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post #1 of 9 (permalink) Old 05-19-2012, 07:42 PM Thread Starter
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Thruxton engine quits after warming up

I'm having a problem with my 2005 thruxton (carbureted). The engine cuts out after 5-10 minutes (2.5 miles in city traffic) every time I start it. It stops abruptly as if I hit the engine kill switch. Headlight, oil light, neutral light and starter work fine. Bike cranks but doesn't fire.

I did some research on the forum and tried the following fixes with no success: checked crank position pick up (gap measured 0.8mm), replaced ignition coil with brand new one, changed spark plugs. I let it idle in neutral on the side stand and it cut out after 5-10 minutes, so I don't think it could be the side stand switch.

Some history on the bike: I got it with about 9000 miles on it 2 years ago. I put about 5000 miles on it. I have the snorkel and baffle removed, StainTune silencers. I recently had the air injection removed. Also, I recently started running a home made heated jacket and pants, of about 90 watts. The spark plugs that I removed are black and dirty.

I'd rather not take it to the dealer to fix it, but I'm out of ideas. Does anyone here have any suggestions as to what I can try next?

thanks.
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post #2 of 9 (permalink) Old 05-19-2012, 08:36 PM
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A couple of things I would check are the fuel tank breather hose is clear, athough that would probably cause missing prior to it cutting out completely and give the ignitor box harness under the seat a wiggle while shes running. Could also check all the fuses and battery terminals have good connections as well as the connectors in the headlight housing. Intermittent faults are allways the hardest to find so good luck.

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post #3 of 9 (permalink) Old 05-20-2012, 02:44 AM
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I'm not a motorhead but just a suggestion, you may be running rich at idle and or pilot jet.

Does the bike start easily when stone cold? If so, then try this as an experiment;

Fire it up so it idles and let it sit for a couple of minutes idling. If after it gets warm, it starts to bog out and dies, then you are maybe too big on your pilot jet or too rich on the fuel screw, or both.

As the motor warms up, it needs less fuel than it does when cold. Your black plugs may be another indication of a rich mixture as well.

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post #4 of 9 (permalink) Old 05-20-2012, 03:07 AM
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Sounds to me like everything else checks out, except...

Have you read Forchetto's post on checking the pick-up coil? The air gap you specified (0.8mm) is good, but did you measure the pick-up coil with a multimeter? I've had exactly the same symptoms on my '05 Thruxton - my '01 Bonneville riding mate's pick-up coil disintegrated in-situ!

In my case even though the coil tested as "ok", I replaced it and all was good. I suspect that with age and heat, the magnet in the pick-up coil starts to weaken, so I suppose you could reset the air gap to 0.7mm and see if the situation improves. If it does, your pick-up coil is dying.

I doubt very much if the electrical accessories are the problem, but you can eliminate them as the source by not using them for a couple of days

Your plugs might be black because of a rich mixture, but they won't tell you a lot for as long as you have that misfire.

~'05 Thruxton~Sport Evolution 1100cc~FCR 39mm~813 cams~shim-under-bucket~+1.5mm valves~45mm headers~Ignitech CDI~m-Lock~m-Unit~Acewell CA85~F3 Frankenfork~Hagon Nitros~Pretech caliper~520 chain~wide 17" rims~sticky rubber~

Last edited by H3CT1C; 05-20-2012 at 05:47 AM. Reason: link and a couple of other ideas added
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post #5 of 9 (permalink) Old 05-20-2012, 05:26 AM
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Sounds like you have a set up like i used to run. Mine is 865 bonnie. Same pipes, baffle, snorkel etc. I did open up the entry to the air box, which does give a huge amount of extra area.
What jets are you running? Because black plugs mean extra fuel or lack of air.
Does it start real easy with no choke?
You need to go through things in a systematic manner. Are the plugs new? Gapped right? Air filter clean no mice rags etc or too much oil?
Even things like a sudden onset can be attributed to crap fuel, water contaminated. You can drain the bowls and let the fluid! stand if it seperates you have an issue.
Also what do you have your tickover set at. When you are off the choke she needs to be at at least 1050 or they can suddenly spit the dummy.
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post #6 of 9 (permalink) Old 05-20-2012, 05:51 PM Thread Starter
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update

Thanks for all of the suggestions. I think I found the problem. It seems to be a failing pick up coil. I measured the resistance before starting the engine at 650 ohms, then started the bike and let it idle. It ran at about 1000rpm for 5-6 minutes then shut off. I quickly remeasured the pick up coil resistance and found it to be infinite (open).

I'll order a new pick up coil and update again once I've installed it.

Next, I'll have to look into jetting and determine why the plugs were black. I bought the bike used and I haven't messed with jetting yet, so I don't know what mains, shims, turns etc I currently have.

Thanks to H3CT1C for pointing it out and Forchetto for writing instructions on how to test the pick up coil.
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post #7 of 9 (permalink) Old 05-24-2012, 01:55 AM
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No worries - glad to be of assistance. That's what this forum is here for!

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post #8 of 9 (permalink) Old 05-24-2012, 08:52 AM
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For what it is worth my 96 Scrambler is on its third pickup coil in 12,000 miles.....
post #9 of 9 (permalink) Old 05-29-2012, 10:48 AM Thread Starter
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solved

I finally got it fixed this weekend. The pick up coil (crank position sensor) failed and would only work when cold, once it heated up it would stop sending a signal and the spark plugs wouldn't fire.

a few observations:
1. The pick up coil connector is buried in a frame pocket below the tank bolts. I pried on the battery/air box right side tab to try to free the cable and accidentally broke the tab clean off. This is good and bad because it allows me to get to the connector, but may eventually destroy the battery/air box.

2. I was able to replace the sensor without draining the oil. I moved the bike so that it was leaning on the stand with the crown of the road. This was enough to keep the oil level below the engine case.

thanks again for the help
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