Carb question

[intofx]

The Speed Demon 750cfm on my built 383 has never run well. I bought the "finished" car with 250 miles and now has 800 miles and now I'm faced with de-bugging it. I have the car at a shop now that has sorted a few issues. They replaced the oversized mechanical fuel pump with a smaller one, replaced the fuel filters before and after the pump, and added a spacer with an additional vacuum nipple to allow a port for the PVC. The car runs MUCH better now but at idle with the car hot, the fuel pressure drops to around 3 and tiny bubbles of fuel "jump" out of the vent at the front of the carb. The mechanic thinks the gas is percolating. I've been looking around and it seems that some people have had boiling gas issues as a result of block mounted fuel pumps or even gas lines being too close to the exhaust. The shop is looking at a heat shield for the carb but would that really do that much to drop temps enough to prevent boiling fuel? Any experience here or suggestions?

TIA,
Matt

[Yoda4561]

Incorrect timing can cause or make this problem worse. A heat insulating carb gasket may also help.

[intofx]

Incorrect timing can cause or make this problem worse. A heat insulating carb gasket may also help.

The car seems to be "running great." Wouldn't a timing issue present issues elsewhere?

[Yoda4561]

It will but they may not be immediately noticable with regular driving, especially if you don't have experience with the engine perfectly tuned for reference. Just wanted to throw it out there because it's not something that often comes to mind when you think of carburetor issues. A carb spacer/heat shield will likely fix the fuel percolation, but if you're running with the timing too retarded or lean it will just disguise more serious issues. It's probably not worn out timing components if the engine build is fairly new, but stuff like a defective harmonic dampener isn't too uncommon, and that can throw off timing results enough to cause increased combustion/head temperatures.

[MrQuick]

what are your engine temps? What intake manifold are you using?


vince

[intofx]

what are your engine temps? What intake manifold are you using?


vince

The gauge reads fine but the mechanic said the carb was hot when they pointed the temp gun at it. How hot I'm not sure. I should know more details tomorrow. The intake is a CERAMCOATED EDELBROCK RPM.

[Pro Stock]

If you suspect that a fuel line is to close to a source of heat (header ect.) fix that first, put an insualator between the block and fuel pump and make sure that the fuel line from the pump to the carb is isolated from heat with proper routing and an insualting sleeve over it, also run an insualating gasket under the carb this way you will have addressed all of the heat issues at once. Check your float levels while the engine is running, your Demon carbs floats should be set so that the fuel level is halfway up the sight glass, are you running a regulator ?, if not you should be, set he regulator to about 6 1/2 psi, I would also run a BG regulator with a small guage attached to it so that you will always be able to see what your fuel pressure is if your having a problem. When you said that your fuel pump was to big, were you talking about an electrical pump or mechanical ?, if it is electrical and big enough you can take the mechanical pump out of the system and eliminate that source of heat. As stated above correct timing is critical and incorrect timing can show up as a carb problem,retarded timing will also cause the engine to run hot, unless your running a compression ratio above 10.5:1 and less than 92 octane fuel you should be able to run 36* total timing.

Dale

[intofx]

If you suspect that a fuel line is to close to a source of heat (header ect.) fix that first, put an insualator between the block and fuel pump and make sure that the fuel line from the pump to the carb is isolated from heat with proper routing and an insualting sleeve over it, also run an insualating gasket under the carb this way you will have addressed all of the heat issues at once. Check your float levels while the engine is running, your Demon carbs floats should be set so that the fuel level is halfway up the sight glass, are you running a regulator ?, if not you should be, set he regulator to about 6 1/2 psi, I would also run a BG regulator with a small guage attached to it so that you will always be able to see what your fuel pressure is if your having a problem. When you said that your fuel pump was to big, were you talking about an electrical pump or mechanical ?, if it is electrical and big enough you can take the mechanical pump out of the system and eliminate that source of heat. As stated above correct timing is critical and incorrect timing can show up as a carb problem,retarded timing will also cause the engine to run hot, unless your running a compression ratio above 10.5:1 and less than 92 octane fuel you should be able to run 36* total timing.

Dale


The old pump was mechanical an the smaller replacement one is as well. They added fuel filter near the tank and a regulator between the pump and carb. They say they solved the percolating issue. I have not had much time to drive around yet to test the car,tho... They said they set the fuel pressure to 5 lbs per the gauge. Should I run more (6.5 as you suggested)? Octane here in Kalifornia is 91 unless I go to race gas which is 101 (and $10 a gallon). Thanks for the help!

[Pro Stock]

The old pump was mechanical an the smaller replacement one is as well. They added fuel filter near the tank and a regulator between the pump and carb. They say they solved the percolating issue. I have not had much time to drive around yet to test the car,tho... They said they set the fuel pressure to 5 lbs per the gauge. Should I run more (6.5 as you suggested)? Octane here in Kalifornia is 91 unless I go to race gas which is 101 (and $10 a gallon). Thanks for the help!
Was the regulator on before the fuel pump change? if not the bigger pumps pressure could have been too high, the bigger pump was the better choice of pumps as long as the fuel pressure was set correctly, 6.5 psi is a good number, that way when the pressure fluctuates it will be neither to high or to low. also the filter should be placed between the pump and the carb because pumps are more efficient on the pressure side than they are on the inlet side. You didn't say whether you have addressed all of the possible heat issues that I talked about in a previous post so don't forget to do that, that is probably where your problem is. Lastly, you can greatly improve the gas by using an octane booster or mixing 2 or 3gallons of race gas with it, just experiment with it a little for the right mix, it's not much of an issue here in North Carolina because we still have 93 octane gas available.

ps, I can help you with that Demon if the problem persists after all of your fixes, I do R&D for BG
Dale

[intofx]

Was the regulator on before the fuel pump change? if not the bigger pumps pressure could have been too high, the bigger pump was the better choice of pumps as long as the fuel pressure was set correctly, 6.5 psi is a good number, that way when the pressure fluctuates it will be neither to high or to low. also the filter should be placed between the pump and the carb because pumps are more efficient on the pressure side than they are on the inlet side. You didn't say whether you have addressed all of the possible heat issues that I talked about in a previous post so don't forget to do that, that is probably where your problem is. Lastly, you can greatly improve the gas by using an octane booster or mixing 2 or 3gallons of race gas with it, just experiment with it a little for the right mix, it's not much of an issue here in North Carolina because we still have 93 octane gas available.

ps, I can help you with that Demon if the problem persists after all of your fixes, I do R&D for BG
Dale


Thanks for the help, Dale. I'm from High Point, NC originally so I miss the days of 93 octane.

The regulator was put on after the big pump was replaced. The shop said it was faulty and I needed a new one... I still have the part. Is there any way to tell if the pump is working correctly?

I mentioned the filter being after the pump as that is what I was always told and the tech said he wanted to filter before the pump so it would protect the pump.

I took the car out for a spin tonight and I am still getting a hesitation when I put my foot into it from a stop but overall the car runs much better.

Matt

[Pro Stock]

Thanks for the help, Dale. I'm from High Point, NC originally so I miss the days of 93 octane.

The regulator was put on after the big pump was replaced. The shop said it was faulty and I needed a new one... I still have the part. Is there any way to tell if the pump is working correctly?

I mentioned the filter being after the pump as that is what I was always told and the tech said he wanted to filter before the pump so it would protect the pump.

I took the car out for a spin tonight and I am still getting a hesitation when I put my foot into it from a stop but overall the car runs much better.

MattMechanical pumps aren't near as sensitive as electric pumps are to debris, and the filter will only hinder flow to the pump unless it is a high flow racing filter, so if your guy won't remove it do it yourself, just make sure you have the filter after the pump. the only way for you to check the other pump is to bolt it on the car. set the fuel pressure, and drive it. The hesitation sounds like a carb tuning issue independent of your other problems, don't know where you live in CA but I know a really good carb guy in Ontario. I hear ya on the octane thing, it's funny, I used to live in Southern CA and now I'm living in Davidson, NC.

Dale

[intofx]

Mechanical pumps aren't near as sensitive as electric pumps are to debris, and the filter will only hinder flow to the pump unless it is a high flow racing filter, so if your guy won't remove it do it yourself, just make sure you have the filter after the pump. the only way for you to check the other pump is to bolt it on the car. set the fuel pressure, and drive it. The hesitation sounds like a carb tuning issue independent of your other problems, don't know where you live in CA but I know a really good carb guy in Ontario. I hear ya on the octane thing, it's funny, I used to live in Southern CA and now I'm living in Davidson, NC.

Dale

Looks like the heat issue is still a problem. I went to pick up the car yesterday and they wrapped the headers on the fuel line side but the line is still pretty close to the header in a few places. May have to re run the line. The other issue is that the fuel pressure is steady for the most part at 4.5lbs but if it sits (and gets hot I assume) it drops into the 3 lbs range. It seems that there is just a ton of heat in my engine bay. This is getting ridiculous and expensive.

[Pro Stock]

Looks like the heat issue is still a problem. I went to pick up the car yesterday and they wrapped the headers on the fuel line side but the line is still pretty close to the header in a few places. May have to re run the line. The other issue is that the fuel pressure is steady for the most part at 4.5lbs but if it sits (and gets hot I assume) it drops into the 3 lbs range. It seems that there is just a ton of heat in my engine bay. This is getting ridiculous and expensive.
Do you have the tools to work on this stuff yourself?, if so I would put the other pump back on the engine and set the regulator to 6.5-7psi, set the floats halfway up the sight glass, relocate the filter between the pump and carb and re-route your fuel line to get it further away from heat sources. Unless your mechanic is a friend or someone you really trust I would get away from him and either do it yourself or find another mechanic.

Dale

[My427stang]

The other issue is that the fuel pressure is steady for the most part at 4.5lbs but if it sits (and gets hot I assume) it drops into the 3 lbs range. It seems that there is just a ton of heat in my engine bay. This is getting ridiculous and expensive.

Are you running a liquid filled gauge? Often they will read lower, even down to zero when they get hot, even if there is pressure.

As far as cooling things down, run as much initial timing as the starter can stand, and recurve based on that. In addition as much fan as you can to move air at low RPM.

Finally, if there is any way to run a flat base air cleaner, it'll let some heat escape where a drop makes a good heat "bell"

Ultimately, the more ethanol in our gas, the easier it will be to boil off in the bowls.

[Tech @ BG]

Some good information already posted in this thread. Also want to point out if the fuel itself is percolating in the bowls that while a spacer will help it still may not eliminate the problem and you may have to use a heat shield which comes out and covers the bottom of the bowls .

GM offers them as well as most speed shops keeping them in stock. It is a thin piece of aluminum with gaskets on both sides and as mentioned it comes out past the actual base plate between the intake and the bottom of the bowls.

Also as mentioned if you are using a liquid filled gauge take it off and throw it away as far as possible. As they get hot the pressure shown can decrease and in many cases the tuner will raise the pressure thinking they are compensating. At some point it gets where it will blow past the needle and seat but after cooling the gauge will then read the actual now too high pressure.

[intofx]

Some good information already posted in this thread. Also want to point out if the fuel itself is percolating in the bowls that while a spacer will help it still may not eliminate the problem and you may have to use a heat shield which comes out and covers the bottom of the bowls .

GM offers them as well as most speed shops keeping them in stock. It is a thin piece of aluminum with gaskets on both sides and as mentioned it comes out past the actual base plate between the intake and the bottom of the bowls.

Also as mentioned if you are using a liquid filled gauge take it off and throw it away as far as possible. As they get hot the pressure shown can decrease and in many cases the tuner will raise the pressure thinking they are compensating. At some point it gets where it will blow past the needle and seat but after cooling the gauge will then read the actual now too high pressure.

Thanks for the info. The gauge is liquid filled but it is not just the reading that is affected. The idle changes and the car sputters a bit when it gets hot. The fuel line is in the stock location which is less than 2 inches away from the header at the front of the engine bay - so I'm pretty sure that is a huge part of the problem. Any suggestions on rerouting the fuel line?


Thanks!

[Tech @ BG]

Thanks for the info. The gauge is liquid filled but it is not just the reading that is affected. The idle changes and the car sputters a bit when it gets hot. The fuel line is in the stock location which is less than 2 inches away from the header at the front of the engine bay - so I'm pretty sure that is a huge part of the problem. Any suggestions on rerouting the fuel line?


Thanks!

Agreed, just throwing that out there as well once you get the line issues corrected.

Maybe I missed it but what body is this in?

[intofx]

Agreed, just throwing that out there as well once you get the line issues corrected.

Maybe I missed it but what body is this in?

69 camaro.

[intofx]

So I'm having them move the filter in the rear to the front as suggested here. In the process they put a vacuum gauge on the line as they thought this could also be a tank or sender issue. They said they think this could be an issue in the gas tank as they were getting vacuum on the line as if the sender gets clogged? They are thinking there may be something in the tank causing issues? I have had a recurring problem where the fuel gauge reads empty and when I go to fill up the 18 gallon tank, it only takes 9 gallons.

Thanks again for the suggestions.

Thanks,
Matt

[Tech @ BG]

So I'm having them move the filter in the rear to the front as suggested here. In the process they put a vacuum gauge on the line as they thought this could also be a tank or sender issue. They said they think this could be an issue in the gas tank as they were getting vacuum on the line as if the sender gets clogged? They are thinking there may be something in the tank causing issues? I have had a recurring problem where the fuel gauge reads empty and when I go to fill up the 18 gallon tank, it only takes 9 gallons.

Thanks again for the suggestions.

Thanks,
Matt

Is the tank vent open and working?

[intofx]

Is the tank vent open and working?

It's a good question but I don't know. Is there an easy way to check?

I picked up the car yesterday but it's still stumbling off the line at WOT. They put a larger (same size as my original) fuel pump on the car so at least I'm getting a steady and regulated 6psi of pressure. With the car hot and running I checked the level in the bowls and the secondaries (bottom of the glass) are lower than the primaries (half way). Shouldn't they be the same front to back?

Also, it looks like my alternator is weaker than it should be and isn't pulling it's weight with any accessories on. Could that be causing issues with the ignition that may present themselves as a fuel/carb problem?

Would it help if I posted a video of the stumble?

Thanks again, guys.

Matt

[Yoda4561]

Low voltage at the ignition can be a real problem. I assume by your issues you're running a 1 wire alternator, I suggest switching back to a 3 wire style, like a 12si or more modern cs130.

This article convinced me it was the way to go. I had all kinds of charging and voltage issues on a one wire alternator back when I had a 72 cutlass, at idle it could barely power the lights, and I'm pretty sure it was the cause of some of my off-idle stumbling issues.

http://www.madelectrical.com/electricaltech/onewire-threewire.shtml

[intofx]

Low voltage at the ignition can be a real problem. I assume by your issues you're running a 1 wire alternator, I suggest switching back to a 3 wire style, like a 12si or more modern cs130.

This article convinced me it was the way to go. I had all kinds of charging and voltage issues on a one wire alternator back when I had a 72 cutlass, at idle it could barely power the lights, and I'm pretty sure it was the cause of some of my off-idle stumbling issues.

http://www.madelectrical.com/electricaltech/onewire-threewire.shtml


You would be correct. 1 wire. Is it tough to rewire for 3 wire?

[intofx]

Low voltage at the ignition can be a real problem. I assume by your issues you're running a 1 wire alternator, I suggest switching back to a 3 wire style, like a 12si or more modern cs130.

This article convinced me it was the way to go. I had all kinds of charging and voltage issues on a one wire alternator back when I had a 72 cutlass, at idle it could barely power the lights, and I'm pretty sure it was the cause of some of my off-idle stumbling issues.

http://www.madelectrical.com/electricaltech/onewire-threewire.shtml


You would be correct. 1 wire. Is it tough to rewire for 3 wire?

[Tech @ BG]

It's a good question but I don't know. Is there an easy way to check?

I picked up the car yesterday but it's still stumbling off the line at WOT. They put a larger (same size as my original) fuel pump on the car so at least I'm getting a steady and regulated 6psi of pressure. With the car hot and running I checked the level in the bowls and the secondaries (bottom of the glass) are lower than the primaries (half way). Shouldn't they be the same front to back?

Also, it looks like my alternator is weaker than it should be and isn't pulling it's weight with any accessories on. Could that be causing issues with the ignition that may present themselves as a fuel/carb problem?

Would it help if I posted a video of the stumble?

Thanks again, guys.

Matt

As mentioned the low voltage will cause all kinds of tuning issues and should be corrected prior to trying to make any calibration changes to the carburetor.

In regards to the vent you'll need to locate it and then trace it back to the tank, make sure it is not pinched or collapsed and if it has a filter at the end make sure that is not stopped up.

[Yoda4561]

You would be correct. 1 wire. Is it tough to rewire for 3 wire?

Not at all, the DIY info is all on their website, you can find other diagrams and stuff googling 3 wire alternators. It's really not a complicated hookup procedure at all, you can probably do it in an hours time if you study the diagrams beforehand.

[intofx]

So, in the LA heat this weekend (100+) the car almost left me stranded because of vapor lock. I'm having a new shop look at running a new fuel line next week - then the alternator issues - then either a new carb or try to get this Demon going. The overwhelming majority of people I've spoken with, say ditch the Demon for a Holley HP. Any suggestions as to which one? I could have bought a Holley and a half for what I've spent trying to get this Demon to play nice. I'm so mad at the guy I bought this car from I could spit.

[RUSS68CAMARO]

Ditch the carb. I had a demon on a mildly build 350 and I could never get it to run right. I swore I would travel the earth telling of the junk I got from BG. The front fuel bowl sight glass o-ring was pinched, metal shavings in the aft fuel bowl and I bought on with the Easy Idle adjustment which never worked right. I called them and they said that was for built motors but no where in their advertisements does it say that. Trash It!!!!!!!!!!!!!!

[intofx]

Ditch the carb. I had a demon on a mildly build 350 and I could never get it to run right. I swore I would travel the earth telling of the junk I got from BG. The front fuel bowl sight glass o-ring was pinched, metal shavings in the aft fuel bowl and I bought on with the Easy Idle adjustment which never worked right. I called them and they said that was for built motors but no where in their advertisements does it say that. Trash It!!!!!!!!!!!!!!

Any suggestions which carb to buy for my 383?

Matt

[John Haley]

I have not had a bad Demon carb yet, but have found many other problems that will make you think you have a bad carb. The two most common are, Idle screws are set at about 3 turns out. The other is old rotted fuel line sucking air.

Turn your idle adjustment screws in until they stop. You will think they are in all the way, but they are not. At this point they are about 1 1/2 turns out. Leave them there, reset your idle and go drive it.

If you are still having vapor lock problems and droplets of fuel spitting from the over flow tubes, check for bad fuel line sucking air( usually the rubber one at the tank connecting to the metal line on the frame), also check to see if your fuel sock in the tank is plugged.

[intofx]

I've officially given up on the carb. I'm going to do what I should have done in the first place and have the car EFI-ed. All of the problem parts will be replaced and the fuel lines re run as a result. I just wish I had pulled the trigger before spending a pile of cash trying to fix this car one part at a time...

Matt