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Carburetor Problem?

Posted By charliemccraney 5 Years Ago
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charliemccraney (4/3/2019)
...I started getting the 670 ready.  It's an aluminum Street Avenger, part 0-85670.  It has 4 corner idle.  Holley's instruction manual says we know best, leave it where it is and don't question us.  I'm paraphrasing, of course.  They should know best but the baseline spec would be nice so it can be verified.  The primary screws have 1 1/8 turns and the secondary screws have 3/8 turn.  David Vizard suggests in his book to start 2 turns out on all four.  Should I leave it as Holley set it or would something else be better for the initial startup...

Now that you know what the factory fuel mixture screw adjustment is, you can always go back to it if you decide to start the engine with a different initial f/m adjustment.  I’m in the David Vizard camp on the four corner fuel mixture idle adjustments.  I like to start with all four screws at 1½ turns out and keep all adjusted the same when making fuel mixture adjustments.  When making adjustments whether leaner or richer, I’ll make the adjustments in 1/8 turn increments.



Lorena, Texas (South of Waco)


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Well, I still have work to do tuning but I'm going to give it a cautiously optimistic it's fixed.  I started it. It idled great, surprisingly smoothly almost instantly.  I haven't run it very long, yet.  Just wanted to make sure it would start and idle.  The other carb did fool me by idling again at one point, so we'll see.

I found more problems with the new carb, right out of the box.  After installation, the accelerator pump wasn't working.  I pulled the bowl off to take a look.  It was assembled as incorrectly as it can be assembled.  The spring was on the outside and the diaphragm was upside down.  Without a close look, it seemed like it was working when I was checking it out on the bench because the lever was still pushing on the spring.  I suppose the loose screws should have clued me in to take a closer look.

I decided to leave the idle screws as they were delivered.  Assuming it is deliberate, they must know best about the initial setup for their carb but the issues make me wonder.  I bought this carb years ago.  It could be a fluke or they could be better now... or worse.  I didn't have any of these out-of-the-box issues with the 570.  I didn't even check that one so maybe I was lucky.


Lawrenceville, GA
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I got idle dialed in and the preliminary tuning of the accelerator pump and it's doing well.  The idle screws only needed 1/8 additional turn out.  Any more or less and vacuum dropped off.
For some reason, when one of the transfer slots is approximately square, the other is still slightly rectangular.  I guess that may just be the result of manufacturing tolerance.  I found that I'm able to reach the secondary idle speed screw with a small screwdriver, thanks to the 2" spacer I'm using.
Still need to check/adjust secondary actuation and need to see if my regulator works.

The 570 was really lean for this engine in the out-of-the box tune.  Reving the engine resulted in the rpm staying high for a couple seconds then coming down and it felt really sluggish.  It ended up 7 jet sizes bigger to be acceptable.  This 670 does seem to be a much better match out-of-the box.  My O2 sensor is not working so I'll probably wait until I get the gauges installed with the new wide band sensor to get the mains dialed in.  It should be good enough to drive. If the plugs fowl or mileage is considerably  worse, those should be clues, too.

It's amazing how many things I found that could have been causing this problem, some seemed to be obvious and the problem still isn't obvious to me but was something to do with the carburetor.  Maybe some of that contamination stopped up a part of the idle circuit in the metering block.



Lawrenceville, GA
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I think your story is typical with Holley. We bought a 750 double pump for our dirt 1/2 mile car and before we ran it took it to SMI ((Shawn Murphy) in Huntington Beach well known in the performance carb business. We heard and he showed us everything that would have made a struggle out of the box... It seems like this should not be but quality control seems to be lost today..good luck.

56 Vic, B'Ville 200 MPH Club Member, So Cal.
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I haven't done much more, yet because I noticed that my temperature gauge stopped working so i want to fix that before going out on the open road.

I did check the regulator and it does adjust now.  So maybe the failed pump was putting out enough pressure that the regulator could no longer control it.  I was also able to set it to 6psi and it will go higher but I did not see how far so there may still be something going on because I'm sure I could not set it that high before, which is why I got the higher pressure pump.  I'll keep an eye on that.

I contacted Holley tech to verify the baseline idle screw spec and they said 1 1/2 turns on all 4.  So assuming that the tech guy knew what he was talking about, that's another thing that was wrong out of the box.  I'll try adjusting them equally and see how that does.


Lawrenceville, GA
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Just an update.  I still haven't done anything else regarding fine tuning.  Since it does seem to be running fine, I'm focusing on other projects that I already had parts for and had hoped to be done by now, until the original subject happened.

So far it appears that mileage remains the same and it feels like it could be more responsive.  That last one is hard to be sure, though because it was a little more than 3 months that I didn't drive it.  It  may have simply been the result of getting used  to it again.  Big difference in terms of acceleration between it and my car (stock and slow).  So it seems that a bigger carb is not necessarily worse, if the engine can use it.  It's all in the fuel curve.

6/25/2019


Lawrenceville, GA


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