SRM-225

PTmowerMech

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SRM-225 with an RB-K93 carb.

First question, doesn't Zama label their carbs "Zama?" Because all it has on it
K93
4Za
Then CHINA in the middle.
Then RB 37

That's it.

Also, the main question. It'll start and idle fair. Giving it throttle is a different story. When I get it to full idle, the RPM's seem about 3/4's. So I figured the screen was blocked, so I took the carb apart. And it was about 3/4 block. Diaphragms weren't too stiff, so I put everything back together. And couldn't get it to start.
Just a little bit of the gasket stayed on the carb.
So it not starting now, is that because there's too much of an air leak? The gasket didn't rip or tare. Just a little bit stayed on the carb.

BTW. Good spark and my gauge read 120lbs compression (cold)
 

bertsmobile1

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Despite what 9,999 your tube experts say
Never reuse the diaphragms & gaskets from a cube carb.
The old ones will have compressed and your chances of getting a good seal are not particularly high.

Over the years I have learned not to jump in and rip a carb apart.
Very few problems end up being in the carb in any case.
But because they are there people feel obliged to rip them apart.
I suppose it makes them feel smart.
You can test most function of a cube carb without opening it up

To see if your carb is leaking, take it off .
Fit some hose to the two fuel spiggots then place the carb in a jar of water.
Block the return line off ( I use golf Tees ) and pump the carb to 10 psi
look for bubbles which will reveal where it is leaking from.
move the Tee to the other side and pressure test the purge bulb
 

PTmowerMech

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I'm not a very good mind reader, but something made you mention a carb leaking. I could see how this would not allow the engine to run at full throttle. BTW, the spark plug didn't look like it was burning rich. it was a tan color and not coated. It the inside and out looked like it had been in there for a few years.
 

bertsmobile1

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Unfortunately the big egos go to all sorts of efforts to make sure their videos come up top when you do a You tube or google search.
As THEY are what is important , not your problem the videos are really about them showing you how smart they think they are.

With a hand held that does not rev as high as you think it should , the usual culprit is the exhaust screen or exhaust being blocked and choking the engine.
A blocked inlet screen causes the engine to run lean which on two strokes makes them run too fast and lack power.

If the screen on the carb was partially blocked then the filter in the tank requires replacement or the carb screen will block again.
Cube carbs are a pressure vessel and run at around 7 psi so a really filthy looking carb screen will still pass sufficient fuel to reach full revs.

Nearly every donkey brain on You Tube will show you them blasting carb cleaner through every hole to "clean them out" with gay abandon.
Very few tell you which ones to spray and which ones not to.
Depending upon the exact carb there is from 1 to 3 check valves.
Carb cleaner will destroy many of them and cause them to become stuck in the closed position.
That is why Walbro went to the expense of making the very expensive carb cleaner that is check valve safe.

When blowing out a carb, you should , where ever possible blow the passages out backwards WITH LOW PRESSURE AIR , not a dust gun hooked up to your sand blasting compressor running 150, PSI that will force the crud so tight that you will never be able to shift it.

Like you, when I started out I watched all of the clots on U-toob and did what they did as cube carbs were totally new to me.
However since then I have learned better and never pull one apart unless the diagnosis is pointing to an internal carb problem .
And over time I have come across some of the better informed presenters and of course gotton my hands on some of the better service manuals.

In your post you mention a lump of the gasket stuck on.
This always happens and when it does you fit a new one that is nice & soft so can conform to the surface and make a proper seal.
If you run a cube carb on a wet stone you will see that the machined surfaces are not smooth but contain machining grooves.
This is intentional and is there to allow the gasket to fill in the grooves and thus make a pressure tight seal.
It also provide an escape path for air that gets trapped by the gasket.
It is also why the gaskets stick and break up when you are trying to take the carb apart and also the reason why you should never reuse them.

So now days the only time I jump right in & pull a carb apart is when a customer comes in with a tool that worked fine last season, has not been used for months and now won't start.
On those you can be fairly sure the carb will be gummed up, but only around 1/3 will be able to be cleaned properly.

Around here I have become the "go to guy" for problem hand held tools and this is purely because I replace fuel lines, filters & grommets
So when I have adjusted a tool & sent it back, it works as it should for several years till it is time to replace the fuel lines again.

Now back to you and your problem
Pull the carb off the engine and leave the fuel lines attached.
Double check you have them on the right way round.
When you pump the primer bulb the line that blows out the fuel gets connected to the short fuel line that stops just inside the tank
Turn the carb sideways and blow air through the venturii and watch for atomised fuel mist coming out the engine side of the carb.
It should be a strong dense stream.
If not then you have probably wrecked the check valve, in which case it will be a lot easier & faster to replace the carb.
And Zama carbs have the work Zama casr into the body or pump cover depending upon where the pure bulb is located.
If it does not say Zama then it is not a Zama
 

7394

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Had one doing the same & I changed the tank fuel filter, & runs like new again..
 

PTmowerMech

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Thanks for ALL of that information. I learned my first year, not to use carb cleaner on these carbs. Or compressed air within 6in from the carb. I blew a couple check valves out, and never found them. I do use seafoam spray, lightly. That may be too harsh. IDK. The small engine guy I worked for last year, used it. He was a 2 cycle wiz. Problem was, he wouldn't teach me much, because he was afraid I'd put in against him. LoL.


As far as the youtubers go, I do watch a lot of their video's. But I'm pretty good at filtering out the most of the ones you described. Do you have any youtube video's?
 

bertsmobile1

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No.
I do not have a big enough ego to be bothered and do not see my job as educating the entire planet.
I do show customers what I do and explain why I do it because it should not be secret service mans business.
Most appreciate it .
I am here to learn just like every one else.

As you are setting up I hope you have a pressure / vacuum tester
Seek out the videos by Joe Pace- 2 stroke diagnostics or some thing like that.
That is where I learned the magic of pressure testing tanks, carbs & caps.
Quite honestly at least 75% of the problems are there and not inside the cube.

After that it is getting the EAR

Adjust an engine till it 8 strokes and stalls out rich a dozen or so times
Then do the same thing lean
You will hear the distinct different sound of a lean stall out & a rich stall out.
That makes diagnosing really easy once you have your ear in.

This applies particularly when adjusting the cross over between L & H jets on the older carbs.

When I was starting out I went to the effort of getting a smaller color tune ( Gunsens make them ) and setting up the video camera on the colortune while was adjusting the H jet.
On a few occasions when I have been really stumped I have resorted to doing that again.
I inherited a trimer clamp from the previous owner but found that a Triton Super Jaws works a lot better as it is a not more stable.
I can also clamp the bar of a chain saw in it so adjust the saw , chain on in the vice, really handy.
 

PTmowerMech

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Had one doing the same & I changed the tank fuel filter, & runs like new again..

Ironically, this one had no fuel filter at all. Not even just laying in the tank. Adding one made no difference.
 

jp1961

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Hello,

Check to make sure there is no blockage in the muffler. I had a mud dobber wasp build it's nest inside the muffler of a 25cc Homelite motor that caused it not to reach max RPM, a clogged muffler spark arrestor screen will do the same.

Just curious, why aren't you supposed to use carb cleaner on a diaphragm type carb?

Regards

Jeff
 
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