Flat spot

[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

[sharpy]

get a genuine manual from basscliffs website... http://members.dslextreme.com/users/bikecliff/ and then use that other manual to start a barbie.

[Nev]

I downloaded that today luckily. What's so bad about the clymer one?

[sharpy]

I downloaded that today luckily. What's so bad about the clymer one?

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)

[steve murdoch]

"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?

[Nev]

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?

[steveb922]

From memory, 92.5 is correct. Which exhaust do you have?

[chuck hahn]

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.

[chuck hahn]

Although not a Suzuki style CV carb, the basic principals in this video apply. Note the idle and needle interact as its transitioning from pilot to main circuits.

http://www.thegsresources.com/_forum/showthread.php?249531-Graphic-of-how-CV-carbs-function

[Nev]

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

[chuck hahn]

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

[chuck hahn]

You may also have to place a shim between the E clip on the needles and the slide itself. These guys make stainless as thin as .005 thick. Measure the diameter of a needle and the OD of an E clip and use the selector feature on the left side.

https://www.superiorwasher.com/washers/6/Shims.html

[Nev]

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

[Nessism]

Edit: commented deleted because it was in reference to VM carbs. Sorry.

[chuck hahn]

Should just be the one rear mixture screw on the top...just regular CV carbs far as I see on the fiches.