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Holley SA 670 tweaking. Need advice.
#1
A question for more knowledgeable carb guys.
As most likely know, my 71 M code, with a close to stock rebuild, has a Holley SA 83670 carb. I had minor issues with it from the get-go. Main problem was getting the idle mixture right. I took the car to a very good tuner locally who performed surgery on it by drilling a 3/32" hole in each of the primary plates. This solved most of the low speed problems. It took other tweaks, timing mainly, to achieve a strong pulling motor. However at times, which may depend on atmospherics, I get a "cough" not unlike a backfire (but it's not) when I tramp on the gas pedal. If I accelerate slowly, it does not do that, just pulls strong all the way to 5K.
What I have done so far is to borrow an AEM air fuel ratio gauge, but the tuner guy says it's still on the fat side. He suggested some minor changes that I can easily do. Increase the power valve to an 8.5 as this motor holds 18"Hg at idle. Stock P/V is a 6.5 and maybe one jet size smaller on the primary, from 65 to 64.
I took a video of a test run I did, but I think the file is too big to post. Readings at idle are around 11:1 and 13:1 at 2500 rpm or 50 mph. Tuner guy told me it should be around 14:1 at idle and 11:1 at full throttle.
What is the opinion on ideal readings on an AFR? I want to get a better handle on this carb before I ditch it for a Quick Fuel HR 735. I've always felt the 670 does not allow enough air flow for a perfect A/F balance on this particular motor.
Help and constructive comments welcome and thanks.
Geoff.

I learn something new every day!
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#2
Geoff,
If it were mine,I would start by trying to lean your engine out at idle.
if you have 18'' at idle you should easily be able to do that, watch the vacuum gauge while leaning and set the mixture where it just starts to lose a little vacuum, let the engine tell you what is best.
Your 13:1 at cruise is OK but you could be as lean as 14:1 with next to no load.
Surprised your tuner says 11.0 :1 is ok at WOT there should no reason to be richer than 12.5: 1 at any time period unless you are using a power adder or running boost.
Your primary jetting is NOT the issue here and guessing the power valve is not either, however the power valve restrictor could be.
I would want to watch your AFR while observing a vacuum gauge and see what your afr does when it is supposed to be opening.
When the power valve opens, you should see 12.5; 1 if it is richer than that you need to decrease the power valve restriction.  ideally when the power valve opens it should give you  the same as adding  of 10 jet sizes . in my opinion you are dealing with a accelerator pump issue, either too small of a shooter nozzle or too small of a accelerator pump cam.
You did not state if you are running vacuum advance and mechanical advance or just mechanical, it could make a difference, also gear ratio and rear tire diameter all play a role here.
Boilermaster
The 670 will provide enough air and fuel until you reach a certain rpm, but it is NOT the issue you are dealing with here.
[+] 1 user Likes boilermaster's post
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#3
Good analysis, I agree, the cough is a sign of being too lean, momentarily, when the carburetor is quickly opened. You need a quick, and bigger, shot of fuel when the throttle opens.

I would work on one issue at a time, get rid of the cough first, then work on the idle mixure, and then on cruise and WOT.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#4
(07-11-2019, 04:43 PM)boilermaster Wrote: Geoff,
If it were mine,I would start by trying to lean your engine out at idle.
if you have 18'' at idle you should easily be able to do that, watch the vacuum gauge while leaning and set the mixture where it just starts to lose a little vacuum, let the engine tell you what is best.
Your 13:1 at cruise is OK but you could be as lean as 14:1 with next to no load.
Surprised your tuner says 11.0 :1 is ok at WOT there should no reason to be richer than 12.5: 1 at any time period unless you are using a power adder or running boost.
Your primary jetting is NOT the issue here and guessing the power valve is not either, however the power valve restrictor could be.
I would want to watch your AFR while observing a vacuum gauge and see what your afr does when it is supposed to be opening.
When the power valve opens, you should see 12.5; 1 if it is richer than that you need to decrease the power valve restriction.  ideally when the power valve opens it should give you  the same as adding  of 10 jet sizes . in my opinion you are dealing with a accelerator pump issue, either too small of a shooter nozzle or too small of a accelerator pump cam.
You did not state if you are running vacuum advance and mechanical advance or just mechanical, it could make a difference, also gear ratio and rear tire diameter all play a role here.
Boilermaster
The 670 will provide enough air and fuel until you reach a certain rpm, but it is NOT the issue you are dealing with here.
 Boilermaster, thanks for the detailed reply.
First to clarify. The carb is vacuum secondary, electric choke. I do have vacuum advance, but only about 4 -6 degrees on top of 34* mechanical. I was running it at 16* initial, 36 total, but backed it off to 14, 34 total. It seems to run better at that. At 16* I still have had the odd time it spark knocks slightly. Yesterday was extremely humid here and when my tuner guy and I were testing, it rattled just slightly. To be honest, it's not easy to get the timing light directly pointing at the markers, so just a slight angle off, could make quite a difference to the reading, 16 could really be 18 even though the timing light is set for 16 if that makes sense. The motor has a comp ratio of just shy of 10:1 Melling MTF 2 cam, slightly higher lift than stock. KB 13cc dished pistons at zero deck, quench heads.
Secondly, I made a typo. it should have been 12:1 at WOT, my mistake. Even that is slightly lower than you suggest. 
The rear end ratio is 3.25:1 open dif and tires are 245/60/14 at 26.6" dia.
I'm not entirely sure what you mean by the power valve restrictor. Can you explain in simple terms?
I am today going to install a temporary vacuum gauge in the car along side the afr. I will also re-set the idle by vacuum gauge, but I'm pretty sure it's right where it needs to be by your recommendation.
I did not mention what I had done before, so briefly I changed the primary jets from 65 to 64, not much difference noticed without having an AFR for reference, went back to the 65's. At one time I blew the 6.5 power valve and installed an 8.5 with 17-18" Hg, went back to a 6.5. So I tend to agree, not the jets or the p/v. Played with a different pump cam, white I think it was, but it made it worse. I have a #35 shooter nozzle, but I honestly can't remember if it the original I changed or one I bought and never put in. I also changed the secondary spring to the 'black' heavier one, more for economy than power. There is a 1" thick fiber spacer under the carb to reduce heat transfer, which was a major issue until I blocked the heat cross-overs. 
I have checked the pump lever clearance and that's good as per instructions with the carb. That's about all I think I can offer.
So I tent to agree, with my very limited knowledge on carbs, the pump size and shooter size may be all I need to change.
I really don't want to buy another carb if I don't have to, money is tight these days, so if I can get it performing better, all well and good.
I tried earlier to upload that video, but for some reason I'm having trouble attaching files lately. (posted on that in site feedback)
Again I appreciate your help and time,
Geoff.

I learn something new every day!
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#5
(07-11-2019, 09:46 PM)Don C Wrote: Good analysis, I agree, the cough is a sign of being too lean, momentarily, when the carburetor is quickly opened. You need a quick, and bigger, shot of fuel when the throttle opens.

I would work on one issue at a time, get rid of the cough first, then work on the idle mixure, and then on cruise and WOT.
Thanks Don, that's the plan, one thing at a time...…….. before I spend much more money.
Geoff.

I learn something new every day!
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#6
Found this tuning guide a few years back, gives a pretty good step by step with lots of information.

http://brucerichards.com/tuning.htm


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#7
Hemikiller,                                                                                                                                                      funny you should post the Bruce Richards article, he explains exactly how to modify the PVCR (POWER VALVE CHANNEL RESTRICTION that is exactly what I did to my 750 vs carb, not larger but slightly smaller to achieve what I wanted.                                                                                                                                  Boilermaster
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#8
(07-12-2019, 02:37 PM)Hemikiller Wrote: Found this tuning guide a few years back, gives a pretty good step by step with lots of information.

http://brucerichards.com/tuning.htm
Time to study! thanks

Geoff.

I learn something new every day!
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#9
Okay the latest so far.
I hooked up the more accurate vacuum gauge as well as the AFR so I can see them whilst driving. The new gauge shows 17.5" Hg at idle. With a separate vacuum gauge, I readjusted the idle mixture screws and was able to achieve between 13.2 and 13.8 afr at idle, approx. 800 rpm. Yes I can likely drop the rpm a bit, but for now that's where it is. Rechecked timing is at 14 initial, vac disconnected. I did take it out for a quick trip to get it warmed up before I did any adjustments. No time tonight to road test it, so tomorrow when I go out to RM Restorations for "Coffee and Cars" I'll have a chance to put it through the gears. 
The idle was smoother for sure, but in reality, I only turned the screws in by about 1/3 turn and set them both the same amount. That is also where the vacuum started to drop off. This won't make any difference to the rest of the issue, but the place to start.
More to follow.
Geoff.

I learn something new every day!
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#10
So today, Saturday, I took the car to the RM coffee and cars, when I got home I tackled the idle mixture one more time on a nice hot motor.
What I found was to achieve a ratio of 13.5 - 14 at idle, I needed to turn the idle mixture screws out from just seated to just 1 turn out. The more accurate vacuum gauge I bought was at 17.5"Hg. I found that turning the screws out less than 1/8th turn more, made a huge difference. The AFR would drop from around 13.5 to 12. Hard to believe how sensitive it is. After taking the car out for a blast, I noticed the idle numbers changed to around 12.5. After it cools down, I'll restart it and see if it goes back to the 13.5 area. I have decided to leave the rest of it where it is for now. With the current humidity levels, it's not "coughing" like it did, so in reality I don't think I have as serious an issue that I thought I had. The car pulls strong and fuel mileage is pretty good, so unless the problem reoccurs, it is what it is.
Besides, I need a new exhaust system before another carb anyways.
I'll go over all the advice given for reference later.
Thanks to all for your help,
Geoff.

Update; after the engine cooled down, on restart, the AFR reading was 14.2 ish. That suggest engine and carb temp play roll in the air fuel ratio. Interesting!

I learn something new every day!
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