T500 bucking when throttle closed?

[Ivan]

Anyone who needs to reduce the pilot jet or lean the mixture at small throttle openings on a T500 has worn out needle jets, or they are the wrong size.

Excessive lean at part throttle is this bike's signature.......


Ivan

[Vintageman]

Fritz,

Your additional info is helpful

Changing pilot smaller is probably wrong (not root cause issue)

Check you needle jet and jet needle. You can't get 188 series, but you can get 159 series Jet needles brand new shinny smooth

You can put in the round main(Tz375 would smack me for saying that) or just do it perfect and cross the round jet to a hex for the 159 is a hex.

Is your T500 early or late?

I don't think late jet needle is available new if yourd is worn (call Paul Miller with part number).

Early, The Keyster kits have the needle for the early style. (looks fine to me)

Ivan may have something soon?

Pop and Fart?

That sounds like it could be ignition too and you reduce fuel making it ignite easier
Since yours is a T500 not GT check those points for pits. Even if you just changed them

[Fritz500]

Vintageman,
My T500 is a 73 model that had done 10,600 miles when I bought it. I put in new main jets (correct Mikuni ones) when I first got it so there are no issues with them. The needles are original and I can see no wear on them. Float heights are spot on. It has new points and condensers and the timing has been checked with a dial gauge and a timing light.

It runs really well on part and full throttle. It is now possible to ride at low speeds and have very little, or no jerkiness.

I do most of the riding at 40 to 50mph (I avoid the freeway - speed limit is 60mph - as I hate the vibration) - at those speeds it runs very nicely. I'm not sure, but it does seem to be smoking less too. I'll take a blast down the freeway soon and see how it looks - it used to smoke a fair amount at 55 to 60mph.

I really wanted it to run well at 20 to 50mph and I seem to have gotten there.

For higher speed riding I take my Yamaha TRX850 - now there is a great bike! Wish I'd bought one years ago.

[Vintageman]

Fritz500,

OK,

But you said it

Pop and Fart. That sounded like you are describing miss fire. I know hard to use words.
If it really goes bang out exhaust it is most likely ignition miss fire.

Smoking.

Do you run a JASO FD rated oil. It really is smokeless and why it gets FD rating. I am not embarrassed anymore. My bikes may not smoke, but they sure do smell bad (un- burnt fuel the MO of pure MPG 2 strokes) if you follow it. I pull over for them

I assume your pump setting s spot on. And that is two stroke oil you see/smell

Good news, my 75 T500 is like your 73. Both later style. My bike runs good all throttle positions and all speeds so should yours

Can you relate your slow running problems more to a throttle position versus road speed?
That is helpful to know.

I agree with Ivan,
Needle Jet is worn and INSIDE maybe if not outside. You need to hold it to a bright light and look inside barrel with magnifying glass. It shoot be oh so smooth and shiny inside and round at the exist.
Also verify it is the correct size Also make sure air jet is free in carb body.

Make sure correct needle jet. If someone put a Keytser kit it would have the needle for an early one.

I hope it proves easy for you to find the issue

What year is the TRX850

[Fritz500]

When I first got the bike it would barely run. Wouldn't rev past about 3500RPM and smoked really badly. I thought the air filter looked suspect so I made one out of some 19mm foam from some sheet I bought from my local dealer...it made things worse. I put some thinner foam and it improved things. I then bought a standard foam filter and bingo it ran much better.

Oil pump settings were slightly out when I got it. Also the pump was jamming open for some reason. Did a rebuild and now no more issues. I put on a set of throttle cables from a 74 model as these have the 45 degree metal tube bends atop the carbs. This vastly improved the throttle feel (way lighter too) and I was able to set everything up more precisely.

Needle and needle jet are not an issue as previously explained.

Initially before changing jets etc running the bike with the air bleed turned out about 2 1/4 to 2 1/2 turns made the bike run way better so I figured there was a problem with the pilot jets. I first thought the previous owner and fiddled with them. I couldn't see any evidence of that. The move to #25 from #30 has worked. Like I said I want to go for a cold evening ride and test it...still too warm here though.

My TRX is a mid 1999 build. I understand they have an issue with the needles wearing the jets. Apparently this shows up as a worsening fuel consumption. It gets around 55 MPG now so I'll keep and eye on it. It has only 4000 miles on it so it's as fresh as a daisy. Right now it is such a great motor that it feels like one of my old Ducatis. So smooth above 3000, it is unbelievable.

[ConnerVT]

Initially before changing jets etc running the bike with the air bleed turned out about 2 1/4 to 2 1/2 turns made the bike run way better so I figured there was a problem with the pilot jets. I first thought the previous owner and fiddled with them. I couldn't see any evidence of that. The move to #25 from #30 has worked. Like I said I want to go for a cold evening ride and test it...still too warm here though.

All that you wrote makes sense except this. The #25 Pilot Jet should be much to lean. Then add the Pilot Air Screw was out 2+ turns (further out = more air) would have it running even leaner. It shouldn't been running at all.

A few thoughts come to mind:
-- The Pilot Jet that was in the bike was cleaned by reaming out the holes, making them bigger;
-- The original Pilot Jet was not a Mikuni, and did not meet correct specifications.
-- the original Pilot Jet was a Mikuni OEM Pilot Jet, which fits, but is a different design and runs way too rich (I've made this mistake).
-- There is still a partial blockage in the air passage for the Pilot circuit. (Both carbs?)

If the new #30 jets solve it, and your screws are now 1-1.5 turns out, then one of the first three were your issue.

Best of luck!

[Vintageman]

Fritz500

A corroded needle jet versus one worn from miles of uses from Needle wearing against has slightly different symptoms,
Old bike like these with low miles may not have a wear issue. What does happen and very likely since the needle jet and needle is near the exposed air in our atmosphere is many humidity temperature cycling, The humidity and Oxygen eroded the inside of the barrel, That cause extra surface restriction to fuel flow and poor air atomization with air jet.
I always change this part today for its cheap if you can get it, the jet needle corroded too, but some seem to be a better material and more resilient

The symptoms of corroded needle is subtle, The bike just does run as perfect as it can. It takes very little to make this true. I am not sure what happens if it is really potted inside. I have seen them like that but never ran it

It may not be your issue, but hope I make sense in case you do see it someday.

I suppose you need to figure out why it smokes if still heavy maybe it too (you bring it up as a concern),

You did not state if you ran JASO FD rated oil?

You know what to do and have more advice than needed.

If you do remove carbs again question everything so you ( OK us pessimists) are 105% sure.

Please let us know what you learn. You will figure it out. Bike is not a lemon!

That 850 does look like a Ducati. Sweet.
I would love a Ducati too. Don’t see many where I am.

[Vintageman]

Fritz500

A corroded needle jet versus one worn from miles of uses from Needle wearing against has slightly different symptoms,
Old bike like these with low miles may not have a wear issue. What does happen and very likely since the needle jet and needle is near the exposed air in our atmosphere is many humidity temperature cycling, The humidity and Oxygen eroded the inside of the barrel, That cause extra surface restriction to fuel flow and poor air atomization with air jet.
I always change this part today for its cheap if you can get it, the jet needle corroded too, but some seem to be a better material and more resilient

The symptoms of corroded needle is subtle, The bike just does run as perfect as it can. It takes very little to make this true. I am not sure what happens if it is really potted inside. I have seen them like that but never ran it

It may not be your issue, but hope I make sense in case you do see it someday.

I suppose you need to figure out why it smokes if still heavy maybe it too (you bring it up as a concern),

You did not state if you ran JASO FD rated oil?

You know what to do and have more advice than needed.

If you do remove carbs again question everything so you ( OK us pessimists) are 105% sure.

Please let us know what you learn. You will figure it out. Bike is not a lemon!

That 850 does look like a Ducati. Sweet.
I would love a Ducati too. Don’t see many where I am.

[Fritz500]

Initially before changing jets etc running the bike with the air bleed turned out about 2 1/4 to 2 1/2 turns made the bike run way better so I figured there was a problem with the pilot jets. I first thought the previous owner and fiddled with them. I couldn't see any evidence of that. The move to #25 from #30 has worked. Like I said I want to go for a cold evening ride and test it...still too warm here though.

All that you wrote makes sense except this. The #25 Pilot Jet should be much to lean. Then add the Pilot Air Screw was out 2+ turns (further out = more air) would have it running even leaner. It shouldn't been running at all.

Best of luck!

Thanks for the input guys.

Maybe I didn't make some of my posts clear enough.

When I said I turned the air bleed screw out to 2 1/4 to 2 1/2 turns I did that to get it to run as I couldn't find anything else wrong. That to me confirmed it was running too rich (for whatever reason). When I installed the #25 jets I turned the air bleeds back to 1 1/4 turns and it now runs fine.

This bike was in dry storage (Texas) for most of its life so there is very little corrosion etc. Absolutely no corrosion issues internally to the carbs.

The two stroke oil I'm using is Valvoline "2 stroke - racing oil" rated as JASO FC. I was using Silkolene two stroke oil which had a rating of JASO FB - I think the switch to Valvoline has reduced smoking a fair bit.

Cheers

[Fritz500]

After a few months of distractions and holidays I decide to try and tune the T500 carbs again!!

I noticed that after about 20 to 30 minutes of riding the bike started to sound like it was running slightly lean on low throttle openings.

I checked all the usual stuff (again!) and then I spent more time fiddling with jets. I changed the pilots from #25 to #27.5 and bingo! It no longer runs lean no matter how long I run it. However it still had a tendency for chain snatch on a trailing throttle. I screwed out the air bleeds each an extra 1/4 turn and the chain snatch all but disappeared. I'm happy to ride it around at any speed in any gear.

I must admit I never fully appreciated how sensitive it was to fine adjustments.

[Vintageman]

Great bike when it runs perfect! Street riders really need all throttle position to work well else just not enjoyable as it can be.
But, if like my T500 wait until the temperature changes... you'll then know how sensitive it is.
I have mine tuned spot on all throttle positions/rpm for above 80F. After, I road it on a cool day this past fall 60F and was too lean in certain spots so just put it away versus trying to retune or avoid those spots.

I also find, since the T500 having a long swing arm and relatively small rear sprocket is sensitive to any issues with these items being worn or loose versus just blaming it all on low speed jetting.