Carb Balancing - Why Bother?

[Wagonwheels]

So things progress with the CB400NE (that’s what the engine number says?)

All fluids replaced: Oil/hydraulic brake/forks
carbs cleaned, really cleaned second time around.
engine runs, pick-up a bit hesitant, sounds better but much like a bag of nails
off comes the clutch cover (bottom two fasteners have to come out with cover as they foul right hand exhaust tube on extraction
loads of fun on re-assembly, but we got there)
adjusted balance weight chain, adjusted cam chain, adjusted (again) tappets.
much quieter.

NEXT!

balance carbs/adjust mixture/ignition timing.

Any pointers to previous posts or personal experience on these topics gratefully received

As always Many Thanks

 

[Maraakate]

Balancing carbs helps a smooth idle and throttle response. It can also help with the bike being a bit unbalanced in some extreme cases. It is worth doing.

Btw the E at the end of the engine number just stands for Engine. You have a CB400N.

 

[LongDistanceRider]

Sync/balance of the carbs insures that each cylinder draws the same amount of air/fuel mixture under all throttle positions which in turn produces optimal power.
Consider a wagon drawn by 1 draft horse and 1 pony racing against a wagon with 2 draft horses. Who will win?

 

[Wagonwheels]

Sync/balance of the carbs insures that each cylinder draws the same amount of air/fuel mixture under all throttle positions which in turn produces optimal power.
Consider a wagon drawn by 1 draft horse and 1 pony racing against a wagon with 2 draft horses. Who will win?

Thanks for the response...
OK carb balancing is a good idea. In fact in my youth (now long gone) I did balance a four carb set up on a CB750F..... I know how to balance the carbs on my twin, but how the heck do you get at the thread
and locknut that sits neatly between the carbs and are inaccessable from every angle. Fair enough, I understand the wheel has already been invented and twin carbs do get balanced. I just need to know how cos at the moment it seems impossible?
I guess I could take the carbs off for a third time and check the butterfies on each carb close the same about, but I'd like to adjust the intake to each cylinder dynamically with vaccum gauges, maybe
Many Thanks

 

[Wagonwheels]

Thanks for the response...
OK carb balancing is a good idea. In fact in my youth (now long gone) I did balance a four carb set up on a CB750F..... I know how to balance the carbs on my twin, but how the heck do you get at the thread
and locknut that sits neatly between the carbs and are inaccessable from every angle. Fair enough, I understand the wheel has already been invented and twin carbs do get balanced. I just need to know how cos at the moment it seems impossible?
I guess I could take the carbs off for a third time and check the butterfies on each carb close the same about, but I'd like to adjust the intake to each cylinder dynamically with vaccum gauges, maybe
Many Thanks

More searching the net has thrown up advice from some 10 years ago from this forum on balancing twin carbs, I was sure someone had been down this path before me, however a nice link to Motion Pro shows intake adapters for $4, but I'm in the UK, No problem Amazon do them for $18-19.
Many Thanks

 

[ancientdad]

More searching the net has thrown up advice from some 10 years ago from this forum

Probably some of the same members saying it, but that was likely our old home where you found it (HT)

 

[esmith250624]

I won't say it was EASY, but I used this 90 degree ratcheting screwdriver and found the right combination of screwdriver extension bits, taped them together so they don't fall apart, and it was not too bad. Access for me was from the right side and I removed the triangle motor mount plate for better access. I think the 400n is the same at my 400t. the thumbwheel on this tool is what made it doable, due to the limited space for the ratcheting motion. so something similar might work.

 

[Wagonwheels]

I won't say it was EASY, but I used this 90 degree ratcheting screwdriver and found the right combination of screwdriver extension bits, taped them together so they don't fall apart, and it was not too bad. Access for me was from the right side and I removed the triangle motor mount plate for better access. I think the 400n is the same at my 400t. the thumbwheel on this tool is what made it doable, due to the limited space for the ratcheting motion. so something similar might work.

View attachment 36477

Many thanks for this, I was begining to think I was losing my grip.... again! Yes I also have taken the right side triangular top engine steady off to gain access. I even toyed with the idea of drilling a hole in the frame to give a clear route to the screw using
a long (really long) flat blade screwdriver. Also tested vacuum on each side with a gauge, seem to be 100mm Hg difference. Anyway we have to take the left hand carb as the reference since (I have now learned) the adjustment screw acts on the right hand carb thottle only. So we just need to tweak the adjustment screw so both carbs' vacuum match while running the engine at approx 1200 revs. no problem! Well apart from realeasing the locknut and turning the adjustment screw (without turning the locknut).
What could be simpler!

Many thanks for your help.

 

[Richard Pitman]

Balancing the four carbs on the Hornet is a pain, as access to adjustment screws on the throttle linkage is only available at certain throttle openings. Means you have to turn the motor off each time to make adjustments, then start up again and note changes, makes it a trial and error process to get three carbs sucking the same as the fixed reference one. Be much easier if the screws were accessible with the engine running, make adjustments in real time. Thanks Honda

 

[ancientdad]

Thanks Honda

Yes, you'd have thought they would have designed some access into the situation as they did for the head bolts on (at least some of) the SOHC 400/450 models, especially considering the head bolts on those models would be far less often removed than the carbs adjusted.

 

[LongDistanceRider]

Yes, you'd have thought they would have designed some access into the situation as they did for the head bolts on (at least some of) the SOHC 400/450 models, especially considering the head bolts on those models would be far less often removed than the carbs adjusted.

In Honda/Keihin's defense, the carbs were sync'd before they were installed during assembly and the sync isn't a regular maintenance item on these models. It's pretty much a one and done affair

 

[ancientdad]

In Honda/Keihin's defense, the carbs were sync'd before they were installed during assembly and the sync isn't a regular maintenance item on these models. It's pretty much a one and done affair

Yeah that's true, I rarely had to sync any of my 4 cylinders and never had to sync either of my 6s (not that I wanted to on the latter). They weren't perfect, but not off enough to bother with.

 

[Wagonwheels]

And so I notice petrol leaking from the slow running screw on my meticulously refurbed Keilin carb set. No problem, must be them Chinese spares again. Fixed that leak only to find petrol dripping from the float bowl seal. No problem as by now I have spare gaskets. Off with the float bowl (using my allen key on hex head screws, no more cross head access struggles for me!) only to find the dreaded red dust sitting in the bottom of the bowl!

So I yanked and levered out that fuel filter from the tank that I was too scared to do previously and out came a load of crud.......and have been flushing since. Bought a new tank filter with tap (after market item). Honda original item £60 ~$80 ++ No chance!
Friend says that no-one touches 'vintage' japanese bikes that have carbs these days, I wonder why?
Wife has simple solution....Sell it!!

Many Thanks

 

[Maraakate]

The carbs on these are not very complicated and there are people who will touch them. Your local shop however will not.

Hex head screws are potentially harmful as they promote overtightening and then you can strip the carb body. "Cross head screws" are really JIS #2 screws. Laser tools in the UK sells a t-handle set with removeable bits that works well. Vessel also has a JIS #2 screwdriver. Both are cheap tools that do the job.

You don't need to change the float bowl gasket every time you drop the bowl. You can let the gasket sit out for a bit to reduce swelling or wash it in cold soapy water.

Chinese spares? Are you referring to chinese float valves (i.e. float needles)? If yes, this will contribute to poor running and overflowing, guaranteed. You must use OEM float valves on these carbs or it will never run right. For aftermarket carb gaskets get the K&L ones for a GL1100. The K&L ones seems to be the best aftermarket ones that hold up. The super cheap ones get distorted after a couple of times and never go back to shape... total junk.

 

[Wagonwheels]

The carbs on these are not very complicated and there are people who will touch them. Your local shop however will not.

Hex head screws are potentially harmful as they promote overtightening and then you can strip the carb body. "Cross head screws" are really JIS #2 screws. Laser tools in the UK sells a t-handle set with removeable bits that works well. Vessel also has a JIS #2 screwdriver. Both are cheap tools that do the job.

You don't need to change the float bowl gasket every time you drop the bowl. You can let the gasket sit out for a bit to reduce swelling or wash it in cold soapy water.

Chinese spares? Are you referring to chinese float valves (i.e. float needles)? If yes, this will contribute to poor running and overflowing, guaranteed. You must use OEM float valves on these carbs or it will never run right. For aftermarket carb gaskets get the K&L ones for a GL1100. The K&L ones seems to be the best aftermarket ones that hold up. The super cheap ones get distorted after a couple of times and never go back to shape... total junk.

Many thanks for the comments and experience...duly noted, I did think I was making progress....reminds me somewhat when my BSA 175cc which failed its roadworthy test. Oil leaked from the new folk seals I'd installed. My friendly tester pointed out that if the chrome on the stanchions was worn or uneven (which was true) the new seals would not last long, made sense. So we got some new stanchions, replaced the seals (having stripped the front end....again!) and all has been well since (fingers crossed). The bike I bought was in one piece but neglected, I dont think it had seen much fork oil before me.....

You live and sometimes you learn..

 

[crazypj]

I don't remember having a problem doing carb sync on the 250/400 three valve twins (but, I forget stuff from 45+ years ago )
Many of the mid/late 70's Honda's were very easy to service but as they got newer they became more difficult or even impossible (Was it 2009 or 2010 Hornet didn't even have any adjusters?)

 

[zhuffy]

I won't say it was EASY, but I used this 90 degree ratcheting screwdriver and found the right combination of screwdriver extension bits, taped them together so they don't fall apart, and it was not too bad. Access for me was from the right side and I removed the triangle motor mount plate for better access. I think the 400n is the same at my 400t. the thumbwheel on this tool is what made it doable, due to the limited space for the ratcheting motion. so something similar might work.

View attachment 36477

can't you just put a screwdriver adapter onto a regular ratchet?

 

[crazypj]

Those little 1/4" ratchets are much lower profile than even a 'low profile' ratchet.
I have one of them although almost never use it nowadays.
You can also but a cheap screwdriver extension kit and cut it to the length you need

 

[old timer]

I only sync'd the carbs on a 400 on the bike once. After that they were done before installing.

I never used any carb kits except from Sirius Consolidated . All the 400 kits came with the correct jets for the model I was working on. Never had a problem with any kit bought from them.

Bill H

 

[LongDistanceRider]

I only sync'd the carbs on a 400 on the bike once. After that they were done before installing.

I never used any carb kits except from Sirius Consolidated . All the 400 kits came with the correct jets for the model I was working on. Never had a problem with any kit bought from them.

Bill H

Yeah, the sync on the CM/CB 400/450 series is pretty much a one and done deal. I've checked periodically after thousands of miles and nothing needed.

 

[esmith250624]

can't you just put a screwdriver adapter onto a regular ratchet?

Yes you can, but there is not must room for the ratcheting action on a full size ratchet, This is why I commented that the Thumbwheel on the mini ratchet was very helpful in making small adjustments. Could be just my preference.