Engine misfire in cold temps? : V-Twin Forum: Harley Davidson Forums

cyclenut80 · 09-19-2009, 05:44 PM

I have an 89 FXR with the stock 80" motor. The only mods it has are an S&S carb, and Bub pipes. The bike has run very well over the season (bought it late spring of this year), but now I have an issue. This occured in temps of mid 40's. The bike starts perfectly fine and idles well with the enrichment lever used on the carb. I let it warm up for a couple minutes and the bike seems to take a little snap of the throttle with no problems. Now comes the part I don't understand. You'll start to ride the bike away and suddenly its trying to drop a cylinder and backfiring through the exhaust. This occured several times even after the bike was ridden for a solid thrity miles and then left to sit for about half an hour, maybe 45min. If I pulled the enrichment lever it seemed to help but until the bike ran for at least 10 minutes it wouldn't run right. In my mind on a bike that has been ridden recently you shouldn't need a ten minute warm up. After this period of coughing, spitting, and riding while trying to modulate the enrichment lever for about ten mintues the bike suddenly clears up and runs perfectly. Until, that is you stop, and let it cool off for a little bit. Then the whole cycle started again. It only did this when it was cold out. I started the bike and rode it yesterday and it seemed fine in the warm temps. Any ideas? Maybe intake seals, water in the fuel, ignition? Thanks for the help guys!

myglide · 10-05-2009, 10:30 PM

ignition coil popped into my head. Both cylinders act up? You might be right about an intake leak. Try spraying WD40, carb cleaner around the intake manifold and see if you idle changes.

LowRiderCustom · 10-06-2009, 03:58 PM

You need a bigger main jet, when the air is colder it's more dense, so the bike runs leaner (too much air & not enough gas). That's why pulling the enrichment lever makes it run better.

myglide · 10-06-2009, 10:22 PM

when it is running good and you are in 5th at 60mph give it full throttle. If your bike stutters like it is running out of gas your main is too small.

wdgld · 10-07-2009, 10:55 PM

Definitely sounds like it is running a touch on the lean side.

As previously suggested, go up a size on the carb jet.

Or if it is only happening on acceleration, tweak the accellerator pump a little to pass a little more gas.

Regardless, I would do a plug chop for sure.

Waffen · 10-08-2009, 12:56 AM

Quote:

After this period of coughing, spitting, and riding while trying to modulate the enrichment lever for about ten mintues the bike suddenly clears up and runs perfectly

If it were a jet issue I don't see it clearing up. JMO

I might suggest a float problem. It may be sticking or out of adjustment.

W

LowRiderCustom · 10-08-2009, 02:11 AM

If the float was out of adjustment, the bike wouldn't run well in warm weather either. When the air is cold the motor runs leaner, and pulling the enrichment lever richens it up, so going bigger on the main jet will accomplish the same thing, just on a smaller scale.

Personally I don't like the S&S carb, they are difficult to tune. The Mikuni is a much better carb, and I feel it's the only way to go on a Harley. My brother had an S&S and he always had tuning problems, then he bought a Mikuni and his bike runs perfect now. My bike has a CV carb and I'm having tuning issues with mine too, so I'm springing for a Mikuni over the winter.

Waffen · 10-08-2009, 11:20 AM

Quote:

Originally Posted by LowRiderCustom

If the float was out of adjustment, the bike wouldn't run well in warm weather either. When the air is cold the motor runs leaner, and pulling the enrichment lever richens it up, so going bigger on the main jet will accomplish the same thing, just on a smaller scale.

Personally I don't like the S&S carb, they are difficult to tune. The Mikuni is a much better carb, and I feel it's the only way to go on a Harley. My brother had an S&S and he always had tuning problems, then he bought a Mikuni and his bike runs perfect now. My bike has a CV carb and I'm having tuning issues with mine too, so I'm springing for a Mikuni over the winter.

I agree in theory, but he said "the bike suddenly clears up and runs perfectly". If it were a jet (to small) I would think it would continue to run rough.

Another thought is a dirty jet??, possibly a VOS vaccum problem, but I do agree it seems to be a lack of fuel issue.

W

myglide · 10-08-2009, 11:01 PM

As been said you do have a lean condition. Intake leak probably wouldn't clear up. It would be there when it is warm too. Float possible. Main jet possible. Main jet is mostly used at wide open throttle. That is why you should do the wide open throttle test. You have to hold it wide open long enough to burn off the gas the accel. pump puts in to get the true main jet gas flow. If it stutters let off the gas about an 1/8 to 1/4 turn. If it stops stuttering and catches up your main is too lean. Go bigger. A float that is sticking or not adjusted right can give you similar symptoms. Starves gas. Holding a steady throttle how does it act. Could be intermidiate/slow jet is too small. Cold does lean out your mixture. Try a bottle of seafoam in your gas to clean things up if there is a little blockage. A crack in your coil or bad wire wouldn't clear up with enricher but it might with temp. I have an S&S super E on my evo. Now that it is getting cold it doesn't run the same. A Mikuni CV carb is a good carb. Had one on my other bike. You can make the S&S livable. Getting a new carb isn't necessary.