Where did the "smooth" go - K-Bikes.com - Excellence in Motion
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post #1 of 29 (permalink) Old Mar 30th, 2013, 4:34 pm Thread Starter
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Where did the "smooth" go

Ok here goes. Chicago finally warmed up a bit to go out for more than a 15 minute ride. Over winter I replaced the input shaft and replaced the clutch in my 87 K75s. I have ridden for several 15 minute short trips and tranny seems all good. I decided to give a tune up to de-winterize the ol girl thus checking valve clearances and throttle body sync. This is where I ran into problem beginnings. I had read about the TPS "click" and was getting none. I repositioned the switch and now I get the "click" but it also seemed to put my idle high and not allow throttle to completely close. Re-adjusted the TPS to click and not hold idle positioner open.
I only have some dial type vacuum gauges to try and sync the bodies and all were pretty damn close to one another.
Today the bike performed fine until about 20minutes into the ride and then I am getting pretty dynamic power surges and terrible throttle response sputtery exaust (more than normal) Pops when rolling on the throttle. Last year I replaced the common dried rubber hoses and such so I am feeling like it isn't air leak. Also upon coming back to my toyshop I saw this little cap sitting loose near the fuel rail. Please Please! any help and advice would be greatly appreciated.
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1987 k75s
1979 yamaha xs750
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post #2 of 29 (permalink) Old Mar 31st, 2013, 12:30 am
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Quick and dirty test for air leaks: Start the mc and remove the oil filler cap. You now have a self-created vacuum leak. The mc should get lumpier/rougher. If not, you probably already had one. You didnt mention how the mc is idling. If the problem is only while running: put the mc on the centerstand. I usually put a board under the centerstand to get a little more distance between the ground and the wheel. Put the mc in gear and run until the problem reappears. Then remove the cap and see if there is a difference. Please do not run the mc into your garage wall.

My K75 had similar issues in freezing temps and high altitudes toward the end of its life. Fine in warm weather. Mildly irritating in cold temps at lower elevations. Never figured it out. Best guess was vacuum leak due to worn rubber which only manifes:-)ted under extreme conditions. This was before I knew about the oil cap test and the crank case breather hose.

Re: the TPS. Bosch manual for LE-Jetronic is available for DLing as a pdf at many sites. The TPS has two contacts inside. One indicates throttle off and the other WFO. That is it. Minimum gas and maximum gas. Two switches. The controller gets no info from the TPS when the throttle grip is in intermediate positions. The air flow sensor handles that job. At WFO, fuel economy and emission control are not the controlling factors. The control unit is designed to deliver a fuel mixture for maximum power. One reason for the throttle closed switch is so the control unit can shut down the injectors at the proper time when you go from throttle on to throttle off. Without this, in situations such as from WFO to closed, the fuel injectors would not decrease the amount of fuel in sync with the decreased amount of air. You would get a rich mixture of some sort. This results in the popping exhaust sound. You also get this if you fail to completely close the throttle when rolling off the gas. I did this regularly when I first got my K75. It was duck and cover eveywhere I went.

The point of this is the manual says failure to properly set the TPS can result in the popping noise even if you close the throttle completely when rolling off. In this case, you have set the TPS so the switch which indicates throttle off is not activating. So, the control is not shutting down the injectors in the way designed to prevent overrun. You do get decreased fuel as the air flow sensor sends its signal to the controller. But the specially designed shut off at throttle off does not occur. BUT THIS IS AT ROLL OFF, NOT ROLL 0N. As far as I can tell, TPS does nothing on roll on until you hit WFO. Unless you have misset the TPS so badly the WFO switch activates very early, missetting the TPS should have no effect on moderate roll on. I do not know what the effect of early activation of the WFO switch would be.

The click you are hearing with the throttle grip forward must be the contact closing. That means the controller should be shutting down the injectors to prevent overrun. Presumably there is a click at WFO to indicate that switch is working, but I havent tried that. If I remember I will try that tomorrow to see if I can figure out when the WFO switch closes.

I am not an expert in LE-Jetronic. I have studied the manual to get a basic understanding of the system. A guru may correct me here.

Is the cap rubber\vinyl? I havent run a K75 for 5 years, but I have a vague recollection of two small black rubber\vinyl caps that cover open-ended tube spars that came out of the main frame under the tank somewhere. One on each side? Mostly decorative I thought. Maybe kept water and dirt out of there.

Good luck. I would triple check for a vacuum leak. That is an easier fix.

Last edited by 97tiger885; Mar 31st, 2013 at 12:56 am.
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post #3 of 29 (permalink) Old Mar 31st, 2013, 11:48 am
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I think your little cap is the protector for the end of the throttle shaft at the very front of the throttle bodies.

Some of the later bikes had such a cap and some have found it laying there. I say some....not even in the parts fiche and no such thing on my later bike but yeah may have disappeared somewhere on the road back then.

Shouldn't affect the vacuum unless dirt got in there and wore out the throttle shaft, doesn't seem to be your case if vacuum readings hold steady. Vacuum leaks would certainly show as fluctuations on the gauge or "no vacuum" if a big leak. That may not show at idle the small leaks, you may have to use the throttle a little and see what the gauges are doing.

But popping/sputtering.....how good is the gas? Just asking because my little XR did just that with relatively fresh gas, took a while to clear what I think was water getting sucked in.Also fuel filter? Just had to replace mine at way less than the recommended interval, was plugged almost solid and moisture/water will also swell the filtering media and plug it.Yep been through that with the K100RS, only took a batch of bad gas and sure did not want to run well, with symptoms very similar to yours.

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post #4 of 29 (permalink) Old Mar 31st, 2013, 12:29 pm Thread Starter
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Thanks 97 for the in depth response! Brilliant idea for the quick vacuum test I will certainly be doing that today along with another close inspection of potential leak zones.
To mention the bike at idle, I definitely have a stutter or stumble at idle. also the day was mild about 57 degrees. The surging problem is most noticeable at lower speeds and off the line I have really rough throttle response. Highway speeds its much less noticeable but the bike is much less punchy than it used to be.
I'm pretty certain I haven't grossly mis-set the TPS. I get a click just as freeplay of throttle takes up and another when I close throttle.
Back to the toy shop today. Thanks again

1987 k75s
1979 yamaha xs750
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post #5 of 29 (permalink) Old Mar 31st, 2013, 12:40 pm Thread Starter
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Quote:
Originally Posted by H96669
But popping/sputtering.....how good is the gas? Just asking because my little XR did just that with relatively fresh gas, took a while to clear what I think was water getting sucked in.Also fuel filter? Just had to replace mine at way less than the recommended interval, was plugged almost solid and moisture/water will also swell the filtering media and plug it.Yep been through that with the K100RS, only took a batch of bad gas and sure did not want to run well, with symptoms very similar to yours.
H9, Thanks for the tips. I was looking at the throttle shaft end as well thinking that was probably the home for that little cap. It must have very recently come off so I don't imagine wear yet.
As for the gas, who knows about quality with these city gas stations. Could be half water these days. I filled the tank with Mid the day this problem arrived. I also threw in some HEET to try and wrangle any winter moisture.
I replaced the fuel filter last season but did get a good amount of water into the tank from water sitting under cap when I opened it. But that was several tanks ago. I will try swapping the filter out if I can get my hands on one quickly. Thanks again.

1987 k75s
1979 yamaha xs750
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post #6 of 29 (permalink) Old Mar 31st, 2013, 2:17 pm
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When you synched the throttle bodies, you removed three rubber caps. Did you closely examine them before you put them back on? They can get brittle, crack, and leak. New ones are available at local parts stores, usually in a variety pack but very inexpensive.

Larry Johnson
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post #7 of 29 (permalink) Old Mar 31st, 2013, 4:35 pm Thread Starter
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Quote:
Originally Posted by ljjohns
When you synched the throttle bodies, you removed three rubber caps. Did you closely examine them before you put them back on? They can get brittle, crack, and leak. New ones are available at local parts stores, usually in a variety pack but very inexpensive.

Larry Johnson
El Paso Tx
Thanks Larry, Caps are brand new from auto parts store. at least I can check that one off the list.

1987 k75s
1979 yamaha xs750
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post #8 of 29 (permalink) Old Mar 31st, 2013, 4:38 pm Thread Starter
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Update to missing smooth

spending the day in the toy shop I have just discovered that when the bike is idling and I wiggle the rubber boot on the third injector the idles bogs dramatically. The 2 other injectors toward the front do not have the same effect when manipulated. Any thoughts on this? Thanks again for all the advice.

1987 k75s
1979 yamaha xs750
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post #9 of 29 (permalink) Old Mar 31st, 2013, 5:15 pm Thread Starter
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New update

So I pulled the connector at the top of the third injector that seemed to be shorting out and low and behold it looks like it has gotten a little cooked. Any thoughts on if this could be part of my problem and why it would happen. Thanks tons
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post #10 of 29 (permalink) Old Apr 1st, 2013, 9:11 am
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I found enough greenish/whitish connectors on my much later bike to know about the why.Mostly road salts eventually seeping in there but could be salty sea air or even just normal moisture. You should follow the BMW bulletin that recommends cleaning and protecting all the connectors on their bikes once a year. Old bulletin but still valid.

Pretty small wires also, it may be broken inside the sheating and you can't see the break.May need to find the other end and put the Ohmeter to it and wiggle to look for continuity.

Better stuff and availability nowadays as to dielectric greases and electrical cleaners. Did my bike 2-3 years back, re-inspected this year and still all good.No need to follow the "once a year" bulletin as long as it gets done with some regularity.

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Yamaha RD 200 at 16
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