Slow on take off

[Jacobite]

I finally have my bike on the road. Now just one (for now) more problem to solve.

When I am taking off, the bike ('81 special) stumbles in its acceleration until I get over around 3000 rpm - then hang on! It starts easy, sounds good - but when I let the clutch out it feels very weak until the Rev's start to pick up, and then it seems to work fine. Lots of power - and runs along great.

It also seems to be using an abnormal amount of fuel. I have only put about 60-65 miles on it and the fuel light flickered.

I have cleaned the carbs (twice), so I assume its in the settings - any suggestions on where I should start? I know very little about Carbs, but enough about the local Yamaha shop to try a fix myself.

[2fast]

weak at low rpm

Sounds normal for the power band. Not much there until 2500-3000 area, then it comes on. You get used to it. About the fuel comsumption. did the fuel light flicker and you had to go fill up? Or did it just flicker. If you went and got gas, was it really close to empty? If so, you are running pretty darn rich. Check the spark plug color and see if they are black. More info?

[tab1324]

How far out are your idle mixture screws? I saw 40 MPG this weekend without anything but a bench sych job on my carbs. I think my idle mix screws are about 2 1/4 turns out from a gentle seat. I tried them at a bunch of different settings and at about 3 1/2 turns out, mine did what you're saying yours is doing - real boggy at low RPM but screams at high rpm. 2fast is right though - the power comes on after 3000 rpm. Take 2nd and 3rd gear up to about 6500 rpm and you're flyin!

Tom B.

[TopCatGr58]

Hey there Jacobite,

As Tab said, sounds like you may be a bit too rich at least in the idle circuit. I did a search and found your previous post about what you had done to it. It being an 81, it came with those brass plugs over the pilot/idle screw chambers, did you drill out and pull those plugs out to get to your idle screws? If so, then you can turn them in to a gentle seat, and then back them to 1 or 1&1/4 turns out. However with your description of cleaning, you didn't mention about your float level settings? I know the book describes the friggin' tube to the bottom of the bowl and filled with fuel, etc., but you can still use the older way of measuring from the float bottom to the carb body, I used the '80 settings of ~23mm vs. the 78-79 of 25mm, but whatever it is now, it may be too rich and you may want to slightly adjust them to a 1mm higher measurement than where they are!?

I had mine set to over 3 turns out, from recommendations of bike race shop, it did great at sea level, but was a dog at modest elevation in N.C. mountains, only had W.O.T. or nothing, got back down close to sea level and they behaved again, got back home and readjusted them to 1-1/2, and they worked great in Pa. "Hills" and recently in W.Va. mountains!!! HTH, Good luck.
T.C.

[sidecarKev]

Hey Jocobite,

Give your float heights a check. I had the exacts same problem. Get them set close to 25mm. Also check to see if any of the floats are leaking. Leaking floats will cause alot of problems. That RPM range is affected by float height.

More info here.

Carb Info

[Jacobite]

Thanks for the tips. I didn't have time to work on the bike last night - but will try your suggestions tonight. I haven't drilled out the caps over the idle mixture screws yet - but that will be next on the agenda. And I will check the float heights. Is the method on that factorypro.com website the best way to do it?

The PO said the bike was always sluggish at low RPM - but it is stumbling now. I am at an elevation of about 2500 ft, and will regularly be driving into the mountains, so it will be even more of an issue.

The fuel light was right - I went for a short run last night & came back on reserve! It acted worse after it had warmed up.

On the plus side it starts as easy as any bike I've ever had - just touch the starter and its running.

[sidecarKev]

Well the info on the XS11 site might be better to reference. An '81 should have a little valve in the bottom of the bowl to check fuel height with. Before I said set them to 25mm. That might not be the case. I'm only familiar with my '79 which has no little valve in the bottom. Check out FAQ for more info.

Float Height FAQ

[randy]

Jacobite,

If it helps, I had an '81 special, bought it at sea level in NYC, drove it there four years, then moved to the Poconos, PA elevation 2300ft and no change in operation, so I would doubt that the elevation change would be an issue once you get it dialed in.

[petevan]

I had a very similar problem when I first purchased my bike. I can tell you I feel your pain you almost have everything running perfect but this one problem you can't figure out! If this helps the problem with mine was a piece of rust or dirt came down from the gas tank and wedged itself in the valve that closes when the floats rise.( PO owner did not have inline filters , do you) In result my floats couldn't rise to cut off the fuel supply. So when I was riding it it was very slugish until about 3000 and then it seemed as if I was riding a two stroke and hit the power band. What was happening is the excess fuel that was coming up through my carb was flooding my engine untill the rpm reached a point that it could use all of the excess fuel ( explaining the wierd power band) As a side effect I was also leaking fuel. It would leak up out of the bowl and back into air box and drip out onto my crankcase. Yes, I ended up with fuel in my oil. Are you also leaking fuel? To fix the problem I had to drop the carbs and clean all of the moving parts I took apart float assembly and sure enough there was a piece of rust/dirt in there. Since I removed the piece my bike runs great at all rpms. To tell you the truth it was a pain to do, so I hope this isn't your problem but I wanted to let you know of the possiblity.

[Jacobite]

I don't have a fuel leak - and I just changed the oil again on Sunday - no fuel there. But it looks like I am going to be pulling the carbs off tonight anyway - so thanks for the suggestion, it is something else to look for.

I don't have in-line filters, just the ones in the tank.

I really hope that I can solve this soon - I'm anxious to be out riding!

Off at lunch time to get some more tools! (calipers/machinists ruler).

[ratbyk]

Filters

Pick up a couple of 1/4" in-line filters while you are out. Don't have to go for fancy trick looking ones from a bike shop. Almost any automotive parts supply store will have them in white plastic for a couple of bucks each. Really worth the expense and trouble of installing. After 20 plus years there is a lot of crap in the tank. Stop it BEFORE it gets to your carbs.
Ken/Sooke

[Jacobite]

Thanks - I will. This is the only kind of shopping I like!

[pgg]

You really need to sync the carbs with a gauge because even miniscule tweaks on the butterflys make big differences. Also a dicey sparkplug or not quite perfect plugcap fit causes low down stumbling on take-off. Also a twitching vac advance unit.

[Jacobite]

Picked up a couple clear plastic filters - $3.84 each at Canadian Tire. Installing those will be the easy part of what I have to do. My plugs are new - so they are probably ok. I don't know what a vac advance unit is - so I don't know if mine is twitching. My plan is to set the float height - adjust the idle screws - and then see where I am. Synching the carbs will be next. I don't know if I will attempt that myself or not - I don't have a lot of faith in the local Yamaha shop - I don't have any biker connections here yet to find out who is good ( I already found out who was not - with my son's dirt bike).

[John]

The vacuum advance is under the left side engine cover. The one w/Yamaha on it. Follow the vacuum line from #2 carb, not the boot, to the lower, left side of the engine. Remove the cover. There it is.