Derbi senda heat seizure

[Max]

Hi there all I’ve had a Derbi senda 50 for a couple months now and recently bored it out to 70cc I have changed the carb out and was running fine no issues what’s so ever, I soon then put a new exhaust on from technigas and I moved a single coolant pipe out the way and bike was having cooling issues. I bleeded the cooling system and it was still over heating now I’m not sure what really was the cause of it but I think it may be lack of oil causing lack of lubricant but after 2 days of the fitment of the exhaust the top end blew up. As in the bottom compression ring on the piston is stuck into the bottom of its casing this was caused by a heat seizure.

I need to rebuild it now with another 70cc bore kit any advice of how to avoid this I’m pretty certain it’s not enough oil to the engine as my cooling system is fine, if it is oil is it worth running pre mix instead of injection in the future or can I run injection and pre mix slightly ? 

[Turbodave]

Obviously no one has replied coz no one knows about the Derbi Senda. Is yours a 50 SM for on the road or do you have knobblies on and using it off road?

You still have to run Pistons in Lad! 500 miles or it'll heat sieze. 

I know it's a pain in the butt going slow for even 200 miles but I had to do it every 3 months with my '88 Honda NS125F .

I think those bikes are designed to do 14 laps of a track ,then cool down. On the road it's virtually flat out all the time. So have you taken the electronically controlled carb off and put a manual carb on? 

I don't know about needle sizes or whether you have to go to a bigger size ,but I've heard when you use a bigger size the cold fuel coming in cools the head as well. 

If it's just heat siezed,it will run again once it's cooled and just keep the revs low until it's run in.

 

[Bianco2564]

Welcome to the world of two strokes.

They can be temperamental if not set up right with jetting,ignition, spark plugs exhausts etc.

Did the up 70cc kit people recommend what jet/needle or plug setting to run? Likewise with the exhaust, may need another change.

You say you moved a cooling pipe out the way, is that to clear the exhaust? This will need fixing before you run it up again once its fixed.

I'd go back to a good known running condition, i.e. take the technigas exhaust off ,get it going and run it in, make sure you use a good quality 2 stroke oil, I use Castrol Power. Be careful warming it up, you can get a cold seizure where the piston gets too hot before the barrel gets to temp and it nips up. When I ride my RGV, I religiously warm it up before setting off and the ride for around 10miles before opening it up.

Once it's all running sweet then consider the exhaust again.

I'd stick with autolube, pre mix is a PITA .

 

 

Edited February 2, 2023 by Bianco2564

[Tinkicker]

1.  More power equals more heat.  Has your cooling system got sufficient overhead designed in to cope?

 

2. More capacity equals more gas flow.  You say you changed the carb.  Have you looked at the plug to see what colour it is.  Lean running equals more heat.

 

3.  If your carb is uprated and jetted correctly for your 70cc kit, your autolube pump is not and will be delivering an insufficient oil to fuel ratio.  It is common for people with modded bikes to add a small splash of oil to the fuel everytime they fill up. Helps lubricate the carb and fuel system too.  Not much lubricant on modern fuel.

 

4.  You do not state what oil you are running.  In any tweaked two stroke, I would only ever run fully synthetic oil in it.

 

5.  A thermal ring seizure will cause the piston ring to be affected all the way around the piston.  Next time check the ring gap for proper clearance to ensure insufficient gap is not behind it.  Very suspicious it being the bottom ring.

[Capt Sisko]

6. The big bore kit itself. They're advertised online from under 100 euro. That's for the Barrel, Head, Piston, Rings, Gaskets, the lot! And that's with a retailer making a profit.  - Often things are cheap for a reason.