81 XS 400 stumbles at 5k rpm

[mjewell]

Starts and idles great, when first starting out bike will hit 65 easily, thereafter only 55. Im guessing float level? Did plug chop and plugs are white. Pipes are aftermarket and quite open. Maybe go up a size on the main?
Thoughts?

 

[willem]

you're running lean. Try adjusting your idle mixture screws, but that probably won't be enough. Yes going up one size on your mains is likely the way to go.

 

[16VGTIDave]

Go to http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html and have a good read. Then walk away from your computer for a while so you can digest what you've read. Then read it again.

This is the best set of instructions I've found for tuning CV carbs. Not specific to our carbs, so you will have to do some thinking on your own. Keep in mind that ANY changes to the engine (air filters, mufflers, valve adjustment, etc) will require starting the tuning process from the beginning.

 

[mjewell]

Solved problem rubber boots from carbs to motor were leaking, new pair and she ran like a top

 

[Scorpio1963]

Easy fixes are always the best.

 

[Unkle Crusty]

Go to http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html and have a good read. Then walk away from your computer for a while so you can digest what you've read. Then read it again.

This is the best set of instructions I've found for tuning CV carbs. Not specific to our carbs, so you will have to do some thinking on your own. Keep in mind that ANY changes to the engine (air filters, mufflers, valve adjustment, etc) will require starting the tuning process from the beginning.

This is the guide I have been using. If I was only reading the plugs it would not work. Must be the yicky thing. I concentrated on the main jets only, so did a bunch of WOT runs. Had spluttering and jerking. Went large on mains, got worse, slowly came down in size, 130, 127.5 125, getting better each time. Best is 122.5 with the needles up one notch. Currently have EMGO filters directly on carbs. Deep Purple is now running so well I am in no hurry to change anything. I still suspect my after market mufflers are not breathing well. Likely better mufflers could lead to bigger main jets.

Like you stated somewhere else, the plugs may show lean. But following the guide has worked the best for me.
I still have to do the WOT test in top gear on the freeway.

Unkle Crusty*