General Questions

I was wondering:

1) Once one opens a fuel valve on any moped, does the fuel flow because of gravity DOWN the tube to the carb or is there some kinda pressure action going on?

2) In a Tomos carb, (DelLorto) there is an L shaped "jet" right next to the air filter. It is removable and is covered up by the choke action when the choke is on. What is this for / how does it work?

THanks for any answers. I appreciate all of the help.


Re: General Questions

I'm not sure,but it might be you're describing the oil injection tube.And on almost all mopeds,fuel is strictly gravity flow.

Re: General Questions

This moped doesn't have oil injection. From what I can tell the gas is sucked up the main jet into the brass pipe with two holes (jets of some kind I guess). These two jets in the main cavity (center), I'm assuming squirt the gas into the air and it goes into the carb. However, before it gets to the two jets in the pipe, there is a hole on the other side that goes through the carb and comes out near this other "jet". It looks like the fuel flows down the carb and is sucked up through this jet also. It is seated behind the brass pipe where it came from.

The reason I asked about the gas line is because I noticed that when my rough running ped was going last night that there were HUGE gaps in the gas. When I unplug the line gas flows quickly and smoothly out of the tank. When I plug it in it stops and there are gaps and bubbles. Ahhh, this would suggested that somewhere in the carb, it is flooded with gas and can't take anymore. So I open up the carb and can't find much other than the wierd jet that I previously described. I cleaned that and stuck it back together. Haven't tried it again since but it probably won't be any different. The bizarre part is that if the carb is plugged then the cylinder and plug should be dry, right? Well, their damp. That is why I thought that the timing is off. I think I have a couple of problems going on at the same time. I didn't touch the timing last night b/c I'm not sure what I'm looking at. Afraid of making it worse.


Reeperette /

Sounds like the float could be stuck too.

The float in a DelLorto is really tricksy to put together JUST right...and if ya don't, it sticks.

I know the L-shaped thingy you speak of, as fer what it does, I have no idea.

Take a small bit of fishing line and try pushing it through, see where it comes out, and that might give you an idea of what, exactly, it does.

I've never seen it, or it's condition be a factor in running in over 10 years of workin on em, so you can prettymuch safely look to other causes.


Re: Hmm..

Can you give me any hints on what to look for as far as the float? Putting it together JUST right? Maybe you could explain to me why that might be my problem. I know the float controls the valve/pin that shuts off the fuel when the compartment gets full. However, if the float is stuck open what happens?

Re: General Questions

Ron Brown /


If the fuel runs into the carb when you turn the gas on and then stops, your float valve is ok. Make sure that the vent hole on top of the carb is clear and that your choke mechanism closes all the way when it is off.

The spark plug is the best indicator of how good your mixture is, so check it.


Re: General Questions

Ron Brown /


Your #2 question seems to be describing an "enrichener jet", or "choke", which provides a rich mixture for starting by sucking extra fuel from the float bowl.


Re: General Questions

I would agree. However, the choke plate that minimizes the amount of air going to the inlet sweeps in front of the "jet". Therefore, it is cut off. I don't think it is associated with the choke but I could be wrong. It seems the fuel comes out of the brass pipe through the carb and collects on the bottom of the carb. Then somehow the jet sucks it up and pushes it out. Do you think it is an extra jet for when the throttle is all the way engaged?

Re: General Questions

I'll check the vent hole. That sounds like a really good suggestion. That sounds like it wouldn't let me fuel in if it isn't getting the air for displacement.....

Re: General Questions

Somewhere I have a drawing of several different Dellorto carbs.It must be at home.The carbs on my Sebrings and Cosmo colt are not complicated with anything like that,and my Tomos book only talks about Encarwi carbs with no pics.One of my 4 tomos may have one like that though,so I'm a little interested.I'll try to find those drawings soon.

Re: General Questions

Okay, I've been thinking about your reply...I know what a concept. I know there are two hole in the float cavity at the bottom of the carb that lead up to the top of the carb. However, it looks like the throttle/carb cap (black) covers thes holes so that they can't breathe. Are there other holes that I haven't noticed?

What is the problem?

You are asking all kinds of unrelated questions.. and making bad assumptions.

Do you actually have a problem?... Or not?

If you want to know about carburetor theory... libraries have books on that.

The only problem you seem to be having izzz... You are worried about air bubbles in the fuel line.

Thoe are normal... quit worrying about them.

If the moped doesn't run right... then tell us what is wrong... don't be jumping to conclusions when you dobn't even have a problem.

Re: What is the problem?

Okay, I know I've been jumping around. I started this thread initially to ask a couple of general questions so that I could inform myself on why my moped has no power, sputters when it is going and seems to bog down when I open up the throttle. I think I have many problems with the moped and will have to take it in somewhere to have someone more experienced look at it.

1) I know some bubbles are normal but fuel not flowing is not. The fuel flows into the carb then for all practical purposes it stops. I little must get it b/c it keeps sputtering for a minute. All of the fuel circuits are clear. I've looked it over a dozen times. I like the idea of an air vent hole though.

2) I know I need to clean the exhaust and that this causes power loss. I haven't done it yet but I don't think this is causing my engine to bog down and sputter.

3) There are small leaks between the cylinder head and cylinder block. I see bubbles. I've tried reinstalling the head a couple of times. No change. I must correct this, I know.

4) All lights work, all carb seals replaced, new plug, new gas, new tranny oil, good spark but I haven't checked the timing b/c I don't have ANY idea of what I'm looking at and I figure I'll probably make it worse! The inlet seals around the jug look good. Everything is tight (but I haven't tried the cleaner test recommended by someone here yet). I'm not 100% sure that I got the gas/oil mixture 100% correct but from what I can tell it should still run a little smoother than it is.

5) I've run the engine without the air filter. Still bogs down and sputters to a stop in a couple of minutes.

I'm sure I'll think of more once I post this but I think this is the important stuff. I've read your article many times. I've done a lot of it but like I said I don't feel comfortable yet about the timing. I've learn a ton from posting here. Thanks to everyone. However, it sounds like I'm getting on everyone's nerves. That wasn't my intent.

Stop the leak.

Alright... here is the deal... if the engine is not running... then it is not consuming fuel.. then it will not be flowing fuel into a full float bowl.

(and you cannot "see" the fuel flowing either... it runs 'around' the bubbles)

Second... you are chasing you brain around thinking it is 'timing' (most likely)

You MUST stop the cylinder or head leaks FIRST !!

Forget anything else.. till you do.

Then... you can run with the exhaust pipe unbolted and pulled away if you suspect its clogged. (or remove the pipe and see if you can blow thru it with your mouth... if you can give a fast exhale with your lungs with little resistance... then its not clogged).

If this bike has a CDI and you are getting good spark with a new plug... then your timing is probably set at wherever the factory set it... (should be fine).

Stop the head/cylinder leak before you do anything else.

Re: General Questions

Ron Brown /


I am not familiar with your carb, I did not realize you had a traditional "choke" which blocks the air intake.

Let me try a 2 cent lesson on carburetors and see if you can figure it out.

The float bowl simply provides a fixed level of gasoline for the carb to operate on.

The venturi and slide, the part where the air flows through into the engine, controls the volume of mixture fed to the engine and so the engine power output.

Some carburetors have a tapered needle which fits into the center of the slide and is withdrawn from a needle jet as the slide is raised. If you have one of these, its purpose is to control the flow of fuel from the main jet during partial throttle openings or mid-range. The needle can usually be raised or lowered to vary this mixture by moving a clip which fits in a series of notches in the top of the needle. Many ped carbs do not have this needle.

The brass tube you see, in the center of the venturi directly below the center of the slide, is the main gasoline supply which flows from the main jet, screwed into the bottom of this tube. The tube is sealed to the carb body at the top and bottom and there is a cavity around it between these seals. Air enters this cavity through a passage from the carb air intake side, then through holes drilled in the side of the brass tube. This air is mixed with the gas flowing through the main jet to form a "bubbly" mixture of gas and air which is much easier to vaporize than pure gasoline. This air intake may be what your choke plate is covering as that would make the mixture richer. Old carburetors with a manual mixture control, did so by varying this air flow.

At idle, with the slide in its lowest position, the idle adjustment screw raises the slide to allow just enough air for idling. The fuel for idling is provided in several ways depending on the carb, but they all achieve the same thing. Some carbs have a separate small air intake passage on the intake side of the carb which continues through a smaller passage to the engine side of the venturi. A side drilling provides gas either directly from the float bowl or from the cavity around the center brass tube. The gas supply is regulated by either a removable jet or a precision drilled hole to control the gas/air mixture. Large carbs provide a tapered screw to adjust the idle mixture by controlling the volume of idle air, or of the air/fuel mix. Most ped carbs don't have this.

Some ped carbs get idle fuel/air mix directly from the main jet. When the slide is in the idle position, a channel cast into the bottom of the slide, exposes the top of the brass tube to intake vacuum. This causes the fuel level in the brass tube to rise until the small air flow through this channel can pick it up. I have no proof of this, but I think that the higher fuel level in the brass tube helps overcome a "leaning out" condition as the throttle is opened.

As the slide is raised from idle, the idle passageways begin to lose ther effectiveness and the center brass tube is exposed to the air flow through the venturi. The faster air flows, the lower the pressure of the air. This low pressure above the brass tube allows the aerated gas to flow out of the top of the tube, propelled by atmospheric pressure in the float bowl. The cutaway or angle on the intake side of the slide, controls the speed of the air over the tube at small throttle openings which varies the gas/air mix. As the slide is raised, the cutaway hass less infuence on the mixture and at full throttle, only the main jet controls gas flow.

Cold starting mechanisms are of two basic types, the "shutter over the air intake" type and the "auxilliary carb" type. This type has an extra drilling in the engine side of the venturi, connected to an atmospheric air supply with a connection to the gas in the float bowl somewhere along its length. In operation, the air drawn through the air passage lowers the pressure at the fuel connection, just as the main body of the carb functions, except that the resulting air/fuel mix is very rich. This device is usually controlled by a brass slug which blocks the air and sometimes fuel passages.

A lot of older carbs had a "Tickler", this was a button on top of the carb bowl that allowed you to sink the float and flood the carb for starting.

OK, so my 2 cent lesson became a 25 cent lesson, hope it helps,


Re: Stop the leak.

And... don't forget.. that once the float bowl is full ... the engine will run for a minute or two even after the fuel line is disconnected... it is running on the fuel in the bowl.

Re: General Questions

I have had this problem too. The screen in the petcock was almost fully plugged so not much gas could flow, and there was a pinhole sized leak (a crack actually) in the fuel line at the base where it joined the carb. Fuel would flow but air would come back up the line and the float bowl would empty before the air bubble would clear. A new line and a clean petcock screen (about 10 minutes total time) fixed it right up.


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