Modern Vespa

I have a 76 VBC Super that I have kitted with the following:

Pinasco 177 alloy cylinder, piston and head.

S.I.P. Road 2.0 exhaust.

Clutch upgear.

20/20 carb.

Air corrector 140.

Atomizer BE4.

I started out with a 116 main but that was was way too rich. I came down to where it is drivable but still a little rich.

Top speed on the flat is about 56-57 mph.

I bought a Trail Tech CHT gauge but haven't mounted it yet.

I assume the display should be mounted on the headset and the wires run down the tunnel. I need to pull the fuel tank out for another reason but can I get the wire through by just pulling the headlight and speedo or do I need to remove the headset?

The wire is obviously not long enough and an extension is $20 with shipping. I can cut and splice in extra wire or should I suck it up and buy the pre-made one? If splicing is OK should I add to the gauge side of the connector or the sender side?

I did some searching here to see how to use it but anything to do with head temperature or jetting is a deep hole.

The consensus I seem to be getting is that I should gradually drop my main jet size until the temperature goes up then go up a size. 400°F seems to sweet spot.

Am I correct so far?

I have a VDO temp gauge on it now that uses no electricity or batteries but the needle swings around wildly whenever the motor is running. I tried a resistor plug but that helped only a little. It's pretty much unusable while riding. I have killed the motor and looked at it immediately after a hard run and it has never read over 350°. How fast does the motor cool down after a strong pull?

Any advice would be greatly appreciated.

Thanks in advance.

400F seems very warm. Are you reading temp under the plug or elsewhere?

If under the plug, you really want to be under 300F for longevity.

I use CHT gauges only as an early indicator/warning sign that the the jug is unhappy. I would otherwise jet the scoot by feel, which it seems like you have done. When running normal I get around 275 to 325 on all my scoots. I get nervous over 350 and at 400 I would be fully clinched.

I'll let others comment on your jetting....but have you checked your timing? I wonder if that is contributing to the high temps?

As far as running the CHT cable: I just stick em to the front of the tunnel rubber between my feet. Cord going down the floor boards. I do have one mounted on the spare wheel of the VNB, but that is the exception for the lazy man's mounting I do on all the other scoots.

As said, Under 300F under the plug is the target. However, for rough jetting it's good to prove to the scooter that you are in charge and get it under 280F across the board. Then decide what needs leaning out.

I have a vespa super and am running a 14mm trailtech CHT.
I run it under a nut on the cylinder head with a 24in extension.

It's possible, but unsightly to run it outside the frame channel. I did for a while and then tucked it the next time I had a legitimate reason to pull the tank.

user MJRally was selling some CHT brackets which you could install under an open-bottom 70s headset. I have mine still, haven't used it yet. I'm not sure if he has any left. If he doesn't, I could trace and have one cad cut for you using a service like sendcutsend.com

Someone ran a series of cht tests under different studs (and the plug?) and logged the results: CHT Location
I wouldn't recommend installing under the plug. I'm just running on the SW head nut which suits me fine.

OK, 300° not 400° got it. Glad I checked.

I'm sure I read someone running 400° but I can't remember who and that may have been with a cast iron kit.

To be clear I should aim for 280° first then jet down until it runs about 300°. Would that be a plan?

BTW, I have the sending unit that goes under the plug.

Should I use heat sink compound between the sending unit and the head?

Is your VDO gauge analog? I have a VDO analog gauge on my P200. Mine came with a 2 wire cable with a specific length according to the instructions this length cannot be shortened. I have the excess wire coiled and hidden below the fuel tank. The coiled wire connects to a 12" thermocouple wire with a terminal ring that goes on the plug with the compression washer removed from the plug. The cloth covered thermocouple plug wire insulation is easily frayed make sure that isn't worn through and contacting the head or frame which seems like it could easily happen. My gauge is mostly stable unless I'm heavy on the throttle then I see it spike as expected.

Temps may have different parameters if aluminum or iron cylinders I believe aluminum may tolerate higher temps than iron but generally 350 is baseline for not to exceed. I've seen mine go close to 400 on my aluminum cylinder before I tuned the carb better now I see it in the 350 range if at extended runs above 60 mph and it does spike up to 350 if I'm ringing it out but drops swiftly on gear changes. When heat soaked it drops fairly quick when the engine is calm.

Hey there, Matthew.

We're running virtually identical set ups, except I have a cut crank, opened up ports, and a BGM Sport exhaust. I also have a bellmouth on the 20/20 carb.

Double check your timing. Don't advance less than 20-21°.

You're jetting's in the ballpark, maybe a few points rich.

As for the Trail Tech gauge, an under-the-plug sensor is fine. I have one. The engine temp rarely reaches 300°.

The wire that comes with the Trail Tech is much too short. I mounted the gauge on the turn switch and connected it by running the connector through the headset. I spliced in a long extension of 22 gauge wire that runs back to the sensor through the frame and under the tank.

Hope this helps.

The current VDO gauge is analog but it is a different model than yours. Mine appears to be older.

The sending unit is permanently attached to the garage by two wires covered in a braided metal sheath that goes from the sending unit to the gauge. There are no other wires or terminals.

It gives a steady reading when the motor is off but jumps around when running making it useless.

Pretty sure thermocouple wire is different deferent than automotive wire, so you are stuck buying whatever extension goes with the gauge.

These type of gauges aren't very accurate. Under the plug is the typical place CHT gauges get mounted on air cooled engines. They tend to not hold up that well on Vespas because the plug is taken in and out so often.

These gauges are most useful for trending. When your engine is running well in normal conditions, you know the range of values it gives.

Thanks, again. Very helpful.

I've never substantially modified an engine before and never worked with jetting so without help from all of you I would never be able to do this.

Would it be a good idea to verify the gauge reading with a non contract thermometer?

It's good to know about the extension wire. I was just about to go out to the garage and splice in a piece. Now I'm going to order the correct wire extension. What's another $20 at this point?

I was very proud of myself for being able to fish through a piece of mig welding wire from under the speedometer to under the gas tank. The wire had just the right amount stiffness to flexibility ratio to make it through the tunnel. I'm going to use that to first pull a string through and then use the string to pull the wire through. I'm going to leave the string in there in case I ever need to pull cables or anything else through.

I used a piece of minder wire to pull the sensor wire through the hole for the front turn signal. I left a length of it in for the next time I have to add/repair wiring. You can use an infrared thermometer to double check the gauge, but I'm not sure it will tell you much.

OK, I got the display mounted to the headset.

Like I said getting the wire through the tunnel wasn't too bad. I took off the horn and fished it from both directions.

What was hard that I didn't anticipate was getting the wire up through and out the bottom of the headset. There's a lot going on on there and not much extra room. In the end I have a nice, clean installation.

The gauge is working great. Nice, steady reading.

When riding into a15mph headwind it was reading 366°-368°F WOT and 55mph for 10 miles. Is that too hot? Should I go one step larger on the main jet?

Going downwind it ran 325° at 62mph.

One more question: That was with an ambient temperature of 65°F. If it were 40° warmer would the CHT be 40° higher. That would definitely put me in the danger zone.

Those temps are a bad day for an iron bore. Definitely want to bring them down further.

Aluminum.

Sounds like I should go at least one step bigger on the main.

I'm surprised that aluminum can take more heat than cast iron.

Aluminum dissipates heat better than iron. It helps that the aluminum has the Nikasil layer to address it's shortcomings in the wear department.

Ideally, you want to be under 300F riding in calm air, so you have a buffer when those 15mph headwinds are encountered again.

Bigger Main Jet
Cooler plug
Double check timing

Good looking super too!

If I remember correctly you said I should set the timing at 32°. I got to 30° or 2° less than whatever it was that you recommended and ran out of adjustment on the stator plate. I could get more advance by closing the point gap a little. Should I do that?

Whoa there Matt. 21-22°. Not 32°.

Yeah. I'm just remembering what you said wrong.

It sounds like I need to close up that point gap so I can get a little more advance.

This. Cook with a cast iron pan and an aluminum one, you'll see how much faster the aluminum one cools off. Cast iron holds heat for a very long time.

I would jet to have the temperature under 280f at all times, under any condition. Then decide if it was too rich anywhere, which I certainly doubt it won't be.
The cooler you can get it to run, the faster it will go. Worth trying harder.

For the record, it's official I am a complete idiot.

I did have the timing set to 30° advance. Mike, I must have misunderstood you at some point. I'm surprised it even ran at all but it did seem to run fine.

Reset it to 21° and took it for a run and the highest temp I saw was 310°.

Much better but from what you all have been saying that's still too high. Should I try 22° or 23°?

Should I put in the next bigger main jet and see what happens? That would be a lot easier.

Not that high, depending on what you did to get to 310.

I would try a BE3. See what it does.
BE4 is the (relative) leanest of all stock atomizers at WOT.
Easy to try.

About ambient temperatures, these bikes run richer at high temperatures and the opposite. Not a straight line CHT wise +40.

My mistake was thinking 30° advance was normal.

When I backed it down to 20° I had to start over with the jetting.

SocalGuy suggested 116 so I put that on but it was too rich. The temp was 245 but it was sputtering with a top speed of 45.

I don't have a 114 so I put in a 112 and the temp was ~290° and the speed was maxing at about 56 with no wind.

I'm actually satisfied at this point but I would like to get to an honest 60 with no wind.

It looks like my next objective is to get the timing dialed in. Socal said a minimum of 20°-21°. I'm at 20. I'm thinking that I'll go for 23° and go from there. When I had my VBA and was working with nothing but a Hanes manual I discovered that timing was critical. Half a degree made a difference.

It's a pain that you have to pull the flywheel to change the timing but it is what it is.

Is it better to get your jetting right first, timing or go back and forth?

Thanks one more time everyone.

I'd assume timing first. Being that you'll be chasing your jetting otherwise, know what I mean? I'm not an expert by any means but I'd assume you want the timing done then move on to the jetting.

Thanks. That helps.

I'm new to this (in case you couldn't tell). I've worked with ignition timing on Vespas and other vehicles but I've never jetted anything.