engine cutout at max speed

[Benelli Pepe 50 LX]

Hi Guy's
I have a problem with my Benelli pepe 50 (minarelli, yamaha clone) where any sustained cruising at around 60KMPH causes the engine to suddenly stop. The engine will re-start without a drama but the problem will repeat when top cruise speed is reached.
The scoot is this years model and has only 600km on the dial.
As my 16 year old rides the bike The scoot has the standard exhaust and the variator restrictor in place.
I plan to add a tuned exhaust and remove the Variator restrictor in a couple of weeks when he built up some road time.
The cut out problem manifested when I machined the head to increase the compression ratio. The clearances are good ( I removed .45mm of material) there is no pre-detonation under load, i run 98ron fuel. The head mod really helped the torque, the scoot starts well but now the cut out issue.
My thoughts are as follows;
Because the cut out is so sudden it feels electrical could it be a RPM cutout.
The carb was not retuned after the mod but I did readjust the idle screw setting to lower the idle RPM.
your advice and experience would be really useful

[90GTVert]

It sounds like your engine is soft siezing to me. The fact that this issue started just after milling the head to increase compression and that it happens when you are sustaining a speed for a bit leads me to believe it is a heat issue. More compression leads to more heat and it will build up over time on these air cooled engines. I would almost always hear spark knock with engines that have done this to me. Even if not when cruising, if I tried to change throttle positions I could often hear it once I had been riding for a bit.

Take a look at your spark plug and see if it shows signs of rich, lean, or extreme heat. It would be best to install a new plug and run it WOT for a bit. Then kill the ignition and hit the brakes so the engine will die out. Push the scoot in and check the plug.

I'm thinking you might be able to install a larger main jet and perhaps richen the part throttle by raising the needle (lower the clip) to get more fuel into the cylinder. The added fuel will help to keep the cylinder cool. If that doesn't work out, you might have to add a head gasket to go back to the squish setting you had before or close to it.

I would also advise checking the cooling fan, engine shrouds, and engine for any debris that could be keeping air from flowing. Make sure your engine is getting 2 stroke oil as well. Heat or lack of oil are the main causes of soft siezes. It sounds like yours are relatively minor, although not to be overlooked. In more severe cases the engine will need to cool for a bit before it refires.

[Benelli Pepe 50 LX]

The internet shines when the storehouse of individual wisdom can be shared for the greater good of all.

The advice in the above post was spot on. I checked the plug condition which indicated a lean mixture. I pulled the carby off removed the main jet and went down to the local bike mechanics shop to see if they could drill it out as it was proving hard to find a jet supplier around Perth (Western Australia). The mechanic told me the current jet size was a 58 and after some discussion we decided to open it up to a 68. Took it home reassembled the whole thing without moving the needle and installing a new spark plug set at .7mm gap.
As insurance I added some optima 22 oil to the gas tank.
The bike started up as normal.

Im not sure if i was hearing faint spark knock at idle or if it was just general drive train background noise, anyway it was not there once the engine was revved or driven under load up a hill at full or 3/4 throttle, overall response seemed more torquey than before. I followed the plug chop instructions which indicated the mixture was slightly rich. My only regret was not waiting to add the oil to the gas tank until after I had done the first plug chop.
I let my son take it on a 20km run and the guess what, no engine failure.
When he returned we did another plug chop and the condition of the plug had remained constant, still slightly rich but no signs of excessive heat.

A great result, a big thank-you to the oracle in the post above.

My follow on questions are as follows:

Do soft seizes cause any residual damage to the motor?

Could the faint engine knock like sound at idle be actual engine knock and be fixed by adjusting the needle?

When I fit the tuned exhaust will the jet size need to be altered again, or is that only something that can be answered by another round of plug chopping?

Is there a post on the site which explains the basics of carburettor tuning?

Thanks again for your insight on "2 stroke traps for young players"

[2strokd]

Brent is good isnt he? Dang! A 58 is tiny small @ any elevation
I think a tad rich is better than a ted lean on a daily driver (DD).
If the temp outside drops you should be spot on then :thumb: !
Soft seizes can cause some damage to the cylinder and or piston.
You might have slight scuffing and or little grooves in the piston and or cylinder. They can be lightly sanded with high grit sandpaper (800 grit or better is what i use), and the cylinder can be lightly honed. New rings might not be a bad idea if you do this. In your case tho, since it always started back up and ran well i dont think you will need to go that far. Sound more like yours got hot and shut down before any real damage. You can count on a little premature wear though, sorry it happens. Might be a good idea to pull the head and rotate the engine while inspecting the cylinder for "scars". As far as the noise, it could be the top-end now, or it might be the CVT system chattering a bit like you mentioned? An inspection will tell.
Your jetting could very well need to be changed again with a different pipe. A good plug reading will tell :thumb: .
Of course if you are way to lean or way too rich the scooter will bog and stumble at WOT. Allot of people got confused when an engine will rev right up and run good on the stand but stumble and die when they go to ride it. Simple, the engine dosnt need as much fuel without a load on it. When ya get on and go, its got a load :scoot2: .
Not saying you thought that, just figured ide throw that in since we are on the subject

I think Brent might have a tech article writen up on carb tuning and carb basics (how they work). He can help ya there.


BTW: Welcome to the forum :welcome: ! You live in Australia?
Ide love to visit (move their ) someday!

[90GTVert]

Glad to hear it worked out so well. Just to add a little to what 2strokd posted... I would bet taht you do need to go up on the jet size and readjust the carb with the addition of a good tuned pipe. When you decide to install one, do a plug chop immediately. Results vary, but I had one scoot where swapping pipes made a pretty bad lean condition. As you've learned already, you don't want that... and yours was relatively mild. It can be much worse.

Liek he said, you probably have some scuffing. If everything is running well and you aren't seeing any lack of performance that might indicate compression is dropping from ring wear or cylinder wear, I would leave it together for now. If you just enjoy working on things and wanna pull it apart, go for it... but I don't believe it is necessary just yet. I also agree that it's likely to be the something in the CVT chattering at idle. If the guides on the variator's back plate get owrn it will rattle like crazy. There's other stuff that can cause it in there as well.

I actaully never have got around to writing up a carb tuning guide. There's some info about cleaning and such in the carb section here...

49ccscoot.proboards.com/index.cgi?board=carb

Here is a great link to info on tuning carbs from another site though...

hondanighthawks.net/carb14.htm

[Benelli Pepe 50 LX]

Oh well as my mum (mom) would remind me through my formative teenage years "experience is what you get when you don't get what you want"
So i guess cylinder scuffing is getting off lightly in the scheme of things.

Thanks for the heads up on the pipe install.

What are your thoughts on removing the snorkel from a standard air box when fitting a performance pipe should be done or leave it alone?

BTW:
Be careful what you wish for, Oz is a great country with an unfortunate low scooter tolerance level.
In the land of V8's, straight roads and long distances, 50cc scooters are the enemy of the empire.

Thats why I want my 16 year old to develop road craft on a scooter to help prevent the onset of invincibility that comes with 6 airbags, four wheels, one handed texting and speed.

Not all doom and gloom,
Riding a scooter at sunset on the west coast after a swim at the beach on a lazy summer Saturday. now thats living!

All good stuff.

Cheers

[90GTVert]

Oct 7, 2010 10:49:52 GMT -5 Benelli Pepe 50 LX said:

What are your thoughts on removing the snorkel from a standard air box when fitting a performance pipe should be done or leave it alone?

I'd say it could go either way. I removed the snorkels from mine. I can;t say I see any real difference. You might need to upjet slightly more and it would be a good idea to readjust the idle mixture anytime you change something that could alter airflow.

Oct 7, 2010 10:49:52 GMT -5 Benelli Pepe 50 LX said:

In the land of V8's, straight roads and long distances, 50cc scooters are the enemy of the empire.

As a muscle car/drag racing/scooter enthusiast, you just gave me some pretty big pros and cons all in one sentence. lol

[2strokd]

you can remove the snorkel if ya want move of a throaty sound to it.
ahh man, they dont like scoots over there either...?...DAnG!!!!