Acceleration problems

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Khosatral Khel

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I just got my bike back from the mechanic. They replaced the needles and the ignition coil and set the float. I rode home fine, but I was only on surface streets. When I rode it this morning into work on the freeway I had issues accelerating at 60 MPH in 5th gear, once I got to 70 it suddenly had pick up, almost like I had hit "turbo"or something. If I dropped it down to 3 or 4th gear it would accelerate. Is something not set right, such as the float? Am I getting too much fuel? Am I just riding it wrong as I was used to the old needles/float settings or what??? Thanks!
 
You haven't given any of the pertinent info needed, like what carbs you're using, for anyone to give you much more than wild guesses. Looking at your other posts, you mention having a 1980 model, but are the carbs a 1980 set? Good question and you're not tellin', lol.
 
Sorry, I’m a newbie and although I attempt to do things on my own, I’m not a mechanic. It’s a 1980 XS Special and it has the stock carbs. This all began when the engine was sputtering at approximately 4500 RPMs, but it would do this at random times. Then I took it into the mechanic to have the engine gaskets replaced. When I got it back, the sputter was consistent at 4500 RPMs to the point that I couldn’t really get it over 60 MPH. So, I took it back to the mechanic and they were having a hard time figuring out why it was doing this and eventually replaced the needles, which seemed to solve the problem, but now I’m having this new problem.
 
Since you just got back from the mechanic you probably don't know what needles are it. If you do know give it up so 5twins can help you. I think your riding it wrong myself. :D
 
Since they replaced the coil they must have thought the sputter was electric related. The BS 34 carbs have a non adjustable needle not shure what they put in it. What exhaust are you running straight pipes or mufflers. If it ran fine on the way home and now it's not good on the highway is troubling.
 
Your 1980 BS34s were the first year for those carbs. They share the same jetting specs with the later sets but the floats were different. They used a brass float whereas all the later sets used plastic floats. The setting spec differs between the two types. Here's a chart showing the specs. Note that the brass floats are set around 27mm, the plastic ones around 22mm. If the shop mixed that up, it could be a problem .....

CarbSpecsReducedSize.jpg


It's also possible to "update" the 1980 carbs to the plastic floats. If that's done though, you must use the plastic float 22mm setting with them, not the 27mm 1980 spec.

As mentioned, the stock needle is "fixed" with no adjustments. There is an adjustable one offered as a replacement. It has to be set at the proper height to work right. If the shop put adjustable needles in and set them wrong, that could be a problem.

You mentioned having charging problems in some of your earlier posts. If they have cropped up again, that can cause the ignition to malfunction. In other words, what you think is a carb problem may be electrical. I would clip a volt meter to the battery terminals first thing and verify that you're still charging.
 
It’s got mufflers. It could DEFINITELY be me and the way I’m riding. This is my first street bike, but rode trail bikes quite often in the past. Previously it seemed to cruise well in 5th gear at 55+ and only need down shifting to 4th if I need a quick acceleration, but now it seems like it doesn’t want to run at all in 5th gear unless I’m going over 70. It definitely struggles to get to 70 in 5th gear even when slowly throttling. Once I get to 70 it rides like I feel it should. I think I’ll ride it for a week and see if it is just at high speeds where I’m having trouble. Unfortunately I’m at work and can’t go for a test ride.
 
The speeds and gear you're talking about, 55 - 70 MPH in 5th, usually have you operating in the 4 to 4.5K RPM range. That's in the upper range of what the needle controls and just getting "on the mains". The fact that it runs better as you go faster and get into the mains more seems to indicate they're OK. The symptoms seem to point at those new needles.

Float settings could also be at fault, not too low a fuel level but too high. Too low would run badly at higher speeds because you would be starved for fuel. Too much gas in the bowl could make it too rich and run badly at lower speeds but better at higher speeds. Higher speeds use the fuel faster so could keep those too high levels in the bowl down in an acceptable range.
 
Remember- when checking float heights the carbs are upside down. So as an example. A 22mm allows less fuel and more atmospheric pressure 14.6 in the carb vs 20mm which allows more fuel and less pressure.
Typically, if the float height is off that bad it shows up other places as well. You could have a worn nozzle depending on hours on carbs. not likely tho. You'd most likely would notice it IN ALL GEARS AND SPEEDS.

Sounds to me you're just lugging the bike. When you're at 50 and try to accelerate what is the RPM at that point. As someone about mentioned there is an RPM range the bike likes.....depending on final drive gearing etc.
 
In post #2 Terrible1 linked the carb guide. Print it out and take it with you to your mechanic.
Or read it a few times and do the work yourself.
It has a "Tuning For Mods" section. It may help you understand just how the carbs work.
Leo
 
I'm gonna chip in here..................There is no reference point for anyone to help you....By that i mean no intro, (not really necessary but helps to endear), no info, (year/model), or pics of the bike or any information on what type, bobber/chopper/barn-find/original or resto mod.

All this help others help you. Take it to a mechanic who changes things then it doesn't work out............ come here without any relevant info with an expectation to get things solved.........

Learn to do the work your self, remove the needles and find out what you have, (reference no's on the needles), and read the jet size and then others will be able to help you to sort this out
 
Thanks for all the input. Again I am a noob and so don't really know what relaxant info would be needed. I watched some videos on carbs this weekend and read that guide. I will probably go ahead an take it off and check it my self. I did see one video where the guy said you can check the float hight to get a rough estimate by attaching a tube to the float bowl with the carbs still on the bike. I have a 1980 and I don't see the nipple he puts the tube on. do you just take out the brass screw and attach it there?
 
Time to find a new shop I think. That place doesn't know squat about XS650s, lol. If you look at the chart above, you'll see 17mm was never even close for ANY 650 carb set. What it was speced for is some of the Mikuni VM slide carbs, but you don't have them. You have Mikuni BS CV type carbs, a whole different animal.

When you have the carbs apart, you're going to want to check and WRITE DOWN all the jet sizes, the needle number, and the needle jet number. You'll need to refer to this info to properly jet and tune your carb set.

The nipple on the float bowl didn't happen until 1981. Your 1980 set still has a drain bolt like the earlier BS38 carbs. If you want to use the tube for checking your fuel levels, it's best to make or buy a special fitting to screw in place of the drain bolt. This eBay seller makes them .....

http://www.ebay.com/itm/Mikuni-Carb...rcycles_Parts_Accessories&hash=item3f32674d3f
 
After you get them off, try to get some good close-up pics of the carbs from all angles, and post those pics to this thread. That's probably the best way to positively identify which particular carbs you have. Having the bowls off, with pics of the floats and carb interior will help even more...
 
650Skull said:
I'm gonna chip in here..................There is no reference point for anyone to help you....By that i mean no intro, (not really necessary but helps to endear), no info, (year/model), or pics of the bike or any information on what type, bobber/chopper/barn-find/original or resto mod.

All this help others help you. Take it to a mechanic who changes things then it doesn't work out............ come here without any relevant info with an expectation to get things solved.........

Learn to do the work your self, remove the needles and find out what you have, (reference no's on the needles), and read the jet size and then others will be able to help you to sort this out
Click to expand...

Khosatral Khel said:
Thanks for all the input. Again I am a noob and so don't really know what relaxant info would be needed. I watched some videos on carbs this weekend and read that guide. I will probably go ahead an take it off and check it my self. I did see one video where the guy said you can check the float hight to get a rough estimate by attaching a tube to the float bowl with the carbs still on the bike. I have a 1980 and I don't see the nipple he puts the tube on. do you just take out the brass screw and attach it there?
Click to expand...

Already stated whats needed...................

If you don't know what type of bike you have then post pics and you will get all the info that is needed to identify what you have.

Every thing is a guess, and all the answers are mostly a guess, and the different answers, (guess or correct), are going to confuse you more................Need pics
 

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