Tuning Demon Carb on Stock 460ci Pickup Engine

I've asked a couple other places and can't find the answers I need. I will be long winded to get all the details on the table since I am running out of ideas at this point.

A while back I replaced the stock intake manifold and 4180 Holley carb on my 1986 F350 truck (88k total miles). In its place I put a Barry Grant Road Demon 750 (4402020VFE), a Weiand Stealth 8012 Intake Manifold, and a 1 inch Moroso Poly Spacer. The main purpose was to rid the engine of the EGR emissions stuff that stopped working long ago/partially taken out and the Holley which hasn't been touch since the factory (blown power valve/5mpg max on a good day) and to give the engine a little performance above stock. The spark plugs are Motorcraft Platinums and the wires are some brand I can't remember the name of but in good condition. The factory air cleaner was cut to fit until I can get a better one, filter is untouched and still seals tightly. Cap and rotor have been replaced sometime in the past. All else on the truck is stock.

As of now it will start and run ok and smooth at idle in the drive way. Starting it on a cold morning, I get heavy choke which involves white smoke, like steam, and black specs on the ground outside of the tail pipe. After that phase is done there is no smoke at idle and the tail pipe is sooty (unknown if this was before the swap since I never cleaned it) and dry. At idle she has 17-18hg vacuum with sudden drops and recoveries on sudden throttle usage. Timing is at 10 BTDC and revving the engine will show the advance goes to around 38 BTDC.

Now with the carb, BG shipped with the carb with 76 Primary and 78 Secondary jets (76/78). I put it on and got the above characteristics. Also not mentioned was a few backfires under load that came out through the carb. Also there is some hesitation on a sudden acceleration or starting from a dead stop. With some advice of friends, I went to a smaller primary jet and used a 74 (now 74/78). Backfiring got worse under load and even slammed the choke butterflies shut once (on an interstate on-ramp during rush hour none the less). I could accelerate lightly and maintain a speed of 65 mph but as soon as the engine was stressed or under load, the backfiring started until I let up on the pedal. This happens on all hills and all interstate on-ramps, fast accelerating, etc. I decided to try a stab in the dark and went with a 78 Primary jet (78/78) and this didn't change a thing. A friend recommended I retard the timing to 5 BTDC to see if that changes anything and while the engine runs rougher all around and almost stalls while starting from a dead stop, the backfiring has decreased slightly. He even suggested unhooking the vacuum advance to see if that will suffice the issue until we can find the main culprit but I haven't went that far yet.

A family member with a lot of Ford SB/BB experience in the 1980's has suggested I am running way too lean and recommended I jump to a primary jet size of 90 and start working up from there. I know there is no magical chart to reference jet size to an engine size but I would like to get some insight or even advice on if this is correct or if it is something we aren't seeing. I will add any technical details if someone ask for them. I have a 70-80 jet kit but don't want to buy more jets/kits until I am certain that will lead me to solve the issue.

You're running lean when the secondary opens up. Bump up the rear jet size 6 - 8 sizes (I have 78/84 in my holley 750) and see what that does.

After you test out how it's running (under load) and if there's improvement, then you can make some changes with your timing. Try it at 16 BTDC.

Hope that this helps. Others will chime in with more suggestions. These suggestions should get you closer to what's needed. Well ....... those are the current settings I'm running.

It way to lean! i have a barry 750 and had to go up 5 tp get it up on the converter and have been told by a couple engine builders and they said they went up 5 jets front and back on a dozen BG on the dyno. My car with a 8 inch stall and 501ci would shut off taking off slowly till the power valved opened and took of normal then with full exhaust on it. ED

Thanks for the input so far. I plan on jumping the jets up to 78/82 for now. I know it is a long shot but by chance could a stock air cleaner or stock exhaust be creating this?

unscripted80 wrote:Thanks for the input so far. I plan on jumping the jets up to 78/82 for now. I know it is a long shot but by chance could a stock air cleaner or stock exhaust be creating this?

Doubtful but stranger things have happened. Sounds like a lean condition, go up in jet sizes like as been suggested.

95lightiningguy wrote:

unscripted80 wrote:Thanks for the input so far. I plan on jumping the jets up to 78/82 for now. I know it is a long shot but by chance could a stock air cleaner or stock exhaust be creating this?

Doubtful but stranger things have happened. Sounds like a lean condition, go up in jet sizes like as been suggested.

Already planned on the jets although I will have to order a 80-90 set for the secondaries. I was just bouncing the idea off of those here for responses if there was something I might not be seeing.

Are you sure there are no leaks in the intake, that could make it lean.

Some auto parts stores carry the Holley jets. Try one of them first to see if they have something in the 80-90 range. If so, just pick up a set of 80 somethings (82, 84 or 86) for now. At least that way you can go ahead and test it. Then you can fine tune it later when the 80-90 set comes in.

95lightiningguy wrote:Are you sure there are no leaks in the intake, that could make it lean.

The gaskets I got when putting the intake on weren't the right ones ("performance" gaskets, bore sizes were too big) and did leak. After installation, I went over the gasket area with starter fluid spray and found 2 leaks in the same spot of the gasket on both sides. Reverse the gasket and the leaks would be in the same spot. They have since been sealed and the way the gasket sits I doubt they have leaks on the inside of the oil valley. I figured if there were further leaks that the vacuum would be fluctuating more.

The Pope wrote:Some auto parts stores carry the Holley jets. Try one of them first to see if they have something in the 80-90 range. If so, just pick up a set of 80 somethings (82, 84 or 86) for now. At least that way you can go ahead and test it. Then you can fine tune it later when the 80-90 set comes in.

No stores in this area carry jets, already checked that route. I ordered the kit through Jegs and they should be here Monday/Tuesday.

How is the fuel system? What pressure is the fuel pump putting out? Could be bad or worn spark plugs(Big gaps), maybe even the ignition is weak. Just a thought to check these before going too much further within the carburetor. I hope this helps. Todd

aquartlow wrote:How is the fuel system? What pressure is the fuel pump putting out? Could be bad or worn spark plugs(Big gaps), maybe even the ignition is weak. Just a thought to check these before going too much further within the carburetor. I hope this helps. Todd

I re-read your post and found where you are using platinum plugs, try using a copper core plug instead. I have read on different websites that members state that the copper cores work better or give a better spark in our older ignition systems, just a thought.

I apologize but I forgot about one thing. On the ignition system I did replace the factory coil with a Pertronix Flame Thrower Coil (epoxy filled). The plugs are still gapped with the factory specifications. Also the plugs were put in about 2000 miles ago and still looking fine as of when the carb and intake went in. As for the fuel system pressure, I don't have a fuel pressure gauge handy.

i dont know nothing about race tunning but i spent a lot of years working on farm trucks at the local feed coop. i am here to tell you that holley 4180s always came with jets in the 60s and the power valves were not on the light side. i think ford used a small carb and low rise intake to keep port velocity high at low rpm. a 750 and a stealth on a work truck aint quite the ticket. sorry for stinking up your thread.....bobn

Did you make sure the wiring was correct when installing the coil, + to + and - to - if not it might be cross wired, it will still run but terrible if these are reversed. I hope you're not offended, I'm just trying to help with some suggestions.

aquartlow wrote:Did you make sure the wiring was correct when installing the coil, + to + and - to - if not it might be cross wired, it will still run but terrible if these are reversed. I hope you're not offended, I'm just trying to help with some suggestions.

I traced the wires to make sure when I put the intake manifold back on. Thanks for the help though.

unscripted80 wrote:

95lightiningguy wrote:Are you sure there are no leaks in the intake, that could make it lean.

The gaskets I got when putting the intake on weren't the right ones ("performance" gaskets, bore sizes were too big) and did leak. After installation, I went over the gasket area with starter fluid spray and found 2 leaks in the same spot of the gasket on both sides. Reverse the gasket and the leaks would be in the same spot. They have since been sealed and the way the gasket sits I doubt they have leaks on the inside of the oil valley. I figured if there were further leaks that the vacuum would be fluctuating more.

yeah about those gaskets... if you could just see about getting the correct gasket for your application...
Nobody wants to reinstall an intake, especially in a truck, but its the right thing to do.
My .02 free of charge

I have heard of a few people having problems with the moroso 1 inch poly spacers not sealing you might want tocheck around the carb base andintake to spacer. for a vacuum leak. Alos if you have had backfires through the carb probably should replace the powervalves just to be safe. JMO Todd

the1969fordguyinky wrote:

yeah about those gaskets... if you could just see about getting the correct gasket for your application...
Nobody wants to reinstall an intake, especially in a truck, but its the right thing to do.
My .02 free of charge

New gaskets are planned and the intake manifold being aluminum compared the cast one it replaced will be easy to take out. However it hasn't been above freezing here in a couple weeks and the holidays kind of make time for truck work harder than normal to get. Probably after Christmas she is getting new gaskets.

lghting94 wrote:I have heard of a few people having problems with the moroso 1 inch poly spacers not sealing you might want tocheck around the carb base andintake to spacer. for a vacuum leak. Alos if you have had backfires through the carb probably should replace the powervalves just to be safe. JMO Todd

Thanks for the tip. I'll give the area a squirt of starter fluid to see if there are any leaks when I get the time.

I checked the spacer with starter fluid and found no obvious leaks. I finally got a jet kit and went 80 on the primaries and 84 on the secondaries. Idle didn't change one bit and now I can accelerate under load unlike before without backfiring. Only issue right now is if I am at a higher rate of speed (45mph+) and stomp on the pedal, she still backfires. I'll probably go to 82/86 in the next couple of days and see if that changes the issue.

Appreciate the update! Its the feedback as to what ultimately fixes your problem that will help countless others in the future.

the1969fordguyinky wrote:Appreciate the update! Its the feedback as to what ultimately fixes your problem that will help countless others in the future.

No problem. I will be posting questions and updates until the issue is resolved to where I am comfortable. The other day I had the carb off and set the jets to 82 for primaries and 86 for secondaries. I also pulled two plugs from each side to check for colors. The darker one is #1 cylinder and might be from leak down, the others are #3, #5, and #7. The plugs haven't been cleaned since the carb originally went in so there is crud on them from the original timing and tuning issues.




After the jets went in the truck ran a lot smoother and no backfiring. In the place where the backfiring would happen, there is be a rattling/knocking from the engine. I'm guessing she is still running too lean and might jump up 2 sizes on the secondaries. This sound right?

The secondaries should be 6 to 8 sizes larger than the primaries.

If you can, post what you engine vacuum is when your crusing down the highway. This info will help us to guide you to the correct power valve size.

The Pope wrote:The secondaries should be 6 to 8 sizes larger than the primaries.

If you can, post what you engine vacuum is when your crusing down the highway. This info will help us to guide you to the correct power valve size.

It will be a bit until I can get a hose that will reach the cab. I planned on leaving the primaries alone for the moment and upping just the secondaries.

Maybe useless for your aplication, but I seem to remember these carbs having an adjustment screw in the baseplate between all 4 barrels in the center of the carb. You need to remove the air cleaner stud and use a screwdriver to adjust it. Clockwise should be rich, and counter clockwise would be lean. This works like having the holes drilled in the butterflies without exposing the transition slot. If you have any paper work with the carb, it should be in there. I found this on a carb with similar symptons as yours once I took the baseplate off. Then the owner found the original paperwork and there it was. By jetting it up like you that you are just crutching the lean symptons and increasing fuel consumption.
Travis

TravisRice wrote:Maybe useless for your aplication, but I seem to remember these carbs having an adjustment screw in the baseplate between all 4 barrels in the center of the carb. You need to remove the air cleaner stud and use a screwdriver to adjust it. Clockwise should be rich, and counter clockwise would be lean. This works like having the holes drilled in the butterflies without exposing the transition slot. If you have any paper work with the carb, it should be in there. I found this on a carb with similar symptons as yours once I took the baseplate off. Then the owner found the original paperwork and there it was. Travis

The option you mention in the BG Demon carbs is "Idle-Eze" which the Road Demons do not have. The documentation that came with this carb and available online is sparse at best describing what to do without how to do it. Also this option, if available, only affects the idle and not much beyond that. Thanks for the idea though.

TravisRice wrote:By jetting it up like you that you are just crutching the lean symptons and increasing fuel consumption.
Travis

Explain?