OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions on Other Engines Millers Falls 13hp

Hi have put a Millers Falls 13hp engine onto a spare ride-on i am doing up. Was given thge motor, never even heard of them before. Problem, engine will idle and rev out to full throttle without air filter attached, as soon as i attach airfilter the governor will go off, as in rev out fully back to idle back to rev out fully. Have had to reset valves, which i have checked a couple of times still all good, carby/ fuel tank/ hoses are clean and free of dirt and a new spark plug and new airfilter.
First photo is engine before fitment. Photo 2 is how the engine wants to run , nice and smooth. Photo 3 & 4 is when engine will start to play up quiet hard.



Wondering if anyone can answer why or has had this problem before. Any help or advice appriciated.
Thanks Paul.

From the pictures I'm guessing that is a chonda copy of the GXV390. I haven't worked on one. Remember, with chondas the bits that play up are the external bits that aren't accurate copies of the Honda parts: the governor plate, muffler, air cleaner, starter, top cover/fuel tank, etc. Hence I think you are likely to be correct in focusing on the air cleaner, or even more likely, the air cleaner fouling on the governor plate or linkage.

If it will run perfectly without the air cleaner, but plays up when you put it on, the air cleaner is causing the problem. I'm not clear on whether it runs properly with the air cleaner shell in place, but no filter element installed. If it still plays up under those conditions, and in view of your description of the problem, it sounds like mechanical interference between the air cleaner housing and the governor mechanism. Some part of the housing may be rubbing fairly hard against the linkage.

Please post pictures of the governor arm and governor link, Paul.

Hi Grumpy, have a photo, its not the best, with the air filter inner shell and air hose attached. There is about 1" of space between governor arm/linkage and airfilter casing and about 3/4" space between motor and governorarm/ linkage. Have done some thourough looking around all parts that are near governor arm/linkage but there is nothing close enough for it to rub on or for something to rub on it. Have even double ckecked the governor setting but no problems there. In my second photo, above, having no hose or airfilter attached is the only time it will run without stuffing around, as soon as i attach the hose, with or without airfilter, it will stuff around. Will take top off tomorrow when i get another 5 mins and get some more photos.

We should check some other obvious things if it isn't the most obvious one (friction in the governor linkage). Can you try choking it to see if that stabilises it? If it overcomes the problem, obviously it is lean mixture, and we can go from there.

Hi Grumpy, have done as you suggested. Started engine without air filter/hose attached ran smoothly as we know, attached hose and airfilter, started stuffing around so i slowly started closing the choke. I was able to close choke half way before the engine started to run smoothly. The first photo is of carby and i will let you know what i did.

There was a little metal cap if front of mixture screw so i carefully drilled it out. Did try adjusting screw to different settings, 1&1/2 turns, 2&1/4 turns but made no difference were i put it. At the moment it is out 3/4 of a turn, that is were it originally was. The next photo shows the governor linkage on carby , underneath it is a small metal base that is screwed on, would it be worth taking it off to see what is under it, as there are no springs under governor linkage as there are with other mowers, wondering if there might be one under it. Let me know what you think.

I believe you have been adjusting the idle air bleed, Paul. It is in series with the idle jet, and is strictly a very fine tuning adjustment of the idle mixture. In other words, it does not seem relevant to your current problem.

Can you put the right angle fitting (elbow) that goes on the intake side of the carburetor, in place without the air cleaner box? It works without that elbow and the filter box, but doesn't work with them, so there is nothing wrong with the carburetor, there is something wrong with the elbow and/or filter box.

Hi Grumpy, that is right, am unable to put anything on carby without it stuffing around, have even tried a straight peice of pipe, still no good. There is nothing phisicaly wrong with either the hose or airfilter/box, which has got me completely stumped. There shouldnt be any difference whether it is sucking air directly from carby, nothing attached, or through the hose/airfilter once attached. Oh and yes i will stay away from the carby for now on.

Can you post a picture of the carburetor flange where the air cleaner mounts on it, and the matching gaskets and insulator please Paul? Remember, the air cleaner side of the carburetor, not the cylinder head side.

I'll tell you where I think this may be heading. On many of the Keihin carburetors Honda uses, and the Ruixing and other copies of them used on chondas, the float bowl vent runs to the air cleaner through a port in that flange. If you put the gasket or insulator on backwards, you block the float bowl vent, and probably get a result something like what is happening.

Hi Grumpy, have some photos taken in order.





On the backside of the gasket (photo 4 ) there is a flat part right next to the bolt hole, just wondering if that could be causing a bit of my problem.

Well, I don't think you could put the insulator or gasket on upside down without it being very obvious. I don't understand the part about there being a flat part next to the bolt hole.

It seems to me that the next step is to lay each of the other parts (gasket and insulator) on the carburetor flange, and see if the three holes in the flange are unobstructed. You might also try clamping those two bits to the flange and verifying that the engine runs properly. Then we'll know it is the elbow that is at fault. I'm conscious that one of the moulded recesses in the elbow is very much smaller than the corresponding recesses in the gasket and insulator, and I think it won't reach across the matching hole in the carburetor. Does it actually block the hole? That may depend on how it fits over the gasket and insulator.

If it runs well with gasket and insulator clamped in place, the next step would be looking at the gasket and insulator placed over the flange on the elbow, to see how they match up.

Hi Grumpy, have worked out how to put circles and such onto photos. Sent the photo of gasket/insulator and put a circle around the flat part. This side sits against carby and unsure if it makes any difference.


Have proceeded as you suggested and all holes between carby,gasket/insulator and flange on elbow do actually line up. Have found this though.

Even with the gasket sitting against the elbow you are still able to see a tiny bit of sunlight through the two. I am thinking once attached to carby, because the bolt holes are on the edge, it may not be not sealing properly as all pressure is twisting elbow flange into a slight arc as to say. Hope i have explained this easy enough to read and understand.

I can see that the elbow flange is bowed, Paul. It may be leaking dirty air into your carburetor, and should be fixed in due course, but that is not the immediate problem.



I've copied your picture of the carburetor flange. The red circle is around the idle air bleed. I think you've said it idles properly even when the elbow is in place, and if so, this hole is not being blocked. The green circle is around what seems to be another air bleed: it connects to the inside of the carburetor but only passes air slowly. If that one is nearly blocked, but allowing a bit of air in each time suction builds up, it could cause your problem. That is the cut-out that you put the yellow oval around in the picture of the insulator. Note that the moulded-on rubber on that metal insulator will be crushed when you install it, and it may be squeezed into that recess. Can you try running it with the insulator and gasket omitted, so the elbow bolts directly onto the carburetor?

Hi Grumpy, lead you astray abit, the flattened bit rubber on the insulator actualy sits to the outside of the hole so there is no problems with it. Have ran the mower with only the elbow attached and it was worse, now jumping at both idle and full revs. Tried just the gasket/insulator, had same problem...until i put my finger over the idle air bleed (red circle). Smoothed it out and ran perfeclty at both idle and full rev. Did cover over the other air bleed ( green circle ) made no difference.
Would it be worth attaching all peices again and trying to adjust the idle air bleed screw or will i be making it worse, or worse yet more fustrated. Or would you advise taking carby apart for another thorough clean and inspection.

I'm a bit mystified at present Paul. In an earlier post, you said it will run perfectly without the elbow or insulator. Now you said it won't run with the insulator unless you block the idle air bleed. Can you verify that it will run without anything at all on the intake flange of the carburetor, but it won't run with the insulator on unless you block the idle air bleed?

If you find it won't run either with or without the insulator unless you block the idle air bleed, that will make sense, but at this point I don't see how putting on the insulator alone, can mess it up in such a way that you have to block the idle air bleed.

Hi Grumpy, no doubt i have confused everyone involved, this is how i feel everyday ( sorry readers ). Let me start again.

Photo 1 all attachments on carby: runs but revs high then low etc.

Photo 2 elbow and gasket/insulator attached, no hose or airfilter, runs as above.


Photo 3 elbow only, no gasket/insulator, runs as above.

Photo 4 gasket/insulator only, runs as above.

And with nothing on the carby it will run the same as above.. I think i have forgotten to mention that if i pull the choke approx half way closed while running the motor it will smooth out and run properly, or as i just found out blocking the idle air bleed. Sorry Grumpy, hope this get us up to speed.

It seems straightforward now, Paul. I went off in a different direction in the belief that it worked properly without the entire air cleaner apparatus, but badly if any part of the air cleaner were fitted. However if it runs badly regardless of the air cleaner fitment, and gets better when you partly close the choke or cut off the air bleed, we are down to two main suspects, both of them pretty routine, in happening and in repairing. It sounds as if it either has a leaky carburetor insulator gasket (they often split when the carburetor is removed, and will not work properly thereafter) or an obstruction or misassembly in the carburetor. I suggest you begin by removing the carburetor and inspecting both insulator gaskets (there is one on each side of the plastic insulator). If they are perfect, you need to look primarily at the main jet and the emulsifier. All these things are very easy to put right.

Hi Grumpy, have had a look at both gaskets and plastic insulator, they are all in good condition. Have some photos of them



Have also stripped down and cleaned the carby again, not that it was needed, i blew it all out with air compressor and followed it with carby cleaner for a just incase. Photo of carby parts.


Now all back together, will wait for a reply before i go any further incase i have missed something.

The green gasket looks good - not so sure about the grey one. The insulator looks good, but be sure you put it on the right way around so that the groove in it matches the port in the carburetor flange.

Most people who get poor results from cleaning those carburetors make one of three simple mistakes:
1. They don't use carburetor cleaner on the main jet because they think it looks clean. Blow carburetor cleaner cleaner under pressure though the jet: it is often reduced in size by gum.
2. They aren't nearly careful enough cleaning the emulsifier. Don't just blow carb cleaner through the central orifice: put a finger over the hole in the far end and make that carb cleaner squirt out under pressure from every one of the lateral holes. If any of them doesn't squirt or looks a bit less strong than the others, put a jet drill through the hole backwards, or use a piece of copper wire and a lot of care.
3. They don't clean the idle jet properly. The hole in it is tiny, only 0.014". If carb cleaner doesn't come out of the tiny hole strongly when you put a finger over one end of the lateral hole and squirt the cleaner in through the other end, you need to use a jet drill (usually 0.33 mm, but you use the 0.3 mm one that comes in the set). Note that in your case the symptoms don't match an obstructed idle jet, so it isn't a big issue, but it's never too early to develop good habits.

Hi Grumpy, the way you have explained cleaning a carby is the way i do it, in one of my first post you talked me through it that way and done it that way ever since. But just to make sure did strip carby down and fully cleaned it again.
Did make a new gasket for side of carby, grey one, there was a tiny split in it were the air port is, other "split " was just a mark. Sad to report there is no change at all. Thinking out loud here, could it just be a sh#t carby.

There are still things to look at, Paul. Have you checked the fuel flow rate from the needle-and-seat with the float bowl plug removed? That checks the whole system upstream of the needle-and-seat. It should continue to flow a good stream, steadily: run the test for at least half a minute.

If that is OK, check the float height, idle mixture screw setting, and main jet size.




Does it idle best with the screw set where it should be?

Does the float mechanism operate smoothly with no binding?

Hi Grumpy, have checked the fuel flow rate and there is a nice staedy flow, so all is good there. The float and pivot mechanism both work well with no sign of sticking. Couple of curious questions.
1. When checking the float level height, with the carby in a standing position do they actually mean to measure from the bottom of the float to the outside ring were the fuel bowl sits, with the float held up so the needle would be cutting off fuel supply,,, or do you hold the carby upside down to get measurement. Would it make any difference.
2. Were could i find the right main jet size.
3. The idle mixture screw, is that the one i spoke of earlier and you said not to worry about it, i'm guessing it is about time to start worrying about it.

Hi Paul.
1. It should be 13.2 mm from the sealing ring on the carburetor body (where the float bowl seats) to the top of the float, with the float slightly lifted so the needle is against the seat. Whether it is legitimate to turn the carburetor upside down depends on whether it is a light float or a heavy one - you judge that in each case. Usually it is OK to turn it upside down.
2. #92 means 0.92 mm jet. That is 0.036" For comparison, a GXV140 has a #45 main jet: it is a much smaller engine, compared with the GXV390. There are two reasons to check it. First, it might be clagged up with gum. Second, some previous tenant might have changed it or soldered it up. Find a drill bit that just goes through, and measure the drill diameter.
3. The idle mixture screw (or pilot screw in Honda-speak) is the fine adjustment of the idle mixture. The reason to check how many turns it requires to get the best and fastest idle, is because it is one of the ways to tell whether something is wrong up-stream from there. If you get the best idle somewhere near the specified setting, all is well with the entire idle system. If it just keeps getting better as you turn the screw anti-clockwise, something is badly wrong in the carburetor.

Hi Grumpy, done some measuring and here goes,
1. float level height is 15.08mm, i measured it upside down, was unable to blow through fuel inlet so i hope i have done it right, i went to bring it down to the specified 13.2mm but it started to feel to tight so i stopped. I didnt push it to far as i knew it would of wrecked the needle.
2. the smallest drill peice i have is a 1mm, when measured with vernias it comes to 0.97mm. It fitted through, thats not right is it or is a little bit of difference allowed.
I wont try to run it and adjust the idle screw just yet , will wait for you answers to my findings so far.

Hi Paul. Float height adjustment is done by removing the float first, not by pushing against the needle. If both float and float lever are one-piece plastic, it is not adjustable: if it is wrong, you check the condition of the needle and seat, then replace parts, including the float, until the height is correct. It is a concern that it was intitially a couple of millimetres low, because that makes the mixture leaner.

I'm surprised that the jet was oversized: can you tell from examining the bore of the jet, whether it has been drilled out? If it has, there is a possibility that the carburetor has been extensively interfered with by a previous owner. However in this case the Honda engine was an emissions one, and it may be that the version you have, did not attempt to comply with emissions rules. That is not what has occurred generally with chonda engines, though: their jet sizes are normally very similar to the Honda ones, and are interchangeable with them.

Please go ahead with the idle mixture adjustment. It adds to the information we have: you collect enough symptoms until you can make a guess, then investigate the guess. That is what I do, at least.

Hi Grumpy, have gave the motor a run with the screw out 2&3/8 turns, no good still jumping around, so slowly opened the screw up. Stopped at 4&1/2 turns as it did not improve any were up to that region. Did close it up though and on approx 1&3/4 turns it is running with minimal jumping around but i am getting a small popping out the exhaust. Is this getting worse or is it just me??
Just need to confirm i wasnt trying to adjust the float by pushing on it yesterday, was just getting a feeling of it, so dont stress i wasnt trying to find the easy way out. Only used minimal pressure.

The float i have is plastic non adjustable. Needle and seat look fine. If some one has drilled the jet out it is hard to tell, i cant find any markings or scratches on either end to suggest it has happened. Am i going to have to change the whole set up or can i change a peice at a time, for example a smaller spring to start with and then a needle and or float.

Have found a main jet in my box of goodies, it is 0.94mm, the misses sewing needles came in handy again. Would this jet be alrite or will it still be to big knowing i need 0.92mm.

The popping out of the exhaust sounds rather as if you have a leaky valve, Paul. Let's try to summarise what we have:

1. The engine surges or hunts with the speed control not being moved.
2. If you richen the mixture, either by partly closing the choke or obstructing the air bleed, it improves substantially.
3. Recently you've noticed some minor popping from the exhaust.

These symptoms are a bit disparate Paul. Have you done a compression check?

I suggest you do that before making any further changes in the carburetor.

Hi Grumpy, have done a compression test,its good, sitting on 120psi exactly. Will do some tinkering with carby, fill you in when finished, dont stress wont do anything extrarvigant or over the edge.

Specified compression pressure for the GXV390 is 71 to 121 psi, Paul, so your chonda is right at the top of the expected range. I realise it is an extra test that may not produce a solution, but I suggest you try running it with the muffler removed. If it has a blockage, many things could be explained - just about everything, in fact.

Hi Grumpy, have tried running it without exhaust,smoothed it out just a fraction and no popping but instead had little puffs of blueish grey smoke, i'm guessing i had them instead of the popping as no exhaust. Could this come back to valves, should i check the setting even though i have had to set them or take the head off to check they are sitting right,, but knowing it has good compression theres not any need is there.
Cant believe it,with all the stuff i have got laying around i dont have a float or spring to suit what i need, am going to town tomorrow will see if i can pick something up. I was looking for a float that would be a couple of mm's smaller then the original, is that the right way to go.

Paul, I don't think it is at all clear that correcting the float height is going to make a big difference, given the rather diverse symptoms you have reported, but a greater float height does richen the mixture slightly. However the chonda tuning may be different from original Honda tuning, and the float height difference is fairly small anyway. You said that you have a spare jet that fits this carburetor. You have also said that applying a small amount of choke makes the engine run properly. If you don't mind using up your spare jet, I suggest you drill it out very slightly, and don't change the float height, or the original main jet. The problem is to find the right sized drill bit. You said a 0.97 mm drill bit fits through the original jet. You also said that it was actually a 1 mm drill bit, but measured 0.97 mm. Unless something really odd has happened to that drill bit, I'd suspect your vernier caliper of inaccuracy rather than the drill bit.

My normal approach at this stage would be to try a 0.95 mm jet drill bit in the jet. If it drops through, I'd try a 1.0 mm jet drill bit. If that dropped through as well, the next jet drill bit in my standard set is .12 mm, which is rather a large jump. Living in Melbourne as I do, I'd drive to my nearest industrial tools outlet and buy a 1.1 mm drill bit. I don't know how practical that would be in your part of the world. For reference, the first production GXV390 engines had a 0.95 mm main jet; all other versions had 0.92 mm, including the California-certified version, so it would not seem very surprising if the chonda version, with a different brand of carburetor, had a slightly larger jet. As long as you are working with a spare jet, not your original one, drilling it out to 1.1 mm as an experiment does not seem outlandish to me.

Before you do that though, had your carburetor ever been apart before you cleaned it? If it had, please inspect the tip of the emulsifier and if possible photograph it. If the tip is damaged, or the emulsifier is not quite seating all the way into the carburetor body, major effects on mixture might be the result.