gmeyer316

55 century carbissues

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Thanks to you all. Lets see, first off I cant seem to get it to idle any slower than about 800. The first thing I did was to bend the accelerator pump rod straight, then eliminated the vacuum line to the wipers, then I tried timing, was a little advanced so I brought the lines back together and it idled a little better, that is when I checked the points and replaced them, set them at .016, that brought the dwell from 21 to 29, had the vacuum advance disconnected at the carb and the hole on the carb plugged during timing, again idled a little better but still dies is gear, I don't have a vacuum guage. Im thinking I need to pull the carb and go through it myself.

Greg

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Benefits of AACA Membership.

Hmm, before pulling the carb, just for kicks, adjust the timing 'by ear' if you can. i.e. bring it back where you have a slow/smooth idle. It could very well be that the dist is off by one gear when it was installed and when timing by the marks it's not matching up to what it's supposed to be.

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When troubleshooting something like this it is best to say "I have a problem with my car." Pulling the carb is like "taking it out of context". Do you know how much trouble is caused by taking things out of context?

Bernie

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Did you verify that the vacuum advance is working?  Also check the sparkplugs.  Whoever neglected to change the points may have done the same with the sparkplugs---or---they installed platinum plugs that will absolutely not work in a carb engine.

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Absolutely I do Bernie, politicians do it every day, totally get your point, plugs and advance are my next targets and I will leave the carb alone for now, thanks.

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You must get the idle back down before you can adjust anything, back off the idle screw.

X2.

Let's work with what we know is wrong. The idle speed is twice what it should be to take an accurate timing measurement and the speed screw is bottomed out.

You stated that you brought the marks back together. At what idle speed? How about putting a timing light and tach on the car. Drop the idle rpm from 800rpm in 100 rpm increments and monitor the timing and try to get the timing back to base 5 deg at 400 rpm or less.

My interpretation of the findings in the post is if the timing is "right on" at this high of an idle , given it is difficult to idle below 800, then cranking the idle down is retarding the timing to the point of not running as the advance comes off. Maybe advancing the timing incrementally as the idle comes off of 800 will do it. Let's see how low we can get it to idle in P before it dies.

Don't pull the carb yet. There's a few more easy checks before we disturb the carb. One thing at a time. Pick someone's advice on this thread one step at a time and go from there. Do each step accurately so we get good information. If you did do any changes since it was running right and "just started acting up" tell us back to Paul's point.

Stay on it.

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I'm not familiar with the distributor's advance curve on the Nailheads of that vintage, but I know that for a middle-60s era motor, the centrifugal advance usually starts at about 1000rpm, but then idle speeds were usually about 550rpm in gear, or thereabouts.  I'll have to see if I can find an older service manual online, just for general information.

 

Platinum plugs in a carbureted engine?  I've been doing that for a good while, even Iridium (which has superceded platinum in many spark plug applications--even Buick 3800 V-6s OEM).  I'm not sure what issues Old-Tank might have had, though (which I'm interested in finding out).  Be that as it may.

 

You DO need to get it to idle at the factory spec rpm FIRST.  Point dwell WILL change with increasing rpm, too, which again, affects distributor spark timing.

 

WITH the engine at specified base idle rpm, is there any sensitivity to the adjustment of the idle mixture screws?  One or both?

 

Getting back to the "flow of fuel in the idle system" (mentioned in my earlier post), the FINAL point of calibration in the idle circuit is the idle mixture screw.  This screw seats against a hole in the throttle body, even might stick out of it a little.  The SIZE of the hole is important in how rich you can make the mixture with the screw.  In about 1971 or so, for example, GM/Rochester decreased the size of that hole from its earlier size so that you couldn't make the mixture richer than a certain level (for exhaust emissions reasons).  In this case . . . it's the manifold vacuum which is acting upon the idle system to pull fuel from the float bowl, into the venturi cluster, and then down to the idle mixture screw level AND the transition slot in each throttle bore.  IF those holes might have a little corrosion on/in them, even if everything else is "as designed", it would make the idle too lean and could result in the "dieing" described.  Something else to check when you disassemble the carb.

 

Also, on the pointed end of the idle mixture screw, the pointed end will be smooth before the threads start.  Check for the smoothness of that pointed surface, where it seats against the carb throttle body.  Sometimes, the screws can be tightened too much in seating them for the initial "turns-out" mixture setting, which can affect how things work a little bit.  A "shiny line" is acceptable, but an "indented line" would be marginal or need some attention.

 

Hot base idle to specs, then base timing setting, then dwell (already done), then adjusting the idle mixture/speed for "best lean idle" and "highest manifold vacuum".  "Best Lean Idle" is an emissions' era orientation, but can work on most anything.  When the idle speed and mixture are optimized, to verify that, you can turn the mixture screw in to get a 20rpm drop, then return it to what it was . . . on each one.  The manifold vacuum should not change that much, if any, but this ensures that an "over-rich" idle is replaced by a "cleaner idle".

 

As mentioned, too, the vacuum advance needs to work for best drivability and cruise fuel economy.  Typically, it is hooked to a "ported vacuum" source (significant vacuum happens when the throttle valve is opened, with a very small amount at base idle, if any).  Vacuum advance usually needs about 5" Hg to start things moving .  .  . separate from the mechanical advance system.

 

Thanks for the updates.  Please keep us posted . . .

NTX5467

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I'm kind of curious why post 43 seems to have just been an exercise in my typing skills.

Bernie

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Sorry Bernie, Im just trying to take all this in, so many ideas Im not sure where to start, gonna check mixture screws for scoreing and make sure the plugs are good, I think Im finally catching on, been a lot of information to process, will do my best to lower the idle speed using all of you guys ideas, and then take it from there, I sincerely appreciate all the time all of you are investing in order to help a stranger solve his problems. I haven't messed with engines for years except Harley engines, Im trying to catch up.

One question, the vacuum line from the carb to the distributor is made of steel and is very very small, Ive always seen much larger rubber hoses used there, could that be an issue. Bare with me Im not totally clueless but I have much to learn.

Thanks again for all of you guys time.

Greg

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Ok,sorry Bernie I am kinda overwhelmed with ideas from a number of directions, anyway I figured start over with the easy stuff first. Finally got it to idle just a little under 500 if you can call it idleing. it barely keeps running but seems to be pretty smooth. the vacuum advance is disconnected with the carb hole plugged, timing is advanced about a degree, starts to lag if moved either direction, adjusted idle screws according to the shop manual, dwell is at 29. Is it supposed to keep running is gear at 450 rpm just putting it in gear drops it 100 or so. None the less I think we re headed in the right direction

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Update, I finaly got it to idle nice at 500 timing a tiny bit advanced dwell 29-30, took it on a road test couldn't hear it running while at a light, was nice and smooth, only problem now is a slight hesitation when you first give it gas. Maybe accelerator pump issue?

Thanks to everyone who helped me struggle through this. Got plenty more I want to do to this car and will come to you guys when I need help.

Greg

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Thanks for that update!  Sounds like things are coming together!

 

You can check the accel pump "shot" by operating the throttle . . . engine OFF . . . and looking down the primary throttle bores.  Might need to verify that the pump arm is shaped like it needs to be, for good measure.  From my experiences, Carter 4bbl pump shot always seemed a little "lazy" compared to a similar Holley 4160-style carb.  But no real issues with tip-in "sags".  Adjusting the pump volume is what the extra holes are for in the accel pump arm.

 

Some of the older 4bbls used a leather accel pump plunger, rather than a more modern neoprene pump cup.  Leather can shrink with age, just as the accel pump bore can wear from use.  Might need to pull the pump plunger out and roll it backwards to get it fluffed-up some.  Some prefer the leather to the neoprene in those older carburetors.

 

Keep us posted, please.

NTX5467

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Try fine tuning the mixture screws as NTX suggested.  On todays fuel they may be turned out about 2-21/2 turns - least thats where mine cooperates best.  Some of Carter WCFBs have a bit of a flat spot in them coming off idle.  If its not too bad consider leaving well enough alone.

 

Good job.

Edited by KAD36 (see edit history)

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Thanks Ken, I think you might be right, a little hesitation I can learn to live with and it seem that the mixture screws are a little further out than you might think they should be, seems to be running pretty good.

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Good work and congrats on a good running nailhead.  Thank the guvmunt for the ethanol laced fuel that causes the off idle stumble.

Willie

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another update, I think I solved the accelerator pump issue, I noticed that the bend on the arm was so that it was keeping the pump arm from returning all the way down to rest position, making it so the pump was not getting a full stroke and the stroke was starting in the middle instead of the beginning or bottom. Make any sense, seems to have improved, much less hesitation.

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Thanks for those updates and congrats on your progress!

 

NTX5467

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There is a steel plate and a gasket that goes between the intake manifold and the carburetor. Which one goes next to the manifold the plate or the gasket?

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A comment on the mixture screw setting:

 

There is a MAXIMUM setting beyond which no further good is accomplished. Check the original manufacturer's literature for a range. On carburetors built prior to 1968, over 1 1/2 turns generally is futile, but there are a few exceptions.

 

What happened in 1968??? Smog emission. So, the angle on many idle mixture screws was changed; as for smog emissions, it was desirable for finer tuning with the screws. Some carbs in the early 1970's still have adjustment at more than 3 turns.

 

Contrary to what many believe, the idle mixture screws do not adjust the idle mixture! Rather, they meter the preset mixture. The mixture is preset by the diameters of the idle jet, idle air bleed, and idle air bypass.

 

If it seems impossible to acquire sufficient mixture within the manufacturer's range on pre-1968 carburetors, remove the idle jet(s) from the carburetor and drill them oversize. Start with 0.002 inch and then in an increment of 0.001 until a good mixture is acquired OR a maximum of 0.005 inch. Making this modification on 1968 and newer carburetors being used on licensed vehicles is probably unlawful under the 1966 Amendment to the Clean Air Act.

 

Jon.

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I had the same issue with my roadmaster until I rebuild my carter 4 barrel. But then it flip flopped and know driving it spits and sputters around 30mph

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