Hawk Rebuild

[Brook]

Good morning my dual sport friends. I have a 2021 Hawk 250 RPS with a soft seize. As I felt it going I applied the clutch and shifted into neutral, which kicked the engine off automatically as the piston stopped moving in the cylinder. The kick start is still capable of moving the piston with only medium effort.


That said:



Am I correct in the notion that replacing the cylinder and piston/rings would be the best course of action?


I plan to re-gear it to comfortably cruise around 60mph with throttle left over (preferably faster if possible). Before the engine seized it would max out at 52-55mph. Would it be worth it to put in a 300cc big bore kit or should I just stick with a stock replacement? Does anyone have a link for something with quick shipping one way or the other?


What gear ratio should I use to achieve this so I don't have to replace the chain? (I literally has less than a hundred miles on this new chain!).


I don't want to go through this again, so this is a pressing question as well. When I bought it second hand there was little to no oil in it and the "oil" that was there was like water. I changed the oil and everything seemed just fine. A couple weeks later, my battery died because I left the light on. I started it with the kick start and let it run for around an hour but adjusted the idle screw slightly to make it idle just high enough to be effective. When I went to shut off the bike it was extremely hot. A couple days and not many miles later the engine failed. Which of these is the most likely culprit in destroying the engine?

As always I appreciate y'all. As a single mom I don't always know where to turn and you folks are amazing <3 !

[65cabriolet]

First off, don't assume it was the piston/cylinder seizing. Make sure of the problem before fixing it.

[Sport Rider]

yeah, step one is to get it apart to see what the real problem is.

[TominMO]

When you were riding it and it seized, were you redlining it for an extended period? Like highway riding with that low gearing?

Idling the air-cooled engine for an hour with no air flow probably didn't do it any good either. I'd say that both these mistakes together did the bike in. Lessons learned. Keep it cool.

[Megadan]

Gearing question. At least a 17 tooth front sprocket, OR a 45 rear. Most of us run both, stock chain is still ok. I personally don't trust the stock chains and they rarely last long. I would do a New chains with the new sprockets.

It is not a good idea to idle an air cooled bike for an hour unless it is very cold outside. From a cold start, no more than 15 to 20 minutes. If already warmed up, no more than 10, 5 if it is hot outside.

Also, do you know if the carburetor is stock or not? Lean jetting can cause a soft seizure by overheating the piston.

[Brook]

Originally Posted by Megadan

Gearing question. At least a 17 tooth front sprocket, OR a 45 rear. Most of us run both, stock chain is still ok. I personally don't trust the stock chains and they rarely last long. I would do a New chains with the new sprockets.

It is not a good idea to idle an air cooled bike for an hour unless it is very cold outside. From a cold start, no more than 15 to 20 minutes. If already warmed up, no more than 10, 5 if it is hot outside.

Also, do you know if the carburetor is stock or not? Lean jetting can cause a soft seizure by overheating the piston.

It's definitely not the stock carb but it works reliably enough. Though the proper idle point seems to jump around 1/8 to 1/4 of a turn here and there depending on temp and weather, so I should probably find something better.

The big questions are is it worth it to rebuild with the 300cc big bore or simply stick with stock 250 replacement jug, piston, and rings? Also do you happen to have any links to the proper stuffs?

[Sport Rider]

FWIW, I think it's worth it to go 300. There are several threads here about doing it, so best to take a look for those.

[Megadan]

Just my opinion on the big nore cylinder. Being "worth it" is a matter of opinion. Will it provide a gain? Absolutely. On it's own without some head porting and/or a cam upgrade, it likely wouldn't be a whole lot. Plenty more torque on the bottom end to take advantage of, but no real top end improvement.

The other real issue being cost. Motocult offers a machined and decked 70mm bore jug ready to install with the piston and new gaskets for $250. If the supplier in Aliexpress ever gets them in stock again, there is a source there that runs about $150 that already has the sleeve machine, ready to bolt on. It's not decked.

A regular bore cylinder can be had on ebay for as little as 60 to 70 bucks.

[Brook]

Originally Posted by Megadan

Just my opinion on the big nore cylinder. Being "worth it" is a matter of opinion. Will it provide a gain? Absolutely. On it's own without some head porting and/or a cam upgrade, it likely wouldn't be a whole lot. Plenty more torque on the bottom end to take advantage of, but no real top end improvement.

The other real issue being cost. Motocult offers a machined and decked 70mm bore jug ready to install with the piston and new gaskets for $250. If the supplier in Aliexpress ever gets them in stock again, there is a source there that runs about $150 that already has the sleeve machine, ready to bolt on. It's not decked.

A regular bore cylinder can be had on ebay for as little as 60 to 70 bucks.

https://www.ebay.com/itm/250cc-Cylin...5-e0bc64292688

Will this one do? its a link from the good ol boys youtube channel but mine is a 2021 so i dont know if these engines have changed at all.

[Bill Hilly]

Originally Posted by Brook

https://www.ebay.com/itm/250cc-Cylin...5-e0bc64292688

Will this one do? its a link from the good ol boys youtube channel but mine is a 2021 so i dont know if these engines have changed at all.

I think that's the stock size bore. The kits can be deceiving. The 300 kits with a 70 mm pistol is actually going to equal 250 cc on these motors, and if you do buy one,it either has to have the outside diameter of the "sleeve" to be at a 73 mm, if I remember right, and it has to have a 16 mm wrist pin.