Hi,

Bought a used BG Might Demon 850 cfm carb to replace vintage D/P holley.
Engine was running well with the holley, apart from being a bit on the rich side.
Basically yanked off the holley, bought a 1 inch open spacer to go on top off GM LS6 manifold ( to aid in linkage clearance and the BG fuel rail)
I set the carb up to Lar's paper.
Nothing else in the engine has changed save for the carb and spacer.

On first attempt on start up floats were set too high, engine coughed and backfired, fire and all.
Re adjusted float levels to the center of the sight glasses, all idles screws are set on turn out.
After a day to dry out and ( was very wet) recharge battery, again tried to start, now nothing, engine cranks, has spark, pump the gas nothing.......
Any ideas?
Fuel pump is stock 69 vette BB, ( allmost new)
Short of refitting the holley, must be a tuning issue

All thoughts appreciated

Cheers Richie

 

Stock mechanical fuel pump and you adjusted the fuel bowl levels without it running? Pull a spark plug and check for flooding. If you have spark and fuel, it may be you still have too much fuel. My first guess would still be fuel bowl too high. This is why I prefer removable fuel bowl sight plugs. Set bowl level low, get engine running, remove site plug, adjust level up until just at bottom of sight hole, install plug. As a used carb, it could have some problems and may need to be broken down and inspected/rebuilt.

 

Put the Holley back on and see if it starts, many will sell a problematic carb and buy a new one instead of trying to fix it.

The Race and Mighty Demon carbs are prone to go lean at WOT so you need to be aware of that, the power valve restriction channels are not big enough to feed a high HP motor which is what they are made for.

 

I would also check plugs for being gas fouled.Some plugs never recover
from this issue.

 

I work on a lot of used "buy it on eBay" Demon carbs, and the reason people sell them is because they have completely screwed them up to the point that they won't run. I just tested one the other day, and it had a list of problems that was simply unbelievable. If you bought it used, you need to tear it completely down, inspect, check, and fix everything, and set it back up from scratch.

As Motorhead stated, the Demon carbs run lean at WOT - every one I've had on the dyno has required bumping up the primary & secondary jet size from 2 to 4 jet sizes. This, of course, won't prevent you from getting it started and running reasonably well, but while you have it apart, you may as well increase the jetting to fix the WOT lean condition.

If you can't get it right, slap the Holley back on, send the Demon out to me, and I'll test it for you for the "New Carb Setup" fee, assuming the carb does not need significant gaskets and parts.

Lars
V8FastCars@msn.com

 

Hi,

Yes I had thought this maybe the case......
Secondary Jets are 2 sizes larger than stock specs.
Yes sending it to you Lars would be an excellent option, however I live "down under " in New Zealand, sadly the freight would cost as much as a new carb ! ( we do have excellent beer and food here, should u visit )
Will pull it off and have a good look

Cheers

 

I buy a fair number of "new" Street Avenger and Performer carbs for dirt cheap on CL, and without exception someone has "tuned" them into being completely useless. An hour or so going through them nets someone a nice carb at a great price. I think the best so far was a Performer where someone had removed the metering rod springs.

I'm no carb guru, but I can read a service manual and I have an LM2

 

as stated above, a new set of plugs will probably fix the problem....however if its too much fuel being pumped into the engine you may be right back where you started with the carb

 

I have a Demon. An earlier model before poor Q/A set in. My booster were all straight and square with the veturies. A later race Demon was sent to me and it was sad. Had to machine the booster slots............

Anyway with air bleed tuning on a flow bench and test mule motor. I now have 3 jet sizes smaller on both front and rear that what it came with stock. It also on the test mule dyno does NOT lean out through 7800 rpm on the big ci test mule sBC

 

Does anyone know what happens to the plugs, the current ones are only a few hundred miles old.....? You would not expect them to go bad

 

That's not an out of the box Race Demon if you have been tuning it, especially down to point of air bleeds. Mine has screw in boosters you need a special tool for and I changed mine from downleg boosters to annular boosters.

I would not trade my 825 Drag Race Demon for any carb on the planet

 

The plugs probably just real fouled with all the gas running in there and then sitting for awhile. If you have an HEI in there...they just about won't ever fire a really wet fouled plug. Great for keeping them clean..but terrible when they are already drenched. You can pull them out and clean them with carb cleaner or something. Once you get it fired up and running they will clean off usually.

JIM

 

Hi Jim,
No HEI, MSD pro billett dizzy, 6AL box.

Got it going, cheated used ether, had to set the idle mixture screws half a turn out......., but hey running wont idle yet, but there is hope

 

You adjust the 4 corner idle screws while watching a vacuum gauge.

double pumpers need the throttle plates to be near perfectly even on both front and rear. It is hard to do with the carb on the car. You have to look at them from below. Then you adjust your idle speed to over 1000 rpm turning both front and rear screws an equal amount.

start with all four idle screws the same starting at say two turns out each. adjust them one at a time on a warm motor until your vacuum and idle speed goes up. continue to turn the the idle rpm back to the base 1000 rpm. I have a radical cam so I actually use 1200 rpm as my base number.

Adjust all 4 idle mixtures screws to the smoothest idle and highest vacuum reading. Demons in my experience were always way rich at idle, so I drilled two holes in the primary throttle blades. it induses an equivolent to a vacuum leak and then you can dial in the mixture screws. I think that my Demon had the screws shut at idle and it was still to rich. The holes brought my screws back ou to @ 1 1/5 turns

Depending on the motor they really run the best at @ 16-18 degrees initial advance. Which requires limiting the mechanical advance to not get too much timing over 3200 rpm

 

Followed the Lars set up paper, so throttle blades are all set the same, primary and secondary.
Started with idle screws at one turn out, it would not run, they are about half a turn out now and it runs, ok
Will spend some time and sort today, float levels at lower mark on the sight glass, may need to come up a bit. Cam id 250@50, 108 centres, so still should idle aroud 1000rpm.
Have no vac advance, have all in by 3000rpm, will have to check initial.
This carb has ezy idle screw in the centre.

Thanks for the input

 

The EZY idle screw in the center is the equivolent of drilling holes in the throttle blades. If you open it up it will lean out the mix and then you have a greater range of adjustment on the screws.

With a 250 cam I would do the adjustments up around 1200 rpm. bring up the initial to the 16-18 mark and turn down the idle if you gained rpm. then adjust the screws turning down front and rear idle screws the same amount.

Double pumpers often are out of adjustment and stumble off idle when you are driving. It is usually traced to two things: Mismatch of the throttle blades and not having the squirter adjusted for instant dribble even at the smallest throttle movement. That you watch with the motor off and just move the throttle a red hair

 

Well the rain has stopped ( winter down here) managed a post tune test drive...wow drives like a different car, more responsive, will light up the BFG'S in first 3 gears........
BUT then, final test run was down shifting to turn into driveway, stopped dead!!
Opened the hood, the primary bowl spewing fuel everwhere, it the breathers ect....
I am asuming a stuck needle and seat?? fuel is way up above the sight glass......
any other thoughts ........bugger !!!

 

Pull the needle valve and blow it out and reinstall. Take the secondary screw only off and judge the number of threads it is screwed in, assuming the secondary float level is where it should be and setup the front the same. Keep an eye out for the same problem happening again, it is trash in the seat and could be coming from the carb itself or fuel inlet. Change your fuel filter to good one I use Earls.