T500 - No/Weak Spark at Left Cylinder

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DaytonDave
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T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

My T500 has been misfiring on it's left cylinder. I have narrowed the problem to electrical - getting little or no spark at the left cylinder. Because of this, I replaced both points and condensers and reset the timing. I have a brand new fully charged Yuasa 12N7-4A battery. I replaced the spark plugs with new B77HCs. The bike runs strongly on the right cylinder with occasional firing of the left cylinder. The cylinder temperatures after a brief running are 250F on the right and 170F on the left. Laying the plugs on the cylinder head test gives me a whitish orange spark on the right and no visible spark on the left cylinder. Input voltage at the coils with ignition on and not running reads left cylinder coil (problem side) is 10.38v to .32v while running through crank rotation. Right cylinder coil (the cylinder that fires ok) reads 9.9v to 0.0v for same test. Separating the points while ignition on does not generate any spark between the points for either side (new condensers working it seems). The original coils are ST217. I suspected a bad coil. I had several 3402 coils in my parts bin. I subbed a 3402 for the left cyl coil - still no noticeable spark on cylinder head test. I tested all the coils. Two failed (One original ST217 coil for the misfiring left cylinder failed testing specs and one of my used 3402 spares failed. I currently have the other used 3492 that passed installed for the left cylinder - still no spark. In all, I tried 3 different used coils (with their associated power leads) for the left side with no improvement. As an aside, I noticed that with the ignition on, and the bike not running, both coils warm up on the ends with the power leads (left cylinder coil reads 132F, right cylinder coil reads 118F) - don't know if this is normal? I checked the harness connection plugs and cleaned them with contact cleaner they looked okay, no corrosion. I'm kinda running out of ideas and appreciate suggestions. Is it an indicator of something that the offending side coil input does not go quite fully to zero voltage in the cycle? Rectifier? Regulator? Bad new condenser and bad old condenser too? Am I mistaken that I can't sub a 3402 coil for a ST217 coil? Other? Thanks.
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

I should have mentioned that the points are set at .013" and the plugs are gapped at .018".
I also mis-typed 3492 for 3402 in one spot.
thanks again
Last edited by DaytonDave on Fri Jul 01, 2022 1:08 pm, edited 1 time in total.
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

Problem Solved!
Many thanks to jabcb for his excellent points diagram that was posted on Jun 07, 2020. I had saved and printed it for my notes and it caused me to re-check my setup. Sure enough, I had the new condenser wired to the wrong side of the insulator on one set of the points. My tired old eyes are not what they used to be. Anyway, fixed it and now have good spark for both cylinders! Very happy indeed. :up: :clap: :clap: :lol:
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Alan H
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Re: T500 - No/Weak Spark at Left Cylinder

Post by Alan H »

The voltage to the coils is low.
Check the main fuse for 'tiredness' or just replace it, and fuse clips for tension. I've replaced the original fuse carriers on all my bikes with modern blade type which helps.
It might be an idea to go through the harness connections and see where the volt loss is - you should have at least 12.5v at the battery, so clip the negative test lead to anywhere on the engine and red lead to battery + to prove the battery earth connection and then 'move' the red test lead to fuse, ignition switch, kill switch and coils. I always use non resistor plug caps for none electronic ignition too.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

Thanks Alan. Good ideas, and I will pursue them. :up:
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jabcb
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Re: T500 - No/Weak Spark at Left Cylinder

Post by jabcb »

The T500 has a pretty small battery. The nominal voltage of 12.5 or so is at rest. Voltage will drop under the load of supplying the coils when the bike isn’t running. Plus it’s normal for the wiring to have a 1 to 1-1/2 volt drop between the battery & coils.

So it’s not surprising that you measured ~10V at the coil when the bike isn’t running. You can improve the #s a bit by going through the switches & electrical connectors. But if it’s running fine then …

On Alan’s suggestions…
No telling how old the fuse is if you haven’t replaced it. A 40 year old fuse is working on borrowed time.
My 69 T350 broke down a few years ago because of fuse holder problems.
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

Thanks for the suggestions jabcb. The fuse is the factory original - so it's 54 years old. I got my money's worth on that one! :wink:

I believe the electrical is pretty good now, but am still ironing out some roughness/mis-firing below 4,000 rpm which I think is fuel related. May have to take the left carb off again and recheck/re-clean the jets and passages. I gave them a good ultrasonic cleaning with a Simple Green solution and brushed and blew out all the passages but might have missed some gunk somewhere.

It's all coming along nicely though and I am very pleased. It seems to be improving also with more use.
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Re: T500 - No/Weak Spark at Left Cylinder

Post by Alan H »

Misfiring is rarely carburation. Low voltage at the coil/points, bad points faces or dry points cam/points heel/condenser starting to break down would be more likely. On my 550J, I had a bad misfire above 'about' 3500 rpm and after testing/changing everything timing/ignition wise, found the issue to be a wrong points cam. Link here - viewtopic.php?f=2&t=12751&hilit=Yeti&start=15
Just a thought..... 8)
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

Alan,
Thanks - thinking you are right. I am noticing something odd is occurring with the electrical system. After running the bike at idle for about 15 minutes while trying to sort out the misfire, the battery voltage was down 38% from fully charged, rather than fully charged. Seems like the bike is only running on the battery for ignition and not charging? And, the engine is mostly running on the right cylinder only, with only occasionally the left cylinder firing. However, when riding and accelerating at around 4K+ rpm, the left cylinder kicks in strongly all of a sudden, but below that rpm it only fires occasionally. I am wondering if it is a charging issue or a regulating issue and the spark is only coming from the battery at low rpm until/unless the revs are high and then the ignition is getting a higher voltage (and better spark) from the alternator/rectifier/regulator circuit.
I am still making my way through checking and cleaning all connections and I have improved a few. I would rather not change out the rectifier and regulator to a modern unit, but wondering if that will be needed. I trimmed the plug wire 1/4" and the coil tests good. The points and condensers and plugs are new.
Recommendations are welcome. Perplexed.
Dave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by jabcb »

The twins have a tendency to run on one cylinder when the voltage is too low because of a bad battery or other issue.

The electrical specs are on pages 15 — 18 of: https://ozebook.com/compendium/suzi/suzidata1.htm
Fully charge the battery.
Then check battery voltage with bike off.
Then check battery voltage with lights off at idle, 2k rpm & 4k rpm. Repeat test with lights on.
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

jabcb,
Thanks for the additional information and the link. I will be doing testing and measurements accordingly.
Dave
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

jabcb,

As suggested, I fully charged the battery over nite, ran some tests and got the following data:
12.77 v @ battery fully charged, on the bench
12.76 v @ battery installed
12.14 v with ignition on, but not running
11.94 v with lights (and ignition) on, engine not running
12.34 v with ignition and lights off, after the above test cycle
So, .43 volts were consumed by the ignition being hot for less than a minute and lights being on for about 10 seconds

With engine running (mostly on one cylinder) and the lights off:
5.21 v @ 2,100 rpm
4.46-5.0 v @ 1,500 rpm (voltage varied)
5.61 v @ 4,000 rpm
3.95 v @ 1,200 rpm

Then, after shutting the engine off after about 4-5 minutes of total running time:
3.35 v at battery at rest

Left cylinder temp = 104F, Right cylinder temp = 174F

Amazing to me that the battery at the completion of these tests had lost a charge of (12.77 - 3.35) = 9.42v drop !! :shock:

I was pretty shocked with these results and forgot to get running readings with the lights on. I assume that the results would have been even worse though.

As a side note, even with the battery at 12.34 v at the onset of the running tests, the left cylinder was still missing.

Thanks for any thoughts?
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Re: T500 - No/Weak Spark at Left Cylinder

Post by jabcb »

“Not running” numbers are fine. “Running” numbers are bad.
No point in worrying about the ignition at this time — could be perfectly fine after the voltage issues are resolved.

What all have you done to the wiring?
My first guesses are that (1) the alternator+rectifier are wired up incorrectly, (2) the rectifier is shot.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
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DaytonDave
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Re: T500 - No/Weak Spark at Left Cylinder

Post by DaytonDave »

Thanks, glad the non running numbers seem okay. The rectifier and regulator wiring are undisturbed from the factory except that I took apart the bullet connectors one by one and cleaned and connected them. If the rectifier is shot, I am guessing I would need to install a Rex's solid state unit. I hate to go away from the factory stuff, but do want to make the bike to run properly. Thanks again.
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Re: T500 - No/Weak Spark at Left Cylinder

Post by karl pa »

A few years ago the regulator went bad on my GT500, I was getting around 5 volts while running. Ended up burning out my charging coils till I found it.
I know you said you would like to keep stock charging, but the original regulator system on the twins was not very good, you will get as high as 16-17 volts, at that voltage you will be replacing lights quite often, if you have stock headlight it will be expensive and hard to find. And you can not use modern battery.
I changed to modern regulator/rectifier on all my twins, but I ride them often and want more trouble free.
To check voltage regulator, you can just unplug it and check voltage while running, the regulator only dumps excess voltage to ground, and yours could be dumping anything over 5 volts and overworking the charging system. I would keep lights off while testing this way, although some twins didn't even have regulators.
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