My Zenith carb adjusting / synching process

Question regarding vacuum connections

For the sake of clarity I want to make sure I understand the following statement from this thread:

' I took Jerry's advice and have linked the vacuum ports of each carb to the corresponding port of the other via vacuum tubing - both ported and manifold connections - this smoothed things out considerably on its own.'

1.Does this mean that the intake (passenger side) of the carbs should be tee'd together the run to the distributor?
2.Should I do anything with the drivers side vacuum ports or just leave them capped?

Constructed a sync tool using the clear tubing/ATF method. Also used coolwhip tubs and a vacuum gauge I had on hand (Tee'd with shut offs) going to the top of each carb.

So I am in the final stages of the carb adjustment sync process. I just want to make sure I am getting the best possible performance and running setup.

Thanks for the help!!
 
You link the driver side ports. I used the small port at base of carb and used vacuum hose. You can also use the smog/egr port at base of manifolds which is larger. I can't tell if it made a difference though.
 
1. No, not necessary.

2. Yes, connect. The theory or practice is to smooth out any intake pulses, improve carb vacuum signal for better mixture, and equalize the two banks of cylinders 1,2,3 to 4,5,6 as in practice they rarely see the same mixture strengths due to manifold design. ( Unequal length runners.)

DYI- carb sync tools- make sure they seal even if you have to tape them on. Mind the plastic cap or lack of one for the choke linkage- air can bypass there.

As you get closer to optimum tune, small adjustments make differences. ( If not, something's wrong.) It is not exactly explained well by the repair manual as it was written for trained mechanics with new parts.

I cannot overstate the importance of making sure the ignition, timing, etc. is optimized first. Use a dwell meter, timing light, tach etc. A cheap vacuum guage is your friend to show you how you can get the most vacuum and then try to make both sides the same.

Now with all the contraptions, listen. Listen to the exhaust, look for the smoothest running state. I happen to focus on sprak plug wires at the cap to get them to stay steady and not vibrate or shake. There should be no shake.

Now, I can agree with Chris- if you're engine is in an optimum state, balancing the manifolds might not get you a noticible smoother running engine. It will get you a better signal ( intake charge/ fuel/air/ vacuum) signal. For me it did something positive and it was cheap. I had the hose and didn't have to worry about caps deteriorating/ leaking. As a matter of fact, my manifolds had the egr tube crimped and silver soldered. One was leaking for god only knows how long. Found it with a stupid piece of vacuum hose and fixed it. Another trick is to go back over all the screws, nuts, etc. on the carbs/ manifolds. A small leak here, another there makes a huge difference in Zeniths operation. You'd be surprized how much vacuum you pick up and that means fuel economy too.

Now, lastly. I keep saying that with two or more carbs it's mathematically probable it's one over more. I use the air mixture hole that's next to the main air jets on top to close them with my finger or a pencil with an eraser. That changes the mixture to a richer idle. based on that- gets rough, speeds up, or stays about the same tells me what to go fix. Rich, lean, just right.

Just keep asking. It's a journey until you understand.
 
1. I did because I like overkill :-P but it probably doesn't make a difference. I teed them together and ran a line to the distributor. I also removed idle retard and capped at the distributor.

2. Drivers side are linked, this made a difference for me, I used the egr pipe capped in the middle after checking for connection tightness on the manifolds and ensuring no leaks anywhere. Capped the manifold vacuum ports used for retard. Otherwise thes can be linked if you've capped the old egr fittings.

PS. Consider the Carbmate. No risk of sucking oil into one carb from the monometer. Use with air meters for precise synching.

PPS. Yes, Jerry is the man.


gdub0717 said:
1.Does this mean that the intake (passenger side) of the carbs should be tee'd together the run to the distributor?
2.Should I do anything with the drivers side vacuum ports or just leave them capped?

Constructed a sync tool using the clear tubing/ATF method. Also used coolwhip tubs and a vacuum gauge I had on hand (Tee'd with shut offs) going to the top of each carb.

So I am in the final stages of the carb adjustment sync process. I just want to make sure I am getting the best possible performance and running setup.

Thanks for the help!!
Click to expand...
 
Stevehose said:
I also took off the 4 linkage swivels and adjusted the 2 to stock length (50.8mm) and the others to equal length which made it zero out with the main accelerator linkage
Click to expand...

Steve, I'm getting ready to adjust my carbs following this procedure and I have a question. What does "...and the others to equal length..." refer to? Which "other" linkages?

Any other changes since you wrote this? It's a phenomenal thread, thanks for your contribution.
 
Hmmm, if i recall correctly the 2 black plastic linkage rods have a spec in the blue book at 40mm center to center, the 50.8mm number below must be the outside diameter spec which I must have found somewhere. I recall seeing a pic of a micometer gauge on it but can't find it.

Correct me if I am wrong, aren't there 2 other metal rods, the knurled one being at the rear carb? It's been a while so I don't exactly remember. If so these would be equaled as a starting point.

Things I would do differently or that aren't as important in hindsight:

1. Lose the cheapo Craftsman vac gauges. I am not sure they are accurate enough to use as a comparison, and the flutter makes them pretty useless. Maybe some higher quality dampened gauges would work better.

2. Just tune with the digital tach, Synchronmeter and the Carbmate.

3. Don't cap the retard port on the distributor, leave it open because I think capping it can affect the ability for the advance side to work. Still not to be used-only the advance.

4. There will be a compromise between idle and 3k rpm sync. Hard to get both perfect, I tend to err on perfect at idle and ok to be off a little at 3k, result is smoother idle and transition and not much if any issue at higher rpm's. But see what works for you.

5. Jerry's tip of covering the idle jet hole on the cover to test for rich or lean idleing is a good one.

6. Take the play out of the accelerator pedal linkage, adjust at both the firewall and the rotating plate on the block so that there is no wasted movement before the carb linkage moves. This will give you a nice smooth response and feedback to the foot with no lag etc.

7. Get a good flexible screwdriver for the mixture screws - 3rd degree burn warning.


Keep us posted.




Luis A. said:
Steve, I'm getting ready to adjust my carbs following this procedure and I have a question. What does "...and the others to equal length..." refer to? Which "other" linkages?

Any other changes since you wrote this? It's a phenomenal thread, thanks for your contribution.
Click to expand...
 
mtiger_dds said:
On the Carbmate TS-111: it isn't readily available and seems to have been replaced with the TS-110? There's a host of accessories listed with it. What would I need or do I need for Zeniths? TIA!

CarbMate – TecMate

tecmate.com tecmate.com
Click to expand...
No accessories needed, just the 2 hoses - one for each carb. I replaced the short hoses with long ones that reach into the cabin so I can observe any synch issues while driving through all rpm's under load.
 
HB Chris said:
Steve, So you use manifold port on driver side?
Click to expand...
Yes - manifold ports for synching. For my sidedrafts I have a 6 port device. If you have popping or hesitation on acceleration this will also show if one carb is off as you push the pedal.
 
Stevehose said:
After a couple years of adjusting my own carbs and with much valuable input from 61porsche and others on this board, I have finally worked out a system that gets my Zeniths pretty darn smooth. For the experts' consideration:

Parts used:

Digital rpm/timing light - critical for seeing small changes in rpm from mixture adjustments - my old analog gauge I now realize was useless.

Vacuum gauge(s) - I have 2 but only use one for this procedure.

Carbmate TS-111 synchro device - shows vacuum balance electronically between the carbs - basically an electronic manometer with more accuracy and without the risk of sucking fluid into an unbalanced carb. Adjusts for sensitivity. This thing is great.

STE Synchrometer with home made adapter to fit on top of the carbs - had one for each carb but one blew up from a backfire last year so now have to switch it from carb to carb - no big deal.

Here's the procedure (feel free to suggest improvments to the process)

1. Ensure valves are adjusted and timing is set. Check for vacuum leaks and repair if so. I have a pertronix so I have it slightly advanced and have plugged the retard function.

2. I took Jerry's advice and have linked the vacuum ports of each carb to the corresponding port of the other via vacuum tubing - both ported and manifold connections - this smoothed things out considerably on its own.

3. With this in mind I teed the tube between the manifold vacuum ports (driver's side of carbs on mine) so that I have a fitting to attach the vacuum gauge and cap it when it's not used.

4. Attach vacuum gauge to the tee.

5. Disconnect the linkages to the carbs so they are independent.

6. Close the butterfly valves via the set screws all the way (counter clockwise) so that no more valve movement happens when you turn the screws - then screw it back in just enough to detect the slightest of movements. The idea here is to get the carb to stay on the idle mixture circuit as much as possible.

7. Close the idle mixture screws until they stop all the way in (gently) - record how many turns it took for each carb so you have that to fall back on. Back the screws out 2 1/2 turns. I use a flexible driver from Sears with a screw driver bit taped in - this helps to access the screws behind the linkages when things get hot. A folded towel on the valve cover helps insulate.

8. Start the car, if it dies you can open the butterflies equally a little but only do so just enough to get it to idle - too much and you will suck gas into the venturi and mess everything up. Ask me how I know.

9. At idle, adjust the mixture screws (out is richer, in is leaner) and watch the rpm gauge and the vacuum gauge - small increments - one carb at a time and back and forth - I turn it out until the rpms and vacuum max out then back in until it stumbles then back out a little rich till smooth again. I watch the corner of the hood up in the air to determine the point of minimal shake. Combination of highest rpm and minimal hood shake. Using the vacuum gauge connected to both carbs minimizes gauge flutter and for me makes seeing small changes easier than having separate gauges - and is also visible proof that the connection between the carbs cancels pulsing vacuum and results in smoother operation. Using this technique I improved my vacuum reading from 15 to 18.5. My rpm here was about 800 +- 10 rpms but this probably will be different for everyone.

10. Once the idle mixtures are set disconnect the vacuum gauge and connect the Carbmate - it has a hose for each carb. Adjust the butterflies equally, keeping the LED display in the middle until you reach your desired rpm - I have A/C so I set it at 1050.

11. Check each carb with the Synchrometer to verify equal air passing through and adjust accordingly while keeping the Carbmate in synch also.

12. Idle should be smooth now with minimal missing/hood shake and puttering happily with a nice exhaust note.

13. Reattach the linkages - making sure that when you do there is no movement of the butterflies and no change in rpm. Put a little white grease in the sockets before popping them back together. Adjust linkages accordingly if change is detected.

14. Rev the car to 3500-4000 rpm and watch the Carbmate - if one carb is out of balance at higher revs, twist the knurled linkage adjustment piece at the back carb linkage until it zeroes them out. This ensures balanced carbs throughout the rev range, not just at idle and makes a big difference when driving.

15. Carbmate should show balance at both idle and through the rev range if linkages are correctly adjusted. Revs should be smooth and responsive with no missing.

16. Disconnect devices, plug the vacuum tee, check all other vacuums for tightness and take it for a drive!
Click to expand...
This is the best summary and sequence to remove all the mystery and concern about Zeniths. This would have saved me many frustrating hours while working on the rebuilt carbs on Patricia Meyer's project car. (I am going to go buy a Carbmate). The one thing I would add is a 13.5 -when reattaching the ball to sockets on the carb arms, these arm lengths are critical. Cycle the bell crank (at lower block) while watching the exact moment that each rod (attaching to the carbs) starts to turn the arm of the butterfly shaft. Watch one and then the other to ensure that they are both rotating the butterfly shafts at the exact same moment. It only takes a moment to alter the length of one of the shafts to get them to 'pull' at the same time. Then you are truly synchronized.

Zeniths are wonderful carbs and immensely gratifying to drive when this work is completed.
 
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