Anybody out there tried to setup a 625 cfm Road Demon on a stock Buick 350. I am running a TA alum. intake on an otherwise totally stock motor. Distributor has been swapped out with a GM HEI but other than that all is stock. The folks at Barry Grant advise advancing the timing to about 12 degrees BTDC which is the top end of the hash marks on the harmonic balancer. How the heck do you set this Demon up???????????????? Car just sitting but would like to get it running. Am just 19 but am trying my best with what little I know. Would sure appreciate any help. Some people have told me this is too much carb, but the tech folks said that's not true. Help.
Too much carb???uzzled: It may be a smallblock but it is still a Buick! If anything, it's not enough carb. I wouldn't have gotten anything less than 750 cfm. Have you tried to start it yet. If your timing is even close, the engine will start and run. You probably wont need to do anything to the carb to get decent results. The key for a good idle is the idle mixture screws, they will probably be pretty close out of the box. But you will need to play with them a little, as well as the idle speed screw. If you have or can get a vacuum gauge it can help, just attach it to one of the unmetered vacuum ports on the carb--there should be one on the front of the carb near or on the baseplate (should have a diagram with the carb). Once you get your timing set, just start messing with the idle mixture screws a little at time from side to side. You should hear the engine speed up and slow down as you mess with the screws. You want to adjust them to achieve the highest idle speed, if you have a vacuum gauge you want the highest vacuum you can get (should be between 15 and 20--the higher the better). Once you have your mixture set you need to reset your idle speed. For a stock 350 it should be about 600 rpm in park. It sounds a lot more complicated than it is. Good luck:grin: :TU:
Setting Up the Demon (625) Your tips on idle mixture are really helpful. But the tech guy at Barry Grant told me to start up the engine and set the idle to 16-20, then another guy told me 12 was OK since that is as high as the marks go. Where do I set the timing so that I can then adjust the primary and secondary - not to mention the idle which comes later right??? Sorry if I sound like a confused amateur, but I am. I would really appciate any help you could keep giving me. Dan
Are you asking about vacuum at idle, or are you asking about initial timing. o No: The Barry Grant figure (16-20) sounds like a vacuum recommendation. (similar to my recommendation) The guy that says that 12 is OK, sounds like an initial timing recommendation. (I think the timing marks on the front cover stop at 12.) 12 is not a bad point for initial timing. Do a search on this site (buttons at top of page(s)) for specific information you want to know. Tons of info here. Do some searching/surfing. We need to get you educated so you know what questions to ask, and how to ask them.:TU:
timing The first guy I spoke to at Barry Grant said to set the initial TIMING at 16-20 degrees BTDC - the next guy said 12 was OK after I told him the marks on a Buick 350 only go to 12. How do I know if 12, 16, 20 whatever is the right advance - go by highest RPM, use a vacuum manifold gauge hooked up to the diagnostic port on the carb, or what? Thanks for any help - I need it.
Well...you're talking two different things here, but both are important, and somewhat interrelated. FIRST, you need to get your ignition set up, 'cause if it's not dead on, you'll never get the carb set up right. Make sure your cap, rotor, wires, plugs, etc are in tip-top shape, or you can run around in circles forever...then get it to run as best you can on the carb, adjust the ignition, then come back to the carb. FIrst, you need to mark your harmonic balancer to show more than 12 degrees advance. I believe that 1.75 inches up from tdc will correspond with 30 degrees advance, so file/make a mark there, and then you can measure halfway back and find 15 degrees, then split the difference between 15 and 30 degrees for 22.5 degrees, etc. You'll want to start at 12 degrees of initial (at idle, with vacuum disconnected and plugged), as set by rotating the distributor. Then you've got two advance mechanisms that need looking at, 1. the mechanical or centrifugal advance, and 2. the vacuum advance. Let's do the mechanical first. Disconnect and plug the vacuum advance line from the carb to the dist. Hook up your trusty liming light and a tach, and have a helper rev the engine up in maybe 200 rpm increments, and you note the ignition timing as indicated at each rev point. Subtract the initial 12 degrees of initial timing, and you've got your mechanical advance curve, and at somewhere around 3500 rpm, it should stop increasing. NOW, what you want is for the mech advance to be adding a total of 20-22 degrees difference from start to finish, and you'll want it "all in" by 2200-2400 rpm. You change / tailor this by changing the weights and springs in the top of the distributor. If you've got an HEI distributor, do a search on posts by "Ignitionman" and "HEI" "Weights", he's posted some spring/weight combos that work well. If you've got a points dist, I'm not sure which ones do what, but you should be able to search and find info on them. Once you get the mech advance set up, you'll need to limit the vacuum advance, or else you'll end up with so much advance when you're driving around that the engine will ping like crazy. I think it's Crane that makes a good adjustable vacuum advance cannister, costs about $20-$25, but it's well spent $. Again, search on "Ignitionman" and "adjustable vacuum", you should find the good specific info. I seem to remember that you want about 8 degrees of vacuum advance. So, you want to end up with 12 deg initial, and 22-24 deg mechanical, and 8 deg vacuum. When you floor it you'll have 12+22=34 total, because the vacuum goes away at wot, and cruising down the highway under a light load you'll have 12+22+8=42 degrees for better economy. If I remember, my stock points dist setup was something like 6 deg initial, 14 degrees mechanical (all in at a very late 3400 rpm), and 14 deg vacuum. Changing the ignition timing curve REALLY wakes up a Buick engine!!! THEN you can work on setting up the carb. I don't know about Holley-based carbs, but normally you set the idle mix screws to get the highest possible manifold vacuum, the more the better. Often, you'll have to adust the idle mixture, then move the idle speed, then go back to the mixture, and maybe back to the idle speed until you get it dead on. (personally, I'd drop the 625 cfm carb and get a 750 Q-jet, but that's my opinion, and I hate Holleys...too many design flaws that need "upgrading") Search this site for posts by "Ignitionman", he posted good solid info on setting up various timing curves. Or, you could send him your dist, and for $175 he'll convert it to HEI and set it up for you. His website is www.davessmallbodyheis.com, check it out. Good luck, and let us know how it's going! :beer
