trouble maintaining constant RPMs (RPMs dipping rhythmically)

â™£ McKraut â™£ · February 12, 2009, 05:19:59 AM

i've recently (within the last week or so) noticed that while under slow & steady acceleration, or while maintaining a constant speed, such as on city streets going about 40mph, my RPMs seem to dip almost rhythmically (maybe a little faster than every second)... maybe wavering or shakey are better descriptors...i'm not sure if calling it surging is accurate. it's fairly subtle... i.e. if i'm attempting to maintain a constant 4800rpms, the RPMs will dip to maybe 4775, back to 4800, back to 4775..etc. so it's fairly slight...almost gives me the feeling like i'm going over a series of small dips in the road, but again, there's a constant rhythm to it. does that make any sense? i'm not even sure what i would check or where to start... everything seems to be working just fine otherwise; it always starts, spark plugs and yuasa battery are new as of about two months ago.

any advice on this would be appreciated

Howie · February 12, 2009, 05:31:45 AM

From your description, sounds like typical closed loop surge. Sometimes cam timing, valve adjustment and throttle synch (base settings) can eliminate or reduce the problem. Sometimes more drastic measures like the DP computer or a Fat Duc
http://fatduc.com
are needed. First, check the base settings.

ScottRNelson · February 12, 2009, 05:49:07 AM

I noticed something like that when my chain had stretched unevenly. Check the chain tension at different points in the chain to see if that might be the cause.

Speeddog · February 12, 2009, 12:33:20 PM

Is it a Desmoquattro S4R, or a Testastretta?

What year?

â™£ McKraut â™£ · February 12, 2009, 01:00:16 PM

it's a DQ. 2005 S4R.

re: chain tension... my last check seemed to be right where it was supposed to be, though i didn't check the entire length of the chain for any possible slack spots. timing was checked coming up on 6k miles ago...so that shouldn't be too bad at this point i wouldn't think.

it's currently at about 21k miles, stock air intake/filter setup, stock ECU, and full arrow system. engine-wise everything is stock.

ScottRNelson · February 12, 2009, 01:09:03 PM

Quote from: â™£ McKraut â™£ on February 12, 2009, 01:00:16 PM
re: chain tension... my last check seemed to be right where it was supposed to be, though i didn't check the entire length of the chain for any possible slack spots.

But that's what you need to do. You can't check one point and determine that a chain has stretched unevenly. You should check the whole chain so that you can rule out a possible cause.

Speeddog · February 12, 2009, 01:11:34 PM

Valve clearances may be a bit loose, if your last service was 6k miles ago.

â™£ McKraut â™£ · February 12, 2009, 01:43:48 PM

the chain i'll check this weekend for sure. the valves will have to wait a couple weeks or so, depending on how schedules work out.

if it doesn't end up going away after those two things, are there any other things i can check? i'll see about throttle sync; haven't done it before... would it be a culprit even if i haven't changed anything in the system?

thanks for all the suggestions

Speeddog · February 12, 2009, 01:47:00 PM

Throttle sync would be a good thing to do.

The shop doing the work should be able to adjust the trim as well.

â™£ McKraut â™£ · February 16, 2009, 09:33:44 AM

checked the chain the proper way the other day...turns out it is much too loose. with the top section of the chain at it's tightest point, i was finding some mega slack on the bottom... now i'm just trying to locate my chain adjustment tool...

Major Slow · February 16, 2009, 09:59:47 AM

Quote from: ScottRNelson on February 12, 2009, 05:49:07 AM
I noticed something like that when my chain had stretched unevenly. Check the chain tension at different points in the chain to see if that might be the cause.

You're going to have to explain this one to me. How can the chain have different tension at different points? The total length of the chain is constant. The diameters of both sprockets are constant. The distance between the sprockets (without changing the adjustment) is constant. I am not seeing how the deflection of the chain can be different anywhere in the chain.

ScottRNelson · February 16, 2009, 10:22:12 AM

Quote from: rose_guy on February 16, 2009, 09:59:47 AM
You're going to have to explain this one to me. How can the chain have different tension at different points? The total length of the chain is constant. The diameters of both sprockets are constant. The distance between the sprockets (without changing the adjustment) is constant. I am not seeing how the deflection of the chain can be different anywhere in the chain.

There are two different things that can happen.

1) The chain wears where the links rotate relative to each other so that the chain in effect stretches, but some parts of the chain wear more than others so that some parts of the chain are tighter or looser than others.

2) The links get tight spots where they don't bend as easily in some parts causing the links to ride up the sprocket at some points with the same effect of the chain being tighter at some positions than at others.

The total length of the chain might be constant, but the length of one 20-link section is not necessarily the same as the length of another 20-link section.

I had to replace the chain on my ST2 because of this very issue, even though it looked perfectly good and was not stretched overall by a large amount. Once you get a loose section in the chain, besides the surging feeling that you feel at low speeds, it puts extra stress on the chain and sprockets whenever it transitions from the loose part to the tight part, hastening the degeneration of the chain.

muskrat · February 16, 2009, 01:28:38 PM

I too have the same issue I just noticed today. While at idle it simply quit. I just got it back from it's 6,000 mile service. Any thoughts? I'm calling AMS tomorrow because it was not doing this before.
Thanks.

Howie · February 16, 2009, 01:56:35 PM

Quote from: muskrat on February 16, 2009, 01:28:38 PM
I too have the same issue I just noticed today. While at idle it simply quit. I just got it back from it's 6,000 mile service. Any thoughts? I'm calling AMS tomorrow because it was not doing this before.
Thanks.

The bike should go back to the dealer, as you said.

â™£ McKraut â™£ · February 16, 2009, 04:29:06 PM

couldn't find my OEM adjustment tool...so ordered this one: http://cgi.ebay.com/ebaymotors/Ducati-chain-adjuster-tool-748-916-996-998-S4R-S2R_W0QQitemZ260360087209QQihZ016QQcategoryZ35618QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

hope it works... basically all i need to do is loosen those pinch bolts and then tighten with the tool, right? then tighten them back up to something like 30ft/lbs?

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trouble maintaining constant RPMs (RPMs dipping rhythmically)