Carbureted Thruxton Stutters/Misfires When Warm - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 29 (permalink) Old 09-25-2018, 01:32 PM Thread Starter
Minitwins
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Carbureted Thruxton Stutters/Misfires When Warm

Hello All,

Sorry in advance for the long post, but trying to provide all information I could. I have a friend with a Triumph Thruxton that Iím working on, and itís having some issues once it warms up. Iíll start with the back story, explain the work I did, then explain the current status, which is better but not by much.

Iím a little hazy on the ďbeforeĒ details, but this is my general understanding: The bike is ridden lightly and does not have many miles (maybe 12,000??). The owner moved from elevation of ~1,000ft to 5,000ft but rode the bike without issues for ~1 season at 5,000ft. I believe the bike had some sort of running issues at one point and it was taken to the Triumph dealer where they gave it a full tune up, and if I recall correctly, rebuilt carbs and did the valve adjustment. (I have trust issues with most mechanics and all dealers, so Iím doubting they did any work at all). At any rate the owner says the bike rode great for the remainder of the season (a few rides) and this spring when it was taken out, the bike warmed up and would drive great for about 20 mins before missing and sputtering to a stop. The first time it happened he took it to the same dealer who told him it was just out of gas (insert additional trust issues). Obviously the gas issue was not the problem, and the bike would sputter and die repeatably after 15-25 mins of a cold start. At this point my friend called me to get my opinion and help him out.

The first thing I did was clean the carbs (first thing I always do on a carbed bike with unknown history), but the symptoms remained the same, and I was skeptical since it ran so well before it completely warmed up. Next I pointed my finger at the coil since Iíve has similar issues before, and replaced it with a Nology coil. No change so I turned to the internet for research, which told me that a common issue with these bikes is the pickup coil. First I checked resistance and set the gap with no success. I changed out the pick-up coil and once again had no success. We figured the last resort was to replace the CDI/igniter as research showed that being a potential problem as well, so we purchased a ProCom computer and once again had no success.

At this point I was a little stumped, so I pulled the carbs off again to check the settings. Float height was fine, the jets were the same on both carbs (and it ran great before so Iím not going to re-jet yet). I did notice that one fuel/air mixture screw was all the way out, and one was just ľ turn out. I set them both to be ~1.5 turns out. Re-assembled and while the bike seemed responsive of the change, it still has a horrible misfire and tries to die after about 20 mins. It typically doesnít completely die like before, but I suspect this is just due to the carbs being in better shape now.

At this point I feel like the valves could be out of adjustment, but Iíve never experienced a bike that runs great when cold, and horrible when warm and had it be the valve. Plus they were allegedly done at the service less than 1,000 miles ago. My other thought is that the new pick-up coil is bad.

Suggestions?

-Alex
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post #2 of 29 (permalink) Old 09-25-2018, 03:36 PM
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I woulda said pick-up coil, but you've already been there. What did you set to be? It was .08, but Triumph then changed it to 1.0. There's lots of threads here on pick-up coil issues and the measurements, certainly sounds like 'electrical.'

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post #3 of 29 (permalink) Old 09-25-2018, 03:41 PM Thread Starter
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Hmm, I was under the impression it was the other way around (see here). If anything, it maybe be a hair tighter than 0.8.
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post #4 of 29 (permalink) Old 09-25-2018, 03:53 PM
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Originally Posted by nnamssorxela View Post
the bike warmed up and would drive great for about 20 mins before missing and sputtering to a stop.
OK, this is just a WAG...

Maybe the fuel line/filter is partially clogged? It's open enough to slowly fill the bowls if you park it for a while. Once it starts running it's drawing gas from the bowls faster than they are filling, and it just runs out of gas after a while? Let it sit for a while, and the bowls gradually fill to the point where it will run for another 15 minutes?

If you run it till it dies, shut off the petcock, and open the carburetor drain screws do you get a good bowlful of gas out of each one?

Grant
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post #5 of 29 (permalink) Old 09-25-2018, 04:39 PM
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Quote:
Originally Posted by nnamssorxela View Post
Hmm, I was under the impression it was the other way around (see here). If anything, it maybe be a hair tighter than 0.8.
You're absolutely correct, didn't feel right so I went back to check - Should be .8 from somewhere near 1.0 originally.

Here's Forchetto's thread on the subject: https://www.triumphrat.net/air-coole...nd-boring.html

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post #6 of 29 (permalink) Old 09-25-2018, 05:02 PM Thread Starter
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Good suggestions. I should have mentioned that I checked fuel flow and found it to be more than sufficient, and I drained all of the gas and filled with fresh. I also sprayed around for vacuum leaks and found none, and I replaced the vacuum caps. I will sync the carbs soon as well, in case that could be causing issues, but have not done that yet.
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post #7 of 29 (permalink) Old 09-26-2018, 01:27 AM
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Quote:
Originally Posted by nnamssorxela View Post
Good suggestions. I should have mentioned that I checked fuel flow and found it to be more than sufficient, and I drained all of the gas and filled with fresh. I also sprayed around for vacuum leaks and found none, and I replaced the vacuum caps. I will sync the carbs soon as well, in case that could be causing issues, but have not done that yet.
Regarding the pickup coil, I am aware that you checked the resistance but did you check both hot and cold? Try heating it up with a hot air paint stripper or leaving it in a jug of boiling water for a couple of minutes then take another reading. It should be around 600 ohms cold or hot. If the resistance rises significantly when hot then that's a sure sign of a bad coil. Air gap should be 0.8mm. On the fuel starvation side of things, check the tank vent tube has no kinks in it - there have been lots of posts on here with the same symptoms which have been solved by finding a kink or blockage in the vent tube. The tank cap is sealed on these bikes and the vent tube is the only way for air to get in as the fuel level goes down. It should be terminated at the front of the airbox with the rollover valve. Check that the ball in that moves freely too.
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post #8 of 29 (permalink) Old 09-26-2018, 09:52 AM
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I believe the vent tube for the fuel tank is on the under side of the fuel tank on the right hand side. Where the tank goes over the frame. It then runs back towards the air box area and down through to exit under the bike. The ROV [roll over valve] is inline mid-way. Many just eliminate the ROV to avoid issues.
One other thing is to verify the vent tube coming of the "T" of the carb balance pipe are clear. That tube routes over to the left side exit behind the left foot peg area. That vent tube can cause similar conditions and drive you crazy trouble shooting.

Good luck.

ďIts all about motorbikes. Always has been, always will beĒ

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post #9 of 29 (permalink) Old 09-26-2018, 01:22 PM Thread Starter
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I checked the resistance when hot with the old pick-up coil and I recall it as being in spec, but replaced it anyways. You are reminding my of all the details I left out: I did open the tank while the bike was running bad to see if it was a vacuum issue with no success. I also removed the ROV and blew through the line which was clear. I want to say I checked the carb lines too, but I will definitely check that "T" line. I plan on working on the bike this weekend and here are my plans:

-Check valve clearance and adjust as needed
-Check all vent tubes for blockages
-Check resistance of pick-up coil when cold
-Start and run bike to see if it is miraculously fixed
-Once bike is warm, sync carbs

Anything else? Tempted to check the float height again. Are there any special tricks to this or additions to the manual?
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post #10 of 29 (permalink) Old 09-26-2018, 07:02 PM
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Quote:
Originally Posted by nnamssorxela View Post
I checked the resistance when hot with the old pick-up coil and I recall it as being in spec, but replaced it anyways. You are reminding my of all the details I left out: I did open the tank while the bike was running bad to see if it was a vacuum issue with no success. I also removed the ROV and blew through the line which was clear. I want to say I checked the carb lines too, but I will definitely check that "T" line. I plan on working on the bike this weekend and here are my plans:

-Check valve clearance and adjust as needed
-Check all vent tubes for blockages
-Check resistance of pick-up coil when cold
-Start and run bike to see if it is miraculously fixed
-Once bike is warm, sync carbs

Anything else
? Tempted to check the float height again. Are there any special tricks to this or additions to the manual?
Anything else???

Why....Take it for a but of a fang.
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