Dear forum,
I took apart my '91 GS500E this weekend to clean and rejet the carbs. Because I had no 125 pilot jets available, I drilled my 120s out slightly. I also placed #4 washers (one per gas needle) and turned the fuel mixture screws 3 turns out.
The bike starts, idles and runs beautifully, no loss of power whatsoever (in fact, it feels slightly more responsive). The only problem I have is that the bike stutters a lot when applying full throttle and I have not been able to determine the cause so far. The stutter occurs seemingly independent of RPM, just applying full throttle causes it to happen. The stuttering also occurred before taking apart the carbs, though not as pronounced.
- The bike is completely stock, other than the jets and washers
- The air filter and spark plugs are brand new
- I cleaned the carbs, replaced gaskets and float needles
I am going to sync the carbs coming weekend to see if that helps. The only other thing I can imagine is possibly a bad ignition coil, but I would think that would cause a lot more problems.
If anyone has an idea, I'd love to hear it.
Thanks in advance
Get on the freeway, make it stutter as long as you can, pull in the clutch, hit the kill switch, pull over to the side of the road and read the plugs, they will either be dark (rich) or a light color (lean). You might even have a vacuum leak, spray some WD40 around the carbs as it idles...if the Rpm increases as you spray an area, you have a Vacuum leak.
EDIT: change the plugs to rule them out, and it's not the Sync, the farther you get away from an idle, the less carb sync comes into play. Yes it could be ignition/coils as you stated.
You drilled out your 120s? How did you drill them out? These are brass jets, are in very fine increments (thousandths of an inch steps), and need to be incredibly smooth and straight.
You probably ruined your jets.
I'm sure you mean the main jets, too. The pilots are the tiny little guys, I think 40s are the common size, the flanged jets are the mains.
http://www.affordablegokarts.com/Drilling%20Main%20Jets.php
Reference this chart. I don't think it's particularly accurate, but it goes to show that just making the hole a "little" bigger might actually be WAY bigger.
People generally only drill jets to ballpark a size to buy. Not to actually use it.
But since jets are WAY cheaper than a number drill set, and especially with the GS500 where the wiki is charted for jet sizes and requires NO guess work or trial and error, I don't see ANY reason to drill these.
Just buy some new jets.
The main jets only really come into play when there's a lot of airflow, so high RPM and/or wide open throttle, so I'm thinking you just made your jets way too big.
Do as Stevo suggested with reading the plugs. It'll give you a better indication.
Even if it turns out not to be the problem, if I was you I'd be buying proper jets anyway.
But it IS something that was changed before the problem developed, process of elimination and all.
Quote from: Watcher on September 26, 2016, 02:14:37 PM
You drilled out your 120s? How did you drill them out? These are brass jets, are in very fine increments (thousandths of an inch steps), and need to be incredibly smooth and straight.
You probably ruined your jets.
125 jet would be very close to 3/32" (if you don't have metric drills... would be 1.25mm if metric) and one MIGHT have a drill of that size. In a drill press you could easily drill that straight and smooth if you knew what you were doing.
And I doubt most people who own a 3/32" drill don't know how to drill a straight, smooth hole in a brass fitting that already has a nearly identically-sized hole in it to use as a pilot hole.
But anyway, I tend to agree. Jets are pretty cheap. I have not ridden 10 miles on my bike and I have three sets of main jets in addition to the ones in the carbs. I'd say replace the jets too, just to remove doubt.
Quote from: mr72 on September 26, 2016, 02:40:05 PM
125 jet would be very close to 3/32" (if you don't have metric drills... would be 1.25mm if metric) and one MIGHT have a drill of that size.
1.25mm is 0.0492in. 3/32 is 0.0938in or about 2.38mm.
3/32 is about two times too big.
I think most people won't have a drill small enough.
You'd need one from a number set. No longer in fractions, listed in decimals into the ten-thousandths.
But I don't know the OP or what manner of drill set they own or have access to. He might very well be a machinist.
Right now only the OP knows what size drill they used...
@Watcher I meant the main jets, you're right. I left the pilot jets (small ones) untouched.
I used a 1.25mm drill which I borrowed from my local motorcycle shop. As the bike starts, idles and runs just fine (when applying anything but full throttle), I don't think I made a mistake in drilling out the jets. However, should checking for vacuum leaks and replacing the ignition coils fail to solve the problem, I will install new jets to rule it out.
In the meantime, I can still get it up to 170kph indicated (with stock gearing) without any problems. It is only the last bit of throttle that causes problems.. makes me wonder if something weird is happening with the gas cable..
Quote from: Watcher on September 26, 2016, 11:39:32 PM
1.25mm is 0.0492in. 3/32 is 0.0938in or about 2.38mm.
3/32 is about two times too big.
My bad I meant to type 3/64. However, doing the math over, I find that 3/64 probably won't enlarge a 1.20mm main jet. 3/64" is about 1.19mm which would probably clean deposits from a 1.20mm jet but wouldn't open it up any. Next size drill in a commonly available (and reasonably affordable) bit set is going to be 1/16" and that would be more like 1.58mm, way too big. Well maybe not way too big in a GS500 with extremely open exhaust and no intake air filter at all, but way too big for a stock GS500.
Quote
I think most people won't have a drill small enough.
You'd need one from a number set. No longer in fractions, listed in decimals into the ten-thousandths.
I agree, which is why I said if someone has a drill that size, then they probably know how to drill the jet without ruining it.
BTW I think you could also use a hand-operated pin vise, which a hobbyist may have. A set of microminiature bits and pin vise together is like $15. Still cheaper and easier to get the right jets but I think if I already had the tools on hand I may try reaming existing jets, especially if I was trying to tune and wanted to step up in jet size until I knew what would work, then replace with properly machined jets. Speaking of which, I think I'm going to buy these tools for myself.
Anyway, just a different way of approaching the problem. Some folks tinker, a lot, and don't worry much about what others think about it. I personally would have no qualms with drilling jets, and I'm no machinist, just a determined tinkerer.
Update:
Checked the float height and swapped out the jets. It runs much better now, definitely more top end response and maybe slightly more power. It still hesitates slightly when snapping on WOT, but I reckon thats just due to it having carburettors rather than fuel injection.
Turns out that in Mikuni carbs, 125 is not the diameter but rather the flow rate.. oops :icon_rolleyes: