performance decrease when warm

Nikolas · April 12, 2009, 12:20:02 PM

When cold this thing runs like a raped ape, accelerates smoothly and climbs through the revs, can maintain freeway speeds at partial throttle. Once it warms up it has trouble maintaining 70mph, and even on surface streets has to be run at WOT just to avoid getting run over by lead footed SUV drivers; my Vespa is seriously faster. I mean its rideable, and if I hadn't experienced riding it when cold I'd probably just think that was normal for such a small bike.

I thought it was all in my head until yesterday, when it happened while I was riding on the freeway; going with the flow of traffic in socal cruising in high gear, then all of a sudden started slowing down and had to downshift to 4th just to do 70mph.

The bike has just been reassembled after paint. Carbs have been cleaned and rejetted (125/40 w/ #4 washer), tank boiled and lined; since I was having fuel issues before I took it apart. Valves are in spec. I'm thinking either an air leak (there is some decell popping) or some kind of compression issue as parts heat up and expand. Any thoughts?

I'm also getting really terrible gas mileage, ~30mph. I'm thinking that's related to having to be at WOT all the time.

jrains89 · April 12, 2009, 01:14:42 PM

i think you're running too rich. the warmer it is, my bike seems to be getting faster.

fred · April 12, 2009, 01:32:05 PM

Quote from: jrains89 on April 12, 2009, 01:14:42 PM
i think you're running too rich. the warmer it is, my bike seems to be getting faster.

Warmer weather should actually decrease your engine performance... Warm air is less dense and if you're putting less air into the engine, you'll make less power.

What color are your plugs? Are they showing obvious signs of trouble? The valve clearances are what will change the most when the bike gets warm, so it might be worth double checking them to make sure you don't have any problems there... Also might be useful to see if the bike is still running on both cylinders when it is warm. Just ride it around until it starts to have problems then pull a spark plug wire off. If the bike dies, you've got problems, if not, try the other plug wire. I've heard talk of coils failing in a way that causes them to work when cool then not function when warm, but that's just what I've heard, I don't have any actual experience with that particular problem. Plug color, double checking valve clearance and testing to make sure you're running on both cylinders is where I'd start...

Nikolas · April 12, 2009, 02:04:23 PM

left plug is leaner than right but they're both coffee brown.

fred · April 12, 2009, 02:22:41 PM

Quote from: Nikolas on April 12, 2009, 02:04:23 PM
left plug is leaner than right but they're both coffee brown.

Well, double check the valves and then make sure you're actually running on both cylinders when the problem occurs. It also might be handy to do a compression test on the bike. If possible, do it cold and warm when you're having problems to see if something weird happens like a loss of compression when the engine gets warm...

The Buddha · April 12, 2009, 05:33:33 PM

Air leak will run bad when warm and much much worse when cold.
I think you have a slightly high float. Good when cold but soft and under powered when warm = rich. 125/40 is right for stock, so my guess is that your floats have gone up ... or you have a needle jammed in the up position ...
Cool.
Buddha.

dwkfym · April 12, 2009, 07:16:31 PM

Quote from: fred on April 12, 2009, 01:32:05 PM
Quote from: jrains89 on April 12, 2009, 01:14:42 PM
i think you're running too rich. the warmer it is, my bike seems to be getting faster.

Warmer weather should actually decrease your engine performance... Warm air is less dense and if you're putting less air into the engine, you'll make less power.

What color are your plugs? Are they showing obvious signs of trouble? The valve clearances are what will change the most when the bike gets warm, so it might be worth double checking them to make sure you don't have any problems there... Also might be useful to see if the bike is still running on both cylinders when it is warm. Just ride it around until it starts to have problems then pull a spark plug wire off. If the bike dies, you've got problems, if not, try the other plug wire. I've heard talk of coils failing in a way that causes them to work when cool then not function when warm, but that's just what I've heard, I don't have any actual experience with that particular problem. Plug color, double checking valve clearance and testing to make sure you're running on both cylinders is where I'd start...

I'd think if his bike was running lean, warmer air would cause the fuel mixture to go closer to optimum.

Nikolas · April 20, 2009, 06:00:59 PM

Well the floats were low. I raised them to be in spec, and its better, but still not completely so. I'm going to fiddle with it more next weekend. I hate how I have to take the carbs out every time I need to make changes. Give me FI and a laptop any day...

The Buddha · April 20, 2009, 06:05:09 PM

OK floats being low will have made it run slightly better when hot and horrible when cold as well as feel like its gasping and sound dry when you are opening the throttle ...
You're prolly losing a cylinder when hot due to a bad crank trigger.
Otherwise you're running so super hot the bike is on the verge of heat seizing ... though typically it makes more power then ... just before it locks up it makes an extra horse or 2 ...
Cool.
Buddha.

Nikolas · April 20, 2009, 06:07:04 PM

oh sorry, they were "low" when upside down, so they were high? right, confusing. they were on the rich side before, and now are within factory tolerance.

The Buddha · April 20, 2009, 06:15:51 PM

U tube the floats to the gasket level.
Cool.
Buddha.

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performance decrease when warm