Acceleration Misfire

[K2-K6]

I do think its worthy of debate here ... thank you to the contributing debaters in following this topic through.

If a statement of this type can't be usefully examined, cross checked and verified ... then really its probably myth in quality.

Things like this must be able to withstand reasonable scrutiny to hold a validity in making adjustments to these carburettor and engines.

There is ... in official workshop manual of 750 ... an engineering drawing, in section vertically through the carburettor and with noted key parts.
Subsection of that drawing is an expanded view of the idle circuit junction/interface showing decent enough detail.

It's also split in vertical plane (important here) and not derived into plan view, usually signified in technical drawing by 45 degree extraction notification to make clear that has been accepted. It's important here as that expansion diagram could be inadvertently "read" as an airscrew illustration if it was indeed plan view. Its not.

What that shows is a side view, sectioned vertically of the idle jet, installation and emulsifier tube in situ. This has an air passage disecting it around the emulsion tube height. There's no modulation drawn for adjustment of that fuel supply route, at all. It's just plain through from idle fuel jet orifice, up the tube and through the emulsifier toward venturi.

If that drawing was mistaken for an adjustable compartment (the sectioned jet lookin like a needle type adjustment screw, because it's section illustration has similar form) then someone could make the incorrect interpretation, based on that detail, that it is a different component.

I'd be honestly surprised if it could be shown that this design could work in tbe opposite of that which we understand it.

[TIM1954]

Generally speaking the Coils tend to fail as the bike gets hotter. Worth checking them out.

[Seabeowner]

Always fascinated me this discussion on mixture. When on tickover or very low revs how much of the air goes past the the carb sliders and how much goes via the air mixture screw (not much)? Where does the air that passes the screw feed in? Still have an open mind.

[Bryanj]

There is a small brass orifice below the slide on filter side of carb

[McCabe-Thiele (Ted)]

Here is an observation I've made on my 500.
When the engine is started from cold (Summer time) I do not like to use the choke so it's soon off. I made the error a couple of days ago of screwing down the main throttle bracket screw 1/4 turn to make it idle when only part warm. As I rode off it started popping and banging like crazy. I returned home thinking something was very wrong  - by this time it was well up to normal running temperature so the idle was 2k plus hanging throttle. Returned the throttle stop  screw to idle again at 1100-1200 rpm as engine was warm. Blipping the throttle now no popping or banging, no hanging idle. Went back out for ride all was good. Using the choke plus it's fast idle from cold is different - no popping or banging. The only difference I could work out was the hanging throttle here was caused by the sliders being prevented from closing to the correct position.

This has led me to believe that hanging throttle issues are as a consequence of the sliders not being sufficient closed in the shut throttle position. If the sliders are in the sufficiently closed position any idle speed adjustment should be made via the air screw only.

I think the trick might be knowing what the sweet spot is for "sliders  are sufficiently closed".  Open for debate

[Nurse Julie]

To this day Mark Paris is still insisting screw in for more air, screw out for more fuel. Anectodal and personal experience on 750's and all other non PD SOHC /4 carbs, is that out gives more air, in gives more fuel. If we had set the screws on our own builds and bikes as per Marks advice, the bikes wouldn't have run. Regular riding of all our bikes by ourselves and all our customers bikes, prove we have done it correctly.

[McCabe-Thiele (Ted)]

Sounds as if Mark Paris is getting confused between when an engine is running rich and when it is running weak.
If he has those concepts reversed then he is correct.lol

Back in my youth I used to think a hot tail pipe exhaust meant it was running too rich my Dad put me right it took me a while for the penny to drop. After all too much fuel it would burn hotter no?

He was right of course when it's rich the exhuast gases are cooler at the tail pipe - in a way it defies logic if you ignore what's taking place inside the engine and how the petrol actually burns.

[Saesneg Shaun]

Haven't been out much on the bike but got the chance to ride today. I tried turning the air screws in a bit but no difference to the bogging down unfortunately. Slightly richer means it won't idle cleanly unless I up the tickover speed to nearer 2000rpm so i'm guessing 1 1/8th turns out is about right. Also tried spraying the inlets with brake cleaner but no air leaks either.

The I rule out the more i'm beginning to think its electrical and the boyer ignition is to blame. Its happening between 2000-3000 rpm in all gears.

[Skoti]

On the other hand you may have the same problem that I had.
Needle jets worn and guttered out, giving a massive flat spot when accelerating.

Honda part number 16012 392 004

[Saesneg Shaun]

On the other hand you may have the same problem that I had.
Needle jets worn and guttered out, giving a massive flat spot when accelerating.

Honda part number 16012 392 004

I hadn't considered that. The bike has 33k miles on it but their probably the originals in there. Is there any obvious visible signs of wear I can look for or just replace?

[Johnwebley]

If the needle jets are worn,it's likely to make it to rich,

Try dropping the needle by one groove, see what happens

I assume you have the standard air box and filter?



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[McCabe-Thiele (Ted)]

Have you eliminated the possiblity that its coil or plug lead related rather than suspecting the Boyer unit?

As has been said if it was worn needles that would make it run richer then you might expect that to be reflected in the spark plug colouring. Are you using resistor plugs or just resistor caps. Are the spark plugs relatively new?

If you have the points base & advancer might be worth returning the ignition back to standard temporarily - easier than having to take the carbs off. (Never seen a Boyer Unit so not sure they are)

[Saesneg Shaun]

Have you eliminated the possiblity that its coil or plug lead related rather than suspecting the Boyer unit?

As has been said if it was worn needles that would make it run richer then you might expect that to be reflected in the spark plug colouring. Are you using resistor plugs or just resistor caps. Are the spark plugs relatively new?

If you have the points base & advancer might be worth returning the ignition back to standard temporarily - easier than having to take the carbs off. (Never seen a Boyer Unit so not sure they are)

Yes, I checked them all and trimmed back and loose ones. I checked the connections on the boyer ignition and just to be sure the kill switch and ignition switch. I'm running resistor caps and non-resistor NGK plugs.
Unfortunately I had issues with the points cam running out of true hence going to boyer electronic.

[Skoti]

On the other hand you may have the same problem that I had.
Needle jets worn and guttered out, giving a massive flat spot when accelerating.

Honda part number 16012 392 004

I hadn't considered that. The bike has 33k miles on it but their probably the originals in there. Is there any obvious visible signs of wear I can look for or just replace?

The originals in mines had done 23k.
Held up to the light along with the new ones you will probably easily see the wear difference.

Good luck.

Skoti