[70_ragtop_ss]

Hey Guys,

I am trying to tune my 461 motor and am at my witts end. I'm running an 850 Demon Carb and can't get the carb to lean out. No matter what I do, it runs rich as all hell. Here are the problems I'm having and I would appreciate any help / suggestions you can offer.

Set-up: 461 BBC, Performer RPM manifold, 850 Demon Carb, MSD ignition, Oval ports with 225/188 valves, Super Comp Headers, AC Delco 43 plugs) Timing: 37 degrees total and 16 inital.

Issues:
* Backfires out of the passanger side header when it comes down off of throttle.
* Black soot along the entire exhaust ports by the head.
* Runs totally fat no matter how much I turn the needle and seat down
* It idles for a little while then it loads up and wants to shut down unless I keep giving it gas
* After the motor sits for a few mins. it is hard to get it started (almost like the carb is leaking into the motor and flooding)
* Fouling the plugs
* Gas dripping near the throttle shaft

My question is should I just buy a new carb and which one and size for my set-up.

Thanks!

 

[SWHEATON]

Your timing looks ok,but base 16 deg btdc could be slightly retartded esp if your running a lrg cam.

So lets start here,what type cam & specs (dur @.05/gross lift/LSA) are you running in your 461 along with what power valves are in the prim & secondary if running one in secondary along with front/back jetting too.

Are the idle mix screws adj properly and are the secondary blades also properly /seated ?

I am assuming your running proper fuel pressure,floats are adj correctly & not bad taking on fuel & being heavy,and there are no issues with needles & seats and or dirt in them or the carb.

Also,remember when you fuel foule plugs to a sooty black color like you stated the motor will start hard and run like crap or not start at all if thet are fouled bad enough no matter what you do so you must get new plugs for when you do some tuning to try to fix this issue. Find someone with a spark plug sand blaster or buy one from an auto prts store of JCwhiney,i have never seen them in summit /jegs. But be warned after cleaning the plugs you must spray carb cleaner into the plug and on the threads to clean off any sand/carbon/soot residue and then a good hit of compressed air deep into each plug to ensrue all sand and debries are out of of it post cleaning and blow the threads off too.

The plug cleaners dont cost that much/approx $25.00-$35.00 and its a great tool to have when going through the tuning process like you are where you could easily go through 4-5 sets of plugs at $ $16-$20 per set = $80-$100. You could get our money back on the cost of the plug cleaner in just one tuning/dialing in process.

Some people will likely say dont use the plug cleaner but i have been using one for over 37yrs without one problem ever saving me & customers of mine tons of $ on plugs when dialing in motors with issues like your has . But you must be sure to do proper post clean of each plug like i stated or you could possibly have a problems in debries from the plug cleaner gets into a cyl,it's extreemly important to do a proper cleaning,takes me approx 15-20 mins to blast/clean a set of 8 plugs & it saves $ .

You will need an air compressor that will pump up to a minmum of 100psi to run the plug sand blaster .

The R43's are a pretty cold plugs ,not that they cant be run on the street but you need to have a well tuned/dialed in motor to handle those cold plugs so dont use them when trying to get this sorted out as the bad plugs can send you chasing your tail later on in this tning/dialing in proess so keep that in mind.

Scott

 

[Dave427]

Sounds like you have picked some junk in the needle and seats. Take the needle and seats out blow them out. Usually when gas is running out the throttle shafts the floats are either too high or by-passing because of dirt.

Dave

 

[70_ragtop_ss]

Your timing looks ok,but base 16 deg btdc could be slightly retartded esp if your running a lrg cam.

So lets start here,what type cam & specs (dur @.05/gross lift/LSA) are you running in your 461 along with what power valves are in the prim & secondary if running one in secondary along with front/back jetting too.

Are the idle mix screws adj properly and are the secondary blades also properly /seated ?

I am assuming your running proper fuel pressure,floats are adj correctly & not bad taking on fuel & being heavy,and there are no issues with needles & seats and or dirt in them or the carb.

Also,remember when you fuel foule plugs to a sooty black color like you stated the motor will start hard and run like crap or not start at all if thet are fouled bad enough no matter what you do so you must get new plugs for when you do some tuning to try to fix this issue. Find someone with a spark plug sand blaster or buy one from an auto prts store of JCwhiney,i have never seen them in summit /jegs. But be warned after cleaning the plugs you must spray carb cleaner into the plug and on the threads to clean off any sand/carbon/soot residue and then a good hit of compressed air deep into each plug to ensrue all sand and debries are out of of it post cleaning and blow the threads off too.

The plug cleaners dont cost that much/approx $25.00-$35.00 and its a great tool to have when going through the tuning process like you are where you could easily go through 4-5 sets of plugs at $ $16-$20 per set = $80-$100. You could gety our money back on the cost of the plug cleaner in just one tuning/dialing in process.

Some people will likely say dont use the plug cleaner but i have been using one for over 37yrs without one problem ever saving me & customers of mine tons of $ on plugs when dialing in motors with issues like your has . But you must be sure to do proper post clean of each plug like i stated or you could possibly have a problems in debries from the plug cleaner gets into a cyl,it's extreemly important to do a proper cleaning,takes me approx 15-20 to blast/clean a set of 8 plugs.

You will need an air compressor that will pump up to a minmum of 100psi to run the plug sand blaster .

The R43's are a pretty cold plugs ,not that they cant be run on the street but you need to have a well tuned/dialed in motor to handle those cold plugs so dont use them when trying to get this sorted out as the bad plugs can send you chasing your tail later on in this tning/dialing in proess so keep that in mind.

Scott

Scott,

Thanks for the response. The cam is a Crane hyd. .553/.571 234/244 @.050. The power valve (only in the primaries) is 65 and the jets are 83 / 93, which are stock BG sizes.

I have pulled the needle and seats to make sure they aren't clogged and they look good. As far as the secondaries being seated, I assume you mean they aren't cracked open at idle. I haven't checked them so that is a great suggestion.

Do you think the 850 is too big for my set-up? Also, it seems that no matter what I do, I can't get the float level down. I have to look at the float levels again and make sure I'm turinging them the right way

Thanks for the suggestions.

John

 

[Dave427]

Turning clockwise they go down.

Dave

 

[Dave427]

Fuel pressure ok? No more than 7-8 psi?

Dave

 

[SWHEATON]

No,an 850 should not be too big,its very likely the fuel calibration and timing that needs to be dialed in better.

I would run a 5.0 power valve front & in back in running one in secondary with that cam in your 461,the 6.5 currently in the carb is for a milder cam'd setup not that your cam is that lrg in a 461 which it's not.

You know its so stupid that demon carb installs 6.5 p/valves in carbs that they know are going on hot motors with lower intake vacuum due to longer duration perf cams.

The 83 prim & 93 sec jets are pretty darn rich too and you should go down too 72-73 prim & 78-80 in sec as a good starting point and go up from there if need be but make sure install the 5.0 p/valves too at the same time,that 6.5 p/vale has to go with your setup /cam.

Make sure your running approx 18 deg base timing and 18-20 deg mech in the dist all in by 2800-3k rpm.

Then 18 deg base + 18-20 deg mech in dist = 36-38 total which is a good starting place for tuning.(will liley need to run 93 oct fuel with this perf timing curve)

Getting the power valve/s & jetting corrected along with running this proper ign timing curve and you will be well on your way to getting this squared away.

You may need to replace the idle bleeds too but try getting p/valves/jetting/and timing better dialed in 1st before going after finer tuning with idle bleeds.

Again,I am assuming your running proper fuel pressure (6-7 psi maybe 8 psi tops),floats are adj correctly & not bad taking on fuel & being heavy,and there are no issues with needles & seats and or dirt in them or in the carb.


Scott

 

[mr 4 speed]

How much vacuum does your motor make at idle? That is the best way to determine what powervalve is needed
That cam is basically like an LS6 solid,but hydraulic and has more lift.
Go with less jet as Scott mentioned,like 10 jet sizes less..73/83 or 74/84 would be fine..but I don't what happens when you try to tune a Demon like a Holley

 

[DougJ]

I had the exact same problem with my Demon. You need to jet down the IFR's. Hopefully yours are screw in type, if not put in some .015 wire into each IFR to start with. Bend them over at 45 degrees to keep them in place. I used strands from a telephone wire...they were .010 and worked out perfect. After the wire trick I knew exactly what sized IFR jets I needed to order. My AFR was perfect after this... 13,idle with screws 1 1/2 out, 14 at cruise and 12.5 at WOT. It is amazing how much the IFR's impact the AFR at all speeds. I went from a IFR of .036 to .026. in the Demon. Main jets dont start coming in on my demon until 3500 RPM, below that the IFR's control everything.

 

[SWHEATON]

I figure that 235 dur @.05 cam (not knowing the LSA) should have approx 10" maybe 11"" vacuum at idle when running 18 base timing in 461 BBC .

Thats where i came up with the 5.0 p/valve because starting at about 1/2 the idle vacuum is a good place to start.

But as Chris stated its best to have the idle (auto in park with 18 deg base timing) /low engine speed idle vacuum reading when choosing a p/valve but the 5.0is definately closer to where you need to be then the 6.5.

Scott

 

[Tom Mobley]

Read carefully what DougJ said. Don't try to take 10 jet sizes out of your 850, won't work. Those are jet sizes for a 750, not an 850. Forget about the power valve, is won't affect your idle no matter what number it opens at. All it does is add fuel to the main wells which are already full, holding much more fuel than the idle circuit could ever pull.

As Doug said, the main circuit won't even start operating until 3000 or so RPM, below that (which is most driving) the car is running on the idle circuits. That's why changing the jets doesn't help much (as you've correctly noticed). The IFR (Idle Feed Restriction) orifice size is what controls how much fuel is available to the idle circuit. Try the wire trick or just order smaller IFRs from BG. These are not the air bleeds visible on top of the carb, the IFRs are screwed into the metering block.

 

[70_ragtop_ss]

Thanks guys! Much appreciated. I am running 7psi for fuel pressure, and I really don't know what my vacuum is. I am planning on testing that when I get back to tinkering. One more thing, I've heard that the Demon carb jetting is different then the Holley in that they need alot more jet then a Holley does. With the suggestion to go down to 73/83 or 74/84, do you think that is a little too low considering BG puts a mich higher jet in from the factory?

Thanks again!

John

 

[Tom Mobley]

Oops, I type too slow.

 

[Dave427]

Leave the factory jetting,,,, When fuel is running out of the throttle shafts its usually float or needle and seat issue. The power valve will not cure it either leave it alone, whats the fuel level look like when you look in the sight glasses.

Dave

 

[Skeeveman]

There's a lot of solid advice in this thread, especially from Scott about the plug cleaner. I use it all the time, as long as you clean out the plug good (20 seconds of work), it will work just fine. I got mine from harbor tool here locally, cost 10 bucks and comes with a small amount of abrasive, but you can buy a larger bag for a few bucks. But, I did have to fiddle with the lever that controls the abrasive right out of the box, it was well... you know how the quality control of their stuff is, lol. http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=32860

Go to www.4secondsflat.com and buy his carburetor tuning book, it's really worth it, he puts a lot of things in a new perspective. Also, watch this video of his as well, very useful for baselining your settings. http://www.4secondsflat.com/DEMON_0001.wmv

 

[SWHEATON]

I must have missed the fact fuel was supposedly comming out of throttle shafts unless it came after my post/suggestion.

But anyway the p/valve has nothing to do with general carb flooding if thats the case it's things like dirt/too much fuel pressure,wrong float lvl,bad float or needle & seat,or motor/intake /carb getting too hot that can cause general carb flooding.

That's the problem with these forums becasue you cant see what's going on for yourself and things get missed or plain mis-stated by the person asking for help when they are exlpalining the problem/s.

Scott

 

[Todd1312]

Well I too had the same problems with my BG Speed Demon 850 cfm carb. My motor is a 496, single plane manifold, flat top pistons... According to BG the carb is too big.. If I had domed pistons to produce the higher compression rate then the carb would be adequate.. (even though I asked them which one prior to buying) The carb that they are suggesting now is a 750 speed demon.. So my 850 is now a $549 paper weight in my shop.. I traded my brother a guitar for a used 725 Holley double-pumper for now. My velle is running okay now

 

[Dave427]

Buy a 950 HP your problems will be gone!!!

Dave

 

[540 Olds]

Well I too had the same problems with my BG Speed Demon 850 cfm carb. My motor is a 496, single plane manifold, flat top pistons... According to BG the carb is too big.. If I had domed pistons to produce the higher compression rate then the carb would be adequate.. (even though I asked them which one prior to buying) The carb that they are suggesting now is a 750 speed demon.. So my 850 is now a $549 paper weight in my shop.. I traded my brother a guitar for a used 725 Holley double-pumper for now. My velle is running okay now

I have a buddy in Maryland who went through the exact same situation with his BG850; they told him had a stumble in it because his 243 @.050 cam was "too big" in his 454. He went and bought the one they rec'd and it was also a junk carb out of the box. He did a complete tune on his own using an O2 sensor and it runs great now, BUT it is nowhere near the carb he pulled out of the box. I have known others with BG's that ran filthy rich out of the box, or had "holes" that could never be fixed.....all the car mags rave about 'em, but I have talked to too many who had problems considering all the money they paid. JMo from what I've seen.

Bob

 

[SWHEATON]

Yes,it seems we see alot of tuning issues here in t/chevelle with BG/Demon & holley,but the dig difference is the most all the time the holley's can be corrected with proper tuning but the BG/Demons are often not improved at all or only a little but never completely fixed.

There are some guys here in t/chevelle that seem to have good handle on them like TRMNTR but most many people that seem to be decent carb tuners stillhave issues with them.

And as already mentioned even with the BG/Demon techs help they cant get the carbs squared away.

It will be holley for me all the way if i have a need for an application my favorite Q-jet carbs can't handle like race only all out max performance application where holley's really shine.

Scott