Holley street avenger help

Discussion in 'Engine Topic' started by delaware67, Nov 24, 2020.

  1. delaware67

    delaware67 Veteran Member

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    I’m running an edelbrock EPS intake, initial timing at 18. Ordered an accelerator cam kit and a 6.5 and 9.5 PV. Seems to me when I mash the pedal the lean condition comes in when the PV should be opening. Which makes no sense since that should be adding fuel. Carb is jetted up to 72 on primary, runs a bit rich so might drop down to 70’s, orange cam in #2 hole with a 8.5 PV. Cam was in #1 hole but seems better in #2.
     
  2. mrluckies

    mrluckies Veteran Member Lifetime Gold Member

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    An important thing to understand (and contrary to popular belief) is that an oxygen sensor is exactly that - a sensor of oxygen. It does not detect air fuel ratio. The air fuel ratio is determined by a mathematical formula on the gauge and based on the fuel used. The reason I bring this up is because your engine can be running so rich that it misfires and the oxygen sensor will detect excess oxygen because of the misfire. As a result the gauge will say the air fuel ratio is really lean, when in actuality it’s because the engine is so rich that it is misfiring. I found this out the hard way back in 2007. I would have the same off idle stumble you are referring to and I had just read the car craft article on changing the idle feed restrictors. I thought for sure based on my o2 data logging that the problem was a lean stumble. I kept richening the circuits and changing squirters and pump cams. Sometimes it seemed better, sometimes not. But the car never got better. Eventually someone else told me about the issue and also asked if the car really stunk when idling. It did and would make my eyes water, but no black smoke.
    The point is, the o2 sensor works really good when you’re close and not so good further out and will confuse you more. Out of the box, any carb should be fairly close if it’s the right size for your engine. You shouldn’t have to go up or down too far. If you do, there’s usually a different problem. Granted, you can get a bad carb occasionally, I’ve had one (not the carb I mention modifying above, it was a Barry Grant Demon that had machining chips in it, which was a common problem back then). I cleaned and rebuilt it and it’s been great since. But usually, it’s something else. In my case, my timing was way off because my timing mark on the damper and the tab were off. I used a piston stop and found true tdc and put a bunch of base timing in. And that solved my off idle stumble.

    pull your spark plugs, how do they look? Black is obviously rich, white is lean and speckles on it means it’s really lean. this is the idle circuit. The mid circuit and wot circuit effects are seen further down the porcelain inside the plug, not at the tip. Also, look at the ground strap. It should have a heat indication on it. The heat indication should be somewhere above the threads, but below the 90 deg bend on the ground strap. That indicates timing is about right. Heat range of the plug is about right when the first two or three threads of the plug are heat affected compared to the ones further out of the cylinder.

    The idle and main circuits need to be close (and really should be right on) before you mess with the transition circuits (pump shot and in your case vacuum secondary).

    16 inhg at idle sounds good for an engine with a mild cam that has some overlap, but for a near stock engine, that could be a little on the low side.

    with the engine warmed up and idling, carefully and slowly try pushing the lever of the accelerator pump under the float bowl (don’t move the throttle! We only want to see the effect of fuel, not air on the engine). By adding this extra shot of fuel, does the engine pick up speed? If yes, it is lean and wants more fuel. If instead it stumbles and slows down, it’s already rich and adding excess fuel makes it worse. Another thing you can try at your own risk, if your engine doesn’t backfire, you can carefully try to place your hand over the top of the carb, but don’t totally cover it or else it will stall (or with a manual choke you can try to close it some). As you do this, if the engine starts to pick up some speed, again it could be lean, because you are now limiting air flow. Be careful the car doesn’t back fire and burn your hand. Also you don’t want something sucked into the engine. This will help ensure your idle circuit is good. Assuming your main jets are close, you can try the transition. For a mild engine, it should idle around 700-800 rpm. If so, you need the standard hole on the pump cam. The other hole is only used for cars that idle above 1000 rpm as previously mentioned. Because they are already idling at 1000 rpm, the pump cam would miss some of the ramp action because the throttle is already open further. The second hole delays the onset of the pump shot to account for this.

    is your engine/carb new or used? A common issue on old carbs is the idle and main air bleeds getting gummed up and causing circuits to run rich.

    sorry for the book. Let me know if you have questions.
     
  3. delaware67

    delaware67 Veteran Member

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    Thank you for the info, probably won’t get time to work on it again for a few weeks but will report what I find
     
  4. Crusty75

    Crusty75 New Member

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    Hope I’m not too late to the party here, but I actually just finished tuning one of these carbs for my stock(ish) 350.

    Before you spend even another iota of a second on that carb, replace that primary metering block with a standard 1850 block. Long story short, you will never get that lean transition sorted out with the stock metering block. I bought an allcarbs refurb block for $30 and it runs like a dream. After the swap I found my transition circuit was now overly rich, so I used an .011 guitar string in both IFR’s. Your mileage will vary.

    Let’s talk about the accelerator pump setup next. Stock for my carb was a #31 shooter with the orange cam in hole #2. IMO, for the type of engine this carb is meant for, this is WAY too much. I had a bad rich stumble with that setup. Went to a white cam in hole #1 and a #28 shooter and that did a lot, but I think it really wants a #24 or #26, still a little too rich. You shouldn’t need over a #31 unless you have something real stout.

    Best of luck to you getting this carb dialed in, these 670’s really are great carbs, but it takes close attention to EVERY circuit to truly be the best it can be...

    Cheers! :bowtie:
     
  5. mrluckies

    mrluckies Veteran Member Lifetime Gold Member

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    Yes, the idle circuit on the avenger and other new street carbs is leaner than you used to get from an out of the box universal carb. If you look at the idle air bleed on the primary side it’s huge compared to the secondary side and the high speed air bleeds as well as what they used to be on the out of the box carbs. But that is what most mild engines want on the street and lead to better fuel economy and drive ability. The idle circuit being lean likely isn’t causing his stumble issue as Holley got them right for the street. More likely it’s timing or tuning or a vacuum leak somewhere. As stated before, I wouldn’t solely rely on the o2 sensor to say that the engine is running lean. When you get too far from center it can get confusing. I can’t stress enough how important it is to verify the tdc mark by using a piston stop, never trust its right as there’s too many variables. Then set base timing to around 12-16 deg. Disconnect all vacuum consumers that you can and still be able to drive the car (ex: the vacuum lines for the ac vent actuation). Obviously leave the power brakes hooked up (do they work good btw?). Tune the idle mixture screws 1/16 of a turn in the same direction on all four corners and see if vacuum improves. Do this a few rounds to get it tuned in. Unless there’s junk in the carb, it should be fairly close out of the box.

    What vacuum do you get at idle and what are your cam specs?

    You can do this tuning with the vacuum brakes disconnected too and then before taking a drive hook them up and see if it changes the vacuum reading much. The key is to eliminate as many variables as possible and deal with each circuit individually. Carbs can be 4 jet sizes out and still run somewhat ok.
     
  6. Crusty75

    Crusty75 New Member

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    Well... Yes, with enough primary jet this problem will disappear, but it will compromise other parts of the curve... The real problem here is that the SA670’s primary metering block has zero emulsion bleeds in the wells. That’s why I went with an 1850 block, because once I cracked this carb open and looked I knew I would never be able to get the primaries right. Using the 1850 metering block really makes this carb run the way it should IMO. The other common “fix” on these is to enlarge the idle feeds from .028 to .031, but again, I’d be afraid of it being a bandaid, rather than a fix.

    You do raise a good point with the O2 gauge. I didn’t use one in my tuning and I feel like it sped the process up tenfold. The car told me what it wanted, I listened, and now it’s just an absolute joy to drive. Gets better mileage than my Qjet did too, and it didn’t do bad itself! The thing is though, is nearly every aspect of this carb had to be adjusted and tuned. It was so badly out of whack before I touched it, it’s amazing it left the factory that way... And my combo is exactly the kind this carb was meant for... Chances are, the OP’s combo is hotter than mine, and the problem(s) will be even more aggravated than it was in my case.

    If OP is still listening, you should drop some more details on your combo. If we’re close on that I’d be happy to give a more detailed spec on my tune to give you a good starting point, because the factory calibration sure ain’t it...

    EDIT: Thought I’d throw in some useful timing stuff too... Probably not the “right” way per se, but worked like a charm getting my combo dialed in... I didn’t trust the timing mark on my balancer, or the light for that matter, so I over advanced the distributor and drove the car, backing off until it drove and felt right, and took a schoosh more out when I got to that point. Worked like a charm, and I’m confident my balancer would show 25+* at idle. If you have an MSD box, be very wary of what your timing light is telling you.
     
    Last edited: Dec 21, 2020
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  7. G72Zed

    G72Zed Veteran Member

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    Wow, talk about the QC going down the tubes, maybe it's just on Holley's economy line. The only metering blocks I have done that did not have any emulsion bleeds was the pair of 390 TR carbs with the 7204 metering blocks but used double kill/anti-siphon bleeds, did these just last week.

    I have rebuilt and tested/tuned 7 different Holley's the last few weeks, all performed as they should, some needed work, or repairs/mods to get them to the next level. The worst ones were the shiny aluminum ones, over/under drilled, bad N&S, but after repairs ran great everywhere, made 734HP on a dual quad street Hemi.

    But I have not seen missing emulsion holes altogether yet, that's a new one.
     
  8. Crusty75

    Crusty75 New Member

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    The really sad part is it isn’t even a QC issue, it’s a crappy design issue. I’m fairly certain all SA670’s are set up like this. These metering blocks really are hopeless, save the occasional success story on a lucky few builds. I was not one of them :(

    The metering block I replaced it with wasn’t even a good one, THAT block was a QC nightmare. But yet, it still runs night and day better than the original!
     
  9. G72Zed

    G72Zed Veteran Member

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    Interesting, in the SA670-770 I have seen kill bleeds not drilled through or missing, and Emulsion holes bored but not drill. But not even present is a new one.

    I always buy the Original/Zinc or HP models for my customers, the last 4 new carbs where OK and ran great on the dyno.
     
    Last edited: Dec 23, 2020
  10. delaware67

    delaware67 Veteran Member

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    Thanks for all the input. My setup isn’t radical, ZZ4 with an edelbrock EPS intake. Carb is mounted with a 1/2 open spacer. I have had other carbs on the car, started with a holley 750(because that is what it apex’s for from gm) but had problems with it running consistent. Took the carb apart and found a lot of debris in it. Sent it back and got an edelbrock avs 650. Car ran good but felt like it didn’t have the power it should, seat of the pants feel. Went to the holley and power feels a lot better. Car runs great except for this stumble. Will do it in park or from a dead stop when the peddle is mashed. If I give it alittle gas there is no problem. All the other carbs I’ve had seemed to want more fuel to get the car to run good with no hesitation. Currently have base idle around 18, seems to like there, no hot start issues and no knocking. Carb is currently setup with 68 primaries, 75 secondaries, 8.5 PV, 35 nozzle and the original orange cam in hole 1, and yellow secondary spring. Car pulls about 18” vac in park and about 15 in gear. I’m not the best carb tuner so not sure how to tell the difference between a lean or rich stumble, that’s why I go by the wideband. Mash the pedal in park and no smoke out the tailpipes so I don’t think it is going rich. I must be overlooking something simple, overall car runs great it’s only this one small issue. I ordered a pump cam set in case that was the issue but like all the rest Of my mail it seems to be lost.
     

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