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Help with Holley Tuning

Discussion in 'The Garage' started by mogulmasher, Mar 14, 2007.

  1. mogulmasher

    mogulmasher Registered Member

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    My new to me '77 K20 just isn't tuned right. Truck was built by somebody else, but it has a pretty fresh looking 350, with headers, edelbrock performer eps manifold, msd hei distributor, holley 670 truck avenger carb, and '70s vintage iron heads. I don't know about cam or compression. Throttle response just is not there and it smells of raw fuel....not much black smoke though. I know the truck won't be any fuel miser, but think I got like 7 mpgs on my first tank with it (of course it was running on choke most of the time!) and I wasn't pushing it real hard.

    So far I've dropped the timing back from the 14 degrees original owner had it at to 7 or 8 degrees. He had choke at 2nd from richest setting, dropped back to one less lean then halfway (still pretty cold in the morning here). I messed with the air screws yesterday, seemed to be best (but still pretty crappy) at 3/8 of a turn out. I found out that it has silver secondary spring, will change to heavier black spring to see if it cures any of the problem. Have not checked floats as the sight plugs have already been swapped out.

    Maybe he doesn't have full 12 volts to electric choke and its not working right.....will check this too.

    I have a edelbrock #1406 600 cfm w/electric choke on my '71 Nova with a 327 and I had same 1406 carb on my old '79 K10. I found this carb to be very easy to tune and very responsive yet decent on fuel at cruise.

    What are you guys all running for carbs? This is my first holley and I am less then impressed. Is it just me? Keep screwing with it and it'll work good or what? I'm just looking for a decent running honest 300 horse motor that may pull down 10 or 11 mpg.
     
  2. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Well, your not in that boat alone, I don't like Holleys either. They are extremely difficult to tune for all conditions. I can tune mine for any given day, but if the temp changes, humidity changes, or the moon is partially obscured, it runs different.Holleys can be great on the track, but I think they make horrible carbs for a daily driver. Just my expierence and humble opinion. Those in the south with less weather variations seem to have better luck. I've tuned the piss out of 2 of them over the years, the last one I took to a carb shop and paid over $100,and that didn't help either.

    Also, did the PO leave all the 'emmisions' stuff? Alot of that stuff was not emmisions related, and was drivability related stuff. Thermostatic switches to control vacuum to distributor timing, etc.

    Anyhow, about your problem, it seems like it choking on too much fuel,especially on acceleration.
    Things to look at would be:
    • The throttle cam (little plastic colored cam), there's a bunch of them that change the fuel curve for accelleration. This is purly an acceleration tune.
    • Your jets may be too big. Lots of people love to throw big jets in because they think it will give them more power. The truth is there is only 1 or 2 sets of jets that will make your engine run good throughout the whole range.
    • Your power valve may be blown or sized wrong. You need a vacuum gauge to size it properly. Holleys website has instructions for this, should also be in the instructions for your carb.

    Finally, one of the best tuning tools I've used is the Air/Fuel guage. I bought the Summing Racing cheap one with a cheap one wire O2 sensor, this has proved the most helpful to me in getting my carb to run right.
     
  3. mogulmasher

    mogulmasher Registered Member

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    Thanks for the tips man. Yeah its bad at idle, just off idle, and seems to acclerate very poorly until you hit 3000+ rpm then it starts ripping pretty good. Unfortunately I don't drive at 3000+ rpm on the street and really need the off idle to 2500 to be more responsive.

    I will check into throttle cam to see what options I have. Are they IDed by color, if so I have a red one.

    Jets could have been changed who knows. Should have come with black secondary spring and he swapped that for a silver spring for secondaries to open sooner, so swapping out jets seems possible too.

    I'll check into the power valve. This is a very low mile build, would it blow so soon, or is this something that blows when something isn't adjusted right?

    That air/fuel ratio meter sounds great. That would be a great tool for tuning for sure.:D

    I did a little research online after I posted this and found many stating exactly what I'm starting to think. Holleys are great racing carbs or for real high HP street engines, but not great for DD use. Starting to think going to an Edelbrock 600 is the way to go.
     
  4. 80' 427

    80' 427 1/2 ton status Premium Member

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    I personally like holleys alot. It not that I don't like the others but I find you can get holley parts any where and the carter/edelbrock and Q-jets other than a rebuild kit are hard to get parts for. These are the things I would check/change. First go get the list number off of your carb then go to the holley web site and find out how holley set it up from the factory. Many would be carb tuner start swapping jets, power valves and pump cams thinking they are smarter than holley is. Return everything to stock that is different. This will set the base line. To help you tune the carb you need to get vacuume in gear to get the correct power valve. 6.5 is the most common but you need one that will have a number about 2 points lower than the reading in gear at idle. Holleys are very simple to tune you just need to know how they work. If the idle is poor is is most likely the idle screws, timing, or float level. If it back fires or hessitates when accelerating at lower speeds it is the primary accel pump system (cam or squrter size) if it hessitates when opening the 4 barrels it is the spring size or rear accel pump (in mech secondary carbs). Leave the original jets alone to start with.
     
  5. mogulmasher

    mogulmasher Registered Member

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    I'm begining to think PO might have swapped out stuff from stock. I did print out the manual off Holley's site already. I haven't gone through to see that everything is stock yet, but will be.

    I'll give this carb a chance. Once I baseline it and go from there we'll see how it goes. I'm not knocking them, I've just never owned one is all. Of course if others tell me that no matter what a Edelbrock (or even a built quadrajet) is a better carb for my driving conditions then I'll swap it out.
     
  6. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    The ones made in the last 5-8 years are much better than they used to be. They redesigned the power valve to protect against blowouts. Before that, one low key backfire would blow them.
    It's more likely it would be sized improperly than blown. You need to check your vacuum with a gauge and look at Holleys chart, probably in the manual for your carb, it was for the last one I got, and pick the appropriate valve.
     
  7. 80' 427

    80' 427 1/2 ton status Premium Member

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  8. rcamacho

    rcamacho 1/2 ton status

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    Sounds like you are trying to tune all circuits at once. I'd sugges picking up a good holley book and getting down to tuning once circuit at a time. For a street driven vehicle I'd suggest:

    1) Choke - make sure it fully disengages at operating temperature but still operates when cold

    2) Tune idle mixture first - leverage a vaccum guage to get the highest vacum at any given initial advance and then turn in the mixture screws 1/4 turn to get a nice lean mixture. Re-adjust idle speed as necessary.

    3) Tune primary main circuits next - cruise RPM tune for highest vaccum or use a A/F guage or seat of the pants. Check plug color.

    4) Tune the primary power valve - somewhat related to mains. On a truck avenger the main circuit is pretty rich so you may be able to delay PV engagement down to say 6.5". Again, vaccum guage is your friend.

    5) Tune accelerator pump circuit. cam/shooters. Stumble too lean, bog too rich.

    6) start secondary side. If the jets are stock I'd just tune the spring.
     
  9. mogulmasher

    mogulmasher Registered Member

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    Great info, thanks man!
     
  10. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    At first glance by your discription, sounds like you may need to jet it down and get a bigger accelerator pump cam. Also maybe lower your floats.

    Check out this link for more tuning info:
    http://www.holley.com/data/Products/Technical/199R10268-4.pdf

    Also feel free to PM me if you want to know more info on tuning.
     
  11. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Here's a write-up I did on tuning a Holley Truck Avenger: http://www.4x4mecca.com/forum/vbarticles.php?do=article&articleid=12

    Personally I like the modular carburetor design like the Holleys and Demons. I've done a lot of reading and research about them and I really understand them much better.

    Personally I think you made a mistake tuning the initial timing back. That's probably why it idles too rich. Granted I live at a higher altitude so I can advance further than some, but I run initial timing at 15* BTDC. I would also run the distributor vacuum advance from a manifold source.

    If it were me I would start by setting the idle speed screw correctly. Pull the carb off and look at how much idle transfer slot is exposed. The butterflies should be adjusted so that the transfer slot exposed below the butterflies looks square.

    Bolt it back on and set the initial advance. You can even use a vacuum gage to set initial advance. Advance the distributor for best vacuum, then back it off a little to drop the vacuum about 1/2inHg. Take it for a test drive and see if it pings a little if it does drop the initial advance a little at a time until it stops pinging.

    Now adjust the idle mixture screws for the best vacuum or idle. I like to use a good handheld tach to tune idle mixture for highest idle speed. I generally start with the screws 2 turns out. Turn them in or out 1/8turn at a time and go the direction that yeilds an increasing idle speed. Stop where the idle stop increasing.

    Now the only thing that may be left to figure out is an off idle stumble if your Truck Avenger behaves like mine did. I covered that procedure in my little write-up.

    I will say this is probably not the best way to tune your carb for emissions and mileage.

    If you read through those links I referenced in my write-up you will learn a lot of what I wrote. thirdgen.org is a good source for performance carb tuning.
     
  12. mogulmasher

    mogulmasher Registered Member

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    I'll have to join up with 4x4mecca to read that article.

    Funny I was thinking on the way home today that maybe it was a mistake to back timing down so far. It did ping a little going into 4th gear at low rpms with a bit of a load. Maybe I dropped it back too far. It was at 14 degrees, think its about 8 or a little less now. Will try setting timing with vacuum gauge and see what I end up with.

    I'll look into adjusting idle speed screw correctly. If I don't understand what your explaining to me by looking at it expect a PM:o

    I messed with the idle mixture screws last night by adjusting then looking at tach in truck. I went back and forth about 20 times I think. When I got highest rpm, throttle was not real responsive and blew a little black smoke. This was at about 5/8 of a turn out. Had to lean them in to about 3/8 of a turn to clean up idle, below this a little bit and it would die. Its definitely got the eye tearing richness I read about constantly with these carbs....and I was tuning outside in the driveway!
     
  13. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Try this link: http://coloradok5.com/forums/showthread.php?t=174116

    I think someone listed earlier, maybe you have bigger jets than you need. Should be 68 on the primaries, 89 on the secondaries. I bought my first one used and it had different jets in the secondaries.

    Also make sure you secondary or primary floats aren't stuck open. Had that happen on the first carb too. Fuel was dribbling out of the secondary nozzles at idle. Tapped the secondary float bowl a couple of times and fixed the problem.

    Only 3/8turn out seems too low. You should look at bumping the initial timing up.

    Also pinging or knocking like your describing maybe because you have the vac advance hooked up to a non-manifold source, name escapes me right now. The vacuum source should be full vac at idle. Timed vacuum is what the other port is called, full vacuum comes in as you open the throttles.
     
  14. mogulmasher

    mogulmasher Registered Member

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    I was thinking that 3/8 of turn out wasn't much too.

    Vacuum advance is connected to the vacuum port the Holley instructions say it should be, so should be all set.
     
  15. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    If you're only getting 3/8turn at sea level it's gotta be either initial timing or too large jets, or stuck float IMO.

    I came to Holley from Demon carbs, basically. Demon carbs recommend running initial timing as high as 22*. That initial timing can really help off the line response. Like I said it's discussed a lot on thirdgen.org but I don't think I linked threads that discussed initial advance. You do a search over there and you'll find tons of info, though.

    That pinging in drive gear under load has a solution, I just don't know what it is. I've read that as a symptom somewhere in troubleshooting charts.
     
  16. mogulmasher

    mogulmasher Registered Member

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    Back when I had my '79 K10 w/ a sm465 I built a stout 355 for it and had to contend with the same issue. I managed to get most of it out by playing with various timing curves and tuning the adjustable vacuum advance it had. Of course this K20 is a little more weight then that old stepside shorty I had, so might be a little bit more work to get ping out of it. On this truck there is a MSD distributor and I haven't even really looked at it yet....I just have too many irons in the fire at this point. Just trying to get it running decent so I can tackle one thing at a time. I have an oil pan leak to contend with, fuel gauge doesn't work, driver's window falls out of channel when window is down all the way, PO never installed a stereo after restoring truck (man my drive into work is boring in the morning!!!!), need to get tires balanced, etc. Aren't old trucks fun!!!!:haha: :waytogo: :truck:
     
  17. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    You could also try running a higher grade gasoline to help eliminate some pinging. I run 93 and have never pinged.

    Also just noticed that you aren't a paying member so you can't send PM's. I highly suggest throwing down the $25 and become a member and plus I can see that you live in the New England area :D
     
  18. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    If you have the same HEI drop-in distributor from MSD that I do, you can change the springs on the centrifugal advance. That'll change your curve. There are bushings you can change out to limit centrifugal advance, 18-28*. The vacuum advance is a max of 10*.

    Here's a link the instructions for the distributor I have: http://www.msdignition.com/pdf/8360_frm%2023793.pdf
     

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