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Newest update to carburetor stumble

Aron Gleason

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So the only major change I made was to go from .067 pvcr to .078 pvcr. And now my plugs are darker. But afr's are a little more stable at cruise. 12.7-13.2 depending on throttle position. When I'm cruising I stab the throttle it jumps to 17.0 for about 3 seconds before it picks up. I did go from a white secondary pump cam to a orange cam in the 1st hole, and dropped the squirt
20221017_114549.jpg
20221002_151702.jpg
er size from 42 in primary to 33. And 37 in secondary. First picture is with the. 067 pvcr, and 2nd picture is with the. 078 pvcr. I did read where going that big of a pvcr is a 39% increase in fuel. And I have a 6.5 power valve.
 
When are you reding the plug? If it's after a full throttle hit with a clean cut, it's to rich. 39% is a huge increase. If its checked after cruise it's to rich but pvcr has nothing to do with that. If it falls on it's face the pump cams/squirter size is wrong.
Doug
 
When are you reding the plug? If it's after a full throttle hit with a clean cut, it's to rich. 39% is a huge increase. If its checked after cruise it's to rich but pvcr has nothing to do with that. If it falls on it's face the pump cams/squirter size is wrong.
Doug
IMO.....knowing what the A/F ratio for a carb fueled application is nice to know but it is not gospel or definitive. There are just too many variables: barometric pressure, throttle position, manifold absolute pressure (which is not vavuum), RPMs, air temperature and density, plus the engine variables (cam, advance characteristics, compression ratio, etc.) and if both exhaust streams are measured and averaged. A/F tuning on a dyno, with repeatable conditions initially then fine tuning on the track using ET slips and spark plug readings....but it depends on the use of the vehicle. Electronic fuel injection system can be programmed and data logged to do this relatively easily. I'm sure others will dispute this and have their own preferences and opinions. Just talking out loud....
BOB RENTON
 
Oh I know. My carburetor guy told me awhile back not to be concerned what the afr's are, and to go by what the plugs look like,.
 
So, overall, has the PVCR increase helped the stumble?
 
Never seen any engine need 33 and 37 on the Steet for squirters. Let alone a 42.
What size motor do you have ?
What is the combo included gear ratios
 
Never seen any engine need 33 and 37 on the Steet for squirters. Let alone a 42.
What size motor do you have ?
What is the combo included gear ratios
440 with a 284 484 cam . 10.5-1 compression 906 heads with a M1 single plane intake 950 modified quick fuel carburetor. 67 coronet 3700 lbs. 4.57 gears. 4500 stall. Runs really strong. The only major problem with the carburetor is the stumble while cruising.
 
440 with a 284 484 cam . 10.5-1 compression 906 heads with a M1 single plane intake 950 modified quick fuel carburetor. 67 coronet 3700 lbs. 4.57 gears. 4500 stall. Runs really strong. The only major problem with the carburetor is the stumble while cruising.
I know many times people say they don't like the Mopar 484 cam , the 509 would work better withe M1 single.plane .
We had that same set up way back never had issues like that .
I am currently running a m1 on my 493 with a 4sp and 4 10 gear and it has weird cruise sensation as well . But my af is 12.8 all the way to 6500 and have 250 to 260 duration cam with 630 lift . 850 demon with annular boosters double.pimp 31 squirters on both sides
I chalked up to some trade off do you want to have top end power or cruise ?
M1 is Mopar top of the line race manifold back in the day . Not really a Cruse manifold.
 
The power valve channel restriction will have no effect on an off idle stumble.
It will only effect the mixture on the primary side of the carburettor after the power valve opens and the fuel starts to flow.
It is a jet that effectively tunes the WOT mixture of the primary 2 barrels when they transition from cruise mode(primary main jets) to WOT. The power valve opens and the PVCR comes in to play.
The fuel flow through the primary boosters which is fed by the primary main jets and the power valve/PVCR is not instantaneous and the accelerator pump covers up this hole.

Have you tried a "known" good carb on the engine to make sure the carb is the problem?

I had a problem similar to yours in a new 750 Holley on a Blueprint engine and I played around for a day with all sorts of things and could not sort out the problem fully. I made it better but could not eliminate the stumble.
Replaced the carb and instantly fixed.
I came to the conclusion the carb had an internal fault of some description perhaps in the metering block.
 
I've had everything from a 750 vacuum secondary carb, 750 dp, 830 dp, 850, and this 950 dp that's been modified by Dominic thumper. And they've all had the stumble at cruise no matter what I did.
 
I know many times people say they don't like the Mopar 484 cam , the 509 would work better withe M1 single.plane .
We had that same set up way back never had issues like that .
I am currently running a m1 on my 493 with a 4sp and 4 10 gear and it has weird cruise sensation as well . But my af is 12.8 all the way to 6500 and have 250 to 260 duration cam with 630 lift . 850 demon with annular boosters double.pimp 31 squirters on both sides
I chalked up to some trade off do you want to have top end power or cruise ?
M1 is Mopar top of the line race manifold back in the day . Not really a Cruse manifold.
My carburetor guy has a 9 second street car that he can cruise on the street, with top end power to boot. And he runs a M1 tunnel ram intake with 2 750 double pump carbs. With about a .680 lift roller cam.
 
4 different carbs and no change indicates to me you may have a problem with something else.
 
You said you have a stumble now I am the bad guy because I said you are you using race parts and trying to cruise the street same as a stocker?
Have you rode in the carb guys car on the street or driven it ?
My car can Cruise on street all day, is it perfectly seam less No .
Maybe you should have the carb guy tune it perfectly.
 
You said you have a stumble now I am the bad guy because I said you are you using race parts and trying to cruise the street same as a stocker?
Have you rode in the carb guys car on the street or driven it ?
My car can Cruise on street all day, is it perfectly seam less No .
Maybe you should have the carb guy tune it perfectly.
That would be nice, but unfortunately he lives in Utah. And at a much higher elevation.
 
The M1 has a huge plenum. It is not a street intake & expect less than perfect manners.
One of the problems with the POS brand H stuff [ & clones ] is the tortuous path the fuel has to take to get from the float bowl to the booster when the PV is activated. Some later carbs have the PVCR holes lowered in the PVCR recess so that not as much fuel has to fill up the cavity before it goes into the main channel & out the booster into the air stream. All this takes precious time.
Compare this to a metering rod carb where the fuel is sitting above the main jet, ready to go.
Because of the huge M1 plenum, you are likely going to need even bigger PVCR jets. I would also use the highest number PV [ example, 10.5 ] that will not open at idle. It will activate fractionally sooner & may help.
 
The M1 has a huge plenum. It is not a street intake & expect less than perfect manners.
One of the problems with the POS brand H stuff [ & clones ] is the tortuous path the fuel has to take to get from the float bowl to the booster when the PV is activated. Some later carbs have the PVCR holes lowered in the PVCR recess so that not as much fuel has to fill up the cavity before it goes into the main channel & out the booster into the air stream. All this takes precious time.
Compare this to a metering rod carb where the fuel is sitting above the main jet, ready to go.
Because of the huge M1 plenum, you are likely going to need even bigger PVCR jets. I would also use the highest number PV [ example, 10.5 ] that will not open at idle. It will activate fractionally sooner & may help.
When the boosters are flowing fuel, the emulsion channels are already full. The power valve channel restrictions are parallel to the main metering jet passages as the main metering are controlling fuel flow st this point. The power valve channel restrictions will feed additional fuel, when, and if, the vacuum operated power valve opens, dependant on its calibration point. The power valve channel restriction's orifice sizes are predetermined; race type Holley carbs have replaceable power valve channel restrictions as well as emulsion well air bleeds for both idle mixture and boosters venturi, allowing for additional adjustment points. Some booster venturii assemblies are annular discharge type reputed to provide "better or more efficient" fuel atomization, which is based on air flow velocity or venturii depression or negative pressure as a function of air velocity. Some users will remove the power valve, blocking the power valve channel restrictions, rendering them inoperative. The main jet orifice AREA must be increased to allow for orifice AREA eliminated by the power valve plug. AREA is the determining factor; not just diameter of the orifices as to the flow thru them. Fuel flow is usually measured in pounds/hour or some common measurement. Whether one is a Holley or Carter/Edelbrock (aka Eddy) person is immaterial. Let the word wars begin......."my way is better than your way"....or ....."I know more than you do"...... (notice the lack of abbreviations)....
BOB RENTON
 
By the time the PV opens on a Holley, any fuel that was above the discharge holes in the PVCR round cavity has gone...drawn into the engine. There is now a delay.....
Additional fuel has to come from the float bowl via bends/passages to get to the PVCR cavity & the fuel has to get to the level of the discharge holes in the cavity before any fuel flows. These holes are horizontal & the fuel has to make a 90* turn to get into the main channel to get to the booster. All this takes time...a time lag.
On metering rod carbs, the fuel flow is more direct with less bends/interruptions to flow & hence quicker response. Probably why only Holley used the PV method of enrichment.
 
Ok one more question what Rpm are you cruising at when you stab the throttle and you get the stumble?
 
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