Topic: Sudden loss of power

[janne_83]

Last weekend I was riding my Honda CB 500K Four (1978). Suddenly as I was accelerating normally after exiting a corner I lost basically all the power. As I opened the throttle normally I was losing rpm but when I downshifted gears and opened the  throttle just a little bit I was able to keep my speed steady.

I noticed that I was able to accelerate very, very, slowly when I opened the throttle very carefully but it was clear that something broke. Any suggestions?

I ride about 20km with the problem, then turned off the engine and had a really big problem starting the warm engine. With full choke and a little throttle I managed to start the warm engine but I had to keep the throttle open just a little bit to keep the engine running.

I just don't know... Any ideas?

[BomberMann650]

Sure you weren’t running out of gas? 

[PeWe]

Point plate came loose?
If fuel tank will not breathe it will stop like fuel starvation. I had a rusty tank fuel cap on my CB750. Fuel started to flow with tank cap open

[Deltarider]

CB 500K Four (1978).

If yours is a K3, it's very easy to check if enough fuel reaches the carbs: clear tube test. You don't have any extra inline fuelfilters, do you?
Any strange sounds when it happened? Does it still run on all four? Touch the headers to check if they're equally hot.

[TwoTired]

Any history of the routine 3000 mile (5000km) tune up checklist being performed?

[Gene]

Sounds like you either were fuel starved or flooding. Check fuel flow, check your throttle cables and the linkage.

[ekpent]

 As mentioned start it up and check the exhaust header pipes to see if any are hot or cold to start your troubleshooting. Sounds like you may be running on two or three cylinders maybe. At least that test will get you pointed in the right direction.

[janne_83]

Thanks to ya'll!

I had gas, I always take care of that.

Earlier two years ago I had a problem with the tank cap so I also checked that right away; the problem was still there even with the cap open.

No strange sounds were involved in this so the power-loss came out of the blue. The exhaust sound changed a little bit after the problem;  maybe it's a sign that it's running with only 2 or 3 cylinders?

There no checklist for the maintenance and I've owned the bike for just two years now.

It didn't cross my mind to check the heat of the headers so that's obviously the next think I'll do. I'm not a mechanic by any means but I agree that it's either a problem with ignition or fuel flow and testing the heat would be a good way to start diagnosing.

If there's something positive about this, it happened while I was taking my bike to the garage to store it for the winter 

[AirCanuck]

interested to see what the issue is...

[TwoTired]

No strange sounds were involved in this so the power-loss came out of the blue. The exhaust sound changed a little bit after the problem;  maybe it's a sign that it's running with only 2 or 3 cylinders?

There no checklist for the maintenance and I've owned the bike for just two years now.

If you've had the bike for two years and not done the checklist, you are officially abusing/neglecting the bike. 3/6 mos regular attendance routine.

You'll probably find the problem while doing the checklist. (And, you just might have avoided the problem if attended.)

“An ounce of prevention is worth a pound of cure.”
― Benjamin Franklin

Cheers,

[Deltarider]

Never knew Ben could be so serious on matters. We had an American teacher in high who had us read 'Fart Proudly' and ever since I think of Franklin as a comedian.  But wait... didn't he also fly kites to catch lightning? Must have cured his flatulence once and for all. Poofff! Amazing chap, Ben. Although, I myself am more of a Thomas Paine man. Both feet on the ground, tought us (US) a bit of modesty. Great man, Thom.

[janne_83]

interested to see what the issue is...

Ok, changed the spark plugs and the bike runs normally again. Great news! It seems that at least one of the carbs is leaking a small amount of fuel; this might also be a reason for the broken spark plug. This is something I have to investigate next before the summer starts.

[dave500]

if your plugs fouled up check the caps resistance,old caps can go very high and can cause plug fouling.

[janne_83]

if your plugs fouled up check the caps resistance,old caps can go very high and can cause plug fouling.

Thanks, have to check that too!

[dave500]

if the caps are as old as the bike just replace them,go over the entire ignition aswell.

[janne_83]

if the caps are as old as the bike just replace them,go over the entire ignition aswell.

The caps have been replaced at some point and they look pretty new. Is there some specific value that this should be?

[Deltarider]

Caps (separated from HT leads) should read 5-7 kΩ. > 8kΩ should be discarded (as recommended by Honda France).

[BobbyR]

I have a feeling you dropped two cylinders. You did not mention if you had points or electronic ignition. When it acts up, just pull a plug wire if it does not change you have dropped two cylinders. Points do wear out and two years is a long time between adjustments. You could have one coil crapping out. It is a point by point diagnosis process.

[Gene]

^^^^ It's almost always the points . . . or in your case, maybe the plugs. plugs, points, oil. Check those ALL THE TIME