Yamaha tri-z 250 issue

[Karhu]

Dear everyone,

A year after my rebuild now and I figured I'd take my Tri-z 250 from 1985 for an other spin to adjust the carb and make it run well. Now it just doesn't want to do it......Tried everything from mesuring the ignition, cdi, etc. Ofc a new spark plug. (all turned out well)

Now when I bought this bike the original piston was broken (at the carb side) So i went to the local dealer here in the netherlands and he sold me a original yamaha z250 piston (note about 300$ ***** ) Now this one has a bigger intake for the gasoline.

As I remembered with the old Zundapps back in the days we made these bigger intakes ourselves. But As we did that little to far it didn't want to go in its rpm's at 4/5th gear.......

Now the issue with my trike is the same..... so could it be that this piston just won't work in the tri-z? Or what do you guys think? Id really hope that there is someone who can help me out as im afraid to go to that super expensive dealer here again

Thanks already for your input!!

Cheers,
Piet

[nd4speed]

Did it run well after the rebuild and new piston was installed?

Did it sit for a year since you started it last?

A new piston can make the carb jetting richer.

Letting gasoline sit in the carb for year without draining it can cause problems. The gasoline evaporates leaving the oil in the carb to harden. The the hard pieces of oil get stuck in the jets and holes of the carb (unless maybe your Z is oil injected, but gas can still varnish the carb)

Does the intake filter have too much oil on it?

The piston is the last place you want to go.

Is it the same temperature outside as when it ran well a year ago?

[Karhu]

It had waaaaay less power after the rebuild, it did run in the 4/5th gear tho but without any ''real'' power. So after the rebuild I figured that that would be just a fine tuning job...

No I cleaned the carburetor checked all and filled it up with fresh gas...

I googled these two pistons so don't check for the numbers given on the picture. But the original came out and the other one went in... could it be that that one opens up the jets earlier? I never took measurement as the dealer advised me that it would work... ( me)

I've also played around with the gas needle but don't really give me back the power..

[nd4speed]

The new piston rings will seal better and create more suction which will pull more gas and cause the mixture to be rich go down a size or two on the main jet, you can access the main jet with out taking the carb off usually by removing the bowl cap and using socket.

usually you want the needle to be at a richer setting (lower like 3-4 notches from top) because most riding done in that range and not where you want a lean condition.

main jet is brass at the top of carb leftside of picture.

I have never owned a Tri Z

Do you mix your gas and oil?

[Karhu]

The gas oil ratio is 1/34, after putting full throttle and break till it die, the spark plug was ''nice brown'' bit rich but nothing shocking imho. Always mixes the gas before I go driving.

Okay, well thats a affordable try, I will check it tomorrow as its now 11.15pm here. So You'd say, pick up one/two sizes lower then the current one and give it an other try?

I think its not a specific TRI-Z issue so any two stroke knowledge will do the job. Starting with the main jets seems like a cool strategy to start with. So I'll keep you updated how it goes.

Edit; it also won't start easily (like 4 kicks and have to have the throttle half open)

[nd4speed]

That mix sounds fine, I dont what carb you have but yeah I would start with the main jet.

[350for350]

Maybe this is dumb question, but is the piston the right size? if you put in a piston that's too small, you won't have much compression.

[onformula1]

Before you try anything do a compression test.

[Queef Chief]

The gas oil ratio is 1/34, after putting full throttle and break till it die, the spark plug was ''nice brown'' bit rich but nothing shocking imho. Always mixes the gas before I go driving.

Okay, well thats a affordable try, I will check it tomorrow as its now 11.15pm here. So You'd say, pick up one/two sizes lower then the current one and give it an other try?

I think its not a specific TRI-Z issue so any two stroke knowledge will do the job. Starting with the main jets seems like a cool strategy to start with. So I'll keep you updated how it goes.

Edit; it also won't start easily (like 4 kicks and have to have the throttle half open)

Your Tri-Z looks great!

[Karhu]

One good point of the dealer is that he was able to measure which piston should go into the trike, in fact I do know the whole history of the bike of the moment it was sold in 1987. It hadn't driven much about 20hours now with me owning it for about a year like 21. lol

So the compression is like as it came out of the factory in 1985 so no worries there. I don't have a fancy compression tester but im very sure it has enough as i can't start it on my normal sneakers lol

Bit of on update on the main jet: it has a 470 in it, will go to the dealer tomorrow to buy a few smaller ones. Hope that is it so i won't need to buy a new piston. Eventhough I found out they are quite affordable on eBay usa.

Thanks for the help guys! Keep y'all updated

[dave ytz250s]

Hi Karhu, did you check your exhaust? A leak at the cylinder head or a clogged silencer can cause problems like this.

[Karhu]

@dave thats a rather interesting thought, with the rebuild I decarbonized the exhaust but the silencer was drained with 2stroke oil. Now perhaps you know this, but with the exhaust of the Tri-z I only have been able to take the back end out and lighted that up and it burned for a while, is there a better way to do this or can you get the inside of the silencer totally out (like in my regular dirtbike)?

Also the sealing at the cylinder has been renewed so that should be fine..

[nd4speed]

I always forget about the plugged exhaust problem. It would be a good idea to make sure silencer is not clogged. I have seen people use small propane torch to burn inside of OEM silencers. It takes some time and they just let it sit and burn for awhile.

[dave ytz250s]

I hate the oem silencer, unfortunately I need one to be legal in the trail system here. There is a section you can remove behind the rear cap, you need to cut the welds with a cold chisel or sharpened flat screwdriver and be careful not to bend the inner tube too much because there's a tab welded inside and you need to break it by pulling and pushing that tube with some locking pliers. It does help, but still doesn't compare to my answer silencer, if you want to remove the rest I think you need to cut it open and reweld, I just can't jet it right with the oem silencer, but the pipe is fine. Honestly my favorite setup is oem pipe with aftermarket silencer, if you want more high end get a pipe too but you might lose alot of your low end grunt.

EDIT: if you try to remove the rear part, keep in mind it's not risk free, you can ruin your silencer.

[Karhu]

Hi Guys, So im able to give you'll an update on this matter: tested the smaller main jet and felt like it got worse. So I decided to take the whole engine apart and would just measured the piston. As it turns out its just the wrong piston, I mean too small intake so no wonder why it wouldnt give me any power right. So I guess im going to order a new piston this week and give it an other try. Its still the original size from 1985, so this cylinder has long time left

Thanks for all your suggestions, and im sure I'll update you'll when I've got everything back together!