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Topic-icon R75/5 Surging at steady RPM

  • Subman
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4 months 1 week ago #3735 by Subman
R75/5 Surging at steady RPM was created by Subman
'71 R75/5...bought late last year, rode once & put to bed for the winter. (All fuel completely drained, carb bowls included). This spring, reinstalled carb bowls with new gaskets, filled with fresh fuel. Started instantly and appeared to run fine on the first short ride.

Now, the bike still starts fine...idles well...accelerates fine...but riding at a steady speed/RPM produces noticeable surging. Riding at, say, a steady 3000 RPM results in a "bucking" feel...pretty prominent. Again, if I then roll on throttle to accelerate, it's smooth & pulls hard. The surging only occurs when trying to hold a steady speed/RPM.

All the research I've seen points to (a) checking the valves, particularly seeing if I have tight exhaust valves; (b) verifying ignition system is 100%; (c) then and only then looking at the carbs. Thoughts?

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4 months 1 week ago - 4 months 1 week ago #3740 by Wobbly
Replied by Wobbly on topic R75/5 Surging at steady RPM
1.Valve clearance is paramount, And since Airhead valves tend to tighten over time, zero valve lash is a perennial suspect. The clearance doesn't have to be perfect, but there MUST be some amount of free-play to allow the valves to completely close.

After you've owned your Airhead a while, you'll know how many miles you can ride before adjustment is required, but if you have very little history with the bike it's always a safe first step.

2. All the functions of the points, auto advance, and the ignition timing itself can be easily checked with the aid of a strobe lamp and the "F" mark. No need to remove any engine covers, just clip on the lamp and pop the rubber flywheel plug. Takes 3 minutes.

3. Despite all the pre-work, your "surging" sounds like a classic case of torn carb diaphragm. I suspect the older material was not ethanol-proof, so if you rode the bike for a week then put it into storage, the ethanol would have been "eating" on the old synthetic all winter long.

You can do this job while leaving the carbs on the bike. You only need to disconnect the throttle and choke cables, then remove the top carb cover. After lifting off the top cover, the slide/diaphragm assembly will be right there. Make sure the diaphragm's outer edge is not stuck to the carb and lift it straight out. Stretch the diaphragm in front of a bright light and look for holes or tares. Do one side at a time so that parts don't get interchanged.

There are several things on the bike that are always replaced in pairs, and diaphragms are one of those. You may only find 1 hole, but replace BOTH diaphragms.

Note that the outer periphery of the diaphragm skirt has a "key" or "boss" molded in at one location. The position of that "key " in respect to the slide is important. Furthermore, that "key" fits into a specific divot in the carb body upon reassembly. Don't do anything until you locate the "key" and mark its position on the slide with a marker.

Reassembly will of course require re-balancing the carbs, since the throttle cable adjustments were disturbed.

Hope this helps.

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Last edit: 4 months 1 week ago by Wobbly.

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3 months 3 weeks ago #3814 by Subman
Replied by Subman on topic R75/5 Surging at steady RPM
So just to close the loop, did the valve adjustment. Left cylinder was fine as-found...but the right cylinder had zero valve lash, both intake and exhaust. Reset both, test rode....ta da...issue resolved. Happy camper.

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3 months 3 weeks ago #3818 by Wobbly
Replied by Wobbly on topic R75/5 Surging at steady RPM
Hey, that's great news. You fixed your bike, and what's more you learned an important lesson.

Valve adjustment will have changed the carb balance settings. I encourage you to buy or make a carb balance tool and take the time to make those adjustments. The bike really will run smoother.


Make-It-Yourself carb balance tool

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