Bonneville America Forums Forums Technical Forums General Tech Rough cold idle

So...I've been having a problem with my Thruxton (figured I'd ask here since it's essentially the same as the BA) for the past few months and have finally decided to give it some attention.

OK...when I start 'er up with a completely cold motor and choke full out, it'll idle relatively smooth at around 1300 rpm, then after about a minute or 2, drop down to 700 and stall out. It's becoming a bit of a pain and I'd rather not just run the bike with a cold motor.

Now I don't think it's a low idle issue because when it's at running temp, it's set at 1100 rpm and starts and idles perfectly. If it makes any difference, I'm running Epco pipes, 130 mains, and stock pilots (40's).

At first I was thinking it may be as simple as raising the pilots up a size, but seeing as it runs & idles like a champ warmed up, I'm not sure about it.

I'd try tossing a set of plugs in. Could be they're rich fouled from the choke. This is one draw back of carburation. If this turns out to be the case, it could be running to rich, but I wouldn't think to much about it. I've seen plugs foul and underperform many times due to excessive choke time.
Good luck,Paul

I'm having a similar problem with my Bonnie which I attributed to the hidden filter at the carb junction. Seems the cold weather caused the impurities to jell restricting the fuel flow, I have a Wix filter inline now and when it gets cold again I'll know.

Hey Mark,

This sounds counter intuitive, and I sure can't disagree with anything anyone said. I also had "cold start rough idle", using the choke full out. It always started fine, but took time to level out. On a whim and a Triumph, I started starting without touching the choke. Now it fires up smooth and idles smooth. Of course, this noon, after a 30dF night that theory might fall apart.

standard disclaimer: "Your results may vary"

jh

It does the same without the choke...only about 300-400 rpm lower

Mark,

what do you have for intake , stock airbox or no box and K&N.
it sounds like I want to agree with chopperpaul.
it may be to rich.
though with the stock pilots I would think not.

start the bike cold with the choke pulled ,
just let it idle , do not touch the throttle.
if it dies , it dies.
then check the plugs.
this will let you know if it is a rich condition.
if not , I do not know what to think without more
up close and personal tinkering.

sounds too rich to me. ever try to fire up @ full choke & then back the choke off a little?

Along the same thoughts as everyone else, try using less than full choke. I start my TA with the choke only half way out. Just enough to get it started and running smoothlier.

As far as intake, I'm running a stock airbox with a K&N drop in filter.

If I start it with half choke, it'll run for maybe 30 seconds before it dies. With no choke, I'll run for a few seconds before cutting out. Starting with full choke then backing it off will kill the engine too.

When I get home today, I'll let it sit for a few hours to get the engine cold and then start it and check the plugs.

I checked my plugs and they're weren't fouled from the same symptoms, that's what lead me to the filter to begin with. Have you tried letting the carb warmers work for a minute or so before turning the bike over?

I had a very wise man tell me that the carb heaters only work while the bike is running.