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Discussion Starter #1
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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Why are your primaries with the PV jetted richer than the secondaries with no PV? Is that a typo?
 

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Discussion Starter #3 (Edited)
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. :shrug:

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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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, it’s 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 289’s running no larger than a 650DP. I’m only at 350hp and use a custom 650DP.
 

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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 don’t 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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(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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Discussion Starter #8 (Edited)
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.
 

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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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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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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?
 

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Discussion Starter #13
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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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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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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A carb is always a compromise so if its rich at idle that is one of the least worrisome problems to have and in my mind to be expected with those sizes. You mention both above so if its a choice, choose rich always.

These sight glasses came installed? They used to be meant for tuning only then replaced with a solid plug in case of breaking or leaking so i assume they have improved. Either way i think lower is better until its too low and then you notice its too low while running hard. When they look low, rock the car and see if you see fuel slosh into the sight hole.
This "fuel cooking off the carb" could be from the expansion of fuel from heat or residual high pressure from the pump overcoming the needle. You see it dribbling out of the venturi? I had to add a regulator to stop it from pushing passed the needle or i would tap the needle with a small hammer and it would stop but I am using ancient stuff.

Within the thread i read idle screws and idle mixture screws so make sure you're thinking is square and separate on those. The idle screw should be all the way off and barely touching the throttle lever ideally and yes, a slight opening of it should produce quite a noticeable change.
 

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I'm in the same boat. I installed a QF HR-735-VS last week and am currently running through some of the same issues with the 4 way idle circuit. After a few jet changes and minimum air rate changes on the secondary throttle plates I ordered "David Vizard's How to Super Tune and Modify Holley Carburetors" hopefully it will set me free on how all the circuits operate on a 4150.
 

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Discussion Starter #18
Within the thread i read idle screws and idle mixture screws so make sure you're thinking is square and separate on those. The idle screw should be all the way off and barely touching the throttle lever ideally and yes, a slight opening of it should produce quite a noticeable change.
Interesting. Was watching a Holley tune video (youtube is awesome) and they mentioned the idle screw. Prior to adjusting the idle mixture screws - they said to run the car to temp - and back off the idle screw until it "almost" cuts off. They then say to shut car off - set the 4 corners to 1.5 turns off base and then turn the car on. With car running - begin you idle mixture adjustments.

Only thing I didn't do in that process was backing off the idle screw prior to tuning the 4 corners. Kinda left it as is since it came off a decent tune already. I wanted to "improve" it. :pirate:

So I should go look at the idle screw and how it sits on the throttle lever. Not a big deal to go back and redo the process. Practice makes better.

Thanks
-J
 

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Discussion Starter #19

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I've adjusted enough to realize its too rich at idle and has decent 1/8 throttle driveability or adjust the 4 ways to the highest vacuum and then it runs really lean off idle with an uneven cruise. Each barrel has a screw in low and high speed air bleed as well as an idle jet in the primary metering block to get the transition correct. I'm not ready for the Titanic yet "John_in_SC" My Autolite 4300 float issue has sailed on that ship though.
 
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