Demon carbs?

[LanceB]

I read an article in Corvette Fever talking about the technology of the Demon carb and was wondering if anyone has put them on their stock C3 and what was their experience? My Dad contemplated putting his original carb on his 75 in storage (needs a rebuild) and putting a new one on, but I don't know how an aftermarket would fit or if it is better just to rebuild the old one. I am pretty happy with my stock 81, but it shows its age sometimes. What perked me was the improved throttle response on these Demon carbs and the overall reinvention of the carb.

 

[baxsom]

if the current carb is a quadrajet. you should stick with that otherwise you will have to change the manifold from a spread bore to a square bore pattern.

now that the obligatory keep the quadrajet is out of the way. my 72 BB came with a square bore manifold and a tired holley.
i swapped it for a 750 speed demon. i set the idle and never had a problem with it.

 

[Vettehead Mikey]

If you're asking is this carb better for performance than a stock Qjet, the answer is no. Different, not better.

 

[GTR1999]

We used a 750 speed demon and I hate it. I won't attempt to tune it anymore, it's my son's vette now so he can play with. It's about 4 years old and we get it to run good one day and have to go back later on to work on it. I've have discussions with experts on these and I guess some were prone to poor QC,maybe we have one of those? When it runs good it's great just doesn't stay that way for long.

I ran Holleys, have a 670 SA out of the box on my 72 that is much more drivable, I still have to re jet it but it's close.

I've run QJ's as well and once properly setup they are very reliable and I like them the best.

Never used an Edelbrock carb to compare.

 

[Hib Halverson]

If you have a Quadrajet now, unless your engine is seriously modified, keep the Quadrajet but do some performance tuning.

The Quadrajet is a much better carburetor than a Demon on a stock, near stock and even many mildly modfied engines.

 

[baxsom]

We used a 750 speed demon and I hate it. I won't attempt to tune it anymore, it's my son's vette now so he can play with. It's about 4 years old and we get it to run good one day and have to go back later on to work on it. QUOTE]


are you running the choke.
i noticed that mine ran a lot better once i removed the choke. for some reason until i took the choke off mine would do the same thing.

 

[Jimmy C]

If you're asking is this carb better for performance than a stock Qjet, the answer is no. Different, not better.

Only a question: You are saying that a Demon carb is no better than a quadrajet f/ performance? Only different, but no better?

 

[73shark]

IMHO, a properly tuned Q-jet is hard to beat on a street driven car and in my experience, works well for performance driving.

I've tuned a Holley on a 427/425 '66 and the good news is that they have a lot of tunable features. The bad news is that they have a lot of tunable features.

 

[GTR1999]

We used a 750 speed demon and I hate it. I won't attempt to tune it anymore, it's my son's vette now so he can play with. It's about 4 years old and we get it to run good one day and have to go back later on to work on it. QUOTE]


are you running the choke.
i noticed that mine ran a lot better once i removed the choke. for some reason until i took the choke off mine would do the same thing.

No choke, never ran with one.

 

[AKRAY4PLAY]

We used a 750 speed demon and I hate it. I won't attempt to tune it anymore, it's my son's vette now so he can play with. It's about 4 years old and we get it to run good one day and have to go back later on to work on it.QUOTE]

i have a 750 race demon on my 406 and love it. i'm just curious to what carb (road, speed, race, demon) you have and what application you are putting it on. also what are the problems you are having with it?? the only problem i had with mine was the idle feel restrictors. they were way to big and i could not tune in the idle mixture to save my life. bought a blank jet kit and some real tiny drills, made my own jets and she runs like a top.

on a side note, i love q-jets for stock motors, they perform very well with lots of vacuum and milder cams. they get real finicky with big lumpy cams, but 99% of street cars will do just fine with them.

if you guys and gals like carb tuning and have $350 to throw at tools, i would suggest you buy an LM-1 tuner. it is an O2 sensor that clips on the exhaust pipe and you can monitor your air/fuel mixture realtime. it's a great tool and makes tune-ups supre fast and very accurate. after buying one, i was able to tune my idle feed restrictors and get my MPG to increase from 8 to 26 at cruise speed of 65 mph. so for me it paid for it'self, and i tune my buddies old rides for post cruising beers.

 

[GTR1999]

It's a 750 speed demon,no choke, mech secondaries. I have not played with it in a while. My son does now, he's rejetted it a few times got it running great then it goes back to choking.

I would be interested in the LM-1, I've heard about them but never used one. I see they're about $300 online with free shipping are they as easy as clipping on the exhaust and rejetting?

Thanks

 

[AKRAY4PLAY]

LM-1 Wideband O2 Digital Air/ Fuel Ratio Meter | Lambda Sensor Controller

i see they increased the prices a bit. looks like $350 for the basic kit and an additional $75 for the exhaust clamp. i think it's still a good deal, especially if you like tinkering with carbs.

to answer your question about ease of use, it's WAY EASY!! all you have to do is plug the power cord into a working cigarette socket and hook up the O2 sensor. let the sensor heat up (it's a heated sensor) and stabilize and hit the calibrate button. it will dial itself into normal oxygen content in the air. then install the O2 sensor (after shutting off and letting cool down) into the exhaust clamp. the system will keep the calibration for that sensor after powering down and unplugging. when you fire up your car, you can have the system up and running or not. when using, just let the sensor heat up again and the read-out will tell your lamda reading. super simple. just a warning though, this thing is super sensitive and you can get high O2 readings from misfiring cylinders. so make sure you are hitting on all cylinders. if you get a passenger to read the meter, you can drive and read mixture real time for adjusting transition mixtures and full load mixtures. it's amazing how much timing also effects mixture, it's fun to play with that as well and see the results. it's a cool tool and i highly recommend one if you can afford it. it greatly increased my depth of tuning knowledge.

 

[AKRAY4PLAY]

It's a 750 speed demon,no choke, mech secondaries. I have not played with it in a while. My son does now, he's rejetted it a few times got it running great then it goes back to choking.

sounds like the issue is i was having with the idle circuit. are the idle mixture screws super sensitive and only work around 1/2 to 3/4 turn out from seated?? if so, you may have way to big of idle feed jets. the Demons usually come with 0.039" idle feed restrictors, but they make so many different carb apps that it is hard to tell without measuring them. anyway, i had to drop mine to 0.028" IFR jets and the idle mixture screws became usable again. no more loading up at idle and having raw fuel burn my eyes one day and run super crisp the next. Jeg's sells some small pin drills for drilling out jets and jet blanks for cheap. i think i got a complete kit, drills, pin holder and jets for about $50. way more jets than i will ever need, but it has been usefull for tuning buddies' cars as well.

 

[GTR1999]

Thanks for the info. I'll have to have my son look over your Demon spec's could be the same deal. I like the LM-1 as well. I only work on my cars but we have 3 carburated vettes here. I spend most of my "vette time" machining and building the diff's,T/A's and boxes.

 

[AKRAY4PLAY]

glad you liked the info on the LM-1, hope you can get the Demon figured out. they are great carbs, but a little interesting to dial in some times. as for machining diffs, you cutting them to build 12 bolt carriers?? i have been pondering a Tom's Diff 12 bolt set-up with 1480 u-joint kit for a couple years now. i'm scared to run drag radials with the stock rear-end and my 406. the street tires are my relief valves right now, most likely why my diff is still alive.

 

[GTR1999]

Yeah I built a couple of 12 bolt vette diffs recently. I use Tom's parts and add some of my mods as well. I also added in the 3.5 shafts and 31 spline axles in the T/A's. Real nice stuff but a lot of machine and hand work.
The last one is 1/2 way around the world now in a 800hp 78.

 

[73shark]

Does the $75 exhaust clamp allow the O2 sensor to be placed in the end of the exhaust pipe instead of having to weld in a bung?

Also noticed that there is a LM-2 now for the same price.

 

[TMPJoe]

I run a 770 Holley SA right out of the box on my 80 with my 383 motor... very extreme cam and heads... wierd thing is it ran fine with no tuning issues out of the box so leave it alone.... Love the comments on here too... the one about drag radials... I run L88 size TA's for the same reasons... i don't want to do more axle shafts! or god forbid shred the 3.73 in the back...I would be interested in what happens with the 12 bolt