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BBD too rich at low speed, too lean at high speed
https://www.slantsix.org/forum/viewtopic.php?t=66117		 Page 1 of 2
Author:  rich006 [ Sun Aug 08, 2021 3:20 am ]
Post subject:  BBD too rich at low speed, too lean at high speed
I'm running a Super Six intake with a new replica BBD carburetor. No matter how I adjust it, the air/fuel ratio is either too lean at high speed/high load (say 17 AFR, leading to weak acceleration and ping) or too rich at low speed (say 10-11 AFR, leading to hesitation on acceleration). Is there anything else besides carburetion that I should look at? I do have the BBD manual and I do understand how the step-up piston and metering rods are supposed to work.

Background: Last year I rebuilt the engine; the whole 12-page thread is here. I had ping under load before rebuilding, and still had it afterward. I've eliminated the ping by (1) reducing the distributor mechanical advance using the springs from the same donor car that the Super Six intake came from, and (2) enriching the mixture. However, now it's too rich at low speeds.

Here's a recap of what I've got:
Compression ratio 8.9:1 static, 7.7:1 dynamic
Camshaft: Oregon 2106r (a bit longer duration than stock)
Exhaust: stock 2"
Rear diff: 2.7:1
Ignition: stock electronic
Timing: 15 degrees initial plus up to 8 mechanical (all in by 1700 RPM) and 19 vacuum
Emissions: all stock 1974 except OSAC bypassed

I have a vacuum gauge and wideband AFR gauge in the dash. Typical AFR readings are 10-11 at light-throttle cruise and 15-16 at wide open throttle. I can adjust leaner to get 12-13 light throttle cruise, but then I'm at 17+ at WOT.
Author:  emsvitil [ Sun Aug 08, 2021 5:04 am ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
You might need different metering rods.
Author:  Greg Ondayko [ Sun Aug 08, 2021 6:10 am ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
Can the bbd use afb/ avs edelbrock/carter metering rods?
If so you should have a fairly diverse selection to choose from.


Greg
Author:  Jase [ Sun Aug 08, 2021 6:23 am ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
Quote:

I have a vacuum gauge and wideband AFR gauge in the dash. Typical AFR readings are 10-11 at light-throttle cruise and 15-16 at wide open throttle. I can adjust leaner to get 12-13 light throttle cruise, but then I'm at 17+ at WOT.
What are you adjusting/changing to achieve these changes? Idle mixture screws?

Do the spark plugs agree with the AFR gauge?

It looks like you are lean all the time at WOT. That needs fixed.. ether with a larger jet, or metering rod change.. Or with a stronger spring to get the metering rod out of the the jet with more authority, if the jet and metering rod are actually the correct size.

Once you get the WOT in shape then start trimming down the idle, and transition circuits to lean out to where you want to be.

If you do not have access to jets-rods-springs from the place you got the carb, you could try raising the float level, which will richen things up a tad.
Author:  rich006 [ Sun Aug 08, 2021 2:35 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
To enrich the mixture I tightened the screw on top of the stepup piston (and re-qualified the piston) and I raised the float level. I've also tried using metering rods from 2 different carburetors (the new one and the one from the donor car). The idle screws are all the way in, and I'm still idling rich at 12.5-13 AFR. The weird thing is the mixture is rich all the time except mid to full throttle. I feel like I would need extremely tapered metering rods to correct this, and I wonder if something else is wrong with the engine build or if I've somehow built the carb wrong.

The spark plugs do agree with the AFR gauge. They were light before I raised the float level and now they are darker, see pic.
Image
Author:  Jase [ Sun Aug 08, 2021 6:05 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
It seems to me that metering rods, their springs, and main jets are the players in WOT AFR, well Float level has something to say as well.

I wonder if a stiffer spring would get the metering rod farther out of the jet? I just looked at some of the BBS literature in the engine build matrix, and it appears as if there are single, double and triple step metering rods for the BBS.

Do you know what jet is in your new carb? How about the metering rod size at the tip?
Author:  matv91 [ Sun Aug 08, 2021 6:34 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
https://www.slantsix.org/forum/viewtopi ... 19&t=35294 see this thread for metering rod adjustment instructions changed 1978. Step up spring to soft or short piston always at lean stop. Or spring to long and stiff piston always at rich stop. Edelbrock springs fit, rods do not.
Author:  matv91 [ Sun Aug 08, 2021 7:12 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
https://www.slantsix.org/forum/viewtopi ... 64#p481764 Idle circuit
Author:  matv91 [ Sun Aug 08, 2021 7:58 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
Wonder if idle circuit is blocked and you are idling on main circuit,wrong gasket at bottom of bowl idle down channel blocked. BBD has 2 idle air beeds, first one top of venturi screw[ they are not all the same size] second in carb bowl body. Dirty restricted or to small will cause rich idle and off idle. Swap venturi screws just do not get them mixed up.To blg idle tube opening will cause rich runing stock size was .0295. Idle tube is in venturi cover which also contains main system air bleed. Maln system air bleeds did vary in size, might be interesting experiment.
Author:  SlantSixDan [ Sun Aug 08, 2021 8:51 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
Quote:
I'm running a Super Six intake with a new replica BBD carburetor
I wish you good luck. Those Chinese carburetor-shaped trinkets are rather less than equal to the originals. Turns out for the durn thing to work, it has to do more than just look like a carburetor!
Author:  Joshie225 [ Mon Aug 09, 2021 7:58 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
You cannot raise the float level and expect good things. The fuel level is critical to when the main metering circuit starts. So if the float is too low the main circuit will come in late making the transition circuit will seem to too lean. When the float level is too high the main metering circuit starts early, overlapping transition and you get a rich mixture until transition is out of the picture with increased throttle opening. When you're at that state it's a balance between main jet area, emulsion circuit characteristics and the air bleeds. The step-up circuit coming in should make for a noticeable enrichment and a given vacuum level. This is a balance between engine vacuum and step-up piston spring strength. Idle screws all the way in with the engine bale to run shows that the screws don't do what they should.

With the 'reproduction' carburetor you don't know what the float level should be. If the main well tube was made right. If the step-up spring is right, etc. You've signed up for a lot of experimentation and calibration work by buying this carburetor. I would lower the float level, reestablish some kind of baseline and then plug the main air bleeds. It should then go rich at high engine speeds. Or buy a good original carburetor that benefited from calibration by carburetor experts and fine tune that.
Author:  rich006 [ Tue Aug 10, 2021 12:03 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
I do have the carburetor that came with my intake manifold (I posted about it here). I guess I'll try to clean it up and see if it works better. I've already put the stepup piston, spring and metering rods in the new carb and all of that seems to work. I work with a machinist who might enjoy doing the welding as matv91 showed in my thread linked above.

I tried the Chinese replica first because it wasn't much more expensive than a rebuild kit. From the factory, the float was set almost at the bottom of the bowl. The engine actually ran OK like that, except it was lean at WOT. I raised it to something like 3/16" from the bowl top.
Author:  Jase [ Tue Aug 10, 2021 7:57 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
Quote:
I do have the carburetor that came with my intake manifold (I posted about it here). I guess I'll try to clean it up and see if it works better. I've already put the stepup piston, spring and metering rods in the new carb and all of that seems to work. I work with a machinist who might enjoy doing the welding as matv91 showed in my thread linked above.

I tried the Chinese replica first because it wasn't much more expensive than a rebuild kit. From the factory, the float was set almost at the bottom of the bowl. The engine actually ran OK like that, except it was lean at WOT. I raised it to something like 3/16" from the bowl top.
In my opinion there is no harm in futzing with the replica carb to see if you can get it to produce a fuel curve that follows the trajectory of the engine's requirements. It's not working correctly now, so how can it be spoiled any more....? If you are ultimately unsuccessful in getting it to work, then use replica carb as a parts donor for the BBS you have, or combine the best of both, and see if the BBS will produce anything.
Author:  kesteb [ Wed Aug 11, 2021 4:13 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
IMO, it is a waste of time trying to make the BBD work. Your best bet is to buy a Rochester 2GC to Holley 2300 adapter plate. Then find an AMC version of the Autolite 2100 with 1.08 on the throttle bores. Rebuild this with a kit from an early '70s Jeep with a 304. Change the jets to #46. Bolt it on, adjust/make linkage as needed. Then drive the car. This is the best carburetor you can use.
Author:  rich006 [ Wed Aug 11, 2021 4:41 pm ]
Post subject:  Re: BBD too rich at low speed, too lean at high speed
I appreciate all the ideas. I see United Automotive Manufacturing has a slant BBD (and also that Autolite 2100). Can anyone vouch for their workmanship?

How would I go about plugging the air bleeds if I decide to try that?
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