Yuminashi 164cc keeps over heating

[Hobbs469]

So I have put about 500 miles on my 164 kit in my 2011 pcx 125 and it's starting to have some issues. If I run for wide open for longer than a minute or so it will start pushing coolant into the overflow. If I ride slow it won't have an issue just when I am being heavy on the throttle. I have the 1.6 cap and everything else could this be possible head gasket issue???

[BPT7594]

Sounds like you are too lean on WOT.
Are you the guy who milled his head down a bit? That can cause increased compression. Yuminashi kits already come with very very very high compression ratios already. However, I have a guy running the same kit without problems and he squeezed 20000km or 12500 miles on it before it finally gave up. To think about it, he oil starved it so cannot blame the engine but in his case, no overheating on WOT, at least not that I know of, but I doubt it because he always comes to me with his engines.
Did you use the recommended torque specs on the 4 nuts? Should be 28Nm or 20 foot pounds. Make sure to tighten them diagonally and kinda slowly too.

[lolofigo]

So I have put about 500 miles on my 164 kit in my 2011 pcx 125 and it's starting to have some issues. If I run for wide open for longer than a minute or so it will start pushing coolant into the overflow. If I ride slow it won't have an issue just when I am being heavy on the throttle. I have the 1.6 cap and everything else could this be possible head gasket issue???

I have overheating problems on WOT with 164 kit. I think head gasket doesn't match 125 head cooling ports. I'm going to put 150 head conversion. I've done 4000 miles with the kit and it's been Ok . Had to cut down that plastic covering radiator cap to allow coolant top ups.

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[Hobbs469]

Sounds like you are too lean on WOT.
Are you the guy who milled his head down a bit? That can cause increased compression. Yuminashi kits already come with very very very high compression ratios already. However, I have a guy running the same kit without problems and he squeezed 20000km or 12500 miles on it before it finally gave up. To think about it, he oil starved it so cannot blame the engine but in his case, no overheating on WOT, at least not that I know of, but I doubt it because he always comes to me with his engines.
Did you use the recommended torque specs on the 4 nuts? Should be 28Nm or 20 foot pounds. Make sure to tighten them diagonally and kinda slowly too.

Yes i did torque down to their specs. And I did mill my head down it was warped. I took about 6 thousandths off of it. It doesn't necessarily over heat while it is in the high rpms it actually pushes the coolant into the overflow​ at high rpms and then wants to overheat because their is none in the radiator

[Pcxdemon]

If you're using the 125 with big bore cylinders,its advisable to port out the water holes on the head to match the gasket ports for the cylinder. A quick dremel work is required for it to be done. That'll allow a maximum water flow and cooling effect the way its supposed to be. Using a 150 head has no such issues. It is what Takegawa is actually advising when selling their 170 kits for 125 heads but no one ever went that far to modify the head and prevent the overheating issues...
Also, for a extra safety regarding a gasket seal, there is a copper gasket spray on the market that you could use to spray on the gasket for that more tighter seal. Works quite well and its not messy...There is a youtube vid of one motorbike mechanics using it on a two stroke water cooled bike thats has chronic gasket issues...

[Hobbs469]

If you're using the 125 with big bore cylinders,its advisable to port out the water holes on the head to match the gasket ports for the cylinder. A quick dremel work is required for it to be done. That'll allow a maximum water flow and cooling effect the way its supposed to be. Using a 150 head has no such issues. It is what Takegawa is actually advising when selling their 170 kits for 125 heads but no one ever went that far to modify the head and prevent the overheating issues...
Also, for a extra safety regarding a gasket seal, there is a copper gasket spray on the market that you could use to spray on the gasket for that more tighter seal. Works quite well and its not messy...There is a youtube vid of one motorbike mechanics using it on a two stroke water cooled bike thats has chronic gasket issues...

This is the first I have heard of dremling out the water passages
I did use copper spray gasket on the head gasket though. So do I need to buy the 150 head conversion or just port mine out which is better ???

[Pcxdemon]

going 150 head will give you more power and torque too so id incline and say 150.. but you may have to adjust the fueling litte bit. Dremeling water ports on 125 head is the cheapest option.

[chicaboo]

Since you're talking costs - don't you also need a 150 exhaust to suit the exhaust port location for the 150 head?

[lolofigo]

going 150 head will give you more power and torque too so id incline and say 150.. but you may have to adjust the fueling litte bit. Dremeling water ports on 125 head is the cheapest option.

I was thinking about some dremel work on mine. But if I have to go through all the hassle of removing plastics and taking the engine apart I might put 150 head conversion. Accidentally I bought esp takegawa exhaust from Japan that won't fit my old head. Decisions decisions.

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[Pcxdemon]

Good choice for Takegawa exhaust. They are good performance pipes. Looks like you have already made the decision..

[BPT7594]

If you do go with the 150 head make sure to buy a proper 150 head gasket as well. Also check for proper valve and piston clearance to make sure they don't touch. Can't be too careful.

[lolofigo]

If you do go with the 150 head make sure to buy a proper 150 head gasket as well. Also check for proper valve and piston clearance to make sure they don't touch. Can't be too careful.

There is a yuminashi kit. 150 esp conversion with coolant hoses, domed piston with cut outs for bigger valves and even larger exhaust gasket. I think it comes with proper head gasket as well.

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[Pcxdemon]

Domed piston makes a good power with 150 Yuminashi head coupled with 31mm tb, cam and Takegawa exhaust. Probably same or slightly more than I'm making now with my current set up...

[Hobbs469]

Hmmmm decisions decisions has anyone ever used the GPI radiators ??? Also do I have to buy a new cam or will the 125 cam work in the 150 head???

[Pcxdemon]

This is how Yuminashi 164cc domed piston with 150 head goes...

https://youtu.be/k3Rv7UwGAgs

[MadSteamer]

I just bought Yuminashi 164cc Power Pack with gear seat. I will be installing it in the next few weeks so I will keep you update if it overheats or not. I think that the powerpack is designed to bypass overheating issues? It doesn't require 150 head though.

[BPT7594]

The 125 stock head is pretty prone to overheating because the gasket meat is very thin when you go 60mm. That's why yuminashi sells copper gasket, because last time I check they make their cylinder's coolant holes matching with the coolant holes in the 125 head. Takegawa and Charmo relocate the coolant holes totally but they will mismatch as a result.
If you are upgrading from a 125 be very sure to torque it down to spec slowly, carefully bleed out the system and get the 1.8 cap or something like that. I'd personally go for a 164 with 150 head, never have any overheating issues with 150 head even on a bigger 62mm piston, let alone a 60mm one.

[Hobbs469]

So is the 1.6 psi radiator cap the biggest they make I am still pushing coolant into my overflow when running high rpm