Topic: '71 CB500 NO POWER!

[bvander]

Picked up a clean '71 CB500 with 4k miles. Unfortunately, there was gas left in the tank for a few years.

So I flushed and acid washed tank, cleaned and rebuilt the carbs, and reset everything (timing, jets, floats, etc) to factory specs.  The only change I made was to add fuel filters to the lines as I am still worried about residue in the tank.

Now that the bike is running I feel like I am not getting sufficient power and seems to be the result of a lack of fuel. I can see that the filters are no where near full when the engine is running. Yet when I pull the lines from the pet cock, fuel pours out.

Is this an issue with insufficient vacuum/pressure coming from the carbs or maybe a gravity issue in the lines? I really have no clue at this point.

Thanks in advanced.

[HondaMan]

I'd suggest the following things, not knowing the bike first-hand:
1. Remove and clean the carbs. The jets are likely crusty and partially clogged, and the O-rings that seal (and seat) the jets may be cracked, or not sealing anymore, since they are about 35 years past their lifetime. They are 1.2x4.0 size, or you can get them in the better carb kits: DO NOT change the jets if the kits come with them, as most kits now have incorrect jets in them. You can find kits of just the rubber and float valves.

2. Get a new PAPER (not foam) air filter element for it. The old one is likely damaged and plugged from humidity: they have an expiration of 3 years if not kept in a plastic sealed bag.

3. Check the ignition timing with a strobe light. With 4k miles on it, the points are likely fine, but their gap may have closed up a bit. You should be able to simply adjust the timing back to the "F" marks by slightly opening the gap on the points, as their rubbing foot (on the spark advancer cam) and pivot always wore a little bit during the first 3000 miles or so while the points settled in.

[Scott S]

 Try running without the inline filters.

[TwoTired]

Try running without the inline filters.

+1

[calj737]

You have no baseline for what “power” this motorcycle makes I suspect. These motors, even after 40 years, want to be revv’ed up. If you are riding around at under 3,000 RPMs trying to gently nurse this bike, you will be deeply disappointed. Shifting should be above 4,000 minimum, closer to 6,500. Lower than those ranges and you are lugging the engine.

Go for a spirited ride and let us know if you still have issues. And if the tank has it’s filter above the petcock in tact, remove the inline units.

[Deltarider]

The CB500/550 doesn't like inline filters. Stock length for the fuellines on your model are 18 and 30 cm resp. Later models with newer petcock have 17 and 28 cm. Only the correct length guarantees no kinking and good flow. Inside ⌀ ideally is 5,5 mm.

[1952paul0]

You have no baseline for what “power” this motorcycle makes I suspect. These motors, even after 40 years, want to be revv’ed up. If you are riding around at under 3,000 RPMs trying to gently nurse this bike, you will be deeply disappointed. Shifting should be above 4,000 minimum, closer to 6,500. Lower than those ranges and you are lugging the engine.

Go for a spirited ride and let us know if you still have issues. And if the tank has it’s filter above the petcock in tact, remove the inline units.

Agreed, however I would add that taking a compression reading and basic timing check also make sure the advance and retard are functioning correctly. These bikes do like to be revved to get the best from them. But basics first.

[flybox1]

Picked up a clean '71 CB500 with 4k miles. Unfortunately, there was gas left in the tank for a few years.

So I flushed and acid washed tank, cleaned and rebuilt the carbs, and reset everything (timing, jets, floats, etc) to factory specs.  The only change I made was to add fuel filters to the lines as I am still worried about residue in the tank.   Did you CHANGE jets?  What brand did you use?  Do you have stock airbox and exhaust?  What about a vacuum sync?

Now that the bike is running I feel like I am not getting sufficient power and seems to be the result of a lack of fuel. I can see that the filters are no where near full when the engine is running. Yet when I pull the lines from the pet cock, fuel pours out. its an air bubble.  there will always be an air bubble inside a fuel filter unless it is perfectly vertical...but this bubble is not preventing flow.  as fuel is used, fuel is supplied from your tank.  continue to use it as-is, or keep rinsing out  your tank until it is debris-free

Is this an issue with insufficient vacuum/pressure coming from the carbs or maybe a gravity issue in the lines? I really have no clue at this point.
Well, vacuum sync your carbs after you are comfortable timing, advance, and valves adjustments are right.....then, start with an idle plug chop to see if your IMS need adjustment.
Thanks in advanced