fuel or electrical problem?

[mottyl]

Hello,

The bike is a 1983 Katana 750 with 55k on according to the odometer.

-compression is around 150 across all cyclinders
-cylinders 1 and 2 are running a little lean (tan/white plugs) 3 and 4 are a tan color
-has a 4 into 1 aftermarket exhaust
-original air filter box with aftermarket filter
-has a nice strong pale yellow spark on all cyclinders

My problem is a misfire and subsequent loss of power that starts right around 5800 rpms. The misfire is present under load or not and continues up to redline.

I do not know if the carbs have been rejetted for the exhaust and air filter.

Any suggestions how I determine if this is a electrical or fuel problem?

I may be away from the computer until tomorrow so thanks in advance for any replies.

C

 

[C.V.]

"Nice strong pale yellow spark" doesn't jibe...you want a nice bright blue spark. Check the voltage at the coils.

 

[doctorgonzo]

Yeah, you want better than pale yellow, should be bright blue. Your symptoms can also mean fuel starvation.

 

[mottyl]

Thanks for the replies.

I was being sarcastic with the "strong yellow spark" comment. That doesn't always come through in electronic format.

I should be able to check the voltage at the coils this afternoon. After several other mandatory jobs.

Will update later.
C

 

[almarconi]

I was being sarcastic with the "strong yellow spark" comment

I don't get it .. is that canadian humor??

 

[Guest]

EH?????????????
Too many factors to help you.
Could be ignition or fuel.
Need more info of what has been done.

 

[don_gibb6512]

Thanks for the replies.

I was being sarcastic with the "strong yellow spark" comment. That doesn't always come through in electronic format.

I should be able to check the voltage at the coils this afternoon. After several other mandatory jobs.

Will update later.
C

If they're OEM coils, check the resistance of both Primary and Secondary windings also.

 

[mottyl]

update

Checked the coils just now and secondary impedence is 35k and 33k. Primary is 4.2 and 4.6. According to the manual I downloaded from Basscliff's site (Thanks Basscliff) those numbers should be in the ballpark. I am using a 1983 GS750 manual. Not exactly the same but it seems pretty close as far as the engine goes.

However I am only getting 10 volts at the coils so I guess that is my first problem to troubleshoot.

Thanks for the replies so far.
Chris

 

[mottyl]

misfire is gone

It turns out I was missing a part on the airbox. The top opening which I thought the seat sealed was supposed to have a screw down cover on it. I was removing the tank off the 82 parts Kat I have and saw the airbox cover. Light bulbs started going off. Put the cover on the 83 and the misfire went away.

Right now I am somewhere between happy and embarassed over such a stupid problem.

Thanks for the suggestions. I still need to get higher voltage to the coils but I think that is not insurmountable.

C

 

[don_gibb6512]

misfire is gone

Thanks for the suggestions. I still need to get higher voltage to the coils but I think that is not insurmountable.

C

Search for "Coil Relay Mod". There's several threads with a lot of good information there. Sounds like if you do that, you'll be good to go. Congrats!!!

 

[mottyl]

Well it turns out the misfire is still there. My short test ride down the backlane yesterday wasn't good enough. Rode to work today and it now misfires lower at around 5k.

My next step will be to get sufficient voltage to the coils, probably on the weekend.

If I get 12 volts to the coils, the spark improves and the problem is still there should I start looking at fuel issues?

Thanks
C

 

[don_gibb6512]

If I get 12 volts to the coils, the spark improves and the problem is still there should I start looking at fuel issues?

Thanks
C

Either that or you've got an air leak causing a lean condition. Once you're sure you're getting a good, strong, solid spark then focus on the carbs and air intake. Leaking carb manifolds, between the carb and the engine are very common. If you haven't thoroughly cleaned out the carbs and replaced all the O-rings in them yet, that's probably you're next step. Good luck.