After all I have done, the left plug on my RD350 is still wet and oily. It is better but isn't right. The right looks real good.
Is it even possible that the left could pick up oil from the transmission? I am out of ideas what else it could be........again.
It's possible; it would have to pull oil through the mating surfaces of the case. A leak test oughtta show it.
That possibility is all I can think of after all I have done to track this down.
How would I go about a leak test on that cylinder?
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very very very unlikely. back to the ignition
If it was the right, then definitely. Sounds like timing/spark might be off. Definitely check that all before jumping into jetting. Been there too many times before.
Quote from: RDDave on August 13, 2025, 02:00:18 PMHow would I go about a leak test on that cylinder?
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The usual vacuum/pressure test of a two-stroke cylinder as described elsewhere on this forum. And as Mark pointed out, it's not likely in this situation, just possible.
only way it would be if it was leaking thru the case. if it was coming through the crank then it would share the same issue left/right. (since the lab seal isn't air tight in the crank anyway)
check the coil. recently we went thru a bad sv coil, (probably due to it being left on too long w out running and cooking it). Resistance from the plug lead w the cap off to the terminals was around 18k ohms as 'ok', 8-9 k was bad. :twocents:
Yeah, I have switched a pair of the new Suzuki style coils back and forth along with their wires and plug heads more than once.
This problem preceded my ownership of this bike and is why it has been passed around.
I am out of ideas and feel that I have done EVERYTHING I can think of that is unique to that cylinder, including using everything from the right on it. I have a couple more things to try tomorrow.
Thanks guys, I really appreciate your patience with me on this.
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I bet the issue lies within your ignition system. Your symptoms are of incomplete combustion within your LH side cylinder in conjunction with your pilot circuit of your carb. There are many factors that can contribute to this condition, one of them being dwell, improper or worn horseshoe arm of the CB point, worn cam, type of plug cap etc....also check to see if there is any leakage via the LH side cylinder head plug hole. A hairline crack on the threads can cause mild leakage/compression drop.
double check your ground for the coil on that cylinder, also connections to the coil all the way back to battery source. does not take much resistance to reduce amperage available for the coil. Use a multi meter to check, be patient and thorough. It is the cumulative combination of multiple connections that can really reduce current draw to a component.
you dont need to ground a coil like that just fyi. not on a points system.
what was the charging voltage? its either the harness or the charging system.
Agree with Mark here.
Quote from: 350GUY on August 13, 2025, 09:24:51 PMI bet the issue lies within your ignition system. Your symptoms are of incomplete combustion within your LH side cylinder in conjunction with your pilot circuit of your carb. There are many factors that can contribute to this condition, one of them being dwell, improper or worn horseshoe arm of the CB point, worn cam, type of plug cap etc....also check to see if there is any leakage via the LH side cylinder head plug hole. A hairline crack on the threads can cause mild leakage/compression drop.
Not likely to be the points cam since that would affect both sides. He's already ruled out the points and caps since those have been swapped several times and the issue remains on the left.
I have the coil's positive on a relay direct from the battery. It has 12+v. (The bike runs the same with the lights on or with them off.) And I just disconnected the harness from the points for the left cylinder and jumpered direct from the points to the coil.....again.
Remember that this is after I have swapped the coils along with their wires and heads side to side more than once.
I also went ahead and drained the gear case and refilled with 2 stroke engine oil. I also put a new Iridium plug in the left cylinder.
The left cylinder runs now. I pulled the plug; it looks better, not as soaked, but still wet around the rim. So, I checked the resistance on the points wire to the coil and since it was negligible, I removed the jumper and reconnected the harness wire. Then took it for a 10 minute ride.
The left cylinder still runs good but the plug is still a bit wet around the rim like it was with the harness out of the picture. The right cylinder has a very good, clean burn.
I am going to do a few adjustments to the carbs and ride it some more. The left plug looks much better now that it did. Not soaked and fouled, just wet around the rim.
I guess I better hope it isn't the engine oil in the crank case that is what is impacting the burn.
who split the cases last? I've seen 1 or 2 cases in many decades of either the wrong sealant or a gouge allowing a leak. the BEST way I have found, is to use some fuel scent (cheap on amazon) and add it to the transfer pil. I prefer strawberry but grape is potent. IF the exhaust suddenly smells like that, you have ab incursion. also.. what is the charging g voltage? I don't care what the battery tests at statically. yes, it matters.
Quote from: m in sc on August 14, 2025, 01:50:57 PMwho split the cases last? I've seen 1 or 2 cases in many decades of either the wrong sealant or a gouge allowing a leak. the BEST way I have found, is to use some fuel scent (cheap on amazon) and add it to the trans oil. I prefer strawberry but grape is potent. IF the exhaust suddenly smells like that, you have an incursion. also.. what is the charging g voltage? I don't care what the battery tests at statically. yes, it matters.
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I don't know the history on this bike. It shows 5,000 miles, the cylinders were still on the stock bore with the stock rings until I threw on a set of the Chinese cylinders chasing this issue.
I have ridden the bike about an hour now and the left cylinder is still running strong. With the 2 stroke oil in the gear case, the left plug looks much better, still wet as you can see, but much better.
sparkplugs.jpg
I like the scent test to further help define the issue. We used to use that back when we street raced our cars.
As to the charging, I can check that again. But it maintains the battery at voltage with the lights and a turn signal on. And all of my riding has been with the lights on.
I will drive the bike as much as my time allows the next few days and see how it does.
By the way, does that right plug look too lean? The main jet is stock. I have the needle raised up 1 notch because it looked a little lean to me. Everything is stock except for Bassanis.
Thanks for all the help. The bike may be far from fixed, but at least I can ride it enough to nail this down.
right plug looks ok. pic below is what my 72 w the same cyls (which doesn't matter) looks like. different ignition though
Rode it off and on all afternoon. In town stop and go and on the hiway at 60+.
So far, so good. I ordered some strawberry scent. It won't land until next week.
The generator puts out over 12v at idle and hits 20v by 3,000.
While I REALLY want to know for sure what the issue is and right now it looks like it's sucking gear oil on that cylinder, I don't know whether I hope the scent confirms that or not given the final fix.
Anyway, thanks AGAIN for all the help.
Is the gear case going to be happy for a while with engine oil in there rather than gear oil? I guess I could put gear oil back in and see if it starts fouling that plug again. I would rather wait for the strawberry scent. It's just a matter of how much I ride it between now and then. I would like to put some time on it to see if anything changes.
Again, thank you!
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your regulator is shot if its going over 14v. the trans is specced at 10w30 or 10w40. I run Castrol synthetic motorcycle oil in all my transmissions. they aren't specced for gear oil.
Quote from: RDDave on August 14, 2025, 01:10:25 PMI also went ahead and drained the gear case and refilled with 2 stroke engine oil.
:eek: Not a good idea! Two-stroke oil is not designed to be used in a transmission. Stop riding it with that oil in the trans. I would highly recommend you drain the trans immediately and refill with a proper motorcycle-spec engine oil designed for wet clutches. I use BelRay Gear Saver 75w (which is equivalent to a 10w30). There are many options out there.
Quote from: RDDave on August 14, 2025, 06:00:00 PMThe generator puts out over 12v at idle and hits 20v by 3,000.
Quote from: m in sc on August 14, 2025, 06:13:47 PMyour regulator is shot if its going over 14v.
+1, time for a new regulator. 20v is crazy high!
To be honest, I think I was testing the direct output of the generator rather than the 'regulated' power. If 20+ volts were getting to the entire system, I think I would have seen some results of that by now.
The 2 stroke oil in the gear case was recommended to me by an old (very old) Yamaha mechanic who has seen this issue before. The strawberry scent will be here next week. Had I gotten that recommendation first, I would not have gone with the 2 stroke oil. But, it does seem to have had an impact. So, I will leave the 2 stroke oil in until then to test with it since it seems that the plug is not fouling with it. Adding the strawberry scent is the most definitive test.
I won't put much in the way of miles on the bike with the 2 stroke oil in the gear case. But it's starting to look like it will need to come apart anyway and I have several spare engines, so it's not that big of a deal in this case. But I appreciate the opinion just the same.
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I think the old yamaha 2s or r was transmission compatible. chuck q told me that but I don't remember which one.
its an alternator. where were you exactly checking the voltage? what wire? it needs to be checked, running, at the battery terminals at 2k.
It was the solid red wire that is in the same bundle with the 3 white wires.
With the lights on and a turn signal on, the battery holds at 12+ volts. And I have rode it a lot with the lights on. No problems and the battery is always over 12 volts when I spot check it.
Is there something more to be concerned about given that?
And again, when I first started on this journey, the coils were only getting about 10.5 volts. I wired them directly to the battery through a relay and consequently now have over 12 volts to the coils. That additional 2 volts has not changed anything with the left cylinder.
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the fact it was seeing 10.5 prior tells the the charging system has an issue or the harness as a whole is reducing voltage to an an alarming level and it hasnt been diagnosed. .
you still haven't answered the question on what its doing at 2k at the battery & i'm not sure why. either way approach it how you want, but its not right how it is. The non relayed power is controlled by the output of the regulator, including power tot he relay (depending how you wired it). I get powering it that way, my KZ has its dyna and coils powered trough a relay in a similar fashion.
There may be a parasitic partial short or draw at rpm you arent catching. Ive seen that before as well.
fwiw in the tech library there is a post i made about using a ford regulator to replace the stock regulator. its an easy simple cheap upgrade/repair.
if you don't smell fuel scent in the exhaust from the trans whenever it shows up, (and i doubt you will), but if you don't, then you're right back here. If you do, then its case splitting time. even then, i still would question the charging system. you can run these for a decent amount of time on a battery, basically total loss. this can be and is common, when 1 diode out of the 3 takes a dump in the rectifier. this would NOT be caught in your alternator output test (as an fyi).
A dodgy electrical system is a very, very common issue with these bikes, especially at this age. Heck, it was 20 yrs ago tbh. as has been said many times in the past: most jetting issues are electrical.
I hope you find your smoking gun. If NONE of that works, than something's off in the carb physically, somehow. Ive seen float level, a worn jet tube, a bad pilot jet etc cause this issue as well.
I will try tomorrow to run the test in the service manual for the 2,500 RPM test. The only reason I haven't went further with that is because every time I check voltage anywhere it is above 12.4 volts. With lights on or not. But, if you think something that could be amiss with that situation, I will run that test tomorrow.
I have several rectifiers and regulators so I can test run them if there is any doubt. Of course, the Ford replacement is good option, too.
And just to be clear, I have removed the bike's harness from the ignition circuit and jumpered directly to the coils. And swapped ALL ignition parts from one side to the other. Of course, that still depends on the bike's charging system.
I have also swapped a different carb on the left cylinder. The only thing I haven't done is to swap the carbs side to side.
Thank you again for not giving up on this/me. I am determined to find the cause. With your help.
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Just throwing another check if your electrical gremlin just isn't. What's your float height at in both carbs?
I had wet plugs before when the float on the left carb was set too deep for fuel.
^ yup. we discussed that recently. amazing what a mm or 2 can do
My service manual says to check the voltage at the red wire coming out of the rectifier. I only have a red/white wire. The wire I checked before the battery had 13 volts at idle and 20+ by 2,500. The battery has 12.4 with the engine off, with the engine idling, with the engine at 2,000 to 3,000 for several seconds, with the lights on or with the lights off.
Those are the readings that I have always gotten. Is there something I am not testing correctly? As I said; I have other regulators and rectifiers. They just look to a bit of a pain to get at. If you think I should try other components, I will. But what other tests on what wires can I run?
I have had the carb apart several times and have even changed it out for the only other good one I had which was a right carb. But, it still ran the same. I can pull the left carb again and set the float lower.
I will be gone all day Monday. I will wait until I get the 'strawberry scent and have a chance to ruin it a bit before I do anything else at this point. If the exhaust smells of strawberries, that is all I need to know. And if it doesn't.....................
Thanks again. And I will check back on Tuesday or so.
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check the voltage at the battery, when its running, at 2000 rpm.
12.4 volts at the battery terminal.
With lights on, with lights off.
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that makes zero sense. that completely contradicts Ohms law.
Something here isn't adding up.
Educate me on what you mean. I don't understand. The voltage at the battery is 12.4 volts at 2,000 RPM. I just don't get what you are trying to tell me.
The battery and the coils are getting a steady 12.4 volts. Actually up from the previous 10+ volts. And holds that voltage with the additional load of the lights.
Please explain to me what I don't understand. This is not an argument, I truly do not understand why the right cylinder ALWAYS runs just fine with anything between 10 and 12+ bolts while the left will not. Even with the right's components and separated from the bike's wiring harness.
Please take this as a plea and not as an argument.
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by flipping the headlight on, and off, you'll see at least some change, usually .2 ish v. even if its an led headlight and gauge light, etc. also, it should be closer to 13v.
regardless,
if this has been a recurring issue since before you got the bike, then it needs a complete go thru for diagnostic to be reliable. its something simple. since you say the carbs were COMPLETELY stripped and cleaned (all passages clear), that you've bypassed the wiring (did you run it straight from the points or at the alternator junction?) . you say the coils were swapped. were the plug caps swapped out w new as well?
I find it highly unlikely its a case leak, but its possible.
I've owned over 20 of these, I've never had one stubbornly drag a cyl unless one of the things I mentioned up top was off.
check your point ts wiring atthe points. make sure they are insulated from the base correctly, and that the flags aren't close enough to arc over to the alternator housing.
I have seen a bad rectifier cause a similar issue, breaks down at speed and the weird thing is, at just low enough voltage, one coil will drop before the other.
thats all I got. if you're anywhere near Charlotte nc I'd be glad to look at it you one Saturday or Sunday afternoon.
The voltage may change a bit when the lights are switched on, but it recovers very quickly. Does that address the question?
I totally disconnected the bike's harness from the ignition. I unhooked the wire at the points and ran a jumper wire directly to the coil. And that was with the points from the right cylinder. The coils are getting 12+ volts through a relay again with the bike's harness disconnected. I swapped the coils with their wires and heads. I swapped the original coils and I ordered new 'Suzuki' coils and have swapped them with their wires and heads, too.
Now, I do have a question on wiring these 'Suzuki' coils; Which lead is the positive? The one in line with the plug lead, or the other one? I have wired it both ways. I know there is a correct way, but it ran the same which ever way I wired them.
The previous owner sold this bike for this reason. He didn't say anything about it, but when I got home with the bike and went through the items that came with it, there were about 20 fouled plugs.
I would dearly LOVE for you to look at this thing. I am clearly overlooking SOMETHING! No doubt.
I have other rectifiers. I can change one out, I guess if you think that is worth the effort. And I can pull the carb and change the float setting and even swap the floats and needle and seat from the right just in case. I would just swap carbs, but the oil line on the right just won't reach.
I agree that 12.4 is a little on the low side. It always hold right in that area no matter how I check or with what meter.
It is time for a fresh set of eyes on this. I just don't have ANYONE anywhere near me that knows a darned thing about any of this stuff anymore. The old guy that suggested the injection oil in the gear case is just not able to offer any physical help.
I am going to stay at it. I really, really want to find the issue. And for more than one reason.
Thanks again for you help and you patience. It will be a couple of days before I can get back on it, but I will stay on it.
the smaller terminal on the sv coil will be the - side.
Check your timing between cylinders my guest is the right side is more advanced than the left. I had a similar problem years ago and it turned out to be the timing was not synchronized between cylinders. The specs are pretty liberal on this but I have found the closer you can get them to be timed exactly the same the better it will run and less likely to foul a plug. That said the point plates like to move as you tighten them down so it can take some time to get it just right. Set your left side timing and the match the right side to it as closely as you can get it. Just my :twocents:
I use a dwell meter to set the points so that I KNOW they are the same; 45* dwell on both sets of points which I came to after a little experimenting with the feeler gauges. That is on the 4 cylinder setting on my old automotive box. It also tests the points; I have filed and cleaned the points to get the lowest resistance I can get and swapped them between the cylinders.
I set the timing with a timing light. And I have tried a few different settings around the recommended timing.
I will check the wiring on the coil.
Thanks!
its good to be that picky about it. i also understand the value of a dwell reading, especially with aftermarket points. (yes i actually have and use a dwell meter myself ) Modern replacement points... the heels are always too long. if you set the gap and its at the max of the range, sand the heel down some .
But to be honest, i've seen these things run exceptionally well with timing specs pretty far out of whack, surprisingly so. its actually amazing what they can tolerate spec wise. I do follow the logic of get that base right, then diagnose. so theres no harm there.
:thumbs: