Throttle Position Indicator

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IanSpain
Posts: 12
Joined: Mon Jun 26, 2017 12:21 pm

Throttle Position Indicator

Post by IanSpain » Wed Jan 10, 2018 8:27 pm

I need some help and I think it's TPI related.

Carbs are balanced, valves re-shimmed to correct spec. Bike still stutters/stumbles around 3000 rpm when I gently role on the throttle. The TPI is set to 500 as per the manual and it's much better than when I bought it at around 800 ! It's got after market exhaust cans and it's much better with the baffles in than out. I think that's because it has never been re-jetted to suit.

I've tried to test the TPI off the bike. It reads around 400 then as I start to turn it goes down to around 370 before going up to 430 and onward. Very difficult to get a stable reading or for the reading to go up in a stable linear way. Is this normal ?

Has anyone got any readings for a good TPI or can suggest any other solution ?

This is the last mystery to solve !

Many thanks !!!

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VTRDark
Posts: 19335
Joined: Sun Mar 18, 2012 9:24 pm

Re: Throttle Position Indicator

Post by VTRDark » Thu Jan 11, 2018 3:01 am

This is all down to tuning and is a combination of jetting / mixture, TPS and carb balance and doing things in the right order. The problem here is that one can end up chasing their tail, you fix one problem that then introduces a new one somewhere else like a chain reaction. It's a good thing that you have had the valves done but by getting these done it then has a knock on effect on fueling. By stumble this could mean a lean or even rich stumble. How does the bike idle. Have you done a blip test.

https://www.vtr1000.org/phpBB3/viewtopi ... 14#p238528

It could also mean that your not giving it enough throttle your borederlining where it goes from the pilot circuit onto the needles there so it's a very temperamental part of the fuelling especially at low speeds. These bikes don't like low speed fuelling.

It's a lot easier to set the TPS with the carbs off. I would be tempted to remove the carbs and then either richen or lean out the carb mixture screws 1/8 of a turn depending on the result of a blip test, Set the TPS while the carbs are off, refit carbs and then balance them, check TPS once more to see how much the balance has affected it and see how things run. I'd take a guess that it needs to be a tad richer as you say it runs better with the baffles in so that would mean turning the A/F mixture screws out to allow a little more fuel through. Another option would be to shim the needles which will give a little at both ends ie the pilot and main's as well as giving some more in the middle. Also what airfilter are you running and is all the jetting standard. I would suggest starting with the mixture as this is the easiest to change.
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IanSpain
Posts: 12
Joined: Mon Jun 26, 2017 12:21 pm

Re: Throttle Position Indicator

Post by IanSpain » Thu Jan 11, 2018 8:41 pm

Thanks for the thorough advice. Gut feeling tells me you're right and its the mixture screws. Plugs are quite sooty so I think maybe the idle mixture is a bit rich. I've tried more throttle during the transition phase and it makes it worse than when feathering. The air filter is also standard. I tried taking the shim out from under the needle to weaken it, but no improvement so must be the idle circuit. May have to save up for the right angled mixture screw tool !

Turn in to weaken or out ???

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VTRDark
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Re: Throttle Position Indicator

Post by VTRDark » Thu Jan 11, 2018 10:27 pm

Clockwise to lean it out and Anti-Clockwse to richen. Think of it as a plunger that goes into a chamber ooo errr so as you turn the screw in it closes off that space so less fuel can pass. Even with the right screwdriver it can be extremly fiddly epsecially the rear.
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IanSpain
Posts: 12
Joined: Mon Jun 26, 2017 12:21 pm

Re: Throttle Position Indicator

Post by IanSpain » Sat Jan 13, 2018 3:33 pm

Well, I decided to take it all apart (Again !!!) to check the pilot screws. Front one was 3 turns out from fully in rear was 4 turns out. I think someone had tried a quick fix when they put on the open exhaust cans. I've adjusted the front and rear back to the factory suggested starting point of 2 1/4 and 2 1/2 respectively. The bike immediately started better from cold on the choke. Once it had warmed up the idle was much more even and it revved more crisply. It's pissing with rain so haven't ventured out for a test ride. Should be dry tomorrow so I will post results. Fingers crossed this sorts it !!! Thanks again for taking the time out of your day to help someone you don't even know !

IanSpain
Posts: 12
Joined: Mon Jun 26, 2017 12:21 pm

Re: Throttle Position Indicator

Post by IanSpain » Tue Jan 16, 2018 5:53 am

Test ride complete. No change to the symptoms whatsoever :-( My gut is telling me TPI or some other electrical....

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