'81 XJ550 Maxim - Stumbles and poor performance hot....

XJ Folks,

My XJ 550 Maxim has begun to show signs of trouble once it gets hot.

The bike starts decent with a little pain in the butt playing the choke a little than once warm sits an idles very nicely. A very short 4-5 mile ride is no issue. Good Low, Mid, and Upper Range performance.

However, once truly completely warmed up it stumbles and loses power pretty bad. Last longer trip out I had to turn around and milk it home. I was about 20 miles of 55mph away from home when I had to turn around....

• I was suspicious of the coils but they check out....
• Compression is good and level across all four.....
• Carbs are clean and fuel is good.... as stated runs good until truly HOT.
• Valves are adjusted properly.

Where would ya'll start looking next???? Stumped trying to figure out what other component (probably electrical) is going to act up once hot?

Thanks for the help.....

KURT

 

you could have a plugged vent in the gas cap, next time open the cap and see if it helps
cold engines like rich mixtures, hot ones like leaner. look at your plugs then watch the choke linkage on the carbs and see that they close all the way
the hose to the fuel valve can get hot, soft and collapse. check the routing, get a new rubber one

 

Valves too tight, plugged gas cap vent, TCI giving up the ghost or the pick-up or ignition coils:

From one of the true technical experts, Dwayne Verhey:

"Avoid disconnecting just one spark plug and cranking the engine, as this procedure can create permanent damage to a coil, as this procedure create a very high resistance in the entire secondary circuit of a coil. Voltage will rise inside the coil without a path for escape. Eventually it rises high enough to exceed the break-down voltage of the insulation within the coil, and then current will flow via this newly created path. In the process, it creates a "channel" or weak point through which the insulation value is reduced. Every time this occurs, the insulation value at that point is further reduced, until it drops to the air gap value of the distance between the two internal conductors.

So long as this internal resistance is greater than the resistance of the spark plug gap, the coil will continue to function when the spark plug is reconnected. Bear in mind that the resistance of the spark plug gap increases when the gap is subjected to compression -- known as "spark quench" -- so the plug may still demonstrate a spark outside of the engine, but not inside, leaving one with the mistaken impression that the coil is fully functional. If the internal damage is severe enough of course, then we say the coil has failed and search for a replacement.

The truth is that the coil didn't fail, it was destroyed by cranking or running the bike with one plug disconnected.

Fouled plugs are a different matter. Why do fouled plugs fail to spark? Because the carbon build-up on the plug electrodes is a conductor and the thus short-circuits the spark plug gap. Same with flooded plugs. Gasoline is also a conductor. So in these situation, we have not increased the resistance, we have actually lowered it, and therefore there is no great voltage build-up inside the coil to cause damage."

 

Just to give you food for thought. Are you running stock airbox or PODS ? Are your jets correct ? This can effect how hot the engine runs as well as tight valves. You stated carbs are clean but have they been to church full tear down and butterfly seals replaced? Fuel level set correctly? To low can cause a lean condition , are carbs synched correctly? Popping in the exhaust at idle or when letting off throttle when coming to stop from speed is also where synch being off can show up. Not trying to second guess you in any way , but these are somethings to check.

 

You say the ignition coils check out good, did you check them cold or after the problem presents itself? Did you check the pickup coils both cold and hot/when the problem presents itself? Heat changes resistance and can be the difference between a borderline component functioning properly or not. My dad's 68 cutlass died in Madison one night on the way to dinner. Let it sit and cool in the parking lot while we are, fired right back up. Soon as it warmed up again, no fire. Bad coil. Not saying that is a for sure diagnosis but when it was tested cold it was within spec and as it got it it went out of spec

 

Thanks for ALL the replies. This was my first bike back in the saddle after nearly a decade without a 2-wheel ride. I've owned this Mini-Maxim for about 6 years and it has always been a perfect machine. Had to replace a few cosmetic items and a leaky tank, performed all the overdue maintenance (fork oil, valve adjustments, etc.) Turn the key, push button, go for a ride.........NEVER an issue...Until Now.

The bike is absolutely stone stock with original exhaust, air box, jetting etc. The valves are set correctly and carb sync was correctly performed with the YICS tool installed. Carb boots are nice and there is no evidence of an air leak. The plugs look a little rich like unburned fuel fouling, definitely not lean.


I tested the coils COLD but I have not tested them promptly following a hot run. I'm back to suspecting these coils (thanks to all the encouragement that direction). Before I spend the $$$ on a new set, I'm gonna get her nice and hot and test them again HOT. The fact that these are 36 years old and I've had to replace the coils on a couple of my XS11's of the same vintage tells me that's the most likely spot for an issue and they're way past due anyway I guess.

 

Why are my plugs carbon-fouled (dry, sooty black deposits)?:

- Carbon fouling is the result of incomplete combustion----for any reason. It is most often associated with an overly rich fuel mixture (whatever the cause), but can also be caused by an overly lean fuel mixture (or poor spark, etc.) In a lean-mixture condition, most of the un-burnt mixture gets pumped out the tailpipe, but some fuel droplets remain in the cylinder and add themselves to the next intake charge. That's not a very precise way of metering the mixture, so when it's finally rich enough for a spark to ignite, that particular charge may be too rich, resulting in incomplete combustion and plug fouling. So carbon-fouled plugs can be due to rich or lean conditions.......your Colortune spark plug will tell you for sure. If you have a light blue or white-ish flame, intermittent flame, and/or intermittent flashes of yellow within an otherwise white-ish flame, then your fuel mixture is too lean.

And by the way.........if the spark that happens to ignite this overly-rich mixture is the "wasted spark" (which occurs in each cylinder at the top of the exhaust stroke, it will occur JUST AS THE INTAKE VALVE IS OPENING ---- so the backfire pressure wave may be directed back up the intake tract!

 

This is exactly what my bike was doing when I bought it and was a result of the carb bowls overflowing. Seemed find when cold (Wanted rich) and got crappy when hot. My commute is 30 minutes and a few miles from home it would start acting up. Turns out it was also masking throttle shaft leaks as when I got the levels close to correct, the bike rpms would start running away when you take the choke off...

That's when I sent the rack to hogfiddles . Been ok for about 1K miles now....