How-To: Bench Synch Carbs

[@polymath4fun]

this worked flawlessly! the bike runs beautiful. no popping on decel! been trying to get this cv carbed with pods cb750 running for a while. Definitely can start the build now I know the engine is good. thanks

[@vasillevsky6327]

WOW. This is really was brilliant and after weeks of trying to understand what was going on with my carbs, this really saved me (along with the fact that I had to turn my fuel air screws in more due to my elevation - or at least thats what I think) . This is a really great technique. Thank you!!!

[@TysonNeil]

Thank you for the clear and straight-forward explanation and no shaky cam. Have a 81 CB750 due carb work, this will help.

[@HTMR]

Lesley, doesn't matter what size. I use 1/8 or 3/16 since I have a ton of them laying around. Remember, both have to be the same size.

[@HTMR]

Start with 1.5-2 turns out as baseline. Adjust to highest RPM when you are up and running.

[@monochromaticlightsource9153]

Just done this on a Honda 400/4 but used a piano wire instead of a drill bit. The difference was incredible! A rattly unstable idle, where you could hear the cam chain and primary chain rattling; suddenly became quiet, and quite even, at about 1200RPM. This is just the bench synch, and using vacuum gauges, I'm expecting the idle to be even smoother.

[@HTMR]

YUP, nice job.

[@jesusber3997]

1000 thumbs up man. Congrats and thanx for not to much bla bla

[@HTMR]

It will. Your #2 carb is the base carb. Insert 1st drill bit into #2 and adjust the remaining ones.

[@HTMR]

For those carbs, adjust slides so idle screw is no longer touching the slide and slide is all the way down. Adjust idle screw so that it just touches the slide. Go to WOT and turn idle screw in 2 turns. Any further adjustments will have to be equal for both carbs.

[@qoph1988]

Nice nice... I've heard guitar string for feeler gauge, about to try this with the drill bits, I like that idea. Gotta get a baseline on carbs for a V4 Honda Magna... Most annoying carbs in the world to get off and put back on, hoping I don't have to do it again..

[@HTMR]

Great! Are the both the exhaust ports and spark plugs oily?

[@warovskie]

Ok, I just bench synced the carbs! This is what I did 1.) loosened all slide adjustment screws and turned out the idle adjustment screw until it was not touching the throttle lever. 2.)adjusted all slides in the closed position. Tightened all lock nuts. 3.)WOT turned the WOT screw in 2 turns. 4.) Turned the idle set screw in until I could slide a .075mm pin under the lowest set slide . Then set the remaining three the same. 5.) set the A/F screws! The air screw set at 1 1/2 turns and the idle s

[@HTMR]

No. I thought you had VM slide carbs which do not have synch adjustment and are not banked together. You synch those carbs with the idle adjust. For your carbs, take 2 equal sized drill bits. Select one carb as base, and put drill bit under slide. Adjust other 3 slides with other drill bit until there is slight drag under slide. Adjustment will be under carb top cap.

[@warovskie]

No black smoke, there is some light white smoke that looks to be expected. It did blow a big puff off black smoke once this morning after I messed around with the idle screw when I was installing the top covers on the carbs. Once I got that set back up again it started up fine and runs fairly good! One thing I did notice is if you rap on the throttle really quick the engine will stall if you hold it @ W/O. Other than that it seems to have pretty good response.

[@keeperlit.leterrip7722]

I used to use feeler gauges. Then the last time I did it I decided to try a different method. I lay carbs in a plastic tub with butterflies facing directly upward. I poured equal amounts of fuel into each carb with butterflies in closed position. I adjusted the carbs until the flow rate of all four were equal. It appears to be the most accurate method I have tried. Better than feler gauges. I don't know if anyone has ever used this method before. I just took a notion to try it and I liked the results.

[@jaredj631]

Dude awesome method better than the manual says. I'm going to do my 81 cb750 tonight. Just build a vacuum sync too.

[@HTMR]

Thanks!

[@jorgecanale6660]

MATTHEW, YOU ARE GENIUS!!!

[@HTMR]

OK, disregard my last comment. There might be a base carb in which you need to stick the drill bit under. Get a second drill bit and adjust other 3 slides to have same height. If there is no base carb, pick one and synch the 3 to that one.

[@HTMR]

Hold up a sec. What year make model are you working on?

[@HTMR]

Thanks, hopefully it helped you out.

[@zakoday]

I have the same carbs. Prior to bench tuning, what have you done with the throttle adjustment screw? Thanks

[@HTMR]

Turn until slides are just cracked open. Hold them up to light.

[@warovskie]

Ok, What should the idle Air & fuel mixer screw be set @?

[@kasunchamara9328]

Is this 100% ok. I like to try it and looks easy

[@HTMR]

81. I would try carb clean first, pinesol dip second before messing with pressed in jets.

[@pauldavis4508]

clever,going to try on my 94 seca2/Diversion here in Ireland :-)

[@obfuscated3090]

If you have the diaphragms/dash pots off for cleaning or needle adjustment you can aim the venturis at a BRIGHT light source and get a very good bench sync by SLIGHTLY opening the idle air adjustment so you can barely see light.

[@HTMR]

Good tip. I've done them that way also. Make sure your eyes don't play tricks on you...LOL.

[@nedinsonmejia1811]

thanks for all your videos ive been able to do alot of work on my own bike from your videos i have a 84 CB650 with 5k miles haha its a baby.. was wondering if you could show a video how to adjust that air/fuel on these types of carbs. my bike over heats quick and i think is because when i took the carbs out to clean them must have just screwed in all the jets main jet screws with out manual procedures

[@HTMR]

No problem.

[@Veep_LMc]

Hey Matt what size drills ??

[@warovskie]

Matt, That worked awesome! after I set the manual sync I hooked up the sync gauge up and all four carbs did not need any adjustment what so ever. I noticed while the bike was running that exhaust ports 1 & 3 are very oily. Any idea why? or is this normal with these engines? Port 2 is very dry and 4 has a very little trace of oil on the out side of the port.

[@HTMR]

The base carb will have no adjustment. Verify in the service manual.

[@HTMR]

Good be valve guide seals? Does is smoke when running/

[@HTMR]

No prob!

[@FreziodClipsTwitch]

Cool method but still didn't understand what you were doing with the drill bits. Were you seeing when they pulled out easily or hows that go? Newbie here.

[@HTMR]

Yes, I used them as a "feeler gauge" to make sure all slides are at the same height.

[@warovskie]

Ok, so let's see if I understand this correctly! I turn the idle set screw all the way back out.. Make sure it's not touching. Then adjust all four slides to fully closed. Then go to WOT turn the idle set screw in 2 turns any further adjustments to each carb will need to be done equally...meaning if you adjust the slide on one you need to adjust the other three the same! Is this correct? When using the carb sync on the bike does the adjustments rule still apply? If u turn one adjustment screw c

[@frankjames2712]

I check a lot of bench checking yours was the best. I did mine the way you did on utube a and it runs better then it ever has .. Thanks a lot ofep up the good mechanic tips your good?

[@HTMR]

Your welcome.

[@warovskie]

If there's no base carb, from what I stated on how I synced them is that correct and what should I set the A/F screws @?

[@aladinoreyes946]

When doing that.do you turn idle up or turn it down Or slightly.which way..

[@theDaftman]

can anyone please give a make and module for them carburettors, i have a 1979 Honda cb750 with a set on it that need work.

[@bpago]

Great idea and technique

[@TallGuySmallBike]

So this will not work on fj1200 carbs? since its, 1(screw between the carbs)2 sync 3(screw between the carbs)4 sync then 1/2 (screw in the middle of the set) 3/4 sync

[@keath1963]

Nice video. Very clear!

[@HTMR]

Thanks!

[@jarnosaarinen4583]

You didn't tighten the lock nuts?

[@warovskie]

Ok, The manual does not state that there's a base carb. So going back to my previous steps that I did, was this correct?

[@huimotorco]

Just curious if you have a video or advice on how to bench sync mechanical slide carbs?? Mine are Keihin PD50a style and the butterflies only seemed to be controlled by the choke, the slide itself is controlled by the throttle...any help appreciated!

[@HTMR]

***** I'm familiar with those carbs, although haven't synched a set in 2 years. There should be a base carb, probably number 3 that all have to be synched to. If there is not a base, then pick any carb, stick a drill bit under the slide, then adjust the remaining 3 to match using same size drill bit. The adjustment is on top of the slide under the little cover.

[@huimotorco]

Its the number 2 that is the base carb. I just wasn't sure if the process was the same for mechanical slides vs the butterfly/diaphram style. Thanks for the response. Lets see if this CB650 will respond midrange and up now lol

[@HTMR]

***** Do you plan on synching with gauges eventually? Bench synch just gets you close. Also, I notice that synching becomes more important as the slide nears the closed/idle position. If you have a midrange and up problem, this may or may not solve it. Worth a shot though.

[@jamiechristine001]

81 or 82? You should post a video on how you clean your cb650 carbs Ive got 2 of the same set of carbs and am wondering how everyone else cleans the pressed in jets

[@warovskie]

Is this the same for the vm 26mm mikuni slide carbs?

[@juancordova4301]

hey Matthew should I do the same thing with my 85 kawasaki LTD 700?

[@HTMR]

+Juan Cordova I would inspect them first before considering bench synching them. Did you degang the rack or something?

[@frontcentermusician]

I just tip them up and loosen till the drill bit drops out of the carb. Perfect!

[@aicrag11]

Matt do you know what the springs are called. I am looking up "synchronization screw and spring" in google and nothing is coming up. All of the Honda parts list websites are showing that part as not available? Any help would be greatly appreciated.

[@HTMR]

What year make model. Watch this video:kzbin.info/www/bejne/m6LIZmyZfrOebaM

[@architude]

@@HTMR For Christ sake, their called throttle adjust springs. Either upper or lower.

[@TallGuySmallBike]

Ok cheers will test it out

[@troyperez3822]

Is it the same with the GSXR 1100 carb? I will try to do it

[@HTMR]

Same procedure yes.

[@troysomething2097]

MathewMCRepair how does it work for a snowmobile since the carbs have slides?

[@HTMR]

Same. Slides or butterflies can be set up the same way.

[@nedinsonmejia1811]

bro YOU ARE THE MAN!!!!

[@warovskie]

What should the A/F screws be set @?

[@louielimbaro-refresher7576]

when is the best time to re-synch the carbs? or there are symptoms ? my unit is a cb400 S4

[@HTMR]

Anytime the carbs are serviced is a good time to check synch.

[@deb-kenjohnson9497]

I've ways used feeler gages at closed but this looks way easier.

[@reinaldostrobel81]

Great video!

[@MrGodgdgd]

hi matt i have a 82 nighthawk 650 ,i am having a problem with the plugs going black after 10 mins of running it.but i am new at this shit ,so how do i clean the carbs the right way.thanks for your time .help please..

[@HTMR]

I don't have a detailed video on your model, but if you browse some of my vids, it should give you some tips on cleaning your carbs.

[@marks6663]

when my butterfly valves are at rest, they are both totally closed. Does that means they are already syched?

[@HTMR]

It means they are close enough to get the bike to run OK, you need to vacuum synch them on the bike to get them perfect.

[@marks6663]

I tried your method and with one drill bit holding one open, the other one had a big gap between the drill bit, and no adjustment on the screw would get that gap to close. But once I removed the bit, both closed totally.

[@HTMR]

Is a spring holding it open or something? What year make model?

[@warovskie]

How do I tell what one is the base carb?

[@danamuise4117]

excellent advice!

[@connorhudalla3905]

what is the big brass screw sticking out the top right of carb 2 i broke mine off and cant figure out what it is.

[@HTMR]

Vacuum port for the petcock.

[@amayhew94]

What drill bit did use?

[@HTMR]

Any 2 of the same size. Use small ones like 1/16".

[@AlvaroGarcia-kf3ih]

Im gonna try this on my 94 katana. My cylinder 2 and 4 are not getting hot. i clean the carburetors like 10 times

[@YOUTUBEFELON]

Nice ... a very smart way to do it... :-)

[@cyrielgaemers]

Thanks sooo much!

[@domoD5]

really helpful. thanks!

[@warovskie]

Spark plug 1 & 4 are dry 2 &3 are oily.

[@HaleTruman]

holy shit i would have never thought of that haha

[@warovskie]

ya sure! how can I attach them on you tube?

[@everready19373]

Brilliant!

[@aladinoreyes946]

Does that work on a 95 katana?

[@HTMR]

yes

[@alen91071]

So, why do they even have adjustung screws if all have to be the same?

[@HTMR]

After you synch them with a gauge, I doubt they will all be the same due to all the tolerances. Bench synching them will get it close enough to run before hooking up gauges.

[@jayfbee]

Liked and subscribed

[@vlademirbenevides9850]

Show. Faz menos lambança que água......

[@robertgrant008]

Thanks

[@kiankristopherm.labarda1955]

i want to try it in my bike HONDA CB75cc CUSTOM

[@juancordova4301]

I cleaned them up and everything but those butterfly are not the same

[@HTMR]

+Juan Cordova They should be very close to the same. Bench synch gets you close, still need to vacuum synch.

[@kamonix2000]

very easy thank you very mutch

[@shelbyhilliard8677]

Mine are wide open at idle

[@arvincheech]

tnx!!! cheers!

[@HTMR]

send them to matthew at howtomotorcyclerepair dot com

[@donnafinlayson4430]

why wouldnt you put the bit at the top? easier to feel and remove....just saying

[@HTMR]

See me vid - "Pilot Air/Fuel Screw Adjustment Explained"

[@warovskie]

Ok I just checked the plugs, very strange! Plug #1 is dry oily exhaust port Plug#2 oily dry exhaust port Plug#3 oily oily exhaust port Plug#4 is dry some small traces of oil

[@MarkLoweLowebrau]

You either need to replace your valve stem seals or your piston rings are going

[@HTMR]

Want to send me a pic of plugs?

[@warovskie]

1977 Suzy gs750

[@warovskie]

Still do not have my carbs setup. Please help!

[@gerardodonaire699]

Crack 👏🏻

[@warovskie]

No!

[@troyperez3822]

Is it the same with the GSXR 1100 carb? I will try to do it