Demon carb issues?

need4steed · June 4th, 2011, 10:07 AM

Hey guys, i have a 67 mustang that has a modded 347 motor in it. I have a demon 750 cfm carb on it and have been having some issues with it.

When I try to start it after sitting for a few days it will not start without some starting fluid shot into it. The electronic choak works as it should from what i can see. It closes and opens slowly when it gets warm.

Next, at 2500- 3500 RPM's (doesnt matter the gear) there is a slight miss. You can feel it and hear it. Nothing crazy, but just enought to feel and know somehting isnt right. When i get on it hard it misses just a bit and then takes off. Same thing happens when i get on it hard from a stop, shift at 6500 rmps, go into 2nd and there is a bad hesitation and then it takes off.

Last, when i am high into the RPM's and let off the gas, it backfires like a bunch of gun shots...normal?

Thanks for the help guys. Should i just get a holley carb instead?

69fastback · June 4th, 2011, 10:25 AM

I've never had any luck getting a Demon to work. You could always call tech........oh wait

It's hard to connect all those issues to one problem. It sounds like it may be lean if it's popping up high, and if it's hesitating when you stand on it, it may need more accelerator pump shot.

If you're going to buy a new carb, and you have a modded engine, I highly suggest giving Pro-Systems a call. You'll get a custom built Holley HP for about the same price as an OTB one. On top of that, you get the best customer service in the business. Pat can help you tune anything over the phone, but his stuff is usually spot on out of the box.

mrtleavitt · June 4th, 2011, 11:32 AM

Sounds a lot like a vacuum leak somewhere. Make sure all lines are hooked up correctly without any leaks and that any capped off ports have a good cap on them. Could also be a gasket that is sucking in air. I have gone through a bunch of carbs on my 428 FE. I started with a holley street avenger and that sucked. Then went to an edelbrock...huge mistake. Finally ended with a Holley HP and it is so sweet! I was able to fine tune it and it runs amazingly! It helps give me 450 hp with 12 mpg on the freeway (pretty amazing for a FE engine that is driven hard!) It's been 2 years now with no problems at all. The HP Holleys don't have a choke on them so it's fine here in Las Vegas but any cold weather and you might not want it.

need4steed · June 12th, 2011, 09:42 AM

Thanks for the help guys. When I get back home I will check everything again and make sure all things are plugged and sealed.

ultrafast · July 11th, 2011, 06:50 PM

The motor sounds lean. Fatten up the low speed and see if it cleans up.Maybe even a little more timing. I have a mighty demon and overcame a hesitation. Unfortunately Barry Grant is no more. I am sure parts availability will go away. Mine starts immediately with no choke horn at all. Accellerates like a rocket. 427 windsor - 580 horse 540 tq. Oh yeah and check for vacuum leaks like mentioned before. Could be the whole problem!!

June 4th, 2011, 10:07 AM	#1
need4steed Regular Join Date: Aug 2006 Location: joliet, IL Posts: 141 iTrader: 0 reviews	Demon carb issues? Hey guys, i have a 67 mustang that has a modded 347 motor in it. I have a demon 750 cfm carb on it and have been having some issues with it. When I try to start it after sitting for a few days it will not start without some starting fluid shot into it. The electronic choak works as it should from what i can see. It closes and opens slowly when it gets warm. Next, at 2500- 3500 RPM's (doesnt matter the gear) there is a slight miss. You can feel it and hear it. Nothing crazy, but just enought to feel and know somehting isnt right. When i get on it hard it misses just a bit and then takes off. Same thing happens when i get on it hard from a stop, shift at 6500 rmps, go into 2nd and there is a bad hesitation and then it takes off. Last, when i am high into the RPM's and let off the gas, it backfires like a bunch of gun shots...normal? Thanks for the help guys. Should i just get a holley carb instead? __________________ 67 Mustang = 492 HP/478 TQ 07 Mustang GT = JLT CIA with SCT Tune, Drops 95 MarkVIII= Project/ resurrection

June 4th, 2011, 10:25 AM	#2
69fastback Premium Member 1969 Mustang Mach 1 6.824@101.61 Join Date: Aug 2006 Location: Oklahoma jackass, get to driving Posts: 5,572 iTrader: 0 reviews	I've never had any luck getting a Demon to work. You could always call tech........oh wait It's hard to connect all those issues to one problem. It sounds like it may be lean if it's popping up high, and if it's hesitating when you stand on it, it may need more accelerator pump shot. If you're going to buy a new carb, and you have a modded engine, I highly suggest giving Pro-Systems a call. You'll get a custom built Holley HP for about the same price as an OTB one. On top of that, you get the best customer service in the business. Pat can help you tune anything over the phone, but his stuff is usually spot on out of the box. __________________ ./___ _ ____ ___ \. ( ]]] _ _ O _ _ [[[ ) \_ o _ _____ _ o _/ \|___\| ......... \|___\| http://www.youtube.com/watch?v=HyYU3...e_gdata_player

June 12th, 2011, 09:42 AM	#4
need4steed Regular Join Date: Aug 2006 Location: joliet, IL Posts: 141 iTrader: 0 reviews	Thanks for the help guys. When I get back home I will check everything again and make sure all things are plugged and sealed. __________________ 67 Mustang = 492 HP/478 TQ 07 Mustang GT = JLT CIA with SCT Tune, Drops 95 MarkVIII= Project/ resurrection

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June 4th, 2011, 11:32 AM	#3
mrtleavitt Regular 1967 Coupe 20 @ 80mph? Join Date: May 2010 Location: Overton, NV Posts: 81 iTrader: 0 reviews	Sounds a lot like a vacuum leak somewhere. Make sure all lines are hooked up correctly without any leaks and that any capped off ports have a good cap on them. Could also be a gasket that is sucking in air. I have gone through a bunch of carbs on my 428 FE. I started with a holley street avenger and that sucked. Then went to an edelbrock...huge mistake. Finally ended with a Holley HP and it is so sweet! I was able to fine tune it and it runs amazingly! It helps give me 450 hp with 12 mpg on the freeway (pretty amazing for a FE engine that is driven hard!) It's been 2 years now with no problems at all. The HP Holleys don't have a choke on them so it's fine here in Las Vegas but any cold weather and you might not want it.

July 11th, 2011, 06:50 PM	#5
ultrafast Newbie Join Date: Jun 2010 Posts: 28 iTrader: 0 reviews	The motor sounds lean. Fatten up the low speed and see if it cleans up.Maybe even a little more timing. I have a mighty demon and overcame a hesitation. Unfortunately Barry Grant is no more. I am sure parts availability will go away. Mine starts immediately with no choke horn at all. Accellerates like a rocket. 427 windsor - 580 horse 540 tq. Oh yeah and check for vacuum leaks like mentioned before. Could be the whole problem!!

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