You said you are using a 2100 series carburetor. I have found that to be the best for street use. If everything is clean , the float level is set correctly, and the needle and seat is working properly, it seems to me that it is the power valve not seating correct, or is no good.
not trying to ruffle any feathers here but I'm curious why some would spend that kind of cash on that little carb? While I can certainly understand using it from an originality standpoint(or on VERY small/stock motors that rarely if ever get revved).. it just seems like such a waste to spend that kind of cash on a brand new unit when you can nearly buy a good used small 4bbl setup.. intake AND carb. Should also keep in mind that the airflow ratings for 2bbls are taken at twice the depression as a 4bbl so it's considerably smaller on the primary side compared to a 4bbl than you may think it is. Using some quick math tells us that the 350cfm 2bbl actually flows only 248 cfm when tested at the same 1.5 inch figure that the 4bbls are. There are also several different 2100 series 2bbls with differing bores and venturi sizes as well. As far as mileage is concerned.. a correctly sized/properly tuned 4bbl will get you at least close to the same mileage as a 2bbl. So long as you can force yourself to keep from ahving too much fun and staying out of the secondaries all the time. But.. if you can't find the self restraint to do that consistently?.. then that little 2bbl will definately keep you throttled back and save mileage in the long run. Like I said.. not trying to flame anyone using that unit.. just curious as to why you think it's better than an upgraded setup, is all. PS.. a smaller 390 or 450 cfm carb is a damned nice upgrade to smaller/milder motors too.
I'll be double-checking the float level this weekend. What symptoms do you get if the power valve isn't seating correctly? It would be nice if that was the source of my problem with vibration around 2000RPM.
I understand your reasoning, but considering my car is a daily driver with the majority of the 70 miles a day being highway speeds (65-70mph which keeps my tach at about 3000-3200RPM), I would probably be close to the secondaries being open a lot. I know I could adjust the secondaries to open later, but I never really drive my car like that (high rpms). If my car suddenly had a lot more power, I might be tempted to play more and destroy my 18mpg average, which I really enjoy.
CFM is CFM, can compare apples or oranges. 2V are better for low and low mid range, but 4V is good for high mid rang and high range. Same 302 motor and same car with 2V vs 4V. 370 CFM vs 560 CFM. Zero to sixty for the 2V is 6.9 sec and quarter mile in 16.10 @ 87.2 mph. Zero to sixty for the 4V is 7.19 sec and quarter mile in 16.0 @ 87.3 mph Plus the 2V gets way better mileage because its booster venturi system uses “annular discharge” — the most efficient and effective system ever developed to atomize the fuel charge in a carburetor.
Also, if the power valve is sticking (is that even possible?) or if the float level is incorrect, could this explain my engine vibration and why hitting the carb sometimes drops the RPM?
You may be good at some things, but you're clearly lost on carb sizing. The reason Holley 2 bbls are rated at different vacuum pulls is at WOT, a 2 bbl that's smaller will allow more air to flow thru it than a 4 bbl carb will with 4 holes open. This is why I posted the info about the 500 cfm being exactly half of a 750 cfm 4 bbl. If you take half a 740 cfm 4bbl carb, put it on top of an engine that's capable of pulling 750 cfm thur it at WOT, then suddenly close off two of those 4 bbls, the remaining 2 will flow more than 375 cfm thru them. This is why Holley uses a different vacuum pull to rate 2 vs 4 bbl carbs. On my 3x2 setup, all three carbs are rated at 250 cfm. At the point where the two secondary carbs begine to open, that 250 cfm carb is actually flowing about 365 cfms of air thru it. What any carb actually flows at WOT is directly dependent on the engine it's sitting on top of. It may be rated at 350 cfm, but that does not mean that's what's going to flow thru it when you bolt it to say a 460 cid engine. The cfm rating is just that, a "rating" used to compare carbs, nothing more, nothing less. It's a guideline to use in choosing a carb. Some people do not want to nor may be able to afford to do a complete intake/carb swap. That is why I supported the 2 bbl swap. I personally have used the 500 cfm Holley on 5 different engines and never once had any reason to regret that decision. Now, you do make sense when recommending a 390-450 cfm 4 bbl as their throttle bores are really tiny compared to the bores on a 750 cfm 4 bbl (or the 500 & 350 cfm 2 bbl). But, he would then have to buy and then swap the intake, in addition to buying the carb. And just FYI, the 350 cfm 2 bbl is exactly half of a 600 cfm 4 bbl. If you took a 600 cfm 4 bbl and disabled the secondaries, you would have a 350 cfm 2 bbl.
With todays fuel prices.. I completely "get" the mileage concern. I was just curious as to the reasoning, is all. I and tens of thousands of others have gotten over 20 mpg on even heavier cars with the old 600's(with their crappy straight air-robbing boosters).. so it's been done many times. I've even seen tunnel ram setups get near 20 mpg and it's definately all in the parts match and tuning. I can understand the "bolt on and go" mentality too and the stock stuff will usually give you that. And just a friendly FYI.. you'll never run into the vacuum secondaries going down the highway at steady state speeds with a 4 bbl. Even with the lightest spring it won't happen because the engines airflow requirement is throttled way back for constant cruising speed. The carb is load sensitive and even constantly cruising at 110 mph won't tip secondaries in until you really push it down to accellerate. That's also why they won't even begin to budge at 7,000rpm under no-load. As a "purely scientific experiment".. I've tested it on almost every car I've ever owned through the years. Only a HUGE motor with a TINY 4bbl could possibly do that due to larger airflow requirements at crusing speed exceeding the primaries flow capability. Like maybe a 632 Chevy with a Holley 450 cruising down the highway at 80 mph with 4.88 rear gears and no overdrive? Sometimes faulty power valves can start to have seating problems over time to give issue like this.. but generally speaking if they start to crack or blow the diaphram out of them.. they go out pretty quickly. They're not at all like acellerator pump diaphrams where they can slowly go out and leave you with that nice gassy smell before they start leaking like a siv onto your hot manifold. Some of the older models didn't have blow out protection though.. so you may have just popped it. If the valve was bad.. you'd have no mixture adjustment whatsoever.. and the car would run and stink pretty badly. Hitting it surely wouldn't help either. Check the needle/seat/float for debri/stickiness(even new ones can catch sometimes), power valve, and float level. From there it gets more complicated with full disassembly and cleaning while checking for any obstructions along the way.
You just shot down your argument for a 4 bbl with that last sentence. If you're not going to use the secondaries, then why buy an intake and carb that have them ?
sigh.. as usual you're right on target and have it all figured out for us. yeah.. I know more than my fair share about Holleys as I've run the 500's several times too. But that's really old school "backyard hotrodders" mentality and we usually did those kinds of swaps because it was low budget and many other parts options weren't available at that time. The aftermarket has changed and you don't need to make so many compromises these days was the main point. As for my knowledge level?.. lol.. you may think you've got many things over on me with age here bud.. but you obviously have no clue as to what comes out of my garage. I don't even "swap Holleys" anymore.. or "bolt on and go" for that matter. I totally reengineer the designs by chopping, hacking, grinding/polishing, drilling/taping, tapering/stepping boosters, and anything else I can think of to improve on the antiquated cookie cutter designs they're so famously derived from. There's far better designs and manufacturing techniques out there these days. Holleys vary so much during manufacturing.. it's not even remotely funny. Except maybe to the one's capitalizing on those shortcomings and running to the bank after making you pay over a 1,000 bucks for a top-line precision machined and blueprinted unit. I just redid my "super cool top-o-the-line" 750HP and replaced the downlegs with larger race-style double stepped boosters while it was being refreshed.. and was amazed at how far out of whack the original spun in boosters were. Crooked as hell/off center and still tightly seated where they had been put from the factory. Nice work. And if you say you're little carb flows almost twice what it's rated at while sitting on top of your massive behemoth of an engine?.. well.. who am I to argue. Either way.. it usually never hurts to dream.
man.. again with the misinterpretation. read back for once and see that I was answering his concern that he'd be running down the highway on the secondaries.
As typical, you dodge the issue with pointless BS. You got called on something yet refuse to admit you're wrong. You question the guy's reasoning in using a 2 bbl, yet now you agree with it. Which is it ? He didn't want to shell out the money it would cost to do what you suggested, so obviously those options that are available now are irrelevant. Not everyone can afford to do what you do. I'm pretty sure I didn't misinterpet what you were saying. I've read it several times over, just to make sure.
His concern about running down the highway on the secondaries was in response to you're questioning why he chose the 2 bbl over a 4 bbl. Then you tell him he wouldn't be using them, ergo, he'd still have a 2 bbl carb, which you questioned to start with ? And I'm misinterpreting something here ?
lol.. you really like to get on here and quible about the details don't you? here you go.. hopefully it helps to put my reply in proper context. anyways.. what's the point of all the posturing? at least my posts aren't full of bicker and actually deal with his question as to why hitting the carb can sometimes change idle chracteristics. you're obviously a smart guy.. so, help him actually figure it out instead of just pushing an old school "500 swap" on the man. That's way much better than me telling him to just swap to the 4bbl setup?
personally I put on the 350 because I saw no point in upgrading a motor I plan to replace. It seemed stupid to throw money at a stock 74 302 with 150k miles and zero issues in the block. Also, Since my motor wouldd NEVER use the full draw of a 4bbl or a larger 2bbl, and its my daily driver. I dont need to go through all the engineering of it, you all know already, but the 350 cfm and its 30cc pump can be tuned up to the mid to high 20s for mpgs on a basically stock motor with little effort. I love my 350 it does great. when I buy that crate motor im drooling over a 600 and 4bblmare in store for the same fuel efficiency reasons. then again if I was getting a tach reading of 3k at 65 I would be pissed, 3k is more like 80 in my car.
