[pbody]

Hi all,

I've got a situation that's stumped me, but more importantly, has stumped my uncle who has 50+ years of experience with small engines.

Here's the situation:
I have a surge in my 316. It has the Onan B43G in it. The engine is fresh. It was originally a generator and never got used (the electrical part of the generator never worked properly). The engine was installed two summers ago and ran like a top until this spring. It developed a serious surge over the winter.

We thought that the floats were sunk. The intake manifold and carburetor were both swapped. Switching the intake and carb definitely improved the problem but didn't fix it. The engine still surges but only under load and briefly after opening the throttle. It will mow fine as long as I go very slow on flat ground. If I go up a hill, accelerate beyond half speed, throttle up quickly, or otherwise put any strain on the motor, it surges very badly. At idle, it doesn't surge, but it misfires occasionally, at varying intervals.

Here's what I've done:
The first thing I did was try and reseal the intake. Because the new carb has a variable high-speed jet, I was able to create a surge at idle and then I confirmed (at least I thought I did) the need to seal it by controlling the surge by spraying carburetor cleaner on the intake manifold. This did not help.

The points were calibrated to the appropriate spacing. No change.

I then put the old carburetor back on to see if I could get any different result. The float has either not sunk, or it dried out in the months that it's been sitting. I preemptively ran some seafoam in it (through the gas line) and let it sit for a good 20 minutes. It seemed to help a little bit, but I suspect that this may be due to the larger throat on this carburetor.

I did a visual inspection of the fuel lines, no apparent leaks. There doesn't seem to be any shortage of fuel reaching the carb.

Because I have the parts for 2 engines lying around, I switched the coil and condenser out one at a time to see if there was an issue with delivery of current. Neither of these tests had any effect on the surging or misfires. I also checked the continuity of the spark plug cables and made sure the spark plugs were gapped appropriately. No improvement.

NOTE* when I pulled the plugs, they were covered in soot. My research says that this is an indicator of running rich, but I never see any dark exhaust to support that idea.

In an attempt to rule out air leaks, I installed new gaskets on all three openings of the air intake manifold, and the one between the spacer and the carburetor. All of them were installed with gasket sealant.

This last effort got the most reaction for me, but it seems to be moving the wrong direction. Now, instead of surging, the tractor will sputter and almost die under load.


Any suggestions?
I'm all out of ideas. Has any one else experienced a surge so stubborn?

 

[lodogg2221]

I noticed you didnt mention cleaning the carburetor, if you havent, and maybe if you have, Id bet thats where your problem lies.

I cant say for certain, but I can tell you what happened to me and what I did.

My 318, P220 engine, was running great. Didnt use it much over the Winter, but I did use it. Late Winter it developed a bad surge. Not so much at idle, but at mid/full throttle. I was stumped. The engine was new not 200 hours before. Carb and all. Fuel looked ok, no points, intake good.
Knowing all that, the only other option was the carb itself.
So I pulled it, took it apart, and found all sorts of nasty stuff in it. I cleaned it out, drained the tank, cleaned the tank, cleaned the fuel pump, replaced all fuel lines, fuel filter, replaced tank pick ups and grommets, and all was/is well.
Well, it was after I did the carb the second time. First time I cleaned the carb and put it back on. Ran ok for a few, then started its stumble again.

The problem I had came from the ethanol in the fuel. It likes water. When the temp changes drastically and you get lots of humidity quickly, like what happened several times the week this developed, I think the moisture in the air can affect the fuel either by way of getting into the tank, or the carb itself due to condensation. I think now that keeping the fuel level higher may help with this.
I could be way off on all that, and it could just be that 25 year old grommets and fuel lines contributed to moisture in the fuel, or the fuel going bad.

Due to that, and a few other things over the years with other engines, I now try to get ethanol free gas for all my small engines. Regardless of what type of fuel I end up with, that fuel now gets 1-2oz of Walmarts TCW3 2 stroke oil (because its the cheapest) mixed with it and 1oz of Mercury Quickleen. Those amounts are for 5 gallons of fuel.

The TCW3 eliminates issues due to ethanol. What happens in some carburetors is that when shut off when hot, the ethanol can evaporate very quickly, causing corrosion on the internal parts of the carb. Same thing happens when it sits for a long time too. The oil leaves a film behind preventing this from occuring. That tip I learned when I used to have a Kawasaki Concours. They had very small passages that didnt react well to that problem, and more than one carb guru recommended that to stop it. Nobody that used it had issues. I never did after that either, and havent on my small engines since I started either.

The Mercury Quickleen is to keep things clean. And get them clean if they arent already. I know lots of guys swear by Seafoam, and I used to, but the truth is, this stuff is worlds better, and cheaper. Check a boat forum, and do the reading on it that I did, and youll see that they like Seafoam too, but for problem engines, nothing beats Quickleen. The fact that its cheaper is why I like it, and its 1oz for 5 gallons of fuel, unlike Seafoam where you need 1oz per gallon. If I recall, a 16 oz bottle is about the same price as Seafoam, but you use 1/5 as much. Now, Quickleen isnt a stabilizer but they make one, and next time I buy more Quickleen, Ill get the stabilizer too.

Some guys dont like adding stuff to the fuel each time they fill up, but I dont mind. Takes an extra few minutes before I fill my 5 gallon gas cans, and is good insurance (for me at least) against issues with the fuel system.

 

[dhager4692]

Jim, I too doctor my fuel but I use Seafoam because I didn't know about Mercury Quickleen. But I encourage treating the fuel for our small engines. I have a Ryobi trimmer that I use it in and it has started the same the last three years I have used it. It even started that way on gas I left in it over the winter. Adding stuff to the gas each time just means I won't be fighting a problem. I agree that ethanol fuel is a problem for these small engines because of the size of the gas passages. My 318 is also easy starting and good running because it gets this treatment also. Knock on wood but I have yet needed to clean or adjust my carburetor.

 

[lodogg2221]

Just in case you want to tackle the fuel line stuff I did, heres a link to the thread I made about it:
http://www.wfmachines.com/forums/sh...b-cleaning-to-fix-stumble-surge-at-all-speeds

Doug,
In my 2 strokes, I have used Opti-2 for nearly 20 years. It seems like Opti-2 is somewhat controversial, but I love it. Ive got a Deere XT120 trimmer and BP60 backpack blower that were purchased in 2002 and still start on the 1st or 2nd pull. They both still have original fuel lines, pickups, and spark plugs. Ive been told thats impossible, but apparently its not. Anyway, the point was, its got a stabilizer in it too, and even after sitting all Winter, they start the same. Sure is nice!
After all the work on the 318 to get it going again, Ill never put untreated fuel in it again if I can help it. My best guess is that what I do adds maybe 30 cents per gallon. Not worth worrying about considering the relatively small amount of fuel we use. Heck, even with the 318, 300, Exmark Lazer Z, and all the other small engines (Generator, edger, pressure washer, etc), I use more fuel in 3 weeks in my car than I do the whole year in the other stuff.

 

[pbody]

Thanks for the suggestions, Jim. And nice work on the fuel line replacement! I will plan to include this in my current repair.

The carb with the variable jet was cleaned prior to installation. I soaked the fixed carb in cleaner, but couldn't remove the high-speed jet because it's incurred some damage that prevents me from being able to remove it. I realize that this may be a source of an issue, but I haven't seen any negative change when using it compared to the variable speed carb.

Currently working on a second cleaning of the variable carb to give it another shot.

 

[lodogg2221]

I rarely remove jets to clean them. Generally I take the good old Gumout Jet Spray carb cleaner with tube attached and stick that tube in every orifice of the carb and spray until I see it coming out somewhere else.
Cant say thats gotten 100% of the crud out 100% of the time, but its always worked well for me. And as a bonus, I dont have to worry about goofing up the slots on the jets by removing or installing them, because if theres one thing I know for certain after turning wrenches for almost 30 years, its that if it can be damaged, I can find the best way to do it!

 

[pbody]

Update:
I invited my wife's grandfather (another accomplished mechanic) to look at the B43G. He pointed out some things that my lack of experience prevented me from knowing.

1) The fixed-jet carb was spitting gas upward when surging. Apparently, this is bad. Also, he felt that the high-speed jet had too large of a bore through it which makes me wonder if there isn't a piece missing there.

We cleaned the variable jet carb and installed it. Still no improvement in how the engine runs, but he pointed out that the variable jet carb functioned better. It still surges and misses, but doesn't spit gas out the intake.

2) With the variable-jet carb installed, there is a significant amount of white vapor that drifts out of the intake after running the engine. His advice to me is to purchase a rebuild kit because the vapor indicates that the needle valve is leaking. So that is my next step. He graciously offered to remove the fixed jet from the old carb for me, but encouraged me to move forward using the variable-jet carb after rebuild.

Thank you all for the continued input! I'll update after rebuilding the carb. Will probably be a few days before an order can be delivered.

 

[mitchdaly]

Keep in mind that all of the carburetor cleaner/spray won't get to all of the internal passages in the carburetors. The ethanol residue that is left behind (usually a green film) will still block the passages and some of the cleaners won't even touch it. I've talked to a lot of people about it and most of the shops at the dealers are using an ultrasonic cleaner to get into all of the passages AND they are using a strong Simple Green solution to do the cleaning. I can't get over how dirty the solution is after cleaning an already clean carburetor. I use one of the ultrasonic units from HF and usually run a carb through 3 cycles of a very hot solution.

Mitch Daly

 

[pbody]

Hi all,

Here's an update. I ordered a rebuild kit for my Walbro carburetor. I sprayed the carb thoroughly through all passages to clear any gunk, and applied new gasket, needle valve seat, needle, and spring I also adgusted the float according to the manual. No improvement on the surge. At least I feel like I can mark that off of my list of potential causes.

This morning, I adjusted the valve clearances with the guidance of an experienced mechanic. This did not help with the surge, but definitely reduced the backfire issue at idle.

At this point, I have three areas that I consider next steps worth trying. The first is to renew the fuel system like Jim mentioned above.

Second, I plan to inspect the governor. If nothing else, verifying that there is no damage will rule out another variable.

Third is to replace the intake manifold completely. I believe I have it sealed sufficiently. I can't spray carburetor cleaner on any part of it and cause a difference in the idle, but, I believe replacing it would be the prudent thing to do.

Do you guys think I'm on the right track here?

 

[KB]

I'm thinking its a failed flyball spacer at this point. I use propane for intake tests, but passing is passing.


Sent from my iPhone using Tapatalk Pro

 

[chuckv]

pbody,

There is one more thing to look into regarding the carburetor and a surge. All fuel based surges are caused by a time variant change in the fuel to air ratio of the mixture, and the governor "chasing" the change in speed to keep things at a relatively steady RPM. As you note the most common cause is an air leak in the split design of the Onan manifold, but there can be other sources.

Look for a poor seal on the manifold gaskets where the carburetor mounts to the manifold and where the manifold mounts to each head. Another very subtle source of air leaks can be the throttle shaft itself -- if it is sufficiently worn air will leak past the shaft into the manifold (which would of course lean out the mix and cause a surge...)

Beyond an air leak causing a time dependent variation in the mix and engine speed, a fuel blockage can behave the same way. Again it is the governor that is trying to set a throttle position that gives a constant RPM -- but if all the jets and carburetor fuel circuits are not working as they should due to blockages then a hunt condition is set up.

The third and least common cause of surging/hunting is improper "gain" in the control loop of the governor. Have you tried adjacent spring settings on the linkage? This illustration shows the factory default positions, but changing to an adjacent one will alter the "gain" and the frequency and extent of the hunt behavior.



Chuck

 

[pbody]

How wold you identify a failed spacer? I looked at the governor today. All appeared to be in good condition as far as I could tell. No grooving visible anywhere, no flat spots or lines on the balls, no indication of linkage binding.

 

[pbody]

pbody,

There is one more thing to look into regarding the carburetor and a surge. All fuel based surges are caused by a time variant change in the fuel to air ratio of the mixture, and the governor "chasing" the change in speed to keep things at a relatively steady RPM. As you note the most common cause is an air leak in the split design of the Onan manifold, but there can be other sources.

Look for a poor seal on the manifold gaskets where the carburetor mounts to the manifold and where the manifold mounts to each head. Another very subtle source of air leaks can be the throttle shaft itself -- if it is sufficiently worn air will leak past the shaft into the manifold (which would of course lean out the mix and cause a surge...)

Beyond an air leak causing a time dependent variation in the mix and engine speed, a fuel blockage can behave the same way. Again it is the governor that is trying to set a throttle position that gives a constant RPM -- but if all the jets and carburetor fuel circuits are not working as they should due to blockages then a hunt condition is set up.

The third and least common cause of surging/hunting is improper "gain" in the control loop of the governor. Have you tried adjacent spring settings on the linkage? This illustration shows the factory default positions, but changing to an adjacent one will alter the "gain" and the frequency and extent of the hunt behavior.

View attachment 161193

Chuck

Thanks for the suggestions, Chuck! I'll definitely run through those when I get the engine back in.

 

[pbody]

Problem resolved!!!

There was a two-fold fix to this problem. The first was inside the governor. I have to thank Boomer and his vast knowledge of Onan for this part. The cam shaft in my tractor has been replaced. The replacement shaft came from an Onan generator set and had 10 balls in it which is 5 more than it should have for the RPM you want on a mower.

Taking the balls out helped some, but didn't solve the problem. The second part of this surge issue came from the governor control linkage. I was correct in my assessment that there was no binding, but I didn't think to look for extra movement. My uncle looked at it with me and identified that the governor was kicking in as it should, but the extra play in the linkage between the governor arm and the carburetor caused a delay, and therefore, a surge. To solve this, we first moved the throttle rod to a less-worn hole. Second, we took a spring and placed it on the rod in a concentric manner, like a sleeve. We drilled a small hole in the throttle lever to hold one end of the spring, and hooked the other end of the spring into a preexisting hole on the governor arm. This may be worth looking into for anyone chasing a surge.