Category: Common Problems
Type: How-To

Article Name: Throttle Position Sensor adjustment
Author: mamisano
Description: This modification will improve low end response and smoothness on the VTR. It is especially effective if you have rejetted the carbs.

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I incorporated this recalibration into my winter service. It work as advertised. Pull from a stop is much smoother. it's amazing how helpful it is to be on the bitmap. I would encourage any and all to take the time to check the resistance of the throttle position sensor.

 

I haven't seen this mentioned in any of the TPS posts…
I’ve had a recurring pinging problem when warm in the lower RPMs (< 3.5k). I, and the local Honda dealer, had tried everything we could think of to correct the problem. All the common things were tried like adding octane booster, richening the jetting and even uncommon things like filling the cylinders with liquid carbon remover. (Very messy, but I was desperate to find a fix.) Well I just want to let you all know that adjusting the TSP was just the ticket to correct my problem. My TPS was set at 843 Ohms. With a bit of fiddling I was able to adjust it to 501 Ohms. If your VTR was like mine and pings all the time, take the time to adjust the TPS. It’s worth the effort.

 

Just FYI, you don't have to drill the factory screws out. If you have a small, sharp chisel, they can easily be removed. Just use the chisel on the edge of the flat factory screw to turn it counter clockwise. Once removed, you can then replace with the 5mm screws, or modify the head of the factory screw and re-use them.

 

The TPS makes no difference at idle. It's only function is to indicate to the ignition when to advance the timing. This happens much later in the rev range.

 

So, what then is the factory recommended setting? Is it 850, or 400-500? I have not seen a manual. Also, is it measured between the top and center pin? This is the only place that makes sense, but.....

 

4-6K ohms at 68 degrees at the sensor terminals.

Normally resistance increases (when its all connected) as the throttle is opened, so a high reading might be due to a butterfly that doesn't close all the way.

The ignition advance change over point is 3500 rpm. If you disconnect the sensor at this speed or above, the speed should drop. It should increase when you reconnect the sensor.

 

I believe the spec in the factory manual is incorrect. 4 -6K ohms is 4-6 thousand ohms. I set mine at 514 ohms. if your machine is in otherwise proper tune the difference should be very noticeable pulling off from idle.

 

Why. It doesn't do anything at idle at all. It's purpose is to tell the ignition when to advance the timing.

 

Because if the TPS is off calibration at idle, it will be off at all throttle positions. Just because the TPS is used for the advance doesn't mean that the computer isn't digitizing the TPS position even at rest position and 500 ohms will digitize differently than say, 800 ohms.

 

And doing what with it?

 

I agree. My understanding is it is for timing advance for higher rpm. I sure don't notice any difference in low end power. Timing advance is negligible below about 3500 rpm, no?

 

If the TPS setting is supossedly only effecting higher rpm timing why did the bike idle MUCH smoother after reseting from 900 down to 500 ? At the original 900 setting the idle sounded like it was running with hotter cams. Sounded great but pulling out in 1st was a PIA.

 

Impossible to know the answer. Maybe it was all the other stuff that was tried. Maybe it was resetting the idle speed. Who knows.

 

The difference in the resistance will put you on a different point on the computer's bitmap for the timing curve. Better understanding may come if we can determine what variables are inputing data to the computer. It has to be more than just the TPS or the computer would not be needed. As for results, my TPS adjustment was performed only after a complete tune. The difference off idle was as significant as the installation of A Dynajet kit and a K&N was to overall performance. No maybe!

 

No other stuff was tried. I like to make one change at a time.
The TPS change made a significant idle change and off-idle performance by it self.

 

Yes, I feel the differences I saw were due to synching the carbs and adjusting idle. The TPS change did not seem to make much difference. Also, I think that the temperature was much higher than the manual specified 68 degrees, which would have had an impact too. I just found it odd that there was a change from 460 last year to 843 this year!!!

 

500 Ohms is for throttle open 0 I think.
It should change and increase with throttle opening...

So it could impact idle if the TPS is not calibrated and cause the computer to apply an advance corresponding to an higher RPM ... Does it make sense ?

I have to admit, I never have checked my TPS nor noticed bad advance timing so far ....

 

Theoretically, yes. Practically, no. It could not be so far off as to advance the timing at idle. If it was, it would be broken far out of the range that could be fixed by adjustment.

While the symptom you describe could exist, it could not be fixed by rotating the TPS.

 

yes, it should not get so far out of adjustment without actually removing it and elongating the holes or something. Those are designed to be adjusted within a range. The range is generally not large enough to cause major problems. Even at either end of the range, the biggest problem would likely be emissions

 

Last night I looked at the reading on my TPS, never touched. 896 OHMs.
With out messing with the holes, the lowest setting I could get was 600OHMs.

I didn't want to mess with the holes....so that's where I set it.

I did find idle to be a little better when cold......and better yet when hot.
It was the only change I made and the bike used to idle rough when at operating temp. Also....the mid range stumble appears to be less.

But then again......every SH has a mind of it's own!

BTW....no drilling required to loosen screws....that's what Vise Grips are for!

Mike

 

It's not fuel injection. I understand it perfectly. It has nothing to do with idle.

 

For those who have ever worked on computer controled Rochester Q-jet carbs of the mid-80's.......A TPS used in that application does affect idle.
A good read on that is a book by Doug Roe.

Mike

 

Are you saying you believe the TPS is used as a pollution control device?

 

Well.....to make this short......it monitors how open the throttle is. Inturn adjusts ignition spark to efficiently burn the most fuel. In that sence.....you do get better emissions and at the same time more power. But...I could be wrong............

Just my opinion......I hunt subs for a job.....not emissions engineer


Mike

 

Good thought, but not at idle. Advance is fixed at idle. It further advances at 3500 rpm.

The functional test for the TPS is to disconnect it, rev to above 3500 rpm and reconnect it. If it is working, revs will increase.

You can test your theory by disconnecting it and reconnecting it at idle. Let us know if your theory proves to be true.

 

Old school car mechanics that have worked with vacuum and centrifugal
advance know an engine needs more advance at small throttle openings
(cruising) for cooler running and better fuel mileage.
Less advance at large throttle openings low rpm to pull smooth under load
and reduce detonation.
More advance at large throttle openings high rpm for more power.
So the throttle position sensor would have to work in conjunction with the
ignition module to select the right advance.

When we adjust the TPS from 800 to 400 it shows the module a smaller throttle opening and gives advance though a larger range of throttle position.
(at lower RPM) when knock could occur. Then @ higher RPM you'd get full advance regardless of throttle postion.

 

I'm definitely puzzled.

My service manual says "4 - 6 K ohms at 68F", and my TPS reads 5k ohms (brandy new 2004). Of course, it's 98F out right now...

Should I rip it off and try to tweak it down to 500 ohms? Will it adjust that far?

Keith

 

Update: The service manual says 4 - 6 k ohms at 68F, true...but has you measure across the outer terminals. And that value never changes, regardless of throttle position. Maybe an ambient temperature resistance?

The lower terminal to the center terminal on my bike is currently at just a bit over 800 ohms, and definitely changes when the throttle position changes. Gonna set it at 500 ohms for "closed throttle" and see if that helps.

Keith

 

Correction to my last post...upper terminal and center terminal. Doncha hate when you have the carbs upside down on the bench....