King Lear

I called BG because I am running the Mighty Demon 750 vac sec annular carb on my 383, th400, rpm air gap, 3.36 gears, Crane HR 224/232 dur @.050 and .543/.563 lift., 200 cc RHS heads and Hooker super comp side pipes with 18* initial timing 36 total.
I have all of a sudden developed a serious hesitation going from idle or part throttle to wide open throttle. I have talked to the tec and he told me I should be running the 650 Mighty Demon vac sec downleg boosters.
His reasoning made sense since it has smaller venturis and bore but he claims the 650 Mighty Demon flows actually around 750 cfm to 800 cfm and I find that hard to believe has anyone run this carb and flow tested it to really see how much it can flow?
At this point I am about to get the Holley Street HP 750 vac sec and call it a day. I really like the Demons with the idle-eze with my camshaft but I am sick of all the multiple answers I get from their tecs, Jegs has offered to replace the MD 750 ann carb but none are due to be shipped until 7/25/07.
Yes, I have checked the PV and its reading right at 7 to 8 psi at idle in gear, I also went up and down in squirters with no success and also rechecked the throttle plates transfer slots , the vacuum housing is tight not torn. Everything I have checked seems fine. HELP!
I really liked this carb but dont want to wait for a month while its on backorder!

68coupe

Relax, that carb will work on your car.
do you have stock jetting?
Do you have stock idle air bleeds?

When you say: part throttle to wide open throttle - what RPM do you consider part throttle?
Are we talking about an off-idle lean spot or is it falling on it's face when you punch it at 2500 RPM?
Thanks,
Joe

Gordonm

That carb will be fine on your engine. I am running a 825 Mighty Demon on 385 with no trouble or hesitation at all. I did have to make the idle circuit smaller. I got some blanks and drilled them myself. That did clean up the idle a lot.

You say you checked the PV and it is reading 7 to 8 psi in gear. I assume you mean the vacuum at idle is 7 to 8 inches of vacuum. That is kind of low for the duration of cam you have. I am running a 252/260 @ .050 solid cam in mine and get 8 to 9 inches at 1000 rpm. This is where it idles best. I also run about 22 degrees initial with 36 total. Your PV might be blown but it would idle quite poorly with a blown PV. What number PV are you running. I run a 6.5 in mine and that seems to work very well. The Demons run good but they do require some tinkering. When you get it right hold on!

gkull

My 750 speed demon measured right at 820 CFM with down legs. The annulars would be closer to 800 CFM.

The CFM is not the problem. You have to have the squirt cams set to instantly dribble at even the smallest movement and squirt on fast movement. Lots of initial timing and the four corners idles set for the highest vacuum. Have the secondary open slightly more to set the idle speed.

You can make a 1000 cfm work on a 350. You just have to know how. The air bleeds also come into play. A small cfm carb will also run bad if it is not adjusted.

King Lear

Thanks for the responses I have run a 3.5pv and a 6.5 pv and the 3.5 runs best and I had to jet down one size on each primary and secondary to get it perfect, it has run awesome for the last three months and then all of a sudden it has started to hesitate, I replaced the PV with a new one so that is not it. Maybe I just need to jet one up since it has gotten so much hotter and humid here the last two weeks.

As far as off idle or at 3000 rpms when you hammer it, it dies! Then you here the secondaries open and it stumbles a bit more then catches and takes off.

I would like to keep my Mighty Demon because compared to the speed demon it runs much better to my cars configuration.

King Lear

The cams have been set that way and re-checked again after the problem started, that is not the problem. Yes the carb was set 4 months ago to the highest vacuum and again, I know how to tune a carb I just wonder why after three months it decides to act up?

TheSkunkWorks

That's the thing that really bugs me about BG. You'll get different answers from different techs, and they are all vague about actual flow capacities. Why can't they just designate their models by actual cfm rating and be done with it, or at least give a straight answer to a direct question about cfm.

I'm forced to spend extra money on the "RS" version of BG's King Demon vac. sec. (no other source) for my 496, so I can change cfm without buying a whole new carb if I get it wrong the first time. And, no thanks to them, I probably will even though I know my cfm requirements up front.


Back on topic, have you tried simply chaning the spring on your secondary diaphram?

King Lear

LOL, Doug told me to put on a 650 or 750 Speed Demon vac or mech, Scott is the one who told me to put on the 650 MD vac, I cant remember the name of the guy who told me to put on 750 MD vac. I agree they do need some consistency in their answers.

Yes I have tried all the springs the lightest one gave the best response.
For the moment I have cured the problem, I put my Q-750 back on to see if it was something other then the carb issue and it ran great so I think I am going to have to inspect the vac housing with it off to see if their is a tear in there delaying the secondaries from opening on time.

It just makes no sense that it runs like a champ for 3 months with no flaws and then starts hesitating with nothing being done to it.

68coupe

Your carb is vacuum secondaries, so I am a little surprised that you are getting the lean spot. Vacuum secondary carbs open based on engine load, so CFM is really regulated by engine load and how much vacuum drop exists. (You can actually put a rubber band on the rod of the diaphragm to figure out how far your secondaries are opening.)

Anyway, when you wing the throttle back, a massive amount of air rushes down the venturis, it is almost like a human gasping for air. The problem is that the fuel is not present, that is what the accelerator pump is for. As mentioned above make sure the pump is properly set, it should start shooting as soon as the throttle is touched. Also, make sure it is not bottoming out. (there should be .015 play when the throttle is depressed.)

Also, transition slots should be set to no greater than .020, if they are overexposed, then there may not be enough left for off idle transfer. Keep in mind that you should have both the primary and secondary set to the same amount. Also, keep in mind that when you adjust the idle speed screw, you are changing the transfer slot exposure.

If the accelerator pump is setup properly and the transfer slots are correct, I would start by increasing the squirter 2 sizes on both the primary and secondary sides.

If that does not help, increase the primary and secondary jets by 4 sizes and see if that helps decrease the hesitation. Keep an eye on the tailpipe after that change and make sure you do not have any black smoke.

I am not really sure why you had to jet down….carbs that are too big, run lean…..

Thanks,
Joe

King Lear

thanks, but yes I already did all that, and the reason why I had to jet down was because it was to rich, backfiring and puffing black smoke at cruising speed, and the transfer slots were set to exactly .020, the only thing I used to adjust the idle was the idle-eze screw.

Everyone is missing the point that it just started this past week after running perfect for 3 months. So it has nothing to do with the initial settings because they were dialed in perfect for the last 3 months. So it has to be something that has gone wrong since, anyway I have it off now and am going to check all the baselines again and the vacuum housing to make sure the diaphragm isn't torn, I think this is where I am leaning towards at this point.

TopGunn

Are you sure you haven't picked up some foreign material in a fuel passage somewhere?? The fact that it ran perfect and now is giving problems is suspect of something like that. If it's backfiring thru the carb it sounds like a lean condition not rich.

TheSkunkWorks

That's the next thing I was thinking, maybe the diaphragm...

Personally, my experiences with FOD in a carb have been with the needle & seat becoming unable to close off, but something could well get past it and cause a clog elsewhere.

Good luck, and let us know what you find.

68coupe

Yeah, I guess if it is all of the sudden, I would start with vacuum leaks. That would cause a lean condition.
Thanks,
Joe

pws69

Just out of curiosity, what are you running for ignition components, particularly the distributor (i.e. points/condenser or some type of "electronic") ??

MotorHead

If it ran fine and now it doesn't that means there is nothing wrong with the size or model of the carb. Once you figure out what is wrong the carb will work fine on your combination. A stumble is usually a lean condition and usually caused by a vacuum leak, check the four nuts that hold the carb down as a starting point. Also I hope you used a new gasket when you put the Demon carb on as they have a different "footprint" than a Holley and you can get a vacuum leak if the old gasket has the Holley footprint in it

King Lear

I'm using a MSD corvette tach drive dist with the 6al box.

Matt Gruber

maybe the weights got stuck?(dist)

King Lear

Nope, found it, the primary pump housing loosened up and was leaking a little and causing a lag on the initial pump shot, it was pushing the front of the housing down instead of the pump arm. Every time you hit the throttle it was leaking gas out the front of the housing and loosing pressure.
It looks like whoever had it before me stripped out the two front bolts and just tried to teflon tape them in there, guess thats what I get for buying a remanufactured carb!

TheSkunkWorks

Bubba is omni-present. Glad you found it.

King Lear

Yep, last time I try to save 100 bucks on a remanufactured carb or any part for that matter.