Uh oh, losing power above 5K

So today was 70 degrees here in mass! I was leaving work and halfway home i had a nice long stretch of open road. I put it to red line in first and second and third, and shifting to 4th, it stayed at 7k and wouldn't accelerate. And its an SR so redlining 3rd isn't that fast.

After whoaing it up, it wouldn't accelerate well after 5K. It seems like the bike is not running on all 4 cylinders after 5K, or under any real load. I can drive up to 4K no problem nice and smooth. Definately on all 4 cylinders and running smooth. This bike has been fast and reliable for years now with me. It was in great running condition and never had a problem with it. I've also had it out of winterization for a while now. It was just after that moment of hard accelerating that its acting up.

I changed the spark plugs thinking one may have burnt out but that did not help.

Any ideas???? Thanks

Sounds like it's running lean when you hit the higher rpms. Any chance of plugged main jets?

check the mech advance, may not be working,, also fuel filter

I personally would try and determine the basic cause for the power loss. Take the bike out and after warming the bike and experiencing the problem, stop and take out a spray bottle with water and squirt all four head pipes. See if water VAPORIZES (normal) or ROLLS OFF (cylinder not firing). I would then check compression and valve clearances to attempt to resolve the issue. Noting WHICH cylinder(s) were not firing will help pinpoint a possible electrical problem. If compression is good and you have good spark, the tip to clean your mechanical advance is good and then I would pull the carburetors and check for sediment in the bowls... If you have an inline gas filter, I would REPLACE that piece first and use one from a bike shop. DO NOT use a lawnmower filter because while they LOOK about the same as a motorcycle filter, they don't have enough flow rating in many cases.

freebyrd24 wrote:

So today was 70 degrees here in mass! I was leaving work and halfway home i had a nice long stretch of open road. I put it to red line in first and second and third, and shifting to 4th, it stayed at 7k and wouldn't accelerate. And its an SR so redlining 3rd isn't that fast.

After whoaing it up, it wouldn't accelerate well after 5K. It seems like the bike is not running on all 4 cylinders after 5K, or under any real load. I can drive up to 4K no problem nice and smooth. Definately on all 4 cylinders and running smooth. This bike has been fast and reliable for years now with me. It was in great running condition and never had a problem with it. I've also had it out of winterization for a while now. It was just after that moment of hard accelerating that its acting up.

I changed the spark plugs thinking one may have burnt out but that did not help.

Any ideas???? Thanks

I had similar symptoms once after getting bad gas that rusted my tank. I flushed it and put in good gas, but rust particles would sometimes get into the main jet and it would lose power.

I'll check compression tomorrow when I have the chance to. The thing that is just odd is how it happened all of a sudden. I too thought it was lean so I gave the carbs a little bit more fuel with the adjustment screw, and a new set of plugs and that did not help the issue. The advance mechanism also popped in my head, and i'll have to check that tomorrow as well. How do I test the advance?

I may have to take the carbs off as well. which is a royal pain with stock airbox! So I'll try to rule out most other things first.

UPDATE

I finally got some time to tinker around some more with the bike. I was thinking maybe something with the coils so I decided to check the resistance as specified in the book.

The primary (winding?) resistance to my understanding is the resistance of the leads that go to the coils. a yellow/red wire and green on one side, and a yellow/red wire and black on the other. They are supposed to be at about 4.0 ohm resistance as the book states. The side with the green wire has a resistance of 3.0 ohms while the side with the black wire has a resistance of .8 ohms.

What does this mean??

Oh and compression was excellent, as was the advance mechanism

If not already done, could try running with fuel tank cap loosened (to see if clogged vent is causing the problem). And also check for ample free flow of fuel through petcock (to assure petcock is unobstructed).

Sometimes crud hovering inside the fuel tank settles around and clogs the petcock intake tube screens which go up inside the tank.

While stopped trying to figure things out, enough fuel may trickle into the float bowls allowing normal operation for a short distance before again "running out of gas."

Hoping for a cheap and easy fix.

Good Luck!

Excellent, I'll have to try that this week. Currently I have a coil out and disconnected and kind of a mess to slap it together and take a ride. Weekdays get kind of hectic. Any significance to the resistance readings?

another update

the fuel system is completely clean. the filter is extremely clean, its not a fuel issue

the bike has has new points, new condenser, new plugs, new wires and its running off of cylinders 1 and 4 only. I switched the coils to see if it was a coil, and it did the same thing, so not a coil problem

any ideas on why its running on only two cylinders?

Take a timing light and see if you are getting spark on the non running cylinders. If not, check the voltage to the coil and continuity to the points.

If you have spark, pull the plugs and check for fuel in the dead ones. if you have spark and feul, it should run. You should also check compression across all four.

I checked compression the other day and it was excellent on all 4. I checked the resistance at the coils and the coils checked out. However resistance going TO the coil through those wires was different. One set to one coil was 3 ohms while the other was only .8 ohms. I dont know what that would mean.

I'll try to check voltage to the coils and continuity to the points. whats the easiest approach to checking both of these?