eniine peeters out at app 80mph

[Chris]

This started some time a go the bike got upto app 80mph then seamed to run out of go would not go any faster as if there was no gas in the carbs
I am only getting 80 to 100 miles per tank ,If I rev the bike it bogs down and pours black smoke out the back
I have checked the float levels cleaned the carbs replaced the air filter replaced the spark plugs and leads checked the coils
any suggestions??
Chris

[strom]

Had the same problem on my kawa... went up to 100-110 mph, then like sbd killed the engine... as speed dropped, some more gas seems to have dripped in the carbs... and it pulled again...
The tank vent was OK,.... never figured it out.

LP

[sixtysix]

Black smoke is a rich condition....have you checked the air filter?

Check this tip for more ideas..

How to improve gas milage

[dragxs11]

Just a thought,
You might look at your carb diaphrams. The rubber holds up for a long time but these bikes aren't young anymore. The rubber can eventually wear against the inside of the carb. I noticed this about mine and changed them out. At $112.00 a piece, you'd better be sure the're really bad though. I thought they might not be getting up all the way if they're leaking air. Just an idea though. I'm tuning in my carbs now, so I've got them on the brain at all times.

[Chris]

Yea Gary black smoke is rich and the first time I noticed this I ran out of gas about 5 miles after I passed a fuel station on a club run It cost me a few beers. I had put in a new filter and I thought it might be Too thick (a Uni I think) so I took it out on the ride home. The bike went better not so much gas used but still breaking down under load .
The bike is at an auto electrician now he has tested the coils with an oscilloscope and said it has a good pattern on the primary side but he did not seem to know how to check the output on the secondary side, he said that the coil which feeds 2 and 3 could be breaking down under load
If you open the throttle quickly to w.o.t. it will break down but if you open the throttle slowly it will rev higher before if breaks down I do not know the revs my taco is not working I may try another set of coils (second hand $70 to $100 NZ new around $450NZ)
The bike only has 7000 miles on it, it had been stored in a farmers shed for 20 years. He could not get it going, had the carbs plumbed up wrong, gas going into the over flow. It had 2 miles on it when I got it. I have stripped the carbs down 3 times and the diaphrams are ok thanks

[dragxs11]

Hey Chris,
I don't suppose you've done any mods or put rebuild kits in the carbs? I'm kinda out on a limb here to try to help you out, but you haven't changed the main jets or anything right? If you have, they're too big. The only way what your describing can happen, is if either the mains or pilots are too big.
I was dumb enough to change my pilots and threw away the old ones. The ones in the rebuild kits from Yamaha were to big, so I've had to go through about 5 different sizes to get to the point where it will idle again; but thats not like your problem. you're getting to rich when the diaphrams lift. If you have free air flow, it's got to be the mains. If you did change them, the numbering system is different from one manufacturer to another. So 20's from one company aren't the same as 20's from another!
I have often seen people suspect the coils when they can't find anything else wrong. Very seldom do they go bad. Though it is possible, most of the time they're fine. But they're not too expensive, so you can try it.

[dragxs11]

Just had another thought:
Maybe check the timing advance?

[Chris]

Matt
the carbs are jeted for a 79 xs and I have not changed them. it was going ok, then it was not! I have backtracked as much as I can to find out what I did wrong ?? I did check the Valve timing but it looked ok to me but have asked the auto electriction to check it again It could have been just a bit retarded and when I adjusted the cam chain it may have altered it more?? the marks may be wrong ??

[dragxs11]

if it's like my '80, you have a hose going from #2 carb to the left engine side cover. I wonder if that hose got pulled off inside down below. That would take out your timing advance. But that wouldn't explain the richness issue.

[Snow]

Chris, you say the carbs are jetted for a 79 XS, but what model are the carbs from? If they are from a 80-81 XS, you will have huge problems if you put main jets from a 78-79 in them. The standard mains in 78-79 are 137.5 and between 110-120 for 80-81 depending on what the Yamaha engineers ate that day.

[Chris]

Matt
I thought of that on my last ride I wasn’t sure if I put it on the right place? on my bike it has a brass tube on the 4 carb boots and one on No.2 carb one is for the vacuum fuel tap (witch I by passed) the other is for the vacuum advance, I can not remember witch one is witch so I swapped them around and a slapping noise came from the left side like a cracked piston I traced it to the ign pickup so I swapped them back

Peter
The engine according to the number on the case is out of an '80. is there any way I can identify the carbs?? They have brass floats and do not have a drain holes under the float bowl but have a brass bolt

[Snow]

Here's an easy way to identify, when you had the float bowls off, were the float valves screwed in or did they have a retained?

Take a look here to see what I mean:

http://www.xs11.com/forum/images/tip...0-carbjets.jpg

Do yours look like the top right or the bottom right? Bottom right is 78-79, top right is 80-81 as far as I understand it, as long as you have a US model.

Being that you have brass floats, there is a good chance you have 78-79, but the floats can be swapped out, the float valves cannot be interchanged.

[Jeff]

I agree. With the brass floats... it sounds more like the 78-79 carbs. If this is the case..... your vac lines for the fuel petcocks should go to the fitting on the carb boots of 2 and 3. The fitting on the boot for 1 and 4 should be capped. There is a fitting directly on the carb body of #2 carb for the vacuum advance line that taps vacuum for the advance from behind the throttle plate where it is dampened somewhat so it pulls the advance smoothly instead of bouncing it.

When you cleaned the carbs..... did you remove the emulsion tubes and thoroughly clean all the little holes in them... and the bore that they fit into? If there is any buildup between the outside of the emulsion tube and its bore.... you will be very rich in the midrange. In this case.... any throttle past about 1/4 will be very boggy unless you open it very slowly especially if the engine is under a load such as climbing a slight grade or riding into a headwind.

The emulsion tube is the brass tube that the slide needle goes into. It protrudes into the carb throat a little bit and is held into place by the main jet. Once the main jet is removed.... the tube can be pushed out by inserting a small screwdriver into the hole the main jet came out of and pushing it out into the throat of the carb. Be carefull not to damage the threads for the main jet. Once the emulsion tube is out.... there are also 2 small openings in the bore it came out of. One is a passage to the pilot jet and the other is the air passage which supplies the air to emulsify the mix in the tube thru the small holes in the tube itself. Other than these 2 small holes.... and the holes in the tube itself.... the bore the tube rides in is pretty well sealed and no amount of soaking or spraying the carb will clean it out. Make sure both of the small passages in the emulsion tube bore are free of any obstuctions by spraying carb cleaner and air thru them. Be careful putting the emulsion tube back in since there is a small pin in its bore that needs to align with a slot in the tube.

I have found that after sitting for a while...... the small space between the emulsion tube and its bore is very prone to varnish buildup because any fuel that might seep into there cannot evaporate easily due to the small size of the passages leading into it. I neglected this area the first couple times I cleaned my carbs and was amazed at the gunk that was in there when I finally pulled them. I was also amazed at the difference it made in the bikes performance.

[Chris]

Thanks to every one for all the help and tips . The carbs I am sure are off a ’79 but it is good to know the floats can be changed, one of mine has a lot of small holes and has been repaired twice
I talked to the auto elec today and I could hear the bike running in the back ground . He had checked the valve timing and it was OK so he was going to check the vacuum at the carbs and the air flow . He removed the air box . when he ran the bike it went ok. He thinks there may have been a restriction at the intake (crushed rubber?) I had checked the rubber intake for obstructions like a rag or something like that but it was clear I didn’t notice anything that looked like it was crushed! But never mind as long as it is going ok!
Once again Thanks
Chris

[dwales]

I had the same problem with my 78. Turned out it was the exhaust sooted to the point of blockage. I have a SuperTrapp 4 into 1. I removed the end cap that holds on the baffle plates, and the bike once again ran great! Check your exhaust.