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3Vd Running Condition Help


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#1 nsr250

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Posted 04 November 2018 - 11:24 pm

Need a little help with my '92 TDM. Was on a ride this past spring when suddenly the bike started surging while at a steady speed. Sometimes actually bucks a bit. If I give it a bit of throttle it accelerates fine but goes back to surging/ stuttering as soon as I go back to a steady cruise. I've taken the carbs apart twice for cleaning, changed jetting, plugs, wires, coil, rebuilt the petcock, cleaned out and coated the gas tank, new fuel filters, etc. Most of this was just done for maintenance sake since the previous owner neglected it. But there has been no change in the surging. Any ideas as to what I might be missing?

 

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#2 Bjørge

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Posted 05 November 2018 - 08:22 am

May be a problem with the fuel pump. If it's this problem, it's better with a full tank (higher pressure from fuel weight alone), and worse if you drive fast with open throttle.


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#3 fixitsan

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Posted 05 November 2018 - 08:34 am

As you ride along with the problem happening, steadily pull out the choke, does it run better or worse ? Does it start to improve and then as you pull the choke out further it dies away again ?

 

the main phases of carb operation are low opening of throttle, mid range and wide open.

 

Wide open is controlled by the main jet, mid range is the needle profile/needle height, and low opening and or slow speed running is the idle circuit.

 

If you're running along at a constant high speed, but a steady speed, does it surge then, or is it only at low cruising speeds, like around town ?


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#4 nsr250

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Posted 05 November 2018 - 12:19 pm

May be a problem with the fuel pump. If it's this problem, it's better with a full tank (higher pressure from fuel weight alone), and worse if you drive fast with open throttle.

Brand new fuel pump and has done the same with the stock pump and electric.


As you ride along with the problem happening, steadily pull out the choke, does it run better or worse ? Does it start to improve and then as you pull the choke out further it dies away again ?

 

the main phases of carb operation are low opening of throttle, mid range and wide open.

 

Wide open is controlled by the main jet, mid range is the needle profile/needle height, and low opening and or slow speed running is the idle circuit.

 

If you're running along at a constant high speed, but a steady speed, does it surge then, or is it only at low cruising speeds, like around town ?

It happens at any speed but at a  steady throttle (no acceleration).

 

And this all started while on a 4 hour ride. Ran fine to begin with and slowly got worse.



#5 Pedro

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Posted 05 November 2018 - 12:58 pm

mine did the same when i first bought it when about 1/4 tank of fuel. it turned out to be fuel tank sealant the original owner put in from new. they were flaking off and sitting at the bottom of the tank. when low or near reserve it kangarooed.  worth peeking inside the tank with a bright torch in the dark or draining it completely in case there's flaked off sealand or crud in there. sometimes water can build up over time as well and sit at the bottom. could be other things like fuel pipes weak and collapsing which happened to mine as well. joys of a 26 year old bike!



#6 nsr250

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Posted 05 November 2018 - 01:35 pm

mine did the same when i first bought it when about 1/4 tank of fuel. it turned out to be fuel tank sealant the original owner put in from new. they were flaking off and sitting at the bottom of the tank. when low or near reserve it kangarooed.  worth peeking inside the tank with a bright torch in the dark or draining it completely in case there's flaked off sealand or crud in there. sometimes water can build up over time as well and sit at the bottom. could be other things like fuel pipes weak and collapsing which happened to mine as well. joys of a 26 year old bike!

Happened before and after I sealed the tank. Inside of the tank is now spotless. Also, brand new fuel lines.



#7 Studley Ramrod

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Posted 05 November 2018 - 03:28 pm

Have you set the air mixture screws/pilot screws and balanced the carbs.  Also worth checking you have the correct play on the throttle, too tight and it will surge at steady throttle.  Same goes for the drivechain.


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#8 fixitsan

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Posted 05 November 2018 - 07:04 pm

I had a similar problem once with a death trap FJ1100. After repairing a load of problems and the bike ran well for a few days I started to get a similar issue. It turned out that whoever had put the loom back in around the headstock had pulled some of the wires tight and they had been rubbing  around the edge of the frame, occasionally shorting to earth. When i accelerated the harness moved a little bit away from the frame but when running steadily, combined with vibration, one wire would touch the frame edge.

 

i would look at anywhere the harness passes around a bracket, and every single plug/socket, remake them all, and look for evidence of previous repairs. Leave the bike idling with all the plastics off and push/pull the harness and plugs and see if the bike dies when you manipulate one particular area


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#9 nsr250

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Posted 06 November 2018 - 01:34 am

Have you set the air mixture screws/pilot screws and balanced the carbs.  Also worth checking you have the correct play on the throttle, too tight and it will surge at steady throttle.  Same goes for the drivechain.

Everything is set up per the manual except I upped the main jets a little. They are synched spot on.



#10 Studley Ramrod

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Posted 06 November 2018 - 11:48 am

Well, settings for carbs in the manual are really only a factory setting and the pilots and balance screw may need adjustment using some suitable gauges.


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