Hitchcocks Forum

I have 2017 euro 4 bullet it runs great in first 4 gears BUT when in top it will only do70 mph unless you run on three quarter throttle then will go to to 80 mph slowly at full throttle engine dies go back to 3/4 throttle and engine picks up any ideas dealer is a bit baffled

Has the dealer got computerised diagnostic kit?

I've had a similar problem with my year 2004, iron barrelled, 350 Bullet. I could tell it was running weak at full throttle from the way it held back and then from plug "chops". So I fitted a richer main jet. The problem got worse. I then noticed that if I backed the throttle off as soon as it misfired it would pick up again. Experimenting in more depth, I turned the fuel tap off as soon as it misfired and the engine just died to a stop, but it would easily start and run normally as soon as the fuel was turned back on - until the next time I held it on full throttle for more than a few seconds. The engine was obviously using more fuel at full throttle than could flow through the fuel tap to the carburettor and the float bowl was running dry!

I confirmed it wasn't an air lock from a too tight fuel tank cap (fitted a long pipe from the tap into a petrol can and checked the fuel drain didn't slow down with the cap firmly on - it just seemed rather slow. Next, having drained the tank I removed the fuel tap and confirmed there was no debris in it (or on the filter in the tank) and then I decided to drill out the internal fuel galleries to a slightly larger size - this had no effect.

Seeing as the fuel tap had a push on "stackpipe" filter inside the tank, and a lower bowl filter PLUS an OE canister filter in the fuel line I next reckoned it could probably do without one of them. The most logical one to remove was the lower bowl filter - so I cut away the nylon gauze/mesh, leaving just the rim, which allowed a noticeably quicker fuel flow. The bike is now running a lot better, but not completely cured so (until I can be bothered to experiment with a different type of fuel tap) I just avoid holding full throttle for more than a very few seconds. It will now cruise slightly above HGV speeds on the motorway, so that's almost good enough on trials tyres..

I don't think 2017's have a fuel tap

My 2017 euro 4 bullet is the same, goes flat at full throttle but when you close throttle picks up again, I have recently fitted gold star type silencer, but because of bad weather have not done many miles with silencer but does seem to run better with this silencer, So I think my next move will be to change the front pipe for something more strait thru, the exhausts on these euro 4 bikes are very restrictive so could be the original exhausts holding the bike back.

At full throttle the EFI's are outside of closed loop adjustment. Modify inlet and exhaust too far from stock and there's no electronic compensation for the changes made so it just runs weaker with less power.

Presumably this is why EFI owners fit Power Commanders alongside their ECUs, or Mikuni carburetor conversions with a really good fuel tap...



A.

I had similar symptoms on a Laverda GTL 750. Cured by going one size down on the main jets. The problem wasn't lack of fuel, It was an over rich mixture at full throttle. That's why it was faster if I backed off the throttle a tad. Counter-intuitive it may be but that was the case. I wondered at the time if this was Laverda's way of preventing a potential to overheat on long full throttle blasts.

I agree with Dave p. On a carburetted bike, that would be a classic symptom of running way too rich at full throttle, you'd drop the main jet size. However, yours is fuel injected. So symptoms are consistant with the same issue but perhaps with a different cause/solution.



Some physical causes I can think of would be something blocking the air inlet or having fitted a different exhaust system. Or if there is a cold start mechanism of some sort sticking in the cold start setting. The solution is probably electronic though.



Another possability is fuel starvation either the fuel pump not running, a blocked tank breather or a fuel line obstruction.



Have you modified the bike from standard in any way?

Silly me!!! I 'd forgotten about the fuel injection aspect.