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stevecFri May-20-05 02:13 AM
Member since Mar 18th 2009
11 posts
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"slight backfire"


  

          

Hi There,

Riding my Mk11 850 after about 6 months, accelerating up a slight incline in third gear trying to avoid pigs, dogs, chickens and errant pedestrians in my little country, I did what I always dread I could do in a moment of non focus and dropped it into second instead of fourth.

It was momentary, slowed me and proberly red lined the engine.
After a few minutes continuing I turned around and started back home.

I noticed a slowly worsening cackle/backfire when I throttled off.It never did this before.

The bike still ran ok but the dynamics changed ever so slightly and seem to also develop a slight lag in the chain takeup when going from the throttle off to throttle on.

It problem is not a severe backfire just a continual in between plop plop and a little louder than the normal exhaust tone.

I'm wondering if any one else who may have done something similar that might like to own up!!!! and had similar symptoms. The bike starts ok but doesn't idle that well now.

I'm wondering if I've just stretched a chain or dislodged a setting in the timing slightly.
Avoiding comments that allude to my stupidity I would appreciate some help if there is any.

SteveC

  

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HenrikFri May-20-05 03:35 AM

  
#1. "RE: slight backfire"
In response to Reply # 0


  

          

Your intake or exhaust valve could have collided with the piston and got damaged, is the compression still as god as before?

B.R.
Henrik

  

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dynodaveyFri May-20-05 06:36 AM

  
#2. "RE: slight backfire"
In response to Reply # 0


  

          

I agree you may have had valve clash from over reving. I have several heads with evidence of valve damage/bent stem, including one I got off Ebay where the seller claims it came off a running bike(just bolt on and run), yet you could see a bit of light by the intake valve since it no longer would seat all the way.
It might show up on a compression check.

  

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stevecWed May-25-05 10:31 PM
Member since Mar 18th 2009
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#3. "RE: slight backfire"
In response to Reply # 2


  

          

Hi Guys,

Thank you very much for your replies.

In kicking the engine over it seems to have the same feeling of compression.
I can get hold of a tester.
What sort of pressure parameters could I expect on a standard 850 motor.
The motor had an overhaul about 9K ago.


Regards
Steve

  

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HenrikThu May-26-05 04:08 AM

  
#4. "RE: slight backfire"
In response to Reply # 3


  

          

Hi Steve

I think the normal readings are around a 145 psi, but if it is a bit lower I don´t think it is critical.

B.R.
Henrik

  

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stevecTue Sep-06-05 03:39 PM
Member since Mar 18th 2009
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#6. "RE: slight backfire"
In response to Reply # 4
Tue Sep-06-05 03:46 PM by stevec

  

          

I'm back again on this same topic (sudden reving by pushing the gear to 2nd instead or 4th)as I haven't had a chance to run my bike since the last replies. Thanks again for giving me direction.

I did change the spark plugs and found the bike improved it's demeanour a little. After another run it still had a little exhaust backfire but not to serious.It started,ran and pulled well.

The main thing seems to be at idle, it runs with a subtle miss in the engine. It revs out ok and sounds smooth but at idle (it won't idle properly, almost but not quite)it eventually stops and coughs. Cold or warm.

The plugs look ok. The spark looks ok. The battery is fully charged.


The compression tested out at 150psi pretty close on both cylinders.But can't tell if there is bad or slightly damaged valve/seating through.

Because the bike was going so well prior to this I'm not sure where to head, ie electrical,Mechanical or fuel. Any more assistance would be greatly appreciated.
A thought though, could the strain through the running gear by sudden drop in speed as I went into second gear cause a change in a setting has altered something.


Regards
steve

  

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janSun Sep-11-05 06:18 AM
Member since Mar 18th 2009
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#7. "RE: slight backfire"
In response to Reply # 6


  

          

check for air leaks between carb and head . check balance tube . are the exhaust pipes tight ?

  

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dynodaveyThu May-26-05 06:46 AM

  
#5. "RE: slight backfire"
In response to Reply # 3


  

          

A leak down guage would reduce the guessing factor about the compression check results....

  

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