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Makotosun

More tales of an LT2 resto

  • bdub_lt2
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More tales of an LT2 resto was created by bdub_lt2

Well, at the last post I was wondering why the carburetor for my 1972 LT2 was painted in the first place and, honestly, I'm still wondering.  I've never looked at a carb, even on a naked bike, and thought to myself: "That carb clearly needs some paint on it, and it's a good use of my time to put it there."

I'll probably never know unless I overcome my anti-social tendencies and actually interact with other LT2 owners, as the Chem-Dip ate that stuff right off.

Stepping back, this is the third update where I share the ridiculous tomfoolery that is me trying to fix up an old bike that had been rotting away on my brother-in-law's farm for a long time.  The carb was so nasty that the slide was entirely gummed up and immobile - much like the tubes going into my petcock, which broke off in a solidified mass of oily crud and varnish at the bottom of the fuel tank when I removed the petcock.  An hour in Chem-Dip didn't help.  Four hours in Chem-Dip didn't help.  So out comes the ultrasonic cleaner.

Just barely enough brain cells have survived the initial Chem-Dip fumes for me to realize that I should do this in a glass jar immersed in water rather than heat and ultrasound a whole vat of Chem-Dip, so I find a glass jar that just barely fits the top half of the carb, put some Chem-Dip in there, dropped the whole assembly in, put the lid on, heated it up to 50C, and shook it for 30 minutes.  Once I pulled it out, the slide was easily movable with a little force at first then loosened right up.  Other than that, there was no organic material on the carb that hadn't been eaten by Chem-Dip.

I did the usual blasting with carb cleaner and compressed air and convinced myself that we were good to go. Opened up the carb rebuild kit which thanks to some detailed Ebay reviews I was able to supplement with the right missing pieces, and put everything together.

At this point with my shiny carb, shiny reed assembly, and shiny air cleaner I was starting to feel optimistic about getting this thing to start. I confirmed that I could get spark, pulled the magneto cover and sandpapered the points a bit anyway. Got a 14mm threaded dial gauge holder and - without rigorously checking dwell and exact open time - confirmed that the timing was roughly in the ballpark of where it was supposed to be. And in the meantime, I got my first good look at the piston which was enormously grotty. Whew. Feeling optimistic though so hooked up the plastic aux tank of premix without spilling too much on my garage floor, pushed the bike outside, installed the spark plug, and started getting my right-leg workout for the day.

Ignition! See video below (if it actually attached). First time in at least a decade, probably more, that any internal combustion has happened here.

Now - it won't run for more than about ten seconds or so, no matter what I do and how I adjust air jet and idle settings. I'm confident that I have spark, and I know I have a lot of work to do yet so I'm not too stressed about it - next step is to pull the top end and figure out whether I'm replacing rings, rings and piston, or honing / getting re-bored. As I mentioned above, the piston is grotty as hell and I'm sure the cylinder and ports are no better. Could also still be a number of things in the fuel system despite the carb rebuild. Just too much going on to experiment systematically, I think, until I pull the top end and see what craziness is in there.

That said - I can't resist speculating a little as an engineer - why am I seeing exactly this happening? - so I have two questions for y'all. I'll learn the answers soon enough and will share as I learn.

First - any thoughts from those of you who play with small bikes on whether 125 PSI is really enough? (I have a proper small-engine compression tester with a Shrader valve right down at the adapter, and it reaches 125 after a few kicks.) The shop manual and owners' manual strangely don't mention what the target PSI is, and the compression ratio is a mere 6.9:1 which implies something more like 100 PSI anyway, although I know the calculation isn't as straightforward for two-strokes, right?

Second - guesses on why it will only run for 10sec or so, then take a little while before I can start it again? My top hypotheses are:

- Too-rich mixture craps everything out, or bowl runs out (although it feels like it might take more than 10sec for these to happen...)
- Crap on piston and cylinder causes a bit of a seize every time things heat up even marginally

Next - look at plug after today's fun, and dive into top end!  Will continue to post. Onward.
Last edit: 28 Nov 2023 00:47 by bdub_lt2. Reason: My newlines and CR/LFs didn't work right and everything was run together
28 Nov 2023 00:44 #1

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Replied by Gr8uncleal on topic More tales of an LT2 resto

"Well, at the last post I was wondering why the carburetor for my 1972 LT2 was painted in the first place and, honestly, I'm still wondering.  I've never looked at a carb, even on a naked bike, and thought to myself: "That carb clearly needs some paint on it, and it's a good use of my time to put it there."

I'll probably never know unless I overcome my anti-social tendencies and actually interact with other LT2 owners, as the Chem-Dip ate that stuff right off."


Almost certainly was painted black.

Many Yamaha carbs of that era were and, final confirmation for me, is that the part number for the float bowl ends in "33", which means painted black.

Funnily enough, a Yamaha Technical Bulletin (which I haven't read in full yet) mentions the paint, or method of painting, causing issues on the RD350 carbs! 

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28 Nov 2023 02:19 #2

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Replied by RT325 on topic More tales of an LT2 resto

I'll guess that despite all your carb cleaning the problem still lies in there.
Did you have every jet out--miniature pilot jet, needle jet [emulsion tube] which i think screws out on those.
Take the main jet out of the side holder in the bowl as crud get behind it--that's jet 'out of' the jet holder.
Poke any airways leading to the center from the carb mouth.
Choke jet is a fixed jet down the cavity in the bowl.
Pilot outlet up into the venturi-two outlets one angled forward.
Just stuff like that as can be a pain.
Also most importantly is the Oring that seals the needle jet into the bowl so it sucks from the main jet.
If that was leaking she'd run but just be super rich although likely keep running.
Which is why i think you have a blockage or restriction somewhere causing it to go lean then stop.
I'm guessing you are starting it on choke so it's running on 'that' small mouthful for 10 seconds.
Mind you i can often rubbish my own theories because if choke is working but other things blocked i think it should still run at not much more than idle solely on the choke while choke is in operation.
Talking timing [if i read it right] using a dial indicator won't be accurate on an angled plug hole.
I'll go read again.
28 Nov 2023 17:50 #3

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Replied by darinm on topic More tales of an LT2 resto

125 on a small two stroke of the era is pretty good compression, typically below 100 is when they stop idling.

I doubt it's seizing from the carbon build up but the coil is a suspect, when going bad they love to cut out once warm and return to normal as they cool off.

It takes more than ten seconds for the bowl to run out at idle. On my '72 CT2, when I park it and forget to turn the petcock off half mile from home, it's shocking how long it will idle off the carb bowl with the petcock closed.
1972 Yamaha CT2 175
1972 Suzuki TS185 Sierra
2000 Suzuki RM100
2003 BMW F650GS
2009 Yamaha WR250R
2013 Yamaha XT250
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28 Nov 2023 18:01 #4

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Replied by Ht1kid on topic More tales of an LT2 resto

RT325 maybe on to something with the carb you said carb rebuild kit what kind of kit did you buy?
28 Nov 2023 20:26 #5

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Replied by bdub_lt2 on topic More tales of an LT2 resto

Thanks, all!  The carb kit was the Keyster kit specifically for the LT2, although I put in a 130 main instead of the 120 in the kit.  eBay reviews of the Keyster pointed out that there were a few things missing from Keyster so I ordered from Babbitt's.

I would have thought the carb too, no question, but wouldn't it only be using the idle circuit / pilot jet / choke in this test?  Doesn't rule out a carb problem at all, just narrows it down a bit.

Appreciate the tip on the coil crapping out after a start.  I'll keep an eye out for that.

Plug looked virtually unused after the couple short periods I was able to get it to run - had a little oil on it but not super fouled I think.

Glad to know compression might be ok!  Pulled the top end anyway - it came apart easier than expected - and there's a decent amount of carbon and crud but a little better than expected.  Now I'm going through all that in detail before putting anything back together and trying more starts.

Thanks again!
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28 Nov 2023 21:27 #6

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Replied by Enduro nut on topic More tales of an LT2 resto

A bit of a long shot here but pop the gas cap open just in case of vapor locking. Also double check to make sure your fuel is flowing good from the petcock.
Been a while since i checked my LT but as i recall its got 110 compression and she runs good. I need to clean my tank out better because every once in a great while a piece of crap makes it into the carb and its very easy for something to plug those jets so it could be that too.
02 Dec 2023 19:23 #7

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Replied by bdub_lt2 on topic More tales of an LT2 resto

Thanks, Enduro Nut!

I wish my tank was usable enough to worry about this.  I'm in hanging-plastic-aux-tank land at the moment.

I do think there's probably some crap in the idle circuit of the carb that I didn't manage to get out.  I've got the top end open right now and assuming I don't drop anything into the crankcase that shouldn't be there I'll be done with that later this week (yes, I work at "go out to the garage fifteen minutes at a time between dad duties" speed), and then will go over the carb again before proceeding!
03 Dec 2023 13:13 #8

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Replied by Ht1kid on topic More tales of an LT2 resto

Keyster brassware isn’t calibrated like mikuni brassware what did you use from the kit 
03 Dec 2023 14:11 #9

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Replied by bdub_lt2 on topic More tales of an LT2 resto

Ooh, that's good to know.  In particular, potentially related to this problem I used the #25 pilot jet and the starter plunger from the kit, but they may be completely out of whack with respect to the original Mikuni parts.  I kept the originals and can clean them out completely and try them as well.  Thanks for the tip.
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04 Dec 2023 16:17 #10

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