Six Pack Fuel Slosh?

[EngineerDoug]

Hello all,

I posted this a few days ago in the A12 forum, but thought I would add it here as well.

I have a six pack setup on a 493 stroker, backed by a 727. Power brakes. Cam is very similar to Mopar 528, with wider lobe separation. Fuel and ignition are pretty well setup at this point, as I manage 15" of vacuum in gear at about 900 rpm idle. Initial timing is 20 degrees.

Installed a vacuum gauge and an AFR gauge recently and have been debugging an issue. A firm stop - especially one facing downhill - and the car wants to falter/stall. Watching the AFR gauge reveals a spike in AFR above 16 when this happens.

I've checked the power brake booster and it holds vacuum well. Note that I have a vacuum reserve tank between the intake port and the booster, so there are effectively 2 check valves in the system. Braking when the car is not moving does not give any issues, so I wouldn't think the brake booster is the issue.

My theory is that the fuel in all 3 bowls sloshes forward, leaning out the mixture. The jets are perhaps not uncovered, but the levels in the emulsion wells would drop.

I've got some more experimenting to do - will try raising float levels a little, and running a little richer idle mixture. I'd like to get this sorted out, or at least reduced, as it feels rather unsafe. I haven't tried a panic stop yet, but I would hate to have the engine stall if I do.

It's been awhile since I have spent much time driving a carbureted engine, but I don't recall this being normal/acceptable behavior.

Has anybody else seen this? Any other ideas I might have missed?

Thanks!

 

[RJRENTON]

Hello all,

I posted this a few days ago in the A12 forum, but thought I would add it here as well.

I have a six pack setup on a 493 stroker, backed by a 727. Power brakes. Cam is very similar to Mopar 528, with wider lobe separation. Fuel and ignition are pretty well setup at this point, as I manage 15" of vacuum in gear at about 900 rpm idle. Initial timing is 20 degrees.

Installed a vacuum gauge and an AFR gauge recently and have been debugging an issue. A firm stop - especially one facing downhill - and the car wants to falter/stall. Watching the AFR gauge reveals a spike in AFR above 16 when this happens.

I've checked the power brake booster and it holds vacuum well. Note that I have a vacuum reserve tank between the intake port and the booster, so there are effectively 2 check valves in the system. Braking when the car is not moving does not give any issues, so I wouldn't think the brake booster is the issue.

My theory is that the fuel in all 3 bowls sloshes forward, leaning out the mixture. The jets are perhaps not uncovered, but the levels in the emulsion wells would drop.

I've got some more experimenting to do - will try raising float levels a little, and running a little richer idle mixture. I'd like to get this sorted out, or at least reduced, as it feels rather unsafe. I haven't tried a panic stop yet, but I would hate to have the engine stall if I do.

It's been awhile since I have spent much time driving a carbureted engine, but I don't recall this being normal/acceptable behavior.

Has anybody else seen this? Any other ideas I might have missed?

Thanks!

Holley use to market jet extensions, that are short lengths of tubing that fit in the counter bore of the jet by a light press fit. These extensions did not effect the metering jets calibrated orifice size, but prevented forward fuel slosh in the bowl during extreme braking or cornering maneuvers. Increasing static fuel levels should help, but will need to be empirical determined.
BOB RENTON

 

[EngineerDoug]

Yup I have considered the jet extensions. I have the center carb apart anyway - can't hurt to try. The end carbs have metering plates, so no jets there.

Thanks.

 

[RJRENTON]

Yup I have considered the jet extensions. I have the center carb apart anyway - can't hurt to try. The end carbs have metering plates, so no jets there.

Thanks.

Oh contraire......I've installed the Promax metering plate modification to my end carbs....with re-moveable jets, just like the center carb. By converting to screw in jets, you'll be able to stagger jet the carbs for more uniform mixture distribution. If you are striving for an authentic look....no one will be able to see the revision. I run my carbs with the fuel level slightly low....an RCH below the sight hole threads. Make sure you've installed the bumper spring on the float arm. Just my opinion of course...
BOB RENTON

 

[PurpleBeeper]

Hmmm.... I've never had that issue. Why not put in those jet extensions? (couldn't hurt). Bob is a better resource than I am, but doesn't Holley have those press-in plastic, rectangular, 2-1/2" long tube things that press into the metering block? (can't think of the name) that's supposed to help with "fuel slosh" too?

My 2-cents? It sure "sounds" like a brake booster leak....even with the reservoir. Hmmm...maybe just re-set the idle at 950 or 975rpm and see if it the issue goes away? Maybe you optimized it for low, smooth idle just a touch too well?

 

[RJRENTON]

Hmmm.... I've never had that issue. Why not put in those jet extensions? (couldn't hurt). Bob is a better resource than I am, but doesn't Holley have those press-in plastic, rectangular, 2-1/2" long tube things that press into the metering block? (can't think of the name) that's supposed to help with "fuel slosh" too?

My 2-cents? It sure "sounds" like a brake booster leak....even with the reservoir. Hmmm...maybe just re-set the idle at 950 or 975rpm and see if it the issue goes away? Maybe you optimized it for low, smooth idle just a touch too well?

SURE, there is always the possibility of a brake booster vacuum leak, especially with a single diaphragm Bendix booster as used with drum brakes. I believe that cars with the A34 super track pack option (mine) and the six barrel engine (also the Hemi) used s Bendix Tandum diaphragm booster, which is less susceptible (but not immune ) to vacuum leaks.
Because of the cam I use (Crane equilivant to the Mopar purple shaft 292° duration @ 0.509" lift), the engine idles at approximately 1200 - 1300 RPM hot idle solenoid energized, stalling during hard braking is not an issue for me. The idle is a little "lumpy".......
BOB RENTON

 

[72RoadrunnerGTX]

Every six-pack I’ve owned or worked on has done the same thing to some degree or another for the same reason described. Don’t recall it being the issue it is these days, back in the day however. With a correctly tuned six-pack, stopping too quickly in a downhill attitude can/will stall the motor as the idle mixture leans out. I have spent quite a bit of time trying to tune this issue out. The only thing I have found that has any impact was the addition of an MSD multi-spark ignition. After the MSD went on, seemed to alleviate the lean condition stall point quite a bit.

Frankly, don’t see how jet extension tubes would make any difference, the jets are not going completely un-covered. The issue is the fuel level in the main wells dropping as the fuel in all three bowls moves in the same direction, forward. All three idle circuits go lean. The extension tubes will not change that, the main well levels will still drop. With a Holley 4bbl configuration, as the primary side goes lean, the secondary side goes rich.

 

[EngineerDoug]

Now that is very interesting, for a couple of reasons. I used to have a 440 six pack back in the eighties, and don't recall wrestling with this issue. But what I DID have was an MSD ignition.

As I bat this around in my head, I also really doubt the jets are being uncovered. The stops are not panic/put your face in the dash stops. But the fuel levels in the idle wells/passages certainly does drop.

I may also have tuned this thing a little too well - optimized for max vacuum and relatively low idle speed. That could be contributing - maybe a little richer mixture and a little more idle speed would help.

Thanks for the ideas & suggestions - once I get this put back together I can experiment a bit.

 

[RJRENTON]

Every six-pack I’ve owned or worked on has done the same thing to some degree or another for the same reason described. Don’t recall it being the issue it is these days, back in the day however. With a correctly tuned six-pack, stopping too quickly in a downhill attitude can/will stall the motor as the idle mixture leans out. I too have spent quite a bit of time trying to tune this issue out. The only thing I have found that has any impact was the addition of an MSD multi-spark ignition. After the MSD went on, seemed to alleviate the lean condition stall point quite a bit.

Frankly, don’t see how jet extension tubes would make any difference, the jets are not going completely un-covered. The issue is the fuel level in the main wells dropping as the fuel in all three bowls moves in the same direction, forward. All three idle circuits go lean. The extension tubes will not change that, the main well levels will still drop. With a Holley 4bbl configuration, as the primary side goes lean, the secondary side goes rich.

IMO....as the fuel level drops due to fuel movement forward, it becomes harder to maintain an adequate fuel supply thru the jets, main well emulsion channel and thru the idle mixture channel. This results in the mixture becoming leaner....the pullover depression pressures remain the same but the available fuel volume decreases...to the point of a lean stall condition. Perhaps increasing the diameter of the idle fuel restriction in the metering block or jet plate (front and rear carbs) will help but I have not tried this change. Just my opinion of course
BOB RENTON

 

[TN Mopar]

When I had my six pack I followed an old Car magazine article and drilled & tapped the metering plate for changeable jets
I also used the plastic jet extensions and the float with cut outs for the jet extensions
It worked very well
I can no longer find the article on modifying the metering plates for changeable jets but the promax plates do the same thing

 

[72RoadrunnerGTX]

The Promax plates do not address the forward fuel slosh lean condition described in the OP. Ran them a couple of years initially on my current six-pack, did the same thing. Funny, I also battled an outboard opening bog with this set-up as well, could never tune it out. Put the original #35 plates back in, problem solved. Again, forward facing jet extensions will not address the issue either.

 

[383man]

Never mind as I read your post wrong.

 

[EngineerDoug]

Hey I know it has been a long time since this thread was active but I thought I would post an update. In general, I have been slowly tinkering with this setup to improve driveability.

On the subject of fuel slosh, I found that the reason for this behavior was the center carb metering block. Here goes:

I had noticed that my steady state cruise AFR was rich, so I popped for one of the Promax metering blocks for the center carburetor. My intent was to reduce the area of the idle feed restrictor (IFR); this orofice controls the amount of air/fuel emulsion that reaches the transfer slots. Note that the idle mixture screws are upstream of this restriction, so they do not affect the transfer slot operation. The metering blocks arrived with the IFR's installed near the top of the metering block, well above the fuel level. Older Holleys have these installed at the bottom of the fuel well, below the fuel level. I had read that many folks prefer this configuration, so I moved the restrictors to existing locations at the bottom of the fuel well. Note I had to deepen the threads on these holes to seat the screw-in restrictors correctly. I also experimented with reducing the size of the IFR's, drilling out some 6/32 hex brass setscrews. The IFR's that came with the Promax plates were 0.031" diameter; I replaced these with 0.027" restrictors.

The result was a leaner cruise AFR (~13.8 or so), and I noticed the "fuel slosh" problem was gone. As detailed above, I had previously noticed that deceleration/mild braking, especially facing downhill, would lean out the idle AFR to 16-17 and change the tone of the idle as well. My concern was that the engine might stall if I really needed to brake. This behavior virtually disappeared with the reworked Promax metering block, but why? Well I can't say I understand why, but I had noticed that the stock Holley metering block uses a pressed-in tube in the idle well. The low end of this tube serves as the idle feed restrictor. Note this carburetor is a new generation Holley, not an OEM sixpack carb. The upshot is that there appears to be some dynamic fuel level behavior in the idle fuel well that is much more pronounced with the "new" idle well design.

Hope this might help somebody who has seen the same (mis)behavior with the repro six pack setup.