Rough Running, and the Throttle Position Sensor

[ex-viffer]

Hail, O Fellow Firestormers!

I thought those of you who haven't already read this might be interested in the following which appeared in the December issue of MCN.

FROM MCN:

Reader's Superhawk Tip

Hello, I wanted to share with you and fellow Honda Superhawk owners a great
tip I got off of Factory Pro Tuning's web site. Check the adjustment of your
TPS (throttle position sensor). It controls the ignition system's response to
throttle opening.

The most common complaint from Hawk owners is poor gas mileage. Mine got 30 - 32mpg, had poor throttle response below 3,500 rpm and the sparkplugs were always black, despite at least a dozen jetting changes.

Turns out you can't fix an ignition problem with jets! The manual calls for
theTPS to be set to a range of 4K-6K ohms in the idle position. Mine was
factory-set at 7.5K. I reset mine to 5K and the difference is incredible. I now get 38 - 44mpg, crisp throttle everywhere, with a 4000 - rpm sweetspot at cruise that's smooth as silk. And the plugs burn a nice tan color.

This tip could apply to any TPS - equiped bike. I hope this helps some of my fellow MCN subscribers.

Steve Louis
Consulouis@cs.com
Dayton,Ohio

[kingster]

sounds interesting. Where is it, what is it, how do I fiddle with it?

Chris

[RQ]

Dear God, is there something out there that will get me more mileage and only affect performance in a positive way !!??!! The guy in the forecourt said to me that he is convinced I'm siphoning the juice out of the bike at home and filling a car up with it !!

Where is this TPS, PLEASE reveal all !!

RQ.

[kingster]

Ok Boys, did a bit of surfin and found this...

http://www.biohazmat.com/vtr/shim.htm

Not been through it all yet, so don't know if it makes sense...

Chris

[rodon]

Ok Boys, did a bit of surfin and found this...

http://www.biohazmat.com/vtr/shim.htm

Not been through it all yet, so don't know if it makes sense...

Chris

I did the TPS adjustment a few weeks ago. The bike runs like warm butter now!
I didn't disconnect the wiring at the sensor to measure, I did the reading at the connector on the ignition module in the rear of the cowl under the seat.
It was set at well over 800 ohms from the factory. I got it down to 500 (within the range specified in the manual) by lengthening the holes in the sensor housing with a small Dremel bit: didn't have to bend the tabs as mentioned in the writeup. I was able to do this by poking the sensor out the side, while it was still connector to the wiring harness.

To remove the shear bolts, all I did was back them out with a long, narrow, sharp chisel and a hammer, working them out slowly. 1/2 turn and I was able to turn them the rest of the way by hand. A lot easier than drilling. I reused the washers, and used socket head cap screws to make reassembly easy.

There's no reason to remove the carbs. I even left the airbox in place. The hardest part was removing the connector from the ignition module, it is clipped on in a very peculiar way. I snapped some plastic bits getting it off, but it went back on tightly. You can get the color codes for the wires from the service manual (I can post them if you don't have it).

This fix is well worth the 2 hours of time it took (including the drive to the store to get new fasteners). Before, I would always keep engine revs at 4k or above while cruising. Now, I can let it fall to 3k and it is still very smooth and pulls well.

Good luck, and good hunting,

Rod

[delmeekc]

I have played with the TPS on a few VTR's and basically you are changing the ignition advance in relation to the throttle postion and engine rpm. Buy adjusting it as per the link you are effectively applying less advance in relation to the revs/tp this will make the bike smoother at low to mid revs/tp but as a trade off you would have less pull and topend grunt. You could only prove this by being able to view the ignition map of the ECU in relation to the TPS.

Del (ramblin again)

[Neophyte]

I was unfortunately not able to set my tps low enough to the setting stated in the above but after reading the above write-up I saw there are tabs on the throttle shaft that can be bent to accomodate setting the tps low enough. I was however able to set it lower by about 200 ohm and I can already feel a difference. I'll have to wait till the weekend though to retry. Will keep you posted.

[rodon]

I was unfortunately not able to set my tps low enough to the setting stated in the above but after reading the above write-up I saw there are tabs on the throttle shaft that can be bent to accomodate setting the tps low enough. I was however able to set it lower by about 200 ohm and I can already feel a difference. I'll have to wait till the weekend though to retry. Will keep you posted.

I wouldn't bend anything. Just enlarge/lengthen the slots in the sensor housing with a file or a rotary tool and a small bit. It's plastic, and easy to do. A couple of mm's should do the trick.

Rod

[ex-viffer]

Well, having posted the original article, I thought I'd better do something about checking the TPS on my bike. Firstly, the article says the TPS is on the right of the front carb. EHH! Wrong!! It's on the right of the *rear* carb. Secondly, I found it almost impossible to get some connectors on the pins in the TPS to measure the resistance without removing the airbox, so I did that. Then I used two thickish pieces of insulated wire, pushed the insulation out over the ends by a few mm, then (eventually) got one piece on the top pin and one on the middle pin. The resistance at idle was around 900 ohms! Next, I set off to the local electronics store, and bought a set of security bits, as the screws that hold the TPS on are those torx (star-shaped) screws (2.5mm, I think) with the little pin in the centre. Once I used these, it was a piece of cake to loosen the screws, although I had to use a right-angle screwdriver on the back screw, due to its position and proximity to the cam cover.

I rotated the TPS anticlockwise as far as it would go (just a few degress rotation), and the resistance dropped to a bit over 600 ohms. Near enough.

I must say, that the difference in performance, smoothness etc., was pretty much indiscernable to me. If anything, I think it *may* have made the pickup a bit worse when the throttle is whacked open at lowish revs, although I can't be sure. I thought I was pretty much in tune with what is happening in the negine room, but I couldn't notice any noticeable improvement. YTPSMV...

[delmeekc]

Which I basically what I said would happen as you have changed the advance of the ignition and are now using a less advanced part of the map. ie, less advance = less power =quieter engine = smoother engine but still less power. If it is to advanced then the engine will produce more power but will pink and spit back and possible knock and at worst try and detonate itself and put some nice holes in the dustbin lid (pistons). Bike should run smoother now with what you have done.

Del.

[ex-viffer]

Del,

I did think it seemed to be pinking a bit less, so maybe that has been fixed if nothing else. I'd been running 98 octane because it sounded like it was pinking at low revs / under load, so if what you're saying is correct, then it's fixed that up, and I can go back to 91 (RON) petrol.

Before I changed the setting, the TPS was installed with the screws smack-dab in the middle of the slotted holes, so I'm guessing they don't even bother checking this at the factory, just put it in like that. If so, it's reassuring to know they take so much care setting it up right....

[delmeekc]

Another thing that manufacturers do is make the valve clearances really tight to start with do when you ride you bike or ride a demo the engine sounds really quiet. Then at the 600 mile service they get set correct and then it sounds like a bag of bolts.

Del.