Dave
testing vacuum secs
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Joined: Aug 2003
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From: cincinnati oh
testing vacuum secs
i would like to test my vac secs on my sd 750. was wondering if anyone knew a way to see if they were for sure opening. dont feel much of an extra oomph like i could with my other carb. soooo, does someone know of a way to test that part? i emailed sd, but waiting on a reply and it has been a few. been having a few issues with carb as of late, so i would like to check to make sure.
Le Mans Master
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Take a paper clip and slide it on the vacuum actuating rod. It will hold it's place from the tension between the folds in the paper clip wire. Slide it flush against the top of the vacuum pod.
Next go for a drive, Floor it to a given RPM, say 2000 RPM, then go back and check to see how far the paper clip has moved down the rod. Do this every 1000 RPM, and it will give you an idea when and how far the secondaries are opening.
Mark
Next go for a drive, Floor it to a given RPM, say 2000 RPM, then go back and check to see how far the paper clip has moved down the rod. Do this every 1000 RPM, and it will give you an idea when and how far the secondaries are opening.
Mark
Team Owner
Joined: Oct 2000
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From: Washington Michigan
You can also put a dab of grease on the part of the carb base that's the wide-open throttle stop for the throttle shaft lever on the passenger side of the carb, and go out and run it hard. If you find grease transferred to the throttle shaft lever, you know it opened all the way. If it's a small-block, a 750 may NEVER open all the way, but the original 585 probably will. 
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From: cincinnati oh
tis a small block
yea, tis a small block 350 with some add ons., but still a small block. i always kinda felt the 650 was a better carb for this setup, and the 750 was too much. was i wrong? the 750 came with the engine so i kept it. thanks for the advice. will do this asap. nice here. and also thinking of doing somerset cruise july 28. so, i'd like to clear this up.
Race Director
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From: CT
Kind of depends on what HP the 350 has. My stock '70 LT-1 short block with Dart Platinum heads, headers, Torker manifold runs great with the #3310 Holley 750 and the secondarys do open all the way. Depends on what vac. spring you are using. On a stock 350, a 600-650 is probably better. It all depends on modifications or extra stock HP (I think the stock 350 LT-1 engines in 1970 came with a 780 Holley - correct me if I'm wrong)
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Joined: Aug 2003
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From: cincinnati oh
not stock
not stock. really nice engine. gotta check that vacuum spring. will post specs on my cam and heads etc. love the engine, just tweaking the carb. the sec spring needs changed and the choke was screwed up a bit. will post specs later. going to wings game tonight.
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From: Clayton NC
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From: "cutesy" TN
Race Director
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From: Clayton NC
Le Mans Master
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From: "cutesy" TN
Just to give you an idea what size the original equipment was. If they didn't put a bigger carb on there, it was because it was not needed.
Very few street cars need a 750 cfm carb.
Race Director
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From: Clayton NC
, Does anyone want to buy a used 750 cfm carb? 
... Dave
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From: cincinnati oh
specs on engine.
ok. just to stir the pot a bit. here are the specs. i think it is kinda in between both carbs, but i really do value the genius on this site. so, here you go.
trick flow Kenny Duttweiler aluminum heads with 64cc chambers, 195 cc intake chambers, 2.02/1.60" stainless valves and 1.47" springs
flat tappet hydraulic cam with 228*/234* duration @ .050, .480"/.494" lift with trick flow push rods, 1.5 roller rockers, and valve covers. all valve train are Trick Flow peices and edelbrock air-gap RPM intake
trick flow Kenny Duttweiler aluminum heads with 64cc chambers, 195 cc intake chambers, 2.02/1.60" stainless valves and 1.47" springs
flat tappet hydraulic cam with 228*/234* duration @ .050, .480"/.494" lift with trick flow push rods, 1.5 roller rockers, and valve covers. all valve train are Trick Flow peices and edelbrock air-gap RPM intake
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
That 750 vac sec BG carb will run just great on any 327 or 350 - no problem. That's the glory of a vac sec carb.
However, my testing on those shows that the carb will likely not fully open the secondaries with the stock spring. During the Tuning for Beer Tours, we're able to really wake those carbs up by installing the softest possible spring on the secondary side. Using the Holley spring kit, use the short white spring - that's the softest one. This will allow much earlier secondary opening, and will assure that the secondary side opens as much as it should. This little mod has consistently worked very well on all the BG 750 vac sec carbs we have done on 327s and 350s during the Tuning Tours.
However, my testing on those shows that the carb will likely not fully open the secondaries with the stock spring. During the Tuning for Beer Tours, we're able to really wake those carbs up by installing the softest possible spring on the secondary side. Using the Holley spring kit, use the short white spring - that's the softest one. This will allow much earlier secondary opening, and will assure that the secondary side opens as much as it should. This little mod has consistently worked very well on all the BG 750 vac sec carbs we have done on 327s and 350s during the Tuning Tours.
Thread Starter
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Joined: Aug 2003
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From: cincinnati oh
that was it. it is alive
thanks lars. you are a wizard. you know, emailed bg about this several times, no reply. i mean, these carbs are not cheap. i would think some better service is due from them. put the white spring in and bam. the car is zipping all over the place. vacuum secs seem smoother than mechanical secs from what i can feel. thanks a bunch man. you rock. i hope to run into you one day and buy several beers.....
