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Surging At High Revs/speed


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#1 paultdm850

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Posted 01 January 2019 - 09:02 am

Happy new year one and all.

 

My old 99 TDM (vacuum fuel pump) had been off the road for about 8 months whilst I gave it a bit of a freshen-up. Anyway, I've done over 500 miles since putting it back on the road but have noticed that when I get above 90 mph in top gear and go for full throttle, it surges like it's running out of fuel. It only happens when I demand a lot of fuel at high revs in top gear, otherwise it fuels beautifully. First time it did it I thought perhaps it was carb icing, but now I'm not so sure. Is this typical symptoms of a fuel pump begging to fail or something else?

 

Thanks.

 

 

 

 



#2 Studley Ramrod

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Posted 01 January 2019 - 09:50 am

Difficult to know for sure but you've mentioned the most probable cause, although a carb clean and balance might solve the problem.  Also worth draining the float bowls as water can accumulate there if the bikes' been stood for a long time.


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#3 fixitsan

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Posted 01 January 2019 - 10:10 am

Blocked main jet or low fuel level in the float chamber ?


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#4 paultdm850

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Posted 01 January 2019 - 01:25 pm

Thanks guys. It pulls up through the gears fine, it's just when you demand high fuel flow after it's already been cruising at around 90 mph for a while. I guess it could be a partial blockage in the tank restricting the flow. I will report back when I find it.



#5 wicklamulla

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Posted 02 January 2019 - 12:44 pm

most loikely not this,     but please check the tiny small bore fuel tank vent / breather beneath the filler cap.  I use a length of strimmer cord and push it down thru the hole from the fuel tank end,  you will see the strimmer cord pop out underneath the engine (or where ever the rubber vent hose terminates).


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#6 dablik

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Posted 02 January 2019 - 01:52 pm

most loikely not this,     but please check the tiny small bore fuel tank vent / breather beneath the filler cap.  I use a length of strimmer cord and push it down thru the hole from the fuel tank end,  you will see the strimmer cord pop out underneath the engine (or where ever the rubber vent hose terminates).

 

Did the same but used a small syringe and watched the water flow out the bottom..


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#7 TKH

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Posted 02 January 2019 - 06:32 pm

I know someone'll post this if I don't, so.

 

Might not be the best thing to be admitting cruising at 90mph.....and then opening the throttle.



#8 Nog

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Posted 03 January 2019 - 09:20 am

Might not be the best thing to be admitting cruising at 90mph.....and then opening the throttle.

 

Obviously when he was on holiday in Germany and testing it on the autobahns <cough>



#9 paultdm850

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Posted 04 January 2019 - 07:25 pm

most loikely not this,     but please check the tiny small bore fuel tank vent / breather beneath the filler cap.  I use a length of strimmer cord and push it down thru the hole from the fuel tank end,  you will see the strimmer cord pop out underneath the engine (or where ever the rubber vent hose terminates).

 Very good call my friend, I will definitely check this when I get around to it.


I know someone'll post this if I don't, so.

 

Might not be the best thing to be admitting cruising at 90mph.....and then opening the throttle.

 

I didn't say I was on a public road...



#10 Favs

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Posted 06 January 2019 - 11:28 am

Generic fuel filter is a couple of quid & PoP to fit - cured my 850 from surging when demanding more fuel.


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