1977 GT550 left cylinder misfire

[Kevgt550]

I have been tracking down a left cylinder not firing for quite a bit now. I thought I’d write out my situation to see if anyone has some insight.

First of all: Compression test showed a quick ~110 PSI, not what i would consider to be a huge issue

Second: all new ignition coils, wires, and plug caps, tested at roughly the same resistance so they should be good.

Third: carburetor cleaned multiple times last time with ultrasonic cleaner everything appears clear and fuel line to that carb is clear.

Fourth: Air filters are new on original air box, brand new boots from carb to motor.

Five: oil injection visibly in the lines all fresh oil

Sixth: tested with multiple new spark plugs including working ones from other cylinders.

So i am leaning toward weak/intermittent spark issues. I cleaned the ignition point which didn’t seem to do much. I’m thinking i should replace the points and condensers if possible (so hard to find parts for this bike). I will try to attach videos. The points all spark a lot which i don’t believe is optimal or correct. The cylinder does spark as i tested it with a ignition tester.

The new carb boots, ignition coils and plug caps, and carb cleaning were all done after the cylinder stopped firing. So i don’t believe they were the problem but they definitely help everything run better.

Center and right cylinder fire fine, may still need tuning but I’d like to have the left running before i get it dialed in. Is there any suggestions?
I can’t attach videos so here is a link to my videos:
https://youtu.be/MFxvmZffTCY

[jabcb]

I have an old induction timing light from back when cars had points. Use it to see if the plugs are actually firing when installed in a running motor.

Check for voltage drops with the points open & closed: battery + to coil, across coil, coil to points, & points to ground. This may clue you into the problem.

You have a Kokusan ignition. Parts seem to be harder to find than for Nippon Denso. You can swap out the entire assembly for a Nippon Denso.

I upgraded to an Accent ignition on the GT550 cafe build. East to do & highly recommended if you want to go that route.

[Kevgt550]

I have an old induction timing light from back when cars had points. Use it to see if the plugs are actually firing when installed in a running motor.

Check for voltage drops with the points open & closed: battery + to coil, across coil, coil to points, & points to ground. This may clue you into the problem.

You have a Kokusan ignition. Parts seem to be harder to find than for Nippon Denso. You can swap out the entire assembly for a Nippon Denso.

I upgraded to an Accent ignition on the GT550 cafe build. East to do & highly recommended if you want to go that route.

Thank you for the tips on checking voltage drop, hadn’t seen that anywhere else. And as far as electronic ignition, I’ve looked through the electrical parts sources thread and it appears none of them sell kits for the 550 anymore, unless I’m missing something. I would definitely go down that route to make it easier in the long run.

[jabcb]

The GT550 cafe build:
viewtopic.php?f=21&t=13106&start=70

Accent ignition on the GT550:


Accent’s website:
http://www.accent-electronic.com/ELZ2Coil.html

The website lists only the GT380 & GT750, but they have a separate product for the GT550.
The points cam spins in the same direction for the GT380 & GT750, and the opposite direction for the GT550. So the ignition also differs.

Send them an email asking about it. They understand English. And use PayPal for payments.

[Kevgt550]

The GT550 cafe build:
viewtopic.php?f=21&t=13106&start=70

Accent ignition on the GT550:


Accent’s website:
http://www.accent-electronic.com/ELZ2Coil.html

The website lists only the GT380 & GT750, but they have a separate product for the GT550.
The points cam spins in the same direction for the GT380 & GT750, and the opposite direction for the GT550. So the ignition also differs.

Send them an email asking about it. They understand English. And use PayPal for payments.

Thank you, i was confused by the website to be honest. I sent an email and will definitely look into it if I can get a kit. Thanks again!

[Alan H]

Well, I still use points on all my 550s (and other GTs in the past) and they all run well.
Electronics like a strong battery at full charge and tend to work less than optimally if it's even slightly low voltage at the pickups.

[Kevgt550]

Well, I still use points on all my 550s (and other GTs in the past) and they all run well.
Electronics like a strong battery at full charge and tend to work less than optimally if it's even slightly low voltage at the pickups.

I am considering keeping the points for now. Do you usually use a timing gauge or just the marks. I think i can figure out the rest, but as far as the marks go I see the rotating plate with a cylinder mark like “L” “R” or “C” but I cannot figure out what the line on that plate is supposed to align with. If i can attach a picture i will, not sure if it will work, but I’m sure you know what I’m talking about anyways.

Edit:
Forgot to address the battery strength, as it has been only tested and not driven i have been keeping it charged between working on it so it usually is putting out a full 12.6V and is a year old battery.

[Alan H]

Lots of info on here - https://www.oldjapanesebikes.com/index.html .
And timing here - https://www.kettleclinic.co.uk/kcforum/ ... ng#p101046

[jabcb]

The service & owners manuals should help. See: https://www.oldjapanesebikes.com/mraxl_ ... /index.php

[Kevgt550]

I have an old induction timing light from back when cars had points. Use it to see if the plugs are actually firing when installed in a running motor.

Check for voltage drops with the points open & closed: battery + to coil, across coil, coil to points, & points to ground. This may clue you into the problem.

You have a Kokusan ignition. Parts seem to be harder to find than for Nippon Denso. You can swap out the entire assembly for a Nippon Denso.

I upgraded to an Accent ignition on the GT550 cafe build. East to do & highly recommended if you want to go that route.

I finally got around to checking voltages. I’m trying to figure out why i keep getting full battery voltage which is 12.7V currently as it was just charged. I try to get the exact moment where the points are closed but i keep reading 12.7 from battery + to coil or battery to points

[jabcb]

The points are open for 180 degrees. And the points are staggered 120 degrees apart.
So through the 360 degree crank sweep, either 1 or 2 points are open. They are never all closed or all open.

—Voltage drops to measure —
voltage drop in the wiring between the battery + and the coils = should be a small drop

voltage drop across coil primary with points open = should be 0V
voltage drop across coil primary with the points closed = should large voltage drop

voltage drop in wiring between coil & points with points open = should be 0V
voltage drop in wiring between coil & points with points closed = should be a small drop

voltage drop across points with points open = should be a large drop
voltage drop across points with points closed = should be 0V or really small

— to do —
1) make a sketch to record your voltage readings, make two copies of that sketch

2) position crank so that right points are open & other two are closed (this occurs for 60 degrees — anywhere in the 60 degrees is fine)
record all of the voltage drops on a sketch, mark sketch with "right"

3) repeat step 2 for the center points, mark sketch with "center"
4) repeat step 2 for the left points, mark sketch with "left"

— results —
compare the three sketches. this can highlight a problem that is hard to find


Sorry for the PITA explanation. This is fairly easy to do but a PITA to explain.

[Kevgt550]

The points are open for 180 degrees. And the points are staggered 120 degrees apart.
So through the 360 degree crank sweep, either 1 or 2 points are open. They are never all closed or all open.

—Voltage drops to measure —
voltage drop in the wiring between the battery + and the coils = should be a small drop

voltage drop across coil primary with points open = should be 0V
voltage drop across coil primary with the points closed = should large voltage drop

voltage drop in wiring between coil & points with points open = should be 0V
voltage drop in wiring between coil & points with points closed = should be a small drop

voltage drop across points with points open = should be a large drop
voltage drop across points with points closed = should be 0V or really small

— to do —
1) make a sketch to record your voltage readings, make two copies of that sketch

2) position crank so that right points are open & other two are closed (this occurs for 60 degrees — anywhere in the 60 degrees is fine)
record all of the voltage drops on a sketch, mark sketch with "right"

3) repeat step 2 for the center points, mark sketch with "center"
4) repeat step 2 for the left points, mark sketch with "left"

— results —
compare the three sketches. this can highlight a problem that is hard to find


Sorry for the PITA explanation. This is fairly easy to do but a PITA to explain.

Thank you very much for the explanation. It really helps and hopefully helps future members troubleshooting something similar. Thanks again I will give that a go.