Carb gods step in please
Carb gods step in please Hey guys, I recently rebuilt the small block 350 that is in my nova.
Forged dished pistons, forged rods ,etc.. demon blow thru carb 750cfm.
The problem im having is I cannot get it to stop going lean during high boost. I've swapped jets many times with not much effect. I know the bigger I go the better. Just had 72/84 and it wouldn't go far without going lean. Yet the exhaust fumes and spark plugs tell me its running pretty rich. Max boost is about 10psi.
I've had 86/96 jets in and I can get it to run pretty strong up to wot again, then goes lean. Will run great 1/2 to 3/4 throttle around 11.5 afr.
Sorry for the dumb questions, pretty new to boost, and this is the first carb'd engine I've had. Any suggestions?
Forged dished pistons, forged rods ,etc.. demon blow thru carb 750cfm.
The problem im having is I cannot get it to stop going lean during high boost. I've swapped jets many times with not much effect. I know the bigger I go the better. Just had 72/84 and it wouldn't go far without going lean. Yet the exhaust fumes and spark plugs tell me its running pretty rich. Max boost is about 10psi.
I've had 86/96 jets in and I can get it to run pretty strong up to wot again, then goes lean. Will run great 1/2 to 3/4 throttle around 11.5 afr.
Sorry for the dumb questions, pretty new to boost, and this is the first carb'd engine I've had. Any suggestions?
TECH Resident
Joined: Sep 2013
Posts: 994
Likes: 120
From: Mechanicsburg, PA
Yeah, check the basics out (as Loud Mouse stated) and then throw that Demon as far as you can and get something else, lol. Admittedly mine is roots blown and draw-through, but these Demons have kicked my carb-experienced ***. I have found flaws in them as well. One of my IFR's was never even drilled! Mine goes lean when you start to stand on it. Part throttle at about 5psi it's at 13.9 AFR. They have too many emulsion holes and act like no other carb for sure. Keep us posted. Best of luck with yours!
Launching!
Joined: Jun 2012
Posts: 249
Likes: 36
From: Pennsylvania
This is the benefit of having boostbrefrence power valves.... when you get into boost it’s not relying on main jet and adds fuel through the power valve.... this is why having a carb done by c&s, CSU, or other reputable companies makes a difference.... everyone I’ve seen use a shelf carb from big names regrets their decision.....
Fuel pressure is about 7psi. Instructions that came with carb say 6.5 psi is max. I do not have a boost referenced regulator yet, is that my problem? Floats are set right in the middle.
This is the benefit of having boostbrefrence power valves.... when you get into boost it’s not relying on main jet and adds fuel through the power valve.... this is why having a carb done by c&s, CSU, or other reputable companies makes a difference.... everyone I’ve seen use a shelf carb from big names regrets their decision.....
lol 
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
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Staging Lane
Joined: Jul 2016
Posts: 61
Likes: 0
Yes having a boost referenced regulator is a must. Once you get into boost @ 7lb u have no fuel pressure without it.
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You have 2 problems, no boost ref reg. and a Demon carb.
Once your boost matched fuel pressure, fuel cannot flow into
the carb-like kissing your sister, lol.
Someone like CSU building a carb will help you reach your goals.
Do not use boost until a new reg is installed, unless you like burnt
pistons. There are little things you can do to the Demon, smaller hi speed
air bleeds, larger PVR holes, etc, but w/o the boost ref reg, nothing will help.
Once your boost matched fuel pressure, fuel cannot flow into
the carb-like kissing your sister, lol.
Someone like CSU building a carb will help you reach your goals.
Do not use boost until a new reg is installed, unless you like burnt
pistons. There are little things you can do to the Demon, smaller hi speed
air bleeds, larger PVR holes, etc, but w/o the boost ref reg, nothing will help.
Needle/Seat Size of .140" Hi GTO, the Boost referenced regulator is a MUST.
I did a few SC Panteras AND always installed .140" needle/seats (Steel) in those days, before EFI.
Lance
I did a few SC Panteras AND always installed .140" needle/seats (Steel) in those days, before EFI.
Lance
Launching!
Joined: Jun 2012
Posts: 249
Likes: 36
From: Pennsylvania
I used the aeromotive regulator (13204 which is for the a 1000 fuel pump with boost refrence).....make sure you drill and tap your hat and hook that port to it and no where else. If the inlet of the hat is at 12 o’clock drill and tap 1/8npt at 6 o’clock so flow goes directly in it. Which fuel pump are you running?
This is the benefit of having boostbrefrence power valves.... when you get into boost it’s not relying on main jet and adds fuel through the power valve.... this is why having a carb done by c&s, CSU, or other reputable companies makes a difference.... everyone I’ve seen use a shelf carb from big names regrets their decision.....
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
Demon carbs aren't known for their blow-through performance. CSU does an incredible job on their blow-through carbs.
Get the regulator and go from there, but you might want to start lining up a new carb now just in case your current one doesn't work out, that way you already know exactly what you need instead of being stuck dead in the water.
Get the regulator and go from there, but you might want to start lining up a new carb now just in case your current one doesn't work out, that way you already know exactly what you need instead of being stuck dead in the water.
On The Tree
Joined: Dec 2011
Posts: 151
Likes: 6
From: Houston Tx
I"ve done many blow throughs. I used to buy CSU...now I build my own. A1000 pump and Mallory 4309 regulator works perfectly, boost referenced of course. Also a nice easy way to richen it up top with small jetting is to add vent tube extensions hose clamped to the vent. I use aluminum fuel line from Summit, but brake line works well too. Aim them into the air stream and tie wrap them together. I've even flared them and gotten down to tiny main jetting. Look it up, there are many threads on it on TheTurboForums. Also, you can raise the floats a touch and install smaller high speed air bleeds. But trust me....the extensions work.
I"ve done many blow throughs. I used to buy CSU...now I build my own. A1000 pump and Mallory 4309 regulator works perfectly, boost referenced of course. Also a nice easy way to richen it up top with small jetting is to add vent tube extensions hose clamped to the vent. I use aluminum fuel line from Summit, but brake line works well too. Aim them into the air stream and tie wrap them together. I've even flared them and gotten down to tiny main jetting. Look it up, there are many threads on it on TheTurboForums. Also, you can raise the floats a touch and install smaller high speed air bleeds. But trust me....the extensions work.
Funny how most people get hung up on fuel jets and ignore the air bleeds. Then again, blow throughs have a unique issue with throttle shaft seals that most carb guys don't have to deal with.
Launching!
Joined: Jun 2012
Posts: 249
Likes: 36
From: Pennsylvania
Building a blow through carb to perform well has challenges, but it is not as impossible as you make things sound. Doing so without a boost referenced FPR is, however, impossible. Then again, the only one I ever did was only running 27# boost on a low compression SBC build with a Procharger.
It is much more involved than a normal carb build, but the info is out there. As I recall, CSU was one of the information resources I used in the previously mentioned build. I have not messed with a blow-through in years, so the details are a bit sketchy in my mind at this point.