowls. I may readjust them when I get a min. Haven't started the car since yesterday so will update you all if I experience the same issue.

Running PVC - fuel definitely squirts out of the nozzles.

I have annular booster and carb is not on a spacer. When I had the air cleaner off - I did notice fuel cooking off the carb.

Regarding primary and secondary transition slot - I'm not sure that is the issue for idle being lean as the idle screws are very responsive. Almost too responsive. I seem to be unable to turn the idle screw 1/100th of a turn. =)

With the carb tuned - car runs crazy good. Lots of power - great idle.....just idles rich.

@Jsams - could you PM me some carb dyno tune guys in the AO?

Thanks
J

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post #14 of 23 (permalink) Old 07-08-2019, 12:11 PM
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Sent you a PM.
Do you have the timing adjusted with enough advance?
BTW your idle mixture screws don’t really “lean” out the idle mixture. It just gives you more or less of the mixed fuel that is controlled by the idle and idle air bleed jets.
With 13” if vacuum, your engine likely is milder (with respect to Cam overlap) than it was designed. Btw are you using manifold vacuum to the distributor?
You should be able to tune it to where you don’t get the teargas smell at idle

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post #15 of 23 (permalink) Old 07-08-2019, 01:30 PM
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Those sight glasses can fool you. Having the fuel line above the glass will also look empty. You may need to lower your floats a bit and verify that your fuel pressure at the carb is no more than around 6. QF says set the fuel level half way up the sight glass. If they are high that will make the idles too fat.



The 4 corner idles run a bit fat on my carb too. If your idle mixture screws are still responsive you are probably not too far into the transfer slots. You can also close them off too much which will set up a hesitation when you come off idle or that is how mine act anyway. They need to be exposed below the throttle blade(squared) to make them draw in quicker when you are getting into the throttle. At a 900 rpm idle they are probably exposed.



I also have to square up my transfer slots and then tune my idle with additional initial timing but my cam is 2 or 3 notches bigger than yours and my idle vacuum is lower. I need 1100 rpms to idle clean. Also had to drill the throttle plates to get enough idle air for 1100 rpm and had to put the 2 biggest springs in the advance kit in the distributor to hold off the advance. My initial is set at 24. Not saying you have to copy this but you may have to do all of it to some smaller degree to get your idle to behave.


If your carb came with those pink cams you may need to consider that also. Those are at the bottom of the accelerator pump flow chart. I needed a bit more shot so I put in the orange cam in the front to help it transition to the primaries smoother.


If there is a science to all of this I don't know what it is exactly. It has all ben try this, nope, try that, eh little better, go some more that way, try something else. It can get frustrating finding the right combination of shooter size and pump cam.
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Last edited by macstang; 07-08-2019 at 01:48 PM.
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