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  1. #1

    Default Holley Truck Avenger 670 CFM

    Good morning everyone,

    Like many Truck Avenger carb owners, I have been fighting to tune my carb so it runs properly for nearly 2 years. My reoccurring issue is a bog/lack of power starting around 2,000 RPM under WOT conditions. I've called Holley Tech Support numerous times and each time they would provide some recommendations none which fixed the issue. As a result of their speculation over fuel pressure, vacuum signal, etc, etc, etc I ended up building a data acquisition system that pulls the Air-to-Fuel Ratio (AFR), Fuel Pressure, Tachometer signal, Vacuum, and Throttle Position (video included). The last call to Holley Tech with the supporting data led the Tech person to tell me something to this extent: "The carb is not designed to do what you are doing. This carb is designed only for rock crawling and hill climbs. Sorry that our info isn't clear about that." My jaw about hit the floor when I heard this. I've purchased the jet kit, the spring kit, the cam kit, and nozzles all to find out after numerous calls my carb isn't designed to go WOT?! By the way, the primary use of my vehicle is rock crawling and hill climbs with the occasional runs across the sand dunes at mid-high range RPM in gears other than 1st.

    I have tried numerous things from changing out the nozzle sizes (31 to 35), secondary springs (lightest to heaviest), jetting to achieve proper cruise mixture, power valve (2.5 to 6.5), etc. I even shipped the carb back to Holley where it was wet-flow bench tested. The only compromise I can get to work is running the primary jets so rich (10:1 AFR) that I can mask this lean spot. The last recommendation by Tech Support was to try bringing in the secondaries sooner (which I already tried this and made the condition worse). Otherwise the Tech guys recommendation basically was that I should get a different carb in order to use WOT on occasion.

    Lastly, I've confirmed that it is the secondaries causing the lean condition by tying them shut and recording the resultant AFR. Also, the video below shows that there is no fuel entering the secondaries when they are open for nearly 1,000 RPM.

    Tuning Data and Video:
    This video shows me trying to tune the carb doing a 3rd gear pull WOT (eliminates transient spikes and holds ranges longer). You can see that the secondary throttle plates crack open around 2,100 RPM and no fuel enters the venturis until around 3,100 RPM. The AFR trace shows I miss my bogey of 13.5:1 WOT by 2,500 RPM and things get worse from there. The AFR graph is bad but would be worse if I actually held WOT for the entire range.

    https://youtu.be/uGqjOs2_mIQ

    Info:
    Vehicle: 1967 IH Scout 800
    Carb: Holley Truck Avenger 670 CFM ("-2 model")
    • Fuel Pressure: 6.75 PSI Idle
    • Primary Jets: 67
    • Secondary Jets: 97
    • Power Valve: 6.5 in-Hg Standard Flow
    • Secondary Spring: Black (stiffest)
    • Nozzle Size: 35
    • Accel Pump Size: 30cc

    Fuel Pump: Holley Mechanical, Keith Dorton Series, 170 GPH
    Motor: Chevy 350 CI (Bored 0.040" over to 357 CI)
    Cam: Mild RV style
    Heads: Double-Hump Heads
    Intake: Performer Dual Plane
    Exhaust: Block hugging headers w/ full exhaust and dual Thrush mufflers

    Does anyone have any other recommendations for me to try? My next step will be to get rid of this carb, and move on with my life.

    Thanks in advance for the help! I hope this data helps shine some light for other Truck Avenger carb owners.

  2. #2

    Default

    First off. Let me say, great troubleshooting on your part. Tons of details and a video with throttle position, rpm, AFR. That is a great way to ask for help. So I guess I will be the first to offer it.

    Not sure why your making a pull from 3rd gear at 1500rpm to WOT. I didn't hear a down shift on the transmission either. Your putting a hard load on the motor by doing that. That's like taking off from a stop sign in 3rd gear, it loads the motor up.

    I think you have exaggerated the problem by putting the heaviest spring possible. Notice right at the 30 second mark when you go full throttle, the secondary throttle blades are just fighting it as much as they can to open. Its fighting the stiffest spring you put in. Notice that when your secondaries do start flowing right around the 40 second mark your AFR starts to enrich up pretty quickly. That's the big 97 main jet you have installed. Putting in a larger main jet wont get fuel to start flowing any faster. It will just let a bunch a fuel flow when there is enough air to start pulling fulling from the boosters.

    For mains (primary or secondary) to start flowing gas, air has to be flowing through the boosters. The venturis and booster have a "squeeze" affect on the air to induce faster flowing condensed air. This helps the primary and secondaries to get fuel to start flowing. Take your camera and drive around easy. (Make sure not to open the secondaries). Video only the primary boosters. You will see that they don't start flowing either unit around the 2000rpm range. Again air has to be flowing before fuel will start to flow. That is why you have and idle, transfer, and accelerator pump. These 3 circuits compensate with fuel. Air will always react faster then fuel in a carbureted setup.

    Also at about the 1:40 mark you get a quick lean spike with just a quick blip of the throttle. That's your accelerator pump linkage and cam. It is very essential that linkage is adjusted properly so that the very instant you move the throttle your accelerator pump is squirting fuel. If your linkage is setup properly with no slack and it operating as stated above then get a more aggressive pump cam. Your nozzle size of of 35 should be pretty good.

    You gotta get the secondaries to open faster.
    Your secondary main jet is way to big. One thing I learned while messing with carbs. Make 1 change at a time.
    Make sure timing is properly set. 16-18 degrees initial and 35-37 degrees total. All in by in 3000 rpm.
    Make sure accelerator pump linkage is adjusted properly. Get more aggressive cam if necessary.
    Quit going to WOT from 3rd gear at 1500 rpm. Either get your downshift linkage working properly or manually dropped to 2nd before you go WOT.
    Last edited by 69cam; 08-24-2015 at 11:13 PM.

  3. #3
    Join Date
    Nov 2014
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    Ont. Canada
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    1,384

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    Doesn't make much sense, you have a carb with secondaries, but it wasn't made to go to WOT. Then what are the secondaries there for?
    All very good suggestions by 69cam, which need to be addressed.
    My thought would be to not necessarily to get the secondaries to open sooner, but to get the secondary booster to flow sooner. When the secondaries open, they draw fuel from the transfer slot. When the flow limit of the TS is reached, the boosters should take over and start to flow. This is not happening. The only way I see to fix your problem is to adjust the HS air bleeds, kill bleed or emulsion bleeds. Have you compared the air bleeds on the primary side, to that of the secondary side?

  4. #4

    Default

    Thanks for the reply 81 TransAm. I completely agree with your question. When I shared this information with co-workers who are really into this stuff, they couldn't believe what I told them. We've all been super surprised by the amount the secondaries are open with no fuel flow. I guess I need to try 69cam's suggestion of taking video, of low throttle applications on the primary side.

    Unfortunately, the Truck Avenger has no HS air bleeds, and this is what the Holley Tech pointed out as why it wasn't designed for what I'm doing. I did compare primaries to secondaries, and there is a big difference in size for the idle air bleeds (primaries are much larger).

    I've tried adjusting the throttle blade on the secondaries, to see if perhaps I'm pulling too little/too much air in at idle. No luck there either. I'm running out of ideas (for this carb).

  5. #5

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    The reason for the low RPM WOT run was to mimic a dyno run and to do so the kick-down cable was removed on purpose. Running the carb this way exposes the lean spot over a much longer band where the transients of the accelerator pump, power valve circuit and primaries mask the condition. The lean condition does exist even if I perform a 1st gear WOT Run (see the first photo).
    I agree running the carb that way does mimic a dyno run, but it's not practical for street driving, and really exaggerates your problem. I can't see your pictures. I know the admin of the board has to approve posts with links, so your links to picture may not be site friendly. That being said, I get your reasoning behind doing it, and agree it is a problem.

    If you were to search the internet for issues with Truck/Avenger carbs, I think you would see this is common complaint. I'm not sure if this is a design flaw with the metering blocks or what, but I think we can get you better then you current situation.

    tried the black vs. white spring as well on matching setups (see the two pictures at the bottom). Most of the enrichment you are seeing around 40 seconds is due me to dropping throttle significantly. FYI, the reason I let up on the throttle was my buddy's farmer neighbor was out front and he complains about people ripping down the road . If you bump through the video slowly you'll see that my drop in throttle perfectly times the significant increase in the visible fuel rate
    If you notice when you drop the throttle is shows 100% down to 60% but the secondaries don't blink 1 bit. They are still right in the same position they were when you are at 100%. I watched this over and over and they might move a little but no where near the amount vs the position of throttle change. This is consistent with the spring you have in. That spring being so heavy causes the secondary to have an average open rpm at about 2800 rpm and never fully open. Below is pic of Holley spring rate chart
    If you look range the springs open over an rpm range i tend to favor the purple spring due to the fact it opens over a shorter rpm range. It opens over a range of 5000 rpm vs 6000 rpm. I don't like the fact of how early it opens. I'm wondering if you got the secondaries to open a little early while your primary was on the rich side if that would help.

    Click image for larger version. 

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    I see that your spring is opening quicker. Did you have the black spring in when you did this video?

    I think what you need to do is try to break the carb down into 2 sections. Primary vs secondary. Lets get the primary side working really well and then work on the secondary side. Remember full throttle is combination of all circuits working together. I think you can get the primary side to compensate for the lean area till the secondaries start flowing fuel.

    Drive around watching your AFR gauge and do not get into the secondaries. We only want to work on the primary side. I think what you could try is to bring the primary main jets up to about 70. Let's get that circuit running a little on the rich side. Remember that cruising and low speed acceleration will come from idle circuit and transfer circuit. Anytime you change your primary mains you will need to readjust your idle mixture screws. Get your idle mixture screw to get you best vacuum at idle in gear. I would then back each screw out about 1/8 turn just to enrich it up a little.

    Drive around making some tests with about 35-40% short burst throttle. Disconnect the secondaries if you have to make sure they don't open. Make sure the initial opening of throttle doesn't lean spike. You don't want an initial lean spike and have to compensate for that as well. Once your rpms are getting around the 2000 pay close attention to AFR. If its climbing go up 1-2 main jets until you can get your AFR to quit climbing and at least maintain a steady rate or start enriching it up. It probably would be better if you go up on main jetting until you see the AFR start to enrich it up. With your carb there is going to be a transition phase that when the secondaries open up air will be flowing first before fuel starts flowing. It is going to start to lean out during this transition. Your secondaries have no high speed air bleed and no accelerator pump to help compensate during this transition.

    Once you get that sorted out, double check your secondary throttle rod and linkage. Make sure all of that is moving freely. Also while digging around the internet I saw many many post about people finding casting flake or just junk in their metering blocks. I would disassemble and use some compressed air to blow through all of the passages on both primary and secondary metering blocks. Im not saying there is something wrong with your but just be sure.

    Also, adjust your secondary float adjustment so that the fuel level is high in the bowl. Not so much to where it is dripping out the booster but close. You want fuel to start flowing quickly and easily. You can adjust this out later if you get to the point that your secondary is running rich during the transition phase. Leave your secondary main jet alone for now. We know its big once fuel starts flowing but lets see how it works initially after to making the other changes.

    Something I found on another forum was some interesting reading to how the user took his carb to a shop and the mechanic fixed a very similar issue to yours. He advised the same as I have but also did something different with the accelerator pump circuit. He used an accelerator pump cam that had a long gradual increase. Put in an #25 pump shooter. I believe this was done to make the accelerator pump have long consistent pump shot over time. (relative to other pump cams) Effectively trying use the the primary pump accelerator to help with the transition phase into your vacuum secondaries. He did raise the guys primary main jet up 4 size from stock. I'm assuming that helped some as well.

    Found some Holley vac. secondary info with spring kit. I assume you have this already.
    http://documents.holley.com/199r8219-2rev.pdf
    https://www.youtube.com/watch?v=4xOiJQj6CSU

    Hope this help some.
    Last edited by 69cam; 08-26-2015 at 04:35 AM.

  6. #6

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    Hopefully the admin approves my pictures. I have many more screenshots I can share to help with the questions.

    The video that I posted was using the black spring.

    Previously, I have ran around 2,000-2,500 RPM steady state as my test range for cruise. With the current setup of 67 primary jets at 2,250 RPM I am producing a AFR of 11.5-12.0. If I go up to 68 jet sizing, previous runs have shown a AFR of 10.5-11:1 and that was with the secondaries tied shut. The best I've had it running was with a 72 primary jet and that was causing my 2,000-2,500 RPM range to read sub 10:1 AFRs (meter only reads to 10:1). Is my testing at too high of a RPM range? The link below has been my guide to tuning so far, which recommends 2,000-2,500 RPM as Holley cruise circuit testing.
    http://www.stangtv.com/tech-stories/...y-carburetors/

    I will look at increasing the secondary float levels to see if I can move it up at all.

    I've read a lot about the metering blocks having casting flakes. Holley sent me set of brand new replacement metering blocks which yielded the same results. I've also read a lot of complaints about the Truck/Avenger series carbs not tuning very well. That was partly what drove me to capture this data and share it with the community. I'd love to see Holley produce a Street Avenger carb with high speed air bleeds but Truck Avenger modifications as well. I see a need in the off-road community to have a vehicle that both performs in off-camber situations as well as on the sand dunes. I know a lot of people that run both scenarios and this type of off-roading is becoming very popular due to the Ultra4 off-road circuit.

    Interesting concept for the accelerator pump. I don't currently have a #25 shooter laying around, but I'll see what is the smallest I can try with a long duration cam in place.

    Yep, I have the spring kit, cam kit, jet kit, and larger nozzles. I've also moved up to 6.5 power valve.

    Any thoughts on using a high-flow power valve to supplement until the secondaries catch up? Thanks again!

  7. #7
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    22,683

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    Quote Originally Posted by 69cam View Post
    I can't see your pictures. I know the admin of the board has to approve posts with links, so your links to picture may not be site friendly.
    Quote Originally Posted by delvin View Post
    Hopefully the admin approves my pictures. I have many more screenshots I can share to help with the questions.
    It's not a moderator approval picture problem. His pictures are not in my moderation queue. The only thing I can do as a moderator (picture-wise), is enlarge the existing picture (meaning it's already posted on the forum). You're somehow not posting the pictures properly. Pictures need to first be hosted on a picture hosting website. I like using Photobucket.com. Then use the "Insert Image" icon above.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  8. #8

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    Thanks for the reply 69cam. I'm going to try and respond to each section as best as I can.

    The reason for the low RPM WOT run was to mimic a dyno run and to do so the kick-down cable was removed on purpose. Running the carb this way exposes the lean spot over a much longer band where the transients of the accelerator pump, power valve circuit and primaries mask the condition. The lean condition does exist even if I perform a 1st gear WOT Run (see the first photo).

    I tried the black vs. white spring as well on matching setups (see the two pictures at the bottom). Most of the enrichment you are seeing around 40 seconds is due me to dropping throttle significantly. FYI, the reason I let up on the throttle was my buddy's farmer neighbor was out front and he complains about people ripping down the road . If you bump through the video slowly you'll see that my drop in throttle perfectly times the significant increase in the visible fuel rate. I have noticed that a reduction in my WOT request will fatten the mixture and provide better performance but who want's to do that?!

    I agree on the accelerator pump circuit. It could use a bit of tweaking. If I remember correctly, I am running the blue cam but I'd need to confirm. I don't think the accelerator pump will mask my lean spot even on a 1st gear WOT request because my lean spot is 2 seconds in.

    I agree on the secondary jet being way too big. This was my last ditch effort to see if magically I could make things better. Nearly every change you see from stock was made 1 at a time with data runs between. Not one yielded good results except for in the second photo below where secondaries were tied shut. You can see the nice consistent AFR reading, albeit too rich, through the entire RPM band.

    Lastly, I have verified timing at some point in the process. Of course, at this moment, I can't remember what it was but the numbers you stated sound very familiar.

    Not sure if any of the above additional information changes your recommendations? I feel like I have tried nearly everything listed based on countless hours of reading/researching through Holley tuning recommendations and the data.

    Key for attached photos:
    Top data section:
    • Vacuum signal (Green)
    • Throttle Position (Red)

    Bottom data section:
    • RPM (Green)
    • AFR (Red)


    1st Gear 4WD High WOT Run with Lean Spot:
    Attachment 1357

    3rd Gear 2WD High WOT Run with secondaries tied shut (NO LEAN SPOT, yay!):
    Attachment 1358

    Black Springs vs. White Spring with same motor setup:
    I did try to use the bookend technique by running the carb with the black spring first and then the white spring second. I've tried springs in between (without the data) and the black spring 'felt' the best. You can actually see the motor nearly dies with the white spring if I didn't lift throttle. This was very repeatable.
    Attachment 1355
    Attachment 1356

    Thanks again!

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