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Thread: Newbie To Carb Tuning

  1. #1
    Join Date
    Sep 2015

    Default Newbie To Carb Tuning

    Hey all. I have a Truck Avenger 670 I'm having issues with. A little info first.
    The 670 is on a AMC 401 transplanted into a 83 Jeep CJ7.
    The Jeep sits on 33" tires, has 4.11 gears, and is an automatic with a manual valve body.
    The 401 has been upgraded to a HEI ignition system, and when it comes to the actual internals, I'm at a lost. I bought it from a guy who bought it from another guy that "rebuilt" the engine. It does sound like the cam might not be stock, but don't take my word for it. I'm also running the stock "hi-flow" exhaust manifolds.
    I've had this carb for over a year now. Last year, I was trying to get the right jets installed by reading the plugs after a hard pull, every time I looked at the plugs they were white, so I keep putting in a bigger jet. Do a hard pull and shut it off while still at full throttle, and the plugs kept showing it was lean. After a month of knowing I'm doing something wrong, I decided to have someone with experience tune it for me, he came highly recommended. I got the Jeep back and it ran great, no problems with acceleration or any bogs or stumbles. Only problem, it smelled rich the entire time I drove it. I even asked the "expert" if he replaced the jets, and he told me that they were fine, just that my float bowl was set too high.
    This summer I started to replace the exhaust and decided to install an WBO2 sensor installed. Well guess what I noticed, the Air/Fuel gauge barley moved from a 10 reading. Oh and the gauge only goes down to 10 Yep it's running rich. I pulled the jets and they are the ones I put in last year #75, well so much for that "expert".
    Now I'm trying to fix it myself and learn as I go, so bear with me and dumb it down, a little so I understand it.

    I adjust the air/fuel mixture and the best vacuum reading I can get is 13, this is after it's warmed up and in gear.
    The A/F gauge reads 13.2ish. The screws are backed out only a 3/4 turn.
    The main jets are now a 66 and the secondary are still stock 89. The stock main jets are a 68.
    The power valve has been upped to a 6.5, the stock is a 2.5.
    The pump discharge nozzle is a 28.
    The accelerator pump cam is red.
    The secondary spring is a black.

    At cruise, I and holding steady at 1900 rpm, my vacuum stays at 16, and my A/F gauge reads 12.5.
    On hard acceleration from cruise, the A/F gauge drops to 10, but then rebounds back to 11ish.

    From a dead stop, if I mash the pedal, I stall with no pop or anything.
    From a dead stop, if I half pedal to 3/4 pedal it, it stumbles for a quick second and the pulls hard.

    Now before I changed the main jets, I started with the stock jets first, 68, and it didn't stumble or stall.
    I don't know if I got the stumble or stall when it was at the stock jets.
    (This just dawned on me, I had the wife in the Jeep and she, let's say, "complains" when I act like a 16 year old.
    I need to put everything back to the stock settings, and see what it does on when I floor it from a dead stop.

    Is there anything I'm missing on my info, that would be helpful in me getting this thing tuned for my engine?

    Once I get everything back to stock, and take it for a run I'll report back.

  2. #2


    Hey Swampbog. In short, the off-idle bog/dying could be caused by your accelerator pump. Check to make sure it is properly adjusted, and fuel comes out immediately upon pedal/throttle plate movement. Hopefully someone else will chime in with the handy link to Holley's procedure. Also, I have read several people recommending a 35 nozzle, and blue cam for mild built 350 CI motors, so you may need more shot sooner to help cover the opening of the throttle plates.

    Before you got too far with this carb, you may want to read my post a few done from this one about my 670 Truck Avenger. I have a lot of data, and a YouTube video that shows the issue you are likely having, is outside of your off-idle bog. I found that running sub 10:1 AFR at cruise, was what was required for making my 350 run better. The more I tuned the carb properly, the worst my mid-range & WOT performance became. After I took the video and gathered the evidence, I called Holley and after 2 years of talking with them I finally received a response that made sense. The tech told me this carb is not designed for WOT, but instead for rock crawling & hill climbing. I'm hoping that perhaps you can get it to work for your application, but what I have found is without major modifications to the metering blocks, I can't get it to work for me. Hope this helps and good luck.

    Feel free to ask any questions you might have, and I'll do my best to respond with what I have tried out in the past. 69cam should hopefully chime in, as he has some good recommendations if you are just starting off with this tuning.


  3. #3
    Join Date
    Sep 2015


    Delvin, I did read your thread before I posted mine. I'm also concerned that this carb wasn't designed for WOT. I got this carb so I can go play in the mud, and tackle the trails & rocks on the way to the good mud holes.

    I put everything back to stock. I did finally get a chance to take my Jeep out again. Unfortunately, the weather has gone from 90° down to 65°, and cooler in the mornings. I hope that the temperature doesn't skew my results.
    After a long warm up on the way to work in the morning, I tried it again from a dead stop. I got a stumble, but not a stall, and the stumble was a quick recovery. I did leave a couple of marks in front of the workplace.
    It rained most of the day, so I had slightly wet roads on the way home. After a good warm up, I tried it again from a dead stop. No stumble at all and the tires lit right up. In fact, they spun real easy to easy. I'm not 100% sure if the wet ground also skewed my results.
    You know it's kinda fun to drive Swamper tires on wet pavement with a locked rear end. I had a hell of a time keeping it straight at the traffic lights where the roads were much wetter.

  4. #4


    I remember having a very similar issue on idle-to-WOT runs. If I remember correctly, running the blue cam & 35 shot seems to be better for me, but I can't guarantee that it's a fix. See the chart below, and you can see the red cam doesn't produce as much fuel initially. Have you noticed if your AFR is leaning out when you stab the throttle? I've also noticed that if I roll on the throttle very quickly, the hesitation is basically eliminated. I suspect this is because I'm allowing the engine/carb the extra milliseconds to catch up, instead of the accelerator pump doing the proper fueling.

    Also, I saw that you're from MI, so I can say wait 3 more days and we'll be back in 90's. I checked out the forecast for later in the week, and you'll be able to do a comparison run then .

    I love taking my Cherokee out with the 31's on it, and welded rear axle in the rain. It makes the old 4.0L feel like a race engine. Now if I was getting too much wheel spin in my IH Scout, I would put it in 4WD to make sure the load is fully transferred from the engine. This is also why I was running my data in 3rd, High, 2WD, at low RPM (hard to spin 36s with that load even with 5.89 gears).

    I can tell you that my next step is likely throttle body injection, so that I can get the benefits of the Truck Avenger without the WOT issues. Hopefully I can get it in this winter and functioning properly.


    Click image for larger version. 

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    Last edited by delvin; 09-12-2015 at 09:30 PM. Reason: Added info

  5. #5
    Join Date
    Sep 2015


    I hate that this site doesn't do email notification.

    I don't recall my A/F gauge reading lean, I usually see a rich issue. I don't have my A/F gauge in line of site when I'm staring at the road. I should do a video, so I can record all three gauges, tach, vacuum & A/F, so I can see what happens at full throttle from a standing stop.

    My MI forecast says the high will be Thursday at 84°, and then drop back off again.

    I won't put my Jeep in 4WD on any pavement, and definitely not on wet pavement. I have a locker up front also. On wet pavement, I'll go diagonal, depending on which side of the crown of the road I'm on.

    Everyone of my buddies keep mentioning to me about going EFI, but I want that reliability of a carb. Even a badly tune carb can keep an engine running, but one broken sensor or wire can stop an EFI. Of course, if I had a couple of buddies that ran carbs, I might be closer to fixing this problem.

  6. #6
    Join Date
    Dec 2009


    Quote Originally Posted by swampbog View Post
    Everyone of my buddies keep mentioning to me about going EFI, but I want that reliability of a carb.
    My Holley EFI system has been more reliable than any carburetor I ever owned.

    Even a badly tune carb can keep an engine running, but one broken sensor or wire can stop an EFI.
    That's not true. It depends on which wire breaks (I've never had one break), and some of the sensors will not stop an EFI engine if it's defective.
    Also, it's easy to simply plug in a spare sensor and let it dangle in place, than repair a carburetor on the road. I'd never use a carburetor again! (Related Forum Thread)

    Also, for me it was a safety issue, in terms of my trucks power brakes & power steering capability at low RPM: (This post explains why in more detail.)
    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.

  7. #7
    Join Date
    Sep 2015


    Quote Originally Posted by Danny Cabral View Post
    My Holley EFI system has been more reliable than any carburetor I ever owned.

    That's not true. It depends on which wire breaks (I've never had one break), and most of the sensors will not stop an EFI engine if it's defective. Also, it's easy to simply plug in a spare sensor and let it dangle in place, than repair a carburetor on the road. There's no way I'd ever use a carburetor again! (Related Forum Thread)
    I was just saying it in a general exaggeration.
    I know the EFI will require less adjustment than any carb, but we all have our own personal choices, and right now mine is to stick
    with a carbureted engine. I'm not going to 100% dismiss the idea of an EFI system, because one day I might just decide to do it.

  8. Default

    My advice, if you are at all contemplating EFI, is to sort your carburetor properly and have it working at it's best, before you go to injection. That way you have a definite baseline to compare one to the other.

    I speak from experience. Not having ever run a carb on a brand new engine, and then having nothing but troubles with the EFI system I fitted, I'm absolutely amazed at how well it ran, once I went back to the carburetor, and tuned it properly. Now at least when I get the courage to put the EFI back on the car, I'll know what minimum standards to expect from it.

    Cheers, Robert

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