Topic: Thoughts about points

[Deltarider]

About air screws...like a paint gun, less air will bring less paint to the nossle and spray. I finally understood it when Hondaman explained it. The pilot circuit will work like a paint gun, more airflow, more fuel will flow here.

Not on my bike...

[BomberMann650]

About air screws...like a paint gun, less air will bring less paint to the nossle and spray. I finally understood it when Hondaman explained it. The pilot circuit will work like a paint gun, more airflow, more fuel will flow here.

Not on my bike...

Doesn’t the idle air screw INCREASE vacuum pressure in the circuit by closing the hole in the idle circuit?  Forcing the circuit to pull MORE from the jet than the throat?  That would be opposite of the paint sprayer function.

[AirCanuck]

About air screws...like a paint gun, less air will bring less paint to the nossle and spray. I finally understood it when Hondaman explained it. The pilot circuit will work like a paint gun, more airflow, more fuel will flow here.

Not on my bike...

Doesn’t the idle air screw INCREASE vacuum pressure in the circuit by closing the hole in the idle circuit?  Forcing the circuit to pull MORE from the jet than the throat?  That would be opposite of the paint sprayer function.

Uh, are we talking about idle screws or what? I’m totally lost

[TwoTired]

Hondaman and I have a difference of opinion regarding the idle screws on the earlier, non PD carbs.

I did exhaust analyzer testing of a 74 CB550, that clearly indicated that the air screws on those carbs made the mixture richer when turned inward.  Further, when I turned those air screws outward for leanest exhaust gas readings (ended up being 6 to 8 turns), the off idle engine response was dismal to  non-existent, even though it idled beautifully.  Certainly un-rideable as I just couldn't get the engine to rev up with the bike in any gear.
Gradually and incrementally turning the air screws inward eventually restored the throttle twist problem until it was street acceptable.  The Exhaust gas analyzer once again indicated it was rather rich during idle operation.  And, surprise surprise, my stock configured bike exhaust and intake ran best at Honda recommended air screw setting.  Who'da?

Hondaman explained the exhaust gas analyzer was being fooled, and the bike was getting too rich to accelerate.  My tuning experience just doesn't support this.

Too rich bikes blubber going in and out of too-rich-to-support-combustion mixtures.  Too lean mixtures have a fine border edge of tune where they either fire or don't.  Nothing to "clean up" when going in an out of combustion range, no blubber/no burble.  The Cb550 example had a throttle that acted more like a switch.  On idle it ran fine, off idle under load it didn't, like the coils were turned on and off and on.  No blubber, no burble to me means not a too rich indication.  I simply can't believe my exhaust gas analyser was being fooled, as engine behavior matched indications.

Finally, Honda's own tech article explains, inward rich and outward leaner for air screws...  Which Hondaman also says is wrong.

Therefore, as a result of my findings, I have to disagree with Hondaman's assertion, and maintain that turning air screws outward leans the idle mix and turning them inward makes the idle mix richer.  Of course the Idle Mixture Screws (IMS) found on PD carbs work the opposite way.

I can infer that since the CB750 Carbs have the same physical pilot/idle circuit arrangement as those found on the CB550, they would function in the same way as the Cb550 early carbs.  But, I have no early model 750 carbs to characterize.  If there is some subtlety about 750 carbs I'm unaware of, well believe what you wish.  But, I don't see any sort of venturi in the pilot circuit, the air screws meter the amount of air available to mix with pilot jet output, and regulate the pressure presented to the pilot jet output.  Opening the screw reduces the pressure in the pilot circuit, making it closer to outside atmospheric pressure, which equates to less fuel draw from the jet orifice while simultaneously providing more air.  This makes for leaner pilot circuit delivery.  Closing off the air screw, allows less air availability for mixing with the jet output, while simultaneously allowing the low pressure source (the carb throat) to exert more draw on the pilot jet exit. E.G. the mixture gets richer.

Pity there is controversy about this.

Cheers,

[BomberMann650]

The inverse can be true on other carb designs.  Relative to position of the screw in the circuit.  Right?

[TwoTired]

The inverse can be true on other carb designs.  Relative to position of the screw in the circuit.  Right?

I hesitate to generalize for all carbs in existence.  But, for the SOHC4, it follows that the air screw is on the inlet side, and the IMS is on the engine side.

The engine side screw regulates the combined air fuel mixture to the engine. It both allows more engine vacuum to effect delivery and allows the passage of more mixture to flow past the needle.  Open for more rich, close for more lean, all the way to completely shut off. PD carbs have this feature.

The air screws on the early carbs operate in the reverse, in my experience, and directly control the air entering the pilot circuit, along with providing the higher pressure of near atmospheric from the filter box at the inlet to the carb.

Cheers,

[Deltarider]

The picture TT shows is from the Honda Motorcycle Carburetor Instruction Manual. If only all of us would read the comprehensive and easy to follow explanation there, it would save a lot of confusion. https://www.4shared.com/office/jC4Zzkqhce/Honda_Motorcycle_Carburetor_In.html

[PeWe]

Adjust the air screws so idle smells correct from too rich can be one way.  When I used my std CB750 K6 carbs it was easy to adjust them.
Everything std: mains 105, std pilots (probably 40), needles in middle. Air screws so it did not bang and bubble during decelarations.
4-4 off, 4-1 in: I changed main jet only, 115. I might have tried 120 too. (still std airbox)

836, ported head, cam, small conical pods, loud 4-1. Still std pilot jets, needles 1 notch richer (lower step), mains until it stopped run better when releasing throttle a little at full speed (Speedo at 210 kmp) Or no obvious too rich and sooty plugs. I have seen my old carbs, there are 147.5 jets in them!! This was with the fuel from the 80's.
Air screws still so it will not bubble at throttle deceleration. I guess I started with the recommended in users manual.

Ignition was stock Honda

Exhaust can make the carb tuning tricky too
I noticed this year that it is almost impossible to find a proper tune on low speed with a wide open exhaust.

Work better with diffusers in (not too restrictive though) exhaust back pressure. Maybe in combination with a hot cam that has relatively long overlap. Exhaust Lotus Root 4-4.
I think this is the difference between street and race use. A race bike has no need of smooth and even slow riding, but need the extra power it can get. Not too unrestricted, I got more max power with No numbers 4-4 (HM 300 replicas) than the wide open Lotus Root, both without diffusers proven on dyno runs. This surprised me so I change ignition, advanced and retarded which did not change the outcome on the dyno with wide open Lotus Root 4-4 which had better power between 4200-6800 rpm only.

I have been in contact with Ripple Rock Racers about their 4-2-1. The longer muffler (16") gives more power than the shorter (12") and is not that loud either.

[rotortiller]

I have a difference of opinion regarding the idle screws on the earlier, non PD carbs.

So do I. The pilot air screw theory pretty basic, like taking a straw and drilling a calibrated hole in the side while sucking eggs LOL. I find reading/following the manual and understanding a specific system much better than spraying paint. Glad you cleared up some BS sir, that is always a good thing.

rt

[Deltarider]

I have a difference of opinion regarding the idle screws on the earlier, non PD carbs.

So do I. The pilot air screw theory pretty basic, like taking a straw and drilling a calibrated hole in the side while sucking eggs LOL. I find reading/following the manual and understanding a specific system much better than spraying paint. Glad you cleared up some BS sir, that is always a good thing.

rt

The link I posted previously, seemes to give some problems here and there. Honda4Fun used to have it on their site, but I can't find it anymore there. So here's another one: http://cincinnaticaferacer.com/wp-content/Manuals/HondaCarbManual1975.pdf Download takes some time (31 MB). Although there's more interesting stuff in the 34 pages, the chapters AIR-BLEEDS and IDLE AND LOW SPEED SYSTEM matter most in this respect. Unless you have a particular interest, you can leave the pages that deal with CV carbs for what they are.

[Geeto67]

These engines can run at 20:1 A/F mixture with today's fuels, as they are knock-resistant in order to cope with auto EFI systems, which can be slow to respond to sudden driver's foot requests. This was not the case when they were first designed, so the carbs were set conservatively rich to prevent warranty claims: sparkplug replacements were cheap as compared to pistons. And yes, you can get more torque with richer mixtures, a racer's favorite carb tweak - but racers go to the track with boxes of sparkplugs handy!

This is pretty fascinating. I had not considered the consistency of fuel because there were only a couple of years I had for the lead fuels before they disappeared. In the 90's I had the valve seats hardened and the compression lowered from 10.5:1 to 9.5:1 in the GTO just so I wouldn't have to deal with pump gas issues so it never was a concern for me. On my cb750s I detonated in manhattan summer traffic and started running premium to stave off it during the hot season, but otherwise never had a fuel related problem.