Modern Vespa

My old exhaust wasn't worth trying to preserve any longer, I couldn't afford a new Piaggio exhaust and I saw a cheap 2nd hand steel exhaust (an LV Grandturismo) for sale. It fitted easily enough but I completely lose ooomph at 3/4 throttle and above. It's fine to start and at all throttle settings up to this point. My guess was I needed to change the main jet.

I just took off the carb - not as difficult as I was fearing but I have some questions.
A) this looks like it needs a tube attached but it doesn't have one - is it just for draining the carb?
B) this is just an open tube - what is it?
C) I assume it's a walbro carb with the metal cap on top with 2 screws?

I bought some minuki jets but the ones I've bought are way too big. I took out the existing main jet and it appeared to have a small hair in the jet. I've cleaned it but not put it back together yet. Would cleaning it out fix my problem at 3/4+ throttle? if not what jet should I get? and would swapping the jet to a larger one fix my issue?

I cleaned around the float bowls etc with a paper towel - it wasn't too bad considering it's not been touched since I bought it in 2003? Anything else I should do while it's out?

These days I only ever ride the bike 5-10 miles at a time. Am I likely to damage stuff if it's running too lean? how do I tell when I've got it right? - I was just going to buy a few random jets and see what worked.

Mike

PS Just in case it sounds like I know what I'm doing I'll warn you I don't.

Well I cleaned the old main jet up and put it back in and it does run better but still has issues above 3/4 throttle. The jet fitted into a brass holder that then fitted into the carb - did I need to take that out and clean it also? I should also of measured the old jet before putting it back. In addition to the above questions, I know the standard jet is a #95 but does anyone know the rough size of jet in mm I should be ordering? or have a link?

A) is the drain port. The screw just to the right in your photo opens it up. Tubing is optional.

B) is an overflow port. When the inlet valve needle cannot seal, the carb bowl fills with fuel continuously. Bad float, bad needle, dirt in needle seat are a few of the problems that could cause an overflow.

The main jet fits into an atomizer, a brass tube with many small holes. Sometimes called and air nozzle, or needle jet. You should take that out and clean as well. Strands of copper wire are good for that.

Somewhere on that carb are numbers or names that will give you the clue to which jets you should be ordering. The jets themselves also have numbers stamped or etched (sometimes very faintly or microscopic in size). This will clue you into what size you might want to change to. Delorto jets are sized in increments of millimeters. A size 75 jet has a 0.75 mm hole, a 120 jet is 1.20mm. Mukuni jets seem to have no logical numbering scheme.

Thanks for replying!!

I'm certain the standard one is a #95 but I have a different exhaust (a little more free flowing, cheaper, lasts longer). The exhaust was built for the bike but my guess is the engine needs more fuel at higher revs. The workshop manual says likely to be "Main jet dirty; lean carburetion" when "Engine Tends to Cut Out at Full Throttle". I figured just putting in a larger jet might help.

Bigger jet won't help if carb passages are clogged.

Go buy a can of carburetor cleaner and some nitrile gloves. Spray it in the jets, internal passages. try to avoid spraying on any rubber/plastic bits on the carb. Carb cleaner will eat right through. Buy a small can of compressed air, and blow out everywhere you sprayed the cleaner. Let it all air dry for a bit, and put it back together. Try and keep it off your skin, you likely with go through a couple of pairs of gloves.

Assume I only need to clean the bottom half of the carb (actually it looked surprisingly clean). Doing the top half would mean I'd have to disconnect the carb from the bike. The floats etc seemed to move quite freely. I'll clean the part the jet goes into. I have been using an ultrasonic cleaner for all this - assume that's ok. How likely is it I need a bigger jet - I don't mind buying as they are cheap but obviously if it's pointless I won't. I currently get about 78mpg on the bike.

What solution are you using in your cleaner? How long are your running it? What temperature?

If it were me, and I suspected anything might be clogging up the carb, I would pull it completely.

All things being equal, a bigger jets means you are using more fuel. May or may not have any real bearing on your mpg...

edit: Just went back and re-read your OP. The hair in the main jet yes, will affect your your carb at 3/4 throttle. Keep in mind that is throttle position, irrespective of speed or rpm. Generally speaking, your idle jet works all through the throttle range, but directly responsible for 0 - 1/8 throttle. The needle and needle jet (nozzle, whatever) handles between 1/8 - 1/2 throttle position. Your main jet doesn't even come into play till after 1/2 throttle. You can even take out your main jet and ride around all day long at 1/4 - 1/2 throttle with no issues whatsoever. You will bog out rich if you much past 1/2 throttle. Driving around with no main jet is one old school 2 stroke tuning trick to make sure you are not too lean while breaking in a new top end.

It might be that you need a smaller main jet since you have a free(er) flowing exhaust, not so dramatic on 4T as 2T but still you might be getting too much fuel at full throttle, and now just too rich.

What does your spark plug look like after running around with no ooomph at 3/4- full throttle?

water? 10 mins. I used the cold tap I didn't actually suspect I had anything clogging if it wasn't for the 3/4 throttle issue with the new exhaust. I'll see how easy it is to get the carb off completely. This is all very enlightening! Not sure what you mean by "no ooomph at 3/4- full throttle?". Do you want me to look at the plug after running on idle and at 3/4 throttle? The engine feels great all the way from idle to the 3/4 point then feels like it's run out of fuel, if I release the throttle then she's fine but it just makes overtakes a bit dicey at times. Should I run the engine at 3/4 in the garage or should I go to a nearby hill then switch off and glide back home? I'll pull the plug straight after in either case.

"no ooomph at 3/4- full throttle?" Those were your words from the OP.

Loss of power (no ooomph) can either be too much fuel, or not enough. The color of your plug will tell you which one it is. Or get a hand full of main jets and experiment.

Your ultra-sonic cleaner with only water will not dissolve any gummed up fuel, only some solvent of some sorts will do that. After several years of small engine repairs (think the lawn mower left out over the past winter), I use my ultra-sonic cleaner almost daily. Through much trial and error, I have found that Pine-sol works well on carb bodies, and jets to dissolve gummed up fuel. If the fuel has completely dried into varnish/nail polish then chemical bath is about the only way to deal with that. Vinegar works well on the brass bits, and CLR Remover (calcium-lime-rust) works well on any bits that have mineral deposits or calcification. I use a 5:1 solution with plain water. I also have found using salsa jars, or any re-sealable wide mouthed jar works well to keep the different solutions in, and keep only water in the tank. Doesn't require a tank cleaning when I want to change fluids. I can usually fit two or more salsa jars in the tank, and clean different parts in different solutions at the same time. The ultra-sonic waves pass through the glass with no problems. I strain out all the larger particles of each jar of solution with a paper coffee filter and a wire strainer. I can re-use some solutions for 6 months ore more.

I find the ultra-sonic cleaner is a time saver since non metal parts must be removed for chemical cleaning. I disassemble the larger components and plop them in the Pine-sol solution for two, 20 min cycles, while I work on something else. If a paper gasket is stuck on, I leave it. The time saver is not having to completely disassemble every single little non metal part typically required for a chemical dip. Most good ultra-sonic cleaners have variable temperature, and I find 50C to work well.

Pull your plug after you have run at 3/4 or whenever it seems to loose power. Dark colored plug will indicate a rich condition, lighter color will indicate lean. Try not to overdo it, but keep the idling to a minimum, maybe hit your kill switch, push it to a safe place, let it cool and pull the plug. You want to understand what is happening at that slice in time when it is loosing power. You plug color will tell a story.

Sorry for the delay in replying. It's difficult getting the bike up to 3/4 throttle within range of my home but managed to run at 3/4 for about 100 yards before emergency braking and pushing it into the garage. I did have to suffer a few stares from the neighbours. The bike wasn't cutting out during this but it didn't feel 100% either. It was only a quick trip to the shops and back so only a mile - not sure if that makes a difference. It's also quite warm at the moment so I don't think the choke would come on. This is a picture after the plug afterwards. It looks too rich to me but I'll wait for your opinion. I'll get the carb off the bike tomorrow.

The float bowl are of the carb looked quite clean to me but I'll post a pic when I get it off again.

Looks sooty, and too rich to me as well. BTW, your choke is electric and normally engaged until it sees 12v, then it slowly closes off the choke (enriching) circuit of the carb. If you know how a car thermostat works, (wax plug that expands and contracts ), the choke mechanism is similar. Outside air temp will not have that much bearing.

My suggestion is to get a new plug or two, and a hand full of main jets on either side of your current. You currently have a 95? Buy 80-85-90-100-105-110. Start with the biggest and see how that compares. With a new plug, both in performance and color, you need more than a few miles to get any color on a plug. Performance will immediately tell you if you are going the wrong way.

If your are too rich (which I suspect you are) move down to the 90, then 85, then 80 to find the spot where you do not loose power at 3/4 throttle. When you find the spot you want to be, check your plug after 100 miles or so. Then put another new one in, you're on your way.

Have a look at your air filter too. If that is clogged up, it can foul the plug up in a similar fashion, but should not have that consistent loss of power at a predictable throttle position. A really clogged up filter will create more problems throughout the throttle range, more so at idle. Probably not the problem, but good to eliminate that as a cause.

So I pull the air filter and clean/replace if necessary - if that fixes things then obviously I'll stop there. Next I'll pull the carb out completely. Check what size jet I have (I think it's 95 on the Walbro and no-one would have changed it). Clean the jet and the thing the jet and sits on with with the water/vinegar solution (I may have some carb cleaner from way back). In truth it all looked pretty clean though. I'll also order some different jets and experiment.

Is there a chance I'll burn a hole in the piston if it goes too lean? I've heard of this happening but have no idea whether that's a real risk or how to detect it before it happens.

Finding jets that fit seems to be tough. The first pair I bought definitely don't fit. It was suggested that Minuki small jets might fit the Walbro but I must off got larger ones. I might just get the smaller ones to start because I'm a cheapskate.

Thank you for you help in this it's been very educational.

when I installed my LV 4Road, I went up to a 105 main jet, also running that on my new one with the pm tuning PM62.

I've just bought a 105 jet and the engine sounds like it's running well but I haven't had time to take it out for a full test yet. I tried reving the nuts of the poor thing in the garage but I always get that funny feeling it might just shoot of the centre stand and take me careering across the garage. The air filter was quite dirty so I wondered about getting one of those Malossi red air filters - opinions? I also just noticed that the carb heater thing wasn't attached to the carb - does anyone know what type of screw/nut I'd need to fix it back on?

The #105 jet seems to have fixed the issues! I also replaced the air filter so that may well of contributed too. The bike has noticeably more oomph but that might be because the exhaust growls now as I accelerate and burbles and pops as I come to a halt and I've been testing the bike out at full throttle as much as possible. I guess a more free flowing exhaust, more air from the clean filter and more petrol from the larger jet would mean more oomph. In all honesty I didn't really want more oomph so I might try a smaller jet as I suspect it's a little too big at the moment, maybe a #102?

I don't have any figures for MPG as yet.

RESULT: It's certainly no longer quitting at 3/4 throttle but it is using too much fuel. This might be psychological but the fuel dial does appear to be dropping a lot faster than before. The exhaust also sounds a bit too loud now and is always gurgling and popping which I suspect is unburnt fuel. I'm going to try #100 next.

QUESTION: it's a walbro carb but small Minuki jets fit. I used a small head jet before which exactly matched the original. Could I use a slightly larger headed jet? I have some Minuki jets that fit fine, that are #100 but the head on the jet is about 1mm larger that the old jet - it doesn't seem to interfer with anything but I thought I'd check here first.

This is the jet I want to try installing.

Extra question: does anyone know what type of bolt holds the carb heater onto the carb? my carb heater isn't attached at all so I wondered about fixing that while I was doing this.