AFB Blower Carbs

[Tic's60]

Been running AFB's for a long time but have never seen this and my carb guy who built these seems to have either moved or permanently left the physical realm. Older gentlemen from New Mexico so you never know.

Anyway .. was doing some tuning on the 496 / 6-71 Moonham blower / Cater AFB 750 Comps built for blowers, and had ordered some new metering rod springs in preparation for some fine tuning.

Pulled the metering rod caps to compare how far they where being pulled down while watching my vacuum gauge and to my shock... There are no springs pistons or metering rods there.
Now being these are the 750's which are known to have a lean spot issue I am wondering if this was done on purpose and had seen a post long ago where some people actually pull them out completely on blower setups.

Anyone seen this before?

They run ok just seem a bit rich on the idle side. Also these are not new untested carbs I ran them on my 427 with a blower just never got into any fine tuning before I pulled everything. It's also easy to fix if needed.


thanks

Mike

 

[MRHP]

I have heard of removing rods and springs. Kinda like a Holley with no power valve, just jet it up. My 600 Edelbrocks have rods and springs and were spot on when we dynoed it.

 

[Tic's60]

Interesting! Makes sense just wonder how you could clean up the idle with this setup. Might have to dig out some spare pistons and rods and see what this changes. While running you can see the fuel spray is pretty heavy and anything above 1 1/4 on the screws it starts fouling bad and anything under 1 and it starts leaning out way to much. Anyone remember what the jet size to drill bit size on the jets? Might just try and smaller jet first to see if that calms things down.
and thanks for the info!

 

[Don Jacks]

Tic,it sounds like you need to lean out the primary a bunch.Think of it this way;in stock form the engines jetting is determined by the amount of fuel that passes through the jet with the rod in place.Removing the rod makes the jetted area a lot larger[richer].Another thing to check is if the vacuum source for power piston has been plugged off to avoid uncontrolled air in the carb circuits [vacuum leak].I would put back in the piston,rod and spring.Your combo,while roudy.will not need anything more than a good tuning with the right rods,springs,and jets.it'll just take a little tinkering to get there.

 

[Tic's60]

LOL I like that word "Roudy" You may be onto something there as the vacuum is unsteady though I was thinking large cam and I already did a leak test to make sure everything sealed up. My Dad has a few crates of these carbs so he's digging some pistons up. Looking through some notes from others looks like .116" mains, .110" secondaries, and .073" x .047" metering rods did well on a similar build.
and thank for the tips to check!

 

[MRHP]

I was thinking about the vacuum leak with the piston out as well. I would install a light spring, piston, and no rod. The spring is just in there to push up the piston so you can easily get it out. See what your idle does then.

 

[Tic's60]

Sounds good! Where would he have blocked the vacuum source for power piston other than the air bleed?

 

[Don Jacks]

Look at the bores where the piston goes,there should be a passage,then follow that down into the base of the carb.Air bleeds are a separate circuit.

 

[MRHP]

Don knows his S H I T !

 

[Tic's60]

he does :-)

 

[Phil Reed]

Dr. Don!!!!!!! As opposed to Dr. Phil!!!!

 

[Tic's60]

Finally got back to the carbs. Seems he left the hole to the carbs base open and plugged the metering rod hole So there's the vacuum leak Mikes Carb service has the parts on the way so maybe this weekend I can fix her right.
Did a test using some lead plugs and the vacuum at the carbs is staying a solid 12 at idle and it runs better too.

 

[Tic's60]

Dropped in the new jets, metering rods and some 8hg springs per my vacuum reads and everything's running allot smoother. One thing I am using now that may or may not interest others is a a product by 14point7 called iDash that allows you to monitor your motors AFR, Oil, temp EGT and more. My reasoning was I want to keep my stock gauges yet have some modern features that can keep an eye on things and capture logs. https://www.14point7.com/products/idash without breaking my budget .. if I had one lol! The other point is I can take my cheap tablet and work on the car, idash is wireless, and then put it back in the car where is will also do double duty as my mp3 player.