Zama RB-K75 Adjustment

[KE4AVB]

Anyone knows what tool is use to adjust the RB-K75? It is a new carburetor and I just can't see down hole to determine if is a hex, spline, or what.

[bgsengine]

Anyone knows what tool is use to adjust the RB-K75? It is a new carburetor and I just can't see down hole to determine if is a hex, spline, or what.

Which adjustment?

The center adjustment within the rotary valve rarely if ever needs to be adjusted unless someone's already screwed around with it. Have yet to see one that needed it.

The other one adjusts with a standard slotted screwdriver (but you got to remove the plug, which can be tricky) and that's the only adjustment I've ever seen need to work with.

Have the tool for the RB carbs (Single D) K75 tells me you've got an Echo.. I'd first make sure of everything else before determining carb needs adjustment - Those rotary valves have been *highly* reliable and indeed, have never even had to remove the valve or adjust the screw, except in cases where someone else was messing with it and lost the roller (very tiny part,not even 2mm dia.) because they didn't understand the carburetor.

First thing to do is make sure when you open it to WOT, the bore in the valve lines up *exactly* with the bore in the carb. if it "sticks down" a bit, you're missing the roller, so replace that (P005001000) and then likely won't need an adjustment.

[KE4AVB]

Tnx for the reply Brian. Yes the an Echo HT series hedger. It is a new carburetor and I just having a problem at WOT where spits and sputters like the lack of fuel or too much but of course it could be that it is near freezing in shop. The hedger starts and idles fine. And before someone laughes about it there is a reason why my current shop is unheated but I can't go into it here as it is something I got resolve resulting from my mother's death.

I thought I would try adjusting the mixture first before going inside it to check the metering lever which have I received a few where it was set wrong from the factory. The anti-tamper plug has yet to be install; I just overlooked the adjustment by the primer.

It is starting to snow outside so wait until later so I can strap it down to outdoor test bench while making the adjustment; where adjustment is at makes it unsafe unless I do so especially with that blade so close.

I will also do a check for the roller as someone may have had the carburetor and return it. As you say these rotary carburetors are usually trouble free.

The reason I am having to replace the carburetor another person had cleaned it with carburetor and destroyed the check valves. If they had only check the rest of the fuel system they would had found a clogged fuel filter was their main problem. It one of those die-cast type where they corrode shut. It no telling what else they did while they had it. It is just frustrating when I know else has messed it up and I got to straighten out their goofs first before I can get to the real problem.

[bgsengine]

Tnx for the reply Brian. Yes the an Echo HT series hedger. It is a new carburetor and I just having a problem at WOT where spits and sputters like the lack of fuel or too much but of course it could be that it is near freezing in shop.

As you say it idles fine, you DO NOT need to adjust the idle (The screw that is down in the rotary valve is the idle mixture)

make sure muffler, spark arrestor, exhaust port is completely clear,. If it is a newer emissions compliant unit with catalytic converter muffler, and poor quality mix oil, it can be a problem there (backpressure) I'd also make sure it warms up at least 3 minutes before any adjustment is attempted.

Imma assume you got a HC-150 , so the instructions that follow cover that (if it is some other machine, post the model and serial like we're all supposed to , and I'll dig out the right info)

From Echo Service Bulletin 06-25:
General Adjustment Rules
Before starting the unit for adjustment, check the following items:
• Check for correct spark plug must be clean and properly gapped.
• The air filter element is clean and properly installed.
• The muffler spark arrestor screen and exhaust port are clear of carbon.
• The fuel lines, tank vent, and fuel filter are in good condition and clear of debris.
• The fuel is fresh ( > 89 octane : R+M=2) and properly mixed at 50 : 1 with “ISO L-EGD” or “JASO-
FC” 2-stroke oil.
• The gear case assembly with blade set must be installed for proper engine loading

Start and run engine for 5 minutes alternating RPM between Wide Open Throttle (WOT) and idle
every 5 seconds. Adjust idle speed screw to 3,000 +/- 250 RPM. If engine does not run correctly after
this adjustment, follow guidelines for carburetor adjustment with limiter caps removed.

Adjust
Hi mixture needle (D) to obtain maximum
(WOT) engine speed before for lean drop off.
(WOT) engine speed should be in the range of
9,500 to 11,500 RPM.

Start engine again and make sure engine runs in
the range of 2,750 to 3,250 RPM at idle and
accelerates smoothly to 9,500 to 11,500 at
(WOT)

There's more to it, including idle mixture adjustment, but as I said, if it idles fine, you should not need to adjust that mixture.

Note- the spark plug must only be an NGK BMP8Y or BPMR8Y - There is no alternative cross reference plug that works *correctly* and wrong plug will result in as much as 1200 RPM drop in performance.

Lastly, if needs be, I can email you a copy of the whole TSB.

[KE4AVB]

Just needed my memory jarred and yes I should posted the make and model of the hedger (ECHO HC-150) but my main question was directly related to the Zama carburetor. From here I should be able figure out the rest. For me at least 2Cs can have some strange problems which can be either fuel or ignition. I still remember that Homelite saw from last year where it wouldn't run at top end which turned out to be an ignition problem.

The muffler spark arrestor screen and exhaust port are clear of carbon.

One of the first thing I thought of...that one has bit me as few times when I was first learning two cycles. I even went as far check the ring and cylinder condition while I had the muffler off. Some light scoring on the intake side and exhaust side of piston is clean, no scoring.

I post the solution once I get it to run properly.

[KE4AVB]

It was running too lean. After the carburetor adjustment it now runs at full speed now. I will wait until Wednesday when temperature will in the fifties to do the final tuning.

The crank seals do show some leakage but I expect that when it near 35 degrees outside and as old as they are.

[Echofan]

I realize this post is over a year old, but I'm working with a RB-K75 on a GT-200R trimmer.

Completely rebuilt carb with kit, and had to remove the high mixture plug and needle to clean. The idle RPM screw was adjusted before I got it, but the idle mixture plug has not been touched. This unit sat with gas in it for several years.

I have the unit running with a new dry plug I can get it going. As the unit sits I have high amounts of gas in the combustion chamber. Main jet is out about 1.5 turns from seating, and the idle RPM screw is in about 3 turns, once the idle screw touches the throttle shaft.

The only entry of fuel into the carburetor air stream is through the main mixture torch that comes up the throat of the carb right?

Muffler has been pulled--Clean
Cylinder Inspected--No chrome missing-- through exhaust port.
Top of piston-- Black Ash covered.

Compression-- No gauge yet, but seems like this was a low use unit, as it was in my family since new.

I've been up and down the Zama troubleshooting guide, and yes the roller is in place, and the rotor ramps up smoothly.

When I get it running I can get it to idle and run up pretty smoothly. Kill it, wait 2-3 minutes and it will not restart without drying the plug.

When it's running it seems to be pulling a large amount of fuel through the primer bulb circuit. More than other newer Echo's that I have.

I think this unit may have the variable timing coil, which probably doesn't help my troubleshooting.

Any ideas other than a Carb? I'm really trying to master the RB carburetor. As the next piece of equipment will be my new Echo Blower when required, and I wanted something to practice on.

Anyone have access to the GT-200R specs? Serial Number 08xxx something.

[KE4AVB]

have the unit running with a new dry plug I can get it going. As the unit sits I have high amounts of gas in the combustion chamber. Main jet is out about 1.5 turns from seating, and the idle RPM screw is in about 3 turns, once the idle screw touches the throttle shaft.

Did set the metering lever height to spec. With it set too high it will cause flooding

[Echofan]

Thanks for the response. I ordered a GND-106 for this carb, based on Zama's web page, yet the diaphragm on the metering needle side of the carb appears to take more up and down movement than the one I replaced.

Granted the one I replaced was as firm as cardboard, and I had assumed that maybe Zama did a model refresh and would have the right info.

I'll check the needle lever and height. If the needle is staying open that would account for so much flow through the primer bulb. Something is not right with that carb. It appears that there's a main jet check valve integral to the body of the carb, behind the main jet mixture needle. If this is stuck open then I'm screwed.

Thanks for the thoughts. This thing needs to sit for a little while, and I'll try it again. I can do standard cube C1U's in my sleep, but this RB series carb, is a different beast.

[bgsengine]

Check your fuel tank vent , Check carb metering lever height - If you have a hand operated vacuum/pressure tester (Like the one I use in the article I wrote here: http://ppeten.com/two-minute-drills/pre ... nd-simple/ ) I can walk you through a complete fuel system test that takes just a few minutes.

Heck, I can do that now, as others may want the details if they have the same suitable tester. Works on just about any fuel system where you can gain the needed access to lines, etc.

Step 1: Fish out the fuel filter and inspect - if it shows deep staining, dirt, etc - replace it, it is dirty.
With filter out, purge all fuel from the carburetor back into the tank, using purge bulb, then With fuel filter off, hook up the tester to the fuel line and set it on "pressure" and pump up pressure to 10 PSI (NO MORE) and observe gauge - if it holds pressure for 1 minute, your fuel lines, metering needle are all good. If it does not leaks will be either carb or fuel line so hook tester to carb and repeat pressure test - if it still leaks down, carb is leaking fuel (metering lever height, needle not seating, leaking diaphragms or gaskets, cracked carb) and if it does not leak down then, your fuel line is leaking.

Step 2) Then press and release the Purge bulb *one time* - needle should drop to no lower than 7 PSI and then hold steady. If it does not hold steady and continues to lose pressure, then the inlet needle is not re-seating and it is leaking fuel. if it does NOT drop pressure at all, then needle is stuck closed.

step 3) Switch tester to vacuum , it will release pressure - then use the PURGE BULB and pump it to draw 5 Inches of vacuum on the tester which should HOLD for 5 minutes. If it does not you have a leak in the carburetor (metering nozzle check valve, leaking diaphragms or gaskets, etc.)

Step 4) Disconnect tester, disconnect the fuel return line from carburetor and hook tester to that fitting set tester to read pressure, Use the PURGE BULB to pressurize to 10 PSI which should hold for 1 minute If that test fails, you need new purge valve body.

Step 5) Tank vent testing: First , unplug tank vent from line, plug it in to the tester, set tester on pressure and pump several times - Gauge will go up and drop back down, and tank vent should HOLD 1-1/2 to 5 PSI of pressure - if it does not, you need new tank vent. Then set tester on Vacuum and test vent - tester should pull a slight vacuum and then drop back to ZERO every time - if it does not, you need a new tank vent. (You can also pull out the tank vent line and test that for leaks too using the pressure test as noted)

If all tests pass, your fuel system is 100% correct

All done in 5 easy steps

As to your fuel loading issue - Likely one of those tests will quickly narrow things down to the cause of the problem.