SC71LT1

I spoke with BG today and they recommended a Speed Demon 650cfm for my 71 LT-1 (eventhough it came with a 780cfm Holley). I'd appreciate some input from those who have swapped a Speed Demon (or the like) from a Holley. Will the factory throttle cable set-up work on the Demon? Should I go to a mechanical secondary versus vacuum? (BG says you get better throttle response with a mechanical), is the fuel line set up compatable with the stock dual feed system? Any other issues I should be aware of regarding this swap. Thanks for your help.

LFZ

in my opinion..i had the same carb..and i HATED it! if you do decide to go with a demon...hope you like smelling gas and black smoke. i'd stick with a holley or edelbrok.

SC71LT1

What didn't you like about it? Was it adaptable with the factory throttle cable provisions etc?

LFZ

you have to get a different throttle bracket. any of those chrome mr.gasket or spectre autozone specials will work. as for the carb...it was hard to start, choke tower side screw kept getting caught on the air cleaner base..the screw for the adjustments of the back butterflies broke off...i adjusted float, mixture screws, accelerator pump...always had a hesitation and kept my spark plugs black! some guys on the forum swear by them, however; my neighbor went through 3 demons and finally went back to a holley. i myself went back to a holley and am considering an edlebrock.

dman535

Carb size sounds right. Demons flow a bit more than they are rated for.

I would go with a vacuum secondary, they are a little bit more forgiving in the tuning department. The fuel lines for the speed demon series are about $70, they are not the same as the holley lines.

You need to pay attention to the setup on the carb. It has a very sensative 4 corner idle circuit and needs the floats set correctly. I have run a number of carbs on my Corvette and have found the Demon to be the best by far.

gkull

The 650 is a better choice for a 350ish ci. The discussion about vacuum seconds or mech seconds will go on forever. Throttle responce on either is up to your tuning ability. Nothing out of the box matches.

I like the BG fuel log to attach a Demon carb. the cable setup brackets can be bought from places like Summit or Jeg's

QuickVet

who says?

Alwyn678

Dude I have never seen any black smoke or smelled gas..Looks like you had a bad carb or a bad set-up they are by far the best carb I have run on my 74 and it is built very close to LT1 specs.........GREAT CARB by the way the holley and edelbrock did not even hold a candle

Sigforty

:iagree: I love the 650 on the zz4 in my 71. I opted to go with mechanical secondaries since I have a 4 speed.

Doug Brandon

I have the predecessor to the Speed Demon and Race Demon, a BG Silverclaw 750 with mechanical secondaries. The carb has been blueprinted by BG within the last 6 months (They did it for free). It works great, but it is definitely set up to run rich. I reduced the size of the IFR (similar to adding a wire to the IFR on a Holley DP). After many hours of tuning and trial and error, it is working great. The carb you are looking at may not require all this adjustment.
This is the first mechanical secondary carb I have had on the street and I love it.
If I had it to do over, though, I might have bought an HP Series Holley. It took me a long time to get this carb to work right with my combo.

SC71LT1

Thanks for your input. I'll take a look at the Holley products as well.

Marks69BB

Amen brother; I'm very pleased with my speed demon! The previous owner had a Holley, replaced it with a Demon, and swore by the Demon since it kicked butt right out of the box! Ask Lars here on the forum about them since he is a carb guru in my book. Good luck with whatever you choose! :chevy

BTW, if you prefer a more traditional look than the fuel log, get the chrome adjustable dual line with built-in filter and gauge from Summit with adapters which looks like the original oldschool chrome lines. :cool:


[Modified by Marks69BB, 11:37 PM 7/15/2003]

Graemeinvette

:iagree:

Mystery Machine

I have a Speed Demon 650 on my ZZ4 (actually it's the one that supposedly built for the ZZ4 motor) and am pretty happy with it. It performs better than anything else I've had but I did encounter some "issues" with the choke. Turns out BG had a whole batch of them that had choke issues so they sent me a new choke. Everything seems to be fine now.

BG has a good racing reputation but there are definitely other choices that will perform well. For example, I've heard the Edelbrock Thunder series is a good one.

Good luck! :thumbs: