Verified my problem, but how do I fix it?
6th Gear Member
Joined: Apr 2007
Posts: 10,468
From: California
RE: Verified my problem, but how do I fix it?
First of all, vacuum secondary carbs don't open based on engine vacuum. They get their vacuum signal for operation from the high velocity in the venturis, the same way that fuel is metered. You could have 0.3" of intake vacuum, but if the velocity in the barrels is high, the secondaries WILL open. That means you need a carb that's the right size, or a titch smaller, to generate strong signal(just like for metering fuel) to open the secondaries. Even an oversized carb should STILL open the secondaries. The primary reason for running a DP in a high performance application is that the instant you hammer the throttle ALL 4 barrels are open, and you get even fuel distrobution to the front and rear cylinders on the engine. If you rev the engine in neutral, the secondaries will NOT open about 95% of the time or more. How do you know they aren't opening? Put the tightest spring in and drive it, then put the lightest spring in and drive it. If they're opening, you will notice a major difference on how it drives. What is the car doing and how is it setup right now? Also, how is your ignition setup?
Thread Starter
3rd Gear Member
Joined: Mar 2006
Posts: 971
From: Apison, TN
RE: Verified my problem, but how do I fix it?
Well, I would reply to your questions, but if you would read the previous posts, then all of your questions will be answered. And trust me, they do not open consistantly. Even the demon tech representative told me that it was due to the lack of vacuum.
6th Gear Member
Joined: Apr 2007
Posts: 10,468
From: California
RE: Verified my problem, but how do I fix it?
The demon tech is an idiot then. There are assloads of people running around with 347's that are more wild than your's and are running vacuum secondary carbs without issue. Double pumpers have advantages, yes, but there is no reason that the secondaries should not be opening unless there is something wrong with the carb. If there is enough airflow to draw fuel out of the carb then there's enough airflow to open the secondaries, they use the SAME SIGNAL to work. If you even look in the barrles you'll see little holes in the side of them where the vacuum canister draws it's signal from. Also consider that you have a larger stall, so you're well above the minimum vacuum the engine makes when you're loading the engine.
How exactly are you testing to see if the secondaries are opening? What spring is in it, what gears etc. Have you checked the vacuum canister for cracks and the diaphragm for tears? If there's a vacuum leak in the canister then the secondaries won't open. There is no reason other than a broken carb that a 347 with a 650cfm vac 2nd carb shouldn't open the secondaries. Your cam is not that large for a 347.
How exactly are you testing to see if the secondaries are opening? What spring is in it, what gears etc. Have you checked the vacuum canister for cracks and the diaphragm for tears? If there's a vacuum leak in the canister then the secondaries won't open. There is no reason other than a broken carb that a 347 with a 650cfm vac 2nd carb shouldn't open the secondaries. Your cam is not that large for a 347.
Thread Starter
3rd Gear Member
Joined: Mar 2006
Posts: 971
From: Apison, TN
RE: Verified my problem, but how do I fix it?
I do not have a vacuum canister on it now, because I doubted that it would even help the situation. I did indeed to check the diaphragm for tears. I took it off, and visually inspected it. Then I put the cover back on and then pushed the rod in by hand and covered the vacuum hole to make sure it wasn't leaking... and it was fine.w It has the "regular" spring that came in it. I havent been able to visually see them open or not. Like you said, they wont open unless there is a load, so even if you floor it by hand they are not going to open. I dont think its possible for somebody to look at them while Im driving it down the road, or I would do that. I tried to figure out a way to check them with a paper clip or something, but I couldn't. The only way that I know that they are messing up is because if im driving along and floor it, sometimes it will rev up to like 3600, and then start screaming and pulling hard as ever, and other times, it will rev to like 5200 before it starts to scream and pull hard. When they do open, its like you hit nitrous. One time, took my grandfather for a ride in it, and I would pull out of the street and get up to like 25 mph and punch it, and they wouldn't open. It wouldn't bark the tires, and it wasn't pulling hard at all. My gears are 3.00s- stock ratio. I am planning on adding some 3.55s and a trac loc here in the near future. I also agree that the demon tech rep was a complete idiot and also a complete ***.
One more thing, if I switch over to a mech sec speed demon, or mighty demon, do you thinnk that it will flood it out when I stomp it from a dead stop?
One more thing, if I switch over to a mech sec speed demon, or mighty demon, do you thinnk that it will flood it out when I stomp it from a dead stop?
Joined: Jan 2008
Posts: 5
RE: Verified my problem, but how do I fix it?
Ok, let's clear up a few things as there appears to be a lot of name calling and mis-information given out in this thread. At no point was the customer called dumb but was infact told the vacuum carburetor was not what we would recommend for his application nor would a Speed Demon. The correct carburetor would be a Mighty Demon with mechanical secondary. As he has stated, his secondarys do work but are not consistant and that would be the case given the carburetor choice and engine combination. He was also asked and told that the secondarys cannot be checked at idle and the car has to be under load for them to operate and that they opereated off a small hole in the side of the venturi as the signal goes past it.It was explained to him about trying the lightest springbut that he may still have the same issue.Putting a smaller carburetor on it to make them function correctly would onlyband-aid the real problem at the expsense of some performance as well.They do Not work off "vacuum" . It was further explained that a vehicle with a stick or a converter of 3000 and above will have a different signal/load at the carburetor than one with an automatic or a tighter converter . Hope this clears up a few "issues" as we are only here to help.
Joined: Jan 2008
Posts: 5
RE: Verified my problem, but how do I fix it?
ORIGINAL: tyler72
I do not have a vacuum canister on it now, because I doubted that it would even help the situation. I did indeed to check the diaphragm for tears. I took it off, and visually inspected it. Then I put the cover back on and then pushed the rod in by hand and covered the vacuum hole to make sure it wasn't leaking... and it was fine.w It has the "regular" spring that came in it. I havent been able to visually see them open or not. Like you said, they wont open unless there is a load, so even if you floor it by hand they are not going to open. I dont think its possible for somebody to look at them while Im driving it down the road, or I would do that. I tried to figure out a way to check them with a paper clip or something, but I couldn't. The only way that I know that they are messing up is because if im driving along and floor it, sometimes it will rev up to like 3600, and then start screaming and pulling hard as ever, and other times, it will rev to like 5200 before it starts to scream and pull hard. When they do open, its like you hit nitrous. One time, took my grandfather for a ride in it, and I would pull out of the street and get up to like 25 mph and punch it, and they wouldn't open. It wouldn't bark the tires, and it wasn't pulling hard at all. My gears are 3.00s- stock ratio. I am planning on adding some 3.55s and a trac loc here in the near future. I also agree that the demon tech rep was a complete idiot and also a complete ***.
One more thing, if I switch over to a mech sec speed demon, or mighty demon, do you thinnk that it will flood it out when I stomp it from a dead stop?
I do not have a vacuum canister on it now, because I doubted that it would even help the situation. I did indeed to check the diaphragm for tears. I took it off, and visually inspected it. Then I put the cover back on and then pushed the rod in by hand and covered the vacuum hole to make sure it wasn't leaking... and it was fine.w It has the "regular" spring that came in it. I havent been able to visually see them open or not. Like you said, they wont open unless there is a load, so even if you floor it by hand they are not going to open. I dont think its possible for somebody to look at them while Im driving it down the road, or I would do that. I tried to figure out a way to check them with a paper clip or something, but I couldn't. The only way that I know that they are messing up is because if im driving along and floor it, sometimes it will rev up to like 3600, and then start screaming and pulling hard as ever, and other times, it will rev to like 5200 before it starts to scream and pull hard. When they do open, its like you hit nitrous. One time, took my grandfather for a ride in it, and I would pull out of the street and get up to like 25 mph and punch it, and they wouldn't open. It wouldn't bark the tires, and it wasn't pulling hard at all. My gears are 3.00s- stock ratio. I am planning on adding some 3.55s and a trac loc here in the near future. I also agree that the demon tech rep was a complete idiot and also a complete ***.
One more thing, if I switch over to a mech sec speed demon, or mighty demon, do you thinnk that it will flood it out when I stomp it from a dead stop?
Thread Starter
3rd Gear Member
Joined: Mar 2006
Posts: 971
From: Apison, TN
RE: Verified my problem, but how do I fix it?
So which carb do you recommend me to get? The mech secondary speed domon or the mighty demon? It is cold natured now even in the summer, but I have a clip on the choke to hold it open, so it simulates a chokeless carb.
Joined: Jan 2008
Posts: 5
RE: Verified my problem, but how do I fix it?
ORIGINAL: tyler72
So which carb do you recommend me to get? The mech secondary speed domon or the mighty demon? It is cold natured now even in the summer, but I have a clip on the choke to hold it open, so it simulates a chokeless carb.
So which carb do you recommend me to get? The mech secondary speed domon or the mighty demon? It is cold natured now even in the summer, but I have a clip on the choke to hold it open, so it simulates a chokeless carb.
Thread Starter
3rd Gear Member
Joined: Mar 2006
Posts: 971
From: Apison, TN
RE: Verified my problem, but how do I fix it?
OK, so get the mech sec Mighty Demon, 650 CFM, correct? Just out of curiosity, could the lack of not enough idle cicuit be the reason for hard cold starts now? Sometimes, when it starts now it wants to pop out of the carb (rare occasion), but like I said, it has 18 degrees of timing now, and once its warm it will idle fine andhas a nice lope to it.