'78 vespa bravo new carb still wont idle

Hey guys, im new to the forum as well as new to vespa mopeds. I inherited my moms 1978 Bravo, or atleast i think its a 78. Anyways ive been trying to get it to run right. Before it was difficult to start, wouldnt idle, and was impossible to start after it stalled. I just replaced the stock carb with a Dellorto 12.10 with a #46 main jet. Pretty sure what was in there to begin with. new fuel line, new spark plug, new air filter. It starts and runs great now but I still cant get it to idle even after adjusting the screw. Sometimes it will for a minute then die, when its hot it seems to die immediately. Any suggestions? Much appreciated!

Re: '78 vespa bravo new carb still wont idle

so i just looked at the old carb, it is a 12.7 that has a #39 jet, my new one is a 12.10 that has a #42 jet. I just swapped in the 39 and now it wont even run. Think i need one inbetween?

Re: '78 vespa bravo new carb still wont idle

You running a air filter?

Re: '78 vespa bravo new carb still wont idle

yeah new air filter, it came with two of the mesh filters, i put both of them in, wasnt sure if one was a spare or not

Re: '78 vespa bravo new carb still wont idle

> Colin Onda Wrote:

> -------------------------------------------------------

> yeah new air filter, it came with two of the mesh filters, i put both of

> them in, wasnt sure if one was a spare or not

Pic?

Re: '78 vespa bravo new carb still wont idle

I put the #42 jet back in, messing with the throttle stop and idle screw to no avail

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Re: '78 vespa bravo new carb still wont idle

put the bigger jet in, also poke a small wire thru idle hole underneath throttle slide. the slide cut angle affect idle richness from memory stock vespa should be pretty flat. make sure u running the screens and the airbox. could be low compression, does it spin over really easy? decompression leaking like crazy? sometimes bikes need to run for a while to burn off the spider webs, run it around a few times and see if that clears it up. other things to check are sticky clutch/ variator/starter clutches etc. clean n set points too. clean and grease cam.

Re: '78 vespa bravo new carb still wont idle

Bigger jet is back in. I just looked at the plates, both have a completely flat bottom, my original had a little smaller taper for the idle screw so I put that one back in, no change. I’ve been riding it good the past 2 days, runs great except for idling. As far a compression I don’t think it turns over too easily. How would I tell decompression leaking? This is pretty much my first dive into things like this so forgive me. Thanks for the advice!

Re: '78 vespa bravo new carb still wont idle

Try about a 47 jet. The 12.7 carb was a 7mm carb and the 12.10 is a 10mm carb. That's a huge difference.

Re: '78 vespa bravo new carb still wont idle

Alright I’ll give that a go. I just read on treats that the 12.10 was the stock on most vespas so I went with it. Spose I should have checked mine first.

Re: '78 vespa bravo new carb still wont idle

I am with the others on this. I have never seen a Vespa carb with less than a 49 jet

Re: '78 vespa bravo new carb still wont idle

Dirty30 Dillon /

You are most likely lean, and there's always the chance that the 40 year old seal is bad

Re: '78 vespa bravo new carb still wont idle

Alright thanks guys, ordered some bigger jets last night. What seal are you referring to? Even if it isn’t bad I would like to change it

Re: '78 vespa bravo new carb still wont idle

He is referring to the crank shaft seal. You will need to remove your motor and buy a flywheel puller.

Look here for help with the motor removal.

Re: '78 vespa bravo new carb still wont idle

Awesome! Thanks a lot

Re: '78 vespa bravo new carb still wont idle

All the bravos I've had come with a 12.10 carb and a 45 jet.

Re: '78 vespa bravo new carb still wont idle

all my bravos have a jacked up wire hidden in the rear fender bead.

Re: '78 vespa bravo new carb still wont idle

I’ve read on here that faulty wiring could keep some models from running. I also watched a YouTube video of someone taking apart a danish model bravo motor and he said that they came with a 7mm carb and a smaller intake. I know this bike was bought in the US and that I’m the first one to do any work to it. If my intake is smaller will the 10mm carb not make a difference unless open up the intake?

Re: '78 vespa bravo new carb still wont idle

I have opened 4 or 5 vespa intakes using a professional mill and found that you can only safely open the throat of the intake to 12.7mm or the same as a 1/2" bit.

Any more and you are risking snapping off the intake from just the weight of the carb it is supporting.

Drilling down the throat is only half the battle as you cannot drill all the way through because as you near the inside of the case near the rotation patch, the bit will punch a hole through the side and create a leak.

The rest of the opening of the intake has to be done from the inside using a chainsaw file

You mention 10mm carb. I thought you had 12mm?

Re: '78 vespa bravo new carb still wont idle

Yeah the Video I watched mentioned the 1/2 bit and having to use a chainsaw file. I would like to do this eventually, but I want learn more an get it running right in it’s stock form first. But the replacement carb I bought is a SHA 12.10 which would be a 10mm correct?

Re: '78 vespa bravo new carb still wont idle

You need to measure the actual throat of the carb at its narrowest

Re: '78 vespa bravo new carb still wont idle

Also, any ideas as to how I’m leaking gear oil from the mounting bolt in the rear hub? I just changed it and it’s not coming from the drain/fill bolt or the seal. Just that bottom bolt. Must be damaged?

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Re: '78 vespa bravo new carb still wont idle

take those out clean and thread lock em in

Re: '78 vespa bravo new carb still wont idle

Bolt leaking grease is pretty common cuz it's real easy to over-tighten. Time to pull off your wheel, pull the hub and you're gonna need to silicon seal the inside where the bolt holes leaks. Just do em both and they should hold for a good while.

Re: '78 vespa bravo new carb still wont idle

Maybe a lot easier to just apply a healthy dose of thread tape .

Re: '78 vespa bravo new carb still wont idle

Thanks guys. I put the 46 jet in, wouldn’t start. I’m assuming too much gas. I’m going to grab a gauge tomorrow and check the compression but I think I’m going to break down the motor anyways. Replace the crank seal, replace the condenser and coils, set the points, clean er up. Do you guys this I should split the case and do the crankshaft bearings while i have it apart? Also any other things I should look for while I’m in there?

Re: '78 vespa bravo new carb still wont idle

> Colin Onda Wrote:

> -------------------------------------------------------

> Thanks guys. I put the 46 jet in, wouldn’t start. I’m assuming too much

> gas. I’m going to grab a gauge tomorrow and check the compression but I

> think I’m going to break down the motor anyways. Replace the crank seal,

> replace the condenser and coils, set the points, clean er up. Do you

> guys this I should split the case and do the crankshaft bearings while i

> have it apart? Also any other things I should look for while I’m in

> there?

Did you pull the head and clean the decompression valve? If the crank spins smoothly I wouldn't worry about crank bearings. But if you really feel as though you want to change them you need heat to get the cases apart and back together. I don't know why but I'm betting your decompression valve is leaking and causing your problems.

Re: '78 vespa bravo new carb still wont idle

The bolts that hold the subframe to the transmission are a very specific length. The reason for this is because the transmission has blind thread bosses installed in the aluminum cover. The tread bosses are blind so that they won't leak fluid.

What you did was install longer bolts that punches out the blind/sealed end of the boss so now fluid will always seep out along the threads,

You have also spit the metal slug that was sealing the boss into the fluid cavity where it now will await a perfect opportunity to jump into the teeth of your meshing gears and total those.

Here is a picture of the inside of a normal sealed thread boss.

This thread boss has had its blind seal punched out by too long of a bolt.

It is very common for people to make this mistake.

You can only use teflon tape to seal in the seeping fluid and hope that the metal slugs that are now down in your fluid don't get churnned up and float into your teeth.

Re: '78 vespa bravo new carb still wont idle

Alright I’ll take apart and clean it. I’ve been trying to learn about lapping, I’ve never done this before. Is it pretty straight forward?

Re: '78 vespa bravo new carb still wont idle

What is it you feel you need to lap? No offence to you but you seem to be thinking too much. Not every part that is dirty need to be tossed ans replaced. You should not have replaced your perfectly good mounting bolts.

Here is a pictorial about tranmission over haul.

overhaul

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