BS38 carb adjustment using "Syncrometer"

[Ipa62]

Hi out there. Anybody ever used a "Syncrometer" (carburetor airflow meter) to adjust their carbs on an XS? A friend of mine loaned me this device, claimed it works well. The idea is you plug this into the inlet side of a carb, then adjust the mixing screw for the specified airflow, then move it to the second carb and adjust the mixing screw so that the airflow matches the first carb, seems simple. Only problem is that I don't know what the air flow should be. My thought would be to adjust the first carb, lets say the left, using the dead cylinder method, then check the airflow with the "Syncrometer", then adjust the second carb (right) using the "Syncrometer" to match the first. Any thoughts on this?

 

[mrtwowheel]

Here's a guy that bought one that didn't include instructions. I didn't read the whole thing. Maybe there's something here that can help.

https://www.thesamba.com/vw/forum/archive/index.php/o-t--t-475972--.html

Scott

 

[peanut]

I saw one being used on Wheeler Dealers series and as far as i remember you simply match each cylinders inlet flow rate at a fast stable idle. I imagine you could use any rpm point as the throttles once synced should give a matched flow rate throughout the throttle range

 

[Ipa62]

Interesting, some good insight. The write up on the link from Scott says they first matched the airflow at idle using the stop screws, then adjusted the mixture screws and matched them as low as possible till stumble. Will give it a try, would be easier with two meters though.

 

[peanut]

google is your friend

https://classiccarbs.co.uk/Synchronometer-instructions_and_adapter_list.pdf

 

[5twins]

Yes, that's basically all you do - jump it back and forth between carbs, tweaking them until the readings match. It's a pain in the ass. I started out using one many, many years ago. Graduated to dual meters or a manometer and never looked back. Their only real benefit is they can be used on carbs without vacuum ports for regular gauges.

 

[peanut]

yes they are pretty crude and damn expensive for what they are. Most experienced tuners can sync carbs just as well with their ear and a bit of hose pipe .Its how I have always tuned my twin and triple SU's
Unless your engine is in good tune to start with,.. the synchronometer needles will be bouncing all over the place and hard to read if at all on a M/C twin.

 

[azman857]

With that sync, one would need to do it at idle and above idle for cable tension. Or are the carbs on a common shaft?

 

[JRay77]

I have ATF in my tubing. I use this method on my Bs38 carbs.

 

[Ipa62]

My bike is a '71, no vacuum ports to connect manometers. There are two throttle cables and yes they need to be adjusted the same, which they are. As well the slides are very close to each other when closed. I just got the bike running last fall, put about 50 miles on it, prior to that it hadn't been run in maybe 30 years. It was running rich, so this winter I took the carbs off and rechecked all the adjustments. When I get it running again in a few weeks I will go through the running adjustments again and see if the Synchrometer provides any help. Thanks for all the reply's.

 

[5twins]

With individual cables, syncing is a two step affair. First you must sync the idle speeds, then the "at speed" or throttle opened sync. The 2nd step syncs the cable adjustment so both carbs begin to open at the same time. When you set idle speed, adjustments to the linkage add or subtract cable freeplay. That will alter the "at speed" sync. That's why the idle speed and sync must be done 1st.

These adjustments on CV carbs have no effect on slide position. The slides always close completely and the same amount. Turning the idle adjustment screw or adjusting cable play won't move the slide position at all. The slides are moved (lifted) by the vacuum created as air flows through the carb. Air flows through the main bore but also through the carb top above the slide. It flows faster above the slide and that creates a vacuum which sucks the slide open.

 

[Ipa62]

5twins, you are correct, my mistake. What I meant were the butterflies which I adjusted on the bench so that they were equally closed, I then made other slight adjustments when running along with free play of the cables. In the end I will use the adjustment procedures that you and Grizld1 wrote, great manual BTW.
Thanks for all your advice.