R75/5 suddenly lost 20% of power and idling running poorly

To give the full story I'm in the middle of a cross country road trip on a 1973 R75/5 with a Velorex 562 sidecar. It's hauling me, my wife and a fair amount of luggage so we're's definitely pushing the bike but we've been comfortably cruising at 60-65mph. We were in New Orleans a couple of days ago and stuck in stop-and-go traffic where I had to frequently turn off the bike from fear of overheating. Since we left New Orleans the bike has been struggling at 50-55 mph. It's now also harder to start and it's idling very poorly and often cutting out when coming to a stop if you don't give it some throttle. The idle is uneven but there's no noticeable misfires or pinging while riding, it's just underpowered. It does seem to make a bit of an undulating noise when nearing its limit (hard to explain, it sounds like the load is on a sine curve but the RPM is steady).

I pulled the plugs and they were looking white and crusty. I figured this might be residue from the stop-and-go so I put in new plugs, rode another 50 miles, pulled the plugs and they looked good with a slight light tan.

The carbs were rebuilt 3k miles ago with new diaphragms. I did the valve clearances at the same time. The valves still sound good so I haven't tried adjusting them again as it also doesn't seem like something that would happen overnight.

Things I've checked so far:

1. Spark looks good and strong
2. The ignition timing is spot on using a inductive timing light (I recently added dyna3 ignition to the bike)
3. I've done an aerosol test and can't find any vacuum leaks
4. I checked float levels (in case it's running lean and 50 miles wasn't enough to get a read) and one bowl is at 21m the other at 25mm so could definitely be better but doesn't explain the overnight power drop
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I have not yet tried adjusting the idle or syncing the carbs as that seems more likely to fix a symptom than the cause.

It could be a red herring but the only thing I really have to go on is that the bike might have overheated before this happened (but no glowing pipes of anything). Any ideas for other things to check? This seems too consistent to be caused by bad coils but maybe I'm wrong?

It runs so I'll keep riding and try to figure it out along the road but not really sure what to try next other than getting another plug read after a longer ride.

Thanks for any suggestions.

Maybe check the brass screens on the fuel taps? And also check that the vent for the fuel tank is working.

I'd recheck the valves since they were last checked 3K miles ago. The side with the chair is probably running hotter since the air flow is not as good.

I would give the engine a compression test, both sides. It is possible that the bike has a burned exhaust valve from overheating.

I would also recommend that you re-visit the valve clearances. Especially if the heads have not been re-built and corrected for un-leaded gasoline. Poor idle is a telltale sign of valve seat recession. I had that experience with my R75/5. The hard starting not so much.

I checked the in-tank filters a while back and I've checked the tank cap (just popped it open while riding). I have inline filters but they look good.

I, will do the valve clearances again first thing in the morning. If that doesn't work I'll do a fuel flow test. Thanks for the suggestions.

One thing I just remembered from setting the ignition timing is that the F would at times suddenly disappear just to a moment later appear again. It didn't move up or down, it was just gone then back again. I assumed this was a side effect of the second spark (from not having a distributor) but maybe that was a bad assumption? I only checked the ignition timing timing on the left side.

James.A said:

I would also recommend that you re-visit the valve clearances. Especially if the heads have not been re-built and corrected for un-leaded gasoline. Poor idle is a telltale sign of valve seat recession. I had that experience with my R75/5. The hard starting not so much.

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Exhaust valves can close up very quickly with a hard run if you have valve recession. The "harder to start" is the give-away as it has no compression as the exhaust valves are partially open all the time.

So adjust the exhaust valves and then when you get home, send the heads out to Ted Porter for new valves and seats. He has the excellent permanent fix for your problem.

You were all right! Adjusted the valves and the bike started idling MUCH better and regained most of its power. Another read on the new plugs showed that it was running lean on the left side and rich on the right side. Synching the carbs by ear improved the idle even more and gave the bike a little bit more power but I think it's still a bit short of where it used to be. At this point it could all be in my head though so I'm going to call it done.

I guess it needs the valves adjusted every 2.5K miles or so which seems frequent. I should definitely try to do something about that when the road trip is over.

Hopefully that will get you on down the road. But keep an "ear" on it!!

With valve recession, you may need to adjust them every 1000 miles....

With valve recession, which your bike is exhibiting, you are headed toward a valve job to correct for the absence of tetra-ethyl lead in modern gasoline. My R75/5 was corrected 10 years ago at a cost of $800. It involved new valves and seats and guides. I fitted new piston rings while I had it opened up. 80K at the time, 120K presently. My motor has been bullet proof since the work was done.

ebeeby said:

With valve recession, you may need to adjust them every 1000 miles....

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That's where my R100 was when I got new valves.