How do adjust the mixture on a mountfield 470

There are 2 adjusting screws, one is idle speed and the other is mixture. Adjust the idle screw until it keeps running. That may be all you need. But to check that the mixture is right turn the mixture screw, quarter turn at a time, keep count! until it flutters or speeds up. If it speeds up stop and adjust the idle back a quarter turn at a time until it runs without the blade turning.

adjust the idle mixture screws first and then adjust the idle speed screw to compensate
adjusting the idle mixture screws will alter the idle speed but when properly adjusted then adjust the idle speed screw,

I have a Mountfield HP 470 Petrol Mower 8 years old and I have started it the for the first time and it runs but seems to be hunting or missing as I cut the lawn.

Hi allenplum,
There are 2 mixture screws, 1) the idle speed screw, 2) is the idle speed mixture screw. The idle speed screw is located on the carburetor, nar the throttle arm where your throttle cable is connected. There is a screw that you will find there that you screw in or out to speed up, or slow down the engine idle. The idle speed mixture screw is located lower down on carb and has a spring on it. There should be 2 screws side by side, one is the idle speed and the other is the high speed mixture screw. They are marked on the carb body, with the letter H or L . To adjust, remember the screw position before you do any adjustments. Start by turning the screw in (clockwise) all the way in until lightly seated, so no damage is done to the seat or needle. Turn out (counterclockwise) 11/4 inch, this should be OK to start the engine. Along with the idle speed screw, you should be able to get the correct idle required. With all the modernization happening, there might not be a high or low adjust as they are ported inside the carb. You might get just the high or just the low adjust needles. I hope this is helpful to you. All the best and good luck. Denis.

Hi montex124, I will try to help. In the past few years carb makers have been making changes. Some carbs have no adjustment at all, the ports are all fixed inside the carb. Other carbs have only the idle speed mixture screw that can be adjusted, like yours seems to have. Is this screw you mention at the top and completely outside of the carb to adjust the throttle? If it is it's probably the idle speed screw which can be adjusted to any idle speed you want and has nothing to do with the air fuel mixture but If it is right next to the idle mixture screw, it might be the high speed mixture screw but is capped so it can't be adjusted. The low and high speed mixture screws should be marked next to each screw H and L.
The 1 1/2 turns suggested is sometimes just right but other times has to be adjusted, so you can try adjusting it to get your idle but maybe adjust the idle speed screw first to get a higher RPM so it doesn't stall on you. Like I said earlier it just adjusts the throttle, and is like when you are using it, and giving it gas as they say. I hope this helps just a bit. Regards and good luck. Get back to me if you need more help or just let me know how you did. Denis.

The only way to know if the jetting is correct is to see how well it runs - and check the spark plug color.
You can start by warming up the engine and adjusting the idle mixture. Do this by turning the idle mixture adjustment screw (there are only two adjustment screws on the carburetor: one is the idle speed adjustment, the other is the idle mixture adjustment). Turn the idle mixture adjustment screw in or out until the fastest idle speed is attained. Then adjust the idle speed with the idle speed adjustment screw.
Now ride at full power and see how well it performs. If the engine cuts out or surges at full throttle, that means the main jet is too small. If the engine produces black smoke from the exhaust and doesn't seem to produce enough power, then the main jet is probably too big. Confirm the main jet correctness by looking at the color of the spark plug: white or light tan = too lean (main jet too small). Dark brown or black = too rich. Dark tan to brown color = perfect.

ADJUSTING THE IDLE SPEED AND MIXTURE
On some float-type carburetors, you can adjust the air-fuel mixture and engine speed at idle. Check for an idle speed screw designed to keep the throttle plate from closing completely, and an idle mixture screw that limits the flow of fuel at idle. If your carburetor contains these screws, proceed below.

1. With the engine off, remove the air filter and air cartridge.
   
2. Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns.
3. If your carburetor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube. Then, turn the screw counterclockwise 1 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburetor adjustments.
   
4. Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Turn the screw in the opposite direction until the engine again begins to slow (image A). Finally, turn the screw back to the midpoint.
   
5. Using a tachometer to gauge engine speed (image B), set the idle speed screw to bring the engine to 1750 RPM for aluminum-cylinder engine or 1200 RPM for engine with a cast-iron cylinder sleeve.
   
6. With the engine running at idle (image D), hold the throttle lever against the idle speed screw to bring the engine speed to "true idle." Then, repeat the idle mixture screw adjustments from Step 4 to fine-tune the mixture (image C).
   

Hi Guest,
My name is Dennis and I also have the model you are having difficulty with and I can only repeat the instructions from the owners manual (with personal comments) because they have always served me adequately I hope that will serve your needs.
IDLE ADJUSTMENTS:
There are three adjustment screws on the carburetor labeled
H (for High speed MIXTURE)
L (for low speed MIXTURE)
T (for idle SPEED SETTING)
They are located just above the primer bulb. If your saw floods so soon in the starting process the idle mixture setting can be set too rich and the speed may be set too slow. Remember the chain will not move when the idle speed is set correctly with a WARM ENGINE!
Adjust the idle speed to just below chain movement WITH A WARM ENGINE this is just below the activation RPM of the centrifugal clutch. Let the engine run at a higher speed than idle until it is warm -- 3 min -- you may have to keep goosing it until then.
Please don't exceed the limits of the plastic limiters when making adjustments.
If you haven't adjusted the mixture screws you may just have to adjust the idle speed and that can start my turning that screw all the way in to the stop and then back out 4-1/2 turns for a good place to start. FAST IDLE -- Adjust the speed until the engine runs with out the chain moveing. Adjust clockwise to increase speed. Cclockwise to decrease engine speed.
Accelerate the engine and if it stumbles and makes too much smoke it's too rich turn the LOW SPEED MIXTURE (in) clockwist in 1/16 a turn incriments NO MORE at a time because a twocycle needs to run at a setting foe a short time to realy prove the setting. dont run it wide open to warm it up but it should be at a fast idle. Nothing you do will be proven correct untill the engine is warm.
Good Luck
Glad to be of assistance - please rate the solution because I will learn from your rating. Thank you
Your IDLE MIXTURE may be too rich causing the engine to flood.

Hi 1tgriffin,
My name is Dennis and I hope that will serve your needs.
IDLE ADJUSTMENTS:
There are three adjustment screws on the carburetor labeled
H (for High speed MIXTURE)
L (for low speed MIXTURE)
T (for idle SPEED SETTING)
They are located just above the primer bulb. If your saw floods so soon in the starting process the idle mixture setting can be set too rich and the speed may be set too slow. Remember the chain will not move when the idle speed is set correctly with a WARM ENGINE!
Adjust the idle speed to just below chain movement WITH A WARM ENGINE this is just below the activation RPM of the centrifugal clutch. Let the engine run at a higher speed than idle until it is warm -- 3 min -- you may have to keep goosing it until then.
Please don't exceed the limits of the plastic limiters when making adjustments.
If you haven't adjusted the mixture screws you may just have to adjust the idle speed and that can start my turning that screw all the way in to the stop and then back out 4-1/2 turns for a good place to start. FAST IDLE -- Adjust the speed until the engine runs with out the chain moving. Adjust clockwise to increase speed. C clockwise to decrease engine speed.
Accelerate the engine and if it stumbles and makes too much smoke it's too rich turn the
LOW SPEED MIXTURE (in) clockwise in 1/16 a turn increments NO MORE at a time because a two cycle needs to run at a setting for a short time to really prove the setting. don’t run it wide open to warm it up but it should be at a fast idle. Nothing you do will be proven correct until the engine is warm.
Good Luck
Glad to be of assistance - please rate the solution because I will learn from your rating. Thank you