Hello all! I have a question: Is my holley 750 DP too small for my Buick GS, 1972, w/ a 455? The engine has an SP-1 intake, HEI w/ upgrade parts, headers, and the cam specs are: 238* 248* @ .050 w/ a lift of "491 on both sides, 112* center. I have a hard time tuning her. She likes to idle at about 22* initial advance. If I set the initial to 10* or 12*, common #'s from the posts I've read, then she idles terribly, eratically, and shakes violently. As soon as I turn the distributor to read 22*, the idle gets "streetable" Thanks in advance. ~Mark
If that's true, can't I compensate for that by loosening the distributor hold down screw and turn the cap until it corrects? I understand what you're saying and thanks for the pointer. I thought that kinda thing can be solved by turning the distributor when it's loose?
First, The carb size wont make it run poorly as you described..It may not have the top end pull that a larger carb has, but should be cool for the street. Your timing is a different issue..possibly the dampney has spun 10*..back up and start with basics. Pull #1 plug and put a thin coat hanger rod down the hole to feel the piston come to TDC...it might take a few back and forth trys to feel where the piston changes direction but it should be fairly evident and you should be able to see the timing mark cross the 0* mark on the cover. When you establish that the two TDC events jive then turn the dampner to 12*..point you rotor to #1 and this should be a good starting point..be sure you vacume advance is not pluged in and that your timing light is set to 0. see what happens.
Thanks man! My only concern is that I keep reading people say "plug up the vacuum advance" ....that's what you mean, right?
Turning the distrbutor will compensate for error, however you will get to a point- especially if you have vacuum advance, that the vacuum canister will be facing a direction that will not allow you to turn the distributor any furthur. Has the distributor been removed recently? Certainly check your balancer by lining it up TDC like above. It is the only way to be certain. If the balancer is correct and lined up to "0" on the timing marks, then I would remove the distributor while the #1 is still at TDC and then re-install. Keep in mind, as the distributor slides in to engage the camshaft, it will turn because the gears are helical. Also, the oil pump driveshaft will have to be in one of two positions to engage the distributor. You will have it in the correct position when the rotor is facing the #1 terminal on the cap exactly with the distributor fully seated. Hope this helps you!
Plugging the vacuum advance means to remove the vacuum line from the distributor and plug it with your finger, or insert a device to prevent the carb from having a vacuum leak.
Thanks to you guys for the advice! I've removed and installed that thing more times than I care to admit, and can say I'm good at doing it.......BUT I will try again.....as I make stupid mistakes as well. Based on my cam specs though.....shouldn't I expect to see a higher advance initially, as well as a "crappier" idle quality? I will also investigate the carb's power valves as well. I've had a few backfires here and there, when I first started doing this. Mark
12 is fine initially, but what is really important is total timing. There are multiple threads from more informed folks about where to set your total timing. As I recall, the BBB 455 likes anywhere form 30 to 34 degrees all in by 2000-2500. And certainly dependant upon your combination of parts. BTW, I run a Holley 770 cfm Street Avenger carb. It sees mostly street duty and rarely drag strip. No power valves to worry about. Good enough performance for me. If you go anymore serious than a cam, Id certainly upgrade the carb to something that can handle more airflow.
Mark, You have the right idea. That's a heathy cam. It needs more advance to idle right. Now you just have to recurve your distributor. Keep something in mind though. You can idle at a higher advance by using manifold vacuum to the vacuum advance. If you've read the article I wrote in the FAQ forum, you know it's really important to get your advance right, and have it in by 2500 RPM. I run 16* initial, and 34* total all in at 2200 RPM. I also run 10* of vacuum advance off manifold vacuum. That means that sitting at idle in gear, I have about 26* of advance. When I stomp it, the 10* of vacuum advance goes away, and the mechanical advance brings it to 34* You can get an adjustable vacuum advance, and use it to your advantage, but you probably need to have your didtributor recurved. What's your total advance? Also, I don't feel your distributor is one tooth off. I see guys mention this all the time. Fact is, if the engine runs, and is one tooth off, what you'll notice is, that no matter how you move the distributor, you won't be able to adjust the timing to what you want without the vacuum advance hitting. In other words you won't have enough adjustability. The fact that you can adjust the timing to what you want, means the distributor is in right. Someone mentioned that you should check the balancer to see if it has slipped. If it is old, you should. It's easy to make a piston stop out of an old sparkplug. Then using the stop, turn the engine until the #1 piston hits the stop, mark the balancer, then turn the engine back the other way until it hits the stop again, and make another mark. True TDC will be halfway between the 2 marks. Hope that helps.
Brian, No it's not normal for a stock motor. But put a nice size cam in a 455, and initial timing, mechanical and total timing become very important if you want it to maximize performance, idle, and drivability. Ask JW how he curves the distributors for a cam with 238* intake duration. 22 degrees or more initial is what's needed. BUT, you need to shorten the mechanical advance to avoid too much total. Also, Automatic Transmission cars LOVE 20+ degrees of initial timing. Take a look at tip #2, of 20 cheap track tricks, posted in September 1999 Musclecar Review.
that camshaft is not big enough to need that much advance at idle. have you checked the cam timing when you put timing at 22 does it run good or does it just run ,does it drive ok? Rod
Now I am starting to think! Thanks for that last one. When I unplug the distributor hose and seal the port from my Holley's baseplate, then the car will run pretty damn good as I turn the loosened distributor clockwise. It'll get to a point where it runs well, very well, then as I go further clockwise, it get sloppier and choppier until turning her kills her. I have to back up a half turn counterclockwise to even get her started again. I can really see the line mark since the balancer has been cleaned and repainted. With the timing light, i noticed that at 0*, it would barely run. at 10* it ran, but shook violently, at 12* little better but still shaky shaky, to the point where you're like "wow, those mounts won't last long". Turning the distributor to when the car idled best was way past the furthest point of that little timing tab, on the car's front timing cover. I kept turning to where the marker was no where near the little numbered casting on the front timing cover, so I couldn't tell where it would go, but it would go pretty far until it would idle nice, and the engine shaking side to side would be light and tolerable. After ordering a timing tape from Summit, I measured and really double checked to make sure that the tape's 45* was indeed 45* from balancer mark, and 180* was 180*. Running the car made me see the marker was at 23-25* on the tape w/ my Sears cheapo light, when she idled nicest. She could still go a little bit more, to say 28* and the RPM would increase, but the idle quality would diminish. Granted, "nicest" was still crummy by a stock engine's standard, but when I flip the holley throttle....the neighborhood knows. Very responsive with throttle blips. When I hook up the system again, it seems to get worse. I didn't realize that the vacuum canister on the distributor was used that way in idle. I thought it played a part in higher rpm's. D'oh! Now what? Do I then back off to 16*, and get the other 8 with the distributor canister? It's an adjustable one from Summit as well. What about when I read guys who don't use a vacuum to distributor? What's the point/benefit of that? Thanks to everyone's ideas and comments. Much appreciated. Just when I think I know everything.....I learn something new. ou: blown455.....When I leave the timing at 23* or so, and tighten the distributor hold down, plug in vacuum on distributor, and take her out, she idles along crappily in drive, and if I punch it, bogs and stalls a little, but then will come alive and lay a patch halfway down the block. All the neighbors boys playing were like, "COOOOOOOOOOL!!!!" One mom called her kid in. o No: But like I said....she idles bad and stinks and smells and makes my eyes burn.
Rod, Probably not, but you can get any cam to idle lower than it normally would, by using more initial. As long as the total is good, it works well. Last I checked, this is the way JW does it. He typically uses a combination of more initial + manifold vacuum advance at idle, and a shortened mechanical advance.
It probably would be a good idea to "play" with that carburetor a little as well. The idle mixture can also cause problems like you describe. If your idle mixture is not good it will be difficult to get your timing dialed in correctly as well, and visa-versa.
