Author Topic: Tuning a performance carb for a truck  (Read 864 times)

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Offline Evenprime

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Tuning a performance carb for a truck
« on: May 9, 2020, 09:33:13 pm »
I'm posting this in here instead of another site that is engine specific because I am wondering if this may be because of the weight of the vehicle. The fact it is a truck, maybe I need to adjust differently? This build was for performance but still wanting low end torque.


76 W100 with a LA360 engine.

I cant seem to get my carburetor set properly and I find the engine likes the opposite of what it says to do online.

The Engine:
Brand new rebuilt bored 30-over 71 block
Hyper-Eutectic pistons, free flowing aftermarket aluminum heads giving me 9.5 compression
Xtreme 232 cam I believe it was. Sorry I forget.
Non rpm edelbrock performer intake, hooker headers etc.
Dynoed at 390 torque/ 360 horses
Not an extreme build by any means but its nice.

Carb = quick fuel 600cfm

Adjustable:
1) Choke has an air adjustment screw
2) Butterfly idle air adjustment screw
3) Air mixture screws (2 of them) - supposed to be 1.25 turns from closed by default
4) Fuel pressure screw

Issue: Now that the engine is broken in, and settled in comfortably, the engine began to bog down on giving it throttle from start. Even if it was a relatively slow start so not like the 4-barrel kicking in too early/late etc.

It says to adjust the 2 screws for the air mixture. Start with 1 and move it to the rpm goes up (if it does) and then move the other until it is similar. Then adjust the idle screw incase its wonky.

Here is the issue.

The RPM goes up as I completely tighten those screws. Its like it idles best when they are closed but then u can barely drive the truck. I back it off to regular position and the truck is much better but bogs initially on take off.

Here is a video of a similar carb adjusting those screws. Im literally doing the opposite lol The engine runs best when its rpm is low and bogging at idle. But then when the trucks on the street, it runs better?
https://youtu.be/70hhpX2tfsE?t=340

Im thinking the choke air screw maybe too open or closed?
I've tried tinkering with the idle air screw to set it up and down rpm but I do want it at 700-800rpm optimally but that doesnt help the bog.
76 Dodge Power Wagon W100 6ft box 318ci "near show room condition"

76 Dodge Power Wagon W100 8ft box 400ci "needs love but solid'

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    Offline Evenprime

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    Re: Tuning a performance carb for a truck
    « Reply #1 on: May 9, 2020, 09:36:06 pm »
    And by Bog Down, I mean it only does it initially and only once.

    It just stumbles and then kicks in fine.

    I just cant seem to get rid of this hiccup
    76 Dodge Power Wagon W100 6ft box 318ci "near show room condition"

    76 Dodge Power Wagon W100 8ft box 400ci "needs love but solid'

    Offline modelcitizen

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    Re: Tuning a performance carb for a truck
    « Reply #2 on: May 9, 2020, 10:03:57 pm »
    Accelerator pump,is it working?Engine off,air cleaner removed,open throttle about 1/2 way while observing for a steady squirt of fuel into each venturi.Holley carbs of this type have diferent cams available to tune the duration of fuel stream to suit different needs. This carb should be similar.

    Check float levels,too low a float level can contribute to same symptoms.
    « Last Edit: May 9, 2020, 10:28:44 pm by modelcitizen »

    Offline KurtfromLaQuinta

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    Re: Tuning a performance carb for a truck
    « Reply #3 on: May 9, 2020, 11:38:17 pm »
    Quick Fuel carbs have a lot of tuning capabilities. Basically a Holley with neat tuning features.
    Anytime you can crank the a/f screws all the way in and it runs better... your too rich on the Idle Fuel Restricters (IFR). Since you have a tunable carb... this makes it easier. The stumble off idle is probably because it's too lean there. Thankfully this can also be tuned out with the Idle Air Bleeds (IAB).
    Your modified engine just needs to tuned to work better. This is the exact problem I had with my Six Pack.
    Read this article I posted a month back... https://ramchargercentral.com/technical-discussion/the-best-way-to-tune-a-holley-carburetor!/
    Read it carefully. It will help get your carb working right. At least you won't have to do all the drilling and tapping because Quick Fuel has already done that for you.
    Good luck. Persistence will pay off.
    « Last Edit: May 9, 2020, 11:40:11 pm by KurtfromLaQuinta »
    500 c.i.- Scat Crank, Scat Rods, Indy Aluminum heads, Comp XR280HR Cam, Six-Pack, Milodon 8 qt. Pan, 727  (reverse-manual, low band apply, 5 clutch disc, 3 qt. xtra. pan), N.P. 205, Ford 9" rear axle (4:11, 31 spline axles, Detroit Locker, converted/ disc brakes). Dana 44 front (converted/ disc, 4:10, Tru-Trac limited slip). Hydra Boost Brake Booster, Gear Vendors OD. 600+ ft. lbs. of torque

    Offline Evenprime

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    Re: Tuning a performance carb for a truck
    « Reply #4 on: May 14, 2020, 08:06:31 pm »
    Thanks everyone for the replies.

    Kurt Im not sure Im happy or sad with your reply lol.
    Thank you for the in depth link, I think you are correct with my issues. The sad part is I was really hoping it was going to be easy hehe.
    Although it does appear easier than your job. That 6-pack sure must have taken a long time to tune.

    I did want to ask one more question though as I did some more tuning.

    I adjusted the air/fuel ratio screws to confirm they were increasing in RPM as they became snug as I had mentioned and I noticed that I can get to about a 1/4 turn before snug and the rpm levels off there.

    From1/4 turn to snug I couldnt hear it increase or decrease.
    Do you think I still need to adjust the Idle Fuel Restricters?

    I set it to the 1/4mark and increased the idle screw to bring the rpm down to 900 and so far the engine seems to like it a lot better than before.
    I used to get a tiny bit of dieseling when I shut it off  and it used to be difficult to start (Id often pump the petal twice and wait 10 seconds) both of those are gone.

    But I still get the stumble upon acceleration although not as bad. I suspect that will be the Idle Air Bleeds (IAB) and I havent had a chance to look into it.

    76 Dodge Power Wagon W100 6ft box 318ci "near show room condition"

    76 Dodge Power Wagon W100 8ft box 400ci "needs love but solid'

    Offline KurtfromLaQuinta

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    Re: Tuning a performance carb for a truck
    « Reply #5 on: May 15, 2020, 10:47:56 pm »
    Thanks everyone for the replies.

    Kurt Im not sure Im happy or sad with your reply lol.
    Thank you for the in depth link, I think you are correct with my issues. The sad part is I was really hoping it was going to be easy hehe.
    Although it does appear easier than your job. That 6-pack sure must have taken a long time to tune.

    I did want to ask one more question though as I did some more tuning.

    I adjusted the air/fuel ratio screws to confirm they were increasing in RPM as they became snug as I had mentioned and I noticed that I can get to about a 1/4 turn before snug and the rpm levels off there.

    From1/4 turn to snug I couldnt hear it increase or decrease.
    Do you think I still need to adjust the Idle Fuel Restricters?

    I set it to the 1/4mark and increased the idle screw to bring the rpm down to 900 and so far the engine seems to like it a lot better than before.
    I used to get a tiny bit of dieseling when I shut it off  and it used to be difficult to start (Id often pump the petal twice and wait 10 seconds) both of those are gone.

    But I still get the stumble upon acceleration although not as bad. I suspect that will be the Idle Air Bleeds (IAB) and I havent had a chance to look into it.
    Surprisingly fooling with things like the IFR's and the IAB's isn't that difficult.
    Read that article thoroughly. Especially the parts pertaining to those two... IFR/ IAB.
    He talks about dieseling problems. And also when you don't get the correct range with your A/F screws. IFR.
    The stumble is here... as per the article...
                    IDLE                                                        IDLE MIXTURE SCREWS
                                                                                       IDLE FUEL RESTRICTERS (I.F.R.)
                                                                           
                   TIP-IN ACCELERATION                                   IDLE AIR BLEEDS (I.A.B.)


    The stumble your getting is a lean condition that can be tuned out by richening the Tip-In (IAB)
    You need to go smaller jets- less air there. You'd be surprised what one jet size smaller will do.
    This is where everybody wants to dump more fuel with the accelerator pump. Don't do that.
    A nice set of jets you can get through Summit or Jegs is a real good way to get the tuning right. Look for the link I added to the article. A little bit of money, but way worth it when you get awesome drive-ability.
    Or, if you already have replaceable jets (I'm pretty sure you do with that Quick Fuel carb) see what sizes are in there and order just a couple of sets smaller for the IAB. Get your magnifying glass out... those numbers are tiny.


    My Six Packs aren't really much different than a 4 Barrel. Most of the tuning work I've done is on the center carb. Which is the primary side of a 4 barrel. So all I've learned from that article applies just like a 4 barrel.
    Setting a 6 Pack up for balancing between the 3 is nothing at all. All you need to do is turn two turnbuckles until they line up with the outboard carbs linkage. Hook a clip to the linkage and your done.
    « Last Edit: May 15, 2020, 10:50:31 pm by KurtfromLaQuinta »
    500 c.i.- Scat Crank, Scat Rods, Indy Aluminum heads, Comp XR280HR Cam, Six-Pack, Milodon 8 qt. Pan, 727  (reverse-manual, low band apply, 5 clutch disc, 3 qt. xtra. pan), N.P. 205, Ford 9" rear axle (4:11, 31 spline axles, Detroit Locker, converted/ disc brakes). Dana 44 front (converted/ disc, 4:10, Tru-Trac limited slip). Hydra Boost Brake Booster, Gear Vendors OD. 600+ ft. lbs. of torque