CHT (Vespa BGM 177)

[Andy Beard]

Been perusing various forums to try & find out what’s a suitable ‘SAFE’ Cylinder Head Temperature but as usual the information put out is so varied & at times so vague I’m perplexed!

Scoot is;
Bgm 177
57mm Pinasco Racing crank
Drt Si 24/24 (55/160, Be3, AC 160, mj 125, both holes drilled out of filter)
Sip Road XL (swapping to Sip Road 3 on delivery)
Pinasco 1.6kg flywheel
Bgm super strong clutch
23/65 primary PX 125 box
Bgm ignition set out 24* static
Kytronik curve 7 (8* advance)
Casings matched to cylinder & carb bowl matched to carb etc...inlet 105*/60*, squish 1.1mm.
Bes9 plug
Temp sensor fitted into Cylinder Head not under plug or on cylinder stud!

I was after a more torquey Vespa (bit if a fat knacker you see) not a high RPM racer & torquey it is! I very recently had it dynoed at 16ps, a little low for the Sip road XL hence the swap to a Road 3 when it arrives. The scoot easily reaches 70mph(GPS) @7,000 rpm on the flat but that’s yer lot!

My main concern is CHT’s jump up from 250/275*f at approx. 5,000rpm to 340*f at approx 6-7,000rpm which makes my arse twitch & in turn ease off the throttle! Only given it a few WOT runs for approx 0.5 miles, I should be doing more now it’s run in but am hesitant not to seize.

My simplistic understanding is that 2 stroke motors run best at (CHT) 350-450*f which if correct would make me a happy chap however information (or opinion) is so varied I’m overly cautious.

WHAT IS THE SAFE ZONE?

[Norrie Bodge]

Is the fuel chamber hole drilled out to 2.5mm??

https://youtu.be/K7TJXHEAfaA

[Andy Beard]

Thanks for the reply!

Yes the fuel chamber has been drilled to 2.5mm & a Sip fast flow tap (v2) fitted, I’ve even gone as far as checking the fuel bowl top was correctly fitted & found I had to remove/file approx 2mm off the alloy to ensure that the top & rubber seal seated correctly, this has alleviated minor warm starting issues!

My main concern/interest is ‘what is a safe Cylinder Head Temp zone’ as the info out there is so conflicting at times. I totally get that every modified motor is configured differently, we all have different riding styles & fuel/mixture, timing, jetting, plug, steel cylinder v alloy, exhaust etc...will influence temps but the basic physics has to be the same i.e. go above a certain temp & you’re going to seize etc...some report a lowly 250*f (WOT) others a high 450*f , some even higher (apparently).

I’m definitely running slightly rich when on the main jet, as sure as I can be that fuel supply isn’t an issue but would still like to factor in extra insurance against seizing or put another way I’d like to use the CHT gauge for it’s intended purpose as opposed to testing the engine to destruction.

[Shaunbond]

I have a set up similar to yours but not quite as flash...
Copied from your post and updated with my setup...
Bgm 177
57mm standard crank
PX200 Si 24/24 (55/160, Be3, AC 160, mj 126, both holes drilled out of filter
BGM Touring exhaust
LML standard flywheel with starter ring removed
Cosa clutch with CR80 plates and 23 DRT cog
Standard LML gearbox
LML standard ignition
Barrel matched to cylinder & carb intake matched. LML ports are big enough.
Bes9 plug
Temp sensor fitted into Cylinder Head not under plug or on cylinder stud!

I have seen 350 at WOT and under load uphill. I have felt the same as you, what's safe? But the engine never seems to complain and just goes on. I've come to the conclusion that I'll just ride it as usual and it'll all be fine... Hopefully

[Andy Beard]

Thanks mate, pretty much my conclusion to be honest.

It’s run in, so maybe time to give it some b@11@x & see what happens (careful b@11@x that is)!