Can anyone tell me the correct or good range of compression for my 1978 Vespa piaggio ciao. its stock with the 38.4 mm cylinder.
I rebuilt with new Piston, Cylinder, carb and had it running once. Cannot get it to do anything. Got spark, Fuel and compression but no joy. Need to know if I have enough Spark, Compression and fuel reaching the right spots at the right time.
Les
i'd probably hit it with a timing light to check timing. The other one you can't really check without a leakdown is the primary compression in the case, that can def be a problem with vespas too since you're rotary case inducted
the cylinder compression probably ok ven at like 75 but it shouldnt be that low. that'd be at least bad rings but prolly a gasket or somethin. you sure the decomp valve is OK and working and sealing?
pretty great benchtest setup
Thanks for the info. Ill check the decompression seal. If its sucking there i might get a pop if i spray some starting fluid in the area while kicking it over.
It does have New piston and new rings.
Ill also check the compression.
Thanks again.
Don't use starting fluid, it has no lubrication and can seize the engine. Just drip premix into the plug hole.
Just to be sure, you don't have a "stock" topend, you have an Athena cylinder which definitely will require more fuel than a stock setup.
What is your jetting?
What's your point gap?
Have you done a leakdown test? Compression test?
Per the manual, the compression ratio on a stock Ciao is 9:1, or about 132 psi.
Thanks for the tip!
Thanks for noticing. I did replace the cylinder with a larger cylinder and piston.
The old jetting on the "stock" was a 45. it did run with the 45 on the new cylinder for a few minutes, although I could tell it wan needing more fuel.
I changed the jett to a 50 but no joy. I went back to the 45 and still can't get it to start.
Going to test the compression act..
Thanks for your help.
Thanks for the info Dillon,
I did the compression up to 100 lb, all my cage would do.
The compression is good.
However, When i did the leak down test, air escapes through the decompression valve.
Would this keep it from starting. the leak is small but definite.
Les
Lap that decomp valve .
The leak at starting could easily be 10 - 20 times what it is for the leakdown test .
So by looking at your wiring it looks like you've removed flywheel and ran a white wire to ground. Is the a replacement for the blue ignition external ignition ground? If so the thats fine. If not then where is your blue ext ignition ground?
Details on what you have done helps.
Did you change the one crank seal or service points? Is that an aftermarket carb looks too clean to be og? id say minimun 56-68 main
Also, when you do get it running it is important to have that air shroud over the cylinder to direct air across those fins and down, otherwise its seizecity.
Yes I did replace the blue wire for this white wire as the blue one was in really bad shape. I have really good spark. I did put in new points and condenser and gaged them properly. I did recently discovered on the leak down test that there is an air leak around though D comp valve. Going to address that today. That is a new carburetor and the jetting on it is 45. It did run with that jetting set up before so it should run again. I do have larger jets to try once I get it started to see if it doesn’t run better. Thank you for all your input and I’ll report back later what I find today and if it helps.
Les
If the decompression valve shows A very small amount of leaking air during a leak down test how would that keep it from starting… Don’t you open the decompression valve all the way when you are starting it anyway?
> Les Clark Wrote:
> -------------------------------------------------------
> Thanks for noticing. I did replace the cylinder with a larger
> cylinder and piston.
>
> The old jetting on the "stock" was a 45. it did run with the 45 on the
> new cylinder for a few minutes, although I could tell it wan needing
> more fuel.
>
> I changed the jett to a 50 but no joy. I went back to the 45 and still
> can't get it to start.
>
> Going to test the compression act..
>
> Thanks for your help.
50 is way too low for that kit. 60s at least. Do you have a pipe ?
Also did that base gasket seal? Sometime they don’t line up correctly.
That jetting Is super low, if you keep running it your going to seize. Make sure you get the engine shroud put back on. Don't wanna overheat.
Did you properly gap and grind the new Piston rings?
Pull the decomp valve out and seat it with lapping compound. You can use a bit of thin tubing to spin it
> Les Clark Wrote:
> -------------------------------------------------------
> If the decompression valve shows A very small amount of leaking air
> during a leak down test how would that keep it from starting… Don’t you
> open the decompression valve all the way when you are starting it
> anyway?
Yes and no. Decompression is exactly what it means. You hold it when you first begin to pedal so that the your starter clutches engage thus spinning your motor. Once the motor spins, keep pedaling and then let the lever go so the compression can build to start the bike. Not super necessary on a stock bike (lower compression) but nice feature, some say necessary on a kitted one.
If the decompression valve shows A very small amount of leaking air during a leak down test how would that keep it from starting… Don’t you open the decompression valve all the way when you are starting it anyway?
> Les Clark Wrote:
> -------------------------------------------------------
> If the decompression valve shows A very small amount of leaking air
> during a leak down test how would that keep it from starting… Don’t you
> open the decompression valve all the way when you are starting it
> anyway?
Because an the bike starts when you close the decomp and compression returns. If you have an airleak there, compression is lost as well as straight air is added to the chamber which leans it out incredibly.
What jet are you trying? If you're not going into the high 50's/60's, you're wasting time and energy chasing down that which is most likely 90% a tuning issue.
Durring compression there is no leak at all. Compression test showed that. The pressure on valve closes the valve very well. no bubbles at all.
I did clean and lapped the valve. I put in a 50 Jett to replace the 45 that was there before. It did run with a 45 but not well.
Here is a couple videos i took today. Sounds like not getting fuel. There is fuel in the carb and flows freely through the filter and hose. Carb is new.
It did run with this set up a few weeks back. I=t has great spark.
I have a few short videos but I can't post here.
Les (edited)
I don't think there is much i can do about timing except gap the points with i already did. do you happen to know or a have a" how to " link on timing the vespa piaggio.
Thanks in advance for your help.
Les
I will try the 60 again, Tried it once but it didn't work. I even ordered some 70s jets I may try.
I did get a good seal on the caskets. I do have a pipe.
Thanks in advance for your help.
Les
Ok, so you have a 12:10 real dellorto (is the throttle cable outside the spring) does it run with choke?, with a kit n pipe stock airbox with mesh filters 3. So jetting should be low 60s to low 70s.
Always double check wires, tightness, grounds, point cleanliness and gas mix.
Timing with a light, read up how to on the wiki. At the end its a double triple check thing, should fire right up.
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