Slant Six Forum :: Alrighty, some more BBD tuning questions

(1) If the distributor is still the stock distributor, put the timing back to the stock setting. Overadvancing the base timing can cause a lag on acceleration due to overadvancement of total timing when the vacuum advance kicks in.

(2) adjust the metering rods as per the factory adjustment procedure see here

(3) make sure you have no vacuum leaks. Distributor advance pod, choke pulloff diaphragm, throttle shafts, intake manifold, etc...Super Six cars did come with a special distributor curve, but the factory dist should be okay. You may need to pull the distributor and lube the weights to make sure they are advancing properly.

(4) Make sure the kickdown is adjusted properly

(5) Idle speed should be 750 in neutral. If you set the base idle speed too high, you will be idling on the cruise circuit in the carb rather than the idle circuit. This makes tuning it a PITA and throws off the mixture badly. Idle speed in drive should be about 600-650 RPM, but the proper way to set it is idling in neutral.

Also, if the throttle is opened too much at curb idle, you will be advancing the timing through the vacuum advance port. This means (a) you won't get any vacuum advance when you open the throttle since it is already advanced, and (b) your timing will be set too retarded overall.

(6) curb idle mixture should be roughly 2 turns out, with a half turn margin on either side. If you need more than that to idle smoothly there is a problem somewhere.

(7) did you make sure to bypass the OSAC and EGR valves?

The small exhaust will detract some from performance, but probably not enough to cause a flat spot, unless the muffler is internally collapsed and causing excessive back pressure.

I need to work backwards, advancing the timing until I get all the vacuum advance out of it, then set it to TDC, and work my way up to around 6 BTDC, until the distributor is re-curved.

(6) curb idle mixture should be roughly 2 turns out, with a half turn margin on either side. If you need more than that to idle smoothly there is a problem somewhere.
i'mim having some of the samproblemsms as thor on my volare ,i have the slight flat spot.. my mixture screws are out 6 1/2 on both sides thats the only way it would run decent. ineutralal its idling at 950 and in gear about 730-750 rpm. my timing is set at 12btc, any ideasi'veve checked for vacuum leaks everywere. sorry fostealingng the thread

Do the same things I recommended Thor do above- check for vacuum leaks at the throttle shafts, vacuum advance pod, and choke pulloff diaphragm; set the metering rods correctly; set the timing back to factory specs; set the curb idle to factory specs; etc...

Author:	THOR [ Wed Jan 21, 2009 7:09 pm ]
Post subject:	Alrighty, some more BBD tuning questions
Just a note, I am typing this more to get my mind to think about what the issues are, so if it seems a bit off, that's why... I installed a BBD w/SS intake on my roommate's '73 Duster. It's a bone stock 225 with 200,000+ on the bottom end. When we had the 1920 on there, it pulled well, achieved 26 mpg freeway from Salem to Seattle, and back again. Anyways, I finally got the LOKAR hooked up, got better tires on the front (ones that dont have nails and cracks in them ) and drove down to my uncle's shop to get a timing light on it, and adjust a few things. When I arrived, the timing I set by ear was at 5 BTDC, so I bumped it to 12. I set the idle mixture so that the car ran smooth, and then set in gear RPM to 750. After making a few "tweak and drive" trips, I've got it set fairly well. The problem we seem to be having is that there is a flat spot on inital acceleration (it has a nice healthy pump shot...) and anything above 1/2 throttle feels a bit flat. The 1 bbl was certainly no monster either, but I was expecting a bit more than what it gives. I noticed at full throttle I was getting some smoke out the tailpipe, so I adjusted the metering rods little by little until that went away. Now, upon full throttle, I get no smoke, but it still seems flat. It may just be me, and the fact that I am used to driving a much newer car. I know that the factory exhaust has got to go, would that be enough in of itself to case the "flatness" of which I speak? Also, the dizzy is stock, and needs a definite recurve. Between these two things I think we'll have it nailed.. but that initial flat spot is still bothering me. Whaddya think? ~THOR~

Author:	Reed [ Wed Jan 21, 2009 7:45 pm ]
Post subject:
(1) If the distributor is still the stock distributor, put the timing back to the stock setting. Overadvancing the base timing can cause a lag on acceleration due to overadvancement of total timing when the vacuum advance kicks in. (2) adjust the metering rods as per the factory adjustment procedure see here (3) make sure you have no vacuum leaks. Distributor advance pod, choke pulloff diaphragm, throttle shafts, intake manifold, etc...Super Six cars did come with a special distributor curve, but the factory dist should be okay. You may need to pull the distributor and lube the weights to make sure they are advancing properly. (4) Make sure the kickdown is adjusted properly (5) Idle speed should be 750 in neutral. If you set the base idle speed too high, you will be idling on the cruise circuit in the carb rather than the idle circuit. This makes tuning it a PITA and throws off the mixture badly. Idle speed in drive should be about 600-650 RPM, but the proper way to set it is idling in neutral. Also, if the throttle is opened too much at curb idle, you will be advancing the timing through the vacuum advance port. This means (a) you won't get any vacuum advance when you open the throttle since it is already advanced, and (b) your timing will be set too retarded overall. (6) curb idle mixture should be roughly 2 turns out, with a half turn margin on either side. If you need more than that to idle smoothly there is a problem somewhere. (7) did you make sure to bypass the OSAC and EGR valves? The small exhaust will detract some from performance, but probably not enough to cause a flat spot, unless the muffler is internally collapsed and causing excessive back pressure.

Author:	THOR [ Wed Jan 21, 2009 8:07 pm ]
Post subject:
Quote: (1) If the distributor is still the stock distributor, put the timing back to the stock setting. Overadvancing the base timing can cause a lag on acceleration due to overadvancement of total timing when the vacuum advance kicks in. Sounds good, I'll do that when I get another chance to work on it. Quote: (2) adjust the metering rods as per the factory adjustment procedure see here Metering rods were adjusted to factory spec when I ran it originally, again, I'll set it back next chance I get. Quote: (3) make sure you have no vacuum leaks. Distributor advance pod, choke pulloff diaphragm, throttle shafts, intake manifold, etc...Super Six cars did come with a special distributor curve, but the factory dist should be okay. You may need to pull the distributor and lube the weights to make sure they are advancing properly. I took carb cleaner and sprayed all around the carbs gasket areas, and manifold mounts, didn't notice any change in RPM. Quote: (4) Make sure the kickdown is adjusted properly I need to fidget with the adjustments on the cable, and probably build a new bracket for the carb, the one on there now doesn't provide enough travel. Quote: (5) Idle speed should be 750 in neutral. If you set the base idle speed too high, you will be idling on the cruise circuit in the carb rather than the idle circuit. This makes tuning it a PITA and throws off the mixture badly. Idle speed in drive should be about 600-650 RPM, but the proper way to set it is idling in neutral. Also, if the throttle is opened too much at curb idle, you will be advancing the timing through the vacuum advance port. This means (a) you won't get any vacuum advance when you open the throttle since it is already advanced, and (b) your timing will be set too retarded overall. I set the idle at 750 to comp for the old alternator.. I'll move it back down. I also noticed that I needed to have the curb idle screw in pretty far just to get the car to idle. I think you are on to something mentioning that. I'll get a vacuum gauge on it as well. In fact, now that you do mention that, I think the timing is too retarded... I was able to put it well over 20 degrees at idle and the car ran smooth as silk.. still does... Epiphany has just occured It's all starting to make sense now! The timing was so far retarded with the 1bbl on there that when I swapped to the 2 bbl, it was too retarded to even get the car to run in gear. I bumped it up a tooth, and then moved the plate to allow for more advance and that got me to 5 BTDC. I thought it was odd that the idle screw still needed to be that far in, but it never actually clicked in my mind as to why... now I know. I need to run the advance up until it just starts to run rough, set the idle on the carb, and then put a timing light on it, set it to approx 12 BTDC again, and then drive it. That's probably 3/4 of the issue right there! Quote: (6) curb idle mixture should be roughly 2 turns out, with a half turn margin on either side. If you need more than that to idle smoothly there is a problem somewhere. Yea! Like retarded timing! Quote: (7) did you make sure to bypass the OSAC and EGR valves? The small exhaust will detract some from performance, but probably not enough to cause a flat spot, unless the muffler is internally collapsed and causing excessive back pressure. Yes, the emissions stuff is completely bypassed, did that when we first got the car... Far as I know, the muffler is fine. It appears to be in good shape on the outside, no dents, scrapes etc... so I think we'll leave that alone for now. I will go out and play with it probably in the next day or so... I'll post my findings here. Thanks a ton! ~THOR~

Author:	Reed [ Wed Jan 21, 2009 10:31 pm ]
Post subject:
"I took carb cleaner and sprayed all around the carbs gasket areas, and manifold mounts, didn't notice any change in RPM. " That won't reveal a blown vacuum advance pod or a blown choke pull off diaphragm. Take the vacum advance hose off the carb and suck on it. You shouldn't be able to suck any air. If you can, the vacuum advance pod is blown. Pull the air cleaner. Detach the hose from the back of the choke pulloff. Depress the plunger on the choke pulloff and put a finger over the nipple on the back of the diaphragm. The plunger either shouldn't move at all or move just a little bit. If the plunger returns back to where it was before you depressed it, you have a blown choke diaphragm. "I set the idle at 750 to comp for the old alternator" That is a temporary fix, not a solution. You need to fix the problem with the charging system. Setting the curb idle this high (a) wastes gas and (b) screws up your mixture and timing etc... The fact that you have to turn the curb idle screw in a long way suggests you have either a vacuum leak or the timing is too retarded. This also means you are most likely idling on the main metering circuit, not the idle circuit. I would definitely put the timing and curb idle speeds back to spec and start tuning all over again. Remember, idle speed is 750 in neutral, all accessories off. The factory timing for a 73 Duster with a 225 is TDC. I would start with the timing there, and then if the car runs decent bump it up to about 6 BTDC. When I set up my brother's 83 Dodge van with a 225, I used a stock distributor pulled from a 73 Valiant with a 225. Here is the advance curve for the 73 225 distributor I used: Distributor tag number: 3755042 Vacuum advance: 8.5R, factory settings start at 7 inches, all in by 14 inches governor: 15L 73 Valiant mechanical distributor curve test Factory base timing = TDC 600 RPM = TDC 750 RPM = TDC 800 RPM = 2BTDC 900 RPM = 8BTDC 1100 RPM = 11BTDC 1350 RPM = 20BTDC 1500 RPM = 22BTDC 1550RPM+ = 22BTDC The distributor in your roomates Duster probably has a very similar curve. I ended up setting the base timing on my brother's van at about 8 BTDC. The van ended up pulling 21 inches of vacuum at idle and getting 20-21 MPG on the highway at 65 MPH. Not too shabby for a big heavy box running a Holley 2280 two barrel. The point is, the distributor is probably curved to get much of its initial advance from the vacuum advance. As soon as you crack the throttle, the vacuum advance provides 8.5 degrees of advance, and by the time the motor hits 900 RPM you have also got 9 degrees of mechanical advance. You probably haven't gone below 14 inches at 900 RPM, so at 900 RPM you already have a combined timing advance of 17.5 degrees with a base timing of TDC. If you start out setting your base timing to 12 BTDC with this distributor, you would get roughly 25-30 degrees of vacuum advance by 900 RPM. This is way too much. All this is mean to caution you against advancing the timing too much past the factory setting until you recurve the distributor. I would set it in the 6-8 degree BTDC range, tops.

Author:	THOR [ Wed Jan 21, 2009 10:59 pm ]
Post subject:
I agree with everything you say. However, this is not my vehicle, and therefore, the covering up of the alternator issue was not my idea. In either case, I'll change it back to spec. I'll check those diaphragms out tomorrow, see what happens. I need to work backwards, advancing the timing until I get all the vacuum advance out of it, then set it to TDC, and work my way up to around 6 BTDC, until the distributor is re-curved. Sounds like a plan. I'll get right on it. ~THOR~

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Alrighty, some more BBD tuning questions https://slantsix.org/forum/viewtopic.php?t=33427	Page 1 of 2

Author:	Reed [ Thu Jan 22, 2009 8:22 am ]
Post subject:
Quote: I need to work backwards, advancing the timing until I get all the vacuum advance out of it, then set it to TDC, and work my way up to around 6 BTDC, until the distributor is re-curved. For now, just disconnect and plug the hose that goes to the distributor when you set base timing. You should be doing this already when you set the timing. Another thing to consider is that the vibration dampener might have slipped. On a motor that old with that many miles, it is a distinct possibility that the outer ring has slipped and the timing mark is actually retarded from true TDC. On my brother's van I found that his dampener ring had slipped and the timing read 8 degrees retarded. Setting the timing taking that into account make a big difference.

Author:	1980volare [ Thu Jan 22, 2009 3:32 pm ]
Post subject:
(6) curb idle mixture should be roughly 2 turns out, with a half turn margin on either side. If you need more than that to idle smoothly there is a problem somewhere. i'mim having some of the samproblemsms as thor on my volare ,i have the slight flat spot.. my mixture screws are out 6 1/2 on both sides thats the only way it would run decent. ineutralal its idling at 950 and in gear about 730-750 rpm. my timing is set at 12btc, any ideasi'veve checked for vacuum leaks everywere. sorry fostealingng the thread

Author:	Reed [ Thu Jan 22, 2009 3:44 pm ]
Post subject:
Quote: (6) curb idle mixture should be roughly 2 turns out, with a half turn margin on either side. If you need more than that to idle smoothly there is a problem somewhere. i'mim having some of the samproblemsms as thor on my volare ,i have the slight flat spot.. my mixture screws are out 6 1/2 on both sides thats the only way it would run decent. ineutralal its idling at 950 and in gear about 730-750 rpm. my timing is set at 12btc, any ideasi'veve checked for vacuum leaks everywere. sorry fostealingng the thread Sounds to me like you have a vacuum leak. If you have to set the carb that far out of whack to get it to run, then you either have a vacuum leak or a timing problem, or both. Do the same things I recommended Thor do above- check for vacuum leaks at the throttle shafts, vacuum advance pod, and choke pulloff diaphragm; set the metering rods correctly; set the timing back to factory specs; set the curb idle to factory specs; etc...

Author:	1980volare [ Thu Jan 22, 2009 4:04 pm ]
Post subject:
timing is set at specs, no leaks at vacuum advance pod or choke pull off. i cant find any leaks anywere. and any less idle speed below 750 or so is very choppyand rough in gear.

Author:	Reed [ Thu Jan 22, 2009 5:18 pm ]
Post subject:
What is your vacuum at idle? How long has it been since you rebuilt the carb? Did you chech the throttle shaft for play? Have you confirmed that your vibration dampener hasn't slipped?

Author:	1980volare [ Thu Jan 22, 2009 5:51 pm ]
Post subject:
pulling a little bit over 16 in at idle in neutral, and about 10-13 fluctuationg in gear. i was going to double check for the dampner slipage. carb is supposidly for a 318 it is a nos carb not reman.

Author:	Reed [ Thu Jan 22, 2009 5:53 pm ]
Post subject:
16 inches is at the bottom end of ok for a normal slant. I think you have a leak or your timing isn't correct. Did you modify the choke linkage to work with your slant six? Did you block the holes in the throttle blades? Did you adjust the metering riods correctly?

Author:	DusterIdiot [ Thu Jan 22, 2009 6:39 pm ]
Post subject:	Whoa there pardner...better get your manual out...
Quote: Do the same things I recommended Thor do above- check for vacuum leaks at the throttle shafts, vacuum advance pod, and choke pulloff diaphragm; set the metering rods correctly; set the timing back to factory specs; set the curb idle to factory specs; etc... Uh Reed... if he has a 1980 Volare the stock timing specs for the 3874876 distributor in a 1 or 2 barrel slant is going to be 12BTDC, and 700-750 curb idle speed... 750 is not a waste of gas either...900 would be a waste of gas, maybe. But this is all academic, carbs and fuel flow are analagous to an example with water... your stock idle circuit is much like a squirtgun (not the super soaker type)...when the main jet(s) kick in it is a garden hose compared to the idle circuit (and if your economizer valve opens you get to add another garden hose...)...'idle' gas mileage is important if you spend lots of time standing idling in traffic in major cities... Timing curve for the 3755042 was set to be used with a Holley 1920 1 barrel...so the timing/tuning spec for the BBD does not officially exist so he will have to do some test and tune to see where it likes it... I had that dizzy in my '74 and with the super six it liked it at 8 BTDC base timing... I have pulled the stock spring combinations out of several dizzys and will be working up a post later that will show the springs, measure the loops, and describe the timing curve so hopefully our users can identify which junkyard dizzy they might have and be able to find out if the mystery springs are stock-ish... -D.Idiot

Author:	Reed [ Thu Jan 22, 2009 10:07 pm ]
Post subject:
Yeah, but 1980volare was saying he was idling at 850 in neutral and 730-750 in gear. Thats too high. It is supposed to be 750 in neutral, not in gear. In gear idle should be 600-650. Yep, stock timing for a 1980 225 would be 12 BTDC, but he might have a slipped vibration dampener or a vacuum leak somewhere else.

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Author:	1980volare [ Fri Jan 23, 2009 3:46 am ]
Post subject:
meetering rods are adjusted to spec. i am not using the thermostatic coil spring choke, i have a manual choke. i am not sure if i have the holes in the throttle blades. i will double check.