Hi just in case anyone might be interested...
I recieved a unknown motor from a guy in sweden, as part of a strange deal that we dont kneed to know about!
Some of this engine is from plans. Some from JJ, and so on. But basically for all intents and purposes it may as well be a JJ motor.

Now it appears to be exactly the same as the latest JJ full autostart with a different coloured (gold) front cover and a different (neater) bullet starter.
They did say many parts were sourced in china...

Now.

Like many of you I was having one or two problems with bad combustion, and hot starts. In some cases it wouldnt reach idle. Next time it did but popped and banged a bit as the revs approached idle before it cleared up at 35k. I discovered it ran much better on parafin (and it smells far less too) and this cured some of the issues.

It was usable but ran a bit hot at 730 degrees @ 160k rpm And it throttled slowly.
I checked the fuel ring, on gas, and it looked fine.
The combustion chamber showed uneven heating though. mmm...

Well I have a few engines here to look at, including some home made (shreckling, kj, and even a solent gas turbine starter - 75 shaft hp!) ones.
I decided to try fitting a new combustion chamber. For what its worth the newest ones from wren (removed from my partly home built mw54) to try fits. It does not use those indented 3d holes in the chamber or the angled short air inlet tubes. It runs hugely better and cooler! And definitely makes more thrust. At least as far as I can tell judjung by how much it bends the box lid! (I use the box it came im to store fuel/fadec/battery/pump etc. So now I need to order another one for my home built...

While I was at it I also fitted the FOD screen and starter housing as well after watching a wasp get sucked in! It fits fine on my hybrid so will fit a JJ as well presumably? It is designed to use 6 bolts and slip over the case. Wrens use a longer case with a single row of screws to fasten both front cover and case.

Well because of this you need to
a) drill some extra holes and tap to m2, 2.5 or whatever your motor uses. I changed all my screws to M3 simply because they are easily available.
b) put a smal flat washer under the cover at every bolt hole since we are butting it up to the case rather than over it to take up the difference.
c) the cover is a few mm (5?) further forward than on the Wren, so you need either a bendix not fitted as far on the shaft, or the motor itself case chamfered so it slips in a little further, Or a longer nut! Or it wont engage and turn the motor properly.

I also fitted the tail pipe because the wren one seems thicker at the base and turned. The one I had was presumably either made in sweden or bought from JJ. but it shrunk! Hard to get on and off. So the tail pipe is wren too. Although this makes no difference to performance presumably. It has (because I shortened it by6.5mm an opening of the same size as the original. But a little longer and with a "step" on the inside that is machined in to fit over the ngv outer to keep gas flow smooth.

I am going to buy a wren supersport motor after seeing these parts! I should have photographed the chamber but I dont think they would appreciate their hard work getting copied. So I thought it was a bit unfair to do so.

Anyway here is a plan built mw54 (in sweden) with a wren new combustion chamber, tailpipe, and fod front end. It now makes more noise, definitely more thrust, starts much more consistently and runs at about 670 degrees flat out on a very hot day!

And yes I am using 5 percent oil and its cheapo semi synthetic silkolene from the bike shop.