SBC 406 & Holley 670 rich idle & poor transition issues? - Team Camaro Tech
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post #1 of 12 (permalink) Old Aug 3rd, 13, 11:38 AM Thread Starter
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Scott
 
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SBC 406 & Holley 670 rich idle & poor transition issues?

Hoping some Team Camaro member might have some suggestions for me. My SBC 406 has a Holley 83670 - 670 cfm Street Avenger aluminum 4-bbl carburetor with vacuum secondaries and electric choke which I purchased and installed a little over one year ago.

Starts fine but idle mixture adjustment and transition from idle is not good. It has a bad hesitation during transition both with and without a load on it. Once rpm’s are up around 1,700 rpm’s, it screams, below that while cruising it’s an erratic combustion taking place rather than nice and smooth. And yes in gear at low rpm’s (such as cruising through a parking lot) it will buck back and forth acting like it’s lean.

This carburetor has front and rear metering blocks with four corner idle screws and the only way it will idle reasonably is if I lean the two rear idle screws all the way in (so they are effectively disabled) and the front two screws set at of a turn (which is a bit shy of where it is claimed they should be at 1 to 1.5 turns open). Anything other than this and it goes super rich with clouds of smoke out the tail pipes at idle. Float levels are set per Holley recommendations (half way up the sight glass).

Engine has a flat hydraulic tappet camshaft (lift .508 intake .509 exhaust, advance duration 306 intake 306 exhaust, lobe separation 106 intake 112 exhaust, manufacturer of the cam indicates power range of 3200-7000 rpm and very rough idle camshaft with poor vacuum). I’m running a Pertronix III ignition module in a stock distributor housing with a Crane adjustable vacuum can. Vacuum advance connected to manifold vacuum; at idle base timing is around 18 degrees advanced with vacuum advance can connected.

With the idle mixture screws set as I mentioned above, it idles fairly smooth (relatively speaking for the cam it has in it) at 900-1,000 rpm with 9-10 inches of vacuum. I replaced the original power valve that came with this carburetor (was a 6.5) with a 4.5, but no detectable change in performance (positive or negative) however fuel mileage did increase. I replaced the stock #31 accelerator pump shooter with a #37 but not much of a difference noticed other than mileage decreased a bit. I’ve tried both holes on the accelerator pump cam that came with the carburetor but no real difference detected.

I’m beginning to wonder if the cam is responsible for this behavior and the carburetor just can’t be tweaked to overcome the poor transition from idle, or is the carburetor the culprit and I just haven’t quite figured it out yet?

Also wondering if adding a vacuum retard delay valve between the manifold vacuum port and the vacuum advance can might be helpful so as to delay the vacuum advance retarding when letting the clutch out from idle - I ordered one to try so will report back on that late next week once it arrives.

Any suggestions would be greatly appreciated.

Thanks in advance, Scott

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #2 of 12 (permalink) Old Aug 4th, 13, 05:47 PM
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

I don't have a lot of experience tuning large cam engines but let me ask some questions and throw out a suggestion or two.

What is your compression ratio? This can be a big factor.

How much vacuum does it take to activate the vacuum advance and how far does it advance? If you are idling at 9-10 inches, it may or may not be enough to have some advance at idle. Large cams like a lot of advance at idle and off idle. If your 18 degrees at idle includes the vacuum advance being full on, you are probably too retarded. If this is without any vacuum advance, you are probably about right. Pay attention to total mechanical and vacuum advance. You are literally playing with fire and its easy to go too far and ping it to death. A delay valve will probably have little effect on idle and off idle quality if the vacuum advance is connected to manifold vacuum.

Another thing to check is your carb. Get it adjusted where it runs the best, remove the carb, and look underneath where the throttle plates are positioned. Ideally the primary throttle blades should be uncovering just a little of the idle transfer slots. If it is not correct, the secondary throttle can be opened or closed to compensate. This is also something that can have a huge effect on idle and off idle quality.

67 Camaro, 96 LT1, 4L60E
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post #3 of 12 (permalink) Old Aug 4th, 13, 08:40 PM Thread Starter
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

Red67Camaro, thanks for your reply and suggestions.

Compression ratio unknown, motor built previous to my taking ownership (found some documentation that stated Performance Engine Design of Tacoma WA did the build, but no idea when or if they would have records that detailed). Based on my time with engine thus far, I believe it to be a low compression build, how low I just can't say. Wish there was an easy way to determine CR without tearing it all down but I don't know of a way?

While at a show today spoke to someone that suggested I try ported vacuum instead of manifold vacuum to prevent vacuum advance from retarding the timing when throttle is opened. So I gave it a try when I returned from the show even though I've read much on manifold vacuum being the way to go (perhaps that's true up to a certain size cam, but then changes when the cam is even bigger?). Ported vacuum is really just manifold vacuum that is being throttled up and down relative to the throttle butterfly position of the carburetor (in other words, ported vacuum will be less than manifold vacuum when the throttle is closed or minimally open but will never be greater then manifold vacuum).

This change did accomplish what I thought perhaps a vacuum retard delay valve would do when inline with manifold vacuum as far as not retarding the timing when the throttle is opened and got rid of the huge hesitation and stumble when coming off of idle to release clutch from a dead stop. I'm paying very careful attention to any signs of pinging and can say the performance with ported vacuum advance is much snappier then it was with manifold vacuum.

So now the remaining issue is what I would describe as rough running when cruising just above idle (I'd say holding the accelerator pedal no more than a 1/8" to 3/8" from idle position) between 1,000 rpm and say up to 2,500 rpm (upper rpm's depend on load on engine). This rough running causes what I guess could be described as a gurgling from the exhaust rather than a steady exhaust note and causes the car to almost buck while the free lash in the drive train is taken up and released. The further the accelerator pedal is pushed/held and/or the higher the rpm's of the engine, the less this symptom is present until it fades away.

It feels like she's lean to me, and I believe I'm beyond the idle circuit due to accelerator pedal position, I'm not in the accelerator pump circuit as I'm holding steady pedal pressure, I'm not into the power valve circuit as vacuum would be much higher than power valve value and I'm not into the secondary circuit so I'm thinking perhaps if I am lean in this window, that it might be the primary jets are too small. If I get on her and really let her go, she runs real strong, very steady and smooth, but of course she's well into her vacuum secondary circuit with help from the accelerator circuit and power valve.

With this in mind, the next chance I get I'm gonna pull a few spark plugs, clean them up real good, put them back in and take it for an easy cruise. While cruising I'll try to stay out of the secondary circuit and only run from idle to primary circuit as gently as I can. Then I'll pull the plugs I cleaned up and see if they have a story to tell either confirming my thought that the primary jets may be too small and creating an off idle lean condition when the accelerator pump and power valve are not contributing or perhaps it will reveal something else?

I'll also do as you suggest and double check the idle transfer slots. Thanks again for the reply and your thoughts on this, I will post an update once I have something new to report from what I try.

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #4 of 12 (permalink) Old Aug 5th, 13, 02:53 PM
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mike
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

Did you check the carb by taking it off and checking the transition slot to make sure they are even. If the back 2 idle screws are closed then something is wrong? I am not a carb pro but everything I have read said to make all 4 screws equal. How are the float level set?
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post #5 of 12 (permalink) Old Aug 5th, 13, 06:17 PM Thread Starter
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

I've not taken the carb off to verify the transition slots yet, but it is on my short list of things to do.

As I mentioned in a previous post, float levels in both bowls set per Holley recommendation, half way up the sight glass.

Tonight while I had a brief moment I pulled the spark plug out of cylinder #3 so I could clean it off before taking for an easy cruise and then read the plugs. Plugs are Autolite AP 25 which is the third hottest plug in their grouping for this head. Also the plug was white so definately running lean.

Gonna pull and clean a few more plugs, clean them off and then cruise and read the plugs again but if they look like they do now, primary circuit definately is lean and I'm thinking it might be worth dropping two steps cooler on the plugs to AP 23 plugs instead.

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #6 of 12 (permalink) Old Aug 5th, 13, 08:08 PM Thread Starter
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

So I pulled the plugs in cylinders 1, 3, 2, & 4 (the front two cylinders in each bank) and they were all white indicating a fairly lean mixture. The carburetor comes equipped with #65 jets on primary side and #68 jets on secondary side (at least that's what the Summit site indicates). I went looking through my supply of Holley jets from years ago and the sizes I have have that might be useful are #66, #67 & #72.

Holley claims the jets shipped with their carburetors should not usually need to be increased more than four sizes and that it is best to jump two sizes at a time. So I could replace the primary #65 jets with the #67 jets and if that doesn't solve the issue, I could move the #68 jets from the secondary side to the primary side and install the #72 jets I have in the secondary side.

Once I try both of those combinations, if the plugs still look lean, I'll have to purchase some additional jets or their complete re-jetting kit. I'm also still wondering about dropping two heat ranges cooler on the plugs but I'm not sure if there is any good reason not to?

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #7 of 12 (permalink) Old Aug 6th, 13, 07:26 PM
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

If you did not set the transition slot and have the rear screws closed you are not going to be able to tune the carb properly imho. When cruising you are basically on the idle circuit only. A good investment would be an afr meter.
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post #8 of 12 (permalink) Old Aug 6th, 13, 07:54 PM Thread Starter
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

Tomorrow morning I have some available time so I'm planning on pulling the carb and checking the primary idle transition slots. If I need to open the secondary idle to back off the primary idle screw to correct the primary idle transition slots, how far do I open the secondaries with the secondary idle screw, the same number of turns that I reduce the primary idle screw?

I'm also planning to bump up the primary jets two sizes to try and correct the lean mix, should I also bump up the secondary jets two sizes at the same time to keep them the same difference Holley manufactured the carb with (I mean the same number of jet sizes between the primary and secondary jets)?

I should probably invest in an AFR meter, any recommendations on some that I should consider?

Any warnings on ones I should avoid?

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #9 of 12 (permalink) Old Aug 7th, 13, 02:34 PM Thread Starter
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

So I removed the Holley (#83670) 670 cfm Aluminum Street Avenger Carburetor to check the primary idle transition slots and sure enough, instead of seeing a small square, I could see about four times more of the slot than should be showing from what Ive read (Photo 1 at link below). The secondary butterflies were completely closed with none of the secondary idle transition slot visible (Photo 2 at link below). So I backed the primary idle speed screw out until I could only see a square section of the slot (Photo 3 at link below) and then screwed the secondary idle speed screw in until I could see the same amount on the secondary side (Photo 4 at link below). I figured this would be the best starting point possible and from this point, any idle speed screw adjustments Id make equally between each screw to keep the transition slots synchronized with each other.

Next I removed the larger #37 accelerator pump shooter I had installed previously and put the original #31 accelerator pump shooter back in place. Finally I removed the stock #65 primary jets and installed a pair of #67 primary jets. While I had the carburetor off and flipped upside down I also took an old #2 screw bit that had a broken tip and machined the end down to become a screw bit which I can use with a box wrench to adjust the secondary idle screw while the carburetor is bolted to the manifold (Photos 5 & 6 at link below). This bit became invaluable once I mounted the carburetor and had to do the final setup of the idle mixture and idle speed to both the primary and secondary sides.

I was finally able to get a good reliable idle of about 950 rpm with all four corner mixture screws at about 7/8 of a turn from closed. The exhaust at idle smells reasonable, neither rich nor lean. The next step was a test drive; its a completely different car now! The off idle hesitation is non-existent. Cruising above idle where the car would buck and the exhaust would gurgle and create a rough exhaust tone has now smoothed out incredibly. Throttle response is the best its been so far with this carburetor on this engine. The only thing Im questioning is if a set of #68 or #69 jets in the primary side might be even better than the #67 jets I installed today?

Seeing as shes a 69, Im leaning towards the #69 jets more than likely being the best size

So Ill drive her a bit and then pull some plugs and see what they tell me. An AFR meter is on my list of things to get but I'm looking for recommendations from Team Camaro members on what to get and/or what not to get as far as an AFR meter?

Many thanks to those that made suggestions to help me try and solve this annoying issue.

Photos 1-6 referenced above at: http://s1301.photobucket.com/user/NH...enger%20Tuning

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #10 of 12 (permalink) Old Aug 8th, 13, 11:54 AM
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mike
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

I am glad you pulled the carb and adjusted it right. I have an Innovative lm2 and it works fine. http://members.tccoa.com/392bird/tuning.htm I got a lot of good info that will help to understand the circuits of the carb and how they work. Cruising is mostly the idle circuit that is why it is so important. You likely had the back 4 cylinders running lean and the front rich before you pooped the carb off. Good luck glad it is running well. Learn about the air bleeds and how they help and you could get a carb to run as good as efi, until you move to another elevation or temperature thais lol.
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post #11 of 12 (permalink) Old Aug 10th, 13, 06:28 AM Thread Starter
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Scott
 
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

Mike,

I'm glad I did it as well, the interesting thing to me is that the Holley Tech video titled "Secondary Speed Screw" leads the viewer to believe the secondary speed screw in most situations won't need to be adjusted and before doing so you should first call Holley to see if your vehicle is a candidate for this adjustment?

Thanks for the recommendation on the LM2, did you also purchase the optional inductive pickup for engine rpm's?

That's a great carburetor tuning write up, thanks so much for that link as well!

Scott

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #12 of 12 (permalink) Old Aug 10th, 13, 09:21 AM
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Re: SBC 406 & Holley 670 rich idle & poor transition issues?

I welded a bung in the exhaust close to the h-pipe. I did not get the inductive pick up. Make sure you have no exhaust leaks an engine miss can also play havoc on afr's,also be sure to check the plugs to see what they read. Be careful with going to lean also. Being lean can cause a surge and if you opened up the iab to go lean closing it back up is not as simple(if they are drill out iab's). I cruise around 13's afr and idle at 14.7 wot is lean for a supercharged car at low 12's but I spray water methanol so I should be good. I like the challenge of tuning a carb. There is another link Bob the oil guy or something like that, good read. Also going lean causes heat in the cylinder because gas also acts as a coolant(heat will cause detonation).
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