Back on the Summit Carburetor topic

If purchasing a new carburetor my first choice would be the 2 BBL Holley 2300 PN 7448 with the electric choke kit PN 45223.
The carburetor is rated at 350 cfm @ 3" Hg pressure drop compared to OEM Carter YF or YFA at 195 cfm at 3" Hg.
To normalize these two carburetors to the 4 bbl flow ratings at 1.5" presure drop must divide by the square root of 2, since flow is proportional to square root pressure drop.
YF rating = 138 cfm @ 1.5"
Holley 2300/7448 rating = 247 cfm @ 1.5"

Compare that to the cfm ratings of a new Summit 600 cfm square bore, Edlebrock 500 cfm square bore, Holley 390 cfm square bore, Holley 80555 650 cfm spread bore.
The holley 80555, based on the photos presented by "Abondoned Bronco", has primary flow area equal to 1/3 of the total flow area of primary + secondary.
So, the flow rating of the primary = 1/3 x 650 =216 cfm @ 1.5"

The Holley 390 primary = 195 cfm @ 1.5"
The Edlebrock 500 primary = 250 cfm @ 1.5"
Summit 600 primary = 300 cfm @ 1.5"

Based on the primary flow ratings the Holley 650/80555 secondary opens at 2950 rpm
Holley 390 secondary opens at 2650 rpm, Edlebrock 500 opens at 3400 rpm
Summit 600 opens at 4050 rpm.

However, the Holley 2300, 350/7448 tops out at 3400 rpm with 1.5" Hg pressure drop and 4700 rpm with 3" Hg pressure drop, having no chance of secondary bog.


Note: rpm calculations are based on VE = 85%, basically stock

Having the secondary open at 4050 rpm for the Summit 600 carburetor seems to be a total waste for the 4.9L I6.

My 4 BBl top choice for the 4.9L would be the Holley 650/80555 spread bore rather than the more popular Holley 390/8007, which is basically designed for a 2X4 tunnel ram running dual quads.

The YF tops out at 2650 rpm, based on 3" hg pressure drop.

 

I installed the shield and will check this morning if the bowls are empty. Over the next few days I'll know if the hard hot start persists. I also switched from the 4 hole plenum to a 1" open type and could tell right away a difference in idle quality, smooth and purring.

Now I'll work on a cold air air cleaner.

 

"Having the secondary open at 4050 rpm for the Summit 600 carburetor seems to be a total waste for the 4.9L I6."

That makes a point that I have tried to make in past cussins and discussins about the 'too big' carburetor. The 'too big' part is what's left unused and unusable by the engine. This is what I have put forth in the past:

"It seems that the misconception comes from looking at the total flow capability of the carb and somehow thinking that the engine is going to use or be forced to use all that capability. It just ain't so. A 4v carb is nothing more than a 2v carb with an auxiliary power circuit of 2 more vs. On a vacuum or air-flap actuated secondary carb the secondaries (when properly adjusted) do not even start to open until about 2500 rpm-3000 rpm and on most of our engines will not ever see full opening of the secondaries even at 4000 rpm-5000 rpm. The carb does not determine how much of it's capacity the engine will use...the engine determines that."

Kevin's engine is going to use all of the carb's (Or any carb's) capability it (the engine) can use with some change left over...which isn't necessarily a bad thing.

 

Something's not right with that. I'd put the carb on top of some paper towels or whatnot and use a squeeze bottle/fuel line to fill the fuel bowls so you can check it for leaks.

 

The sticking throttle has returned. The problem, I have found, is the fast idle cam/arm sticking. Some times it hangs out of the blue while driving. I let up at a light, or start to coast, and can feel the rpm's are 200 or so higher than idle. It is most annoying.

I agree with Harte3. I ran an 800 cfm Quadrajet on my engine. With the small primaries it was peppy, and loved that moan when the secondaries opened. I would add to what Harte3 said: It is the diameter of the primary venturi that will make the vehicle peppy or sluggish.

BVA: You are correct. I almost don't want to know, but have to check it out.

 

"... the fast idle cam/arm sticking..." That happens on my carb on occasion usually in the coldest of weather.

 

Yesterday it was almost 80* in LA. Mine is sticking 5-6 times per day.

 

When I say the coldest of weather I mean from the low 30s on down...I love 80* weather!

 

Harte 3: Anything below 60* is illegal in S.Cal.

I installed the larger air bleeds in the carb w/o tuning with the wide band. I put 2.5 gals in the tank and drove 15 miles, 90% on the freeway. When I returned home the tank was reading empty. Ouch.

 

IMHO...its the brass bushings on the throttle plates...they can get worn and as the carb heats and normal slight flexing occurs, the bushings which hold the rod straight tend to tweak just enough to cause the sticking. They are replacable.

 

1981 stock F150 custom 300 has a heat shield below the exhaust manifold connection.

Might consider applying thermal exhaust pipe wrap on a short section of tail pipe
downstream of the CI exhaust manifold connection.

 

Thanks for the help, guys. I've located the problem. It is the choke fast idle linkage. It is just the dickens to see in there when it's mounted, and off the vehicle I can't replicate. What concerns me is how is the fast idle coming into play when the truck is fully warm and driving down the road?

 

I can't remember, so if you have previously detailed this, I apologize...

Are you powering the choke heater directly off the s-stator on the alternator? If so, maybe the ~7 volts is enough to initially pull off the choke upon start-up, but not enough to properly control the aftermarket choke assembly.

Some others have recently mentioned using the 7 volt signal to trigger a relay which is powered by the battery, thereby providing 12+ volts whenever the engine is running.

 

As directed, I'm using the full 12v to the choke.

The sticking throttle was really annoying me so I spent some hours pursuing the solution. I found the clip that holds the choke flap rod was interfering with the fast idle. Problem solved.

Also, the heat shield seems to be doing the job. This morning there was fuel in the bowls! After several pops to the market, etc., it started like a top.

Recently I changed the idle air bleed restriction from a .041 to the original .046 as a reference. My mileage dropped by a huge margin. So I tried using a smaller primary jet size. I went from a .065 to a .064. It didn't like the 64's before, so I took the 41 restrictions and used a pen drill and opened it .002, and bingo. As soon as it started it purred. Soon I will put the o2 gauge on it and I'll know for sure how it does. I thought the idle air bleed was like the air bleed on a Quadrajet, and the larger the orifice the leaner the carb ran. And then to increase the idle fuel you bored/enlarged another restriction inside the carb, but on this carb that bleed in the booster controls the idle. The larger it is the more fuel at idle.

 

I made an interesting observation: The primary jets for autolite 4100's, 480 and 600 cfm sizes, range around .045. With my Summit Carb, a 600 cfm, the primary jets are in the .065 range. That is nearly 50% larger primary jets with the Summit carb. I will take a wild guess here and say that the Summit carb is designed for a larger engine, designed to flow more.