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

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.

Hi Grumpy, have checked out my vernias with a new battery. Measured the same drill as the other day, 1mm came up 097mm again, so i checked the 1.5 came up spot on, 2mm came up 2mm, checked the 2.5mm came up 2.48mm, so i am thiking it may the drills themselves. They arent dear ones so it anybodys guess now.
Did get my hands on a 1.2mm drill bit, got told if i break it dont come back, hoping it wasnt going to be to big. Carefully drilled out the spare jet and gave the motor a run. It seemed to smooth out just a little bit more. Would it be worth going just a tiny bit bigger and see what happens.
The motor had been sitting in a box since bieng bought, not sure how long ago that was, so my guess is the carby has never been apart till i got to it. The only reason i took it apart was incase there was dust and such in it. Have some photos of the emulsifier. Cannot see any damage or scraps on it.

The emulsifier looks good, Paul.

As for the jet, I'll tell one of my interminable stories to set the scene for the next step. About 15 years ago I had the job of working on a couple of very old high performance cars. One had been rebuilt and ran badly, the other was in pieces. Neither had the original carburetor, which could not even be described, the cars were so old. The previous tenant had tried to get the engine running by modifying a completely different carburetor (from a Tiger Moth engine). My approach to making it run was check that the ignition, compression and valve timing were right, and then blame the mixture strength. It was misfiring too much for things like spark plug colour to make any sense. So, I drilled out the main jet in steps, finding it ran better with each enlargement. Finally, with the jet 0.120", it ran properly (it was originally 0.080"). The trouble was with that huge jet diameter, it wasn't vaporising the fuel, it was just spraying liquid like a fire hose, so in cold weather it went back to running badly. A colleague pointed out that when you need a huge jet, that means there isn't enough "venturi vacuum" generated in the carburetor, so we needed to try a smaller venturi in the same carburetor. The colleague turned up a smaller venturi, and I soldered up the jet and started again, with the result that it ended up running properly with the jet at 0.080". At that point it was vaporising the fuel properly too, so it ran in hot or cold weather. A bit of research revealed that the previous tenant had thrown out the Tiger Moth venturi and made his own, much larger one.

The moral of this story is that there is no great difficulty in enlarging the jet step by step, and observing whether it runs better. The modification is easily reversed by soldering up the jet and drilling a new hole through the solder. Furthermore even your enlarged 1.2 mm jet is only 0.047", which is not a large jet, so you aren't coming to the end of the road with drilling it out in steps. The issue is, unless there is something wrong somewhere, the right sized jet should be somewhere near the original size, and it obviously isn't going to come out that way.

What I would do, is enlarge that spare jet further, until it stops running better each time you enlarge it. Then we can see how well it is running, and whether we can find an explanation of why it needed the bigger jet.

Hi Grumpy, sorry but am going to have to pause here until further investigating, have explained on other thread, RE: Tecumseh one. Thanks Paul

Hi Grumpy, have also done here as suggested re: tecumseh motor, have removed kill wire from ignition mudule, with the ignition off the tester showed 0.00, with ignition on it showed 0.44.. Quick question, if it is the ignition module, re: tecumseh thread, how could it have died so quick knowing it is a new motor, or is it a case of when they feel like it they just go.

We have no strong reason to believe it is an ignition problem caused by putting 12 Volts on the kill wire Paul. The question needs to be cleared up by you checking for voltage on the kill wire of the second, or "Miller Falls mower" until you tell us what it actually is. What I think we have so far, is a mower that runs erratically. You have reported that the fault comes and goes with changes in the air cleaner, but I think more recently you have found that it is erratic all the time. If I heard it, I could probably tell you what is wrong. Erratic ignition is normally characterised by two features. First, the engine cuts in and out sharply: each shot is either fired or not fired. Second, there is usually some popping in the muffler under at least some conditions, due to the fuel that doesn't burn on the missed shots.

The way to find out for sure if it is an ignition fault that is gradually getting worse, is to put an in-line spark tester into the high tension wire. We discussed how and why, and how to make one, in a previous thread of yours:

https://www.outdoorking-forum.com.au/forum/u...amp;Main=9879&Number=59818#Post59818

I think what should be done now, is to try that test on both your engines. Afterwards, keep the tester close to hand when dealing with engine faults. What I hope we've both learned from this thread, is that your descriptions of fault symptoms are so different from what I am used to, that we are at risk of going in completely wrong directions if we don't start with some basic hard data.

Applying 12 Volts to the kill wire is a serious abuse of the module, since it applies a potentially high, continuous current to a circuit not remotely intended for such treatment. I don't recall enough cases of it being done to have much idea how long it takes to destroy a module. It may vary widely.

Hi Grumpy, have done test with the in-line tester, did have good constant strong spark. One little thing, hope i'm not getting off topic, but the motor ran beautifully while idling, on full revs it seemed to surge/hunt just the tiniest of bit. Am wondering if i was to put original jet back into carby it might be right.
The way it was running with no wires connected it has me pionting back to bad wiring on the mower base, but that is something i will look into very shortly.

I recommend that you put the carburetor back to standard condition in every way that it has been changed, Paul.

The objective with diagnostic processes is to find the cause, which is usually a single cause. That implies that everything that has been changed, should be restored to original condition immediately after the test that was made with that single item changed. If you don't do that you can develop new problems due to the changes that have been made, and it can become impossible to isolate the original problem.

It sounds as if we now have evidence that the Millers Falls engine has a good ignition system. We don't know at this point what the problem was, so we want to get the engine into its original condition, except with the electricals disconnected from the mower. If we can get it running like a new one under those conditions, we can start to examine the electrical connections in detail.

Hi Grumpy have changed everything back to original.
Do you want some photos of electrical connections that are on motor and on mower base, or is there something else we do before going to them.

I think we should get the engine running properly before changing topic, Paul. My preference would be to get the engine right, close this thread, and open another on the mower frame. Hopefully we'll be able to find a wiring diagram for it, and check whether the machine is different from the diagram.

Hi Grumpy, have given motor another run while on base, no wiring connected , and it ran like a dream. Other then opening up the mixture screw to just below 2 1/2 turn, didnt touch anything else. If i had of tried this motor without the wiring connected from the start i wouldnt of had to stuff you around and waste your time.
Now on to tackling a wiring problem.
Thanks heaps for your help Grumpy,, special thanks for your patience.