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Discussion Starter #1 (Edited)
Hello!

My 1995 Thunderbird 900 has developed hesitation/"missfiring" when driving with steady throttle around 4000rpm (+-500). The hesitation seems to worsen all the time and with cold engine the engine works better.

I havent made any modifications before this started to happen. During winter I installed fuelfilter to the fuelline, K&N airfilter and EMGO mufflers. But I was able to drive over 2000kms with these modifications without any trouble.

I don't notice any hesitation at lower speed/rpm but the worst it gets when I drive around 90-110kmh. When I get to
4500rpm (110kmh) the bike starts to run great again.

Otherwise the bike behaves as always. MAYBE the exhaust backfire noise has increased over time but not sure...

Best thing is that I'm on the road now for 4 more days with limited tools and time to work with the bike. I'd still need to drive over 1000 kms with this before arriving home... can it be bad for the bike?

Any help will be very much appreciated!
 

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You’ve either got a coil going bad or the in-line filter is causing problems. Take out the filter and see if anything changes if not then try a new coil. Might be an opportunity to swap to tt600 plug coils.
 

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The hesitation seems to worsen all the time and with cold engine the engine works better.
Are you saying it hesitates less or not at all when you first start the bike, and it gets worse as the engine warms up? Are you using the choke? Does the increase in hesitation coincide with reduction in the choke setting? Does adding choke make it better or worse?
 

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Discussion Starter #6
SOLVED

The problem was with the in-line filter that was restricting the fuel flow just enough so that the bike wouldn't run right. It only started to cause problems after the filter had gathered enough fuel tank liner (which is peeling quite a bit) inside itself. The symptoms really felt like that there's something wrong with the ignition, but this wasn't the case.

Thank you all for your help and for the quick replies! My answer wasn't quite as quick...but better late than newer!
 

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Alway like.to hear the outcome of these issues. I wish more people would follow up on thier posts. That is how the rest of us learn from the discussion. ...J.D.
 

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Discussion Starter #9
I changed the paper filter into a mesh filter which is easy to open and clean up (see photo). Thats how I managed to cope until the end of summer. Now I'm thinking of fixing this with some better solution.

WSC: I'm a bit conserned about the fact that before there was no filter whatsoever. Nothing in the fuel tap either! Shouldn't there be a mesh around the main and reserve fuel pipes that are connected to the petcock? Can you recommend me some solution for this?
Is there some kind of filter in the carbuerettor end? Or are my float bowls possibly filled with the tank liner? The bike runs well though and the carbuerettors were fully overhauled just before I bought the bike during summer 2017. I've driven 9 000km with the bike myself (3000km without the in-line filter and trouble emerged only after adding the in-line filter).

What do you recommend me to do with the peeling tank liner? It's comming loose quite a bit and in big chunks. I've been searching the forum for topics about this matter and I'm thinking of the Caswell solution. Do you have some good thread/link/video how to go through with this procedure?

Thanks again!
 

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There should be coarse screens on the petcock. There is a fine screen in the "T" where the fuel line connects to the carburetor rack. These are normally enough. The fact that pieces of the tank coating are getting past the petcock tells me you either have a different petcock, or are missing some parts!

At the risk of stating the obvious, you're going to have to remove all the old coating before you can recoat the tank. Once the prep work is done it's just a matter of mixing the stuff up, pouring it in and moving the tank around to distribute it evenly.
 

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Discussion Starter #12
Problem not so solved after all...

Last summer I faced the same problems as I did the year before, as the engine started hesitating again during our annual road trip. It started with occasional "misfiring" but got worse after each ~50-100km we drove. First I ditched the in-line filter and thought for a while that it helped, but not for long. The engine gradually ran worse and worse and eventually felt like that I was driving with only 2 cylinders 95% of the time. At this point I changed all the spark plugs and it fixed my problem! Spark plug in the middle cylinder was black but the others looked fine. I didn't think of changing the plugs right away since I had just changed the plugs some 3 000 km before.

So now I'm thinking that the engine is somehow defecting the spark plugs (especially in cylinder #2). What can cause the spark plugs to fail prematurely?

Today I took off the carburetors to check the screen filter in the "T" between carubertors 2 and 3. It was so cloaked with the gas tanks coating that I'm surprised that the bike has ever worked at all! Could this be causing the spark plug to fail early?
 

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I would think not. The (almost) plugged filter would affect all the carbs fuel supply and I am surprised it had not made the engine run badly. From your description it sounds like the #2 coil is on its way out. You could try riding the bike before taking it apart and when it starts to miss add some choke. If it starts running better it is a fuel delivery problem and you will need to address that not the coil. If it does not respond favorably to added fuel you probably have a bad coil. Try swapping a coil from one of the other cylinders to see if the misfire follows the #2 coil. If it does you have found the problem. If the problem remains at #2 cylinder then you have a carb issue, possibly debris clogging a jet.
 

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You will need to address and fix the fuel tank and crud issue before you attempt any other fix IMO. :)
 

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First, you need to clean the tank and take steps to prevent it from becoming a problem again.
Second, you need to address the fact that there are no screens on the pickup tubes in the tank.
I ditched the in-line filter and thought for a while that it helped, but not for long.
Right, until the next filter downstream started to plug up. So, after one and two, you need to clean the duck bill filter.
Spark plug in the middle cylinder was black but the others looked fine.
I agree, the #2 coil is suspect, or the emulsion tube for the #2 carb is plugged up. Swap the #2 coil with #3 (or #1) and see what happens.
 

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Discussion Starter #18
Thanks for the advice and sorry for another long post:.

I forgot to give you all the background information. When I ditched the in-line filter, I also built a screen filter in the fuel tap at the same time. So most likely the crud in the duck bill filter has been there for 2 drive seasons already.

I've faced this problem twice now during my 3 years with the bike. Both times this problem started to creeps up during yearly road trip where we drive some 1000-1500 kilometers in 4-5 days, and the driving style is quite much more aggressive during these trips compared to normal use. And now when I think back I would say that what really solved the problem in both events was the change of the spark plug(s).

The first time I (for some reason wanted to think that the problem has to be in the fuel supply, but still changed the plugs "just in case" at the same time as I did moderations in the fuel line. After these repairs the bike worked, so I thought that it was because I changed the in-line filter into another type. The next year when these problems started to appear again I first cleaned the petcock and it helped for a short time. As issues persisted I next took off the in-line filter and built the screen filter in the petcock. After all this the bike was still running worse after each day and very gradually started to lose one of the cylinders more and more. Hard acceleration would make the "missfiring" cylinder to wake up for a while but driving with a steady throttle was getting impossible. So again, as a last measure I changed the plugs and voilá, problem solved (and again I felt so stupid not doing this right away.)

I'm not saying that the crud in the duckbill filter wouldn't cause these problems, but I'm just bewildered how the bike seems to wear out the middle spark plug. A bad coil would still remain bad when I only change the plug, right? Or is it possible that a faulty coil works well with a brand new plug, but starts to act up after some time?

Maybe it's better not to go ahead of ourselves. I will do the tests with the choke and coil swapping as you advised, so then we should know for sure. I'll get back to you, Cheers!
 

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Mikuni carbs correct? Have you verified the each needle is set the same? Have you checked the emulsion tubes that the needles correspond with? I wonder if you have emulsion tube and needle wear on #2. It doesn’t make sense that one would be worn to the point of fouling plugs without the others being too far behind. Have you checked the fuel screws to make sure they are set the same? Jetting?

To start with do what WSC recommends and swap a couple coils to see if the problem moves with the coil. It could be that with weak spark it works ok but slowly fouls the plug to the point it takes more voltage than the coil can supply.

If the problem stays with #2 cylinder I would check the carbs over for wear and/or adjustments.
 

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Did you re-line the tank ? I can't tell from your posts. Have you checked to see if crud from the tank got past the filters & into the carbs.
 
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