Idle mixture adjustment

[tom'sB2B]

Having trouble adjusting the idle mixture.

I have a duel carb aluminum intake from George Asche. With the aluminum intake, I've lost the vacuum port. I've tried to use the vacuum port on both of the carbs and get no reading at idle. The RPM's at idle are at 500.

The vacuum gauge will show a reading when I excelerate. 

[Sniper]

You likely have a venturi vacuum port, which will only show a reading when the throttle is opened up.

 

You need a manifold vacuum port.  If your manifold does not have such a port you can add one.  The factory intake on my 51 has a hole in the #6 runner that is plugged with a pipe plug, I pull the plug and screw in a nipple and connect my vacuum gauge to that.  You can see the hook up in the pic below.

 

Also, did you balance your carbs?  Balancing your carbs is when you ensure both have the same amount of airflow through them at idle.  I would start there then do the mixture adjustments.

 

 

[plymouthcranbrook]

You might want to invest in a Unisyn or a similar tool as they make it much easier to set the carbs together. In ancient times we took a piece of vacuum line and just used it to listen to the air going into the carb by holding one end of the hose over the carb intake and putting the other to our ear. Then adjusted until they sounded the same.

 

https://www.amazon.com/Empi-Universal-Carburetor-Synchronizer/dp/B0075AW6CI

Edited June 8, 2020 by plymouthcranbrook

[Plymouthy Adams]

9 hours ago, plymouthcranbrook said:

You might want to invest in a Unisyn or a similar tool as they make it much easier to set the carbs together. In ancient times we took a piece of vacuum line and just used it to listen to the air going into the carb by holding one end of the hose over the carb intake and putting the other to our ear. Then adjusted until they sounded the same.

 

https://www.amazon.com/Empi-Universal-Carburetor-Synchronizer/dp/B0075AW6CI

two ears, two tubes, two carbs too easy...first sync tool available in stereo.......the Unisyn is a tool that is indispensable for anyone with dual carbs...it is well worth the money...done in 5 minutes what toying all days without will never accomplish...

Edited June 8, 2020 by Plymouthy Adams

[tom'sB2B]

I have a unisyn tool. Syncing the carbs is not the problem.

The problem is that I think I'm running too lean. I have to pull my choke out a quarter or it will die at idle. If I can't read the vacuum, I don't have any other way to adjust the idle mixture except by trial and error.

[Plymouthy Adams]

do you know the history of these carbs and are they matched pair for jetting....older rebuild or newly rebuilt?  sounds like a problem in the primary circuit and sync low end is different step than sync of top end..your air gap the same at idle on each carb throttle plate and throat?  can be checked with feeler gauge if you have good round ones if not the small precision drill bits can be used to balance the plates for initial set up to help determine if internal or external problems with balance...imbalance shows its worse on low end and WOT...

[plymouthcranbrook]

Multiple carbs are cool I guess but give me a good single setup and I will be happy.  I used to own several British sports cars with dual SU or Stromberg carbs.  Always annoying and it seemed that I was always doing something to them.

[kencombs]

37 minutes ago, tom'sB2B said:

I have a unisyn tool. Syncing the carbs is not the problem.

The problem is that I think I'm running too lean. I have to pull my choke out a quarter or it will die at idle. If I can't read the vacuum, I don't have any other way to adjust the idle mixture except by trial and error.

If it were mine, I'd drill and tap a 1/4"pipe thread hole in the manifold and install a fitting for vacuum access. 

But, make the hole in a place central to the runners and carb so that it provides an average reading, not just one port.

 

Does pulling the choke out also put the idle stop screw on a faster idle step on the linkage?  If so, it may not be lean, just too slow.  Every multi carb setup I've dealt with required a slight idle speed increase, compared to stock,  to come off idle smoothly. Probably because the pump shot or idle transition circuit wasn't perfect.  

If not I'd be checking for vacuum leaks at all possible locations, manifold to head, carb to manifold etc.   If it is really too lean, with no leaks the fix should be as simple as opening the idle mix screws 1/4 turn each and test drive.  Thousands of older cars have had that done by ear forever.  Mix screw in until it stumbles then  out ,while counting 1/4 turns, until it sort of 'rolls', (changes rpm slightly up and down).  Back in 1/2 the turn count.  Adjust by 1/4 turns if needed after a test drive.

[Plymouthy Adams]

the problem with manifold applied vacuum to the distributor is the full advance at idle....this is a lean condition and will also tend to allow heat creep....the distributor is setup for ported vacuum and the ability to lean the engine only at cruise/low demand operations.  Granted either way with WOT or moderate to heavy throttle, the loss of vacuum does occur in the manifold and you lose that advance at that time the mechanical preset curve does all the needed work as it is designed to do...if for some reason your timing is a bit advanced over stock...combined with vacuum at full advance at idle...you setting some adverse conditions you are trying to compensate for with other setting, mixture and idle speed is just a couple

[Sharps40]

Couple issues.  

 

Choke at warm idle to keep it running point's to a vacuum leak.   Assuming timing is correct find the leak first.  Or, 500 rpm is too low to signal 2 carbs and pull gas in.  

 

 

Second....no need to use a unisync on a log manifold.   If they are exactly the same carbs, start with factory settings to get it running, then using a tachometer or manifold vacuum, adjust for lean best idle.   Afterward drive it and the plugs will indicate if you need to change main jet sizes.  

 

 

I assume the linkage is perfect and both throttles fully open and close in perfect harmony

Edited June 8, 2020 by Sharps40

[Plymouthy Adams]

30 minutes ago, Sharps40 said:

Couple issues.  

 

Choke at warm idle to keep it running point's to a vacuum leak.   Assuming timing is correct find the leak first.  Or, 500 rpm is too low to signal 2 carbs and pull gas in.  

 

 

Second....no need to use a unisync on a log manifold.   If they are exactly the same carbs, start with factory settings to get it running, then using a tachometer or manifold vacuum, adjust for lean best idle.   Afterward drive it and the plugs will indicate if you need to change main jet sizes.  

 

 

I assume the linkage is perfect and both throttles fully open and close in perfect harmony

 

had a guy locally with a dual carb Plymouth make that argument so I convinced him that in using the UNISYNC how would I mess his adjustments up.....today that man owns a unisync...while his car ran good before...he admits that after I tweaked them the proper manner...it now PERFORMS....

[Sniper]

6 hours ago, Sharps40 said:

I assume the linkage is perfect and both throttles fully open and close in perfect harmony

 

 

Using a Unisync, or similar device, will remove the assumption from that statement.

 

Back when I got my first ride in 81, a Yamaha RD250, I though I knew how to adjust the dual carbs using your method.

 

Unisync proved I didn't.

 

What it does is balance the airflow thru both carbs so that they are equally contributing.  Nothing else you use will be as accurate or precise.  It also has nothing to do with idle mixture. 

[Sharps40]

Ok.  Ya'll seem good to go!

[Sharps40]

OP, just curious.   Did you vet the problem yet?   Wondering as you sort of implied above that the magical Unisink tool probably wasn't the way you needed to go.....

[tom'sB2B]

I have not gone back to it yet. I will check for leaks next. I feel that I'm going to need to go through both carbs and maybe start from scratch with the why setup. 

[Sharps40]

Thanks for updating.  If not a leak perhaps raise the idle since it now has twice as much cfm the signal to pull gasoline is a lot weaker.   Naturally,  all the rest has to be right first (ign, linkage, matched carbs, etc)....but I'm sure you know that.

 

Worst case block off one port and tune one carb to run, then block that port to tune second carb to run, then try tuning with both carbs installed.  

 

(Dont worry with the unisink till its running decent.   The Yamaha example above is apple's to oranges since you have 2 carbs on a common log and the Yamaha was 2 carbs on 2 separate manifolds [and crossover tubes were not available on american models).   You should be able to get yours running well as on a log the common Plenum will tend to balance differences between the high carb and the low carb.  Eg all 6 cylinders get fed more closely to the same on a log and then the unisink will help you balance  both carbs making it all the better.    )

Edited June 11, 2020 by Sharps40

[Sharps40]

Oh yes. to tune by vacuum you will need a manifold port.  This could be a simple spacer and port added under one carb.

Edited June 11, 2020 by Sharps40