Greetings everyone,

I have a 1972 Corvette with a 350 crate engine. I purchased the Vette used and as such do not have any insight as to the history of the vehicle or the engine. I do not know if the cam is stock. It has a fairly even idle and does not lope. It appears to have stock heads. I changed out the stock timing chain to a gear drive and have added a set of Hooker header side pipes. I have also added a B&M 144 Powercharger blower and am using a MSD6A ignition as well as a Mallory Unilite distributor. Both were present when I purchased the vehicle. My additions were the blower and gear drive. Also installed is an Edelbrock 750CFM Carb with an electric choke (not used) which was also there. My situation is that when running the engine shows a slight amount of smoke from the pipes and the smoke is very acrid and 1 or 2 minutes in the vicinity of the vehicle causes my eyes to tear and my clothes to smell of exhaust. At WOT the engine feels responsive but when I check the plugs they are carbon black. Not oily, but a charcoal type of black. I am also having a little trouble telling what the exact timing is as the timing indicator does not go to the 35 degrees that the B&M Tech told me to run at when I called their tech support regarding the blower. I think I timed it fairly close but am not totally sure. I have a MSD Knock sensor installed and get an indication of some knock at WOT but also have a MSD Boost Retard installed to retard the timing although I don't really see moch of a difference when I adjust the boost retard knob. The motor starts without hesitation within a couple of cranks of the starter every time and continues to run without having to tease the gas pedal, even when sitting for a week or so. I asked the B&M tech what the recommended carb size for this set-up would be and told him I had a 750 on it and he said that should work but in all honesty, he wasn't overly helpful. I am wondering if I am overcarbing the engine and if i should drop back to a 650 or 600CFM and if I am using the right carb for the application. Any suggestions would be greatly appreciated regarding the carb selection or anything else I should do to help the situation. Thanks in advance for your time. Regards,
BG

Your running to rich IMHO. If your car starts right up cold and idles with no choke it's already "choking itself". Find out what jets you have and go down till you start getting decent plug readings. I'd jump down 2 sizes to start with. You'll find more power the closer you get to the right jetting. Might as well hook that choke up too so she'll start up easily once you get the jetting right. It won't want to run when you first fire it up with the right jetting without a choke.

Generally, a 750 cfm carb is too much for a stock 350 and yours sounds pretty stock in my opinion. A 600 or 650 would probably much better.

Originally posted by bg528:
I have also added a B&M 144 Powercharger blower Doesn't sound too stock to me!!

Thanks to all for the responses and suggestions. I am going to follow everyones advise and drop down to a smaller carb; a 600cfm or 650cfm. I am presently using the Edelbrock performer carb which came with it. Any suggestions as to which would be the better choice in carbs for my setup. I was looking through the JEGS and Summit books and see that they offer Holleys, Edelbrocks and Demons and that they all offer features but I am not sure which would be the better choice for my needs. I need a carb with mechanical secondaries. That much I know but after that...I'm not sure. Thanks again for the help. Regards,
BG

The easiest and cheapest way to improve your car would be to take it to a performance shop that knows how to tune. Seriously, you need an experienced hand to look at the total setup of carburation and ignition and everything else.

The carb size is NOT your problem. Your stock engine, even before the supercharger, would have had a 750 cfm carburator. There is no way possible that after adding a supercharger that it would need a smaller carburator.

Let me take a wild stab at your real problem. First off I'll guess that you have the blower and carb litterally stuffed up against the underside of your hood. Probably has an extra dinky air cleaner assembly, or using a shorter air filter on a drop base assembly. Big restriction either way. A super common problem with mini-blower setups is that the owner/builder doesn't leave enough breathing room. A very restricted filter setup will do exactly what you are describing.

The B&M help desk is not staffed by engineers or experienced engine builders. Nor are just about most of the aftermarket high performance help desks. Their main problem is that a tremendous number of the callers lack a strong understanding of the fundementals. What you may perceive as a lack of helpfulness by the tech desk is more likely a learned response to the unreasonableness of some calls - can you imagine how puzzled the tech desk is that a caller wants to know if a smaller carb should be used now that a 100 hp part has been added to the engine?

Thomas

Thanks Thomas. I have a larger air cleaner and will give it a try prior to switching carbs.

Letting it breath will be a big step in the right direction. It is not just filter diameter that is at issue, make sure the the top lid of your new air cleaner is not too close to the air horn of the carb - that alone would continue your current problem.

Since you seem inexplicably attracted to the idea of a smaller than stock cfm carb after adding a 100 hp part, let me offer the following caveat: if it does not run as "rich" afterwards it will not have been the cfm drop that did it. Instead, it will have been the differences in particular carb setup.

Tuning, on both ignition and carb, is likely to still remain an issue after letting it breath.

Thomas

I agree, your carb is not to big, just tuned wrong. A few years ago i had a mild 327 in my chevelle. Accoarding to conventional thinking, a 600 cfm carb was plenty for it. But i decided to try a 750 anyway. As soon as i pulled out of my driveway I could feel the improvement. I picked up a half of a second in the 1/8th mile from a carb swap alone. I had no fouling problems and ran that setup for 11 years as a street strip car. i drove it as a daily driver and changed plugs once a year.

You need a carburetor modified for supercharger use. Holley recommends their HLY-0-80572S 700cfm for SBC with 140 series supercharger.

Emil:71SS-502
93 Z-28,Vortech SC

Hello.
I have 750 carter afb(same as edelb older carb,not avs.) over stock 350 and after calibration it works better than 600 vac holley. after all if its too rich now and you put smaller carb with same tuning(jets etc) it will go even ritcher,not leaner. with that carb you should get manual witch tells basic adjustment procedures (well atleast few years ago they were included.hmm 6 actually, where those years go;) )so read it and if you did not get it I may try to find mine...