Triumph Rat Motorcycle Forums banner

T-Bird triple wont rev over 4500?

10K views 28 replies 8 participants last post by  Autotek 
#1 ·
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
 
See less See more
1
#4 ·
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?:)
 
#8 ·
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...
 
#6 · (Edited)
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.
 
#15 ·
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.
Actually, while you have the result correct, and the cause, the mechanism is exactly the opposite. It's counter-intuitive, but, if the slides don't rise it's just like having a choke on. Air is restricted, velocity goes up, and the pressure on the main jet is much lower than normal, so fuel delivery is much higher. The mixture goes extremely rich.

Since the carbs are "constant velocity" the pressure on the main jets is supposed to remain constant regardless of air flow, hence the need for an increase in jet size as air flow increases. This is handled by the tapered needle and its jet.
 
#7 ·
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.
 
#9 ·
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
 
#11 ·
No worries, we're all here to help :).

I found the main jet from the center carb laying in the float bowl
This worries me and sets of alarm bells. I would suggest another person had tried to work on it but didn't do a proper job.

Sadly I reckon you're going to have to pull the carbs off again, but it gets easier with practice!

The air filter box is new, that's good, but still look inside it to make sure there's actually a filter in there and no wasps nest. It doesn't cost anything and it could save a whole lot of work.

If the main jet was laying in the float bowl, I would suggest the old owner tried to play with the slides and emulsion tubes. The Mikuni BSTs have a known problem with wearing those grey plastic slide guides out and as a consequence wears out the emulsion tubes and needle jets. But that's just info at this point, first I would take each one apart and make sure everything is in the correct order.

Here's my photo album from when I rebuilt mine, it should give you an idea of what you're looking at -

Part 1 Part 2

Pay particular attention to all the O-Rings. They may look good but please replace them anyway. There are so many people on a Suzuki forum that I frequent that have the exact same carbs and have wasted so much time and money only to find that a stupid 3mm O-Ring was the cause of their headaches.

They're cheap and readily available, but make sure you get fuel resistant ones of course.

My other concern is that the old owner switched the carbs out for a set from another bike. He could have put the wrong ones on, and although they look the same, the jetting and a few other things are different, especially as regards the slides.

I can't find a good pic of my Adventurer slides (the Adventurer has the exact same carbs as the early Thunderbird), but here is a pic of the "wrong" type so you can compare -

Pic1 - click to enlarge

Your slides should have 2 holes that are a lot bigger than the one in the pic.

The other difference is the air inlet for the emulsion tubes (needle jets - responsible for fuel delivery at higher engine speeds) -

Pic2 - click to enlarge

If you are able to, measure that hole, if it's exactly 1mm then I'd suggest you have the wrong carbs. No biggie, they can be modified.

Good luck.
 
#12 ·
Thank you so much for the info. Tomorrow I am going for an all day ride on my Boulevard 1500, (the only bike I have that runs correctly now, haha) but I will dive back into the Tbird probably late next week due to a heavy work load from my pesky job! I will be in touch after I give it another go.
Tomorrow we are hitting the sky line drive in northern Va. wish more of you all could join us, should be a great ride!
 
#16 ·
About 100 kilometres from Holland and Belgium in Germany :).

Randommech I have 99 legend that had similar problems, you may want to check your petcock and duckbill filters . Fuel starvation could limit your revs a bit.
...also a very good point, which I totally forgot about!

Actually, while you have the result correct, and the cause, the mechanism is exactly the opposite. It's counter-intuitive, but, if the slides don't rise it's just like having a choke on. Air is restricted, velocity goes up, and the pressure on the main jet is much lower than normal, so fuel delivery is much higher. The mixture goes extremely rich.
Interesting info there, I can't quite get my head round it, but I'm trying ;). lol
 
#14 ·
I have 99 legend that had similar problems, you may want to check your petcock and duckbill filters . Fuel starvation could limit your revs a bit. Also I recommend dynojets jet kit. Wasn't sure what jets were in my carbs so I decided to go with a whole new set. Hope this helps.

Sent from my LGL55C using Motorcycle.com Free App
 
#17 ·
WSC said:
Since the carbs are "constant velocity" the pressure on the main jets is supposed to remain constant regardless of air flow, hence the need for an increase in jet size as air flow increases. This is handled by the tapered needle and its jet.
OK, I think I'm getting closer now -

So by "air flow" surely you mean "air volume", as flow=velocity doesn't it, ie the speed of the flowing air?

And "constant velocity" would mean that the speed of airflow over the needle Jets should remain the same at (for the sake of argument), all engine speeds I would imagine.

Therefore if the velocity (speed of airflow) remains the same, then the volume of air at that given velocity must increase to supply the demand of the engine - correct?

And to meet that demand the slides rise, creating a wider hole for the air to go through, thereby keeping the velocity the same?

I'm not trying to be a smart arse, or cause argument, I'm very interested in how these things work. It could help me immensely in the future, and every bit of knowledge helps.
 
#22 · (Edited)
OK, I think I'm getting closer now -

So by "air flow" surely you mean "air volume",
I mean volume per unit time. Maybe I should call it "flow rate"???

And "constant velocity" would mean that the speed of airflow over the needle Jets should remain the same at (for the sake of argument), all engine speeds I would imagine.
Yes. This type of carb is often called a CV carb,or Constant Velocity. The CV part only applies to the main circuit, though; the pilot circuit, pilot screw and transition ports are no constant velocity.

Therefore if the velocity (speed of airflow) remains the same, then the volume of air at that given velocity must increase to supply the demand of the engine - correct?
Correct. And, that cannot happen unless the size of the passage increases, which is what happens when the slide rises. If the cross sectional area were to remain the same then the velocity must increase if the flow rate is to increase. Greater velocity results in reduced pressure. (This is how a "conventional" carburetor works).

And to meet that demand the slides rise, creating a wider hole for the air to go through, thereby keeping the velocity the same?
Oh, I see you already said that... ;)

I'm not trying to be a smart arse, or cause argument, I'm very interested in how these things work. It could help me immensely in the future, and every bit of knowledge helps.
Absolutely not! It is "counter-intuitive".
 
#21 ·
Thanks for posting that link Denny, ya beat me to it. That duckbill filter is quit inconspicuous and I never knew it existed till I stubled upon that thread. Just goes to show the wealth of knowledge other members have on these great bikes. This forum rocks :beer::beer::beer::beer::D:):sly:

Sent from my LGL55C using Motorcycle.com Free App
 
#23 ·
Hi guys,can you throw some light on this?,recently bought 96 Tiger. Bike was cutting out after about 3 miles..traced that to blocked carb inlet filter. But now it runs poorly at low revs..hesitant,but clears when i open throttle to accelerate.Not sure if this was before clearing filter,didnt ride it far b4 buying..mistake!New plugs fitted. Noticed it has k&n air filter and std silencers.Have recently cleaned carbs,checked float heights,checked pilot screws..seems to be flat spot under hard acceleration now? Thanks
 
#25 · (Edited)
Here's what I'd do -

Check the colour of the plugs, this usually gives a good indication of what's going on. Black & sooty means too rich, white means too lean. Black and oily is an ignition problem. They should all look roughly the same colour. If not then the odd one out is the problem cylinder/carb.

Check the carb rubbers and complete primary airbox for leaks, when the PO installed the K&N he or she could have put it back together badly, maybe put some silicone in the joint (there's only one). Oh and make sure both of those hoses underneath are connected too, the breather hose and the overflow hose.

Pull the carbs out again and change the tiny 3mm O-Rings in the idle screw holes, they should also have a metal washer that sits between them and the spring. If it's not there, buy new ones. Set the screws to 2.5 turns out.

EDIT - another thing to watch out for is a second rubber or washer stuck in the idle screw holes, make sure they are empty before re-assembly in the order - O-Ring, metal washer, spring, idle screw. You can also stretch the springs a little to give them more strength, and lubricate the thread to avoid serious problems in the future!!

Change the O-Rings under the throttle slides , they're cheap as chips but can cause big problems.

Clean the idle jets thouroughly, and make sure you can see through them and that the holes are clear. Use a piece of brass wire to clear any blockages.

Check the throttle slides for correct operation, they should rise and fall easily and smoothly. If there's resistance - something's wrong. Maybe not relevant in your case as they are high speed related, not idle, but while you're in there you might as well.

Check the choke plungers for correct and free operation, are they closing properly? Pull the springs a bit to make them a bit longer, this gives them more "closing strength" I've found. And lubricate the shafts.

Check the idle air inlet jets at the rear of the carb - they are the ones in the RHS hole that are removable, the LHS holes are emulsion tube air inlet jets and are fixed but should also obviously be clear.

Look at the throttle butterflies to see if anything is awry. They should all look the same, apart from that I can't describe any known problem. You might look for odd staining on the wall of the carb, that could indicate an air leak from around the throttle shaft.

Balance the carbs. She aint gonna run too well if the carbs aren't balanced. You'll either have to have your own tool for this or take it to a garage. If you're going to buy your own I recommend the Morgan Carbtune Pro, it's cheap and works really well.

You said you checked the float height, not sure how you did it, but this is how I do it. Remove the float bowls and rotate the carbs so that the floats move from the open (sitting low in the float bowl) to closed position (sitting high up in the carb body). Do it a few times and when you see that they reach the closed position (you'll see them stop moving as the carbs continue to rotate, should be at about the 60° point), stop rotating - the floats have a line along their mid section, that should form a parallel line with the base of the carb.

As long as they look near enough parallel, you're good to go. Again, they're not going to affect the idle performance, so I shouldn't worry too much.

The only thing that's going to cost you money is the O-Rings (cheap as), and the balance tool if you don't have one (a bit expensive, around the 60 - 70 euro mark).

Here's everything I have that's carb related -

http://christiane.homeip.net/pub/adventurer/carb-rebuild-part-1/
http://christiane.homeip.net/pub/adventurer/carb-rebuild-part-2/
http://christiane.homeip.net/pub/adventurer/carbs/
http://christiane.homeip.net/pub/adventurer/2012-09-09/
http://christiane.homeip.net/pub/adventurer/2012-09-08/
http://christiane.homeip.net/pub/adventurer/mikuni/


Apart from that I have no more ideas :dunno

Hope it works out :).
 
#26 · (Edited)
Thanks for your reply, I balanced the carbs..aswell as I could,bike does run without hesitation,but idles at 1500k and is slow to rev down. Idle adjust makes no diff?? Im hoping to borrow a set of Davida guages,will try those. Also,noticed that intake ducts are straight thru? Have posted Q on Tiger forum about that? Could K&N be prob? ps,new plugs fitted,and intake rubber boots.
Performance otherwise is excellent. Bike pulls llike a train.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top