Honda 31cc trimmer idle rpm drop when hot.

[Alan0354]

I have a Honda HHT31SUNBA 31cc trimmer. I had to replace the carb for other reason. Since then the trimmer's idle rpm drop after it get hot running for 10 minutes. It has been in the shop and they cannot figure it out. I actually overhalt the old carb and put it back on. To a less extend, it still doing the same thing. I have to adjust the idle higher to make it not stall.

The trimmer runs perfect during trimming, lots of power, just when I let go of the throtle, the rpm get lower when hot. If I let it stall, it would be hard to start. So in conclusion.

1) Never have this particular problem until the dealer change to a new carb for other reason. Since then, the problem start happening.
2) Loosen the gas cap do not help.
3) I clean the muffler, spark arrestor and no improvement.
4) Even when I fix the old carb and put it on, the problem still exist to a lesser extend. Both carb have no problem runing at high rpm and lots of power.
5) I check the mounting screws on the carb to make sure it is tight.
6) I checked the cable for binding.

I am running out of idea. Now I adjust the idle high enough that the head keep spinning to avoid the problem.

Any suggestion?

[Alan0354]

Anyone please?? I am totally puzzled. It is not even heat related in normal sense, it happened at 50 deg F. The carb is separated from the engine by a block of plastic.

[S DIAMOND]

Could be several things, but here’s a list of the most common issues as related to the fuel system, if that is what It is.

1. FUEL FILTER CLOGGED.
2. FUEL TUBE CLOGGED.
3. AIR IN FUEL PASSAGE (TINY CUTS OR CRACKS INCLUDED)
4. INCORRECT OR DETERIORATED FUEL.
5. CARBURETOR NOT SECURED PROPERLY.
6. INSULATOR GASKET FAULTY (DAMAGED OR INCORRECT TYPE)
7. METERING LEVER NOT OPERATING PROPERLY.
8. INSPECT CARBURETOR INSULATOR (DAMAGED OR WARPED); MAKE SURE THAT THE GROOVE IS POINTING DOWN.

NOTE: THIS ENGINES USE A CRANKCASE-PRESSURE-DRIVEN, OIL-MIST LUBRICATION SYSTEM.

Though it may require bit of an air leak, I would think that if the valve cover is not suffiently sealed, you may be getting in some maverick air that’s leaning the fuel mixture causing a noticeable problem at idle but not at top end as much.

[Alan0354]

Could be several things, but here’s a list of the most common issues as related to the fuel system, if that is what It is.

1. FUEL FILTER CLOGGED.New filter
2. FUEL TUBE CLOGGED.new tube
3. AIR IN FUEL PASSAGE (TINY CUTS OR CRACKS INCLUDED)
4. INCORRECT OR DETERIORATED FUEL.Always less than a month old, 91 octane and Stabil.
5. CARBURETOR NOT SECURED PROPERLY.Double checked.
6. INSULATOR GASKET FAULTY (DAMAGED OR INCORRECT TYPE)
7. METERING LEVER NOT OPERATING PROPERLY.Checked, nothink stick.
8. INSPECT CARBURETOR INSULATOR (DAMAGED OR WARPED); MAKE SURE THAT THE GROOVE IS POINTING DOWN.What is this?

NOTE: THIS ENGINES USE A CRANKCASE-PRESSURE-DRIVEN, OIL-MIST LUBRICATION SYSTEM.

Though it may require bit of an air leak, I would think that if the valve cover is not suffiently sealed, you may be getting in some maverick air that’s leaning the fuel mixture causing a noticeable problem at idle but not at top end as much.

I'll check the valve cover screws later.
Thanks

[sanman]

Hi there,

I've had low compression engines do this in the past. Did you check the compression on it? Also, check the spark / coil when the engine's hot enough for the problem to show up.

Al.

[dutch1]

If you have checked out all the fuel related suggestions previously mentioned, I would recommend that you check for a crack in the isoblock(the black block you mentioned--between the carb and the engine block) for a leak. It's not a frequent thing, but I have seen a few. The best method I have found is to spray carb/choke cleaner all around the isoblock when the engine starts to falter. If you notice a change in rpms you likely have an air leak. My experience has been that the crack is often not visible to the naked eye, but the carb/choke cleaner will let you know. Most often the crack originates at or near one of the screw holes in the isoblock.

This may/may not be of any help--just something to check out.

Dutch

[Alan0354]

If you have checked out all the fuel related suggestions previously mentioned, I would recommend that you check for a crack in the isoblock(the black block you mentioned--between the carb and the engine block) for a leak. It's not a frequent thing, but I have seen a few. The best method I have found is to spray carb/choke cleaner all around the isoblock when the engine starts to falter. If you notice a change in rpms you likely have an air leak. My experience has been that the crack is often not visible to the naked eye, but the carb/choke cleaner will let you know. Most often the crack originates at or near one of the screw holes in the isoblock.

This may/may not be of any help--just something to check out.

Dutch

Good idea, I'll even try on the other part of the engine like the valve cover also.
Thanks

[dutch1]

I don't know the Honda numbers but I'm assuming from what you've said that this an OHV model. Unless you have some super sloppy valves I would not think it possible that you're going to experience an air leak in that location. If you haven't checked thet valve clearances, that would be advisable. Check with your Honda dealer for the proper valve clearances and adjust accordingly if they are out of adjustment.

Dutch