So I decided to try to sort my high rpm running issue today. Here's the symptom: bike runs like a scalded dog up to 9500 rpm then kind of sputters or cuts out. I have stock airbox and yoshimura shortened exhaust. 40/127.5 jets.

My test today was to run it on prime, thinking maybe it was the petcock or vacuum problem. Same exact results. And since it runs awesome wot up to 9500 I think I have the main jet right, float levels good. Plugs are brown. Getting 45mpg riding like a hooligan.

So, it actually kinda feels like a rev limiter. That makes me think maybe the ignition is cutting out at high rpm. This is outside my area of expertise. How would I diagnose this?

it could be too cold plug, bad sparkplug wire (ive had ignition problems and this fixed it), or actually anywhere between sparkplugs and magneto

OK, so the plug wires are attached to the coils somewhat permanently, are they not? So if it's a bad plug wire, then the replacement program includes the entire coil, correct?

The reason I bring all this up is because I hate taking the tank off... so badly that I don't want to do it just to check things, if I have to do it, I want to have replacement parts on hand and just replace them. These coils/wires are 28+ years old now.

It would seem unlikely that both coils would fail in the same way, so the fact that it simply doesn't want to go faster than 9.5k rpm might indicate some problem upstream of the coils? I guess the bike doesn't completely stop running or buck at 9.5K, it just sputters and stops pulling, so maybe that's just one cylinder running.

I looked in the FSM and besides checking resistances it doesn't show much of a test procedure for the coils. Testing the CDI requires some special equipment, I think mostly something to drive it with a variable rpm signal as if provided by the "signal generator", some sine wave pulse. I could whip up a way to test that easily enough, I mean an audio output through a transformer would do the job.

I just realized that the PO had replaced the CDI with something obviously from a later model bike (much bigger) just before selling the bike. My guess is it had been doing this before and the PO misdiagnosed a coil failure as CDI? Or maybe a coil going bad nuked the original CDI.

Anyway, the coils on this bike are old and they look it. I'm sure the connectors are iffy after all these years. I think I'm inclined to just grab a new set of coils/wires, new plugs, and then replace it rather than fiddling with it.

So on that note, is there some kind of update/upgrade path here? Or some other alternative to the OEM style of individual coils with integrated wires? I see there are dual coils with plug-wire attachment, which I could use with ordinary replaceable plug wires and reduce the number of parts to break. It's just while I have it apart, might as well do it "right".

stock airbox and yoshimura shortened exhaust <- Stock filter and exhaust ??
I'd try 40/125. (unless you already have and went to 127.5)
Over do by 1 jet size and you typically have WOT problems. Or try taking out 1 washer if you have 2 under needle.
K&N in airbox without restrictor needs 127.5 most of the time.
Electricals act up when hotter or colder - Temperature related - not rpm or throttle position related.

Cool.
Buddha.

Stock filter. I already tried 125 jets, same problem. Going to 127.5 was one effort at solving. No washers on the needles. Bike runs better with 127.5 but gets slightly worse mpg, probably is best between 125 and 127.5.

Nearly positive this isn't fuel or mixture bit who knows. Fact that it is at precisely 9.5k even at half throttle is telling.

Oh wait a second, 1/2 throttle 9500 ? does it always crap out at 1/2 throttle ? What if you were in 6th gear ? 1/2 throttle would be closer to 5-6K - see if it does it at the same throttle position. If so, it is likely a tiny tiny rip in 1 diaphragm.

Cool.
Buddha.

TCI has been replaced ? With one from another bike ? what bike ? And did that bike have a rev limiter ?

Cool.
Srinath.

before I got my Triumph, the GS was running fine, revving above 9.5K no sweat. This is a new-ish behavior. I probably haven't put more than 500 miles on the GS in the past year.

I have no idea what CDI is on there, it's what came on it when I got it. But I used to rev it to 11K or more all the time even though it felt like it didn't really make a lot of power above 9K. I don't think there's a rev limiter. I do have the original CDI, I guess I could try it, but the PO put a piece of tape on it and wrote "BAD CDI" ... might be meaningful. Might not. PO did lots wrong with this bike.

It sputters at 9.5K regardless of gear or throttle as far as I can tell. I only tried it on the road, under load. It's too noisy to rev it in neutral in my driveway, but I might try it just to see what happens with no load. I mean, I just rode it around here, 50mph roads and under, only getting it to 9.5k rpm in 1-2-3 gear, and not WOT except in 1st. Remember I am geared down to a 14t front sprocket.

There are no problems with the diaphragms. Carbs are in basically perfect shape. I just had them torn down recently to switch up to the 127.5 jets. Before I did that, it was accelerating "softer" up to 9.5k, actually felt like it stopped pulling hard after about 8K with the 125s and with 127.5 it pulls really hard right up to 9.5k when it sputters.

Rev it in the dark one of these days and see if something is arcing - not even needed to get 9500, likely it just gets worse at 9500, you may see it at 4-5K.
The TCI is the only device that knows rpm in the bike. And it can be fooled if there is an advancer. If this is an emerging problem - likely it will get worse and you will spot it as it gets worse. But My bet is, TCI. Look up part number on the TCI, it may be from the ex500 or something similar and may have a rev limiter, and built for the EX's advancer's dwell which may be different to the GS even if the advancer hasn't been modded.
Cool.
Buddha.

I had similar symptoms once but lower in the RPM range. Turned out that the wire out of my ignition pickup rotor was pinched by my crash bars. So that's something you may want to check. Could be a small fault in the wiring that is currently only manifesting at high RPMs. May get worse over time.

I'm planning on sorting all of this over the weekend, except I'll have to test-ride between rain and thunderstorms.

However, one question, my research indicates that a "resistor" type spark plug can exacerbate this kind of issue, since at high RPMs it makes the coils discharge more slowly and maybe cannot recharge in time to spark at this rpm. I guess a going-bad resistor plug boot could cause this too.

So this weekend I plan to pull the tank, fix my oil leak, check and thoroughly clean all ignition connections, measure the coils (even though I have an idea one may be bad...) and replace the plugs. So I might also consider putting non-resistor plugs in there, even though R type are specified. Any reason not to? Or I might stick with R-type plugs and replace the plug wire boots with non-R type.

I'm just thinking on a 28 year old motorcycle with 28 year old wiring, the chances of corrosion building up in the ignition coil wiring could slow down the charge time enough to cause issues like this, and R-type plugs AND boots can't help. Maybe I'll think about rewiring the ignition power circuit. At this rate I will wind up rewiring the entire bike one wire at a time. Already done about 1/3 of it.

Quote from: mr72 on April 02, 2020, 02:27:34 PM
I'm planning on sorting all of this over the weekend, except I'll have to test-ride between rain and thunderstorms.

However, one question, my research indicates that a "resistor" type spark plug can exacerbate this kind of issue, since at high RPMs it makes the coils discharge more slowly and maybe cannot recharge in time to spark at this rpm. I guess a going-bad resistor plug boot could cause this too.

So this weekend I plan to pull the tank, fix my oil leak, check and thoroughly clean all ignition connections, measure the coils (even though I have an idea one may be bad...) and replace the plugs. So I might also consider putting non-resistor plugs in there, even though R type are specified. Any reason not to? Or I might stick with R-type plugs and replace the plug wire boots with non-R type.

I'm just thinking on a 28 year old motorcycle with 28 year old wiring, the chances of corrosion building up in the ignition coil wiring could slow down the charge time enough to cause issues like this, and R-type plugs AND boots can't help. Maybe I'll think about rewiring the ignition power circuit. At this rate I will wind up rewiring the entire bike one wire at a time. Already done about 1/3 of it.

nice, we got equally old bikes, tho i havent had any problems with ignition, only sparkplug wires have been changed, tho the old ones was fine, just tried to hunt a bogging problem on wot. idk bout resistors, only that without the r ur bike might do magic things to electronic devices near you 😁

The 89 I sold in 05 and the 95 I have now are on their original coils.
They're pretty robust.
Cool.
Buddha.

BTW the CDI that's on my bike now looks like this:


Image searching on the internet seems to confirm this is a standard GS500 part.

The "bad CDI" in my parts box that came with the bike is about 1/2 the size and has a metal heat sink cover with fins, kind of looks like a reg/rect, makes me think maybe it is a reg/rect and the PO mislabeled the part. I have not confirmed it's actually a CDI or that it was ever on the bike. Could be just some random part. Anyway, sort of a meaningless datapoint I guess.

Yea, the stock GS TCI is actually pretty robust. Those metal TCI's if they are Yamaha issue - well the big metal ones don't go, the plastic ones are the high risk ones. Dunno what the small metal one is - kawi ? or something irrelevant completely.
Anyway the gs pretty much hits full advance by 4500rpm. If you have modded the dwell on the advancer it would even hit it as low as 3500 rpm. The width of the nose determines the dwell, the location of the pin to the nose is the static advance. Bob Broussard's mod IMHO would get you 17 degrees static and 3500 for full 40 deg. 9500 rpm is far too high for the TCI to be the culprit. The TCI is the only part that "knows" the rpm. And sometimes we "tell" it the wrong rpm.
Wiring - again how does it know what's 9500 - as rpm increases the electric power availability goes down (but battery is used to fire the thing via TCI - I really question this theory of mine) demand goes up - so that's my theory there, coil probably not - again maybe the supply of spark power doesn't let the coil fire right - 9500 is where your demand curve that is rising and the supply curve that's dropping seem to intersect LOL.
The bike hits max voltage by ~4000 and the R/R keeps it around 14.7, the current peaks at near 5k past which it drops due to the spark power being pulled out of the battery outweighs what's coming in from alternator.

My vote still is on carbs or air/gas flow. Carbs are throttle position driven, but gas flow out of the exhaust is rpm driven.

Cool.
Buddha.

Oh, check if your advancer is sitting up too much or is too far off one of those pick up's. That would predictably lose it at a certain rpm.

Cool.
Buddha.

Pulled the tank this afternoon and checked the connectors to the coil, measured the resistance. Both coils were between 5 and 6 ohms on the primary, and measuring from the secondary "+" lead to the end of the plug wire measured about 29K on both sides.

However, the connectors that were on the wiring harness connecting to the coils were at least somewhat corroded. And one side, the HT connector didn't really fit that tight. More alarming was that on both coils, the HT secondary wire was corroded under the insulation. I chopped off the corroded connectors and stripped and crimped on new ones, which is how I discovered that the HT side had corroded wiring. This makes me think I am going to eventually have to rewire this whole thing. But maybe fresh connectors will get it done close enough to make it run right.

Tomorrow while the tank is off I'll swap the valve cover gasket (it's leaking, had a new one for over a year) and I might even get ambitious and pull the carbs and put the 125 jets back in. Depends on how early I get up and how cold it is. I'm definitely going to put new spark plugs in. I'm hopeful that the new connectors on both sides will fix my problem.

We have thunderstorms and flood warning all weekend. Doubt I'll get to test ride. But I should have some quality garage time since I won't have yard work to do.

New plugs in, decided to stick with 127.5 jets, tested in the garage and it revs freely to 12k. Still too wet for a test ride under load. But I'm optimistic.

Nice. Did you try revving it in the dark to see if you saw sparks.
Cool.
Buddha.

Well the test ride was not a success. Same behavior at 9500rpm in first or second gear. Can't go fast enough on these roads to get it to 9500 in 3rd. But it sputters and just won't rev.

I'm still concerned about the corroded wiring to the coils. Might try replacing it. Might also pull the carbs again but last time just a few miles ago they were mint. However my problem then was at 8500 rpm with the 125 jets and now 9500 with 127.5. So maybe it needs 130s. Or Maybe I need some uni pod filters and 145s.