I have searched and searched with no luck I have spent hours on hold with Demon and they never answer the phone or emails. Im having a hell of a time tuning this carb to my motor. If any of you guys can help I would greatly appreciate it.

Motor specs
1969 Olds 455 bored .060 over balanced and blueprinted
Kieth black dished pistons 10:1
stock crank
stock heads shaved but I think only to clean them up
offy dual plane intake
Stock exhaust manifolds
stock vacuum advance distributor
Holley red fuel pump 7psi 96gph
mild cam (I don't know the specs)
36deg total timing @3000rpm
Mighty Demon 750 Carb with 84 primary jets and 88 secondary jets

The problem I am having is that the motor surges off idle until 3000ish rpm. The demon tech pages says that surging is due to a lean condition and to realign the butterflies, check the floats and rejet if necessary. I started with a 72/83 jet combo and have made steps up to the current 84/88 combo. It is running the best it has so far but it still surges. Do I keep going up in jets or do I have another issue I am not seeing. I cant imagine that this motor would needs main jets in the 90s.

So do I keep upping the jets or toss the carb

Thank You

 

Curious, did you just build a fresh engine and first run with your new demon? Or did you replace the old carb with the demon and have the same issues. Just a little more info helps.

 

Welcome to the CO forums! I would first double check the following:

Vacuum leaks; i.e. vacuum lines, carb base gasket, PCV system (if equipped) intake manifold, brake booster (if equipped), etc.

Base (idle) timing. Timing should be steady (if not, check for worn and/or sticky dist. advance weights and springs). The base timing may need to be 'fine tuned' for best idle due to the cam and engine mods.

Valve clearances.

Next take a look at this link for some general Demon carb tuning:

Pay extra attention to the float level, ign timing, carb butterfly clearance, and idle mixture adjustment suggestions and tips. And, like another CO member said, you'll need to do some spark plug reading to give you an idea of what jets you need to run.

Good luck, and let us know how you do!

 

A few things come to mind. Vacuum leak which you would have a higher and much more difficulty trying to tame the idol. Bad accelerator pump which is unlikely with a new carb. A poor or not enough vacuum to the advanced unit in your stock distributor. Power valve in the carb. Not sure if they have power valves in the demons. Just to name a few possibilities

 

Is this a new Demon or one from before BG went out of business?

 

Disconnect your vacuum advance and see if it still surges. With a stock distributor set for 36 total, your vacuum advance may be driving your total with advance to way over 50 which will cause misfiring and surging.

 

I'm with Eric on the vacuum advance.

On the carb, for me a real Holley is the best way to fix it.

Also Torker is a better manifold.

Gene

 

I had a carb that idled great but off idl broke up and stumbled. Tried forever to get it right, checking for vacuum leaks, timing, everything I could think of. I Gave it one last try before replacing, figured it was easy enough to jet it up just to see, swapped in a fat jet and it fixed it so I jetted it as low as I could and still run right. It's to easy not to just check, if it doesn't fix it go back to your original jets and keep looking

 

Here's a simple article that should get you started.
http://hotrodscott.blogspot.com/2007...ingtuning.html

It's simple stuff the average person should be doing from the start with any Holley/Demon. This article applies to double pumpers but if yours is not then you can disregard that step.

I would also drop the fuel pressure to 5-6 psi. Since you have those big sight glass windows on the carb you can watch and see if the fuel level is creeping up in the window as it sits there at idle and up to 2500 rpm. It shouldn't be but I have seen Demons do it from the factory. If fuel can be seen dribbling from the venturi boosters at idle then the float/fuel pressure is definitely wrong.
I personally think you are way too rich but we'll see.
As Demon recommended check those butterflies. Are they tight on the shaft or flopping around? Seen this too from the factory as well as them being not centered in their bores.
I spent a lot of money and time trying to un**** my Demons. By far they are the worst carbs I've come across. Much of this I attribute to the earlier Demons straight from Barry Grant. I can't comment on the Holley versions since Holley bought them out.

 

ok so the cluster F continues.
Thanks for the imput guys.


So far I have replaced all of the vacuum lines I have no leak that I can find.
I set the timing to 36Deg at 3000rpm
New plugs
New wires


I kept playing with jetting and went all the way up to a 89 main jet and it did nothing to change the surging. I called holley and they told me to rebuild the carb and go back to a 75/83 combo this made things better but the surging off idle it still there.


compression test was good
leak down test was good
valve clearance is good


last night I removed the demon and installed a edelbrock 650 and still the surging will not go away.


Tonight I will hook it up to a smoke machine, there has got to be a leak somewhere

 

Did you disconnect your vacuum advance and try driving it. It may not be a carb issue, too much timing will also cause a surge at cruise.

 

Now we know it's not carb related.

1) Fuel pressure could still be too high or maybe even low. Do you have a regulator on it or are you just going by what the Holley literature says? You need to be able to see that pressure somehow instead of guessing at it. Are you running it through the stock fuel pump or by itself? Is the pump at the rear of the car or in the engine bay? Do you have a large enough micron filter on it? Have you checked the volume of fuel through that line?
A Holley red pump will do 97gph at max rating or 1.62 gallons a min. Put a hose on the outlet line at the carb and let it go into a bucket. Flip on that pump and see how much actually comes out in one minute. 5-6psi is all you need for any carb. More is not better. Carbs run on atmospheric pressure not fuel pressure. As long as the fuel bowl(s) are being kept full it will run fine. Both pressure and volume are at work here. I've seen many a car with 6psi at idle drop to 2-3psi by mid track and nose over from insufficient fuel.
2) Is the dwell on the points set correctly with an actual dwell meter? Is it clean and rust free inside? Have you checked the wires individually for ohms?

You need three things to run that engine, just like a fire. Fuel, air and some sort of an ignition.
You ruled out the carb (put the Demon back to stock specs). So everything from the carb back to the tank is suspect until verified.
Do you have a decent air cleaner on it during testing or is the hood off with no air cleaner?
Check the distributor and secondary ignition. I bet those plugs are black and sooty. If they aren't then I'd be looking at a fuel delivery problem.

 

Just a side note. My friend related a story about his BMW bike that might be interesting. Seems he had a similar surging problem with the bike. No one including BMW could figure it it.
On a whim he removed the spark plugs and found the gap was huge. He gapped them down to .035 and the surge disappeared. Go figure. Sometimes it's the littlest things.

 

This weekend I removed all the plugs they were showing a little rich, but not horrible. I checked the gap and they were a little tight. I regapped the plugs and cleaned and regapped the points. When I was checking the points I noticed the distrubitor had way too much play in the shaft. it was kinda wobbly now im on the look out for a good used aftermarked HEI unit.


its the only thing left, everything has been checked and rechecked and then replaces and checked again.


if this doesn't cure it, the truck will go to a shop,,,


by the way the 455 is in the bed of a 1974 mazda pickup using a Toronado trans. Its fast as hell when it is running right

 

I see you have a drop base air filter with a 2 inch filter element. Is any air getting into that carb? It might be causing suction over the bowl vents and pulling fuel out of the vents and into the venturis since the lid is so close to the top of the vents.
On a 455 I would use nothing shorter than a 4x14 air filter element especially with that drop base.

 

That filter is gone now, I have a tall K&N with the filter top on it now,,, tons of air flow now

 

The new HEI came in the other day, after quick install. the truck fired up right away and ran perfect.


Holy crap is it fast!!!!!!!!