BG 750

[Purple440]

So I took my 750 speed demon off to redo the butterflies for the 10th time and as I was using my feeler gauge to set the butterflies to .020 open I notice that on the primaries the passenger side was open more than the driver side.  I called BG and told the tech guy about this and he said it was normal.  I also told him that the passenger side liked it richer on the idle mixer screws and he said that was normal too on a dual plane manifold.

So wtf, how can I tune the carb and set the butterflies even.  Looks like a POS to me.

Feedback plz.

thx,

- Doug

[Steve P.]

I can't tell you much about BG's other than some like them and others don't. I have tuned 2 of them with great outcome.   

This you should know. Tuning a carb is not just matching turns on screws. It is not going to be the same as someone Else's tune.   The books will teach you HOW, but you have to have the tools and do as they say to learn to tune well.

Also, many times I see people trying to tune a motor starting off in the wrong places. Your tune up, (plugs, cap, rotor, wires, vac. hoses and timing), all need to be right and in great shape.

(((Plugs in the wrong heat range will have you playing with your carb forever)))..

A small vac. leak will screw with you too..

Last but not least, before ANY adjustments on a carb. I snug the carb bolts just a tad to KNOW they are in and tight. This is mandatory if you use an aluminum spacer. I've had plenty of them come in loose.

[jg68]

I can't tell you much about BG's other than some like them and others don't. I have tuned 2 of them with great outcome.   

This you should know. Tuning a carb is not just matching turns on screws. It is not going to be the same as someone Else's tune.   The books will teach you HOW, but you have to have the tools and do as they say to learn to tune well.

Also, many times I see people trying to tune a motor starting off in the wrong places. Your tune up, (plugs, cap, rotor, wires, vac. hoses and timing), all need to be right and in great shape.

(((Plugs in the wrong heat range will have you playing with your carb forever)))..

A small vac. leak will screw with you too..

Last but not least, before ANY adjustments on a carb. I snug the carb bolts just a tad to KNOW they are in and tight. This is mandatory if you use an aluminum spacer. I've had plenty of them come in loose.

, those speed demons can be trickie, i had to put screw in bleeds to currect mine & it still wasn't right for the appl. Also, with most dual plane manifolds, your signal will be diff. from one runner to the other, so the adjustments may need to be different on each corner of the carb, i have found that the ole' holley DPers or the 3310s to be a straight forward carb, even with the 750 demon, i had it jetted way up (84/90), any lower & it wouldn't run right, at this jetting the plugs stayed dark but never fouled, now with this 3310, my plugs cleared up nice & tan & it runs great with 73/85s, always make sure the rest of the tune is right before blameing the carb.

Yeah, i use lock washers & flat washers on mine so the nuts don't come loose, the carb stays locked in place

[firefighter3931]

Not a BG fan....too finicky for my tastes. Holley carbs allways seem to run better and are easier to tune. The fuel curve seems bang on from my experience.


Ron

[Purple440]

I've been tuning it quite a bit actually and feel like I'm doing the right things.  But my point was that when I try to set the transition circuit to twenty thousandths, the primaries are uneven so it is not possible to set them correctly.  So I'm blaming the carb for that.

Ron, I'd like to try a Holley but I keep hearing that they leak.  I'm going to check my 850 speed demon and see if the slots are even, and if they are I'm going to install it tomorrow and see what happens.  If I see the same problem I'm going with either a Holley or a Carter AVS.  Both Holley and Edelbrock tech recommended a 650cfm which I found interesting...

- Doug

[Steve P.]

wOW..   If Holley and Carter BOTH say 650 cfm then you don't want a BG 850 on it. From everything I have read on them and heard from top carb guys, the comparable BG carb to a Holley 750 is a BG 650. Let me put that another way..


If Holley had the (CONTROL) carb and it was a 850 CFM carb the comparable BG carb would be a 750 CFM.

So you having a 850 BG is like having a 950 Holley on there and they are telling you the right CFM your motor calls for is a 650....   

[Purple440]

lol sorry Steve I got lazy...I should have said that I'm using the 850 as a test to see if it idles better.  I'm not running that carb for sure, been there done that and it was a dog on the motor.  I'm hoping the 750 has an internal vacuum leak which would explain my terrible idle.  Can't find any other leaks.  And yes I've tuned the monkey out of it 

[jg68]

I've been tuning it quite a bit actually and feel like I'm doing the right things.  But my point was that when I try to set the transition circuit to twenty thousandths, the primaries are uneven so it is not possible to set them correctly.  So I'm blaming the carb for that.

You don't necessarily have to have them at .020, if the blades are off, then i would try loosening the hex screws just enough to loosen the blades, then close the throttle all the way & retighten the screws, it may take a few trys to get them even, then see if it runs better, i've had mine off alittle on some carbs & never had an issue with the idle quality, as long as your not exposeing too much of them & staying in the idle circuit, 1.1/2 to 1.3/4 turns (from fully closed) is all you really want on the primary throttle screw, anymore & you may have trouble, i'll bet that carb is not right for your appl.

Anytime you put a larger carb on an engine, it will calm down the signal, so it does tend to idle better (with a perf. cam) because its getting more air through the venturies & too the cylinders, so what i'm trying to say is, if both carbs are working fine, you will get a better idle quality from the larger carb


- Doug

[Purple440]

You're right jg68, I just got done with the carb swap and the header banging has calmed down, but it's still rough.

[jg68]

You're right jg68, I just got done with the carb swap and the header banging has calmed down, but it's still rough.

What is the engine combo, maybe i can give some in-sight

[Purple440]

440 .30 over
six pack flat top pistons
M1 Dual Plane
452 heads.  valve job, 2.14/1.81, no porting.
cam: 230@.050, 480 lift, 109 LSA
1 3/4 headers, 3'' collector, 2.5'' flowmaster 50's

[jg68]

440 .30 over
six pack flat top pistons
M1 Dual Plane
452 heads.  valve job, 2.14/1.81, no porting.
cam: 230@.050, 480 lift, 109 LSA
1 3/4 headers, 3'' collector, 2.5'' flowmaster 50's

Is that a lunadi cam, i ran one similar to that & didn't like it (power wise), if you can find a SD, i would get one & ditch that MI, are you happy with this set-up? with your 2355 pistons, you have room to grow & really wake up that engine.

What exactly is the engine doing at idle, what kind of vacuum are you getting in gear, you should be getting around 12 inches on that 109 LSA, also, what is your timing set at & what plugs are running, you know, all the little details

If you want, i'm am starting a service for my mopar friends out there, i will convert your main body over to screw in air bleeds for you, this will make tuneing MUCH easier, then you can dial in the idle & high speed circuits, my price is 65.00 includeing the bleeds, i can also dial it in for you on my 446 if you send the whole carb, if you just want the outers done, it'll be half that


Joe

[Purple440]

Joe - thanks for asking:

I'm extremely unhappy with my performance.  The engine detonates bad and I recently had my MSD recurved to 15* at 3300rpm to try to run on pump gas.  My understanding is that with my setup I should be ok with premium gas.  That's not the case.  My cranking compression is 170+.

Yes the cam is the Lunati Bracket Master II.  This cam sux for my application and I'd love an Engle or Lunati Voodoo, or the Crower D271.  I hate this cam!

My vacuum readings as of today, and after I've done my best to tune the carb, is 14 in park and 8-9 in gear.  No matter how many times I pull the carb, reset the butterflies, redo the mixture screws, set the idle eez, I can't get it to stop banging the headers on the steering box.  Idle is currently at 900rpm.  Any lower and it gets worse.

With my new curve, I requested 18* initial and 32 total at 3500.  He got me to 33 at 3300rpm total which is ok.  First thing that happened is PING at 3/4 throttle under heavy load (3000rpm or so)  this cam just doesn't like it under 3000rpm and I have a stock stall.

I threw in a much stronger secondary vacuum spring today and helped alot with the ping and I don't feel the secondaries kick in hard like I used to, I'm liken that because it's stronger in the RPM range before I used to feel the secondaries kick in.  It reduced the ping somewhat but still happens alot.  I should be at 9.3 or 9.5 to 1 and I'm apparently not.  Ron (firefighter) and Neil (Chryco) have helped me with ideas in this area and I have some upgrade thoughts 

You said with my 2355 pistons I have room to grow?  I was thinking of the Eddy RPM heads.  Is that what you're thinking or something else?

Thanks a bunch,

oops...Accel plugs...134 is the # I saw on them.
Had it blue printed too so they shaved the heads and deck...

- Doug

[jg68]

Joe - thanks for asking:

I'm extremely unhappy with my performance.  The engine detonates bad and I recently had my MSD recurved to 15* at 3300rpm to try to run on pump gas.  My understanding is that with my setup I should be ok with premium gas.  That's not the case.  My cranking compression is 170+.

Yes the cam is the Lunati Bracket Master II.  This cam sux for my application and I'd love an Engle or Lunati Voodoo, or the Crower D271.  I hate this cam!

My vacuum readings as of today, and after I've done my best to tune the carb, is 14 in park and 8-9 in gear.  No matter how many times I pull the carb, reset the butterflies, redo the mixture screws, set the idle eez, I can't get it to stop banging the headers on the steering box.  Idle is currently at 900rpm.  Any lower and it gets worse.

With my new curve, I requested 18* initial and 32 total at 3500.  He got me to 33 at 3300rpm total which is ok.  First thing that happened is PING at 3/4 throttle under heavy load (3000rpm or so)  this cam just doesn't like it under 3000rpm and I have a stock stall.

I threw in a much stronger secondary vacuum spring today and helped alot with the ping and I don't feel the secondaries kick in hard like I used to, I'm liken that because it's stronger in the RPM range before I used to feel the secondaries kick in.  It reduced the ping somewhat but still happens alot.  I should be at 9.3 or 9.5 to 1 and I'm apparently not.  Ron (firefighter) and Neil (Chryco) have helped me with ideas in this area and I have some upgrade thoughts 

You said with my 2355 pistons I have room to grow?  I was thinking of the Eddy RPM heads.  Is that what you're thinking or something else?

Thanks a bunch,

oops...Accel plugs...134 is the # I saw on them.

- Doug

WOW, you have some cranking pressure there, too much for pump gas, my junk pings & will rattle at 38* total timing all in at 2900 with 150 psi, i'm OK at 34* on 93, If you can afford the eddys, i say get them along with a better cam, or just get a better cam with a good converter, if you stay with the 452s, you need to install a cam that will bleed off cyl. pressure, unfortunately, the XEs are out, they build pressure, i'm sure the voodoos do to with there profile, you need a cam with alot of overlap like the purpleshafts have, the .509 would sure bleed off pressure, but they are finiky & require alot of stall & gears with at least 20* int. timing, if not more.

What springs are you useing?, what you could do is, get the MP .557 solid, this will be easier to tune then the .509, but you'll need to back it up with some stall & gear, at the least a 3400 & 3.91s, then get you a SD intake & use the 850 Demon, yes the 850, tuned right, you'll have an easy mid. 12 sec. ride if not low 12s, i know going to a solid sounds like a big deal, but it really isn't, i did it & loved it, just get you an adj. valvetrain, setting the valves ain't no biggie, once you set them a couple of times, they will stay for awhile.

I'll agree, that cam is JUNK, thats why i pulled mine

If you decide you want to have the air bleeds installed, let me know