Jet up with an MSD box?

[450bench]

On the 9 to 1 aluminum oval port headed 454 with the smallest thumpr cam, streetmaster single plane and 870 vac.sec. Holley, MSD pro billet and 6A box, the car just ran like garbage with my vacuum advance. Sputtered and fell on its face. Without the advance hooked up it ran fine.
After a buddy told me to jet the carb way up, I resisted for months. But I wanted to run the advance so I figured I’d try it so I jetted it up and put in a higher power valve. Monster difference and runs great. Vacuum advance also helps a ton obviously cruising around.

Is it the hotter spark requiring more fuel?
Went to 80s with a 9.5 PV and 90s with no power valve.
Plugs are perfect.

 

[G72Zed]

Is that Holley 870 carb an "Avenger" series ?? as in it's got straight leg boosters but with a 650-800 sized base plate ??

Sounds like the higher PV opening and VA was the biggest help and you were lean, and not surprised on going up in jetting helped on your combo with this carb.

I've had many of these "Avengers" across my bench, let's just say there not my favorites.

 

[450bench]

It’s a street avenger 870. Down leg boosters and a the stock baseplate which is not a 650-800 unit. It’s like an 850 dp baseplate with 180 on the butterflies but for a vac. Sec. part number is 80870 from the mid-2000s.
They’re great carbs for big blocks if you can figure out how to set them up. I usually leave the stock 78 jets in the front and replace the ridiculously low 4.5 power valve with an 8.5. On the secondary side I always remove the 3.5 power valve, plug it and run 90s. If the vacuum pod has a checkball, remove it.

 

[G72Zed]

Ah, an older unit then. I thought sure I remembered the "newer" ones having straight leg boosters and 1.688 base plates, one of those different setups...maybe I'm thinking of the QFT stuff.

Well, if the venturi and base plates measure up like the traditional 850, (1.562 x 1.7500 they are in the 80-82 ranges for the most part. Does it have the vacuum tube port in the primary as well ?

They come with tight power valves because everyone repeats/believes the same thing- "idle hg/2+.5" ...that will never die.

Does it have a high set or low set IFR ????

 

[fbody_mike]

As to changing the jets base on ignition. I have found that a stronger spark is needed to fire a more lean fuel mix. The leaner the mix, the bigger the sparkplug gap will be needed. You should not need to change the fuel mixture for ignition changes unless your fuel mix was wrong to begin with.
Ignition timing plays with fuel mix too. To get away with a leaner fuel mixture, especially at cruise, you will need to have more timing advance. This is where the vacuum advance can really help.

As for going richer and getting better seat in the pants response, that means you are going the right direction, go too rich and it will start to feel lazy. If you desire to get some MPGs back you can then work on going lean when the vacuum is high, and working out the bugs during the transition to acceleration.

I bought an AFR gauge about 6 months ago, and been super tuning ever since.

 

[450bench]

Good stuff.

Yeah G I think it’s an older 2 corner model. It does have the secondary tube as well.
I’ve always been a proponent of high power valves because the cut the idle vacuum in half is bs.

Mike the vacuum advance is such a bonus with this combo. Night and day difference.

 

[G72Zed]

Well, sounds like you got a good one with that vac. tube to the front, nothing wrong with a well setup 2 corner idle either.

I always say give the engine what it wants and what it responds + too, every engine/combo is different.

I've tuned/repaired many Holley's with issues due to using to tight of a PV, especially in manual setups.

 

[1980RS]

Jetting up or down will depent on the engines needs at WOT. The MSD will keep the engines plugs cleaner at lower speeds and after about 3500 rpm or so it just turned into a big CD box at high rpm.