Carb tuners suggestions.

[aaronk785]

I just installed an aft gauge in my dart. Engine is a 408 9.8 comp. 254 260 at .050 cam on a 108 in at 104. Speedmaster heads, torker2 intake and a pro form 850 do carb. Readings at idle are 13.8. Tip in goes to 10.9 then about 12.5. Cruise is about 12.8. WOT is at 12.0. When I accelerate from cruise it goes to 14.0 and push down a little more and it goes to 12.5. I know how to lean out WOT but what bothers me is the 14.0 at part throttle because it pings sometimes. Any suggestions are appreciated.

 

[RustyRatRod]

Sounds like you need to tune for a cruise opening power valve.

 

[Just Send It]

The AFR gauge is just a tool with a margin of error.

What do the plugs say? The plugs tell the real story. I'd be interested to see where your timing mark is on the strap.

 

[Murray]

Timing?

 

[Newbomb Turk]

You jet the primary side for cruise AFR.

So keep dropping the primary main jet until it’s too lean at a cruise and then go back up two sizes and don’t touch the primary main jets again.

You tune for WOT with power valve timing at the power valve channel restrictors on the primary side and the secondary main jets.

You will most likely need T slot restricters as well, but you can deal with that after the rest of the tune up is acceptable.

Also what is your initial and total timing and at what rpm?

Edit: does that carb have replaceable air bleeds? If so what size is it? If it’s bigger than .026 make them .026

 

[aaronk785]

Timing is 22 initial and 34 total and 50 at cruise. HSAB are .032. Total timing is at about 3000. Emulsions are .026 top, .026 bottom and the middle is blocked. Jets are 78 and 86. Air bleeds, channel restrictors , emulsion jets are all replaceable. Thanks everyone.

 

[Newbomb Turk]

I’d start by getting the MAB down to .026. That will delay the start of the booster.

Once you do that, start dropping the primary main jet down until you get it lean enough it stumbles just a touch on relatively flat ground.

Go up two sizes and leave it and move on to other tuning.

 

[aaronk785]

Thanks everyone. I’m getting closer. I ordered some air bleed jets. I did change my jets. Went down to 74 in front and 84 in back. So down 6 all together. New readings are idle 14.0, cruise 13.9, tip in 12.0 and WOT is 12.7. I created one problem. I wasn’t driving far enough on my test drives. The last one I did about a 15 mile run. And reading were a lot better after about 8 to ten miles. My dumb. I’ll still work with the bleeds to get it even better. Maybe go down to 72in front and work on the power valve restrictors.

 

[Illahe]

Thanks everyone. I’m getting closer. I ordered some air bleed jets. I did change my jets. Went down to 74 in front and 84 in back. So down 6 all together. New readings are idle 14.0, cruise 13.9, tip in 12.0 and WOT is 12.7. I created one problem. I wasn’t driving far enough on my test drives. The last one I did about a 15 mile run. And reading were a lot better after about 8 to ten miles. My dumb. I’ll still work with the bleeds to get it even better. Maybe go down to 72in front and work on the power valve restrictors.

Side bar: Nice to see a member post a 408 combination with enough camshaft duration (Engine is a 408 9.8 comp. 254 260 at .050 cam on a 108 in at 104).

 

[TT5.9mag]

With that amount of vacuum advance and a curve that comes in pretty quick, and a good amount of initial timing (all variables you are working with) you might have to slow the curve down through the middle to get it not to rattle. Your afr SHOULD trend leaner when applying the throttle until power enrichment comes online.
What did you do to lean out the idle? I didn’t see it mentioned.

 

[aaronk785]

I did nothing to the idle circuit. It didn’t go up much 13.8 to 14. When it warms up the idle goes up about 100 rpm or so. I think I was reading the gauge a little early at first.

 

[TT5.9mag]

I did nothing to the idle circuit. It didn’t go up much 13.8 to 14. When it warms up the idle goes up about 100 rpm or so. I think I was reading the gauge a little early at first.

Ok cool. Also, the o2 has to heat up for a while before it’s accurate. So that’s another variable

 

[Newbomb Turk]

Thanks everyone. I’m getting closer. I ordered some air bleed jets. I did change my jets. Went down to 74 in front and 84 in back. So down 6 all together. New readings are idle 14.0, cruise 13.9, tip in 12.0 and WOT is 12.7. I created one problem. I wasn’t driving far enough on my test drives. The last one I did about a 15 mile run. And reading were a lot better after about 8 to ten miles. My dumb. I’ll still work with the bleeds to get it even better. Maybe go down to 72in front and work on the power valve restrictors.

Excellent work.

Keep tuning on it. The rewards keep coming.