825 Mighty Demon tuning? 385 SBC

[73c34me]

Carb tuning help. I have cleaned up the idle circuit and the main jet sizing seems good. But- I am struggling with trade offs. When the idle circuit is slightly rich,the transition to the main circuit is without a stumble. To correct, I lean out the idle circuit more, and develop a slight hesitation as it transitions to the main jets. Would increasing the pump shot help with this? I don't have an AFR meter and just tuning up and down to move toward the sweet spot.

825 Mighty Demon
reduced IDF jets
low speed air bleeds increased slightly to delay transisiton to main
float level, set slighty below 1/2 level in window
74 main jets ( 72 is lean, 76 is rich)
Transition slot completely covered
Butterflys have 1/16" ish holes drilled
low speed 4 corner mixture screws approx. 1-1/4 turn out
idle speed 1100 rpm 10.5" vacuum
timing locked out at 35 degrees, full time vacuum advance limited to 10 degrees additional

SBC 385
11.5:1
Victor Jr.- 1-1/2" spacer open
Ported Edel. performer rpm heads
msd ignition.
73 vette
4:1 R.P. and 4 speed manual.

[CGT]

Would increasing the pump shot help with this?

That would be the first thing I would try.

[RevTheory]

Have you tried reducing the upper emulsion bleed a hair or trying just over the line on the float?

[73c34me]

No to the above. Float level easy to do. Emulsion bleed, I would need to study up on.

[statsystems]

Check your vacuum at a cruise and then open the power valve sooner.

As an example, if you have 15 inches of vacuum at a cruise, you can use a 10.5 power valve and clean up the transition because the PV opens sooner.

[73c34me]

I tried this. I don't have my notes in front of me, but I went up on the Power Value vacuum rating a bit and it created a stumble. I didn't have several different ratings on hand to try, but approx. 1 -1/2" higher rating seemed to be worse, so I returned to the 7.0 or 7.5 that was in it. 6.5" is stock i believe.

[jmarkaudio]

Try lowering the idle feed, it will usually clean up things.
https://www.tapatalk.com/groups/racingf ... t-t18.html

And try making the idle feed smaller, and compensate with smaller idle air bleeds. It will keep the transition metering active a little farther in the throttle opening to help that transition to the mains.

One more thing, you can try a slightly higher float level, it will activate the mains a little sooner.

[73c34me]

Reducing the I.F.R. and corresponding low speed air bleed has been done; definitely helps. I will raise the float level a bit and check results. Thank you for providing the link/ info for the site that details relocating the I.F.R. screw location on the carb block. I will definitely do this. I will start w/ the float level increase, followed by an increase in pump shot discharge orfice size. hopefully, this (these) will get me close, until time and the required materials are aquired to move the I.F.R. screws to the lower location.

[RevTheory]

Do you have a pump cam that will hold back on the initial shot a little to leave something there for a mid-throttle stab?

[73c34me]

Posting update results. Installed an 8.5” power valve, raised the float levels to just above the half mark line, and drilled the #31 squirted discharges to a 0.35” opening. Huge improvement!! There maybe be a very slight lean spot, but barely noticeable. I plan to relocate the IDF restrictor jet as suggested, but haven’t made time to do so. If I had a 9” power valve I would have tried it but did not. Interestingly, the engine will idle at 850 rpm, . I did not check the vacuum reading at that level. It seems to be better than before. I did install new bowl gaskets and the increased float levels helped. Before, anything below 1000rpm was unstable.
Note: I am pretty sure that one of the primary side orfices on the squirter was not drilled completely through. Corrected this when I opened them up with the 0.35” drill bit.

[73c34me]

Posting update results. Installed an 8.5” power valve, raised the float levels to just above the half mark line, and drilled the #31 squirted discharges to a 0.35” opening. Huge improvement!! There maybe be a very slight lean spot, but barely noticeable. I plan to relocate the IDF restrictor jet as suggested, but haven’t made time to do so. If I had a 9” power valve I would have tried it but did not. Interestingly, the engine will idle at 850 rpm, . I did not check the vacuum reading at that level. It seems to be better than before. I did install new bowl gaskets and the increased float levels, maybe this helped. Before, anything below 1000rpm was unstable.

Note: I am pretty sure that one of the primary side orfices on the squirter was not drilled completely through. Corrected this when I opened them up with the 0.35” drill bit.

[73c34me]

It has been a while. Most recent update on the 825 mighty demon tuning. I did lower the I.F.R.s to the suggested low position, leaving the top locations open. I also went down to a 0.028" i.d.f. orfice screw jet, this has the 4 corner idle screws 1-1/2 turns out. Vacuum is 11" at 1100 rpm idle. Timing is currently 28" intial with 36 degress total. I know the vacuum advance adds some at idle, due to the increased idle speed with the vacuum hose hooked up. Additionally, I went I reduced the squirters back to the stock size. Primary jetting. Been a couple of sizes above, then below to find the sweet spot, whichis 1 size less than it was. I believe everything else is as referenced in above post.
The throttle response is really good! I am still battling some oil on the plugs. I improved the oil splash guard design under the valve cover and reduced the size of the hose coming off the valve cover, by intalling the P.C.V. directly in the valve cover, enroute to the catch can, instead of after the catch can. This is waaay better than before, with a significant reduction (hardley any) of oil getting into the catch can. The engine doesn't smoke much at all. It will smoke at idle a little bit after it warms up; none that can be seen while driving. Additionally, the smell of "oily smoke" is gone.
I am going to get a camera extension so I can look into the ports for oil contanimation. It could the rings, but for what little it smokes, I hate the idea of tearing it down.
Thanks to all whom have given advice!
Jim

[MadBill]

Totally OT Jim, but do you perchance drive a '73 Corvette?

[73c34me]

Forgot to mention. I made an 1-1/2” thick tapered 4 hole spacer that sits down in the plenum, 3/4” inch deep. I had welded and blended 2” more plenum height, so this was an easy mod. It seems to have helped the low speed response.
73 Vette, why would u guess

I would add a pic, but not sure of the process

[73c34me]

26628C26-3977-42FA-B61E-DB8EE5730F99.jpeg