Hey all, I recently picked up an offie intake and Holley 4160 carb. I am doing a rebuild now (long story), and I am a little concerned that the carb will be a little too big for the engine. I beleive that it is a 550 or 600 cfm carb, and I know most people recomend around 350 cfm for these. I like to four wheel alot and its also a around town/highway truck. Its sitting on 33" BFG's right now. So, 1) is this carb way too big? and 2) if it is, is there anything I can do to compensate for it some? Like, stiffer secondary springs or anything? Oh yeah, exhaust is not a problem, I have truck tags so i am just running strait pipes with a glass pack turned down under the bed. Thanks!
Eric

 

"finding the right carburetor"(I spelled it with an "e", and double checked, who's editing my spelling?): "table figures are for stock engines, for high performance, ...increase the cfm rating by 10%".

550 cfm, 425 cu.in @ 5500 rpm
555 cfm, 400 cu.in @ 6000 rpm
600 cfm, 425 cu.in. @ 6000 rpm
600 cfm, 400 cu.in. @ 6500 rpm

The 300, high performanced (figures reflect a 10% increase/85% volumetric efficiency) -

308 cfm @ 4000 rpm
346 cfm @ 4500 rpm
385 cfm @ 5000 rpm
418 cfm @ 5500 rpm
462 cfm @ 6000 rpm....

It keeps going - but MOST of us don't rev beyond 3500 - 4000 rpm. So... at what speed do you do MOST of your driving? A lot of guys think more gas = more power, while in truth, matching a smaller carb to your driving style will give you better all around performance. "Bigger is better - might hold true for cubic inches, but ALMOST NEVER should apply to carburetion". "Be realistic, you're only hurting the end result of all your hard work by overestimating." As you can see - a 350 2bbl or a (very) small 4bbl (operating more often than not on the front 2bbls only) is a pretty good match for the 300-6.

 

Obviously If you have the choice, go wih the correct carb for your application. If however you are like me, with the same exact setup (300cid, Offy Intake, Holley 600, K&N, RV cam, EFI manifolds), you can expect it to run well but it will definitely need some tuning.

I had to rejet mine and put in a 2-stage power valve to make mine perfect. But I suppose you would have to do that to even a smaller carb to make it perfect...

 

Thanks guys, I am pretty much stuck with this carb, so, I am just looking for ways to I guess tone it down a little to my application. Any ideas or tips?

 

I hesitate to suggest ways to make a bigger carb act smaller. There are probably some guys here with more experience with carbs. I've always leaned towards one that fits the application, as it requires less attention for acceptable service. The Holley guys do have millions of applications and years of experience to base their recommendations on.

That said, if you are going to run a 550/600, GeneStoner (or others) would be better able to advise, as he's running one. If I were going to use what I had (I almost did) I'd also think about running a stronger spring in the vacuum secondaries, as this would allow the carb to run on the front two barrels by themselves - longer. As for jetting - ask Gene, but I'd imagine both the jets and the powervalve might need tuning.

I bought the 390 because I'd heard here it is a good "drivers" carb for the 300. I had to rejet (down 4 sizes - for my altitude) and am now working on the Vac. secs.: heavier spring - as they are opening too soon . So yes, no matter what carb you have, it will probably take some tuning to get it right. There's a carb shop in Denver that will dyno tune my carb to my vehicle - for $140! At least here at FTE you have the benefit of a lot of guys who've tried a lot of different things & are willing to share that - at no cost. Besides, if I finish working on my truck, my wife might find some REAL work for me...


I didn't mean to offend - the 390 4bbl is one of the best carbs for the 300. There are several others that are in that catagory. A 600 (obviously) can work. If you come across something smaller, you should be pretty good at tuning by then...

Hey Gene - what power valve did you end up with?

 

Hey broncr, thanks for all the info! I will be able to see how it runs in about a week and a half when its all in. Is rejetting and changing the springs and everything else going to be something that is costly? I will do it myself, but, parts wise? Also, what are some of the results of having a carb that is too large for the application? Thanks again though, lots of help!
Eric

 

...and no, I'd say those are some of the least expensive things you can do, provided you can do it yourself. I bought 4 sets of jets from jegs for $5 ea. + shipping (ouch) =$28. I'm pretty sure you could narrow that down with a little input from others here. Jegs also sell a vac sec. spring kit for about $30 but again, you could get by cheaper if you knew what you had ( the springs are color coded) and only got the heavier springs. Power valves are about $7-8, so in all, you could get by for maybe $25-50. I just got a Summit catalog and see they offer FREE shipping.

Replacing all of these are fairly easy if you have ANY mechanical knowhow ( or desire). I have changed jets ( in maybe 10 minutes) twice in the last week, and did the VS springs (in about 5-10 minutes) twice today. You have to pull the choke assembly (5-10 mins) to get at the VS. It's all pretty easy screwdriver work... If you already have the carb out, it's even easier, but don't lose the little check ball in the VS housing - it prevents the 2ndarys from "flopping" open.

I was at a guys' place last week - a friend was looking for tranny work - and the guy mentioned that old Holleys are a dime a dozen. He had at least 25-50c worth. I have 3 w/ VS and one real nice old 4150 (w/mech secondaries). Anyways, they are real good for practice tearing apart, and I have some of spare springs, etc. Practice on the spare ( I did - today) & if you mess anything up, it's not your running carb. It's easy to assume someone else "knows how" when YOU do... I still pull out the Holley manuals frequently (about $15 ea.). The pictures alone are truely worth "a thousand words". I have one that is so greasy, you can hardly read some sections.

 

Upon closer inspection, I see they have an $8 "HANDLING" charge... hmmm.

 

Hyey, Genestoner, i got your message back, but, i cant read it because the private messaging for the site has been turned off, so, i have no idea what you have written back. Is there anyway that you can send me the message through email? Thanks!
Eric

 

300 (ci/rev) / 2 (rev/cycle) x 3500 (RPM max) / 1728 (ci/cf) = 303cfm

 

Well, thanks for that, but, the whole reason that I posted was because I knew that the carb was bigger than what I needed for my application, i am looking for ways to tune it so that it would work better. Do you have any sugestions?

 

Get a tube of silicone caulking. FILL ONE front barrel of your 550-600 cfm holley with it ( from the inlet horn to the base). Let it dry for 24 hours ( or 72, depending on the directions on the tube). Do the same to ONE of the rear barrels. Your carb will probably run better on a 300-6 with this modification...

 

BWAAAAAAHAHAHAHAHAHA!!!!!
ROTFFLMFAO!

 

[QUOTE]Originally posted by broncr

"I bought 4 sets of jets from jegs for $5 ea..."

"Jegs also sell a vac sec. spring kit for about $30 (oops - $8)..."

"Power valves are about $7-8, so in all, you could get by for maybe $25-50. "

"Replacing all of these are fairly easy if you have ANY mechanical knowhow ( or desire). It's all pretty easy screwdriver work..."

"I still pull out the Holley manuals.... The pictures alone are truely worth "a thousand words".




... sorry, WBT,I had just lost the NASCAR pool by a raindrop...

 

There's no such thing as too much carburetor, just not enough engine. Build the 300 to make it rev about 6k rpm and it will be fine. On a more serious note, you will save lots of time and possibly money just buying the correct carb to start with. A carb that's too large usually won't hold a tune, so you have to rejet everytime the wind blows differently. I had a customer a while back that we told to get a 625 Demon for his car, which he agreed to do. He fell victime to More's Law and brought us a 750 to install. It cost him several hours of labor (at $68/hr) to get it running well, and then a cold front came through and it ran like crap again. My point is, it can cost as much to force a large carburetor to work as it does to buy the right carb in the first place. Plus you will never get the most from your engine with too much carb.