ok heres the situation.. last year i bought a 750 mighty demon with annular boosters from doug herbert.. i put it on the car and it bogged like a mother.. so i buy a jet kit and open the carb.. ok, here is where the fun begins.. i pull the jets from the carb and guess what they are.. 85 primary and 93 secondary.. IN A 750 ANNULAR!! so me and my dad experiment and get it down to 78/79.. now its still not running spectacular.. a couple days ago my dad asks me to call Barry Grant and get the specs on the powervalve and jets for this carb.. so yesterday i call Barry Grant and they say that it was supposed to have 70/77 jets from the factory.. turns out that only the downleg booster 850's come with 85/93 jets.. any guesses as to what carb i really have?? my dad is gonna look for the part number for me and measure everything.. i should find out in a couple days.. oh ya, just for the record.. I'M NOT HOME WITH MY CAR.. i'm 3000 miles away at college.. this will be the last demon i ever buy..

did you call doug herbert--since I'm assuming they modify demons? Since annular boosters will have a different vacuum signal than downleg boosters. Furthermore are there PVs on the front or on both? If the rears have no PV then you should be 6-8 jets higher on the rears.

no, i didnt call doug herbert.. i dont think they modified the carb.. i'll try call them someday.. i just wanted to figure out the jetting that its supposed to come with.. and there is only a power valve on the front.. back is blocked.. i'll be putting in whatever the factory jets once i find out if its really a 750 or not.. measurements tell all.. but i'm pretty sure its a 750..

ok well my dad measured the carb and its a 750.. its just crazy how Barry Grant could have messed up on the jetting that bad.. its not even the right jetting for an 850 annular.. its for the 850 downleg.. 15 jet sizes too big..

Barry Grant will NEVER have my money ever again.. well at least the Demon carb lineup

i'll be switching to a ProSystems Pro Series XC series 950HP in the near future.. already got it quoted at $690.. and i can get a hookup for less..

ok time to let this thread die..

So because you and your dad can't figure out what jetting to use for your car you're calling it junk? Doug Herbert put the annular boosters in and changed the jetting around probably. Next, you have no rear PV but don't have the rear jets 6-8 larger than the fronts...this will cause tuning issues. The only carbs that have the jets close to "square" jetting don't have PV blocked off, or they have them installed in both metering blocks like a typical 850dp holley.

not necessarily calling it junk.. its just pretty sad that they didnt catch it.. like i've heard before, quality control is pretty bad.. i used square jetting because it was recommended by other racers.. dont think they knew the back pv was blocked.. they never asked.. and i probably never told them either.. after i changed to their recommended jetting, the car went faster, so i figured it was good.. even if doug herbert did it, how could they mess up the jetting that bad?? 15 sizes.. its like a blower carb and it has enough fuel to run 30psi..

its like they pieced it together with 3 different carbs.. 750 mainbody and baseplate, annular boosters probably from an 850 annular, and an 850 metering block that came with downleg boosters..

but whatever already.. i'm just gonna try tune the crap out of the demon and see where it leaves me..


END OF THREAD

having annular boosters changes the jet size requirements. stock carbs with downleg boosters have different jets than annular booster cars, regardless no one sticks with what the factory has in it anyways, you tune it for the motor. Sounds like you need more tuning time and not throwing cash around on another new carb.

i plan to tune the carb better but i'm just not home right now.. college.. it also doesnt help that drags are only once a month.. and even during test and tune, if i play around with jetting, i'll be lucky to get 5 runs in.. its usually only for 3 hours and there alot of cars.. and i still have to get my suspension dialed in..

according to BG, annular carbs use 4-5 jet sizes smaller then downleg.. less gas, more mileage and torque down low.. hopefully i'll be able to tune it to its max this summer.. only from then will i decide for a new carb.. if i stick with it, and i decide to spray later on, i will be switching..

That's why I'm going through every carb I buy, new or used. I want to know exactly what I have. The used Mighty Demon 750 I just got recently has different metering blocks than what it should have come with. I think they're off the race demon series due to the ammout of emulsion holes. Jetting was one size down from stock and I changed out the PV for a new one just incase. Whatever they are, the carb seems to work great.

i run a holley 750dp. i have 83 in the front and 84 in the rear. with no power valves. runs just a tad lean. but very smooth. check the shooters. they are more likely to cause a bog than the jets. too much fuel when you pump the gas?

Have you tried going back to the Stock Jetting yet?

We apologize if the carburetor had the wrong jets in it from the factory, just fax us a copy of your receipt with your address and we'll gladly send you the baseline jets.

I have a BG Demon 750 #1402010. I had overly rich issues but no bog or stumble problems just bolting it on. I checked everything and it was as-advertised. #76P / #83S Jets, #31P/S squrters and a 6.5 PValve. I have been de-jetting it to get it leaner. It used alot of fuel and had a very overbearing rich exhaust. At this moment I cannot remember where I stand on the jets. I remember the Holley 750DP I had on there, #4779 I think, was rich and I was down to #71P/#80S when I relieved it of duty. I see we have a BG tech/rep here. I would like to get some advice on my setup and see if I am going in the right direction. If I have to jet lower I am going to have to aquire the jets. I have 67,68,70,71,77,79,80,74,72,76,83 availible right now. I have a cam kit for the acc. pump and #28,31,32x2,35x2 squirters in addition to the BG stock ones. Funny, out of the box as the #1402010 was setup, I raced a buddy with a LX5.0 '92. I flat out left him by 3-4 cars on the bottom end and by the end of the qtr, he had got me by 2-2.5 lengths. I thought I had a good top-end motor, but about midrange it flattened out considerably.

i guess this is an update that i never did..

a couple weeks ago, i called BG for the real jet sizes and they said 70/77.. my dad put them in and i quote him, "its like a new motor, the response is just amazing.. it has absolutely NO hesitation".. BUT the head gasket sprung a leak so he never got to drive or race it.. topend is currently off and awaiting the head gaskets from summit.. woulda cost $52 a piece for head gaskets locally, so he went with summit.. $35 each plus $10 for shipping and handling.. so $40 each.. btw, my dad is doing all the work because the car is on maui and i'm in oregon..

Mikz86TA, i had a similar problem as yours.. i could still roast drag radials out of the hole but my topend felt lazy.. but i did creep an '02 GT with a vortech in first and second.. he only beat me cause my clutch was being a female dog and wouldnt fully let go of the flywheel..

My 750 Mighty feels lazy at the top end too but still managed almost 114 mph. The best throttle response so far has been with the pink cam and using the #2 screw hole since the #1 spot was actually stripped. 31 tube style squirter front and rear, 77/83 jets but want to step the back up to 85.

Let's start out with a few questions:

1) What combination (engine, driveline) are you working with?
2) Is it rich at idle, WOT, or both?
3) What do your plugs look like?
4) Do you know where your carburetor adjustments are?
----------

Is it flat up top in comparison to something else you had on the engine, or just what you think the car should do? Have you tried going up on your jetting at all yet?

i can answer this too.. but i did a new engine combo that was never really tuned well.. in comparison to an edelbrock 800, it feels a bit lazy.. but i did almost maxed out the jetting and with no powervalves in the edelbrock, she just did not want to go down.. she made her own idle right at 1400 when it was set for 650.. like i said before, i never drove the mighty when it was in good factory tune, only the crazy 15jets high tune and 8jets high primaries and 2high secondaries..

I have a 355ci backed by a 700R4 and a 10-bolt with Auburn and 3.42:1 gears. I have the Demon #1402010 carb on top of a Perf. RPM intake. Heads are Sportsman II with Manley street flo valves. Cam is Perf RPM .488/.510 and 234/244 plus I have 1.6 rollers. Motor is 4-bolt with cast crank, LT1 rods and KB hyp. pistons to the tune of 10.2:1 CR. Full MSD. I could go on and on. I am familiar with this carb. Ive re-jetted it and it has gotten better. Right now I am rich at idle. I want to have good milage, have not too rich at idle if any at all, have good off line jump with a strong midrange and no problem to the 6000RPM mark. It was a weekend racer that was a daily turned to a cruiser that I still want to track from time to time. Is a 750 honestly too much? What is what route to maintaining decent mileage but still being powerful. Plugs I havent checked since Ive been periodically down-jetting.I need to rrplace them. They were very black/ a little oily. also another thing is alot of fumes from the breaters, especially the LF. I have one breather on each valve cover. I wanted to do a PCV but the intakes port is blocked by the rear fuel bowl. No room for a spacer under the hood. And if I use a T on the carb rear port, I loose vaccum for the brakes. I even have a Crane Vaccum Res. Thanks for the help and advice.

Also, my car was a stock carb vehicle. I still have the stock tank and internals with the 3/8 I believe hard line to the front. I tapped a #8 AN hose to a Fram HP filter, to a Holley Blue pump to the Holley regulator then to the carb. I used #8 braided with XRP AN fittings/adapters. I have a one in and two out regulator. So I have each fuel bowl with their own feed. I believe its set at 6.5psi, if my gagues are correct. I have a on-line small one underhood and just put a AutoMeter mech. with isoaltor inside. I am tracking down a vibration on the driveline right now, so I cannot drive it. But I have never experienced any pressure drop B4 if you would need to know. I also think my 2-4 band is shot in the 700R4. So I am in process of getting those two things done. In the meantime. I would like to resume the running rich issues while I have a little free time. I appreciate the help.

Mikz86TA,

A 750 may be a touch big, but we should be able to get it to run properly on this combination. You really should get a PCV valve into the system to prevent from oil fouling your plugs. Edelbrock makes a 90 degree fitting to install in the intake manifold when using a modular carburetor to install another vacuum fitting.

A few more questions:

1) How much timing are you running at idle?
2) Do you know where you have your idle mixture screws, and butterflies set?
3) What RPM are you idling at?

Its an auto, so I am about 850-900 in drive. A little over 1000 in park. Timing was set at 35* total adv. I have a quick curve set to full adv. with the MSD billet dist springs. I think its at 25* initial, but its been a minute since I had set/checked it. Ill have o recheck the mix settings. I am assuming you need to know how many turns out. It was something I was tuning on and cant remember the last setting at this moment. I did not crank on the butterflies. I tried to use the Idle-Ease and idle mix settings instead. I might have turned them a little though. I remember seeing where not to go to far off the transfer slots. THX a bunch for your help

If you can check where you have the mixture screws, float levels, and timing that should help us start pointing you in the right direction. Do you know how much vacuum the engine is making at idle by chance?

Ill have to check these other new questions you have. Bowl level is just under sight plug. I dont remember having to adjust this from the way it came. All 4 idle mix screws are 3/4 turn out from all the way in. The P/S butterfly adjustment screws are 1 turn from where it just starts to move the plates. Squirters are #31 P/S and I believe they are the orig. ones. Pump cams are pink and using the top screw holes. This is the way it came. I have a pump cam set. Power valve is the stock 6.5 one. I also have an additional 10.5 and 4.5 ones at my disposal. Plugs still a little black and are Delco R44TS gapped at .045 and I have MSD 6AL/MSD Billet Dist and MSD blaster 2 coil. I have at my disposal the afformentioned P.Valves, Xtra Needle-Seats, #35x2, #32 and #28 Squirters, #68,70,71,74,76,77,79,80x2,83 Jets in addition to the 67s and 71s that are in there already. I can aquire more or additional sizes if needed. Timing is 25*. THX

From what I remember how my old 750 Speed Demon ran and even the 850 Mighty I had on for a short time, so far this 750 Mighty doesn't seem pull up top like the others. Haven't messed with the jetting other than the one night at the track, just haven't had time to take the car out and run it. Took me a week just to fix the blown out water pump gasket I've been so busy. Next month I'll be hitting the track again and messing with the jetting.

OK... Need you to pull her off real quick, and just check how much of the primary, and secondary transfer slots are exposed right now. No need to make any changes just yet.

How does your MPH compare to the other carburetors you had on the car?

Also, have you got to do a plug check at WOT yet, or are you planning on doing this when you go to the track next month?

Will check tomorrow. THX

The only night out with the 850, it ran 108 mph. The old 750 it did 110 mph a couple times. Since running both of those carbs I've upped the compression from 9.8:1 to 10.5:1.

Yes on the plug reading. Put a brand new plug in, made 1 run at the track and idled down the return road. The plug's porcelain was still as white as new and I'm running a somewhat cold plug. It's just running a bit too lean and ran out of time to fix that.

If she's that lean, even after idling down the return road going up a few jet sizes should take care of that for you.

Ok BG Tech, I pulled her off this afternoon but couldnt find anything to measure such a small hole with. The rear is exposed more than the front, though I turned the side screw evenly on my last adjustment. The front looks like a pinhole and the rear is barely defined as a slot, though it is more exposed. I had a Holley float measuring 'L' in 32nds, but it wasnt enough to gain any accurate measurement. I am going to attach pics I took. They are on Micro MAF and high resolution. First is front (smaller hole) and second is rear (larger hole). Hopefully you can determine if they are close by visual. Let me know what you would use to measure or give me a logical starting point. THX again, Michel

basically you're looking to make that slot look like a square in a perfect world. I'd say where they're at is fine, if you wanted to even them out that little bit would be more ideal as well. If you open the primary throttle you'll see how long that slot really is and yours is setup almost textbook perfect.

MikZ86TA,

The carburetor is definitely acting too big on your combination. Have you recently changed the ignition timing or idle RPM? As far as we’re you’re at on the butterflies like xpndbl3 said your goal is to make the primary and secondary slots look like a square with the butterflies in the closed (or idle) position. With where you’re at, let’s start off by resetting the butterflies so they are both open the same amount, and then opening the Idle-Eze in the baseplate 3-1/2 turns out from shut. Now reinstall the carburetor, and start her up. If the idle RPM (once you get up to operating temperature) is too high or too low open or close the both the front and rear butterflies the same amount at that point. Then we’ll need to see if going in on the mixture screws any further helps with your idle quality. If it does, we can work with you on changing the idle calibration on the carburetor.

Do you have room to install a 4-hole carburetor spacer between the carburetor and intake manifold? This would help increase the air velocity at low speed and could help with your situation.

I will try this method this afternoon. So start with a perfect square on the transfer slot? I will get back and tell you what I had to do and where I am at. I have a stock hood on the TransAm and no more room for the spacer. I like this intake and prefer to stay with the stock hood. Ill get back tonight with some results. THX ,Michel

Yes, squares on the front and rear...

No problem, we can make modifications from where we're at to get you there.

I ran out of daylight and couldnt try it out yet. But here is what Ive got to. I used a feeler gague and got both the Primary and Secondary slots at .020". Then I started the Idle-Eze all the way in and then out 3.5 turns. I had read some Holley manual and BG manual, and decided to check a few other things. I reset the Accelerator arm with .015" feeler at wide open on both the primary and secondary. I made sure there was no slack at rest also. Apparently I had the floats set low. I reset them also using the 7/16 drill bit for .500" with bowl upsidedown. I rechecked the idle mix screws and they are 1 - 1/4 turns out. The PValve is still a 6.5. The squirters are still #31s. The jets are #67 primary and #71 secondary. A local shop has #64,65,68 and some #7_ assortments. Tomorrow I have off, so I am going to put it on and fire it up. I want to recheck fuel pressure. What should I be set at idle(6,6.5,7)? I have a vaccum gague and timing light. I will check timing and see what it is at idle and full adv. I will also see what vac. is at idle. I can use the small vac. port on the baseplate to check this right? I know there is 2 and have marked them manifold and ported. TTYL, and Thanks a bunch, Michel

Mikz86TA,

Sounds like you're going in the right direction. Didn't mention anthing about the float levels since with them low it wouldn't hurt you at this point.
Since you've re-adjusted them, make sure with the engine idling that they're about 1/4 - 1/3 of the window (bottom line on the site glass). Set the mixture screws to just under 1 turn out from seated. As far as the vacuum goes, yes you'll want to use the full time vacuum port.

OH ok. I think I had them just lined up at the bottom of the sight glass. So they may be too high, but I just wanted to start it off all right. My idle mix screws were 3/4 turn out when I checked them. I will leave them be foe right now and go from there. How much vaccum should be present at idle in the most ideal situation? And when I am tuning, shouldnt I be concerned with having the highest amount of vac. possible? I will re-check fuel press. setting but I think it was a 6.5psi. I know with that and float level you just want to make sure the boosters are not leaking fuel at idle right? FYI, the tag on the carb I bought had this info : Employee#34 Idle Flow 80% WOT Flow 230PPH. Ill get started and post info about idle,vac and timing today. THX BG Tech!!

Ok BG Tech heres what Ive got right now. I put er on snd got it started. Even with the electrioc pump priming the bowls it took a few more tries than usual to start it initally. After it srarted I had to give a little throttle to keep it running but it wasnt terrible. No choke and cold I expect a little foot action to get it warm. After it got to operating temp, it idles by itself but it has a slight chop (not the cam lope) and shake. Adding throtle clears it up. It idles at 900-950 jumping around there. Smoothes out at 1000RPM but thats with throttle. I didnt yet adjust anything but figure the idle-eze is what I adjust 1st....correct me if I am wrong? I noticed on slight or medium throttle input there is a slight hesitation or off-idle stumble. I looked at the squirters and ther seems to be a delay on the emmiting fuel...and it starts as a drip B4 a stream. I noticed quite a bit of free play on the acc. pump arm adjuster since I bench adjusted it. I am going to adjust this on the car to get it more responsive. Also I have a A/F Autometer gague. At idle it jumps from mid-rich to close to stoich on the rich scale. When you burp the throttle it jumps to mid-stoich breifly but settles back to mid-rich area. Also when you throttle up and hold it does the same, just a breif mid-stoich. As stated the idle is 900-950jumping and choppy at best, the vaccum is at 10-11inHg at idle and rises when given throttle. I used the MVac port on the baseplate. Timing is 18* initial and jumps to 40* full. I have the MSD kit to reset the adv full and curve if that sounds like too much. F.Press is at 6.5psi and sight plug is showing fuel level at the lower line of the 3 on the carb. No fuel is dripping from the boosters at idle, so I think these are good. Again, what should my Vaccum be at idle and throttle? What should my timing be initial and full adv.? I also hear a slight ticking on the LF valve cover, so I might need to adjust the rocker on the #1. Seems like I had that happen B4. Hope to see you soon and Thank you for you time. Youve already been a great big help and I look forward to getting this going nicely. THX, Michel

Ok, I found an error on my part to add to the last post. I forgot to turn out the Idle-Eze 3 1/2 turns. I did so and it has a less rough idle but it still isnt good. Smell of rich isnt as bad as it used to be but it still is too rich and the A/F stays in the rich range. Idle is at 950 park and drops to 650 in gear. Its on the verge of being too low, so that will need to come up. I just need to know to adjust either the Idle-eze or butterflys? The AF gague is actually showing a 1bar richer mark in park and jumps all arounnd stoich in gear. I corrected the off-idle stumble in park I had B4 with the acc.pump arm adjuster nut. I just carefully and barely took up the slack. Manifold vaccum did not change as B4 (10-11inHg) and I also wanted to note it drops to 7-8inHg in gear. Thats all for now until I hear back from you. THX, Michel

i love how this started off as a thread of me venting my anger with BG to a full on tuning session with a BG tech.. BG gets their point back for customer service.. commence with your conversation..

They really do care about us, really they do! I'm truthfully glad that someone from BG is taking the time to come here and lend us a hand

Im lad....it finally motivated me to do some things

Hey BG...you here?

Mikz86TA,

Sorry about that, missed the replies somehow.

There isn't going to be a set amount of vacuum, that will vary depending on how the combination all comes together.

Is the off idle stumble under real light acceleration (moving the throttle real slow) or quicker acceleration?

Off idle stumble was with a moderate to heavy accel. I must tell you I am not going to be able to do drives right now due to the tranny problems. I would like to get richness and idle up to par as a starting point until I get the trans fixed. It seems to have cured the stumble I had by adjusting the accel. pump arm to free up the slack. I put newer plugs that were clean in it and checked them after putting the carb on and doing the prior adjustments as instructed. They are still black but not as bad and I done seem to have some other popping and missing I had B4. I feel we are in a good direction but still needs some fine tuning. I got the PCV valve in and operating today. Hopefully that will take care of any oilyness that was present previously on the plugs. I gapped the plugs to .055 as instructed by MSD ( range .050 - .060/.065max ). The plugs are temporary to tune out richness and are Autolite #24. I am going to get some new ones closer to getting it tuned right. Over on another thread I was discussing the vaccum and timing. I was at 40* ttl timing B4 and 15* initial. I put it to 35* ttl and 17 initial using the supplied bushings with the MSD dist. I slowed the advance curve slightly but cant fully adjust that until I get to run it. I noticed my vaccum is now at 13inHg which is better and higher than B4. PCV feels strong too. Idle is at 950RPM park and 650RPM drive. I feel it nees to be a little higher...what do you suggest? It smells less rich and smoother. But like I said it still has some way to go. A question that popped into my head was PowerValve. If 6.5 is the opening rate ,as is the one that came in the carb, is that OK for having the amount of vaccum I currently have? I think I have a 4.5 and a 10.5.....should I change this? Just dont want it to open too early or too late. Anywho, hope this is good info and can help you help me. I appreciate it and look forward to your reply. THX Michel

Mikz86TA,

Honestly I wouldn’t do much more adjusting until you’ve got the car where you can drive it. Reason being is that you can get the idle so lean that the car won’t drive properly. We can work on getting it a touch better, but until you see how she performs under actually usage you may be doing work you’re going to have to do twice. Try going in a little further on your idle mixture screws to see if that cleans up the idle some. Another thing is that while you’re having transmission problems it can cause issues with the RPM going in and out of gear. Is there a reason you’re running such a cold spark plug in this engine? A 24 Autolite would seem very cold for an engine like yours.

No particular reason other than they are what I had around to test with. I had R44TS Delcos in B4 but they had seen better days. I think the 24s are equiv to Delco R43TS's. Think I should go with a R44TS or R45TS, or equivilent, heat range? Illl try to adjust the idle mix screws this evening a tad. I can run it down the road and it is strong in gear, like stalling it up, but the problem is I believe the 2-4 band is gone. It has basically no 2 and 4 range. I can at least get an off-line feel. Let me know what you think about the plugs. THX again, Michel

Either the R45TS, or an Autolite 26.

I will pick em up thursday..thx. I am going to tune a little more tomorrow

Tech @ BG, you still on this thing?? what are the real jet numbers for a 750 mighty with annulars?? first i called and they said 70/77.. and my dad called just yesterday and they said 67/74!!!

any input from you would be great.. oh and my dad said the motor just seems to get more and more perky as the jets get smaller..

70/77 would be the baseline jetting for the 750 Mighty Demon... with annular boosters.

67/74 would be the baseline jetting for a 750 Speed Demon... with annular boosters.

May have been some confusion as to what he was working on.

What jets are you guys running right now?