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Persistent left cylinder misfire


Grim
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Hi Grim,

I know I'm late to the party but FWIW my '02 Scura would backfire then spit off the RH inlet rubber after 100 miles. Replaced the inlet rubber & issue resolved, even found the low fuel light works as I now get 180 miles per tank.

Rgds.   Noel

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  • 1 month later...
On 2/7/2021 at 8:45 PM, noel_lemansII said:

Hi Grim,

I know I'm late to the party but FWIW my '02 Scura would backfire then spit off the RH inlet rubber after 100 miles. Replaced the inlet rubber & issue resolved, even found the low fuel light works as I now get 180 miles per tank.

Rgds.   Noel

Thanks Noel,

I did infact replace both inlet rubbers during my troubles! someone on a facebook group kindly donated them to me to see if it helped.

 

Looking at the originals, they were cracked but definately not all the way through, so probably ok...

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17 minutes ago, Grim said:

Thanks Noel,

I did infact replace both inlet rubbers during my troubles! someone on a facebook group kindly donated them to me to see if it helped.

 

Looking at the originals, they were cracked but definately not all the way through, so probably ok...

Did replacing the inlet rubbers resolve your issue?

The effect on mine were dramatic & defo the cause of my issue.

Rgds.  Noel

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Whilst it didn't solve it, it did give me peace of mind that I could rule them out, plus the old ones would have ripped eventually. 
What fixed it was re-setting the TPS, balancing the throttle bodies, then balancing air using air screws and a with vac gauge at 3k.
 

Runs lovely now. Except I haven't ridden it during lockdown so I can't enjoy it!

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  • 3 months later...

It was all going so well..

I decided that with my Mistral crossover and straight through carbon exhaust cans I should richen things up with Meinhof's map.

... But now I'm back with a weak left cylinder! Not sure what happened but it's running weak and fouling the plug. 😭

I think the lower idle uncovered it, my idle was quite high with the Ti map.

Things I had done previously to get to this point:

Set valves to world values.

Reset TPS 

Did a compression test, ok

Tested coil resistance, ok

Air screws all the way in

Synced TBs at idle with rod

Synced TBs at 3k with rod

No exhaust leaks

New intake rubbers

 

With all that done, the left cylinder is missing a bit at idle, if I pull the plug lead from the right cylinder then it stalls after a few seconds, however if I pull the left plug lead and the right cylinder will run happily.

What could make the left cylinder do this, I'm at a bit of a loss.

 

Thanks

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Have you swapped injectors side-for-side? With luck, the problem follows the injector. Are you speaking of the pilot screws closed?

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14 minutes ago, po18guy said:

Have you swapped injectors side-for-side? With luck, the problem follows the injector. Are you speaking of the pilot screws closed?

I have not tried the injectors, I will try that next!

 

Yeah, the air bypass screws closed, that is only because Mienhof recommends it with his map.

 

Thanks

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Just now, Grim said:

I have not tried the injectors, I will try that next!

 

Yeah, the air bypass screws closed, that is only because Mienhof recommends it with his map.

 

Thanks

I know that's supposedly a good map, but to eliminate the air bleeds?

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4 minutes ago, po18guy said:

I know that's supposedly a good map, but to eliminate the air bleeds?

O guess it rules out any differences in adjustment. He says to sync idle on the throttle stops. 

The cylinder behaves well on throttle and the 3k sync is good. It just doesn't want to idle! 

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Which raises the point that the bypass are for idle! (or is that tick-over?). Before disturbing the injectors, you "could" try a good clean of the bypass needles and seats (throttle body or carb cleaner, and Q-tips up the passage to clean the seats. Set them to 1 turn and see what happens. Easy enough to undo. 

EDIT: OK, 1:1,000,000, but could there be a blockage between filter and throttle body? Could the TB shaft bush be worn out of spec? Do yours have the vacuum hoses leading from the intake rubbers inward? All are "potential" sources, even if odd. 

Could pull those vacuum hoses and put a cap on the  intake rubber spigot, or simply hold your finger over it.

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5 minutes ago, po18guy said:

Which raises the point that the bypass are for idle! (or is that tick-over?). Before disturbing the injectors, you "could" try a good clean of the bypass needles and seats (throttle body or carb cleaner, and Q-tips up the passage to clean the seats. Set them to 1 turn and see what happens. Easy enough to undo. 

I agree, I was trying to do it 'by the book' for the map, but I guess its better to have a bike that ticks over.

 

 

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True, that. I recall reports that the injectors are extremely difficult to extract, even on the bench.

@Grim, have you looked at the ignition wire under the boot where it leaves the coil?

IMG_5909.jpg

 

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You are setting idle with the left TB stop screw? Brainstorming here.

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7 minutes ago, po18guy said:

You are setting idle with the left TB stop screw? Brainstorming here.

Right now I am yes. 

I went with the method of backing out the right side and only using the left screw.

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12 minutes ago, docc said:

True, that. I recall reports that the injectors are extremely difficult to extract, even on the bench.

@Grim, have you looked at the ignition wire under the boot where it leaves the coil?

IMG_5909.jpg

 

Honestly, my coils look incredibly crappy, covered in mud and stuff. I guess the resistance was good so I left it at that, I have often dreamt of sticking some nice shiny coils on it..😌

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