Slight surge at cruising speed.

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90MAX

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Joined
Apr 17, 2007
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Location
Des Moines, IA
Good evening all,

I just got back from a nice long freeway trip (About 500 miles), and I noticed some slight surging when I was cruising at around 65 - 70mph / 4500ish RPM (5th gear). It's something I've noticed a few times in the past, but not too often, because most of my riding is 55mph or below. I'm wondering what might be causing this. I've searched the forums a bit, but haven't found much info regarding my particular set of circumstances. I'll lay out what I can below, and I would greatly appreciate any info that anyone is able to provide.

1990 Vmax
-Stock, except for what I think may be a K&N drop in filter.
~37000 miles.
-Starts and idles fine.
-Usually warm enough to turn off choke after warming up for a minute, and 2-3 minutes of gentle - normal riding.
-Once it's warm, it pulls nicely from about 2500RPM on up, with the expected kick in the pants after 6000RPM.
-Temp gauge when cruising around town usually reads below 3 o-clock (less than half-position)
-Temp gauge when cruising at 65 - 70MPH reads about 4 o-clock or a bit lower.
-Occasionally when I'm going past 4000RPM or so I notice a slight backfire.
-Occasionally I put out some gray smoke from the tailpipes, fairly certain I'm not losing any significant amount of oil or coolant.
-I noticed a slight surging at 65 - 70MPH/4500 - 5000RPM, it wasn't consistent, but came and went. :hmmm:
-Bike spent the winter in the garage, with fuel stabilizer in the tank, and run through the carbs.
-I've put about 750 miles on since I got it out for the season.
-I did notice this on a couple of occasions last year, after I purchased it in September.


I know that's a lot of info to digest, but I tried to list everything I thought would help. If I've missed a vital piece of info, please let me know.

I would sincerely appreciate any input that anyone has to offer.

Thanks!

/Sorry if this should be in the carbs/tuning section, I wasn't too sure.
 
Try richening your idle mixture screws a tad. From 4000 - 5000 rpms is where you transisition from idle mixture circuit to needles.

Mark
#1098
 
Hmmm, that makes sense... If I downshifted to 4th, so that I was at about 5200RPM/70ish MPH, it stopped surging.

I'll give that a try once I'm able to get the bike out again. (It's stuck in the garage while they fix my driveway. :bang head: )

Thanks!
 
Your intake rubbers(he he) are most likely sucking air. They will develope tiny cracks with age and will start to breath at highway speeds. this should actually happen at most speeds above 45mph once you settle in on one. I had this on my '86 that tormented me for over a year before finally nailing it down as that.
 
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