Hi All,
First time poster. I have a Yamaha 1986 XT600 that cuts out at freeway speeds. Heres the specifics.

The Bike: 1986 Yamaha XT600 - 4000 original miles, completely stock. Sat for a couple of years with fuel in it.

The problem: It cuts out and sputters and misses when cruising at 60+ MPH. Cruising in town below freeway speed is fine except for a barely noticeable hiccup between gears upon throttle decrease when you pull in the clutch to shift. Idles and starts fine. Strange thing is that it accelerates STRONG all the way to 100MPH as long as it is at FULL throttle. In fact, even while you are at 60+ MPH attempting to cruise and it is cutting out and sputtering, if you give it FULL THROTTLE, it accelarates FINE with no miss right up to top speed. In fact all full throttle acceleration, idling, etc is fine EXCEPT the cutting out at 60+ MPH cruise.

So Far: I have replaced the fuel and cleaned the carburetor including the jets and slides. I have had a Yamaha mechanic clean the carb as well as drill out a plug for a "non-adjustable" jet but the same symptoms occur. They have been re-jetting but they tell me it has little to no affect on the freeway speed cruise problem.

I had thought maybe ignition but discounted that idea completely because it runs up to high RPMs fine and accelerates fine as long as it is at full throttle. It is only the 60+MPH cruise where it stumbles. A little more throttle at 60MPH has little to no effect but nail the throttle and off we go with no miss!

HELP

I've had something similar with a '86 Tenere, which has the same carburettor, but probably a different inlet system / air filter arrangement.

Checks:
1. Make sure your air filter is is very, very clean. Is it washable on your bike? If so, wash 3x times in petrol, let dry, and oil lightly with engine oil or air filter oil.
To test the difference, test it without air filter see if the problem goes away.
2. 'Synchronisation' of the second (RH) carb. As far as I know there has been an official modification from Yamaha. The valve in the mixture channel to cil head should not start opening at 5 mm lift of the primary slide, but @ 7 mm. Adjust by putting 7 mm drill under slide LH carb, and adjust screw till butterfly valve just opens.
3. Do not touch the jetting. Yamaha is king in this, no fiddling! Use stock jets, and make sure the needles are in the correct positions:
- LH carb: needle 5C37, clip in 4th notch from top
- RH carb: needle 4A71, clip in 4th notch from top
Use the original jets.
4. Check the vacuum operated slide of the seconf (RH) carb: does it move easily and smoothly, does it return to it's rest position aftr opening?

Your problem has probably to do with too rich a mixture at constant speeds. Rereading the above, I expect the most from 1 and 4, so try them first, provided your jets and needles are at factory settings.

Auke

__________________
Auke
http://aukemarieke.blogspot.com

My 83 tenere did exactly the same thing!!

After many complex technical consultation with the Yamaha mechanics one thing became frightfully clear!!

We are all very often so paranoid that we immediatly assume that the most technical problem is the issue. After much discussion we too my bike andstarted at the begining of the system,fuel, clean pipes, change filter,spray carb cleaner through the fuel inlet pipe into card.

Reassemble bike , test. Problem persisted. you now know the carb is clean, so start at the other end of the system 'air' check the filter and filter box clean the both well. I found washing my filter in prepsol engin degreaser and giving it a good rinse dry and re-oil yielded excelent results!! Personally i dond feel petrol removes enough of the old dirty oil.

Do this before ajusting jets, needle and seat, etc. You will be astonished at how much a clogged up air filter influences the mixture dyamics. When blocked up badly the bike runs very rich causing major overfueling esspeially when cruising at relativly high speed. Chech your spark plug if it is very black and 'dusty' bike is running rich.

I feel only once all these step have been taken and still no improvement, then start adjusting needles and jets as those are the only things left in the equation and they are the most tricky.

Well good luck

Hope its been of some help!!

Keep us posted!

check the diaphram isnt perished or holed, this might cause the 2nd carb to behave strangely at the transitional point from full throttle to cruise position

Before taking everything apart, check if the inletstubs are still ok. They sometimes fail when a prt of the rubber piece comes off the alloy piece (and then will more or less cause what you described.)

grts,
Joris

I used to have an 89 xt600, and it had similar sounding problems after I did some work on it (Supertrapp, K&N, opened air-box, up two sizes on main jet), it would cut out at certain higher speeds.

It was a hard diagnosis, and a simple fix:

The kickstand safety switch.

Somehow, certain vibrations or something were causing the switch to cut in and out, causing me all sorts of grief.

Disconnected the switch and problems went away.

Maybe this isn't the problem on your bike at all, but I wanted to throw it out there just in case, I never did figure it out, a Yamaha parts guy fixed it over the phone for me.

Any time you have a diaphram operated carb (constant velocity) You have something to check. Any small hole will give you the symtoms described and sometimes the diaphram will just be altered (by age or chemicals) as in stiffer than original and cause wierd problems. Sometimes you just have to replace with a Known good unit and see it that fixes the problem.

I seem to have exactly the same problem with my XT 600 -84, except that for me, sputtering starts when cruising at speeds over 70 kph (45 mph). I would really like to know if rdelta has solved his problems in this matter, and if so, what the solution was. (I wonder if anyone will notice this post, almost two years late)

I have the same problem. It sputters a few times when driving over 90kmh at motorway, then it cuts out, but starts easily when starting again. Also it may do that when coming off from a motorway (100kmh) to the ramp. When its almost time to stop I notice that the engine isnt running and the rew light is on. Hmm I guess it may be because I adjusted the 2nd carb to open earlier. Need to put it back where it was and check without airfilter also..

Hi Ilpoj, your problem seems to bee a bit different, I think. English is not my mother tongue (as for You, I would guess), so I'm not too sure what the term "cuts out" means, but in my case the engine doesn't stop completely, it just sputters and misfires and sometimes it stops for a few tenths of a second, then it comes back again. This happens when cruising at moderate speeds (70 kph and more). At low speed and idle, the engine runs almost fine, but there are occasional misfires even then (barely noticeable). The only time it runs perfectly is when accelerating at full throttle. I believe that this is the same problem as rdelta had , only in my case the condition has progressed a bit further, and therefore I get the sputtering at lower speed and also some (very little) misfiring even at idle. I am the only owner of this bike, I bought it in 1984 brand new, and I know that the carburettors have never been tampered with. The bike has now 45000 km on the meter, 44000 of those came the first ten years. Back in those days, when I was young and beautiful, the bike was used almost every day (even in wintertime), and it always ran perfectly, at all speeds and throttle positions. Nowadays, when all that's left of "young and beautiful" is the and, the bike is only used a few km each summer, and I must confess that I've been a bad boy when it comes to draining the carburettor, so like for rdelta, the bike has been sitting a lot with fuel in it. Therefore I had the carburettors cleaned with ultrasound, but it didn't help. I checked the synchronization of the carburettors, according to Clymer handbook the secondary carb should start opening when the primary's throttle is up 5 mm. In my case it opened at 3 mm, which I changed to 5 mm but it didn't make any difference. aukeboss wrote that it is now supposed to be 7 mm. I'll try that too, but I don't think the sputtering has to do with the second carb, since it (sputtering) happens att speeds and loads when the second carb is inactive. I have run my engine while standing still, bike on sidestand (there is no sidestand (kickstand?) safety switch on my bike, there never has been) and the rubber boots from the air filter removed. If I run the engine at 4000 rpm, it sputters and misfires heavily. The primary carb's throttle is only up 2-3 mm, so the secondary carb is not engaged. If I lift the throttle of the secondary carburettor with my finger, nothing happens since the butterfly air valve of the secondary carb is closed. If I instead with my finger affect the push lever of the secondary carb, causing the butterfly air valve to open slightly, the engine tends to die, probably because it's getting too much air(?). I believe that the engine is sputtering because the air/fuel mixture is too lean, not too rich. I have noticed that if i choke the engine (using the lever on the handlebars), I can get the engine to run much better. Not perfectly, but the sputtering is significantly reduced.

Running the engine without the airfilter will indeed make it run lean. Remember, the airfilter is like a cork in the intake system. For obvious reasons however we need it.
The setup without the airfilter and with are not comparable, and what we want is a bike that runs fine with all the necessary accessories on.

You will never see real action from the 2nd carb without loading the engine. On only a little opening of the first carb you already will reach the red zone, so this is not a good test.

Recommend to really clean the airfilter first. Three times in clean petrol, and then lightly oiling will do the trick. Test, and then come again.

Auke

__________________
Auke
http://aukemarieke.blogspot.com

Hi aukeboss (and everyone else of course). First of all, I should say that I indeed have cleaned the air filter. Not in petrol though, since the Clymer manual warns you that "gasoline will break down the seam glue used to hold the filter together", instead I washed it a couple of times in kerosene, and then a few times in soap and warm water, whereafter the filter elements were rinsed under running warm water and left to dry overnight. They were then sprayed with Castrol foam air filter oil as I put the filter back together again. The sputtering problem was the same both before and after cleaning the air filter (the filter wasn't very dirty). Since I have had this bike for rather a long time, the air filter has grown dirty and been cleaned many times before, and I have never had any sputtering/misfiring, or for that matter noticed any difference in the engines behaviour before and after cleaning the air filter. I see what You mean about the engine running lean without the air filter and that testing without load isn't ideal, but since the sputtering is the same when testing it like this as when you're riding the bike with the air filter connected, I still think it has some relevance. Besides, if the air filter was the problem, wouldn't there be a (large) difference between running the engine with or without air filter? The sputtering and the effect of choking the engine is the same whether the air filter is used or not. Or could it be that with the air filter, the engine gets too little air, and without the filter it gets too much, causing the same behaviour in both cases? Personally, I believe that the problem has something to do with either that the bike has been left unused for long periods (with fuel in it), some years it hasn't been started at all, or simply aging of some components (o-rings maybe, or the rubber on the intake manifolds or something like that). I don't think it has to do with wear, because the problem has turned up in the latest ten years, first very subtilely, then accelerating each year, and during these ten years the bike has hardly been used. All input is welcomed though, so don't hesitate to post.

If you had an airflow delivery issue to your carb (blocked air filter etc) you would be running too rich. Adding choke would make it worse. Since the engine runs better with choke on I would surmise that you are indeed too lean.

I think the clue here is that you let the bike stand for long periods with the fuel still in. This can create two possible problem areas that I would suggest you have a look at.

First off. Petrol when left standing can create a wax like residue that can collect around parts such as the jets and float chamber which will inhibit fuel flow----> lean

Secondly, modern petrol has a higher alcohol content than that which was used when the bike was designed. It may be well worth your while to check the state of your rubber components within the carb for perishing (diagphrams O-rings etc). Even if the fuel alcohol level is within the manufacturers tolerance level, alcohol is corrosive to these parts none the less.

Thirdly, and perhaps something you should check first...is your float level set correctly? If you think you are running lean that is always the first check before carb dissasembly.

My 94 XT600 had the exact same issue. It was a combination of ethanol in the fuel creating issues and float level problem. Once the fuel issue was resolved with an additive, the needle was replaced and all is running well now. Most fuel in the US has at least 10% ethanol in the mix.(at least in Northeast USA) Starting to be a big problem for some boats & bikes. Try replacing the needle and see if that helps. Worked for me.

My 89XT had similar problems. It would just die while riding, mostly when going "faster" rather than slower. I cleaned the carbs twice and no improvement. Air filter cleaned and no improvement. Finally opened up the petcock and there it was...GUNK around the petcock. Cleaned it and it's been running nicely since ... knock on wood!

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89 and 88 XT600.