Tomos goes fast then slow

I have a 94 Tomos Sprint. It has new points, new plug, and a few other new parts- It runs great, starts first try every single time. My husband is a 250 pound man, but it gets him to 30mph.

But, when at full throttle, he notices that it would slow down, then speed up- this is a difference of maybe 2 mph- 2 mph faster, then 2 mph slower. But, when he drives it at a slower speed, such as 20 mph, it stays even and doesn't jerk.

I would assume that it's a soft sieze and the head/cylinder is too hot? If that's the case, how do I correct the problem? Add a little more oil? Thanks!


Re: Tomos goes fast then slow

i'd suggest that there may be a fuel system restriction. Here's the scenario.

At top speed the engine is sucking up a lot of fuel from the tiny float bowl. This fuel must be replaced at least as quickly as it is being used. If it's not, fuel level in the bowl drops.

This drop in fuel level makes the engine run a bit hot and lean for a moment and power output is quite good.

But since the engine continues to demand fuel at a high rate, the leanness is closely followed by a further drop in float level until a there's complete lack of available fuel.. and the engine starves and the bike slows down.

Fuel is still entering the float bowl and refills it. The engine then sucks some up and begins to output power again, and the bike speeds up for a moment.



The engine is hunting for fuel..


Pull the fuel inlet hose off the carburetor and drain some into a clean glass container. Fuel should flow in a thick steady stream. If not, check upstream for obstructions.. (some kinda dirtly fuel screen or filter)

Dirt or rust clogging something in the carb is possible..

Another possibility is that fuel level in the gas tank was low.. fuel splashes around and is not continuously supplied to the carburetor..

I used to have a bike with a carb that just could not keep up with the engine's demand for fuel.. Everything was in perfect tune and clean.. Symptoms were very similar to yours.

After a very difficult diagnosis i ended up enlarging the carb's intake float valve orifice and it was cured.. But this is not a common situation. A clean, properly adjusted carburetor normally supplies far more than required.

long freakin post.. maybe cause im wired on coffee and cant sleep..

Re: Tomos goes fast then slow

Thank you so much, that makes so much sense.

I think you may possibly have solved our problem. My husband will be taking a look at it tomorrow.

Everything on our moped is super-clean, & well maintained, but the gas line is old, and I think there's one area where it's kind of squeezed against a bolt. We'll be replacing that hose just to be on the safe side as well as checking the petcock for any obstructions.


Re: Tomos goes fast then slow

you're welcomed ..

btw, it may make sense and yet it may be wrong.. so it may be wise to keep an open mind while you're in there investigating.

Re: Tomos goes fast then slow

this is exactly what has been going on wiht my 2005 tomos ST...goes from 30 to 26/27 then back to 30, makes a miserable gurgling sound. however, be careful if u get into the carb. it took me a week to figure out that i litterally had to drive it off the engine (gently tapping) while everyone else just wiggles theres off, then i discoverd, contrary to the exploded unlabeled diagram in the repair manual i dnloaded, that hte only air filter is apparently in ht airbox and not in the gasket connection in the carb where it attaches (see my post on here about my carb problems). ive got 2700 miles on it and ride it daily as my general transportation.

also, dont bother the throttle linkage on top of the carb. you can get to everything in it without doing that. mine ran fine after i cleaned out the jet wiht a wire, but now it wont start at all, probably something to do wiht the airbox.

dont mean to be long, just thought you might wanna get a heads up on such since this is exactly whats g0ing on wiht mine. i also had anohter post earlier on the GD forum before i realized it needed to be on here.

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