K&N Air filter - Jetting issue - stuttering at constant throttle

[Harveyz]

Hi

apologies if this is some where else on the forum. - having picked up a Firestorm a month back I've been trying to cure. a problem with it stuttering and surging. at constant throttle (3500rpm) plus back firing on over run - so far. - its got stick coils - all checked out - plugs (new) - checked the fuel tap and vacuum. - works fine with vacuum tester. It got LexTek pipe which are v loud. Over the weekend I stripped the cabs - first clue - the air filter housing has a K&N sticker but had a standard air filter in it. Plot thickens - on opening up the carbs it appears to have had a dynojet kit fitted. - main jets 190's - with both air screws out to 1 3/4 turns. Is anyone got dynojet kit fitted with K&N filter and these setting seem plausible

Sounds daft but is the solution simply to fit a K&N air filter as previous owner(s) has had the bike set up and someone has bunged in a standard air filter or do I fit smaller jets eg 180 or 185 and see if it improves.

[Harveyz]

forgot to mention pilot jets are 48's

[fabiostar]

stick a K&N into it to see if it works with the jetting . or option 2. junk the jets and go back to standard.

[MacV2]

https://www.vtr1000.org/phpBB3/viewtopi ... 20#p500620

[MK_WF]

From your symptoms, it sounds like it's a bit lean in the low / part throttle range.

This is influenced by the needles (which are different for the K&N Kit), the idle jet and the idle "air" screw which actually doses fuel (so it acts in opposite direction to an air screw)
A damaged slide membrane could also be possible.

So my to do list would contain
- check CV slide membranes (caution: need a bit of experience/talent to get them back in)
- try to get the idle mixture screw out (i.e. enrich partial load) in steps of 1/4 to 1/2. If it helps, give it another step until you don't notice a difference. Then turn back to last setting.
- if nothing helps, either get the full K&C stuff, or the full OEM stuff

[xeris]

You need a known base line. Put everything back to stock and start from there. Or, https://www.superhawkforum.com/forums/t ... -up-24769/
As MK-WF suggested, the "surging" would indicate a lean condition. Is it really surging? A feeling of gain and loss of power? Or is the "stuttering" just a sound? Something similar to a rapid misfire, but without the loss of power? Given the jetting discovered in the carbs, and the missing K&N filter I would say that it's a rich condition. Try slowly pulling the enrichment knob out and see if it gets better or worse.

[Harveyz]

Hi Folk's - thanks for all the responses - sorry been away working. Update. I have stripped them and found more. - it has a dynojet kit fitted. - extra hole drilled in the slides - I've gone back to std air filter, front main jet 180 rear 182 - air screws front 2 turns , rear 2 1/4 turns - no more popping and banging on over run - start first touch of the button. Only remaining issue is rev's between 3,000 and 4,000 (not as bad but surging on the throttle) - goes like stink 4,000 to redline. - potter about happily in traffic as sub 3,000 rpm. I read on a US site that drilling the extra hole in the slide to speed up its movement that Dynojet recommend - causes the main jets to come into early. Next job is to expoxy the extra hole in the slide (if any one can tell which are original holes. - that would really help) and try again.

[freeridenick]

Hi Folk's - thanks for all the responses - sorry been away working. Update. I have stripped them and found more. - it has a dynojet kit fitted. - extra hole drilled in the slides - I've gone back to std air filter, front main jet 180 rear 182 - air screws front 2 turns , rear 2 1/4 turns - no more popping and banging on over run - start first touch of the button. Only remaining issue is rev's between 3,000 and 4,000 (not as bad but surging on the throttle) - goes like stink 4,000 to redline. - potter about happily in traffic as sub 3,000 rpm. I read on a US site that drilling the extra hole in the slide to speed up its movement that Dynojet recommend - causes the main jets to come into early. Next job is to expoxy the extra hole in the slide (if any one can tell which are original holes. - that would really help) and try again.

You're bang on. Block the extra hole up and you'll probably be OK. At a guess Mac linked you to the carb tuning guide and that pretty much says the same: the extra hole is not the right way to go. With a setup like yours you can fix the lean condition with jetting and needle clip position. If it was a modified engine I'd suggest blocking the air relief hole, but go back to stock and start from there. Make sure the 'heat shield' is in place under the snorkel too.

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[freeridenick]

Hi Folk's - thanks for all the responses - sorry been away working. Update. I have stripped them and found more. - it has a dynojet kit fitted. - extra hole drilled in the slides - I've gone back to std air filter, front main jet 180 rear 182 - air screws front 2 turns , rear 2 1/4 turns - no more popping and banging on over run - start first touch of the button. Only remaining issue is rev's between 3,000 and 4,000 (not as bad but surging on the throttle) - goes like stink 4,000 to redline. - potter about happily in traffic as sub 3,000 rpm. I read on a US site that drilling the extra hole in the slide to speed up its movement that Dynojet recommend - causes the main jets to come into early. Next job is to expoxy the extra hole in the slide (if any one can tell which are original holes. - that would really help) and try again.

The original holes are both at the top of the slide.

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[Harveyz]

Thanks for the advice - I'll check the airbox - looks standard - once I have epoxy the extra hole - will check again on the main jets - I'll keep going down toward stock - the pipe's are loud and very free flowing.

I had a thought last night - to go stock on main jets and fit some Delkevic cans (worked well on my SV mini twin).

[Harveyz]

The saga continues - had the storm out for a run - start first time on the choke - stopped for gas when it was good and hot - wouldn't start - massive backfire poor woman in the petrol station nearly wet herself. Got it started - cut out twice on way home however runs well on the main jets to redline - always seemed to be as the revs were dropping as I come to a stop. Checked everything today - carb sync/pilot jet screw (2 1/4 front 2 1/2 back). - still hard to start when hot. Here's the strange bit - if I screw the idle screw in (idle about 2,000 rpm) it starts - ahhhhhhhh.

My only choice left. appears to be junk the loud pipes it came with and fit a set of Delkovic cans ( jet down from 180/182 to standard 175/178) or take it to my local dyno shop and pay a wedge to get them to sort it (defeats the object as I love tinkering )


Any help gratefully excepted as currently sipping a beer and contemplating my next move

Cheers

[sirch345]

I hope the beer helped

When you had the carbs stripped did you check the diaphragms for any splits/holes in them

It could be worth trying another new set of spark plugs, I have heard on a rare occasion of a brand new plug breaking down before.

If not already done balance the carbs,

Chris.

[Harveyz]

Hi thanks for getting back to me - beer definitely helped - vacuums in the carbs was ok when I stripped them - TPS set and sync'd. I did fit new plugs but might give that a try. spent the evening reading about the effect the fuel screw has on low range rpm - I'll have another play during the week

[sirch345]

Hi thanks for getting back to me - beer definitely helped - vacuums in the carbs was ok when I stripped them - TPS set and sync'd. I did fit new plugs but might give that a try. spent the evening reading about the effect the fuel screw has on low range rpm - I'll have another play during the week

That's known as dedication

Good luck with it, I hope you can get to the bottom of the problem soon,

Chris.

[Harveyz]

Hi

Had a few other avenues to check. - breath carb pipe routing plus an interesting one - it has manual camchain tensioners - if set to tight they cause hot starting problems - busy weekend