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Idle Issues


mrahc

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After chasing my tail for about two weeks, I guess it's time to ask for some help.  

I guess I'll start off with the setup:  I'm running a 230 with a water-heated Edmunds intake with two of the Carter/Weber 32DFT 2bbl carbs on top with all stock jets.  Langdon split manifold headers running two 2" pipes back to Smithy's glass packs.  Pertronix 12V electronic ignition with a matching FlameThrower coil. Head is milled .060".  Stock bore. 

My issue is that I cannot get the car to idle consistently.  I started by setting everything to default.  Distributor was static timed per the manual, idle air screws 1-1/2 turns out, spark plug gaps set to .035", etc.  The carburetors are synchronized at both the throttle plates and the linkages, so that they're open the same amount at idle and open together with throttle.  Only the rear carb has a choke cable attached and the front is wired open.  At its lowest, I can get the idle to bounce around between 700-1000 RPM.  Vacuum at the manifold bounces from 16 to 19 inches, depending how fast the motor is spinning at any moment.  The venturi vacuum ports I was taking the readings off of (top right of the air/fuel screw) gave me a more consistent reading of about 7.5" of vacuum at around 900 RPM, initially.  This was also where I ran the vacuum to the distributor, per Langdon's recommendation.  The port to the left of this gave a reading of less than 1 inch, but both carbs were the same.  There is a relatively large vacuum port on the side of the carb opposite the motor, which I initially thought was for a PCV setup, so I ran hose between the two carbs with a breather in the middle.

In this state, the car started detonating like a bastard under moderate acceleration.  The motor would idle on its own about 90% of the time, but seemed to be stumbling and shaking.  With the choke pulled, the idle was higher than it should be, but it was very smooth, so I immediately started looking for vacuum leaks.  The intake manifold and carburetor bases appear to be fine, as there was no immediate difference while spraying starter fluid at them.  I disconnected the vacuum advance and there was a noticeable improvement in pinging, so I moved the distributor vacuum hose directly to the manifold.  This alleviated the detonation, but instead caused some stumbling under load at higher RPMs.  The next morning, the stumbling was miraculously no longer an issue, and has not returned since.

Still convinced that I had a vacuum leak on my hands, I looked up pictures of similar setups and found that people had the larger rear vacuum ports plugged.  I removed my breather setup, plugged the ports, and readjusted my idle.  Ported vacuum now reads about 11 inches at the lowest I can get it to idle on its own, but I did notice that the rear carb's vacuum reading seems to fluctuate mildly (+/- a half inch) with the inconsistent engine speed, where the front is steady.  Manifold vacuum dropped some, to between 14 and 17 inches.  Pulling the choke even partway no longer smooths the engine, but causes the RPMs to increase dramatically.  As I appear to have taken two steps back, this is where I'm consulting the elders.

Has anybody had any similar issues?  Any recommendations?  I still feel that a vacuum leak is the culprit, but I can't figure out where it would be coming from.  I'd like to adjust the timing based on vacuum, but without a consistent idle, this has proven to be impossible.  I suspect the timing light I've been attempting to use is malfunctioning, as it was showing 45 degrees BTDC at idle, immediately after I set the advance close to 0.  There has to be something I'm overlooking and I sincerely hope this is just an issue of me being an idiot.

20170524_173022.1.jpg

Edited by mrahc
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when was the last time you checked the balance of the carb and with what style meter?  It is not at all hard to get these out of sync if you ever try to accelerate the engine by hand from any single carb link instead of the common central pull point.  Your vacuum to the Mopar distributor is set to be ported, as it is at the manifold, you pulling max vacuum at idle....this is not what you want...you actually can disconnect and plug the vacuum line till such time you get this balanced out as the car will start run idle and accelerate fine without vacuum advance..that is for leaning out in low engine load conditions only for economy..

 

suggest you start over and establish the air flow as even at both carbs both at idle and mid throttle positions...set your base line timing and also use a vacuum gauge and a sensitive tach to do your adjustments by...am thinking this will get you back to where you can tweak it in.

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I've been checking the balance at every step, with the vacuum gauge in the photo.  The throttle stops are very sensitive, so I've been careful to keep them balanced along the way.  I've only got one port on the manifold, so I've disconnected the distributor each time while taking readings and making adjustments to the carburetors.  I've been using a tach/dwell meter connected directly to the coil to measure RPMs.  I suppose a clean slate is the logical next step.

Do you have any experience with the Pertronix ignitions?  When adjusting the static timing by the book, I couldn't get the distributor to light a bulb, so I instead connected a spark plug and based my timing off of that.  I assumed this was occurring because it no longer has a mechanical contact point, but is this correct behavior?

 

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I've had this problem on two different motors (not flatheads, but still). One was a leak at the carb base... I've actually had a few like that, either hairline cracks in the throat or more commonly a poorly sealing gasket. The other was in need of valve adjustment and cleaning as it had some carbon desposits. Not sure if either are worth looking at in your case, but worth mentioning.

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Timing light should be inductive on the secondary side....as the engine is running the static is close enough to use the timing light.....be sure the vacuum is disconnected...if you are not using ported vacuum on the distributor (linear with the throttle) you should only have about 1degree at idle (5 1/2 to 6 1/2 in of vac.) to ( 9 degrees at 14 inches of vac.) if connecting to straight vacuum you are adding your max advance at idle..NOT WHAT YOU WANT...those large open ports at the bottom shown uncapped in your pic.....they sucking air?

 

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rcb,

I keep thinking the carb base gaskets are to blame, but spraying ether did nothing, so I don't know why I'm stuck on that.  I adjusted the valve clearances pretty recently and just tested compression on all of the cylinders to verify the valves were closing properly.  All were between 145 and 155 psi.

Plymouthy,

The timing light I've been using has an inductive pickup, which I've been putting on the Cylinder 1 plug wire.  I suspect that the rheostat on the timing light is faulty, or at the very least needs to be zeroed.  The open ports at the base of the carb are where the idle mixture screws are hidden, unless I'm looking in the wrong place.  I'll check to make sure there are no vacuum ports that didn't have a nipple on them, and I'll also try venturi vacuum for the distributor again.

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Did you synchronize the carburetor linkages using the pictured vacuum gauges? Correct way is to use a device that sits on the top of the carburetor as pictured below. The pictured device will not work on your 2 barrel carburetors without an adapter. Suggest you contact forum member Marty Boise. He had a similar problem with his twin Carter Weber carburetors. He ended up using intake manifold vacuum on his vacuum advance. I do not understand how that worked but he said it fixed his problem.  

carbtune1.jpg

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Don,

I synchronized the carbs using the vacuum ports at the base of each carb.  I'll try contacting Marty today for more information on his setup.  I'm pretty sure I've come across his threads in my endless searching, but thanks for the contact.

More time spent wrenching over the weekend left me with nothing but more questions.  I removed the vacuum advance and found that the diaphragm is shot, so I need to try to find a new one, or at least a functional one.  So, for now, the vacuum advance is disconnected entirely.  I purchased a new timing light to confirm the old one was malfunctioning, but, to my surprise, it read the exact same thing.  After setting the static timing, the inductive timing lights (connected to cylinder 1 wire) are showing about 50 degrees advance at idle.  This can't possibly be correct, but what would cause them to read this way?  I advanced and retarded the distributor and in each direction, the motor starts to stumble and die, so I returned it to the same position and the car runs and drives just fine, with some slight detonation under heavy acceleration.  I hope I can get that to dissipate by retarding the timing slightly more, but I still can't figure out why the timing light is showing me such an absurd value.

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It's possible that the springs on your advance weights, in the distributor, are broken so that it goes to full advance once it spins a little. Or, if the timing marks are on a damper, maybe the outer ring of the damper has slipped and now the marks are off.

Merle

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Merle,

I'll double check the distributor, but everything looked intact when I had it apart on Sunday.  As for the timing marks on the damper, I found TDC before I even knew that the marks were there, then looked down to mark it and it was already dead on.

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Pull off the dist. cap and see if you can twist the rotor. You should be able to twist it slightly one direction, but it should spring back. If you try to twist the other direction it should be solid. If you can twist it back and forth several degrees without much resistance that would indicate a faulty advance mechanism.

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  • 1 month later...
  • 1 month later...

I suppose I can update with some progress...

Merle, I checked the distributor as you mentioned and everything was as it should be.  I also picked up another vacuum advance from a friend of mine, but the diaphragm was in eve worse shape than mine.  I'd still like to find a replacement, but the only one I've found is $98 from Kanter, with a $45 core charge.  At that price, I'd prefer to attempt making my own diaphragm out of some nylon-backed nitrile.  

I tried for days to find a vacuum leak, as that still appears to be the most logical answer to this dilemma. I still could not come up with anything and the car still doesn't idle the way it should.  

Off idle, the car was great for about a month.  I drove it 1,300 miles with no issues.  Then, all at once, the car started running like garbage...backfiring out of the exhaust and the intake, stumbling, no acceleration.  I decided it was probably time for a checkup, so I readjusted the valve lash, changed the fuel filter, checked the plug gaps, synchronized the carburetors, and (still a mystery to me) reset the timing.  She runs better now, though not as well as before.  The terrible idle persists and it's been detonating.  I tried backing off the timing, which didn't fix the detonation, but instead added backfiring, so I returned it back to where I started.  The plugs look to me like it's not getting enough fuel, which is consistent with the detonation, but I don't know why this lean condition would start all at once.

I'm sick of messing with these carburetors, so I'm in the process of rebuilding a pair of Carter BB.  This is where I've run into an issue that I need some advice with.  The throttle plate shafts are worn pretty badly, giving noticeable play that will undoubtedly result in a vacuum leak.  I've been searching all over for replacements, but it doesn't seem too promising.  Instead, I'd like to ream the throttle bodies and insert bushings, but I can't find any information on people having done this with these same carbs.  I believe the throttle shafts are 5/16", but I'm not sure if a standard 5/16" bushing kit would meet my needs, as the shafts themselves are worn, and not the throttle body itself.  I've considered making a nylon bushing, but I don't know if it would hold up to the combination of heat and gasoline.  Any advice on adding bushings to these carbs would be very much appreciated.

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Oh the joys of dual carbs. I realize they are popular, but after a bit of research I determined the minimal benefit did not outweigh the challenges. I thought of going dual for looks, but run only one for ease of maintenance. Even so, I wish you success. 

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Vacuum wipers???  Check the whole system for leaks. Or hook your gauge to the wiper feed port.  Also ported vacuum signal will be roughly half of single carb as air flow through each carb is half flowing through than the single. You also mentioned crank case breather, if they are direct intake manifold connections, they are nothing but a vacuum leak.  Assuming stock cam, good condition of engine single carb set up should give you 19 to 21 inches of vacuum at apx 500 rpm idle.  Dual carb set up will drop to 17 to 19 inches. Should be steady belaying any I eternal issues.

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