[scooter1]

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I recently purchased a 95 SS 900 sp. It has stage one jetting, an open airbox and Micron high mount exhaust. The original owner had rebuilt the carbs with new floats and seals back in 2005. The problem is that it runs great up to about 6800rpm and then starts to sputter and backfire along with a flattening out of the power. Any advice on what to start looking at?

[pat1098]

Have you replaced the fuel filter in the tank? It's a bugger of a job, but is due at 20,000 kms.
But of course it only takes one load of dirty fuel (or a tank repaint) to clog it up.
Your problem sounds like a lack of fuel delivery, but that's just a guess!

[ELTWIN]

To add to Pats suggestions be sure it has the correct plug style and heat range. I had this problem with another bike some years ago and chased my tail with fuel and jetting for days........ until I discovered it wanted a colder temperature range plug. For some reason the standard plug range would make it misfire as it came into the upper power band. On that particular bike changing to a colder plug was an instant fix.
Also check your coils and engine are well earthed because I have encountered misfire caused by poor current supply to the ignition system in times gone by as well. Good luck with it.

[Shrek]

does that run a fuel pump of any kind.. it does sound like it's leaning out up top..

If there is an actual pump I'd suggest it's not operating as effectively as it should, if it's a vacuum type, same deal but I think it's a diaphragm issue.. could even be a tricky hornets nest built into a breather or similar blockage. especially if the bike was an unkown quantity to you prior to purchase.

Hard to pin it on coils though, they should be an under load thing... stranger has happened.

good luck.

[scooter1]

Thanks for the tips. I'll change the fuel filter first as I have to replace the fuel fill gasket anyway and then go on from there.

[hypo ss]

think your on the money shrek had shit in the breather on a 96 blocked up, same prop! fuel filter is also a good start.

[pat1098]

Never had much trouble with the (in-tank) fuel pump on these - it only has to keep the (slightly 'uphill') carbs primed, not maintain the sort of pressure a fuel-injected system requires.
And the coils are definitely a known problem on these. Probably exacerbated by the richer fuelling I presume "Stage 1" entails.
I have fitted so many stronger coils to '90s 2-valves I've got a substantial collection of the brackets they come with - probably around 30 sets?
I usually recommend a coils upgrade for these models (and I fabricated a bracket to this end, which fits the larger/stronger coils without altering the wiring). It solves a myriad of tuning problems, and they ALWAYS start and idle better.
Of course valve clearances need to be spot-on, and the best carburettors I have found are the pricey Keihin FCRs. But a 900 one of these with ported heads, Keihins, 998-type cylinder studs and some exhaust work (and the coils) becomes a fairly potent machine.
Virtually every bike I have fitted stronger coils to has elicited joy and gratitude from the owner - it really cleans them up. And as EL stressed - make sure you have the correct combination of plug and plug cap as you don't want too much (or too little) resistance.
They often need a hotter plug (7 rather than 8 in NGK) to run cleanly with the stock coils, whereas with the stronger coils (I use Bosch transformer type, but there are many options) they run happily - and slightly cooler - on 8s.

[scooter1]

Replaced the in-tank filter but the problem still exists. It wasn't that hard of a job and I got to check the inside of the tank which was in fact very clean. Before I start taking the carbs apart I will install the coil kit from California Cycle Works which includes coils, plug wires and plugs for $159 and probably a new voltage regulator because I've been stranded twice before when they have failed on other Ducs.

[pat1098]

A pity the fuel filter didn't pick it up, but at least you know that's good for a while.
The "bugger of a job" I was referring to was the tiny grub screws on the inside of the cap housing. Often they get over-tightened to the point where you can't undo them without rounding out the hexagon. And the outer seal can be a pain if it's perished..
I hope the coils help. But your original symptoms do sound fuel delivery related.