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Technical carb stutter

Discussion in 'The Hokey Ass Message Board' started by mcmopar, Aug 28, 2017.

  1. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I have been taking care of little problems since I got the truck running, and now this one is getting worse. I know very little about carbs but I have been reading and asking before turning screws.
    I've had junk in the seat twice that I have cleaned out. I am running a stock 318 and a 600 holley. it has 5 degree btc, and 32.5 total timing. I thought that it was running rich, but then I pulled the plugs and they look lean. From idle to 1500 rpms it stumbles, above that it runs great. The reading I have done says that this is a lean condition. I backed out the air mixture screws a 1/4 turn, to richen it up a little. This seemed to help but not completely. how far can I back these screws out before a rejet would be a better idea. The accelerator pump seems to be working correctly. Point me in the direction and I will turn some screws.
    Tony
     
  2. saltflats
    Joined: Aug 14, 2007
    Posts: 10,394

    saltflats
    Member
    from Missouri

    The idle mixture screws only control idle mixture.....so I would check the power valve and jetting.
    Do you know if the jets are stock size for this carb?
     
  3. Truck64
    Joined: Oct 18, 2015
    Posts: 4,230

    Truck64
    Member
    from Ioway

    Idle mixture should be leaned out as far as practicable consistent with good idle. Then leave it alone you're done with it, unless maybe altitude or seasonal temperature changes. Rich idle mixture tends to load up the plugs.

    Jetting does affect wide open throttle fuel mixtures but here again that is not their purpose. For a street driven car it's not a good plan to tune that way. The cruise fuel mixture will be absolutely pig rich and washes out rings and dilutes oil. You can't look at plugs and determine "too lean" very easily with modern fuel. If the plugs look rich, it's real bad.

    Think of each carb circuit as distinct and separate even if they overlap to some degree, don't try to compensate in one circuit for a misadjustment in another. Get the idle mixture set for maximum vacuum, leanest idle you can get. Make sure wet float height is correct. Jetting is for highway cruise only, they should be as lean as practical too. Level cruise takes hardly any fuel, lean fuel mixtures are OK.

    You could remove the jets entirely and drive around town so long as you kept the speed down, for part throttle and up to 35 or so only the idle circuits and accelerator pump, and power circuits would be employed. That's where the changes need to be made - accelerator pump shot amount and duration maybe, and power valve circuit. Here is where you want to be nice and fat. Going lean there (under load) is bad stuff.
     
    Last edited: Aug 28, 2017
    clunker likes this.
  4. Tim
    Joined: Mar 2, 2001
    Posts: 11,778

    Tim
    Member
    from Raytown Mo

    just throwing it out there but every time I thought I had a fuel issue it was a spark issue.
     
    bobss396 likes this.

  5. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    Tim, I have read a lot of threads that support your idea. I have also thought about that. I did pull the cap, and cleaned it. The only other thing I have changed was the plugs. I went from autolite r44xls to a ngk v power. I have thought about changing them back but I had the problem with them to.

    I called the guy that rebuilt it to ask about the jet size and power valve, he said he would get back to me. He did tell me to advance my timing to around 10 to 12 btdc, that seems like a lot to me but they are just numbers. It did smooth out a little but its not right.

    I put a vacuum gauge on it and I am only pulling about 13#. I thought that it should be around 18#.
    Tony
     
  6. Have you checked the accelerator pump to make sure it is squirting consistently as you open the throttle? Inconsistent or delayed action from the accelerator pump is a common cause bogging or stumbling. With the engine off, open the throttle slowly and looking down the throttle bore. the pump should start squirting the instant you open the throttle, and continue the entire time you are moving the linkage. Check the pump linkage adjustment if there is a delay in the pump discharging.
     
    Terrible80 likes this.
  7. saltflats
    Joined: Aug 14, 2007
    Posts: 10,394

    saltflats
    Member
    from Missouri

    (Jetting does affect wide open throttle fuel mixtures but here again that is not their purpose) Truck64
    I could be wrong but I think that at WOT that is the jets main purpose along with fuel supply.
     
  8. check to see that the vacuum advance is working.
     
  9. Budget36
    Joined: Nov 29, 2014
    Posts: 4,884

    Budget36
    Member

    This new gas makes it tough to read plugs, my 235 a few years ago was puffing and idling rich, but plugs looks fine.

    About the best way to verify mixture is with an O2 sensor, widebands are best, but pricey...besides, there are millions of cars from the 80'sand 90's without wideband O2 sensors, for feedback...they do pretty well..so I'd suggest just a simple AF ratio gauge toat least give you a good idea of what's going on.
     
  10. Truck64
    Joined: Oct 18, 2015
    Posts: 4,230

    Truck64
    Member
    from Ioway

    It is a combination, but as I'm sure you know WOT fuel enrichment is mainly the power valve circuit. When the engine vacuum goes bye-bye the power valve opens and stays open as required for the additional fuel right?

    Technically though the jets should be sized for best steady level cruise fuel mixture in a street driven motor. If the WOT is lean after this, then PVCRs should be enlarged. Jets are easy to swap out up or down as required for race tuning (density altitude and all that) but for everybody else simply increasing jet size for tuning out a hesitation or a bog will most likely run pig rich and definitely waste a metric fuckton of gasoline out the tailpipe.

    This is what I was getting at earlier, to really tune a carburetor each circuit has to be optomized independent of the others, the common error is trying to compensate for a misadjustment in one circuit by a misadjustment in another. It does work, sort of. But it's the wrong way to go about it.

    He noted 13" idle vacuum, that is low for a stock engine at Sea level. Tweaking the idle mixture screws back where they need to be should bring the engine vacuum up some, maybe 2 points. Still low. Late ignition timing will cause low engine vacuum too, bigtime.

    One thing I noticed on most of the auto forums reading carburetor adjustment threads, it's evident that an awful lot of people seem to think this "1.5 turns out" or whatever business from seated (on the idle mixture screws) is some kind of a permanent factory setting that should never, ever, be deviated from.

    Every engine is different. Hell that's why they are adjustable in the first place! Otherwise the OEM would have saved the .50c on the screws and used a fixed orifice size. Once they are set though, for best/highest vacuum or "lean idle drop" they don't really need adjustment again except for seasonal variation or elevation change.
     
  11. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    yes, I checked the advance and it works. Its time to turn in the idle mixture screws in and try that. I should say that it idles fine and in the garage I can rev it up with no problems. the problems start when I try to drive it.

    truck64, and saltflats, learning a lot from your posts please keep them coming.

    slayer, checked the pump and it seems to be doing what it is supposed to.
     
  12. oj
    Joined: Jul 27, 2008
    Posts: 6,247

    oj
    Member

    The carb is responding to the low vacuum signal. You are trying to tune an effect not the cause. Time to analyze why you have 13" vacuum, the 4160 has very little tolerance for low vacuum and needs the 18" it is designed for to get the idle circuit to operate correctly. You can 'fudge' it a little by raising the float level a little to add weight and volume to the fuel so it'll take less of a vacuum signal to get it flowing. But the real fix for the carb is to get the vacuum up where it should be, do a compression test and follow it up with leak-down test to see if the valves are seating or maybe the lifters are on the loose side and need adjusting.
     
    Truck64, mad mikey and lothiandon1940 like this.
  13. oj is right on! Vacuum is to low. Check what he has stated, and for a possible leak.
     
  14. Basics first !
    I agree with OJ
    Tuning is a dance, just like any other.
    It's repeating the steps and tweaks over and over in the right order & gaining towards the optimal summit. You repeat them until the engine falls off the other side of that summit then reverse the last change back to the summit.

    The main tuning dance needs to be done on 3 floors - idle , part throttle, wot throttle. It also needs to be done on the stairs between those floors in the transitions - off idle to part & part to WOT. Then hopefully the Idle to WOT romp will fall into place.

    Timing should be set at TOTAL, then IF the initial is off you re curve. Jets control or limit the maximum fuel that can be delivered they are like a gate. The idle screws are more of a controlled drip. If its cruising or running lean the screws won't change that condition much more than pissing in the ocean will Change the amount of water. It certainly does but it doesn't amount to much.
     
    INVISIBLEKID and Truck64 like this.
  15. Truck64
    Joined: Oct 18, 2015
    Posts: 4,230

    Truck64
    Member
    from Ioway

    I'd call steady level ground cruise "part throttle" right? What works good for a street driven motor - Make sure you're done with timing - good HOT ignition, curve is where you want it at TOTAL, vacuum advance etc. Make sure float height/fuel height is good. Then re-set idle mixture screws for maximum steady vacuum at factory RPM.

    Then adjust jetting for steady level cruise at say 50 mph. Here's where a wideband O2 sensor makes yer money. Lean it out till it squeeks and back off a bit if you don't have one.

    So now the idle mixture and part throttle settings are dialed in. Then tune for WOT fuel mixture. Here's where the PVCRs come in, the power valve # itself sez "when" additional fuel comes in, not so much "how much". In a way this method is systematic "curving" the AFR the carburetor provides at all conditions.

    You may find the WOT throttle fuel mixture is now too lean when the steady, level ground cruise mixture has been dialed in. Lean fuel mixtures are OK when engine load is low, but never under load, under acceleration.

    Tuning this way I went from about 9 mpg hwy with a tailwind to almost 17 on a 900 mile roadtrip, fully loaded. (The truck, not me.)
     
  16. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    31vicky, saltflats, thanks for the tips, this is the stuff I am looking for. Never done it before, and I am sure there is someone else on here that needs the same info. I am putting a clutch in daughters car, so I will be off of my truck for a couple of days, but I will report back with how it turns out.
    Tony
     
  17. Do you have the vacuum advance plumbed into full manifold vacuum or to an off idle vacuum port? Using full manifold vacuum will often pick up the idle speed as well as increasing the manifold vacuum at idle. This can make it a lot easier to adjust for a smooth, steady idle. You can also get the same effect by advancing the initial timing at the distributor, but this may end up giving you too much total advance, as has already been mentioned above.
     
  18. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    claymart, vacuum is plumed in at a idle port.

    I had a chance to do the timing dance, with my truck. I turned every screw every way that I could. I changed timing up and down checked plugs compression. Took the wife for lunch in it and all I got was more frustrated, when she asked if we should take the bike. Well I made it home, and started at the beginning with no change. I believe it actually got worse, that's when I decided to start at the beginning again. I put everything back to factory specs, took it for a drive, and nothing. At this point I decided that it wasn't carb related, so I looked else where. I cleaned the cap and rotor, but not the points the other day. Cleaned and checked the points, took it for a ride and problem fixed. Now here's the funny part, I walk to the house to tell my wife its fixed and we can take it to the car cruise, go back to the garage, and the drivers front tire is flat. Take tire to get fixed and they have no tube until later today. On the bright side truck is running and I'm still breathing.
    Tony
     
    saltflats likes this.
  19. Points can easily be the culprit, what did someone say way above? 90% of carb issues are electrical in nature. Good find.
     
  20. Tim
    Joined: Mar 2, 2001
    Posts: 11,778

    Tim
    Member
    from Raytown Mo

    Don't know that anyone mentioned it yet, but you've checked the fuel filter right?
     
  21. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    tim, checked it, and it was good.
     
    Tim likes this.
  22. 90% of carb problems are electrical in nature
     
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  23. saltflats
    Joined: Aug 14, 2007
    Posts: 10,394

    saltflats
    Member
    from Missouri

    Did the vacuum gauge reading get better after the fix?
     
    31Vicky with a hemi likes this.
  24. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,590

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I'm getting 16 to 17 inches now.
     
    Last edited: Sep 1, 2017
    ClayMart and saltflats like this.
  25. Check your idle vacuum, and look at your power valve. Your power valve should be about half of your idle vacuum. That may be a starting place for you.

    I would also look for a vacuum leak too. If none of that works for you I got a tricked out 600 on the shelf that I probably should finish putting back together, we can get it to you to try if you want.
     
  26. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,309

    Atwater Mike
    ALLIANCE MEMBER

    Methinks them vacuum 'pounds' are inches...and that's not just an equation...:eek:
     
  27. That's what SHE said.....
     
  28. The vacuum gage usually reads in "inches of mercury" (inHg).
     
  29. clunker
    Joined: Feb 23, 2011
    Posts: 1,613

    clunker
    ALLIANCE MEMBER
    from Boston MA
    1. MASSACHUSETTS HAMB

    he he!


    Sent from my iPhone using The H.A.M.B. mobile app
     

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