Vacuum Syncing Your Vintage Honda 4-Cylinder Motorcycle Carbs - Pt. 1 - CB350F, CB400F, CB550, CB750

When your vintage Honda motorcycle is properly tuned and the carbs are perfectly vacuum synced you will find that it fires up cold in an instant, has a smooth steady idle, and smooth acceleration at riding speeds. When it is out of sync you can often find your idle peaky, hanging, overly sensitive, and the perfect recipe for a rough ride. With a 4-cylinder motorcycle like the Honda CB350F, CB400F, CB500K, CB550, CB750, or many more vintage 4 cylinder motorcycles you need to take the extra steps past getting the engine to fire and run, and tune it to a place where it is going to keep you safe and perform properly. In this 2 part series, we go over the why, when, and how of vacuum synchronization as well as dive into some of the differences between the different models of motorcycles we support. Learn how to set up your Common Motor 4-cylinder vacuum sync kit and tune your new cafe racer or scrambler right at home in your garage.

[kawabunga]

You can adjust each individual gauge with the small screw under the clear plastic cap to set them all evenly with each other. Run 4 lines, one from each gauge. Then take the lines from gauges 1 and 2 and connect them to a T, same with gauges 3 and 4 so they meet at a T. Then run a line from each of those T's to another T so all four gauges will be pulling from one T. Then run a single line from that T to one vacuum port, keeping all the other vacuum ports plugged. With the bike running, you can see the discrepancies between each gauge and you can remove the plastic cover and turn the screw to make sure each gauge is exactly in sync with one another. This way you'll know that each gauge is accurate to all the rest and you can use the bank of four gauges as intended.

[murraykriner]

I'm following along. Wished that many who work on their Fours would pay close attention. So many people just do the minimum and never prep for doing the Synch. Great job Brenden!

[iantobanter]

Made my own setup back in '77 using an ex agricultural vacuum gauge, win e making taps and pipes and a lighter gas canister as described in Motorcycle Mechanics magazine. My late father made the take off adapters from brass. More recently I use a mercury tube set up made in Northern Ireland.

[preez]

Good explanation of how sync works on the carbs, however I 100% disagree with using single vacuum gauge method. With carb sync you need to see what the other 3 carbs are doing as you adjust one. Yes it can be done with 1 gauge but seems like it would take extra time and added frustration. Quad gauge setups can be easily calibrated and read very close to each other. Liquid filled DYI manometers are even more accurate and can be built dirt cheap. Just my $.02

[wjyoung]

This was video was RIGHT ON!! I purchased the kit and synced the carbs on my 78 CB550 perfectly!!! It runs like it did the day it came off the showroom floor!! Thanks for the video and tools!!

[ihateemael]

instead you could connect the 4 gauges, 1 at a time to the same vacuum port (engine at a stable idle), and adjust the zero screw so that all 4 read identically. Now that all 4 gauges are calibrated to each other its so much easier to sync your carbs.
edit: ps: you can use good quality air valves from an aquarium shop as pulse dampers and buy 4 good quality vacuum gauges from an auto shop to make your own setup.

[magicdaveable]

I made a custom intake manifold for my CB750. It now uses throttle body fuel injection. I did it so the bike runs well at altitude. I ride in the mountains often. Easy to adjust the mixture.

[dougamclean]

Just bought two 70s CB550s for winter projects..love these videos, I've been binge watching them to familiarize myself with the CBFours. Bought the carb sync/tool kit online and am awaiting it.
Love your presentation and video format...informative without droning on, good camera angles with pointer graphics and no thrasher music in the background!!
I'll likely watch every single video relevant to my machines before the weekend it done..then bookmarking them for reference when the work starts. ;)

[alexbrock]

Most useful YouTube video ever. Thank you

[wayne]

Mine was running on two. I found a drillbit that fit under one of the good running carb's slide. Then matched the other carbs with my drill bit. It runs great now.

[totalutternutter]

Always make sure the valve clearances are within tolerance before you attempt to sync the carbs because correct valve adjustment will often fix carb sync issues, syncing the carbs first will just disguise valve clearance issues but it will never fix them.

[johnnyappleseed]

The gauges that you suggest are poor, actually they have a calibration screw under the cover. Hook all 4 gauges to a couple of Ts and pull vacuum from a syringe, match adjust as required, best to do so at operating pressure for absolute accuracy. Using one gauge and a switching manifold can introduce transient errors as you cannot compare all cylinders in real time. Bourdon tube or diaphram type gauges work very well in the right hands. Carb sync is a rare maintenance item and normally holds for a long time, unless a valve or ring problem crops up. I check mine every few years at most.

[stevenbean]

Recently had the same issue he was talking about using 4 gauges they were top of the line but they all varied a few psi which turned into about 12psi and it even got so bad it wouldn't start. Went to 1 gauge and did the same thing he did and it tuned right dead on. Barely touch the starter and it goes. Great video

[daze]

By the way, the coolest and most ingenious way I've seen to bench sync is using a flash light from the other side. Just adjust the sliders 'till you can't see the light shine through

[stevenlangdon-griffiths]

Wow! A man who thinks like I do. I had thoughts going on about the accuracy of bank gauges too.

[wilfredprins]

Thanks for the video, just what i needed
I have to sync mine soon when putting my 350f back together

[santososornia]

Man it really hurts seeing my old bike had same bike except fully custom. Man that hurts sold it really cheap! Beautiful bike

[CrashHardEnduro]

Greta video!
I just picked up a 1974 cb350f that’s been sitting since the late 80s. I have a video on my channel talking about it, but it’s a super cool story.
I’m stoked to be bringing her back to life.
I’ll be ordering one of your kits soon.

[dungbeetlemovies]

Or use mercury filled hose vacuum gauges - self levelling and self synchronising.

[BurlyNerdGetsTheWorm]

Super helpful, thank you. Especially the specific shots of each bikes ports!