Author Topic: Unused Jets?  (Read 338 times)

Offline Raygun

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Unused Jets?
« on: June 09, 2022, 10:49:57 AM »
Hey all,

I'm not here to start a 'what jets do I need thread' so don't worry! But this is, in some way a jet sourcing thread.

Does anyone have jets that they tested, didn't work, and are sitting unused they'd sell/swap? I'm looking for a range of jet sizes to test but sourcing sets is tough. Otherwise I'm going to continue to try and find a number of jet sizes to build a test inventory.

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The bike in question!

I'm running open stacks and 2-into-2 shorty exhausts (baffled), 069A Carbs, stock jetting (Main #98, Idle #38) : running generally well, but with slight rich symptoms at low revs and lean at higher revs)

Main: I'd like to test both 1 and 2, possibly 3 jet sizes up for the main jets.

Idle: On some research, I'd also like to test one size down, along with some larger sizes. The working theory is that with the level of increased airflow, the carbs may be drawing in fuel from other circuits in addition to the idle circuit, creating a rich condition at idle that becomes lean later on.

Thoughts/advice always welcome, and if anyone has similar issues I'll help where I can

Online K2-K6

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Re: Unused Jets?
« Reply #1 on: June 10, 2022, 12:42:30 PM »
Bike looks really good, no spare jets to offer though.

Setting it up;- generally it's not pulling more air in (that's still controlled by the same head flow and cam timing) and that leads to wrong assumption on jetting to get it nearer optimum. The change when going from std to pods etc is that the same air flow into the engine now has a different vacuum profile (this controls how much fuel is pulled through the same jets) with les vacuum just sucking up less fuel.

How to get back to the same amount of fuelling is the question.  You can give yourself a check of idle jets with a routine in the Honda manual that sets them correctly. With all service items correct (ignition etc) and engine warmed, turn the air screw from about 1/2 turn out very slowly with it running until that cylinder starts to falter and misfire (only just about detected) then turn it back in again to get 100rpm drop on revs.
Now carefully (with engine off) turn the screw all the way in while recording how many turns and fractions of a turn it will go until lightly seated. This figure should differ from manual start point for those carbs. If it's further inward (3/4 turn instead of 1 1/2 for example)  that suggests it needs a larger idle jet installed. Likewise, if much further out it would be asking for a smaller jet.
It's effectively using you (by listening to it) as an oxygen sensor much like a fuel injection system does in reporting to ECU mapping. Just as it falters is too lean to run competently and turning it back inwards brings it into decent range of combustion.

Worthwhile working on idle circuit firstly as that influences the main jetting a long way up into throttle opening. Giving it a larger idle jet will also give more fuel at top end (they don't stop flowing but contribute to total fuel load even if you can't adjust it there) with help in separating what you've got and giving clear indication of direction. 

Offline Trigger

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Re: Unused Jets?
« Reply #2 on: June 10, 2022, 06:43:02 PM »
You have just entered in to a minefield of problems as, you have removed the air box you have taken away the turbulence and are getting droplets of fuel instead of a mist  ;)
This problem with removing the air box has been discussed and debated to deaf ears over the years >>>>>
 http://forums.sohc4.net/index.php/topic,182280.0.html 

 

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Re: Unused Jets?
« Reply #3 on: June 11, 2022, 08:50:10 AM »
For safety while working on jetting use at least an #8 grade NGK spark plug to reduce risk of overheating plug tip which will cause pre-ignition  (very bad in getting combustion chamber too hot) it can also give you a better colour reading of mixture effects to help diagnosis.

Offline Raygun

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Re: Unused Jets?
« Reply #4 on: June 23, 2022, 04:20:54 PM »
All,

Thanks so much for your advice- the forum is always a goldmine for knowledge on these bikes.

Understood re the stock airbox- but it was not possible to keep it with the wider design choices I wanted to make for this bike. I recognise this now comes with a potentially painful journey of plug chops, trial and error and tuning, so I'm just at the start of that process now. Reading that thread has been helpful and I may see if there is some way I can engineer an appropriate solution that is akin to stock as possible.

The stacks, fortunately, seal well and are smooth entry so I hope go some way to mitigating the droplets issue.

I'll get cracking on some of the adjustments suggested and see how it goes- thanks!

Offline Oddjob

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Re: Unused Jets?
« Reply #5 on: June 23, 2022, 04:26:59 PM »
Rear brake pedal looks far too high, you got weird feet??
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Offline Raygun

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Re: Unused Jets?
« Reply #6 on: June 27, 2022, 09:26:50 AM »
Yeah you're right it does look weird!

I think it might be the angle? Currently the pedal is level with the footpeg when I saw the bike yesterday!


Offline Oddjob

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Re: Unused Jets?
« Reply #7 on: June 27, 2022, 01:15:56 PM »
Have you got the adjuster bolt in the frame?

And is the lever touching the bottom of the footrest? Looks very close in the pic.
Don't play stupid with me, I'm better at it

 

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