I’m the proud owner of a Fantic 240 which is displaying a detonation sound. I’ve already noted the post in December 2019, which covered this topic but did not provide a cure. Although the member did say what he’d done had reduced it.

Just wondering if anyone has found a cure for it since that last post.

To set the scene, the engine has had a full rebuild, new piston, rebore, cylinder coated, all bearings and seals. It sounds wonderful apart from the occasional detonation sound. The carb is a new Dellorto, jetted as per the manual. It has a brand new front pipe and WES rear silencer. I’ve just put a new NGK plug in her and some super unleaded fuel at 70:1 mix with fully synthetic Shell 2stroke oil. I’ve got an NGK iridium plug on order. Starts almost first time every time so the timing must be right.

Any thoughts would be appreciated.

Edited March 6, 20215 yr by chappo

Maybe try richening the slide needle up one notch and see what happens after that.

Thanks Bullylover, I’ll try that, but please guys keep the advice coming, someone must have solved this niggle somewhere surely.

Interesting link here might be of some help

http://www.tomdonneymotors.com/2019/06/11/2-stroke-detonation/

I only use gasoline with high octane ARAL Ultimate with 102 ROZ, I believe BP Ultimate has the same properties. 

This will help at first if the engine gets hot and in general. 

Next tipp: Do you have changed the compression using a too thin head gasket or is the piston head differently shaped, compared with a NOS piston. 

Tipp #03: Is the ignition proper set up and not too far from before TDC? 

Tipp #04: Go a bit richer in your carb set up what kind if slide with what cutaway are you using? Dell'Ortos have cutaways from 0, 15, 20, 30, 40, 45, 50 up to 60 depending the model as more cutaway as leaner is the complete set up and this effects the complete carburator from idle screw, idele jet, needle, slide position to main jet. 

Edited March 6, 20215 yr by pschrauber

8 hours ago, timdog said:

Interesting link here might be of some help

http://www.tomdonneymotors.com/2019/06/11/2-stroke-detonation/

Hi , yes I’ve read the article. Thank you.

5 hours ago, pschrauber said:

I only use gasoline with high octane ARAL Ultimate with 102 ROZ, I believe BP Ultimate has the same properties. 

This will help at first if the engine gets hot and in general. 

Next tipp: Do you have changed the compression using a too thin head gasket or is the piston head differently shaped, compared with a NOS piston. 

Tipp #03: Is the ignition proper set up and not too far from before TDC? 

Tipp #04: Go a bit richer in your carb set up what kind if slide with what cutaway are you using? Dell'Ortos have cutaways from 0, 15, 20, 30, 40, 45, 50 up to 60 depending the model as more cutaway as leaner is the complete set up and this effects the complete carburator from idle screw, idele jet, needle, slide position to main jet. 

Thanks pschrauber.

Ive read your Bultaco comments over the years. 
 

There’s a few things there that I can try.

I’ve swapped the normal NGK plug for the Iridium alternative and the normal fuel for Shell V Power that’s 99 RON. Not sure BP is a higher octane in the UK but I will check.

The change of plug snd fuel have significantly reduced the ‘knock ‘.

I’ve used a bore scope camera and inspected the barrel. No sign of carbon build up, or damage to the piston edge.

Further investigation has now taken place.

Ive removed the flywheel and found that the pick up sensor on the stator that’s held in place by two Philips screws was loose and could be moved about 1.5 mm when you applied pressure. So when the engines ticking over it’s going to be dancing around. Also noted that the timing was out, to far advanced.

As a belt and braces I’ve sent the stator off to be rewound while I’m at this stage as some of the solder looked iffy and some wires were frayed. Also it’s had a new blue Ducati coil fitted, so this work so guarantee I don’t end up pushing her back over the moors when it packs up at the furthest point from the car park.

Hope to have it back in 7 to 10 days I’ll report back when I’ve got her running. See if it’s cured it.

The orange wire at the end of that semi-circular valley looks to have been rubbed by the flywheel, and possibly shorting out.

.

18 hours ago, phiggs said:

The orange wire at the end of that semi-circular valley looks to have been rubbed by the flywheel, and possibly shorting out.

.

Hi, yes it is. I’ve sent it off to be rewound and reconditioned. The company rang me after making an initial assessment of the stator and said the ohms reading was well down and believed it would have failed in the not to distant future. So for £66, including returned postage and the fact I won’t be pushing it back to the car park, money well spent I’d say. 😄

Okay, the results are in.

As can be seen from the photo above, the timings marks are quite a way off. So the newly rewound stator was refitted and timed correctly. The company that performed the rewind did ring me after undertaking an initial assessment of the stator and said the Ohms reading was low.
The pick up unit is now properly secure. I’ve drained the tank and carb and refilled with 70:1 fully synthetic two stroke oil and Shell V Power 99 RON fuel. I’ve also fitted an Iridium spark plug.

Well, she stated first kick and ticks over very evenly. I ran it up to operating temperature and during the fifteen minute run, this was static on a stand, it only made possibly two detonation ‘ tings’ that I noticed, 

Ill report again when I’ve ridden her properly.