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Author: Subject: Fireblade carb IDLE JET tuning
indykid

posted on 18/1/17 at 08:07 PM Reply With Quote
Fireblade carb IDLE JET tuning

I'm running 919 fireblade keihin carbs on a 1.6 pinto, but the idle has always been rich, even after a Bogg Brothers rolling road 'setup'. The internet is full of guidance on how to increase the size of the mains, but having reduced the size of the mains 0.1mm, making the mid range much more spritely, I now want to try pull some fuel out at idle too. I've done plenty of reading, but would benefit from some learned conversation.

Left to idle, the exhaust will always develop a soot patch on the floor after a bit and when the throttle is blipped to set off. Fine on my drive, but not so much on other people's.

Bogg Brothers sent me on my way with 150 mains and the standard #40 pilot jets and despite having the gas analyser in it while on the rolling road there was no way I could get the idle emissions anywhere like for the following MOT - I maxed out the meter at 9.9% CO. 140 mains got me just brushing 3.5% CO which passes with a sympathetic MOT tester, but I could really do to sort the sooting issue.

I can order pilot jets for the carbs in a variety of sizes down to #30s, but does anyone have an idea on sensitivity of the steps? Winding the idle screws right out has helped, but even 4 or 5 turns out, beyond their effective range, I'm still getting soot patches. does the full travel of the idle screw equate to a step to the next jet, or a number of jet steps?

I was intending buying a wideband to get some idea, needed for a coming project anyway, but even then, I don't want to waste money on endless sets of jets when an educated stab would have got me 90% of the way there and save time iterating to a solution. I also don't really want a redundant lambda boss in the exhaust.

Cheers,
Tom






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MikeRJ

posted on 19/1/17 at 06:52 PM Reply With Quote
Ignore me, just read your post fully!

[Edited on 19/1/17 by MikeRJ]

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indykid

posted on 21/1/17 at 09:42 PM Reply With Quote
Damn, thought I had a start there when I saw a reply Mike!

Anyone else got any thoughts for the weekend?

Found a set of 5 polini brand pattern jets from #30 to #40 for £8 a set, so ~£35 for a car set posted, but again, a nudge on starting point would be great. 4 individual genuine keihin jets are ~£15 so would soon add up

Cheers,
Tom






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indykid

posted on 8/2/17 at 10:31 PM Reply With Quote
To update this, wideband is now fitted. Idle was running circa 10.5:1 with idle screws wound 4 turns out to try lean it out, because according to all internet wisdom, fireblades have idle air screws, not idle fuel screws.

First major learning, keihin fireblade carbs have idle FUEL screws. Screws now 2 turns in and it's idling at 14-15:1.

I made myself a carb balancer and checked the carb balance. Number 4 was a touch low, but balanced up, it's idling clean at 750rpm and the garage doesn't stink of fuel.

It's still looking rich on the mains from gently ramping throttle, but I want to get it out on the road to see how they sit at more load than just off idle before I make any further changes. That can wait til they stop gritting the roads....

I have data now






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