79 PA50 stalls when throttle applied

Started working on a 79 Honda PA50I with my son. We noticed that at the upper end of the throttle, 4 stroking was beginning and the top speed was about 15 mph. So we followed directions listed in the forums here (which were a huge help!) and dropped the engine, took apart the carb and cleaned everything we could find. Checked and cleaned the air tubes and repaired the sticking choke.

Put everything back together and it started right up. Here are the problems:

When trying to set the idle speed ( after it warmed up) the best idling that we could achieve was with the air adjustment screw all the way in.

Now when we start the engine, it idles OK, but when we attempt to increase the throttle, it starts to speed up but then sounds like it floods and will stall if we don't decrease the speed to idle.

Pulled the spark plug after trying to run the engine, very black. We are running 25:1 mixture as per directions.

Tried spraying starting fluid around the engine and carb with it running, but no increase in speed at all.

Frustration is starting to set in. We are not thrilled about dropping the engine again ( it was so much fun the first time!)

Question about the attached carb picture. What is the disc located just below the air mixture screw? When I was cleaning the carb, and utilized compressed air, I would see several bubbles when air was pushed through the air mixture screw hole.

Any help that is available would be much appreciated. Sorry this is so long.


Re: 79 PA50 stalls when throttle applied

The disk just below the air mix screw is a permanant plug that need not be removed. (It's hole allowed them to drill two tiny idle inlet holes in the carb throat near the throttle plate.. then it was permanantly sealed with the aluminum plug) I've never removed one but there's nothing behind it afaik.. there's no room to put anything.

_and utilized compressed air, I would see several bubbles when air_

Exactly where were the bubbles? Around the perimeter of the plug?? That plug should be air tight and last the lifetime of the vehicle.. i'd double check and make sure that i saw what i saw.

You can clean the passage behind the plug by blowing air into the (removed) mixture screw threaded hole and through the (removed) flat-head cover screw threaded hole .. and through the little tubes in the airhorn to clear that passage in all directions.. the plug can stay.


I don't think it's flooding / four-stroking.. nor do i think the mix is lean .. I think the carb is not pre-mixing air with the fuel. This means big drops of liquid fuel are getting into the engine. These big drops don't burn well.. they wet the plug and cause misfires.


Did you remove the brass emulsion tube above the mainjet and clean it and it's cavity? Above the mainjet is a cavity from which all fuel supply, idle and main, comes. Fuel is premixed with air here. This cavity has two holes inside, one high, one low, that are connected to the two brass tubes in the airhorn. Air enters the emuslioon cavity through the tube on the left side of the bike/carb. Air entering the tube on the right draws fuel and carries it through the idle passage.

The main system draws fuel directly from the cavity through the top of the emulsion tube, visible in the carb's throat.

The idle system draws it through the mixture screw's idle passage discussed above. The mixture screw allows more or less of this pre-emulsified gasoline through it's own idle hole, near the two smaller idle holes near the throttle plate in the carb's throat.


Hopefully you can make heads or tails of the above description.. If not and you are interested, trace the pathways with good lighting and magnification and follow along with the description.. If you're not interested i wouldn't blame you.

I hate to say it but it sounds like the engine needs to be dropped to work on the carb. I've done it (unavoidably) about 25 times.. it gets easier with practice and experience. Takes me about 5 minutes once i'm set to go.

BTW, my idle mix screw is 1/2 (one-half) turn out and any movement has a marked effect on idle.. this bike (A PA50II, 30mph) idles smooth and low and steady. Oil mix is 32:1.

and i'm sorry this is so long too.

Re: 79 PA50 stalls when throttle applied

Wow! Thanks for the information. Yes, the bubbles did appear around the disk when air was applied to the air mixture screw and the flat head screw openings. (This was done with the engine still attached to the frame and fuel in the carb).

When we will orginally cleaned the carb, I removed the main jet but could not figure out how to remove the emulsion tube. (I did spray with carb cleaner and blow out with compressed air) It was up in the cavity a little. How do you reach it to remove it? I tried to bang the carb on a block of wood to loosen it, but no luck. I didn't want to break anything.

Since the ped is old (but a good one), I may need to clean the rust from inside the tank too. I have read several methods on the forum from sand and oil to the kreem(sp) product. Any thoughts on this process?

By the way, thanks for the detail in your response. The theory makes perfect sense. Now to see if we can fix everything.

It may take a couple days to get time to drop the engine, but I'll let you know what we find.

Thanks for your help!


Re: 79 PA50 stalls when throttle applied


I've been following this discussion carefully in prep for my own carb project.

Why did you opt for 32/1 oil mix, rather than the factory recommened 25/1? I'm sure you had a good reason and I seek enlightenment.

Re: 79 PA50 stalls when throttle applied

If it idles fine but will not run with the butterfly open it sounds like it is not getting any fuel thru the main jet. When you open the throttle butterfly the mixture goes too lean.

When you removed the float bowl and float and then removed what looks to be the main jet, did you get the main jet out? It is not screwed in but is held in place by the main jet screw. This might have a piece of junk in it.

When you remove the fuel tap, is there a bunch (lots) of rust on the screen? If there is some but it is not a lot you can just take the tank off and clean it out. The fuel/oil mixture will keep it from rusting any further. If there is a lot of rust you can try buying a box of 1/4" nuts at the hardware store and pouring them in and then make like a martini mixer.

You shouldn't have to kreem the tank unles there is a leak in it.

Re: 79 PA50 stalls when throttle applied

Oil was dripping from the tail pipe .. and a thin blue haze trailed behind me down the road... no big deal but it bothered me.

I wasn't anxious to cut back on oil.. My main concern was inadequate lube in the crankshaft area.

So i removed the cylinder after a pretty long test ride and found that all parts had a thick coating of clean oil. I could probably cut back further but have no reason to. There are no more drips from the tailpipe and no smoke once the bike is warmed up.

Re: 79 PA50 stalls when throttle applied

Remove the mainjet.

The brass emulsion tube is above the main jet.. is held in only by mild friction and perhaps lots of dirt.

You can start it on it’s way out by pushing it’s tip downwards.. the tip sticks up and is visible in the carb throat. Get something thin in there and push it down. Then it chould come out with some coaxing.. Pound the carb into a rag or into your palm. Pound it on a block of wood.. don't break anything. Sudden shock should get the tube to fall out.

Perhaps an "L" shaped piece of thick wire can get in the carb's throat and push the tube down even further into the hole.

If it refuses to fall out, compressed air into one of the air-horn brass tubes should blow it out. Otherwise maybe stick something into the main jet opening and wiggle the tube’s end around a bit .. soak the area in carb cleaner.. pound and fiddle with it some more..

You gotta get that thing out.. i've disassembled carbs where the emuslion tube and it's cavity were packed with crud.. but you wouldn't know it from removing the mainjet and looking in there.


That leaking plug is trouble IF it's really leaking.. they crush those plugs into the hole and use a sealer.. i dont see how it could be leaking and am still skeptical. But if it is leaking it should be fixed.

Re: 79 PA50 stalls when throttle applied

I'm no mechanic, but it sounds like worn rings or cyl. bore. I'm sure you have checked for compression.

I can't find my testor at the moment, but my bike has such low compression that I never need to use the release valve. It starts up with very few rotations. I don't get smoke at 25/1 and plug chop shows dark brown, so I figure the mix is about right.

I should think that an older engine has looser tolerances and can handle a leaner mix.

Re: 79 PA50 stalls when throttle applied

Hey guys,

I ride an 82 PA-fiddy. I probably have the same engine as Jon.

When I got it I had some major idle problems, four stroking and spark plug problems. There was fouling and of course the always embarassing premature engine cut off at lights and stop signs.

After adjusting the gas and air screws time and time again, I was told by a fellow PA-that the oil we have today is better than that of the 80s, so that I should lean my mix out a bit. I went from 25:1 (factory recommended) to a 32:1 mix. I also got special oil that is intended to reduce that white smoke we get.

Anyway, the leaner mix really helped solve my problems. I was wary at first about overheating, but it doesn't seem to be running overly hot.

It might help you

Re: 79 PA50 stalls when throttle applied

A two-stroke burns oil regardless of the rings' or bore's health. The more oil that is mixed in with the gasoline, the more oil that is burned.

Re: 79 PA50 stalls when throttle applied

i tried a couple of the more expensive semi-synthetic "smokless" oils and they do cut down on smoke.

However one time i had a carb problem.. upon investigation i found small blobs of oil at the bottom of the fuel in the float bowl.. the oil separated from the fuel. I tested this in a glass jar and after a couple days the oil did separate. I guess some oils are not meant to sit in a tank for long periods.

anyway, i've found a cheap oil that works for me..

Re: 79 PA50 stalls when throttle applied

Update to the throttle problem.

Dropped the engine ( again) and cleaned everything including the emulsion tube. The trick about the small allen wrench helped break it loose. Cleaned all the holes in the emulsion tube. Also ran a wire through all openings and forced carb cleaner through every opening.

Also cleaned the points by dragging emery paper between the closed points. They didn't look bad and are gapped per spec.

Managed to start it again but it still wanted to bog down past 15 mph.

By the way, the aluminum plug on the carb did leak. Verified that during the cleaning process. Sealed the opening with epoxy.

Final result: Managed about 20 mph after playing with the air mixture ( about 3/4 turn) and pulled the air filter out about a half inch. The filter seemed to have more impact than the air mixture screw.

About the air mixture. I still can not notice a difference when it is about 1 1/2 out or 3 out. The only thing I notice during idle is when the screw is all the way in, the engine revs slightly faster at idle.

I will continue to tweak and maybe change the fuel mix to 32:1 as was suggested.

Thanks to everyone who posted. I think the PA50 is back in the land of the living!


Re: 79 PA50 stalls when throttle applied

Idle mix and speed will have no effect on top speed.. If the carb's clean something else is going on.. maybe clogged exhaust.

BW, is this a PA50I (20mph) or a PA50II (30mph)? The VIN plate on the steering tube will say.


Remove the large bolt under the heat shield on the right side of the muffler .. remove the shield to access it.

Get a long scrwedriver and stick it straight in the bolt hole. You want to clear a small, short 1/2 inch diameter exhaust baffle tube. All exhaust must pass through this small tube and this tube is the first thing that gets clogged up. So, Honda was nice enough to put that clean-out hole there.

If you can’t stick a screwdriver straight into the bolt hole 5 inches or more, you havent completely penetrated the carbon in that tube.. The screwdriver is either hitting the baffle wall or the tube is still clogged. Carbon does not collect to any apprecialble degree anywhere else in the muffler. Make sure that tube is open.

If you want to remove the muffler and look at the header tube and exhaust port, go for it .. i've not seen one badly clogged up with carbon but it can and does happen. I _have_ seen the baffle tube in the muffler clogged almost completely.

Re: 79 PA50 stalls when throttle applied

Walter Kravitz /

We had a similar problem... my buddy's 79 PA50II was stalling like crazy, turns out all it needed is some extensive cleaining of the carb, i mean EVERY teeny hole, EVERY nook, etc. Now it runs like a champ

Re: 79 PA50 stalls when throttle applied

_pulled the air filter out about a half inch. The filter seemed to have more impact than the air mixture screw_

Have you cleaned that sponge? If so did you re-oil it? I guess you inspected the air box for any foreign matter and it's all clear.

I once had a similar problem .. generally low power and bogging right after i cleaned and oiled the air filter element. I guess i used too much oil on it.

Anyway, after cleaning it again in plain soap and water and letting it dry, the engine ran fine. I won't ever oil it again with 90W as they suggest in the manual..

Other people have had similar things happen with that filter.. gotta pull it out to get good power. On my bike it doesn't matter if the filter is there or if the filter holder is completely removed .. same exact performance everywhere as far as i can tell.

Re: 79 PA50 stalls when throttle applied

The moped is a PA50I (20mph) Thanks for the drawing. I did not know how the exhaust was routed through the muffler.

I actually checked the muffler prior to the carb cleaning. Removed the bolt and the screw driver will go all the way to the other side of the muffler. Even took the muffler off completly to see if that had any affect. Other than really loud no real change in performance.

I still am wondering about the air mixture screw... Even when I try to set the idle speed, the air mixture screw does not change the engine speed at all unless it is all the way in. During the carb cleaning, I ran a wire from the tube in the carb intake, through the opening where the flat screw fits(the one below the idle speed adjustement) and all the way to the air mixture screw opening. Ran it back and forth and sprayed carb cleaner both ways through the openings.

Why does the air mixture screw not affect the idle speed on a warm engine? I can see no difference. I must be doing something wrong!

Now the moped is very hard to start and slugish unless I baby the throttle, then it eventually gets up to a 17 mph speed.

Appreciate the help.

Re: 79 PA50 stalls when throttle applied

The mixture screw has no direct connection to idle RPM. But since a more correct idle air:fuel ratio results in more power, idle speed rises a bit if and when the air:fuel ratio becomes more correct than it was before.

One clue to why the mixture screw seems to have no effect might be the air filter anomaly you mentioned before. Namely, it runs better with the filter door opened a little.

Idle adjustment is worthwhile only on a carb/engine that is clean and tight .. no vacuum or fuel leaks of any kind.. Setting idle is the very last thing on the list of tuneup tasks. If that air filter or some gasket or anything else has a problem, idle adjustment can wait.


Begin with the mixture screw perhaps 1-1/2 turns out.

Get the engine warm. Come to a stop and adjust idle speed as low as possible with the speed screw... get it so low the engine almost dies... maybe it misfires a bit but it keeps on chugging away.

Then turn the mixture screw slowly inward.. If idle RPM does not increase before the mixture screw bottoms out, something needs fixing.


You said _pulled the air filter out about a half inch. The filter seemed to have more impact than the air mixture screw_

If the bike idles better without the air filter, not enough air or too much fuel is getting in. It's running rich.


And you said _I notice during idle is when the screw is all the way in, the engine revs slightly faster at idle_

Here again, turning the mixture screw all the way in minimizes idle fuel supply. If this makes it idle stronger, it's running rich.

Re: 79 PA50 stalls when throttle applied

Couple thoughts...

After the carb was cleaned the second time, I replaced the airfilter with a thin piece of foam rather than a piece that filled the air filter holder. I got the engine to really perform great; for about 15 minutes, then it started the bogging down again.

Everything seems to point to a very rich running engine with the problems I am having with the engine.

I guess I will remove and re re clean again. I will use permetex on the seals as everything starts to be put together again to address the possible vacuum leaks.

The fuel line now has a new filter to make sure no tiny pieces of any crud get in the carb. Fuel flow is fine.

I am assuming the flat head screw below the idle speed screw is just an access for cleaning the little openings into the carb...

Your information is helping. At least I am starting to understand what all the little channels in the carb are for.

May be a couple of days until I can drop the engine again.

I'll update as we go along.



Re: 79 PA50 stalls when throttle applied

Joe is totally right.

My filter - when oiled - makes my PA fourstroke like crazy. I can't top 30KPH when it isn't clean.

When it was bogging down a lot I tried fiddling with everything else, muffler - air mix - gas to oil mix - and then I cleaned the filter again. And voila!

I now carry a small container of 90% isopropyl for emergency cleanings.

Re: 79 PA50 stalls when throttle applied

_I am assuming the flat head screw below the idle speed screw is just an access for cleaning the little openings into the carb_

yeah, basically .. But that hole is more than an access point. It was drilled (and tapped) for another reason .. That is where the idle jet was pressed in.

I hope you removed that cover screw and cleaned the area.. Look in there with good magnification and strong light to see the brass jet on the bottom of the hole.

So, there's a very small idle jet at the bottom of that screw hole. Air coming into the right-side airhorn tube draws emulsified idle-fuel up that idle jet.. This fuel goes forward to the mixture screw and through the idle inlet orifices near the throttle plate.

I think you can get a single strand of small wire in that jet .. maybe one strand from a throttle cable or similar. Don't scratch the jet's bore hole when clearing it.


All filter-element foam is not created equal. I carefully cut and fit a piece of green stuff made for a lawnmower (Home Depot) and the bike ran bad .. too much air restriction.

Re: 79 PA50 stalls when throttle applied

i forgot to mention .. it does seem to be running rich, both at idle and at speed, and both of these can be caused by a high float level. Even if the float level is adjusted, the float needle may not be sealing perfectly.

Extra fuel raises float level. The idle system and main system more easily draw fuel from the too-high fuel level. This upsets metering.

All of the carb's parts including main jet size, emulsion tube's length and internal diameters, idle jet size, etc., etc., were chosen according to a certain fuel level in the bowl. If that fuel level is off, either high or low, the carb will not meter fuel properly.

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