1991 Trophy 1200 13,000 miles No issues before I put her into storage but I’ve just changed the needle valves after the lay up because fresh petrol ran straight to the floor (also renewed the engine oil, float height I believe to be set correct). All good so far. When I started her up she started on the button with the usual amount of choke and all was normal. After a bit of heat got into her I decided to back off the choke a bit and try a little revs. The engine cut out dead. No ifs or buts – the slightest movement of the throttle grip and it’s like I’ve hit the kill switch. This happens every time. Any ideas would be brilliant as I’m out of talent. Needle valves and blocked jets are my limit.
@KevofCov Have you let it really heat up before revving. Also check the choke is clear. And is it getting enough fuel to feed the carbs. What i try sometimes is a tube and separate feed bottle. Really if you lay a bike up any length of time you should drain the carbs. Joe
Air leak issues, potentially created on re-install after your carb work ? Will it ONLY run on full choke?
Hi all Currently feeding it off a small tank from above. Ran the engine till it stopped before storage but will undo the drain screws in future. Pegscraper – I will be out there tomorrow and will check Eldon – yes will only run on choke, any movement on the throttle and it cuts straight out
Look for excessive air entering the system, incorrect fitting, perished rubber connections, etc. Can you run it with airbox/tank off? This should show you which slide(s) are responding excessively to any slight throttle movements. What carbs are on this?
Hi Eldon – thank you for your reply Whatever the problem is it appears to be common to all four cylinders. I currently have the airbox off (thanks Triumph). In the morning I’m off work and will check the float level height. I did all four last weekend so this is one thing that is common to all four cylinders – not sure how float bowls could stop the throttle working though... Carbs are Mikuni. Bike ran fine 2yrs ago and has only 13,000 miles
This will be a time problem. Did you drain the carbs last thing before lay up. If not you have a/ 4 blocked main jets and if so your whole carb will need cleaned. To test for air leak, start bike, spray brake cleaner/ carb cleaner all around carbs/ air box. Any fluctuation in revs, is a sign of an air leak.
Hi Sprinter I ran the bike till it stopped with the fuel tap off. Been in the garage today. The correct fuel height in the bowl is 1.5mm above the top of the bowl. Only had time to measure one – it’s at 11mm above. Crazy high so could be a clue.
Hi all Quick update (been super busy) The float bowl heights are all now spot on using a clear plastic tube method. I was so so so sure that this was the issue... Nope still the same. To sum up – 13,000 mile Trophy 1200, completely standard, which ran fine 2 years ago. Mikuni carbs. Sticking float valve so changed all four needles. Bike now starts and runs fine on choke (air box etc is off) but as soon as it is warm and I apply even the smallest amount of throttle it’s like I’ve hit the kill switch. Stops instantly and I mean instantly. Pulling my hair out. I have gone over and over my efforts changing the needle valves and I am sure that I have not cocked up. I am out of ideas and talent. Any thoughts or ideas or where to take it (I’m in the Midlands) Kev
To test for air leak, start bike, spray brake cleaner/ carb cleaner all around carbs/ air box. Any fluctuation in revs, is a sign of an air leak. Too much air is as bad as to much fuel. You need to know which it is. Is your choke an air strangulation or an add more fuel? Put your hand over the intake when running, what happens?
Hi, you haven't mentioned the pilot jets explicitly. Have you checked and cleaned them and the passageways to the pilot jet mixture screw? You probably know, but I'll mention it, as I understand the pilot jet supplies most of the fuel up to approx 3,000rpm, after which the main jet (needle jet) takes over the heavy lifting (simplified, they do overlap considerably). So coming off idle the main jet won't be involved, only the pilot. When you open up the butterfly valve more air is going to be sucked in by the engine and if the pilot jets, passageways or mixture screw is blocked then the engine will most likely stall. Apologies if I'm teaching my granny to suck eggs. My trophy 1200 doesn't like sitting around for a few months without use. It's the carbs that always suffer. Of course all the above comments about air leaks are also appropriate. Plus if you have the airbox off the pilot circuit is going to be running lean to start with unless you've adjusted the mixture screws. I like this guys down to earth approach and explanations, "kev's shed" on youtube he's done some work on a sprint 900 with mikuni carbs, although the poor guy only has 3 carbs..
Hi Whattie Thanks for your reply. I did have a look at one of the main jets when doing the needle valves, seemed fine. It must be something common to all four carbs. I’ll watch the vid now and get back in the garage. I’ll report back either way Thanks again to all for your time
Quick update. All four emulsion tubes out and cleaned but to be honest they were clear – looked like new really. Put it back together and got ready to start her up when fuel began appearing from 2 carbs - from points A and B shown on the attached. Turned off the fuel supply and sparked up a fag to help me think.... This is my latest theory which ties up with the fact that whatever the problem is it is affecting all four carbs. The tank is off and I’m supplying the bike with fuel from an old plastic mower petrol tank. This is suspended out of the way just below my garage trusses – say 8 feet high. With the fuel falling so far is gravity forcing the fuel passed the needle valves, over filling all four bowls? With over full bowls the bike will work on the choke but just cuts out with any revs?
Quick way to check if the fuel head pressure is overcoming the float valves; Using a piece of string fasten your temporary fuel tank at a more suitable height. When I was balancing some carbs last week, my service fuel tank was suspended from a truss screw at around mirror height and all worked well including the petrol tap being within reach.
An update. I am now out of talent and ideas. Got the carbs off (getting good at this at least) and back on the bench. The float bowls fill to the correct height and no fuel emerges from points a and b. I know that when I fit them back on I will still have the original prob. Need to get an adult involved as I have given up. Any recommendations for where to send my carbs?
I am wondering if the problem is fuel at all. You say the problem affects all 4 cylinders. Each cylinder has it's own carb. Could be ignition, but if it is, I would guess it to be the primary side of the ignition [12v]. That is because all cylinders share that part of the ignition. Happy hunting. ...J.D.
Just a thought after rereading your thread and it's a while since I've played with vacuum carbs with problems like this. Is the tap on the proper tank in the PRIME position? In effect you supplying fuel from another receptacle would give the same condition, doesn't cv carbs require a vacuum tap feed of fuel i.e. no vacuum no fuel flow? As I recall, probably 30 years ago, there's a vacuum hose to the petrol tap to facilitate the above; is that how your tank connections are?
My experience with Mikuni carbs was with my 95' Sprint 900. One thing I remember is on Mikuni's the needle height has an effect on the transition from idle to main jet. This can be adjusted by moving the needle up a notch or two for richer, down for leaner. Many years ago, after rejetting mine for a different exhaust, I had to set mine UP to keep it from stumbling when I opened the throttle. Might check and see where they are positioned. ...J.D.
