Runs fine for 10 miles... and then dies

sixfivezip

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The bike starts first kick and runs fine for about 10 or so miles. After that RPMs drop, pops from both cylinders and bike has a hard time going from a stop. Eventually it will stop running and won't start again until its cooled down

I've checked:
-Coils, good
-Plugs, slightly rich but not fouled
-battery, 12 volts

Could this be a carb problem? If it is a carb problem, why would it the bike ride fine at first?
 
Fuel and spark, one is going away. What year or what ignition / carbs are you running?
 
From your profile 79 stock special? still points ignition? get a volt meter on it, hows the charging system working? Are you seeing 14 volts when you rev it?
Tight valve is possible but seems we have both cylinders going down together so I'm looking for something that affects both at the same time. Points iggy is really two nearly independent systems. So I'm not looking there first. Blocked cap vent is possible.
But I'll take charging system issue as #1 possibility. KNOWN and charged good battery? Have the brushes been checked? What has and hasn't been done to the bike?
 
Thanks for the advice all!

I rode a few miles this morning with the gas cap open. Still having the same problem, so that's one thing to cross off the list of possible causes.

The bike is a 79 special, almost 100% stock, only 8,000 miles. Still using the points ignition. I bought the bike from a work buddy who redid the points and timing before selling it to me. The only thing not stock on the bike are the carbs, which are the PWKs from Mike's. The battery is new, but I haven't load tested yet.

Carb jets are clean, but the carbs haven't been properly synced and they are acting like there is a vacuum leak (RPMs stay high for 10 or so seconds after reving). I know the carbs need some work, but since it's running fine at first I don't see how the carbs would be responsible for my troubles.

On the charging system.. I have noticed that headlight brightness corresponds to RPMs, low RPMs=dim, high RPMs=bright. Not sure if that tells us anything? I've never had to use the trickle charger and the battery seems to keep at 12V. However, I haven't tested that battery while the bike was running. Also I haven't checked the brush length.

Seems like the next step would be for me to check the voltage while the bike is running, and hot, since that's when my problem shows up. Will do that and report back. Again thanks for the advice!

-Jake
 
hmmm, with your varying headlight brightness, electrical seems suspect.

carb issues can pop up as the bike gets warmer if its gettin really warm for some other reason.

tite valves will get titer as it warms.i would think. i say tighter but tighter would mean decreased clearences resulting in loss of compression.

do as pp said and git sum voltage readings at the batt.

both when you start it up and when it starts to fail.

check all grounds, heat plays hell with ground contact if its suspect.

my bike is a 79 too. let us know what yu find!
 
Headlight gets brighter when you rev it is a good thing. That's pretty much the dime store charging test. Mikes PWK carbs, well that's different subject. A hanging high idle can easily be a synch issue. Yes check brushes and it's best to ohm out the rotor, a failing rotor can draw a lot of current pulling down the battery at less than highway speeds and generally stressing the electrical system.
 
re the issue of a slow return to idle that sounds like the classic case of weak mixture due to either an air leak or a switch to filter pods without increasing the pilot jet size to compensate for the increased air flow.
I think your pilot jets are likely to be 27.5 try upping them to 30

Have you tried spraying WD40 or something around the carb stubs to check for air leaks ?

I put some red hermatite on a new gasket to seal the stubs to the cylinder, then some high temperature silicon to seal the carbs to the stubs . Bit messy but it pretty much eliminates the possibility of air leaks to the inlets.
 
I tested battery at idle, slightly under 14V. New brushes and rotor was at about 7.5 ohms..

Have payed more attention to what happens when the bike starts to run bad. I've noticed that the neutral light fades especially when RPMs are low. Yet battery is above 12 V..

Does anybody have suggestions on what I should test next?
 
I

Does anybody have suggestions on what I should test next?

what would be the point ?

You don't come back to check the responses for 5x days then you don't answer, acknowledge or thank members for their help and advice anyway? :wink2:
 
that's about the right timing for the coils to get good and warm, stock original coils? any cracks in the outer casings?
 
When you check the ohms on the rotor, do it with the brushes removed. Touch the slip rings directly. Should be around 5-5.5 ohms.
I have a rotor that tests ok cold but as it warms up the ohms increase to around 8.5 ohms hot. This increase in ohms drops the current flow through the rotor. This weakens the magnetic field in the rotor which decreases alternator output.
I figured this out when I mounted a voltmeter to my bike so I could constantly monitor the voltage as I ride.
This is a good idea. I recommend every one should do this.
Leo
 
what would be the point ?

You don't come back to check the responses for 5x days then you don't answer, acknowledge or thank members for their help and advice anyway? :wink2:

Sorry about not getting back to reply sooner. I keep my bike in a friends garage and between work and family it's a struggle to make time to work on the bike. I wanted to run the suggested tests before replying to suggestions.

Peanut, thank you for the advice regarding the carbs. Once I get this electrical issue worked out I'll be sure to follow your instructions. There's definitely an air leak there and that's my kind of fix!
 
When you check the ohms on the rotor, do it with the brushes removed. Touch the slip rings directly. Should be around 5-5.5 ohms.
I have a rotor that tests ok cold but as it warms up the ohms increase to around 8.5 ohms hot. This increase in ohms drops the current flow through the rotor. This weakens the magnetic field in the rotor which decreases alternator output.
I figured this out when I mounted a voltmeter to my bike so I could constantly monitor the voltage as I ride.
This is a good idea. I recommend every one should do this.
Leo

Where did you connect the voltmeter? Voltage to my battery seems okay even after the bike dies. It's like something is retarding the spark as it gets hot.
 
My advice would be to solve the known issues first .

You say that the engine is over-revving and not returning to idle for up to 10 seconds or more. That alone could be the issue right there!. With over-revving and weak mixture the engine has to be overheating to some extent and could cause a range of problems including partial seizing. If you continue to run a ‘tight’ engine with a weak mixture and high idle you’re end up with seized pistons for sure .

Anyway you cannot hope to test the bike properly if your idle is up near 1500+rpm every time you rev the engine.

Use a can of contact cleaner or carb cleaner etc to check carefully around the inlets. Any increase in revs would indicate a significant air leak.(If you cannot reduce the idle at all, this will probably not work.) If it were me I’d just remove the carbs thoroughly clean and adjust them and refit with new gaskets with 3 Bond and apply Hi-Temp silicon to the carb stubs to eliminate any possibility of air leaks.

I’m assuming that with new carbs the bike will now have aftermarket pod type filters ? if so you may need to increase the fuel flow at idle using the pilot (air) screw. If you run out of adjustment you’ll need to go for a larger size pilot jets . The "air screw" meters the amount of AIR in the idle mixture. You’ll need to find a Keihin tuning guide to download. I believe turning the Air screw OUT (Anti-clockwise) will weaken the mixture and turning the screw IN ,will enrich the mixture.

A good starting point is to set both air screws 1.5 turns out from lightly seated and then adjust from there for best idle and throttle response.

Once you have the idle speed sorted you can test the bike properly knowing that over-heating and air leaks are not part of the problem.
 
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