Fuel Starvation - top end / full throttle

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GT

GT

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Hi

I am experiencing fule starvation at over 5000 prm in top gear. It pulls cleanly thruogh all the gears to the red line and in top just fine, then after about half a mile the bike just loses power as if it has run out of fuel (it hasn't). If I throyttle back to about 4500 it rides happily and after a few seconds yuo can pull cleanly above 5000 again and then after another 10 seconds or so the same thing happens. It only really impacts in top gear as that is when the trottle is wide open, you can attain and hold 7000 / 8000 in any other gear, I think because the trottle is open much less as the gearing is higher.

The bike has ben fully rebuilt, runs and idels fine. Plug colour is perfect - light biscuit. Standard exhaust ( stainless with Cox chrome silencers).  Standard carburation, but I do not know what size main jets are in it - drilled out air holes in fuel cap a little to ensure clear air passage.

It does it on a full or low fuel level and on main and reserve tap position. Cavis Benz fuel pipes fitted. Standard airbbox, no filter.

any ideas?

regards
Glenn
 
Have you set the float heights to the recommended settings?
Sounds like the float jets can't supply enough fuel at extended openings.

Also are your fuel taps clean and flowing enough fuel?
 
It also is a symptom of insufficient tank venting, are you sure you have everything correct there after your drilling episode? It is often worse with a fairly full tank (less air volume).
Jetting won?t give you such a long time before it makes its presence felt.
Other culprits could be the fine strainers in the tank and under the carb banjoes.
If you can find a very steep hill to climb you will no doubt find it does it also in 4th etc.

 
pay attention Steve, "he said standard airbox and no filter"

this sound very like the needle valve (and seats) are much too small, if the bike is a 180 it would help to fit the RGS metal cross rail tube, but the float valves must surely need changing for larger ones.To prove that the tank venting is (is not) at fault try riding with the tank cap popped open, but be carefull of fuel slosh especially on braking.
CLEM
 
Ahem... read right to the end Steve. :D
Okay, then I agree with the others re fuel supply, the RGS type rail is much better, check your float valves, flow test them in situ with all normal upstream restrictions.  Could also be affected by vibration when in operation?
Cheers
SteveB
 
Triple in stock tune copes easily with the stock 250 float valves.  The fuel rail, while theoretically being a better design, doesn't help much in this situation either.  Stock fuel taps are more than large enough, but it does pay to check if the sealing rubber hasn't swollen and is blocking flow.

I'd be checking float heights and then tank venting...

piet
 
Hello all, be gentle as it?s my first post.

Here goes, IMHO I suggest you take a look at your float needles. They comprise of two parts; the needle that sits in the jet and part that engages with the float, (anti vibration?). The part that engages with the float is fixed into the main piece of the needle by means of a crimp. Twice in thirty years of ownership of a triple the small pin like part of the float needle has worked its way past the crimp, effectively lengthening the float needle and as a result restricting the jet. The solution is simple tighten up the crimp.

As I say it?s happened twice to me, the first time a trip to Slater?s didn?t sort it. I stumbled across the fix myself (internet wasn?t available to the common man in 1991). A couple of years later when I mentioned my experience to Steve Winterton he knew what it was. The second time the symptoms appeared I knew what it was straight away, one of the advantages of long term ownership; it could have been a roadside fix if I?d had any tools  :(

Tried a quick search of previous posts but this problem doesn?t seem to have been raised before.

Glen noticed you?re other two bikes, are you a member of the Trident&Rocket owners club? I know the membership sec, Sam.

Regards,

Mal
 
Mal,

The situation you describe very seldom arises on 180? triples, hardly ever on the twins.  The 120? triples are the most inflicted, must be the finer vibes, one of the first things I check when I have one on the lift...

Yeah, btw, welcome ! :D

piet
 
Piet,

you've "outted" me unwittingly. I own a 120 Jota but didn't want to let that be known in my first post  :D

Regards,

Mal
 
I had Mal?s malady on my SF2 once in the middle of the night in the bush and fixed it by the lights of passing cars. The bike had cut out on one cylinder.  Dead easy to diagnose as a float-bowl was dry. Once the needle had popped past the crimp the spring extended it and the needle closed with the seat shutting off the fuel to that float-bowl. It was no intermittent fault related to revs or gear. Easy fix to just push the needle back in and use my trusty original Laverda toolkit pliers to re-crimp.
 
Hi Mal, yes I am a member of the TR3OC, plenty of work to do on the 2 T160s, always dropping bits of engine!

Regards
Glenn
 
When rebuilding carbs, I have noticed that the float valve housing seats quite a long way into the carb body. This means that the fuel outlet holes appear to be partially obscured. I say "appear to be", as I am not sure if the flow is actually restricted. There is meant to be a thin gasket in there, so make sure it is there. I guess you could add more to move the housing further down to clear the outlets and maybe improve flow (?), but you would have to reset the float heights..

Pete



 
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