Am I four-stroking? And what to do about it

Hello! New to the forums and mopeds in general so please bear with me.

About two months ago I got a 1983 Suzuki FA50 out of my grandfathers barn. It has been sitting for about 20 years. Changing the spark plug got it to start up, a new clone carb from Ebay ( https://www.ebay.com/itm/201950969646?ul_noapp=true ) got it to start up easily.

(I still have the original carburetor, but being new to this sort of thing, I figured for the price, a new carb was better than taking apart the original and potentially screwing it up.)

The guy who sold me the carburetor said it is completely stock including jets. It idles great and runs great until I open the throttle all the way. When I take it on a good stretch of road to test top speed, it tops at about 20.

BUT: when I let off the throttle just a little hair, it will inch its way up to 25. What gives?

Well I thought maybe the exhaust was clogged so I took it off and threw it in a charcoal grill for a while. I shook out some black flakes for a while, but maybe not as much as I expected, and I scraped out some carbon crap from the exhaust port on the engine. No luck though, but maybe I just didn't clean it well.

A look at Fred's Guide suggests it might be four stroking?

Specifically where he says "and the engine acts like it is choking, like it would like to go faster... and sometimes it WILL go faster if you close the throttle a little bit."

So, my questions are:

Is it my air filter? I bought the foam element brand new from treatland, but maybe I used too much oil... I soaked it and made sure to squeeze it out real good until nothing else came out. Also worth noting is I lost the slider bar that holds the box together on a bumpy driveway and have it held quite tightly with duct tape, but I don't think that would affect much.

Is it the carburetor/jetting? It's supposed to be stock, and I even asked the seller after I bought it about the jetting. He claims its completely stock.

There's a little clip that adjusts how deep the needle can go, should I try adjusting it? The service manual tells me I can make the mix richer or leaner this way. This shouldn't affect WOF, should it? Wouldn't that be the main jet?

What about adjusting the throttle? Is it possible that a wrong throttle adjustment at the handle would cause this issue?

Thank you all, I'm new to this so I'm not sure what to look for and I've tried scouring the internet for answers. It's frustrating because I'm sure that once I sort this issue out I'll finally have it up to stock speed, and I'm too stubborn to take it to a mechanic like my dad suggests.(edited)

Re: Am I four-stroking? And what to do about it

run it wo air filter.

Re: Am I four-stroking? And what to do about it

When I first got it this is actually what I did before I bought the foam element, and it was actually much worse (noticeable deceleration when I opened the throttle all the way). This was before I got the new carburetor and cleaned the exhaust, though, should I try it again? I could try a cone filter too.

I may also try running it without the exhaust pipe to rule that out but I don't want my neighbors to hate me.(edited)

Re: Am I four-stroking? And what to do about it

You might try applying the choke at WOT . If it runs better , you'll know the main jet should be a touch bigger .

These carbs are pretty simple . Tear the original apart and clean it thuroughly .

Note which size jets are installed and compare that to what's suppose to be stock .

Here are a service and owners manuals . Download them and become very good friends with them .

https://www.manualslib.com/manual/999530/Suzuki-Fa50.html

https://www.manualslib.com/manual/998156/Suzuki-Fa50.html

Re: Am I four-stroking? And what to do about it

I will try those suggestions. Now that I have two carburetors I'm a little less skittish about ruining one ;)

I'll try running with the choke on but if everything is stock I shouldn't need to rejet though, right? I worry if I rejet I might just be band-aiding some other issue, like an air leak... Admittedly, I reused the old gasket for the new carburetor.

I'll get back in a few hours but still open to suggestions so I can try 'em all at once. Thanks!(edited)

Re: Am I four-stroking? And what to do about it

Clone carbs rarely come with internal components that are in tune with your bike.

Re: Am I four-stroking? And what to do about it

New carb is probably overjetted. When you said you CHANGED the spark plug,was it a new one?

Re: Am I four-stroking? And what to do about it

Yeah it was brand new, I went out and bought a matching NGK plug.

Re: Am I four-stroking? And what to do about it

Whenever I have suffered 4-stroking it was always over-jetted or a restrictive airbox.

Re: Am I four-stroking? And what to do about it

Jeffrey Strichart /

> Don Ohio Wrote:

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

> Whenever I have suffered 4-stroking it was always over-jetted or a

> restrictive airbox.

It was always removing the stock air filter that fixed it for me. Every time. I bought a brand new Magnum in 1979 or whenever they first came out and it would only go about 22 before it four stroked and would not go any faster. Yanked the air cleaner and went 30 immediately.

Re: Am I four-stroking? And what to do about it

I’m going to clean the old carb and put it back tomorrow cuz I got the day off but I also noticed the carb leaks a bit of fuel out the overflow at the bottom, would this be an indication it’s running rich or some other problem, or is it normal? It doesn’t seem like a lot, it is barely enough to wet the surfaces the overflow tube is resting on, and make my finger wet if I touch the end of the tube. I can get pictures in the morning for a better visual.

Re: Am I four-stroking? And what to do about it

I’ll also run it without the air box and see how that goes, it’s just a pain to find a filter than can fit the FA50 given how close the carb is to the rear tire.

Re: Am I four-stroking? And what to do about it

Jeffrey Strichart /

> Drew Cooper Wrote:

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

> I’m going to clean the old carb and put it back tomorrow cuz I got the

> day off but I also noticed the carb leaks a bit of fuel out the overflow

> at the bottom, would this be an indication it’s running rich or some

> other problem, or is it normal? It doesn’t seem like a lot, it is barely

> enough to wet the surfaces the overflow tube is resting on, and make my

> finger wet if I touch the end of the tube. I can get pictures in the

> morning for a better visual.

How many times did you press the primer button without starting it?

Re: Am I four-stroking? And what to do about it

> Jeffrey Strichart Wrote:

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

> > Don Ohio Wrote:

>

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

>

> > Whenever I have suffered 4-stroking it was always over-jetted or a

>

> > restrictive airbox.

>

> It was always removing the stock air filter that fixed it for me. Every

> time. I bought a brand new Magnum in 1979 or whenever they first came

> out and it would only go about 22 before it four stroked and would not

> go any faster. Yanked the air cleaner and went 30 immediately.

I always just took a drill to strategic places on the air box and thereby increased the flow .

Re: Am I four-stroking? And what to do about it

I don’t have a primer button. There’s a prime lever on the fuelcock but I only had to use that when I first replaced the carburetor.

Re: Am I four-stroking? And what to do about it

Jeffrey Strichart /

> Drew Cooper Wrote:

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

> I don’t have a primer button. There’s a prime lever on the fuelcock but

> I only had to use that when I first replaced the carburetor.

Gotcha. Obviously it should not leak out of the overflow for no apparent reason.

Check your carburetor for plugged jets or incorrect float setting. If the jet(s) are plugged even a little it will not run right and gas would accumulate in the bowl while the engine is running and overflow. If the float is not properly set it would fill up and overflow while the bike is parked (if the petcock is on). Turn the petcock to off and empty the bowl. Then turn the petcock back on. If it does not overflow while parked then the float should be ok. Also a faulty petcock could cause a problem but that is rare.

Re: Am I four-stroking? And what to do about it

An FA50 uses an automatic fuel valve ( petcock ) .

Unless something is not right or it has been replaced with a different style , anytime the motor is not running or being turned over , fuel will not pass until it is switched to the prime position .

If the carb is leaking out of the drain at the bottom of the bowl : the float may be stuck ; the float valve may be dirty ; the drain screw might need attention or the brass over flow tube could be cracked .

In stock configuration , the FA50 runs best with a OEM type foam air filter set up and the carb adjusted per the manual , exactly .

My FA50 started first kick , most often , and held 30+MPH for as long as I cared to ride , when it was totally stock .

Now , technically , it isn't completely stock : less 1 ring and the piston crown has been 'channeled' which lowered the compression a bit slightly quickened transfer and exhaust flow . The carb has been adjusted to accommodate the changes and the bike does as much as 34 - 35 MPH . The piston and rings are due for replacement as both have about 8,000 miles of pretty hard riding .

Re: Am I four-stroking? And what to do about it

Mine is completely stock except for the clone carburetor.

I took the air filter off and it didn’t bog down at WOT like before, but it still only topped at 25. I’m gonna try to clean the air filter first and if that doesn’t work I’ll just put the old carburetor back.

Re: Am I four-stroking? And what to do about it

I guess I’ll have to anyway if it’s gonna keep overflowing

Re: Am I four-stroking? And what to do about it

Is it possible my throttle may be opening too wide? I read somewhere that if the throttle isn’t flush with the top of the carburetor bore at WOT, and goes too far up, it can cause “turbulence”... Is this adjusted with the nut at the handle?

Re: Am I four-stroking? And what to do about it

Slide adjustment is best accomplished via the adjusting ferule atop the carb .

Re: Am I four-stroking? And what to do about it

I took apart the clone carb and the main jet was completely unlabeled and the overflow was a completely different style than the original. Needless to say I’m gonna take my lumps and clean out the old one.

Re: Am I four-stroking? And what to do about it

So I put the old carburetor back after what I think was a good cleaning, and it stopped bogging at full throttle but still only went 25. I noticed an air leak (detected with starting fluid; engine died when I applied it to the intake manifold) which applying some Permatex seemed to fix. Any issues with top speed should be with the engine itself at this point, right?

Re: Am I four-stroking? And what to do about it

Jeffrey Strichart /

> P D Wrote:

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

> An FA50 uses an automatic fuel valve ( petcock ) .

>

> Unless something is not right or it has been replaced with a different

> style , anytime the motor is not running or being turned over , fuel

> will not pass until it is switched to the prime position .

Interesting. Maybe the valve is faulty?(edited)

Re: Am I four-stroking? And what to do about it

Jeffrey Strichart /

Good idea. I don't know why China makes such bad products and not just moped parts.

Re: Am I four-stroking? And what to do about it

> Drew Cooper Wrote:

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

> So I put the old carburetor back after what I think was a good cleaning,

> and it stopped bogging at full throttle but still only went 25. I

> noticed an air leak (detected with starting fluid; engine died when I

> applied it to the intake manifold) which applying some Permatex seemed

> to fix. Any issues with top speed should be with the engine itself at

> this point, right?

Not necessarily . Once the motor runs 'good' ( tuned to factory specs ) , then the rest of the machine has to be made to 'run' as good .

>Stock air filter in place .

>Exhaust tight and clean inside . I use cheap oven cleaner which seems to do a good job . I have burned the exhaust out with straight gasoline , but , that can get real exciting very quickly ( don't do it ) . I've heard that drain cleaner does a decent job .

>Transmission has to be copacetic with proper oil applied . I have tried many different oils and have found that 15w-40 diesel oil seems to produce the best results . It should be noted that the best results may not be seen immediately because of 'deposits' left from previous oils .

>The wheels have to be free rolling/spinning ( correct lubed bearings ) with no brake drag .

Obviously , the rear wheel will have a bit more drag when on stand , due to transmission interaction .

Those last few MPHs are the hardest to have .

Always remember that it's the little things that make the big things run . ;)

Re: Am I four-stroking? And what to do about it

I see. The manual makes a great deal out of cleaning carbon from the combustion chamber and exhaust port as part of the maintenance procedure, and I've already put my stock carburetor and air filter on, as well as fixed any air leaks. It runs consistently at full throttle, but just barely breaks 25 mph. Judging by the amount of carbon I found in the exhaust port while cleaning the pipe, there's probably still a fair amount in the combustion chamber.

So I suppose my next step will be a thorough inspection of the engine and transmission. Maybe a compression test as well? How many kicks will give an accurate reading? Just once? And lastly, the manual only gives a compression ratio of 6.5:1. How does this translate to PSI?(edited)

Re: Am I four-stroking? And what to do about it

The exhaust pipe has to be totally clear / clean . That's why the use of oven cleaner or even drain cleaner .

Go ahead and pull the cylinder and head , along with the piston . Remove all carbon . That includes any blow by on the piston and it's likely there will be build up in the ring landings . Remove the rings , carefully , and use a pick or small screwdriver to clean the landings .

Clean the cylinder ports of any build up .

Pull it back together and do a compression check . 100 pounds would be the low end and a signal that it needs rings , at least .

Hold the throttle wide open while kicking multiple times to get an accurate reading .

"How does [compression ratio] translate to PSI"

This , from reference.com :

To calculate the pounds per square inch (psi) from the compression ratio, one needs the compression ratio and the atmospheric pressure measurement. Then, the first number in the compression ratio is multiplied by the atmospheric pressure, then divided by the second number in the ratio.

Re: Am I four-stroking? And what to do about it

6.5:1 CR

average sea level atmospheric pressure = 14.7psi

6.5 * 14.7psi = 95.55 psig (on a gauge).

if you're at any kind of altitude, you have to take that into account (which is what the paragraph from PD explained).

Re: Am I four-stroking? And what to do about it

Based on the CR being the real compression ratio and not the swept compression ratio.

> ken gilbert Wrote:

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

> 6.5:1 CR

>

> average sea level atmospheric pressure = 14.7psi

>

> 6.5 * 14.7psi = 95.55 psig (on a gauge).

>

> if you're at any kind of altitude, you have to take that into account

> (which is what the paragraph from PD explained).

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