Topic: CB500 Power Loss Diagnosis, Possible Bad Coil?

[Pfraser1]

I am trying to get my 1972 CB500 running right.

I completed a full top end rebuild recently but after riding 200 miles and after a lot of carb tuning, my new head gasket was leaking oil pretty bad so I decided to pull it apart again to replace it and replace the head hex caps and washers as well.

When I took the head off last night, i saw that cylinders 1 and 4 were still pretty clean from my rebuild but cylinders 2 and 3 had excessive but equal amounts of new carbon build up on them. I had soda blasted all the pistons during the rebuild so they all looked brand new.

I have a 4 into 2 stock exhaust and I noticed white smoke coming from both tail pipes when it was running, but I had very little top end power above 5,000 rpm. No stumbling or missing, but i didn't really feel like it was pulling like it should.

Compression Numbers are as follows after the rebuild:

1) 145
2) 150
3) 155
4) 155

I tested both coils (i think they are original and quite old) and with the OHM meter set at 200, the lowest setting, both coils read 6.8 when testing the primary side and 14k on the wire side with the resistor caps removed.

I replaced the resistor caps on 1 and 4 during the rebuild but not 2 and 3, those are still the old ones.

Carbs were rebuilt and they have air pods and 105 main jets installed, needle clip position is set at the 2nd notch from the bottom, 4th from the top (Slightly rich).

I also checked the fuel level in each bowl and they are all about 1-2mm below the gasket surface using the clear tube method.

New points ignition was installed and timed during the rebuild.

Basically the only thing that hasn't been replaced yet are the coils and resistor caps on 2 and 3.

Could a bad 2/3 coil cause this engine to run but have weak spark so the top end is running rich and now power in the 2/3 cylinders?

Or bad resistor caps on the 2 and 3 plugs causing too much resistance and top end power loss?

[flybox1]

Test the resistance in the 2/3 caps.
Coils rarely go bad.
If your spark is good on 2/3, and timing is spot on, then look towards a carb imbalance.
What brand jets did you use?
If it was an aftermarket 'carb kit' what else from the kit did you install?

[Pfraser1]

I will check tonight.

What should the resistance be in the caps?

I used the 105 jets from 4into1.com

I installed new idle/air screws, float needle and seat, main jet leaf springs, slide needles, and #40 pilot jet

[calj737]

I would also verify your fuel level via Clear Tube method. What do the plugs look like for all four cylinders?

[Pfraser1]

I did verify the fuel level in the bowls is correct.

The spark plugs were black, sooty, and wet when I took them out.

However this could be because I had 115 main jets in the carbs until last week. Now I'm running 105 main jets.

When i tried grounding the plugs out on the cylinder head to test the spark, it looked pretty weak so that is why I am looking at the ignition system..

I also want to move the needle clips up one notch to the middle notch to lean out the mid range..

[calj737]

I know many use the spark plug thing the head trick, but I am not a fan. I prefer to use a meter and measure the voltage from the coils. It is proof of the strength of the spark. Previously fouled plugs, dirty threads, greasy covers can all impact the strength of the ground/spark.

I'd agree, 115s are way too big, and your mention of needle in 2nd spot is also very rich. Fresh plugs and new plug chops.

[flybox1]

If your existing caps are rated at 5k ohms, anything higher than 8,000 ohms is probably getting too much for an adequate spark, and should be replaced.  Cut back wires to get the cap into clean copper.

[Pfraser1]

The old caps are at 6800 ohms and the full resistance between the 2 wires with the resistor caps on was 28,400.

I ordered a new coil and wire set to start fresh with the ignition system.

I am also moving the needle clips up a notch to the middle position tonight.

Bike should be back together by Wednesday. Fingers crossed!

[jonda500]

CB500 Four stock exhaust is four into four, stock needle clip position is #4 (fourth down from the top).
I have read in here somewhere that changing to a 4 into 2 system shouldn't require any re-jetting...
Stock air filter?
John

[calj737]

CB500 Four stock exhaust is four into four, stock needle clip position is #4 (fourth down from the top).

This is often an issue of dispute. 4th position, technically would be 1 down from the top, as the needles positions are counted from the bottom/tip/bowl end. I'd say you would want to be 2 slots from the top, not 2 from the bottom. Or at least in the middle position.