little carburetor experiment

vector1 · #1 (**permalink**) 10-01-2010, 02:45 AM

i have my engine up & running, it's a 9-1 383 with a rpm air gap manifold, afr205 heads, & pretty good size solid roller cam.

i had a good carb on it, looked to be a 750 size and it ran well, but i wanted to go with a bigger carb and had a bg 850 rr sitting on the shelf, so everyone knows what happens next.......

the bg carb was already ran on the previous engine, i had changed the pv, pvcr, & main jets. the previous 750 carb builder on his carb told me to cover the t-slots with the butterflies so they were just uncovered when the throttle blades moved. so everyone knows what happens next......

the 750 was made for another engine and had holes drilled in each butterfly to keep the idle with the butterflies near closed. btw, when you close the butterflies and hold the carb up to the light, there is all kinds of area around the butterflies to let air through, don't let anybody kid ya.

so i took the 850 and shut the butterflies so just as they moved the t-slots were uncovered. then i went down to the store and bought the smallest carbide bits available, 1/16". i was considering taking the butterflies out but the screws are hammered on the bottom and not coming out. a person wants to use the smallest holes here and work up, as there is no going back, other then soldering the holes shut and redrilling, which i didn't want to do.

without the holes the 850 would barely idle, with the 1/16" holes it was idling at a little less then 1000, i wanted around 1200. next size up was 5/64--this made for the correct idle rpm right around 1100-1200. keep in mind, when these holes go up, afr correction might need to take place to adjust for the holes and the fact the idle rpm is not pulling fuel out of the t slot. don't get your hole sized and then adjust the afr and end up with a runaway. best to end up a little low on the rpm or small on the hole and drive it around and make adjustments to see where things end up. so everyone knows what happens next.......

i take it out for a cruise. im using a afr gauge as my visual indicator btw as the butt meter is a little fuzzy nowadays. everything works great, cracking the throttle doesn't go lean, stays right on the money. i cruise at 14-1 range, depending on the grade. i can watch the mains come on with pv's and the afr goes to 13-12.5. there is a lean area after cruise on slow acceleration before the mains open that i'm going to have to take care of with the iab's &/or ifr's. no hesitation or anything, just lean into the 17-1 range.

so i would say the experiment is a good one so far. everyone knows what happens next.......

Hotfingrs · #2 (**permalink**) 10-01-2010, 03:17 AM

I don't know of one instance where you will find printed material telling you to cover the transfer slot. The slot should be exposed .020 at idle, and with today's technologh in carb, there should be no reason to drill the butterflies, that's old school. I've probably got one of the most radical engines on the street, and have never had to resort to this to get the idle where I wanted.

stallion112 · #3 (**permalink**) 10-01-2010, 05:01 AM

I agree, the slots should be exposed around 0.020 or to form a "square" shaped slot (width ~ length of exposure).

A much easier way to get more air in rather than drilling holes through the butterflies is to crack the secondary butterflies just a tad. It's worked for me on several engines. The set screw is accessed from the bottom of the base plate under the secondary linkage. It acts as a stop for the linkage.

Just a small adjustment ..... a little crack will go a long way.

vector1 · #4 (**permalink**) 10-01-2010, 06:27 AM

there's more then one way to skin a cat, i wouldn't say one way or the other is correct to the exclusion of the other.

but you have to wonder, why do you want to pull fuel off the tranfer slot for idle?

Jerry Clayton · #5 (**permalink**) 10-01-2010, 07:42 AM

they are called transfer slots for a very good reason

your lean spot on acceleration from cruise is power valve related

why didn't you just adjust the bg idleze screw for idle speed???one of the best features on his carbs( besides the billet plates)

vector1 · #6 (**permalink**) 10-01-2010, 08:01 AM

Quote:

Originally Posted by Jerry Clayton

they are called transfer slots for a very good reason

your lean spot on acceleration from cruise is power valve related

why didn't you just adjust the bg idleze screw for idle speed???one of the best features on his carbs( besides the billet plates)

good question, it doesn't have the idle eze, it has the threaded portion, but nothing to adjust. the bg book also lists the carb as having air holes in the butterflies, no air holes. i don't think i'm far enough into the throttle to get into the main circuitry, not even opening the secondaries, probably right before. i was comparing their other carbs and i'm going to mess with the iab's, i think they are larger on the race series carbs for the guys running single plenum intakes, whereas the road series for dual plenums are jetted a little different, i'm using a race series carb on a dual plenum.