Author Topic: CB400f - carb Air Screw query. Two screws different appearance ?  (Read 983 times)

Offline 400 Cafe Racer

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CB400f - carb Air Screw query. Two screws different appearance ?
« on: September 27, 2021, 09:37:07 PM »
Hi Guys,

Just getting down to adjusting the carbs after the engine, strip, inspection, rebuild.  I have noticed that 3 carbs, 1,3 and 4 have a black largish air screw that will only rotate 270 degrees before it meets a "stop" on the carb casting. (Picture 1)

Carb 2 has a smaller diameter screw with a spring which looks like a conventional type that will screw all the way in or out over several turns.
(Picture 2)

Have you come across this ?
Retired Automotive Engineer   MIMI, MITRE, DMS, MCMI.

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In 1956 at 12yrs old and dirt tracking
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Offline Nurse Julie

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #1 on: September 27, 2021, 10:02:32 PM »
Photo No. 1 is a home made bodge. All 4 should be the same as your photo No. 2
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Online K2-K6

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #2 on: September 27, 2021, 10:15:12 PM »
That type (black plastic with limit stop) is used on small garden machine carburettor and possibly some other application too.

It's ordinarily a cap with limit stop incorporated that is pushed on to a brass airscrew with serated top. They'd be installed and adjusted to specification in "turns out" then the cap put on top to just allow limited adjustments in the field.

Wondering whether it's come from something like that.

Offline 400 Cafe Racer

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #3 on: September 27, 2021, 10:17:12 PM »
Hi Julie,

Thanks for your reply.

Having blown up the photo 1 on the black screw I wondered if this was some sort of plasitc capping fitted to limit the amount of adjustment that most owners would make, ie only 3/4 of a turn from each end of the stop.  They seem softish as you will see from the screwdriver markings on the slot.

They look well made and mate exactly with the protruding stop which is cast into the casing.

I thought I may have read somewhere on a thread that Plastic cappings were fitted, but many haven't survived after 40+ years.

Maybe someone will be able to comment on this. ?

Regards

Dennis
Retired Automotive Engineer   MIMI, MITRE, DMS, MCMI.

Latest    - 1979 CB400/4  Cafe Racer - Parakeet Yellow
              - 1939 Velocette GTP 250 twin port Two stroke

1988-91 - Lamborghini Countach QV replica design and
                 build - 3000hr spare time project

In 1956 at 12yrs old and dirt tracking
              - 1937 Rudge 500 4 valve
              - 1940s Royal Enfield 225sv
              -  Ex WD Norton 500sv

Offline McCabe-Thiele (Ted)

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #4 on: September 27, 2021, 10:18:13 PM »
When I strpped my carbs down three  had the plastic stop - one did not - it looked as thought the plastic cover had just come off - this stop if I remeber right limited the number of turns you could rotate the screw.
I found a used replacement on e-bay.
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Offline 400 Cafe Racer

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #5 on: September 27, 2021, 10:22:37 PM »
That type (black plastic with limit stop) is used on small garden machine carburettor and possibly some other application too.

It's ordinarily a cap with limit stop incorporated that is pushed on to a brass airscrew with serated top. They'd be installed and adjusted to specification in "turns out" then the cap put on top to just allow limited adjustments in the field.

Wondering whether it's come from something like that.

Hi K2 - K6 and Ted,

You both posted while I was replying to Julie,

I think there may be other comments to come in later, but i guess my question would be.  Would you remove these "Caps" and is the normal screw what you find underneath ?. ;)

Thanks

Dennis
Retired Automotive Engineer   MIMI, MITRE, DMS, MCMI.

Latest    - 1979 CB400/4  Cafe Racer - Parakeet Yellow
              - 1939 Velocette GTP 250 twin port Two stroke

1988-91 - Lamborghini Countach QV replica design and
                 build - 3000hr spare time project

In 1956 at 12yrs old and dirt tracking
              - 1937 Rudge 500 4 valve
              - 1940s Royal Enfield 225sv
              -  Ex WD Norton 500sv

Offline Laverda Dave

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #6 on: September 27, 2021, 10:38:17 PM »
The black plastic caps were fitted over the screws to limit the amount of adjustment by the owner as K2-K6 suggested. If you find all these still in place on a 400/4 consider yourself very lucky as they were frequently removed during a carb adjustment and lost (trying to put them back onto the brass screws on carbs 2&3 requires fingers six inches long and as thin as a matchstick)!
They are like gold dust to find s/h, you might find the odd one but never four together!
On a personal note, when I rebuild the carbs on a 400/4 (seems to be a frequent thing I have to do!) I adjust the air screw to two turns out and then replace the plastic caps with the stops at the six o'clock position. This will allow the air screw to be adjusted 1/2 turn either way.
« Last Edit: September 27, 2021, 10:40:32 PM by Laverda Dave »
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline McCabe-Thiele (Ted)

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #7 on: September 27, 2021, 10:42:05 PM »
From my memory you do not remove the cap it's fitted to allow you the range within one turn of the screw - so if it says 1-2 turns the cap will be fitted in turn 1 position allowing  a maximum rotation of two turns before it hits the stop.

I left mine in place - I suspect you could remove them but not sure without going into my garage loft where the carbs are stored to check.
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Honda CB400 four super sport (first money pit)
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Offline Laverda Dave

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #8 on: September 27, 2021, 10:47:51 PM »
The Haynes manual states two turns out Ted. That's why I set the plastic cap stop at 6pm position to allow an adjustment to either 1.5 or 2.5 turns out.
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline Laverda Dave

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #9 on: September 27, 2021, 10:52:01 PM »
They are easily removed, just lever them off using a thin blade screwdriver under the bottom edge just make sure you don't lever from one place as the plastic is soft. They are an interference fit on the brass screw, the screw has a 'spline' type finish as you will see from the second photo on the original post.
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline 400 Cafe Racer

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #10 on: September 28, 2021, 11:12:43 AM »
The black plastic caps were fitted over the screws to limit the amount of adjustment by the owner as K2-K6 suggested. If you find all these still in place on a 400/4 consider yourself very lucky as they were frequently removed during a carb adjustment and lost (trying to put them back onto the brass screws on carbs 2&3 requires fingers six inches long and as thin as a matchstick)!
They are like gold dust to find s/h, you might find the odd one but never four together!
On a personal note, when I rebuild the carbs on a 400/4 (seems to be a frequent thing I have to do!) I adjust the air screw to two turns out and then replace the plastic caps with the stops at the six o'clock position. This will allow the air screw to be adjusted 1/2 turn either way.

Hi Dave and K-K6,

Thanks very much for your confirmation, this is how it appeared to me on close up examination of the photo. I appreciate the tip about how to remove and replace the caps.

At the moment Cyl 1 seems too rich on idle and will not respond to the capped adjustment. It is so rich it prevents combustion. I have set float levels and there is no visible flooding or petrol from the drain tubes. I am working on the assumption that if these are capped they may well have never been removed, so I will remove them all, soak well down the tubes and airways with cleaner, and then give them a good blow through withh compressed air. The pilot jets seem clear.

Cheers :)

Dennis
Retired Automotive Engineer   MIMI, MITRE, DMS, MCMI.

Latest    - 1979 CB400/4  Cafe Racer - Parakeet Yellow
              - 1939 Velocette GTP 250 twin port Two stroke

1988-91 - Lamborghini Countach QV replica design and
                 build - 3000hr spare time project

In 1956 at 12yrs old and dirt tracking
              - 1937 Rudge 500 4 valve
              - 1940s Royal Enfield 225sv
              -  Ex WD Norton 500sv

Offline wildtapholer

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Re: CB400f - carb Air Screw query. Two screws different appearance ?
« Reply #11 on: September 28, 2021, 03:42:49 PM »
Hi Guys,

Just getting down to adjusting the carbs after the engine, strip, inspection, rebuild.  I have noticed that 3 carbs, 1,3 and 4 have a black largish air screw that will only rotate 270 degrees before it meets a "stop" on the carb casting. (Picture 1)

Carb 2 has a smaller diameter screw with a spring which looks like a conventional type that will screw all the way in or out over several turns.
(Picture 2)

Have you come across this ?
I was lucky, my carbs had the plastic cover on all my air screws and I asked what they were on the American site before joining this one, just a cover really with a stopper as to not go more than one turn when in place, he actually asked if I would sell them. When I stripped my carbs down I turned the screws all the way in then out 2 turns then placed the caps back on so can turn out one full turn before hitting stopper making 3 turns out if needed being as my manual says 2 turns out.

 

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