Hi everyone. I've been lurking for a while taking in a lot of information as I recently acquired a pretty rad 2003 LS650 Bobber build.

I have no history on the motor, total milage or what is being run in the carb currently. I can tell you it's a stock carb(jetting was modified) with straight pipe and KN pod filter.

I've put a thousand or so miles on it in numerous temperatures and for the most part, the bike runs fantastic except for three seemingly mysterious situations far apart from each other. Here's some details for background on how it's running.

-The previous owner had no details on the bike for me but did say it could use a new spark plug, battery, tune up etc... which I did before even firing it up at all...The only thing that stood out was I ordered a standard OEM plug and the plug that came out of the bike was a colder one. I've been running the stock recommended plug and pitched the colder one.

-The bike starts easily each time in different temps.
-Surges a bit when cold, but doesn't hesitate or bog at all when at operating temp.
-Idle seems consistent and doesn't stall easily even when cold.
-backfiring is present on decelerations and in between some shifts.

-Has a sportster tank with Sporty petcock so I know that's not the issue.

Here are the three scenarios...They happened far apart from each other and overall engine performance has not changed at all out side of these moments.

First scenario:
Bike was roughly 10 miles into my commute home. Got to an open road and decided to see what she had...in 4th gear giving er pretty good was just about to shift to 5th when bike had an immediate power loss. It didn't shut off completely but giving throttle made no impact. I downshifted and after a few seconds of engine breaking in 3rd gear giving random twists, the throttle came back to life and the bike ran flawless again.

Second scenario:
Similar time into riding as scenario 1...Same 4th gear throttle maybe giving it more than it wants to have. Same power shut down happens right as I was about to shift up to 5th, the engine didn't turn off, again the throttle just became inactive. I did the same thing as the first time, down shifted, used the engine braking and gave it a couple twists of the throttle. This time when the throttle came back it was with a mammoth backfire! Loudest one I'd ever heard. Ran great after that again for a few more weeks.

Third time was different.
This time it was right after a tedious slow speed 10 min traffic crawl. I finally turned onto a road that wasn't going 20mph and just throttling up to normal speed early in third gear it did the same mysterious power shut down...Only this time, the throttle did not come back. The bike came to a complete stop and finally stalled on the side of the road. However it started up very easily after I turned everything off and back on again. Rode perfect the rest of the way home.

I feel like it's a fuelling issue but before I go down the jetting rabbit hole...I wouldn't mind some advice on what else could be causing this since it runs so well side from these freak random instances.

Thank you for any advice. Sorry for the long post! Didn't want to leave out any details.

Does the bike still have the stock vacuum operated petcock?

Dave wrote on 09/15/21 at 03:10:23:



No, Bike has Sportster tank with an HD petcock.

The OEM petcock on HD sportsters has been vacuum operated since around '95 up until '06 when they became fuel injected. They don't seem to cause problems as often as Savage petcocks do but folks still modify them to remove the vacuum feature. The HD valves don't have a prime position like the Suzuki's do, so it makes it inconvenient to drain the tank. Need a vacuum pump like a Mighty-Vac or similar to open the valve without engine vacuum.

If your tank has an OEM HD petcock and no vacuum line running between it and the carb, your petcock may have been tinkered with.



https://www.hdforums.com/forum/carburetor-related/1272430-vacuum-bypass-the-p...


http://xlforum.net/forums/showthread.php?t=524246

Sorry for my poor description of the petcock.
The petcock on it is probably not Genuine HD. But I can confirm it is not vacuum operated. Just a standard on/off/reserve.

verslagen1 wrote on 09/15/21 at 08:36:31:


Because the problem is so intermittent. I wouldn't imagine that taking off the filter would tell me anything since it starts and runs just fine pretty much all the time.

In between these issues I did do a complete new bar swap where I would have address and issues with the handlebar switches.
The only switch I havn't looked at yet is the sidestand safety switch. I'll check that next too.

Sorry All! My reading comprehension has failed me.

I thought you were referring to the air filter!

I assumed that because the bike does not have a fuel filter on it. Nothing outside of the tank anyhow. Fuel line goes direct from petcock to carb.

Thanks for all of your insight so far!

Are these problems happening when you are low on fuel?