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The Vintage Japanese Motorcycle Owners Group: Discussion Forums

Vintage Japanese Motorcycle Owners Group :: View topic - 1974 Kawasaki G4TR 100 resistor spark plug cap?


1974 Kawasaki G4TR 100 resistor spark plug cap?

 
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CavemanNed
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PostPosted: Fri Jul 14, 2017 2:19 pm    Post subject: 1974 Kawasaki G4TR 100 resistor spark plug cap? Reply with quote

Recently bought a 74-75 Kawasaki trail boss. I installed new points and condenser and had the timing dialed in. I installed a new spark plug and the spark seems very strong. The coil only has a metal clip at the end of the wire to connect to the plug and no rubber boot. I was told that since I didn't install a resistor spark plug I would need a resistor cap so as not to burn out the other ignition components. Does any one know if this is correct? I don't know what cap originally came on the bike but pretty sure I have the correct plug.
The bike does not run well past half throttle and dies after a few minutes of riding. This may be due the a fuel issue but I'd like to rule out any ignition issues
Any suggestions?
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spurlock
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PostPosted: Sun Jul 16, 2017 6:26 am    Post subject: Reply with quote

I can't imagine how running without a resistor cap would cause any harm to other ignition components. Anyway, the cap resistance is a tiny fraction of the resistance of the spark plug gap.

-Bill

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1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
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CavemanNed
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PostPosted: Sun Jul 16, 2017 7:06 am    Post subject: Reply with quote

Bill- Thanks for your comments. I installed the resistor cap and the bike would hardly start- removed it and it started on the first kick.

Let me pose another question to you... The bike will start on the first kick and rev just fine but when I take off it runs well until about 3/4 throttle where it will cut out just a little bit and by the time I've gone about 70 yards the motor starts to bog. I can keep the motor running if I back off the throttle and shift to a higher gear then I can nurse it back home. Let the bike sit for 10 minutes and you can do this all over again.
It sounds like fuel starvation to me. Here's what I've done so far:
-Rebuilt the carb, it was very clean to begin with - no residue or dirt.
-disassembled the petcock and cleaned- was very clean to begin with, even the screens looked like new.
-verified that gas was flowing through the petcock and fuel line.
- The gas cap is not correct so I tried running the bike with the cap removed and it made no difference.
- The floats were set at the recommended 28mm so I tried adjusting to 25mm and it didn't make any difference.
The tank is exceptionally clean.
I tried running the bike with the side cover off thinking there might be an airflow restriction but there was no difference.

Could it be that it's not fuel related and maybe the coil is wearing out?

Am I missing any diagnostics? THX!
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spurlock
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PostPosted: Sun Jul 16, 2017 7:35 am    Post subject: Reply with quote

I agree it really does sound like fuel starvation. If you haven't done it already, with a drain can under the float bowl drain, open the float bowl drain screw and the petcock and make sure fuel flows out the drain at a steady rate. If not then there is some restriction in the carb float valve. If fuel does flow steadily then that pretty much eliminates fuel starvation.

Then you are left with ignition or (hopefully not!) piston seizure. If the engine bogs down as though putting on the brakes, but does not sputter or missfire, then I'd suspect seizure. Is this bike premix or oil injection? If premix confirm correct oil/gas mixture or in the case of oil injection check oil pump adjustment and function, clear oil lines, and bleed air from system. Crack open an oil line at the engine end and watch for oil flow.

If the engine missfires when it loses power then that might be the ignition coil developing high resistance when it warms up. Here is a post describing a makeshift coil tester that allows testing for an extended time which can show if there is a problem that only happens when the coil heats up:

Makeshift coil tester

Good luck!

-Bill

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1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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CavemanNed
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PostPosted: Sun Jul 16, 2017 1:46 pm    Post subject: Reply with quote

Kawasaki does not provide a bowl drain plug on this carb so I removed the bowl and opened the petcock. The gas flowed easily and shut off when I lifted the floats. I have the oil injector disconnected and I'm using quality gas mixed 32:1 with Yamalube 2stroke oil.
I don't think it's a matter of piston seizure because it does not feel like I'm applying the brakes and the engine does cough and spit a little bit as the RPM drops.
I will check out your link to the coil tester and see if I can figure it out.
I did a visual inspection of the coil and see a crack in the casing about 5/8'' long with a gap about as wide as a thread- does that indicate a problem?

It looks like the only available replacement coils are from overseas... do you have experience with any of these or can you recommend a particular vendor that I might have the best luck with?

Thanks for your help Bill
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spurlock
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PostPosted: Sun Jul 16, 2017 1:59 pm    Post subject: Reply with quote

I'm not sure that using premix is equivalent to the oil injection system. Maybe it is, I'm not familiar with where the oil is injected in this engine. If just into the intake then premix might be fine. Hopefully someone else here will chime in on that.

The fact that it was hard to start with a resistor plug cap compared to non resistor might indicate a weak coil. Not sure about the crack in the coil but you would normally not expect to see that. Probably any repro coil from ebay would be OK, but could take a while to arrive from Indonesia. Got any friends with similar bikes that can loan you their coil to swap in for a test?

-Bill

_________________
1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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CavemanNed
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PostPosted: Sun Jul 16, 2017 4:02 pm    Post subject: Reply with quote

I hadn't thought about the resistor cap possibly revealing a coil issue... When I installed the resistor cap the spark became weak and orange-ish, it even sparked to the side of the plug rather than to the electrode. When I removed the cap and reconnected the plug with the metal connector the spark was once again strong and blue-ish.
I have a replacement coil for a Honda CT90- do you think it would work for this application if I can get it to hook up?
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spurlock
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PostPosted: Sun Jul 16, 2017 5:36 pm    Post subject: Reply with quote

CavemanNed wrote:
I hadn't thought about the resistor cap possibly revealing a coil issue... When I installed the resistor cap the spark became weak and orange-ish, it even sparked to the side of the plug rather than to the electrode. When I removed the cap and reconnected the plug with the metal connector the spark was once again strong and blue-ish.
I have a replacement coil for a Honda CT90- do you think it would work for this application if I can get it to hook up?


I'm not sure about the CT90 coil but there is no harm in trying it. As I recall (and it's been 38 years since I worked on CT90's) they were 6V DC ignition and I assume your bike has the flywheel magnet/primary coil powering the ignition coil. No harm in hooking it up for a test, but it may not be conclusive if it does not work.

-Bill

_________________
1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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kbts250m
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PostPosted: Fri Jul 21, 2017 10:54 am    Post subject: Reply with quote

I agree it sounds like fuel starvation but could also be a plugged exhaust. Does the baffle come out so you can try without? 32/1 is a lot of oil and could leave some goo in the exhaust eventually. You would be fine with 50/1. Question, is the oil injection pump and lines still on the bike?
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dorT500
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PostPosted: Fri Jul 21, 2017 11:03 am    Post subject: Reply with quote

I don't think I could keep myself from taking the exhaust off and at least have a peek at the piston. Hmm
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dorT500
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PostPosted: Fri Jul 21, 2017 11:06 am    Post subject: Reply with quote

kbts250m wrote:
....... You would be fine with 50/1. Question, is the oil injection pump and lines still on the bike?
Good question.
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GONE.......WITH A PUFF OF SMOKE AND A BLUR OF SPOKE
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CavemanNed
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PostPosted: Fri Aug 04, 2017 4:31 pm    Post subject: Reply with quote

Sorry it's taken this long to reply but I've been waiting for the new coil to arrive from Indonesia... I installed the new coil ( came with a cap), the spark looks good and the bike started on the first kick. I took off on the bike and it seemed to be running smooth( I only took it to about 3/4 throttle) I went about 100 yards and the bike bogged, spit coughed and died. I pulled the plug and it was wet. I checked for spark and there was none. I sprayed the plug with carb cleaner and dried with a paper towel. I rechecked the spark and it looked good again.
The original spark plug was an NGK B7HZ which has been replaced by the B7HS- supposedly the B7HZ had a fatter electrode. I theorized that maybe a hotter plug might help so went the next step hotter to a B6HS. It didn't make any difference...

In reply to the previous question- the bike does have the oil injection unit but it wasn't working. It may just be a matter of bleeding the air out. I went with 32:1 because that's what I read online somewhere but I don't know much about 2 strokes. I have removed the exhaust and the baffle as well. It all seemed pretty clear.
Maybe it's the fuel??
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