Inliners International Forums General Discussion Engines Low CFM 4 barrel carb options?

I have been trying to make a 395 cfm Holley 4 barell run smoothly on my 302 GMC engine. It hesitates at low rpm and during takeoff. I have checked for vacuum leaks and changed the accelerator pump nozzels and cams but the hesitation is still there. The car really goes well once it gets past the hesitation problem, but it is very difficult to drive.

Does anyone I know if there is a low cfm Rochester or Carter 4 barrel that I could use as a direct replacement?

Check the power valve/s. Too weak and it'll flood slightly. Too strong and it leans out.

Is the intake heated?

IMHO there are several options. One would be a TLowe "3 barrel". Basically it's just a bigger Holley carb than you have with one of the primary barrels plugged. Another option, would be a Rochester Q Jet. By limiting the amount of secondary air valve opening the CFM could be adjusted. This was done from the factory on smaller engines like the 305 Chevy. By putting a screw in the air valve the CFM could be adjusted from 750 to well below 500. There is also a Carter AFB. There is a 500 CFM model and a 400 CFM model. The 400 might be hard to find. In the marine world, the 625 model was put on a 262 cubic inch Chevy v6. I don't know much about 302 GMC engines and their air flow needs, just throwing some ideas out there to think over. How about an Autolite 4100 for a 289 ford? I think that they are some where in the 480 CFM range. I hope this sparks some debate and exchange of ideas. Jay

A properly set up and adjusted Q-jet would probably be the most drivable!!

I agree about the Q Jet. A small 4bbl that I forgot about is a Rochester 4G series. This was the GM produced 4 bbl from the early 50s until about 1966. The model used on the power pack 283 should be in the ball park for size. I have only owned one of these carbs in my lifetime. It ran ok but I had someone with 35 years carb experience help me set it up. Very sensitive to float adjustment and drop. I was also told by people who were around this carb when it was new that it was considered "thirsty" on gas. A 302 GMC with a Q Jet or 4G would be pretty unique. In my mind a 302 GMC is pretty unique by ITSSELF in a good way. Jay

The smallest 4GC carbs are still about 485 CFM. The early WCFB Carter 4 bbls. are around 375 CFM.

Are WCFBs hard to find? I wish I knew more about them. From what i have read they were used on 283 Chevys also. IMHO the combo of a 302 and a carb in use from that period would be pretty cool. If anyone here ever ran one I would like to hear how it turned out. From reading at the GM heritage center the primary Venturi had a 1" diameter. Certinally in theory at least small enough for a good airflow siginal at low engine speeds. Airflow signal is one of the reasons that TLowe invented the "3 barrel", trying to move air fast enough to make the carb work more efficiently. If I had a stack of 4 bbl carbs I would gladly loan them to the OP to see what worked best. One question, how much carb does a 302 GMC need compared to the same size SBC or SBF? Jay

No, the WCFB's are not rare by any means. Also, many of the Pontiac OHC 250 engines had Quadrajets on them and were very driveable and had good street manners. The idea of improved air signal is one reason why that design of a spreadbore carb works good because of its greatly reduced primary bore size. Given that, a Holley 650 spreadbore carb would likely be a better street performer on many of these 6 cylinder engines including the 250's and 292's than a Holley 600 squarebore carb. Even though overall it flows a higher total CFM, simply because it utilizes that design of smaller primary vs. secondary throttle bores to be more responsive, efficient and driveable in slow street driving RPM's. One thing that is often overlooked today and in times past with hotrodders is using the right part for the job, many do not. Also, Holley invented the 3 bbl back in the early 1970's, not Tom.

Holley may have invented a 3 bbl before me, but theirs was for a whole different purpose. It was a big performance carb for big engines.
My 3 bbl is trying to provide economy and performance for small engines like ours.

One more small 4bbl option is the old Autolite 4100 4bbl. The smaller 1.08 venturi version has a little better manners, but the bigger 1.12 venturi works good too, it is my go-to carb on my 292. And since they are OEM Ford carbs you can generally re-sell them later, especially the 1.08s, the Mustang guys like em.

That said, I wouldn't give up on that 390 Holley yet!
* What list number is it? A 4150 (2 metering blocks) or 4160 (secondary has no block, just a plate). What number is the metering block (just to see if things match up).
* For now, just TRY locking the secondary SHUT, that rules out the possibility it is opening early & causing trouble.
* What jets are in the primary, what number power valve, and how big are the PVCR holes (Power Valve Channel Restriction) the fuel feed holes when you take the power valve out?
* Clean the carb and metering block in brake fluid or diesel, then blend over to gasoline, then over to laquer thinner, then over to acetone, blow dry at each step, kinda like prepping a Super Model's hair, which is how Holley's like to be treated, they are spoiled cuz they're fast and so fussy.
* Check the body-to-baseplate gasket carefully, is it newish & do all the holes line up - the 390 gasket is different so that can cause trouble.
* Make sure the float levels are right up there at the sight plug hole, you've probably done that already.
Well that's a start anyway.

Deuce, there are plenty of good options out there for these engines. The Autolite is often overlooked, but ideal for inlines because most of the engines they were designed for were about the same size as most inline 6's. Also, the WCFB's were actually intended to be used on the Clifford's and Offy inline intakes as well as the Flathead Ford versions of the Offy, that's what the bolt pattern is without the bolt-on adapter plates. I guess they knew a thing or two about carb sizing back then before we got all smart and started thinking "bigger is better".

Wow! Do you mean that if a WCFB were used on an Offy or Clifford on a 250 Chevy the adapter plate that everyone talkes about that goes with the manifold would not be needed? What other carb would fit without the plate? 4gc Rochester? From my limited experience with the 4100, it's very simple and easy to tune. I ran a 2100 on my 79 250, the 4100 is almost two of them stuck together. I looked on E bay for WCFB carbs. They aren't all that bad price wise unless the word Corvette is involved. I agree that bigger is not always better. That 1.08 autolite 4100 sounds like something I would try if I had the 302 in question. One thing that IMHO needs added to the discussion is what are the airflow requirements of the 302. Does it have head work or a hot cam? Is the head design good or bad? Jay

Many online carb CFM calculators are out there, but a 302 such as this street driven one shows a 399 CFM at most with a redline of 5000 RPM. Of course that is way too much RPM for these engines on the street, so a 390 carb should be more than enough for it.

Jay, most of the Ford V8 engines that used the Autolite's mentioned above were anywhere from 260-302 cubic inches, so ideally the same as the inline 6 Chevy 250 and 292's, and of course most of the Jimmy's from 270 to 302.

That is kinda what I figured. Any small 4 bbl that would work good on a 283 Chevy or a 289-302 ford would be good for a 302 GMC. I know that in the past I have been guilty of putting too big a carburator on whatever I was running at the time, with all the bad manners that go with that kind of desion. I went to a pretty big swap meet today, unfortunately no WCFB carbs for sale. Not trying to hijack the thread, but how about a 2bbl instead? If a good balance could be had between size and low speed air volicity that might work also. Just a thought. Jay

Another thing we are guilty of many times is "tunnel vision". Which quite honestly is failing to see what other engine groups are doing to combat this issue. The Slant 6 guys battle and overcome this problem daily with there engines that range from 170, 198 and 225 cubic inches. By all accounts, even a 390 carb on most anything but the most radical firebreathing 225+ cubic inch Slant is too large for these engines. They most often use a 350 CFM 2 BBL. carb to handle their street driving and racing chores with no problems.

Its also a compound problem as well. The solution starts with the intake, and if the plenum is too large, this begins the spiraling downward of slow airspeed for the fuel charge. At this point many times, even if the right carb is chosen, the wrong intake often is overlooked as the true culprit in this situation. And then the carb gets the bad rap for being the problem. I guess the main thing is the lack of knowledge to make the right chooses for parts selection and understanding how they must be compatible and work together.

I try to read the slant 6 forum when I can. Running intergrated heads, I at least don't have a manifold problem/ choice! The only problem left is trying to adapt a Holley 1100 CFM dominator carb to a 1 bbl hole! Seriously though, are Holley 2bbl carbs rated at 3.0 HG pressure drop like Rochester carbs? If that is the case then the 350 Holley would be a close match for the smallest 2GC which is rated at 352 CFM @3.0 HG. That would be about 248 CFM in 4bbl terms, while the 500 Holley would be About 354 CFM. IMHO the 500 would be a poor choice because if you suddenly floor it at low speeds it would almost be like a 390 Holley as a double pumper! Jay

All the Holleys from the Old Days say pre-1980 or so are rated at 3.0" Hg for 1bbl or 2bbl, and 1.5" Hg for 3bbl or 4bbl.

Some 4bbl carbs were rated at 2.0" Hg even back then and some are even rated at 1.0" Hg today so it helps to be sure.

So yes, the smallest Holley 2bbl is about the same as the Primary of a Holley 390cfm 4bbl.

And, the big Holley 500cfm 2bbl is exactly the same as the Primary of a Holley 780cfm 4bbl.

I will try to throw on some bigger carbs during the next carb shootout. Problem is, at my level about 175hp currently, I can run eg a 600-625cfm Carterbrock, but the floppenvalve basically just doesn't open, so its running on the primary plus secondary transition circuits, about 300cfm.

When I run a 450 Holley, the secondary opens maybe half way, so again its at 300-plus cfm.

On a 600 Holley my bet is the secondary wouldn't open at all so in effect I'd have a 1-1/4" primary 2bbl.

And the 780 would be a good test as it would be the same as running a "500cfm" 2bbl, only heavier. It would be a good test of launch and low-end though.

DeuceCoupe, I remember the carb shootout pretty well. I believe that you were running a near stock 250. IIRC the 390 Holley was able to be stomped to the floor off the line, the AFB could not. How big is the primary barrel and Venturi on a Holley 780? Are the primary and secondary sides of the 780 the same size? One thing that I have read about is a new to me size of the small base Rochester 2gc. The one that I am fimiliar with has a 1 7/16 bore with a 1 3/32 Venturi that flows 278 CFM @3.0. The one I just read about has the Venturi size increased to 7/32 and was rated at 311 CFM @3.0. Has anyone on this board actually seen one? Jay

Thanks for all the inputs. I will look for a replacement for the Holley but I would like to get it to run decent as well.

I'll try to reply some of your comments and questions below in order of your replies.

1. Blackwater. Power valve seems to work well. The car pulls very well from mid and high rpm. But it almost shuts off transitioning from idle to around 1200 rpm.

2. tlowe. Intake is not heated but its runs in mild Calif. climate.

3. integrated j 78. Good options. Are adapters available to mate these carbs to the Holley bolt pattern on the manifold?

Radar, I believe the autolite 4100 is the same bolt pattern as the Holley. There are adapters for the WCFB, 4GC, and the Carter AFB. If you want to try a 2bbl there are adapters for that also. Using an adapter to run a 2bbl on a 4bbl manifold is ok. My boat came from the factory that way. Running a 4bbl carb on a 2bbl manifold is not so good. Same thing with running a smaller pattern carb on a Holley pattern manifold not a problem. Do you have extra hood clearence? Jay

"\]More replies from Radar (Frank Hainey).

4. Blackwater and 5. Blackwater - Sounds like the Q Jet is a good option for me.

7. 7integrated j 78 - The 283 SB Chevy would take more CFM than the 302. It has better volumetric efficiency with 1 intake and exhaust port per cyl and the top rpm is at least 500 rpm higher. 400 CFM is about right for the GMC.

10. Duce Coupe - I have is a factory rebuilt Holley 8007 for six cylinder applications. It is has smaller bores and venturi than the 4610 600 CFM carb with vacuum secondary. I wired the secondary barrels shut but it still dies between idle and about 1200 rpm.

More replies from Radar (Frank Hainey)

12. integrated j 78 - The WCFB and Autolite 4100 look good too. The air flow requirements for a mild GMC 302 are about 400 CFM. It's like an overgrown 235 or 261 Chevy, with a 4 in bore and stroke. The head design is poor with 3 intake ports and 4 exhaust ports. It's ported with larger valves. The cam is a mild Isky 3/4 cam that idles very well. It pulls my 3400 lb 40 Chevy coupe with a T5 and 4:11 rear nicely. Currently run 3 small progressive Webers with about 600 cfm total, but 395 CFM Holley seem to pull better once it gets through low rpm flat spot.

Radar,
I have your 40 Chev in the Gonkulator database as running triple Pinto 2bbl's (the Holley/Weber 5200's). Looks like you swapped intakes? Is the rest of the engine the same ie 228-228 duration cam, headers, etc? With the ported head I had it Gonkulating to about 250hp, in which case it would be happy with 450-500-550cfm on top. A 390cfm should be ok, but what I'm thinking is are the idle transition circuits too small and/or not getting a good signal due to that big cam?

Simple fixes:
1. Turn the idle mix screws OUT as much as you can, not til it slows back down, but get your best idle, then go out another 1/2 turn or so. Maybe more, its easy to try.
2. Make sure the float level is high, gas should dribble out of the sight plug when you take it out.
3. Jet it richer, from the Holley book a #51 comes stock - between our CaliCrap Gas, the cam, and the headers, I'd run at least a #53, maybe #54 main jet.
4. Also make sure you have plenty of initial timing, say 12 degrees, and a fairly quick curve, and that your vacuum advance works. If its not getting enough advance it will complain off the line for sure.

Beyond that the fixes are more complex but I would try those first. Then again its good to have 2 good running carbs anyway so looking for another one wouldn't hurt.

Now and then I sell 4100s but the only one I have ready right now is a 289 HiPo Fairlane carb and its $$$$. I like to get em running first, usually on my 292 six, then make a video of them running & driving. Not a guarantee but a start. But right now I have 3-1bbls on the car so I cant get any carbs ready for a while.

The 4100s aren't too hard to find, if you score one at a swap meet that hasn't been left out in the rain, they usually clean up pretty good. They are so simple you can literally run them with the lid off to check the float levels. And like I said they are pretty re-sellable cuz most of the pre-1967 Fords used em.

Another good pick for that hot 302 would be a 4bbl from a 1970s Ford truck, these were a 550cfm 4bbl, still just about right for your rig. They are good runners, made for a mild 390cid so plenty of idle circuit, and again they are around at swap meets.

Thanks Radar, for helping me learn about the 302. I wish that I had more Holley experience so I could help. DueceCoupe is on the right track IMHO. At some point between idle and man metering you are either running out of fuel or some passage is plugged with dirt ect. On my boat which has a Holley 2bbl it started surging and bucking at about 1300 rpm. Idle and anything else was fine. Took it apart and found a small piece of dirt in one of the passages in the metering block. Now it runs ok. One question, if you open the throttle very slowly in first gear from idle does it still have a flat spot? Jay

The slant six guys have had good experience with the 390 CFM Holley.
http://slantsix.org/forum/viewtopic.php?t=18931

I upgraded from a 9625 AFB to a Quadrajet on my Barracuda.
write up here: Slant Six Quadrajet

Just to mess things up what are your thoughts on something like the small fitech unit. Seems like it might be a good match and not have issues with being over carbed.

the 1985 4.3l v6 were qjet equipped. Thats what I am running now, and love it so much , I hunt them down off fleabay and hoard them....lol

Thanks for all the replies suggesting low cfm 4 barrel carbs instead of my Holley 390 cfm which boggs on takeoff.

I took the time to read "Holley Carburetors" by Dave Emanuel from S-A Design and learned the Power Valve (PV) plays a big role in the transition from idle to cruising speed. The PV adds fuel when the vacuum drops below a set level to enrich the mixture and keep up smooth acceleration from idle to cruising speed.

PVs come with one vacuum setting, but are available from 3.5 to 10.5 in Hg. settings. The trick is to find the right PV for your engine. Valve spring constants determine the PV settings.

My Holley carb came with a 6.5 in. Hg. PV. I change to a 7.5 PV- less bogging resulted. Went to an 8.5 PV and bogging was almost completely gone. I just ordered a 9.5 in. Hg PV and think that will fix the problem. Holley PVs are about $9 plus shipping. Hope this helps others with Holley bogging problems.

Or for a low buck efi - megasquirt II (MS II) with a TBI off of a 4.3 V6 - dirt cheap. The whole deal can be done for ~$500. Full fuel and later add ignition control - use the existing dist. with a pertronix conversion to trigger the MS II.

$9.00 to fix the Holley or $500.00 to put EFI on instead? Hmmmmm.......

If you include the cost of the Holley carb plus the 4 barrel manifold plus my power valve purchases, the EFI cost is probably comparable.

I'm just saying that you're already this far into the Holley setup. Fixing what you have is likely a MUCH better plan, especially since you're already down to fine tuning!!!

You likely don't have the squirter adjusted properly. The bolt on the black plastic arm has to be adjusted so it's touching when at idle.

Try there before spending $500+ on efi.

Thanks for the input. I tried many adjustments to the accelerator pump circuit changing nozzle sizes,cams and arm adjustments but it still bogged. Changing to higher vacuum level power valves made the bog problem get better.

It seems the accelerator pump gets you going over a short rpm range when the accelerator is pressed. The power valve picks up from there and covers a wide rpm range. If vacuum drops more gas is added to keep the rpm up without bogging.

I do not intended change to EFI, especially now that my Holley 390 is working better. I was just thinking I might switch to another carb if the Holley did not work out.

Don't give up yet. I had to really play around with mine to get it right.

Try playing a bit more with the accelerator pump.

If I recall on my 250 my jets are a 49, squirter is a 25, and I have a orange accelerator cam. Yours will likely be different but it should get you in the ballpark.

The pump arm should have no gap at all when at idle and should have some tension in fact. That's what caused my bog. Well that and a vacuum leak.

I have not given up - like the mid range performance improvements even though the stumble on takeoff is driving me crazy.

Changed to Accel. Pump cam from white to orange. The orange cam has the 2nd longest shooter time of the 8 available and the 3rd largest cam rise. Also adjusted the throttle lever to have zero clearance with the pump arm. Adjusted the idle mixture screws to give max. manifold vacuum reading at idle (18 in. Hg.).

Less stumble noticeable but still there. I now plan to increase the shooter from 25 to 28 or so to see if that helps.

The power valve was changed from 8.5 to 9.5 in Hg. opening but that did not seem to do much.

Thanks again for all the inputs. I'm learning something useful.

You said there is no heat applied to intake.

Can you apply heat to it?

Heat would help for sure. I would think with a 9.5 power valve gas mileage would suffer because of how easy you get into power enrichment. Would one of those carb spacer plates that hook up to heater hoses work in your application? Just a thought. Jay

I had a stumble with my Holley 390 as well. After going through the carb and getting it fully rebuilt professionally by a racing specialist of some note, I moved on to related systems that can effect drivability. I discovered that my ignition timing was good and under normal operation, it functioned properly. It wasn't until I had a chance to see the vacuum advance in action that I discovered that on moderate to hard acceleration the timing was dropping due to a loss of vacuum. It was only for an instant, but it caused the engine to bog badly for that instant.

I added a small reservoir to the advance system and the problem went away. Don't rule out a timing issue!!