Triumphrat

Autotek · 09-13-2012, 07:02 PM

I bought this bike about a year ago and have left it in storage. Its a low mileage (8600) bike in what appears to be showroom condition, 1997 Thunderbird 900 Triple. I cleaned the carbs yesterday and found a few obvious flags so I expected a good running bike today when I got it back together. It starts easily and runs very smooth on the stand but when I drove it, it begins to studder at around 4500 rpms, almost as if it is being rev limited. Not matter what gear it is smooth until about 4500-5000. Does this bike have a low oil limiter or something? It is difficult to see through the site glass so im not sure if it is low or perhaps high and I dont want to overfill it. Has anyone had this problem or know where I should go next. Obviously I plan to drop the oil and add the recommended amount with a new filter very soon.
FYI= the carbs are clean and sinked and the air jets are at 2 turns out from bottom. (an average of where they started)

Thanks for any help. Milo

BRG-BIRD · 09-13-2012, 08:25 PM

What jets are in the carbs? Needle clip position? Did you check the diaphrams?

Autotek · 09-13-2012, 09:17 PM

Needles are third ring from the top and jets are factory. Diaphragms are fine. I'm saying it sounds like it has a rev limiter at 4500 rpms.

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BRG-BIRD · 09-13-2012, 09:40 PM

My guess is you may have an air leak somewhere between the carbs and engine, make sure all the clamps are tight. How did it run before? Had you put any miles on before parking it?

Autotek · 09-13-2012, 11:26 PM

everything is tight. again, it runs perfect from idle to around 4500-5000 rpms, then it begens to studder as if it has a rev limiter. Not something an intake manifold leek would do.

ArferBrick · 09-14-2012, 04:34 AM

Wrong - Mikuni BSTs are very susceptible to problems caused by airleaks, and do not take kindly to being messed with.

Check the air filter box, does it have the airfilter in there still? Is the filter clogged?

Problems from around the 4000 RPM mark and above are slide related, ie slides not raising properly or incorrect mixture from somewhere, ie an air leak. Did you re-assemble the needle valves correctly? They should be in the order - small white plastic circle spacer first, then the needle & circlip, then lastly the wider "flying saucer" plastic washer. It's very easy to get them in the wrong order, I've done it myself.

Did you use oil on the slides and guides on re-assembly? Clean it off, it hinders their operation. You can check the slide operation by lifting them with your finger (when fully assembled), they should rise to full open and return to full closed without any resistance. If they resist then there's something wrong.

Also check the emulsion tube air inlets in the rear intake mouths for blockage (the fixed jets in the middle).

I would also replace ALL O-rings with brand new correct replacements, especially the ones under the slide guides and the idle screw rubbers as they cause massive problems.

I had exactly the same problem last weekend when I put a set of Trophy carbs on my Adventurer. It idled beautifully and was great till about 4000RPM and then it just seemed to not want to rev any higher, like it was starving of fuel or something.

I changed the slides out with my original Adventurer ones and it ran great. The only difference being that the Trophy slides have only 1 tiny air hole at the bottom of the slide, whereas the Adventurer has 2 massive (circa 3mm) holes.

These holes allow air to be sucked out by air passing over them (inlet manifold pressure), raising the slides and opening the needle jets, the more pressure, the more they rise, richening the mix and allowing more fuel into the engine. If they don't rise, you get fuel starvation, and limited revs.

So I'd be looking in their direction.

Oh yes, one last thought - check to make sure the o-rings are all there under the slide lids, it's easy for them to fall out during re-assembly and you not notice it.

IrlMike · 09-14-2012, 06:00 AM

Besides what Arfer said, & agreeing with his general point that it's fundamentally a mixture problem, the intake is very restricted so even small gaps/leaks cause big problems of the kind the OP describes - check the airbox carefully for any distortion or poor fitting of the two halves. Over time, the plastic degrades, if there's any sign of deterioration it's best to replace it (bought as a complete assy with new filter).

As regards the sight glass, it's likely the reason you can't get a reading is that the level is already above the glass. With the sidestand down, raise the bike up slowly whilst watching the glass until the oil appears. If it doesn't, it may be low. Get a friend to help lower the bike the other direction whilst you watch the glass.

On mine, the front & rear ride height changes completely altered the sight glass level. After measuring what came out, I refilled with 3.4 litres for an oil and filter change, then positioned the bike with a set height of blocks under the sidestand to give me a level in the glass, so I can check it occasionally. Don't worry about this too much - ime, these engines don't burn oil.

BRG-BIRD · 09-14-2012, 07:49 AM

Quote:

Originally Posted by Autotek

everything is tight. again, it runs perfect from idle to around 4500-5000 rpms, then it begens to studder as if it has a rev limiter. Not something an intake manifold leek would do.

You did not answer if you had ridden it before or if you had checked the carb diaphrams. Keep you mind open to possibilities. Arfer an IrlMike have done extensive carb work and know what they are doing, even little old me has rebuilt a set of Mikunis...

Autotek · 09-14-2012, 09:01 AM

Ok, I am sorry, I guess I am under thinking this. So this is where I am...
I bought this bike a year ago at a dealer only bid sale, (my fiance is a car dealer) and the only drive time I had on it was to the shop where we take all of our cars for service. At the time I had several bike projects and didnt have time to mess with this one. It idled way too high and the shop said it need a new airbox and supposedly replaced it. they said it was hard to find and it took a long time. I got the bike home about 8 months later and it ran terrible. Two days ago I removed the carbs and cleaned and blew out everything. I found the main jet from the center carb laying in the float bowl so i figured I found the source. haha
OK, got it back together and what a night mare getting those boots all on correctly, and I am certain they are and pretty sure that the intake boots are all good. The needles rings are correct, the o rings I did not replace but the are all there. I DID NOT oil the slides but I rem they slide easily by hand.(not good enough?) My air jets are at 2 turns, maybe not quite enough?
I REALLY DO NOT WANT to take these thing out again!!!!

I listening and thank you all, Milo

Autotek · 09-14-2012, 09:04 AM

I just confirmed that the airbox was replace so filter is new.