CB900C breaking up after 5k rpm

I have a 1980 CB900C that the previous owner rebuilt the carbs on. I just recently got the bike running correctly at idle (the carbs weren't seated into the boots properly, a coil was cracked so I got dyna coils, and I got new CDIs) and took the bike for a 5-10 minute test ride. That's where I found out that the bike breaks up badly above 5k rpm. It doesn't matter if I am riding the bike or just revving it up while it's sitting. I pulled the plugs today and 1&2 and rich and 3&4 are lean. What would be causing this? I've attached a picture of the plugs.
 

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teazer

Well-Known Member
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That's odd. Which plugs are on which coils? I thought it was 1 & 3 and 2 and 4 but could be 1-4 and 2-3. Check the FSM to be sure and make sure the right leads were on the right plugs.

The two right plugs look like they weren't firing at all. Could be the carbs need to be stripped and deep cleaned.
 

cxman

Active Member
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do a vacuum leak test
do a compression test and make sure you dont have a bridged head gasket leak causing it
do a voltage test for charging voltage and coil voltage engine not running and engine running
 

teazer

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+1. If voltage is low, it will miss. It's really unusual to have one plug on each coil look so clean, but if teh voltage is low, if might not be firing properly at the plugs. In theory, it is not possible for a double ended coil to fire only one plug but in practice I guess anything is possible.
 

pidjones

Well-Known Member
+1. If voltage is low, it will miss. It's really unusual to have one plug on each coil look so clean, but if teh voltage is low, if might not be firing properly at the plugs. In theory, it is not possible for a double ended coil to fire only one plug but in practice I guess anything is possible.
Unless one plug wire/cap, etc. is breaking down, but I would expect that breakdown to happen at any RPM.
 

cxman

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teaser i have seen a bad wire from a dual fire coil kill one cylinder
 

teazer

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That's interesting but not surprising. How does it ground then? As I understood it, one end of a dual coil is positive and one negative, so both need to be grounded to work. We have been told for years not to reverse the primary circuit because the spark should go from center electrode to side but on one side of a dual coil it goes the other way.

In theory it would be open circuit with a break in a lead or a dead plug. I guess it finds a way to complete the circuit. Maybe it grounds through the insulation to the primary circuit.

BTW, I didn't realize you have such an extensive range of products on your web site. Nice stuff.
 

Maritime

Well-Known Member
you could also have 0 fuel on those 2 carbs. My GL had an issue with float needle seat causing #2 carb to not get fuel and I had the same clean plug. Can you smell gas on them? Do the pipes on those 2 get hot after a min of idle? If there is no gas smell it may be badly adjust floats or plugged inlet etc. Someone else cleaning the carbs doesn't mean they did it right.
 
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cxman

Active Member
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teazer i am so behind adding stuff to my website its silly i have a years worth of new products to post
 
All 4 cylinders were hot at idle and they were all within about 70 degrees of one another using an IR thermometer. I haven't had time to go back and check anything else though.
 

pidjones

Well-Known Member
That's interesting but not surprising. How does it ground then? As I understood it, one end of a dual coil is positive and one negative, so both need to be grounded to work. We have been told for years not to reverse the primary circuit because the spark should go from center electrode to side but on one side of a dual coil it goes the other way.

In theory it would be open circuit with a break in a lead or a dead plug. I guess it finds a way to complete the circuit. Maybe it grounds through the insulation to the primary circuit.

BTW, I didn't realize you have such an extensive range of products on your web site. Nice stuff.
If a wire or plug cap breaks down (not open), the spark jumps to ground through the break. Often it can be seen at night if lucky. Insulation only needs one hight voltage puncture. Once through it carbonizes a path and gets progressively worse. I imagine coils can break down that way, also.
 

Maritime

Well-Known Member
My old Jetta would have that issue every 20K or so miles and you'd have to replace the plug wires. It would start small but if you didn't swap the wires within a few days you'd get a light show under the hood at night with all the cross firing. Didn't matter the brand of wire either, $100 set or $500 set would only last the 20K and then start shorting. If it was raining the car was un-drivable even if it had just started to short.
 

teazer

Well-Known Member
DTT BOTM WINNER
We had a Nissan frontier that had the same sorts of issues - but not every 20,000 miles though.
 

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