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Discussion Starter #1
Hi - I'm new to this board and need some advice.

I have a 68 Camaro with a 396 and 4 Sp M21, Victor Junior intake, long headers / 2.5-in dual exhaust, Speed Demon 850 (mechanical secondaries), MSD 6A / Pro Billet 85551 (12* BTDC @ idle, 28* total advance), unknown details on the cam or rotating assembly as I bought car as described (cam definitely does not sound like a stock grind).

Intended application is mostly for street use and occasional fun runs at the track. Current performance seems adequate until I engage secondaries (bogs down big time). Today I found out accel pump for secondaries was broken - fixed it and made it better, but still bogs down at transition. Float level on secondaries currently too low, I'm sure there are other issues.

I am looking for some comments / opinions - Is there anyone here who runs a similar induction setup successfully with a 396?

My problem is with the carb / intake combo. Suspect it's too much carb for a 396 CID. Your feedback is appreciated.

Thanks,

CM
 

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Carlos, you need to get at least another 10* of total timing, something like 38*. Some old iron heads like as much as 40-42*, but 38* will get you real close. This is a huge part or your problem.

Your carb and intake are wrong for your motor unless its a radical race-type motor, although it should run decent. That carb/intake is too big for most mild 454's. You need an something like an RPM intake and a 750 car. You'll gain alot more response and power.
 

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There are two places that the BG carbs have issues. The first and most important is the emulsion holes in the metering blocks. There are too many of them. The BGs come with 6 e-holes at .033". You need to block the lower two at the very least. I drill and tap the e-holes for 6-32 or 8-32 brass set screws from McMaster Carr. The BG blocks have holes that may take either one. The lower two holes I put blanks in. The upper two I reduce down to .028"-.031". This will make a huge difference on a BG or Holley HP carb. For now, even plugging the bottom two holes with lead shot or anything for that matter will be a big help. The BG tends to lean out as load increases without this mod.

Next, drop the High Speed Air Bleeds down to .031". This will require re-setting the jet sizes some as well due to a slight change in AFR. This should help bring the mains in sooner and provide a curve that won't lean out.

I can provide links if you are interested, to all the drill bits, taps and, the McMaster Carr brass bleeds that work better than any of the Holley or BG do and only cost about $8 for 50 blanks.

And no, jet changes alone most likely won't get the enrichment right.
 

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That carb is to big for a 396 but you can make it work. You will never be happy with a victor jr intake on your current engine for street use because it starts making power at 3500 rpm.http://photobucket.com/Dantesrides

My Victor Jr. has better street manners than my old RPM Air Gap did. They make excellent torque below 3,500 RPM. That is a very generic rating made by Edelbrock, and is meant to be followed loosely.
 

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Discussion Starter #6
Thanks everyone - this is very useful information.

I will finish checking all basic gotchas first (run through BG carb setup instructions, check float, accel pumps, fresh plugs, rotor, cap, eliminate vacuum leaks, etc).

lluciano77, sounds like you have been successful in tuning the Victor Junior / 850 CFM combo. I am interested in the links to info on drill bits, taps and, the McMaster Carr brass bleeds you mention in post #3. Thanks again.
 

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As mentioned already your timing is nowhere close to where it needs to be and that is one of the most common issues we encounter.Also changing the circuitry in the blocks and playing with the air bleeds is still not going to make up for the carburetor being too big and the intake being on the big side as well. Do you have the annular or down leg version of the 850?
 

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[email protected] - Thanks, I believe my carb uses the downleg version.
That carburetor will have the capability of flowing close to 1000cfm then. Typically with a mild 396/427 we like to put annular 750's on them but every application will be different.
 

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Not on a 396.:noway:
You are right. It is on a small block 400.

I am using an 800 annular carb with the Vic. Junior.

This my reply to another member that asked about the bleeds/drilling/tapping.

Here are the bleeds. Note the 3/16" length. This is what works best for all of them. You will need 6/32, 8/32, and 10/32
http://www.mcmaster.com/#brass-set-screws/=21zjqm

The drill bits and taps should come pretty standard. The best thing is to get bottoming and pass through/starting. Your BG carb should have almost everything tapped. You will still need the e-holes on the blocks tapped, the PVCRs, and to relocate the idle feed restrictors.

The idle feed restrictors are in the wrong place on BG carbs. BG put them up high in the block. They should be down low where Holley had them. The fuel restriction becomes more stable in the lower position. You can simply take them out of their top location and drill and tap the lower holes to accept them.

Be careful drilling and tapping. Don't go too far on anything. The PVCRs if you choose them can be tough because they are right up on the powervalve. The idle feeds, and e-holes need to be drilled to their channels, and then tapped with the bottoming tap just enough to conceal their faces below the gasket surfaces. Not hard to do, just turn the tap 1/4 at a time, unscrew the tap, and see where the bleed sits. Time consuming, but the results are worth it. Use cutting oil n the BG blocks, and back the tap off every so often to clear out the aluminum chips.

There is also a ton of info on this site on these mods and how they work. This is where I learned a lot of what I know about tuning carbs. There is a user named TUNER on this board that is the most knowledgeable. Shrinker is also really good too.
http://www.innovatemotorsports.com/f...isplay.php?f=7
 

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You are right. It is on a small block 400.

I am using an 800 annular carb with the Vic. Junior.

This my reply to another member that asked about the bleeds/drilling/tapping.

Here are the bleeds. Note the 3/16" length. This is what works best for all of them. You will need 6/32, 8/32, and 10/32
http://www.mcmaster.com/#brass-set-screws/=21zjqm

The drill bits and taps should come pretty standard. The best thing is to get bottoming and pass through/starting. Your BG carb should have almost everything tapped. You will still need the e-holes on the blocks tapped, the PVCRs, and to relocate the idle feed restrictors.

The idle feed restrictors are in the wrong place on BG carbs. BG put them up high in the block. They should be down low where Holley had them. The fuel restriction becomes more stable in the lower position. You can simply take them out of their top location and drill and tap the lower holes to accept them.

Be careful drilling and tapping. Don't go too far on anything. The PVCRs if you choose them can be tough because they are right up on the powervalve. The idle feeds, and e-holes need to be drilled to their channels, and then tapped with the bottoming tap just enough to conceal their faces below the gasket surfaces. Not hard to do, just turn the tap 1/4 at a time, unscrew the tap, and see where the bleed sits. Time consuming, but the results are worth it. Use cutting oil n the BG blocks, and back the tap off every so often to clear out the aluminum chips.

There is also a ton of info on this site on these mods and how they work. This is where I learned a lot of what I know about tuning carbs. There is a user named TUNER on this board that is the most knowledgeable. Shrinker is also really good too.
http://www.innovatemotorsports.com/f...isplay.php?f=7

I'm going to be nice about this but contrary to what some may feel we did not put the restrictors in the wrong place . After lots of actual dyno testing and tuning we came up with the circuitry that we still use today. On our Road and Speed Demons the feed restrictor is still in the lower position but in the Mighty and Race Demons as well as the King Demons it has been moved up due to the needs these engines have with their larger camshafts and such. Every placement and size of the different orifices in the metering block has a function and not only controls either an amount of air or fuel but also the timing in which that circuit starts. Here is a quote that I posted on another forum that pretty much sums up our thought when people start giving out mis-information in regards to how we designed our circuitry that has been in place for years and worked on hundreds of thousands of carburetors.
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" I guess the problem as I see it is that there are a lot of people out there who think they have a better mouse trap but are basing their thoughts on what they see with their meters and read on internet forums and infact many have never worked with circuits on a wet flow bench or watched actual fuel curves on a dyno. Many give out bad information or have tuning ideas that are band-aids for other problems with a given combination. The carburetor is typically the last item to go on but the first to get blamed for any running problems when many times there is something else at fault. We see it every day when guys call in for carb recommendations whether it be a converter that is too tight, not enough gear , mismatched intakes and camshafts, etc,etc.


A good example of this was awhile back when one of the magazines published a story about a guy using one of our carburetors and how he tuned it with his meter to get a flat fuel curve and picked up 5 or 6 mpg. The story went on to tell how he swapped the idle and high speed air bleeds with each other amongst some other changes to achieve this. We were not a part of this and after seeing it in print advised the magazine they should print a retraction the following month as there would be a lot of guys burn their stuff up in trying to duplicate this. They did but the damage was still done. We have nearly 25 years in working with carburetors and developing circuits not to mention some pretty sophisticated equipment to do such. Like I mentioned in an earlier post , we do use the meters as well but they are only one part of the equation and the calibrations we have developed work the best out of the box for the majority of the users. Another good example of this would be our E85 carburetors. We sent several weeks working out a calibration on our flow bench from scratch that we felt was a good starting point. We were then approached by a racer who had been using E85 and we did some testing with him. After many changes in the circuitry including emulsion , air bleeds, jetting , booster and main well sizes , etc the calibration that we ended up using was the same exact one that had been developed on our flow bench.




When a guy puts too big of a carburetor on a combination he is affecting the signal that is present at the booster and what we typically see is a mixture that is too rich at idle and lean at wot. The richness at idle comes from the lower air speed and the fact that the fuel is not being properly atomized as it leaves the booster and is staying “clumped” up. The lean at wot comes from the lack of signal at the booster and the inability to pull fuel. Guys will often take jet away from a carburetor that is rich at idle thinking they are making it leaner when infact it makes the wot even worse. You stated that resizing and plugging some of the emulsion bleeds as well as shrinking the high speeds gave you a flat curve on the meter and as I stated there was more then one way to get to a flat curve but this method like others simply does not work for everyone and as I posted earlier having to shrink the high speeds was a band-aid for what was being done by slowing down the start up of the emulsion circuit. When air bleeds are resized they are doing more then just changing the amount of air that is introduced into the circuit as it also changes the timing and can delay or cause that circuit to start sooner depending on which way you go. "
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Then how do you explain the improvements in both warm up and idle AFR stability when lowering the IFRs on street driven engines? These changes made have consistently, and repeatedly improved the BG street manners.

Nobody thought the magazine was right on the board you are referring to. In fact, everyone was talking of how lame it was.

It was talked on another board about the emulsion circuit and you finally conceded that e-hole bleeds were designed around some big mountain motor that nobody on the street would be running, and that had nothing to do with the price of tea with what the average Joe runs. By changing mine back to the 4 hole specs I gained a lot of vacuum at cruise on the freeway. Others concur with the same results posted.

I am aware of the effects of air bleed tuning and realize that it is not a 1:1 equation. Resizing the MABs could have an effect either way, they are not like the IABs. Of course the timing is different with different sized MABs. The timing is also different with the IABs as well, although more linear.

Anyway, try it or not it is up to him. The results are there. You build carbs to be tuned easily, but when people consistently have results better than yours, you come on the boards and argue with them. I am not here to argue, just to share my results, and the results that everyone else who has tried the above mentioned have come up with. When people change back to the lower IFRs and change their e-holes back to the 4 hole design they see improvement. I have not seen posts the other way around where performance was worse.
 

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I find it amazing that folks are considering/suggesting taking a drill bit to a carb and not have the timing dialed in first.
 

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You already did in the second post.

The mods I am talking about are with the changeable bleeds in the BG carb. The e-holes only need to be drilled and tapped to make them adjustable if that is what he chooses to do. Nothing irreversible.
 

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Discussion Starter #16
Update...

Took SIDEWAYS recommendation on timimg. I went to 11* BTDC initial, and played with mechanical advance curve to achieve 35* total advance for now. Fixed problem with secondary accel pump not working, adjusted idle circuit screws (1 corner too lean, others too rich), and found mismatch in transfer slot settings for primary / secondary butterflies (followed instructions in BG Speed Demon carb instruction manual here - http://www.barrygrant.com/fromBarryGrant/Demon Instruction Manuals/Speed Demon Manual.pdf). BTW, correction to my previous response to annular / down leg booster question on my 850 - my carb uses annular boosters, not downleg type.

There is significant improvement after making above changes. Even though I don't have the correct carb / intake combination for my "itty-bitty 396" (relative to 850 CFM monster feeding it), driveability and performance is much improved relative to where it was when I first posted my question.

Thanks everyone - the information / opinions in this thread are an invaluable resource to people like me, who are trying to understand what's wrong with their setup.
 

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Carlos, how did you go from 12/28 to 11/35* timing? From your first post, 12/28* suggests a 16* mechanical curve. If your total timing is now 35*, that means your initial should fall to 19, not 11*.

11* initial timing is way to low. If your initial is 19*, that's great. Need to get this straight as this is very important, and should be taken care of before doing anything else. You can see the difference a small change in timing makes, just imagine when its where it really needs to be.
 

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Discussion Starter #18
Vince - I increased mechanical curve (changed advance stop bushing on Pro Billet 85551 distributor) and left initial timing about the same. I'm using a 25* mechanical curve now.

Reason for going this way is that in my case, warm startup becomes very poor beyond 12* initial advance (longer cranking time, high starter load with occasional engine kick-back). Current total advance is about 36* and seems to be there by 2000 RPM. Any suggestions on how to achieve a decent warm startup while using more initial timing?

I have no information on compression ratio or cam used with this motor.
 

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OK you have an adjustable distributor...that's a good thing. But, sounds like you have a weak starter. You really need to get your intial timing around 18, perferably 20. MINIMUM 16 for good performance, especially with anykind of performance cam, which I bet you have. 12 is retarded, and will cause poor low-speed performance. Your starter should be able to handle 16-18*, so check all you cables, connections, make sure the battery is good. I run 36* locked, with a GM ministarter, and she doesnt even hesitate when cranking over - warm or cold.
 

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I find it amazing that folks are considering/suggesting taking a drill bit to a carb and not have the timing dialed in first.
We are not suprised by it but then we see it every day. Lack of timing is one of the biggest issues then followed by guys trying to run too big of as carburetor. Add to that you get guys that had no clue on tuning 6 months ago but then they become an "expert" overnight by reading message boards so it really makes our job difficult. I think in this case it has already been determined that the carb is too big for the application plus the lack of timing but the original poster is aware of that now.
 
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