Carb balancing

[Ken4004]

Hi everyone just been trying to set up carbs with the Carbtune kit, not as easy as I thought and very time consuming.

I have managed to get the carbs synchronised but the revs at idle go up and down after tightening everything by this I mean they are a little unstable, also I expected that the engine should pick up fairly well after balancing but it seems to be a bit sluggish.
I am just wondering if it’s due to the air screw position , I have read that the factory set position is around 1 1/2 turns from fully in and also after this initial setting the position can be adjusted with the bike warmed up and the screws are adjusted to get the highest RPM .

If anyone can advise me as to what sort of  vacuum values they are getting for all the carbs at 2000 revs and what their opinion is concerning air screw settings and adjustment.

Many thanks regards Ken

[Nurse Julie]

The air screws should be set at 2 +/- ½. So start at 2, which is the datum, if running lean, screw in ½ a turn meaning more fuel,  if running rich, screw out ½ turn, meaning more air.

[Ken4004]

Thanks Julie

I’ll reset the air screws, also just to let you know I have fitted the new seals you sent me for the oil pump which seems to have fixed the oil leak many thanks.

Regards Ken

[Nurse Julie]

Thanks Julie

I’ll reset the air screws, also just to let you know I have fitted the new seals you sent me for the oil pump which seems to have fixed the oil leak many thanks.

Regards Ken

Great, nothing worse than an incontinent oil pump 😊😊😊

[K2-K6]

Hi everyone just been trying to set up carbs with the Carbtune kit, not as easy as I thought and very time consuming.

I have managed to get the carbs synchronised but the revs at idle go up and down after tightening everything by this I mean they are a little unstable, also I expected that the engine should pick up fairly well after balancing but it seems to be a bit sluggish.
I am just wondering if it’s due to the air screw position , I have read that the factory set position is around 1 1/2 turns from fully in and also after this initial setting the position can be adjusted with the bike warmed up and the screws are adjusted to get the highest RPM .

If anyone can advise me as to what sort of  vacuum values they are getting for all the carbs at 2000 revs and what their opinion is concerning air screw settings and adjustment.

Many thanks regards Ken

There's a little piece missing, that's in Honda's officially detailed method, in addition to that statement.

It calls for this in turning the screw out to get highest rpm ... then to turn it back in again to get a 100rpm drop in revs .... that'll be its final position.

What that's doing is to bring it to it's leanest sustainable mixture point (this being too lean to give decent response to throttle) then to bring it back from that datum point by adding fuel to bring optimal setting for each cylinder.

[Ken4004]

Ok thanks for the extra info, I am hoping that once I have carried out these adjustments the engine will pick up nicely when blipping the throttle.

[K2-K6]

Hopefully you'll get it on target and responding as it should with careful adjustment.

The idle circuit plays a vital, and quite substantial part in how these engines run quietly at tickover and just as much into low speed response to throttle.

With correct sized jets, clean passages etc, they should be relatively easy to set. Any significant variance that takes that "turns" setting of the screw significantly out of range will usually indicate some residual problems with localised flow in the components.

Classic impairment, of one cylinder for example, will usually give lumpy/bumpy idle, iffy response to throttle opening  .... followed by apparent normal operation as the throttle gets itself up fully into main jet territory.  It'll still be running that faulty cylinder leaner than optimal though (if the idle jet is not flowing) that ultimately gives risk to piston life.

Diligent and detailed work will reap big rewards though, with excellent throttle response over a full rev range.

They are particularly nice engines when all running correctly, absolute marvel of design and production.

[Mikep328]

Totally agree!  My 400 was quite cranky at low RPM.  It didn't take the throttle well and stuttered when held at around 1800-2000 RPM.  I pulled disassembled/cleaned/bench synched the carbs and it improved but not to my satisfaction.  I pulled them again and cleaned/adjusted more carefully, also finding two distorted emulsion jets (probably the wrong term) that I hadn't noticed the first time.  I also cleaned all passages with a guitar string as opposed to carb cleaner/compressed air as I had the first time.  I did another bench sync and put the carbs back on.  AMAZING difference.  Bike was smooth at any RPM and pulled immediately upon opening the throttle with no hesitation. 

[Oddjob]

I find inter dental brushes of great use in cleaning out jets etc. cheap, easy to source and come in various sizes. Being nylon they don’t harm the brass surfaces as well which is something which concerns me with guitar strings.

[Ken4004]

Thanks everyone all good info , probably won’t have time to do any more until the weekend but as you all say it’s best to take your time an ensure everything is correct.

I am using the original rubber joints from carb to engine and carb to air box , I did however soften them with winter green and alcohol which seem to work and softened them further when fitting using a hair dryer to warm them up prior to fitting the carbs to the bike.

There might be a possibility that I have an air leak on the engine side of the carb do will investigate this as well.

[exvalvesetdabbler]

If you squirt WD40 on the joint where you suspect a leak the engine note will change as it sucks in a bit of something else instead of aiir.

Regards
Dave

[Ken4004]

Ok thanks Dave I’ll give it a try