Other 1980 Yamaha TT250

Serf27

Serf27

Hey everyone, new to the forum here and stumped on this bike.
I had the carb soda blasted today, cleaned out all the ports, new pilot jet and main jet and some new O-rings and it still won't idle without the choke

About the bike
1980 TT250, old but rides well.
aftermarket pod filter.
Stock main jet is 165, I put a new 170 main jet. From 1/4 throttle to WOT it runs great.
Stock pilot jet is 17.5, I put a new pilot jet size 20, recommended by the dirtbike shop.

With the choke on/open the bike will start right up most of the time and will idle on its own cold and warm with the he choke on.
As soon as I take the choke off it starts to stumble and dies.
I can take off fine with the choke on.
The choke closes by itself when riding and continues riding fine. When I start slowing down to stop, I need to put the choke on so it will idle and not die when I stop.

The choke is an enrichment choke I believe, because of this, does it run and idle with the choke on because it gets more fuel? Would I get a bigger pilot jet to solve this?
Thanks.
 
Serf27

Serf27

Thanks for the reply.
I just messed with the pilot screw and I went all the way out to 3-4 turns out from it being seated and still nothing.
Still will only idle with the choke on/open.

I put a new gasket between the carb boot and the head, the carb boot itself is in good condition too. Could bad gaskets on the carb cause a vacuum leak?
 
2Wheels

2Wheels

Air leaks could cause that condition yes. Try adjusting the idle speed up while the choke is pulled and then close the choke and see if it drops down or just dies again
 
Serf27

Serf27

Trfsrfr:
To test for vacuum leak you can spray around the carb manifold area with carb cleaner while its running, if the engine rpms change you know you have a leak.
Click to expand...
Thanks, I always forget to try this. I just tried it, sprayed all around the carb, all around the carb boot and head and no idle change.

2Wheels:
Air leaks could cause that condition yes. Try adjusting the idle speed up while the choke is pulled and then close the choke and see if it drops down or just dies again
Click to expand...

Thanks for the reply.
The idle adjuster for the throttle is almost all the way up. Even with the adjuster holding the throttle as open as it can, it won't idle with the choke off.
On the side of the carb on the engine side there was a screw and a washer just plugging the carb, I put a small gasket there to ensure no leaks. On top of the carb where the throttle needle goes, I changed that gasket.
All new gaskets, new o-rings, except the float bowl gasket have been changed.

I cleaned all the passages again and still won't idle with the choke off.
I turned the idle air/fuel screw all the way out to I think 4 turns or something and nothing.

What is the big oval passage for? I spray cleaner through it and can't seem to find where it's coming from.
When I spray small passage next to it, it comes out from the main jet and pilo jet.


Here is the carb, super clean, new jets and all. A clean stream of cleaner comes out from inside the carb when I spray through the idle air/fuel screw passage. When I spray through the pilot jet passage, a clean stream of cleaner comes out of 2 holes inside the carb.




The next thing I can think of is getting a bigger pilot jet.
 
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2Wheels

2Wheels

does your pilot screw have an o-ring? With the idle adj cranked all the way + choke there must be air getting in somewhere, or your pilot isn't clear. But you said its new so.. factory spec?
 
Serf27

Serf27

Yea the pilot screw has a spring, washer, the o ring.
I'll try the carb spray test again. Other than that, I have no idea where the leak could be.

Yea the pilot jet is new, size 20.
Factory size is a 17.5 jet.
The closest size I could find was a 20, and the shop said it would be fine with it.
 
Trfsrfr

Trfsrfr

Are you sure the size 20 pilot jet is accurate?
I could and might be wrong, but that seems pretty small to me. Maybe some of the others here could offer some insight...
I have an XR 200 and I'm pretty sure the pilot is at least a 38 or so, and I would think a bigger bike would require a bigger one than what mine is.
Also, do you have a factory manual for this bike?
If not you need to get one. I bet someone here probably has a link to a downloadable pdf.
M33, Palmer, and Big Dan have supplied me with links in the past.

Also - I know you said it runs, but if you haven't done a compression test I recommend that too, and now would be a good time, just to see where you're at.
Also - what is the make and model of that carb?
 
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Trfsrfr

Trfsrfr

2Wheels:
adding fuel with the choke evens things out
Click to expand...
Right, so it sounds like it isn't getting enough fuel.
Serf27:
I cleaned all the passages again
Click to expand...
Did you blow them out with compressed air?

I see in your pics that you have the throttle shaft removed. Is it possible you're sucking air from the shaft seals as well?
The carb looks plenty clean to run to me unless you have a passage blocked that needs compressed air.
But my guess is that you need more gas at idle...
 
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Redordead89

Redordead89

What is all the black crap in the sump of the carb? How many times have you cleaned your carb? cos it looks pretty dirty all over to me!
Carb sump gasket looks pretty pancaked so start there and CLEAN THE CARB!
The big oval passage is where your fuel and air mix and I'm guessing by you not knowing this that you haven't cleaned your carb properly:poke::P
Merry Xmas btw:smirk:
 
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Mihylo33

Mihylo33

Serf27:
Yea the pilot screw has a spring, washer, the o ring.
I'll try the carb spray test again. Other than that, I have no idea where the leak could be.

Yea the pilot jet is new, size 20.
Factory size is a 17.5 jet.
The closest size I could find was a 20, and the shop said it would be fine with it.
Click to expand...


Clean the carb twice and then once more for good luck. Set the float. Make sure your floats are set properly. If it still doesn't run do a compression test.
 
Serf27

Serf27

Trfsrfr:
Are you sure the size 20 pilot jet is accurate?
I could and might be wrong, but that seems pretty small to me. Maybe some of the others here could offer some insight...
I have an XR 200 and I'm pretty sure the pilot is at least a 38 or so, and I would think a bigger bike would require a bigger one than what mine is.
Also, do you have a factory manual for this bike?
If not you need to get one. I bet someone here probably has a link to a downloadable pdf.
M33, Palmer, and Big Dan have supplied me with links in the past.

Also - I know you said it runs, but if you haven't done a compression test I recommend that too, and now would be a good time, just to see where you're at.
Also - what is the make and model of that carb?

Right, so it sounds like it isn't getting enough fuel.

Did you blow them out with compressed air?

I see in your pics that you have the throttle shaft removed. Is it possible you're sucking air from the shaft seals as well?
The carb looks plenty clean to run to me unless you have a passage blocked that needs compressed air.
But my guess is that you need more gas at idle...
Click to expand...

Yea I'm sure the pilot jet is a size 20. I'm also sure the stock pilot jet size is a 17.5. I'll post a picture of the new pilot jet numbers when I can.
I think the pilot jet is still too small because of the pod filter. I will need to get a manual, I have tried to find it online, but can't find any.
I have blow out the passages about 5 times, cleaned the carb about 3 times and fully disassembled it 2 times to clean it.
I will do a compression check this week.
You may be right, I may be getting air by the shaft seal, I will focus on that area more when I spray carb cleaner around it.
I'm not sure of the carb model, I'm 99% sure it's the stock carb. I know the make is Mikuni and it is a 30mm slide carb.
Thank you!:ride:

Redordead89:
What is all the black crap in the sump of the carb? How many times have you cleaned your carb? cos it looks pretty dirty all over to me!
Carb sump gasket looks pretty pancaked so start there and CLEAN THE CARB!
The big oval passage is where your fuel and air mix and I'm guessing by you not knowing this that you haven't cleaned your carb properly:poke::P
Merry Xmas btw:smirk:
Click to expand...

I have no idea what the black stuff is down there. The soda blaster at the shop couldn't clean it, unless they half assed it and didn't try to clean it that well.
I am going to clean it once again and mostly try to clean out the oval passage. Every other single passage shoots out a good stream of carb cleaner when sprayed through.nthe oval passage I can't figure out where it leads too.
Thank you, and Merry Christmas to you too!:hug:

Mihylo33:
Clean the carb twice and then once more for good luck. Set the float. Make sure your floats are set properly. If it still doesn't run do a compression test.
Click to expand...
I haven't messed with the floats, when the carb is upside down, they hit the carb body. The shop said as long as it didn't over flow they were good..?
I'll clean the carb once again and blow air through each port again. I'll try to get pictures of the streams that shoot out to show they are nice and clear.
Thanks for the help.:ride:
 
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Redordead89

Redordead89

Hey:wave: the little black bits are build up and can be picked off so get a small flat driver or similar and be gentle!
The jets of carb cleaner aren't enough to remove well stuck crud so really you have to dismantle the carb to its component parts and clean by hand any dirt the spray can't!
And before you ever open up a carb always make sure the outside is very very clean to avoid further contamination! Block both sides of the big oval passage with a clean dust free cloth or kitchen roll and start cleaning with a small nylon toothbrush type brush and go to a brass brush if necessary! Then open it up and carry on cleaning, if you have limited experience doing this then please invest in a manual for your bike!
 
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