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Discussion Starter #1
Just started doing it over the weekend, I first thought maybe I had some bad gas or something but I filled up with fresh 90 ethanol free and doesn’t seem to matter. Happens in all gears right around 3500 rpm, almost feels like it’s out of gas or missing a cylinder. Could it be something to do with the carburetor?

Any help is greatly appreciated!
 

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It could be a partially blocked filter in the fuel tap or carb, does it spit back through the carbs (weak mixture) or if it bogs down it could well be flooding due to the carb float needle valve being contaminated.
 

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Just take the pipe of the carb and see how much fuel drops from the tank.Then,if this is OK,check the filters in the carb unions.You will,have checked the vent in the fuel cap of course.
My opinion is the carb union filters blocked.
 

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Hi Terry,

"carb unions" Is this something to do with dual carbs as this bike has a single carb?
I suspect "rambo" means the banjo where the fuel pipe(s) attach to a carb. Irrespective of the number of carbs., all Amals have a filter in the banjo, that also forms the seal between the banjo and the float bowl.

Hth.

Regards,
 

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Perhaps the problem is with the needle or needle jet or both.
 

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Hi Nate,

Happens in all gears right around 3500 rpm, almost feels like it’s out of gas or missing a cylinder.
Are you able to rev. through the miss; i.e. can you get it to, say, 4,000 or 4,500 rpm? If you can, unlikely to be a filter that blocks at 3,500 rpm and clears at higher?

Have you checked:-

. If the plugs have the screw-on HT terminals, are they both tight?

. Each plug cap (if NGK-type) or HT lead terminal is tight on the plug? If plug caps are NGK-type, are they both tight on their HT leads?

. Is each HT lead tight in the coil? If standard Lucas/pattern single-ended coils, to check this, you must pull the leads out of the coils, push each 'boot' up the lead a little way, push each plug lead terminal into the coil 'til you feel a slight click, then push the boot back over the coil HT lead terminal.

. Again if Lucas/pattern coils, are the screws holding the male spade terminals tight? Are the female blades tight on the male blades? Have you given each wire a tug to ensure no loose crimps?

Have you tried a new pair of spark plugs?

Hth.

Regards,
 

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Sounds like the main jet is loose or has dropped out.
Either that or the jet block it'self has come loose.
 

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Discussion Starter #9
I cannot work through the miss. I can rev it up to a little above 4000 but it’s still sputtering and missing. I checked the plugs and wires but will re-check. I’ll also check the main jet to see if there’s any blockage or loose connection.
 

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I cannot work through the miss. I can rev it up to a little above 4000 but it’s still sputtering and missing. I checked the plugs and wires but will re-check. I’ll also check the main jet to see if there’s any blockage or loose connection.
Eliminate all fuel possibilities. R&R the jets, the needle, the banjo and see how clean the carb's interior. Just rebuild the whole carb and you've eliminated one thing.
 

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I am with OCR on this one..........Check your main jet and/or the jet holder have not unwound and dropped into the float bowl. If you remove the float bowl drain, feel up through the hole and see if anything is loose. I had this happen on one carb on my T140, exactly the symptoms that you have described. Another thought, have you changed your muffler? An overly restrictive muffler or exhaust will not let the engine rev out properly.
All the best,
Andy
 

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Hi Nate,
Reading through your original text, and follow up text.
If the problem occurs at a set rpm, but is not dependent on load, I would be suspicious of the low tension side of the ignition first, especially that the coil connections are tight, I have had similar symptoms on lucar connections to the coil, I now cut them off and replace with ring teminals screwed down tight as a matter of course. By pass the ignition switch/kill switch with a direct to battery feed wire to power the ignition , this will eliminate these items and the loom from your investigation. Check the return ground wires to the battery, especially from the engine.

If a heavier load on the infuences the problem, I would suspect the high tension circuit, bad lead, bad plug and importantly bad coil shorting internally under vibration. Seeing as this is a 'rpm' influenced fault static testing might not show up any faults.
If you have electronic ignition check the pick up coils are not loose, backing plate not cracked, wiring connections are good. Ohms Test the pickup coils to the manufacturers spec, with them disconnected from the electronic ignition unit. Check that the ohms readings do not jump around, if the pickup is GENTLY wiggled or tapped.

If you have points, check the faces are clean and the gap is correct, do not mess about with the condensors just replace anyway, it is just good practice to replace them every few years anyway and super cheap - condensors could cause the symptoms that you describe except I would expect poor running throughout the entire rev range.

With a speed/vibration fault I would be mighty suspicious of Lucas/pvl style coils, Insulating the metal case in some cases effect a cure, but substitution is the only way to be sure.


FUEL if the problem is always at the same throttle poition regardles of load, then carburettor might be suspect.
With the main jet fallen out, I would expect excess fuel throughout the entire range, not just over 3500 rpm, and perhaps clearing a little at high rpm.
With the choke (fuel enrichment, sorry) stuck on, I would also expect problems at low rpm, you have not reported this.
Float leaking again I would expect rich mixture throughout the range, I am trying to think of a fault that occurs suddenly to cause a low fuel level but cannot imagine one.
The metering needle coming out of it's clip might give these symptoms, but would be more throttle position dependent than rpm dependent (sometimes it is difficult to distinguish the two).

If I was given one chance to guess the fault on this, I would opt for lucas/pvl type ignition coil (especially the lucar spade connection being loose). But this is a guess only.

There are lots of areas in the carb and ignition that can quickly and easily be eliminated from your investigation.
Work through it methodically and you will find the fault.
Regards
Peg.
 

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Have you checked your battery?

My bike once had similar symptom. It would sputter and bog around 3500 rpm. Turned out to be a bad battery cell in my case.
 

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I had similar sounding problems, my engine bogged at 3500 but if I dropped the clutch I
could get passed it and the engine ran cleanly to 7K.

Eventually I discovered a badly worn/lose ignition switch which was vibrating at 3500rpm,
if I put a rubber band on the key and pulled it to one side the problem disappeared.

Your problem is unlikely to be this, but it shows how you have to look everywhere.

-Steve
 
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