Holley SA 670 tweaking. Need advice.

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.

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.

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.

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.

Click to expand...

 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.

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.

Click to expand...

Thanks Don, that's the plan, one thing at a time...…….. before I spend much more money.

Geoff.

Found this tuning guide a few years back, gives a pretty good step by step with lots of information.

http://brucerichards.com/tuning.htm

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

Found this tuning guide a few years back, gives a pretty good step by step with lots of information.

http://brucerichards.com/tuning.htm

Click to expand...

Time to study! thanks

Geoff.

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.

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!

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!

Click to expand...

Don't get caught up in chasing a number. The A/F gauge is a great tool but it is not the end all be all. Getting the car to run as well as it can within safe parameters is the goal. Seat of the pants (and/or the time slip) and how the plugs look have the final say.

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!

Click to expand...

Don't get caught up in chasing a number. The A/F gauge is a great tool but it is not the end all be all. Getting the car to run as well as it can within safe parameters is the goal. Seat of the pants (and/or the time slip) and how the plugs look have the final say.

Click to expand...

Tommy, yes agreed. It's more that I like playing with a new toy and to get it as good as I can. The idle mixture is of course, just that, has no effect on the overall running under power. Playing with the idle mixture is about reducing the rich idle and loosing some of the gas smell in the garage... to keep the wifey happy!

Apart from the occasional "cough" when I tramp on the gas pedal and that definitely seems to be atmospheric related as well as mechanical, I'm getting a good plug read, light brown, so I'm not worried too much about the overall performance, but more about getting a small bug out of it, i.e. the "cough".

Carbs are not my area of any sort of knowledge, but I learn something new every day..... or try to.

Thanks for the input,

Geoff.

Finally decided to leave it well alone. The idle mixture settings are about as good as it can be now and it's running as good as I need it without spending money to get what, a slight cough at hammer time gone. I can play with a few other tweaks as and when I feel like it, but for now it's on to the more important issue, ridding the exhaust of drone!! See new post!

Geoff.

Today I tried one more "tweak" I changed the shooter, squirter, or whatever it's called, from a 31 to a 35. So far no more "cough" seems to have done the trick.

Was it really that simple!

Geoff.

Great news! Hopefully that’s the final fix for it. But ya, sometimes you don’t think something so simple would work.

Great news! Hopefully that’s the final fix for it. But ya, sometimes you don’t think something so simple would work.

Click to expand...

 I'm going to play around with one more "tweak". My friend with the Boss 302, lent me a Holley 650 double pumper that was on the car when he bought it. It's practically brand new and apart from being incorrect for the Boss, it worked well. At first he thought it was a 750, but alas it's a 650. I do have to take the 670 off so I can adjust the high idle speed on the choke, I just can't reach the screw under the choke body with out pulling it. Along with that, I want to clean and repaint the top of the manifold so it looks fresh for the show next month. All part of the 'plan' 

I had the car out last night for a local A&W Cruise Night and happy to say it was running pretty damn good in hot humid weather.

Geoff.

FINALLY, today I solved the problem!! I have gone through switching my gas to Shell 91 from another ethanol free premium brand, bad gas thoughts, to stripping and rebuilding the carb, to new plugs, to switching out my Pertronix II (only to find the brand new, never been out the box in the 3 years it sat on my shelf P II was defective and NFG) to FINALLY finding a hair line crack in the distributor cap. That's all it was!! So why didn't I think of that sooner?? What I figured out was the 50000 volt coil is too much for a stock cap. There are burn marks on the contacts, but nothing that would scream out at you. In checking, Pertronix have a cap FOR the Pertronix coils, so I think I'd better order one soon.

Thanks to all for your input, 

Geoff.

Interesting, always surprising when it's something simple, but as in this case nearly invisible.

So, is the P II OK, and it was all the cap?

It's possible the cap was cracked out of the box, and the problem only showed up after installing the high voltage coil. When I've found cracked caps it's generally been due to water intrusion, but when we don't expose these cars to water that doesn't happen, takes something else.

I'm glad you got it sorted out, always a good feeling.

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.

Click to expand...

+1 on Boilermaster's post, very sound advice. I'm not a fan of any of the street avenger carbs. They all run very rich out of the box. Confirm float levels and fuel pressure is correct. The idle channel restrictor (idle jet) and main jet size is too big. The idle insert restrictor needs to be removed and the hole drilled and tapped to accept restrictors with a smaller hole (like changeable jets). Doing this requires proper tools and attention to detail. Set the transition slot exposure to .020-.040 with the idle speed screw. If more than about .050 of the transition slot is exposed at idle the stumble may be caused not having enough of the slot left to support the transition from idle (not the accelerator pump calibration). Optimize mixture screw settings (1.0-2.0 turns out). Fine tuning can be done by changing the size of the idle air bleeds (idle air correctors) and the mixture screws. Check the transition and light cruise next (14.5-15.0). If the idle circuit is right, the light cruise should be right or close to it. Light cruise runs on the idle circuit until about 2500 RPM before the main circuit begins to pull fuel through the boosters. If the light cruise mixture ratio gets richer as RPM increases the idle air bleed is too small. If it gets leaner as RPM increases the idle air bleed is too big. 

I'd get the idle ratio to 14.0-15.0 range then get the transition/light cruise in the 14.0-15.0. This may require main jet and idle restrictors changes. After the idle and cruise are right move on to the accelerator pump circuit. You should not need a .035 discharge nozzle. If you go from near idle to WOT and the engine bogs and doesn't recover the nozzle is too small. If the it initially accelerates, then bogs, and then recovers the duration of the shot is too short and a different cam setting or different cam with a longer duration profile is needed. You do not need the black spring in the secondary. The purple or long yellow spring should work for you. The WOT throttle setting will likely require the Power Valve Channel Restrictor to be made adjustable (like the idle restrictor). You are looking for 12.8-13.4 at WOT.


All calibrations should be done with the engine at operating temperature and the O2 sensor installed in headers collector or reducer or H pipe before the crossover. Tailpipe sniffers are not accurate enough for fine tuning. None of the numbers are absolute hard numbers as there are many variables.


You light detonation may be due to having a spark plug that is a heat range too "Hot". Based on your build the plug for a Boss 351 should be close. I'd use NGKs as they are easier to read than the Autolite plugs. Inhope you get it squared away. Chuck