Part-throttle stutter, inconclusive tests.

Hi all,

I've got a stutter at part throttle on my 2001 SV650S, 22,333 miles. At WOT, it runs great, but at part-throttle while cruising or in the turns, it has a bit of a stutter, which thanks to the compression of the v-twin, makes for a potentially dangerous jerkiness in the turns. After some forum-searching, I suspected a lean idle or off-idle mixture. I tweaked that, no improvement. I took the carbs apart, suspecting debris, cleaned everything I saw, put back together, no improvement.

Thinking that maybe I missed something in the carbs, I sent it off to a professional to have them torn down, dipped, blown out, rebuilt. No luck. When I installed the rebuilt carbs on the bike, it started up quicker than it has in a long time, and the front tire comes off the ground again in first and sometimes second at WOT, so I'm hoping the carbs are OK... However, that damn stutter persists.

Here's the things I've checked, and the results. I'm getting mixed info in my searches online. I'm hoping someone here and can point me in the right direction:

• Low idle is a bit... bouncy? It will kind of pulse around a 500 RPM change.
• Voltage at battery at idle: ~14v
• Voltage at battery 2 5k RPM: Drops from ~14v to around 12.5v - 13v
• When the engine is cold, there will be an occasional "chuff", backfire from the front carb., and the vacuum slide seems to move less than the rear carb.
• When I measured the voltage at the signal generator unit (based on Clymer repair manual troubleshooting guide) with a regular voltmeter (not Suzuki ignition special tester) I get about 1.5v, not the > 3v the manual says.
• The various voltmeter and ohmmeter tests I ran on the coils passed.
• When removed them from the head and grounded to the crankcase, the spark plugs seem to be producing a solid blue spark. I couldn't tell if it was missing any though.

The manual says to replace the signal generator unit if it reads less than 3v, but at $270 at the only place I've found it so far, I'm hoping other people have better ideas...

The front carb backfire seems to scream to me some kind of carb issue, but the carb rebuilder I used had nothing but good reviews, and it runs like it did when new at WOT, so it's at least working halfway decently...

Help? Any testing ideas? Any potential fixes less than the $270 SGU? Anything suggestions less than saying "screw it" and buying new carbs for roughly $arm + $leg S&H so I don't have to deal with it any more?

ANY info appreciated!

Cheers,
-Troy

[yorkie_chris]

I don't think it is signal generator, nothing suggests this.

Vacuum leak on front carb or bad carb balancing is where I would look.

[Sid Squid]

An ordinary meter won't display pulse voltage - the Suzuki tester has a peak hold display - this would account for the difference in the reading that you got. As it runs well enough otherwise I doubt that the pickup is the fault.

Other than that it's back to basics, check everything again - twice. The bad news is that faults like this can be surprisingly difficult to track down - don't be disheartened though, a methodical and careful approach pays off.

*sigh* I'll get started on checking for vacuum leaks again. If that comes up with nothing, on to carb balance.

Any particular tricks you'd recommend for finding a vacuum leak?

Thanks,
-Troy

[Spanner Man]

Good morning all.


As said, go back to basics, & check all the obvious stuff including the valve clearances.

Another couple of things to check are the spark plug caps. Make sure they're not cracked, & are securely connected to the lead. Any corrosion where they screw onto the lead isn't going to help at low rpm.

Try clamping the vacuum lines from the rear cylinder whilst the engine is running. If there's a minute split in either the fuel tap diaphragm, or a defective fuel pump. You will have an air leak that can't be found by the normal tests.

A good way to find an air leak is to use a can of GT85, or WD40. Spray a little onto the manifold joint with the engine idling. If there's a sudden change in the idle speed, it means that the engine has immediately drawn the spray in.


Cheers.

Valve clearance too, eh? *sigh* Times like this, I wish I'd gotten the naked bike! Thanks for the tips, I'll definitely give the WD-40 one a try tonight...

Spark plug caps and wires are good... I ran my bare hands over them while cranking it over. (You laugh, but it's pretty effective at finding insulation problems!)

Cheers,
-Troy

[Spanner Man]

Originally Posted by Troy

Spark plug caps and wires are good... I ran my bare hands over them while cranking it over. (You laugh, but it's pretty effective at finding insulation problems!)

Cheers,
-Troy

Unless you have heart problems that is!


Cheers.

If anyone is curious, I found the problem! The diaphragm for the vacuum slide on the front carburetor had apparently come off the little lip in the carb body when the black cover was put back on. The resulting vacuum leak past the diaphragm was causing the slide to open very slowly, if at all. This meant the mid range of the throttle was running very lean, as the needle wasn't being pulled out of the jet. The very lean condition was the cause of the stumbling and missing I was seeing. <Problem>

Now all I need to do is figure out the stumble at the lower half of the throttle range when it is opened quickly... Separate post?