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Carburetor tuning

Kern Dog

Life is full of turns. Build your car to handle.
FBBO Gold Member
Local time
4:47 AM
Apr 13, 2012
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Granite Bay CA
It has been awhile since I have tinkered with the carburetor in Ginger but a few things have changed lately that require me to go back in.
I rebuilt the 440-495 with slightly less compression. I was at 10.1 to 1 before with THICK head gaskets, no quench, a MP '528 cam, 1.6 ratio rocker arms, 2" headers, Edelbrock Performer RPM intake and the trusty Demon 850 vacuum secondary 4 barrel carburetor. For that combo, I had #85 primary jets, #92 secondary jets and a 6.5 power valve. I have an air/fuel meter which is both a blessing and a curse because it often attracts my attention at times when I can do nothing with the information.
The rebuild included new dished pistons with 9.8 compression and .041 quench distance. I went back to an old cam that I saved, a fairly rowdy Lunati with the following specs:
Lunati specs.JPG

It actually idles better than I expected but I think I need to fine tune the combination to get the A/F numbers in line. I have a 5 speed manual trans and 3.55 gears.
I'm running a MP electronic ignition at 19 degrees initial, 33 degrees total timing.
At idle, my A/F in neutral is creeping into the 15s and 16s. In cold weather, I've often had to back out the idle mixture screws to fatten it up a slight amount. I just haven't done that yet. At light cruise, the numbers fluctuate to as low as the mid 13s. Now, with 10% ethanol, I know that the magic target is not 14.7 like with pure gasoline so I'm guessing that something in the mid 13s at idle and light cruise is fine.
Just tipping the throttle drops me down in the 12s, WOT goes even further.
Before when I ran this cam, I used a 3.5 power valve based on the guideline of taking the idle vacuum number, dividing it by 2 and picking a PV closest to that number. My idle vacuum now.....

22 3 N.JPG

Looks like 7 1/2" to me. The 3.5 power valve might be the right choice again.
My question....
I probably knew this at one time and gave forgotten: Does the power valve have that much influence on the A/F ratio at part throttle?
I've tried reducing jet sizes before and often times that only reduces power while barely changing the readings.

616 Y.jpg

I’m by no means an expert with carbs but I just went thru tuning the power valve in the brawler on my 440 and having a vac gauge in the car while driving was VERY helpful. My idle vac is around 6" but cruising I was at 15", I was getting a lean condition under light/med accel, vac was only dropping to about 10" there so my 6.5 PV wasn't opening, switched to a 10.5 and lean issue is gone.

Having the vac gauge in the car will let you know when the PV is opening. You could either change it to open later or maybe change the PVCR's if your carb has them. What wide band are you using?
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Sounds like you should first get your idle A/F number down a bit, bigger idle feed restrictors. I don't mess with the power valve until idle, transition, cruise, and main circuits are good. The old 1/2 of idle vacuum sizing isn't the best thing to go by imo. I like to have a vac gauge in the cockpit while driving normal. What is your cruise vac? If for example your cruise vac is 16", you can put in a 9.5 or 10.5 valve. It won't open at idle, so it can be a higher number than your idle vac. With my stick cars, I like to drive around watching the vac gauge. Then decide at what time you want the extra fuel to dump in and experiment with different valves and PVFR's. I always unhook my secondaries while doing everything I just wrote about. Get all that set and then bring out the big guns with the secondaries.
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The A/F gauge is from AEM.
I have both a dash mount and a hand held vacuum gauge but I never installed the dash version. Isn’t the function of the power valve to open only once the vacuum drops to the number?
I think that The idle feed restrictors are pressed in.
I do have a Quick Fuel primary block with tiny plugs on each side of the power valve.
Power seems great. It doesn’t seem to smoke but it always has had black residue on the plugs so I suspect that it has always run too rich.
The A/F gauge is from AEM.
I have both a dash mount and a hand held vacuum gauge but I never installed the dash version. Isn’t the function of the power valve to open only once the vacuum drops to the number?
I think that The idle feed restrictors are pressed in.
I do have a Quick Fuel primary block with tiny plugs on each side of the power valve.

Ahh ok, I don't believe the AEM has data logging but that made it a bit easier for me (I have an Innovate LM2 wideband) and I just found out today I can run a map sensor to it so I can log a/f, rpm annnd vac, probably overkill but I always go overboard on that sort of stuff.

And yeah the power valve opens once vac hits a set number, from what I have read it feeds extra fuel to the primary booster (this is why it doesn't really affect idle a/f) the PVCR controls the fuel from the PV, they are the 2 jets behind the PV, are you running that primary block on the carb no or just have it as a spare?

hunt2elk for sure knows more than me and his suggestion of dialing in other circuits first makes sense.
I was able to position my AFR gauge, tach, vacuum gauge into a lesser extent the speedometer and was able to take video with phone. This really helped me see the different fuel circuits and when and how they came on or didn't. But after you got in ballpark it's real nice and easy to see the type of driving condition versus fuel ing.
Most newer Holley style carbs have adjustable idle feed restrictors. The Quick Fuels I have all come with 31's from the factory. I always end up leaning mine out with 29's or 30's. There are 4 of them, 2 in each metering block. You are a bit lean at idle now, so it will interesting to see what you have in it now.
The PVFR's are under the power valve. They control how much fuel goes in when the PV opens. They are the reason your secondary jets are always bigger than the primaries. I just run a small tube from my intake to a vac gauge that I hold in my hand or have one of my kids watch while I'm driving.

Another thing to think about. They always say to get your timing and advance curve dialed in before doing any carb tuning. Maybe that can be be adjusted somewhat from your original motor?
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Couple of comments.
- PV half the value of idle vacuum was always a starting point, not an absolute.
- Carter carbs used an Economizer jet, which was patented. It regulated the mixed air + fuel delivered to the T slot. Generally in the 0.045-0.055" range. My understanding is that brand H & clones had a large hole in the met block, which was usually way too big & was/is the #1 cause of running rich/plug fouling.
- the cam is pretty big, do not know what Lunati was thinking with just 125# seat pressure.
- idle timing is much too low. This simple test will prove it: engine idling, turn dist slowly to advance timing. Rpm will increase because the engine is making more hp from the extra timing. Would not surprise me if idle rpm topped out with 40* or more of timing. Achievable by using adj vac connected to manifold vac. This will allow carb idle to be re-adjusted with less throttle opening.
- tune idle & tip in response by ear, not vac or AFR gauge.
That guy at FBO is a proponent of manifold vacuum. I'm not so sure about it. I'd need more convincing before accepting it for my own stuff.
What about having vacuum advance at cruise to get a more complete burn? I'd lose that by switching to manifold vacuum, wouldn't I ?
Would manifold vacuum still work at light cruise?
I agree that it does want to speed up as I crank in more initial timing.
Think of MVA as PVA with benefits. MVA does everything that PVA does with these additional benefits: because it adds timing at idle, it improves idle quality; less fuel is used at idle; tip in response is improved; engine runs cooler at idle.

There is absolutely no reason not to use MVA over PVA. Chrysler missed the boat on MVA.
MV vs PV will probably get you into a heated debate around here. I do know what the answer is, it depends, lol. Some motors like it, and some don't. It is worth trying both ways. I have used it both ways. As you mentioned, Don at FBO is a strong proponent of MV. Years ago he sold me a distributor (setup for my car), control box, and carb. I fought with that thing and argued with him for months with it hooked to MV. It would start and idle fine. The issue was that as soon as I put it in gear and took off, the vac advance would not come back down. It would then idle at 1200 or so rpm. This motor only made 7.5" of vac at idle. He had me send the dist back to him 3 times. After that I was done with it. Hooked it up to ported and it worked fine. I had him make me up the same setup for another car of mine and hooked to MV. That one ran great.....
You see this post^^^#12 is a PERFECT example of misinformation & what happens when MVA is incorrectly set up. No doubt the poster means well.
Let us get one thing clear: if the engine is suitable for MVA [ 90 % are ], then the engine will like it, no exceptions.
What amazes me is people buy these expensive carbs with removable emulsion tube/air bleed jets, have no idea what they are doing, spend hours tinkering with the carb that was properly set at the factory....& in the end there is no difference that the butt-meter can feel. Contrast this to banging in a new dist, hook up the VA to man vacuum....& then complain: MVA didn't work for me!!!!

David Vizard reviewed the Crane fully elec dist in the Nov 04 issue of PHR magazine. Note that Crane went to the trouble [ & expense ] to include vac adv curves as well as centri curves. Think about that...

DVs comments say it all. If you still don't get it after reading this, then you must be really dumb...

" At idle & low speed operation, the amount of advance reqd to MOST EFFECTIVELY utilise the fuel & air entering the engine can be as much as 50 to 55*. This is handled by the vacuum advance: a function many hot rodders believe is not needed because their favourite drag racer does not use it. .....A functional VA unit is the single most effective camshaft tamer you can get. By taking the time to hook up the VA to a manifold source you can get a big cam to idle as it it were about 20* less than it really is...."

The statement in post #12 about the 'vac adv not coming down' is a tuning issue, not a MVA issue & no reason to stop using MVA. Perhaps the linkage was binding, could be a few things.
This nonsense then gets posted on the web as 'another' example of MVA didn't work for me....

The golden rules of setting up MVA on modified engines [ it will also work well on stock engines but less total VA will need to be added ].
- you MUST use an adj vac unit. [ I was unable to get adj Chrys VA unit here for a 440. I ended up modifying a GM points dist, { with Chrys mag pick up fitted } to fit the 440; it is still in the car & working nearly 20 yrs later ]
- factory adj VA units are no good, spring is too stiff
- must be aftermarket. Some such as Crane & Accel have a spacer inside the spring. This limits adjustment. What you have to do is prise open the can & remove the spacer. The cheap Chinese dists have excellent adj VA units, no spacer.
- the adj VA unit can add up to about 30* of timing. Only a few engines will need this much. So a stop has to be fabricated to limit total travel.
- the other adjustment is rate of decay/application, controlled by spring tension. This is done by adjusting the screw with a 3/32" Allen Key. Softest setting is AK fully CW. Always start there.
- any combination of initial + MVA can be selected to give idle timing. If say 40* was determined for best idle, you could have: 10+30, 20+20, 18+22. This is where experimentation & time is reqd.
- determining optimum idle timing is realllllllllly easy: engine idling, in gear if auto, VA disconnected & initial timing about 10*, not critical, slowly turn the dist to advance timing. When you have the highest idle rpm, toggle the dist to make sure highest idle rpm; idle will also be smoothest. Now check the timing. That is what THIS combo needs for best idle. You will NOT find this procedure in any books.
- low vac at idle. Most adj VA units will work done to about 6" of vac. Less than that, MVA will not work, but few engines will have idle vac that low. You could try it. What is crucial is that once the MVA has been hooked up, the timing is steady, in gear, at idle. This where a lot of 'it didn't work for me' comes from. Folks try & use factory VA units or non-adj & the timing does not hold.
- with low vac engines, better to use a lot of static init timing. Example, engine idles best with 45*. Try & use 28* + 17* MVA rather than 15* init + 30* MVA.
Ive been tinkering with MA and PV and one thing that makes some engines (Owners?) not like MV is if you dont have steady vacuum and do want a steady idle. With unsteady vacuum (Worn engine/leaks.) then its impossible to get a smooth idle with MV.
For sure my car got snappier with MV/timing set, though I like a super smooth idle.
Prolly my new engine will run on MV.

This is why you need an adj VA unit. And an aftermarket one that will have a softer spring. You make sure that the spring is adjusted [ Allen Key, AK ] so that the actuator arm does not move/pulsate with the lowest vacuum, in gear, if auto. You also have to factor in the total VA that is going to be used because the more VA that is used, the more the spring is compressed & now may not hold the vacuum with that AK setting.
I use a adj aftermarket VA unit, but even adjusted all the way out the idle was unsteady.
I wish I could find VA pods with even softer springs. At idle I have 20hg.
Generally I try to get the idle and light throttle cruise as lean as I can to keep the plugs nice and clean.
They are light load conditions so a bit lean will not melt anything. If it doesn't surge at cruise you are OK to run it pretty lean maybe around 15 on the AFR meter and it will be no problem.
The lean mixture does like some extra timing from the vacuum advance. I tend to connect vac advance to the manifold these days.
Also the smallest accelerator pump shooters that eliminate bogs.

If you have a lot of initial timing combined with the vacuum advance I have found it beneficial to delay the mechanical advance curve start point to around 1250 to 1400 RPM. It depends on the size of the engine and the compression ratio.
First thing to get your mind around, if your wideband is on a gasoline scale, tune the engine to gasoline numbers you're familiar with no matter what fuel you're on. 14.7 cruise/idle is stoich and say 12.8-13 wide open.

For example, if you converted your car to E85 and tuned it on a gasoline scaled wideband you would still tune it to gasoline numbers because it's just converting back to lambda.

If the above makes no sense, change your wideband to lambda. 1.0 is stoich so target that or slightly richer for idle and cruise.

Then target WOT to .87 to .88.
Being in California with what is advertised as up to 10% ethanol, the numbers will be different.
What about tuning for higher altitudes? I'm considering a move to a locale that is 3300 above sea level. Currently I am approximately 500' ASL.
The higher altitudes require smaller jets to keep the A/F numbers in line, right?
What you need to understand is, the numbers aren't different on your wideband.

If it's on a gasoline scale, stoich is 14.68

If you have E10 in your car, stoich is 14.1

If you have E85 in your car, stoich is 9.76

If you are running E85 and monitoring air fuel ratio with a wideband on gasoline scale, it will still read 14.68 stoich because it doesn't know you're running E85, it's just converting Lambda numbers to stoich values for gasoline.

So again, stoich lambda is 1.0. If your wideband is set to a gasoline scale of stoich equals 14.68 than stoich of lambda 1.0 will always be displayed as 14.68 regardless of what fuel is in the tank.
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