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skeptical old git
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Discussion Starter #1
This will be familiar to many of you: riding with revs around 5000rpm in 4th or5th gear. roll off throttle a bit, then roll on, and you get a couple of seconds hesitation. Perfectly possible to ride through it, but obviously not the correct throttle response.

The standard answer (high Yen!) is an air leak between airbox and carbs. Overtighten the collars and the airbox-carb rubber bits slip off the carbs.

But... what if you've checked that out (and in fact the airbox hasn't been off in maybe 3000 miles) and it's fine?

Air filter fine. Plugs look fine. Engine runs perfectly at other revs; power and fuel consumption seem normal; idle even and predictable; drops from higher revs down to idle speed smoothly; no odd noises, smoke from exhaust, etc.

What to check next? :confused:
 

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It sounds very much like the standard air filter. I had similar responses on my Transalp until I fitted a Piper Cross air filter.
It gives a better air flow and seems to remove the flat spot.
 

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I picked up somewhere that with a DynoJet kit is a drillbit (I think 2.5mm) supplied in order to enlarge certain holes in the carb slides - this gets you a faster throttle response, it did it with mine.

There must be some post about it in this forum.
 

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skeptical old git
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Discussion Starter #4
It may be possible to improve throttle response with modifications, but what I have is simply a failure to respond like the stock bike. I've had the bike long enough to know that this is something different to its normal behaviour.

This has been described before, where the airbox isn't sealed to the carbs properly (usually attributed to over tightening the collars). But that's not the issue in this case.

Last night I found that the outlet at the bottom of the airbox (where the drain tube attaches), which is a separate bit of plastic, doesn't look tightly sealed to the bottom of the airbox. Doesn't look significant enough to cause a problem, but have sealed this with silicon sealant anyway.
 

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skeptical old git
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Discussion Starter #8
The symptons you decribe are similar to my bike.
I changed the fuel pump which cured it
thanks, I'll try bypassing the pump if none of my fiddling has had any effect (which seems very likely)

I assume the fuel line out of the pump will fit the fuel filter, so should be a 2 minute job to bypass the pump. And unplug it of course!
 

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Generalissimo Tea Boy
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thanks, I'll try bypassing the pump if none of my fiddling has had any effect (which seems very likely)

I assume the fuel line out of the pump will fit the fuel filter, so should be a 2 minute job to bypass the pump. And unplug it of course!
By-passing the pump will get the engine running, but high speed riding is a bit iffy, even with a full tank. I found that at 60 or 70mph, if I overtook anything the engine would cut out, I would roll to a stop and have to restart the engine. Not enough fuel getting through on higher throttle settings so I assume I ran out for a few seconds each time (well the carb did anyway).
 

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I had the same when I first got my RD04.

I changed everything on the bike apart from the Airfilter as it looked clean as a whistle.

In the end there was nothing left to do as I had tried almost everything else so I threw a new standard airfilter on and bobs your uncle, it has never been an issue since.

Unless you know your Filter is new and in good order you cannot go by looks alone.

HTH

Chris
 

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skeptical old git
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Discussion Starter #11
... a new standard airfilter on and bobs your uncle, it has never been an issue since.

Unless you know your Filter is new and in good order you cannot go by looks alone.
Thanks for the suggestion, but I put a new Honda filter in just 3000 miles ago.

Then again, maybe I should try it as it's an awful lot cheaper than a new fuel pump :D
If it doesn't work I can just keep the new one in stock...
 

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Hi, I'll go against the opinion of fuel pump problems, as if you were suffering a lack of fuel, be it a weak pump, blocked filter or whatever, you would likely suffer hesitation when you had the throttle constantly open as the carbs would steadily run dry, but then when you shut the throttle, even for a only a second or two, it would allow the fuel that is flowing to start to catch up with the engines needs, i.e. start to refill the float bowls, and when you reopen the throttle there would be no fuel shortage, at least until the carbs started to run dry again.

If the filter is OK then the only other suggestions I can make are choke plunger slightly sticking causing a rich mixture, carb balance, or just possibly the throttle position sensor if you have an RD07A. If your fuel consumption has gone up then choke plungers are most likely, but it could be on only one cylinder and then only sticking slightly. Check a plug on each cylinder and see what they look like, cheaper than a new fuel pump!
 

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skeptical old git
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Discussion Starter #13
I think I've fixed it :D But I don't really know how :confused:

Things I did that could plausibly have had some effect...
  • filled up with that posh Shell petrol that's supposedly has additives than clean the fuel system (yeah, yeah, I know, but worth a couple of quid on the off chance!)
  • emptied the airbox drain tube
  • fiddled with airbox-carb connections (don't think they were loose, but I might just have stopped a minor leak)
  • used silicon sealant to stop leaks around the airbox drain outlet
Only the last item ion that list seems likely to have had any effect, though it can't have been more than a tiny leak into the airbox so I'm having a hard time believing it was responsible...

I also took the cover off the fuel pump. No idea what it looks like when it starts to fail, but others have said "you'll know if you see it". It looked normal to me, so either it's OK or I'm just far more stupid than those people think... :sad11:
 
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