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.
4-Cylinder Vacuum Sync Kit: www.common-motor.com/honda-55...
www.common-motor.com

[@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. ;)

[@wjyoung61]

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!!

[@kawabunga660]

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.

[@Common-motor]

another method! We prefer this way but there's plenty of ways to do it. Whatever gets you to perfect sync!

[@thisone.]

Would using a brake bleeder vacuum work the same to adjust the gauges instead of running your engine?

[@warrenzevonsangryghost6055]

@@thisone. Yep. Just need a vacuum source.

[@darkshadowsx5949]

sounds like a massive spaghetti mess of tubes and T's you'll have to make a video on that instead of that confusing paragraph.

[@deadlysrestorations]

@@darkshadowsx5949 he's basically saying hook all the gauges together and run it off of one cylinder to make sure your gauges are set to proper zero and all four gauges are reading the same. When they come out of the box the gauges can be off. Then you can use the vacuum gauge set.

[@kaiserswaghelmii9361]

That's actually a really clever way of doing it! Thank you for this tip. I'm gonna be using your method of syncing from now on.

[@jayarnold8883]

I always look forward to these videos.

[@wilfredprins9718]

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

[@tylermegehe]

Thanks for the great video. Looking forward to the next one.

[@iantobanter9546]

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.

[@murraykriner9425]

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!

[@kenbarlow2001]

if you dont do the basics you only do part of the job

[@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.

[@BurlyNerdGetsTheWorm]

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

[@kenbarlow2001]

not hard to find a take off point

[@alexbrock9509]

Most useful KZbin video ever. Thank you

[@stevenbean9706]

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

[@edwardhovitz1753]

Real nice. Thanks

[@wags174]

Great video!!!!!

[@AdamMcNutt]

Loving the Gremlin for the animation...

[@stevenlangdon-griffiths293]

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

[@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.

[@michaelkorolev2115]

I’m new to all of this and get nervous trying to do something I’ve never done. I recently got a cb750 DOHC. That has idle issues. I bought a Digi sync and the 8 mm tool but was chickening out but after watching your videos with your very well said explanation you gave me the confidence to do this now I need a fan. Hopefully you have a video on th valve adjustment. I watched one from someone else and I think I did it wrong

[@johnnyappleseed7798]

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.

[@santososornia5741]

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

[@preez77]

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

[@Common-motor]

Just different preferences! We’ve found this way to be quite fool proof and easy to understand but at the end of the day if it gets your bike running like a top it’s a winner to us.

[@daze8410]

I think the accuracy of these gauges is often skewed because people neglect to do the first step in the instructions. YOU HAVE TO CALIBRATE THE VACUUM GAUGES!!!!!! It's tedious but the best way I've found is to hook each gauge up to the same cylinder and set them to the same reading one by one but you should only ever have to do this once. Mercury gauges will always be the most accurate however.

[@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.

[@whatsstefon]

Yep. I was shown that when I was an apprentice, and I was able to reproduce the example many many years later when a friend got a VW Beetle with twin carbs. He sync’d the carbs but the engine still didn’t run great. I asked him if he adjusted the valve clearances? Nope. We checked them and none of the cylinders were within spec or within each other’s measurements.

[@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.

[@Freyr77]

That’s awesome you should make a video on it

[@Enginuitycustoms]

What throttle body fuel injection set did you use?

[@daze8410]

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

[@jayarnold8883]

I would call this a bench calibration. It's a good place to start. Final setup with the guages.

[@ecttreker9816]

Hey thanks for the video, would this apply for an '85 CB750 Nighthawk

[@harryohanson]

Even those gauges can be adjusted I do not like them ether. Plus those little plastic adjustable valves are also prone to lie. Found that out really hard way. I prefer Motion Pro liquid tubes. Hydraulics don’t lie.

[@danh7680]

Excellent video! Would this apply to a 77 CB750a HondaMatic?

[@IrnMaiden304]

Well, makes sense about using one gauge and a gang valve so you know you don't have a variable between your gauges, why not just use a manometer ?. Also, carbs of that style you can adjust individually, while other bikes you have to pair each set of carbs, and then make a final adjustment between both pairs.

[@leewilson4994]

Where can I get the setup you use in the UK and thanks for the brilliant videos.

[@dungbeetlemovies]

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

[@russellhueners8499]

They work great and cheep, just dont slam throttle shut or you will suck merc, if I did this more often, I would purchace an electronic carbstick, old timers use a man o meter or just a rubber hose to listen thru, inline fours comming into the shop with runability issues automatically got the carbs taken off, blown apart and put in the parts cleaner (ultrasonic) 3 hours, reassembled and tuned with a stick, never had a comeback

[@rhodesia1578]

Thanks for this blog .. please can you help me .. can you pass on where I can buy new parts for the 1971 Honda CB500 K . This would really be really so appreciated 🙏🏻

[@jseden]

One of the reasons I'm glad my bike only has one carb.. simplicity was tword the top of my list. Mostly because it's my daily driver. Rebel250. Barely more complicated than a lawnmower!

[@kaarong]

That kit should work on my cb650 1979? the carbs looks very similar pd50

[@davidsquires2802]

I set them all at once with a quad set

[@mancavecustoms13]

Enjoyed the quality of the video and the information, great job! Sub from me :)

[@Winterstick549]

I assume this also goes for Honda flat sixes in Valks and older Wings

[@DatMadeMehDay]

Great video! Do you guys think most of this would carry over to my 1980 CB750c?

[@jayarnold8883]

Absolutely. The principles are the same. Some of the adjustment points will vary but common sense should save the day.

[@cgrobe21]

Would this kit work on my 95 honda cb1000?

[@inder605]

Will this work for my old Yamaha. I have a 1983 Yamaha xj650 inline 4 4 carb so it’s similar the the cb650. Will this kit attatch to the Yamaha?

[@Manuqtix.Manuqtix]

My engine kept on misfiring, idle was erratic, high fuel mileage and had no performance I didn’t know so I changed spark plugs, did compression test But it was sorted after syncing the carbs

[@shanec4441]

You can calibrate the gauges against one gauge to guarantee they all match at the same pressure. It’s pretty simple

[@Common-motor]

another method! We prefer this way but there's plenty of ways to do it. Whatever gets you to perfect sync!

[@vespasandengines]

Do you guys still sell the kit? Seems out of stock on the website

[@nickyborg4729]

You can use it on one Cylinder ?

[@dasboototto]

I'm liking my airhead more..

[@sanjayrana6579]

Hello there , do you have 1973 Honda Cb 750 carb cleaning pdf manual thanks

[@nb7466]

I have a cb900c with the real mechanic book. I swore it said you had to sync everything off of carb number two

[@ThisRandomGuyYouDidntNotice]

well both type of gauges have their issues, having to attach four gauges to one cylinder first and noting the offset between the gauges (and respecting these offsets during adjustment) sucks, but so does a leaky line gang valve do... my takeaway: check your equipment before you do the sync :)

[@davidsquires2802]

I set my idle mixture first

[@samsieder9943]

Big Up To Remy Gaillard for his blind 205 fake :)

[@fatrat137]

Aaahhhh flip its so hard to find the right tools to perform the sync on the bike everywhere i go is all 2 guage and 4 guage setups

[@beycoll8275]

You can sync your carbs at anytime. Timing chain, valves, carb rebuild ....really. Buy a decent set of guauges. You can't sync with one gauge. As you adjust one carb it causes other carbs to change flow. A morgan carbtune set of guages is the best without braking the bank. Dont even bother with those cheapo ebay gauges you have there.

[@deanknight3396]

Ment to have a gadge for each cylinder

[@RSChris666]

I didn't know Brain Posehn was into motorcycles.

[@darkshadowsx5949]

i think my 2-3 ignition coil is going bad. All my tuning helped but im fighting the wrong enemy.

[@mikenixon4637]

Wow. This is the crudest approach to this task I can imagine.

[@seanbarney8934]

craziness! They justifiably criticize the accuracy of 4 separate cheapo gauges and then blow it by trying to use one for all 4 cylinders?!?!? ...Too bad the Mike Nixon youtube channel doe not exist to save us.

[@rushtk2112]

Do you have one for a GL1200?

[@harryohanson]

GL1200 has screw in the intake tubes for synchronizing.

[@drn6073]

Nothing wrong with them I've used them for year's

[@nicolascorredor449]

Poner subtitulos al español gracias

[@daze8410]

The vac' ports are different between the models though.

[@daze8410]

Like my 500 has vacuum ports on the intake manifold.

[@daze8410]

Took me a lot of head scratching but after a rebuild on the carbs I was having a hanging idle issue that wasn't there before the rebuild. I assumed they were out of sync and while they were slightly, this didn't solve the problem. What did solve the problem was using genuine OEM Honda jets because the rebuild kit had garbage jets.

[@daze8410]

Oh you do address that

[@velobob4299]

I agree. Shite.

[@dragon60651]

My four gauge synchronizer works for me you just got to know to use it