I have a studder at initial throttle, enough to kill the engine. I can feather past that point and its fine. I figure its timing related, yes/no/maybe? Also, i moved the brake booster vacuum line from the intake to the carb base... i lost power brakes. Not enough vacuum from there?
If it bogs immediately, the accelerator pump squirter is too small. If it goes, then stumbles, then goes, it's too big. If the timing is set anywhere close to factory spec, it's not a timing issue.
I've seen where a lean idle mixture can cause the same problem. Ok, I'm confused. I just read through 5 pages of this thread where the OP starts out saying he's considering buying a Street Demon and everyone starts talking about 4160 Holleys, too large primary venturie, and vacuum secondaries. A Street Demon is a 3-barrel carb that has tiny primaries and an air valve secondary. I'm running a 625 Street Demon on mine currently. http://www.demoncarbs.com/Types/Street Demon.asp Then the OP buys the carb and it's not a Street Demon. Not going to get into pi$$ing contest about carb sizing, but most of the formulae you see for calculating carb size are very conservative and actually have nothing too do with the air flow in an actual running engine. They can't even agree on how much depression to use as a test standard. I too have had very good success running carbs that were "too big" for my small blocks. Just have to know how to tune them.
Either that or you have a vac leak under carb... May be OK once tuned... With engine off look down throat of carb while opening throttle approx 1/4 way, see if it's giving a shot of fuel just as you move throttle arm... If not you need to adj the accelerator pump linkage or maybe cam, also be sure you have the pump spring under diaphragm and not on outside of it... Yes this assumes you have a lean off idle condition... Overly rich causes a bog but usually only momentary and engine quickly recovers...
You are right sir, I have led this thread quite a distance from the OP. I stuck with it to maintain the conversation and development with the fine gentlemen offering their advice. However, I agree that the OP has been satisfied and now I have new gremlins to chase. I will go ahead and start a new thread for those subjects independently. No harm, no foul.
If all you had to do is "tune" a carb, then carb manufacturers would only make one size of carb, and then "tune" it (or have the end-user do it) for whatever engine size it's being installed on. Think of the money that could be saved if carb manufacturers didn't have to make all those different size carbs and could just make one. Heck, why not just get a 1050cfm carb and then "tune" it to run on a stockish smallblock Ford? Carbs are made in different sizes because different engines have different requirements, and "too much carb" will only cause problems (multiple problems) for a street driven vehicle.
I said "You have to know how to tune them". Problem is most people think that "tuning" a carb means you just adjust the idle speed and mixture, maybe change the jets or the power valve or squirters and that's it. There's way more involved than that. Once you start playing with idle and high speed air bleeds, idle fuel restrictors, power valve channel restrictors, emulsion tubes, etc. you can make a pretty wide range of carbs run on even a small engine. Plus, as I think someone mentioned earlier, you can run a much bigger carb with a dual plane intake than with a single plane because each cylinder only "sees" half the carb. Have you ever seen any of the carb calculators ask if you're running a single plane or dual plane intake? No, they just ask for engine CID and rpm. The weird thing about Holleys, they have carbs rated in 50 cfm increments. Why? No one else needs to do that. Street Demons come in 625 cfm and 750 cfm. Q-jets were 725 cfm (for small blocks) and 800 cfm (for big blocks). The list goes on. Another strange thing, a 950 Holley flows less than an 850 Holley. Figure that one out. That said, back in the mid-70s I ran an 800 cfm Quadrajet from a big block Buick on the 289 in my Maverick. Thing idled and ran smooth, good driveability and great gas mileage. Nothing wild about that engine, stock heads, stock exhaust manifolds. I know of a guy that runs a professionally built carb on his 289 that flows over 900 cfm, runs great and he drives it everywhere.
as Crazy said..."Heck, why not just get a 1050cfm carb and then "tune" it to run on a stockish smallblock Ford?" ...
Why not?.. there are more guys than you know using them on 350 Chevy's in street cars. This is a "little 289 Ford". IIRC.. the car runs a 950 Holley and is also driven on the street quite regularly. Makes about 430 horse last I heard. First the race vid.. then the street vid. If you're patient enough to wait till he tromps on it later in the street vid(around the 2.12 and 2.22 mark for the impatient folks).. you can clearly see that engine size has nearly squat to do with the carb sizing as the power levels climb. Throttle response is certainly not lacking. Horsepower is airflow + fuel. The more power you want to make.. the more carb you'll need and a 600 carb.. no matter what you do to it short of boring it out to a larger venturi size.. is severely lacking potential over a 750 right out of the box. I made around 340'ish on my last 302 and that carb was tweaked pretty hard to get to that point. The next logical step would be to bolt on a 750 and start tweaking on that to move up the power range. As an aside.. all the naysayers really seriously need to go look at what the street racers and street strip cars are running. And yes.. they drive those cars on the streets and also to work on occasion too. I can gauran-dam-teyou they aren't the cookie cutter carb sizing rec's that you guys are dishing out here. Not even close. World class engine builders also don't use tiny little venturi carbs like those either.. and not all of those engines are full race hi-compression stroker type deals either. Some guys live and learn by doing it firsthand.. and some guys read magazines and forums to gain knowledge for which they will never have the opportunity to use or quite possibly completely understand how to fully utilize even if the opportunity did arise. Oh.. and PS. If Nates car wasn't running that parker funnel web single plane manifold?.. it could EASILY run a big DOM on a dual plane manifold. The 430 horsepower potential is far far more relevant to what works than those 289 cubic inches of cylinder displacement will ever be.
Don't carb manufacturers know how to "tune" their own products? That's not too surprising considering how small the primaries are on a Q-jet. Try that with a Holley and it won't work.
Sigh.. Larry.. Larry.. Larry.. you should try to understand that making blanket statements like you've been making here is completely and absolutely wrong. A vac-sec carb is simply an on demand type component in that it only flows what the engine will accept and not one little bit more. Sure you can put a clear secondary spring in and make the thing bog without proper tuning.. but that doesn't mean it's too much carb. Only that you haven't properly tailored the fuel curve to match the airflow capability There's also the fact that just because YOU can't make big carbs run right on your junk.. doesn't mean others can't make them run right on their junk. ESPECIALLY a vac-sec style carb.. which is one of the easiest and most forgiving style carbs on the planet. Another thing that many here fail to completely realize is that it is well proven fact that a bigger primary venturi using less secondary can actually make more mid-range power and torque than a smaller one using more secondary. This becomes much more dominant as the engine gets larger in size but max power potential and rear gear have very huge impact as well. And transient throttle response(in and out of the gas) is improved as a result of not having to bring 4 boosters back on rather than just the 2 primaries. Of course you can get into fuel distribution issues depending on the manifold being used.. but even that can be circumvented with spivy's located in the right spots. As for Holley making carbs that are perfectly tailored for any application based on cfm ratings alone?.. that's laughable all in itself since an entire carb and boutique tuning industry has been built off of Holley's inability to finely tailor each carb for every possible engine combo. Hell.. just changing vehicle weight and rear gears can have a very big impact on what dial in tune works best. There is often 20-40 peak horsepower and greatly improved AVERAGE power waiting to be unleashed if you don't mind paying the extra dough to have someone do the dial in for you. Have you ever looked around on a 5.0 liter Mustang forum? I can tell you anyone even slightly serious about hopping up these little motors doesn't even bother to dump much money into a 600 cfm smog carb. 750's are extremely popular.
all this..."knowing how to tune a carb" is what sent me to... https://www.opgi.com/cadillac/MSD2921/?gclid=CPOqncP8_ccCFQEMaQodrKsE1Q
