Hello all, one of our IL members is experiencing a loss of performance with his R75/6. We are looking for ideas. Original owner with an extensive list of experience with his bike. All stock except for a Dyna III ignition. Earlier this year, the owner noted a decline in fuel economy (MPG). Now the bike has trouble achieving a highway speed much over 60 mph, however, the motor revs to 4000 RPM easily in neutral on the center stand. If anyone has experienced this, or has any ideas about it, please let me/us know.
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Gradual decline in performance, R75/6
- Thread starter James.A
- Start date
manicmechanic
ABC,AMA(LIFE),MOA,RA,IBMW
Greetings, Jim! Simple stuff first, clutch adjusted properly, compression check, valves adjusted properly. When was the last time the carbs were cleaned/rebuilt? Clean air filter? Timing set properly? Something in there should direct one to the fix.
First thing I would check is fuel flow. Check fuel flow coming out of the gas tank, and at the float, which would require the removal of the bowls, and put something to catch the fuel when you open the petcock at the gas tank.
The symptoms described are classic lack of fuel.
The symptoms described are classic lack of fuel.
surfdad1060
Member
Check the advance mechanism.
Also, make sure that the cap vent is open and not creating a vacuum situation not allowing enough fuel to flow.
What I don't know is if Howard W., the owner, rides with one petcock open or 2, I will suggest to him to try using both taps to see if that makes a difference. That doesn't cost anything to try. Howard is a pretty good owner/mechanic and has assured me that the diaphragms are O.K. This is why he and I are somewhat stymied by this.
lmo1131
New member
Extreme, but possible; might be a crank bearing(s). I had a VW that went that route. It would idle fine, rev up okay (sitting still) but a slow decline in performance (at speed) until it finally soze up one afternoon.
But I'm a strong proponent of "all-at-once" tune ups ... not doing the valves one weekend, the points the next, changing the oil the following weekend, etc. Too many "fine points" can get lost in the shuffle.
Air filter
carbs (pull/inspect diaphragms, clean, pull jets, blow out, etc)
valves
plugs (pull check gap as required)
ignition (pull advance (check springs & lube) gap (I still use points (NORIS), condenser, and time )
test ride
sync carburetors
But I'm a strong proponent of "all-at-once" tune ups ... not doing the valves one weekend, the points the next, changing the oil the following weekend, etc. Too many "fine points" can get lost in the shuffle.
Air filter
carbs (pull/inspect diaphragms, clean, pull jets, blow out, etc)
valves
plugs (pull check gap as required)
ignition (pull advance (check springs & lube) gap (I still use points (NORIS), condenser, and time )
test ride
sync carburetors
Doubt if the problem would be crank bearings as I would think this would be accompanied by some sort of noise.
However, my real comment is about above comment. I am not a proponent of an all-at-once method of doing much of anything unless one item is "one the way" when doing something else. The Reason? I think doing only a small thing at a time is better, as it allows for better diagnosis if there would be a change. At least you can narrow any changes to just what you did. If you do a whole ton of things at once, then troubleshooting becomes more problematic trying to determine which thing you did caused the problem.
I agree, do one thing, see if it changes anything then move onto the next. Standard problem solving.
Well, I have mailed a set of known to be good coils. We are grasping at straws here, I don't hold out much hope for the coils to reverse the decline.
The bike is a 1975 R75/6 with 177,000 miles on it (original coils). Howard is the original owner and a competent mechanic who does most all of his own work. If he says the diaphragms are O.K., I have no reason to not believe him. If or when I get a chance to travel, I will meet up with him and we will go over the obvious items. This would not be the first time I have done this with one of our "Airheads". I will report any results here for the benefit of all. I suppose the answer will be revealed "in the bye and bye".
The bike is a 1975 R75/6 with 177,000 miles on it (original coils). Howard is the original owner and a competent mechanic who does most all of his own work. If he says the diaphragms are O.K., I have no reason to not believe him. If or when I get a chance to travel, I will meet up with him and we will go over the obvious items. This would not be the first time I have done this with one of our "Airheads". I will report any results here for the benefit of all. I suppose the answer will be revealed "in the bye and bye".
Just guessing, the bike has been well maintained; but. The top end needs a rebuild/rework. The valves and seats are worn out. The piston and rings and cylinder are probably worn out. Carbs probably need a rebuild. 166,000 miles is a lot for an airhead. This is more than a standard minor or major service. Good luck with your project.
1074
Active member
Loss of power
Throwing out a bunch of possibilities is not the best way to diagnose a problem!
Based on mileage, I'd do a compression and leak-down test first. This will tell you the basic condition of the top end. If you have low compression the leak-down test will tell you where the leak is going. If the valves are leaking then I'd adjust them and see if that makes a difference. If the leak is past the rings then I'd guess that it is time for a top end rebuild.
Without any clues, the basis for all diagnostics is with all valve, point and carb adjustments done and with a fully charged battery.
Throwing out a bunch of possibilities is not the best way to diagnose a problem!
Based on mileage, I'd do a compression and leak-down test first. This will tell you the basic condition of the top end. If you have low compression the leak-down test will tell you where the leak is going. If the valves are leaking then I'd adjust them and see if that makes a difference. If the leak is past the rings then I'd guess that it is time for a top end rebuild.
Without any clues, the basis for all diagnostics is with all valve, point and carb adjustments done and with a fully charged battery.
Another item. Not sure if the bike still has point ignition or electronic, but if the points are closing up that would affect the bikes running.
If the problem has come on suddenly then it is likely something fuel or ignition related, but if it has been a gradual decline then it is likely more than that. The list them becomes much greater from rings, valve seats & so on. That is a lot of miles if the engine has never been touched.
If the problem has come on suddenly then it is likely something fuel or ignition related, but if it has been a gradual decline then it is likely more than that. The list them becomes much greater from rings, valve seats & so on. That is a lot of miles if the engine has never been touched.
Yes, I sent him a pair of N.O.S. coils. He swapped them out but it didn't help. He borrowed a compression gauge and found a deviation in readings from one side to the other. This says to me that a new set of rings might be in order. Next step is to swap out the Dyna ignition and install the points and condensor. I know Howard is competent in his maintenance procedures, but I am going to travel to his garage to go over all the usual items with him. Only things I can think of beyond that are developments that no one really wants to ponder. I am not sure that his mileage, at 177,000 miles should necessarily be terminal mileage
