Carb upgrade needed? Need some help

[Pyrotechnic]

I have a motorized bike with a Tecumseh H25 on it. It's a 127CC 4 stroke flathead rated at 2.5HP from the factory. The motor was off a large push edger, and it has a diaphragm style carburetor with adjustment screws for idle and main mix. Most engines like this I have seen have a float style carb instead. The carb has been rebuilt.

The first problem is, I really have to lean and feather into the throttle gradually as the motor revs up. It runs good this way, but if I just shove the throttle forward to the desired position the motor just bogs and falls flat on it's face. It's much like a carbureted car that has a busted accelerator pump.

The other issue is that it is VERY sensitive to temperature changes. The same adjustment that worked in the cool of the morning, wont work in the heat of the sun later the same day. The motor refuses to run right until it's adjusted.

I'm wondering that since this carb was originally for a stationary application, it may not meant to be used in a vehicle application where the throttle, RPM, and load will be frequently varied? All it had to do before was spin the blade of the edger.

Do I just need to find a different carburetor? I see that a lot of people upgrade the honda clones to Mikuni carburetors.

 

[Doc Sprocket]

Sounds to me like it's running lean. It may need a thorough cleaning, and/or new diaphragm.

 

[Pyrotechnic]

The thing is, by turning the main mixture screw out I can make it run so rich that it runs terrible. Once I get the main mixture screw set right, it will run OK aside from the aforementioned problems.

The carb has been rebuilt by me and the diaphragm doesn't leak any fuel.

 

[Doc Sprocket]

The diaphragm may be too worn to function properly. Sometimes with age they stiffen.

You could try upgrading the carb. Maybe something for/from a 4hp engine?

Got some pics?

 

[Pyrotechnic]

Here's some pics. The fuel filter has since been removed, I found out real quick that you can't use one, or at least an automotive one with a diaphragm carb since it messes up the pressure differential just enough to make it run weird.

The long tube header and the tunnel ram intake gave the motor a lot more power than it had with the stock pancake muffler and tiny runner intake manifold. First I built the header and there was some improvement, then when I installed the intake the motor really woke up.

Keep in mind, I've always had to ease into the throttle as the motor revved up even when the motor was 100% stock and the carb was freshly rebuilt at that time.

Could it be that the carb was OK to feed the motor when it was choked in terms of air flow, but now that the motor can actually breathe, the carb can't give it enough fuel to run right at all RPM's? It's interesting that you say the motor sounds like it running lean, could this be the reason?

 

[landuse]

That is a really cool setup that you have there Pyrotechnic.

Not to sound as if I am questioning your carb cleaning abilities, but did you make sure all the little jets and holes were poked out and free of any debris when you cleaned it? Many people think that they cleaned their carb well, but in reality it isn't so clean.

I would definately say that the new air filter setup that you have will affect the carb. There is a lot more air going in than what it used to get before

 

[OzFab]

Your first problem is commonly referred to as a flat spot which occurs when your engine is not being fed enough fuel. If you heve to feather the throttle to get it to go then something is not right.

Personally, I think toystory might be on the money with the stiff diaphragm. Try sourcing a new one or take toystory's other piece of advice & upgrade to a slughtly larger carb: The better an engine can breathe, the better it will perform

 

[OzFab]

I would definately say that the new air filter setup that you have will affect the carb. There is a lot more air going in than what it used to get before

The only way that would affect anything is it would force more fuel in as well, which would make it run better. The problem is the carb can't feed that much fuel due to another problem

 

[Pyrotechnic]

It's always had the feathering problem, even when the carb was freshly rebuilt with a new diaphragm and the motor was stock. Runs great at a constant speed though.

Another data point about the temperature sensitivity. Today before my ride home from work I tried something new. I primed the carb and started the motor, then I richened the main mixture just a hair until the engine sounded healthy. As it warmed up, I backed it off a hair. The kickstand keeps the rear tire up, so I let out the clutch and let the motor spin the rear tire which does put some load on the motor. I adjusted the main mixture until the motor could be revved without stalling when it returned to idle. The ride home was great and the motor ran without stalling. This doesn't really fix anything but now I've got a method to get the bike up and running at least.

What do you guys think about running a Mikuni 22mm VM carb on this motor? I can get a knock off version for a little more than it will cost to put another rebuild kit in the carb I have. From what I've read they don't have the fine attention to detail you get from a real one but they seem to be well made and work good for the money. If you look at the design, the Mikuni has so much more ability to more closely meter fuel than what I have now. I do like the fact that it is meant for vehicle use too.

 

[Pyrotechnic]

It seems I learn a little bit after every ride so I'll post it here. This is going to be really long but I just want to get all my thoughts out about the whole situation.

When I was getting the bike to leave work, I warmed it up and as it was idling I opened up the main mixture screw to richer than normal. I'm talking rich to the point where the inside of the exhaust pipe looks wet from raw fuel at idle.

On the way home, it didn't run so great at lower RPM's but it had top end power like it's never had before. That was one hell of a ride home. This is different than other days where it's runs better at slower speeds and the exhaust pipe is dry but it's totally flat on top.

The trend seem to be that the main mixture will make a narrow RPM range work good, depending on where I set the screw. If you consider the original application, these motors just run against the governor at 3600 RPM and the governor will move to keep it at that speed. Really all the carb has to do is provided fuel in the governed RPM range, and to idle when the motor is set to slow when the operator needs to leave the machine for a minute but doesn't want to stop and restart it.


All my prior experience with carburetors is with automotive carbs. With an automotive carb, you set your main jets so the engine runs good at a cruise. When you mash the pedal, an additional enrichment circuit kicks in and adds more fuel. If there was no enrichment circuit, the motor would be way too lean under load.

If you look at my simple diaphragm carb, there are two circuits. An idle circuit, and a main circuit. There is no enrichment circuit for high load. There is no accelerator pump. It makes sense that the main jet has to be set rich enough for max RPM and load, even though other situations below that may be too rich. It also makes sense that when I set the main mix so the exhaust pipe is dry at lower RPM, it falls flat on its face when I try to get it to run well at higher RPM.

This simple carb may even be good for a go-kart. With only one gear which means you will ride against the governor most of the time, and a centrifugal clutch that provides plenty of slip during acceleration, it seems like it would be adequate as the application is pretty forgiving and doesn't present many different RPM and load situations.

However with my bike, the belt clutch is operated by hand and can be locked up an any RPM. There are gears to be shifted through, and the motor has to rev through each gear and the belt does not slip like a centrifugal would. The conclusion I've come to is that the simple diaphragm carb just can't meter fuel accurately for every RPM and load combination that my bike creates because it just doesn't have enough circuits to do so.

Now this is all just theory and I don't know how right I am but it does seem to make sense to me after my experiences with the bike so far and taking all things into consideration. I'm gonna order a Mikuni 22mm carb today. These are meant for motorcycles with hand operated clutch and gears and are much more complex in design than what I have now.


Thank you everyone for the helpful replies, you guys were right on about the motor not getting enough fuel just based off what I described and it's helped me get to this point so far.