Modern Vespa

I have been reading up on 2 stroke tuning in an attempt to figure out the high CHT's with my LML/ VMC 187 engine.

Issue: High CHT's with sustained cruising at 50+ mph (350F and rising).

Piston/Cylinder: VMC Super G Traversi

Mazz 60 mm rotary crank

Stock Stella reed block-Boyesen reeds-no modifications to reed block.

22t clutch cog 22/68

SIP R2

Timings:
T=122
E=178
Squish= 1.19mm

VMC lists the timings as:
T=114
E=173

I'm wondering if these are suggested timings or just what they would be if the piston height equaled 0.

Spaco 24/24, 26/26

Pressure tested fine at build, but found a small air leak at reed block stud and exhaust stub. Air leaks never help, but these were quite small and not constant.

Tried all different combinations of jetting from the typical Bald John stack and much richer jetting with Jack 221's ongoing help. Found a number of combos where the bike ran great through the throttle range, but could never keep CHT's down at sustained 50+.

Jack suggested possibly going with a 30mm carb and better, 4 petal reed block. A reasonable consideration, but I wanted to make sure I had ruled out everything else before ditching the Si carb and autolube, especially if it didn't end up solving the problem.

Other things I thought of:

Detuning-lowering the timings

-lowering compression- though I realize that this may not accomplish much after learning what a squish band actually does.

Trying a different head. Wondering if the VMC super G Traversi is somehow a bad combo with a 60mm crank. I'm sure the long stroke rotary crank isn't helping with flow either.

Opening up/flowing the Stella reed block, carb and case including cutting the center bar. I'm not sure if this would make a big difference.

Gearing: going up to a 23t clutch cog to lower revs at highway speeds. My stock P200 engine runs way cooler. I think the gearing plays a big part.

It's a fun engine in a way that the stock 200 is not, but it's useless to me as it is now.

I will have plenty of time to work on this engine over the winter. If sidedraft carb and better reed block are really the way to go, then I can live without autolube. This engine might just find its way to the VBB eventually. I just don't like playing new parts roulette without understanding the problem.

Thanks

goodness. change your gearing to 23/65. you have way too much power for that 22/68 setup. This will change several things, your running temps as well.

but yes, that LML reedblock is limiting. I've ran the BGM177 on the LML block with autolube with success, but i do think the VMC is on another level.

Ignition and squish jump out to me.

Instructions say aim for 1mm +/-0.1mm, 1.19 would be on the big side and I'd aim for conservative ignition timing (bang on 18deg at wot, I'm imagining 7.5-8krpm) because you're running a Road 2.0.

Thank you Mr Gick. I was thinking that this kit might be more power than I bargained for especially compared to the iron Polini. I wrongly assumed that it wouldn't pull higher gearing, but it really should.

yes. take that middle bar out of the reed block. I have bulk Polini reed sheets and cut my own. but i see SM sells the reed pedal and gaskets for $38.00. also, shape the reed block to match your 24mm carb.

The instructions I have list the squish at 1.4 +/- .1mm and timings of:

Intake: 114*
Exhaust 173*

That's one of the reasons I was asking about port timings. There are different versions of this kit which adds to the confusion.

A few considerations based on my tuning adventures over the past six months or so...

First off, you have to be tuning to the use case. I ran into this when I took the Smallie out for some touring a couple months ago. I struggled to keep my temps anywhere near under control. My stoplight racing/around town tune was WAY too lean for sustained, heat-soaked riding.

I went back and re-tuned it with an eye towards heat soaked performance, then repeated that same ride a few weeks later and while it now takes longer to warm up, it will run at 50 MPH+ for as long as there's gas in the tank and the temps have yet to get above about 310F. I call that a win.

Similarly, while it wasn't deliberate on my part, I wound up de-tuning my Malossi 210 after I tore a .5mm base gasket while splitting the cases for a cruciform swap. I decided to just roll with the lower timings and tighter squish band (I think it's now 0.8mm). While I lost some power, the thing still has plenty of oomph for around town, but the temps never go above 300.

The other thing I did on the smallie was replace the flywheel fan with the Egig high-flow fan to improve cooling. I also made sure my cylinder shrouds were nice n' tight at the same time. I think that also made a difference, though I haven't swapped back to stock to be sure.

Finally, as already noted, that stock Stella reed block is like putting a cocktail straw into a Big Gulp in terms of ability to move air. You should seriously consider removing the center bar and opening it up. That'll make a big difference (I think I calculated over 20% increase in opening size once upon a time) in flow, which in turn will allow you to actually get mixture into the cylinder at higher RPM's. I don't *know* that will make a difference, but I strongly suspect it will improve your temperature issues, especially at higher RPM's.

Ah… if that's the instructions in the box then I'd go with that.

Below is the kit I thought you had.

But then why are you at 1.19mm when the instructions suggest 1.4mm?

1.4mm makes more sense for an engine to be used on the road.

I think it is the same kit as far as I can tell.

CM:

I agree with you about tuning for purpose. As it is, it would be an excellent city bike. It's great on back roads too.

Detuning makes sense. It would be easy enough to lower the timings a little.

I think taller gearing would help a lot too. If I went with the 65 primary, I would have a choice of clutch cogs to fine tune it further. I think the tall gearing is one reason the the 200 runs so much cooler. I definitely don't want it to ride like the 200, but I would gladly trade off some of the revvy fun for something in between.

I have a spare reed block so in case I screw it up…Opening it up is certainly worth a try. While I am doing that and have the cases open, I could at least flow the case a little and match the transfers. I would want to do that anyway if switching to a side draft carb.

So that's the plan:

Adjust timings
65 primary
Flow case and reed block

Switch to side draft carb and real reed block if none of the above works.

Thanks!

Both named Super G but ... aluminium or cast iron? 🤨 🤪

It's cast iron.

I understand mate, I have same kit.
And same b&w paper instruction.
And same electronic color version.
Maybe there is mistake and color version is for Stelvio?

Maybe the b/w is a revision?

Ah, the aluminum port version

As 108 asked, what's your ignition timing?

It would be interesting to see if the temp keeps going up. 350 is not so bad to my way of thinking.

Couple of quotes from Patrick -

350F was the red line temp for cast iron cylinders with an under-the-plug CHT.

I recall Jack once saying that 320F was his target for max temps and a chart of the impact of heat on aluminum cylinders & pistons out of an aviation manual that Voo doo once shared indicated that 420F was the temp at which it started materially impacting the life of the cylinder. Obviously, all alloy mixes are not the same, but those are data points that stuck with me.

Personally, I still like to keep my temps under 350 or so with the sender tapped into the head, even though I know that's probably low. I also find that a well-tuned motor will tend to run under that anyway, so even if that's not the safety limit, if I'm running hotter, there's something going on that needs further investigation, ideally before I need to call someone to come pick me up.

Where is the sensor?
Plug or bolt? They have very different readings

I've said this plenty before but just to recap. As I see it, when jetted for road use, the temperature will be less than 320F under the plug. The cylinder can run much hotter but hotter is less powerful, slower and one issue away from cylinder destruction.
As a 177 has basically the same cooling as a 200, the correctly jetted temperature is even lower, less than 280F.
Something is going on with this build. Can only be the small reed block. Everything else is like usual, which we know works.

EDIT: Even before any de-tune 122/178 is already low for port timing, so it's not that.

Thanks Jack.

I agree, the reed block is a bottleneck. The question is whether opening it up would be sufficient. It still has the center bar and I haven't touched the carb box and carb.

I also agree with Mr. Gick that taller gearing would help. You are spot on about 200 temps. 280F is as hot as it ever got lugging in 4th up a hill.

CHT was measured with a screw in sensor and a plug sensor at the same time. When the engine gets really heat soaked, they end up pretty close.

Ignition timing?

I set it to 18* and tried it at 17* too.

This is a long shot but did you ever try a BE4 mixer? Fifty mph should be about 5,300 rpm on your gearing. I don't know exactly where in the rev range the different sections of a given mixer tube come on line but I think a BE4 would richen things up quite a bit around that 5,300 rpm point.

If the squish isn't right, temps will go up.

Smaller squish doesn't mean cooler temps.

higher compression will create higher temps as well.

I could certainly raise the squish from 1.19 to 1.4.

We did try the BE4 briefly. It did seem to help with mid-range CHT's but made it harder to get it to splutter when revving out.

I could try jetting again after opening up the reed block and case and switching to the 65 primary.

At that point, I could switch to a real reed block and side draft carb if the above changes don't help.

Seems like every time I look up the specs for this kit, the squish gets bigger.
I wonder why. 1.8-2mm seems like a big jump. But maybe there's a reason.

Squish is currently at 1.19 on my engine as it sits on the bench. A .5mm head spacer I made would put it at 1.69. I'm going to give it a try.

The bigger the squish clearance, the lower the peak power. Many (not all) companies tend to state the lowest risk options. Optimal clearance at this cc for peak power would be 0.6-0.8mm.
As you have very little interest in high rpm and peak power, set the clearance at 2.0mm and forget about it.

Scooter Center has a link to official looking instructions. Calls for 1 mm squish + or - .1. 2 mm is nuts.

No harm in trying. You'd be surprised…

You are correct. For my current needs, peak power isn't a requirement. Certainly worth revisiting to optimize sometime down the road. At this point, I'll be happy to have it pushing another functioning bike.

I'll give it a shot. Thanks!

Did the statute of limitations run out on responding to this?

Assuming no (I've been known to blow a red light or two) this doesn't match my experience at all.

Your on iron so it won't be as happy to go as high as aluminum.

On my aluminum, when too lean, jetted for light to light screaming, it will climb over 400 when put under long heat soaked duress. When jetted a bit more plump, it settles in around 365/370 and will sit there with strong head winds climbing long hills.

In head.
I've never seen more than 330 under plug.
Surprised ur getting same readings head and plug.

Interested to see what happens when u open up read.
Small leak at block is no bueno.
I would make one change at a time - and start here - ensuring seal and opening up the block.

Yes, still planning on opening up the reed block. It's been resealed and pressure tested.

Under plug readings do tend to read lower than screw in sensor, but eventually ended up in the same ball park with sustained 50+ mph. I'm surprised too.

without EGT the picture is incomplete and it's all guess work

It's definitely on my list of avionics upgrades.

What's the compression on this thing?

Data is always good.

For time being - the question I have is - why so hot with so little load.
Frankly, 50MPH should be a walk in the park - even with your fan, your gearing, your carb, etc. etc. etc.

In simple terms, if throwing a larger main jet on it is not cooling you down - then likely something else is amiss.
Will be interested to see how things go after the reed block opening.
Perhaps this is bottle necking your system.
If so - it should have an immediate and significant effect.

Question: Where is your throttle position when you are cruising and seeing the temps climb?
It can be REALLY hard to tell - so I built this beauty of a bodgery to measure mine.
Was super instructive.

Avionics?! You need me to sign off your work? We should do a new weight and balance when you're done.😉😁

50mph should be a walk in the park. About 1/3 throttle, buzzing along steady at 250f (plug) is nice.

Looks like a Victorian solution. I use snopake. Does the job and wipes off with petrol afterwards. Seems too simple now 😕

It will be better next time