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Flat spot

  • Thread starter Thread starter Nev
  • Start date Start date
N

Nev

Guest
Hi guys, I've just picked up an 1981 550. It has a flat spot early in the rev range, I'm not exactly sure where because it doesn't have a tacho! Anyway I've pulled the carbs apart and soaked and cleaned everything but the flat spot remains. What I did find out was that the main jets are 92.5's, the only modification seems to be a 4 into 1 exhaust. The plugs look a good colour to my untrained eye if anything a little lean.
I bought the bike as a project / learning tool so I am very green at this stage. Any advice at all get me on the right track would be appreciated. I'm currently working off a clymer manual if that helps, thanks Nev
 
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I downloaded that today luckily. What's so bad about the clymer one?
 
Nev said:
I downloaded that today luckily. What's so bad about the clymer one?
Click to expand...

its like 2nd hand news, seems about right but not perfect. Dont get into bad mistakes from reading wrong stuff. Best thing you could do is get ure self a 100 buck computer and have music and all ure motorcycle files/books on it in you garage. (if you have one)
 
"Seems" to have no mods except for the 4-1?
Does it have the stock airbox and filter?
Jets in it now are 150/92.5?
 
It definitely has the stock air box and filter, no plans on changing that. Everything else looks standard as well. The main jets are 92.5, I wasn't expecting them to be that large?
 
Sounds like the top rear mixture screws are off a little. The consensus here is start at around 2 full turns out from gently seated. Sounds like its not getting enough fuel in the pio,lt circuit and as it transitions to the needles it stumbles a little. The stumble may b from too lean or too rich on the idle circuit. Youll have fo fiddle with the screws to get that prfected. the manual talks of the HIGHEST IDLE method and I think there are some STICKYS at the top of the carb section that may help you.
 
I can't see a brand or any numbers on the exhaust, It looks about the same vintage as the bike from the condition of it though.

ill check out the mixture screws today, that would make sense seeing as above and below the flat spot it runs nicely.
thanks men
 
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Heres a little read from NGK on reading plugs. Make small 1/8 turns on the screws. Maybe start by turning them all IN 1/8 and ride it to see how the change affected things. If no better...or worse..return them to the original place and then try turning them OUT 1/8 and test ride again. If it gets better then your on then right track. its gonna take some fiddling but youll get it. have you read the HIGHEST IDLE thing in the manual yet???

https://www.ngksparkplugs.com/about-ngk/faqs/spark-plug-faqs/how-do-i-read-a-spark-plug
 
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I had a play with the mixture screws just now, it didn't make a difference either way I moved them. I'm starting to wonder wether I'm actually dropping a cylinder/s. it will have to wait until after Christmas now anyway
 
Edit: commented deleted because it was in reference to VM carbs. Sorry.
 
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Should just be the one rear mixture screw on the top...just regular CV carbs far as I see on the fiches.
 
You were right thanks chuck Hahn. After some trial and error I have her running nicely, thanks for your help
 
Im NEVER right about everything. I had a few ideas so I was just wondering which he used to fix the problem.
 
It was the mixture screws, it?s amazing what a small adjustment can do
 
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