Stumbling/misfire on neutral throttle or WOT

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cikko
Posts: 5
Joined: Fri Sep 09, 2022 4:37 pm

Stumbling/misfire on neutral throttle or WOT

Post by cikko »

Hey guys,

I have a 98 and it's had some updates - braided lines and Manual CCTs - free flowing end cans.

I have only just bought it and while it starts and runs well.... infact handles MUCH better than expected.... I have an issue that "feels" electrical but I'm convinced is fuelling.

Here's what it's doing:
Starts and idles fine (with an ocassional hiccup) that sounds like it's thro the carbs.

At any speed on neutral throttle - it has a bad misfire/stumbling. This is elminated by knocking down 2 gears and revving harder.

At top end speed - the same misfire/stumbling is evident on WOT and in tucked position to see how fast it will go.

Both of the above feel exacty the same and it's like a complete loss of power then return of power every quarter second or so.... doesn't do it at all on hard acceleration until top gear and flat out-ish.

Whether it's my interpretation or not - it appears to be worse when hot....so if I do cane it - it seems to be more problematic than if I don't cane it.

What have I done:
I pulled the carbs and cleaned them out (they weren't dirty) wish I had an ultrasonic bath :lol: and they'd be gleaming all round.
The slides were moving as expected with pressure both ways to check if diaphragms were ok. I didn't take them out as I usually struggle to get them back in and was trying to find a fault and rebuild/test...could have a tiny split but not obvious.
I mechanically balanced them so the slightest throttle movement and both butterflies were synched. I have lost my carbtune balancer :doh:
I also removed/blanked off the cooling system as I didn't think it would be required.
The air mixture screws I set to same 2.5 turns out from a soft seat (found out after the front should be less).
One of the carb rubber mounts wasn't very tight when I pulled them off - so thought it could be just that once I refitted I ensured it was tight.
To ensure the airfilter housing and carbs are pressed tight into rubber boots - I folded a cloth and trapped it between tank and airfilter - just for good measure.

Result:
Exactly the same as before I touched it 8O

What didn't I do:
Make a note of jet sizes. So I don't know if it's been jetted for the open end cans or what.... but someone at some point has cared for the bike and done a few tasty mods and the end cans don't look recent - so I'm assuming :angel: it was jetted correctly.
I have not changed spark plugs - they are iridium and look pretty new. Colour is slightly leaner than I expected (so might not be rejetted afterall).... I'd rather have it rich than lean. I did think about temporarily blocking the airfilter to see if that causes or rectifes anything if it is too lean.


Because the bike starts and runs ok - I can't imagine this to be electrical - otherwise it'd surely be either more sporadic (breaking down of a sensor at high temp) or it just wouldn't want to pull hard under load if it were the coils. I've checked them and they both have 13k Ohm secondary windings and 0.6 Ohm primary. The leads are tight into coil body and the spark plug cap.

If anyone has experience that sounds similar with ideas of a fix... I'd be grateful.

Stu :)
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xeris
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Location: Bisbee, AZ

Re: Stumbling/misfire on neutral throttle or WOT

Post by xeris »

While the problem sounds electrical. Repeat your load test with the enrichment (choke) pulled out. Don't know if a failing petcock would restrict the flow of fuel and cause what your experiencing.
“I am quite sure now that often, very often, in matters concerning religion and politics a man’s reasoning powers are not above the monkey’s.”
~ Mark Twain
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MacV2
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Location: Grain

Re: Stumbling/misfire on neutral throttle or WOT

Post by MacV2 »

Yep check the petcock diaphragm for splits or pin holes.

Image

As revs rise the vacume pull opens them up a bit reducing fule flow.



The low speed chough...can you feel it like a thump from under the tank ? That'll be a carb fart, thay all do it to a cirtain extent. Carb balance & set the TPS to as near to 500 omhs as you can.
Making up since 2007, sometimes it's true...Honest...
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Duffy1964
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Location: Orpington Kent

Re: Stumbling/misfire on neutral throttle or WOT

Post by Duffy1964 »

Also it could have a blocked breather/ vent on the fuel tank cap area.


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Duffy1964
Yellow 1997 Storm R.I.P. Orange 2000 Streetfighter (Rufus), Red & Silver 1968 Triumph Trophy 650, Blue 2003 Storm Project, Red 2007 Montesa Cota Trials 250 & 1959 BSA D7 175cc Bantam Trials Project
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fabiostar
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Joined: Sat Jun 01, 2013 3:53 pm
Location: sunny belfast

Re: Stumbling/misfire on neutral throttle or WOT

Post by fabiostar »

What airfilter you using? for ease of use a standard Honda one is best. as you noticed the fuel screws should be different, richer on the rear and as the lads said check the tank diaphram :thumbup:

i had kinda the same as you but in the midrange or on a steady throttle and mine was running filthy rich. i was getting to much air which normally means weak? not in a storms case, i was getting to much air which because you have dustbin size carbs was drawing more fuel in causing it to run rich in the midrange, to the point it was un rideable yet it didnt show up on several dyno runs :crazy: :crazy:

i should say if its just cans they shouldnt cause that much problem, iv had several storms with god knows how many different cans and never had to rejet to solve what you are getting
the older i get,the faster i was :lol:
cikko
Posts: 5
Joined: Fri Sep 09, 2022 4:37 pm

Re: Stumbling/misfire on neutral throttle or WOT

Post by cikko »

Thank you for the input you guys are ace!

I'll get "on it" and see what I've done/not done/should have done haha

either way "I'll be back" said a dude wearing sunnies.... 8)
cikko
Posts: 5
Joined: Fri Sep 09, 2022 4:37 pm

Re: Stumbling/misfire on neutral throttle or WOT

Post by cikko »

Here's a little update.

During this figuring out timeframe I lost deflection on the temp gauge. Luckily it was a £10 sensor whoop whoop! All good on that front. The hassle tho hahaha good job I was taking carbs off "anyway".... :roll:

Onto stumbling:
So I had the carbs off... no dirt at all.
Set the airmixture screws to F 1 and R1.25
Checked the sync (mechanical still not vacuum).
I did not set the airmixture screws with it running as I couldn't get to them.

I did check the TPS and it had been moved (and body broken) a long time ago.... it was set to 410 Ohms. After some faff and trial and error got it to 499 Ohms :D
I wasn't sure when I strated her up - seemed very boggy and flat spot right off idle. But I gave as good a thrashing as my mechanical sympathy would allow :beer: and it seemed to be a bit better. I reduced idle speed and left it running... after a few mins it just stopped. Which I think is carb cough and results in stall.

On a quick test run and it is MUCH MUCH better. Still has a stumble but it's occasional and not consistent as it was before. Neutral throttle and it's ok... so it's rideable everyday again whoop whoop.

I'm as happy as can be without doing a vacuum balance and an airscrew reset.

Thank you for all your suggestions and help!

I hope I can be of equal help to others as we go along :D

Stu :)
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mjvw
Posts: 223
Joined: Thu Oct 22, 2020 9:29 pm
Location: North East

Re: Stumbling/misfire on neutral throttle or WOT

Post by mjvw »

I had similiar issues followed lots of advice on the forum, went back to basics replacing the KN with OEM air filter (Big Improvment) Removing the pairs system (less popping back).
Then purchased second hand carbs very cheap off ebay swopped out removing the dyno jetted carbs now runs well on blue flames.
Storming About :thumbup:
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fabiostar
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Re: Stumbling/misfire on neutral throttle or WOT

Post by fabiostar »

cikko wrote: Sun Sep 18, 2022 6:09 pm Here's a little update.

During this figuring out timeframe I lost deflection on the temp gauge. Luckily it was a £10 sensor whoop whoop! All good on that front. The hassle tho hahaha good job I was taking carbs off "anyway".... :roll:

Onto stumbling:
So I had the carbs off... no dirt at all.
Set the airmixture screws to F 1 and R1.25
Checked the sync (mechanical still not vacuum).
I did not set the airmixture screws with it running as I couldn't get to them.

I did check the TPS and it had been moved (and body broken) a long time ago.... it was set to 410 Ohms. After some faff and trial and error got it to 499 Ohms :D
I wasn't sure when I strated her up - seemed very boggy and flat spot right off idle. But I gave as good a thrashing as my mechanical sympathy would allow :beer: and it seemed to be a bit better. I reduced idle speed and left it running... after a few mins it just stopped. Which I think is carb cough and results in stall.

On a quick test run and it is MUCH MUCH better. Still has a stumble but it's occasional and not consistent as it was before. Neutral throttle and it's ok... so it's rideable everyday again whoop whoop.

I'm as happy as can be without doing a vacuum balance and an airscrew reset.

Thank you for all your suggestions and help!

I hope I can be of equal help to others as we go along :D

Stu :)
try the fuel screws out a little further. 1.75 front and 2.25 rear. yours seems a little weak at 1 and 1 and a little bit :roll:
the older i get,the faster i was :lol:
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