Intake manifold hole size

[JohnnyTillotson]

What's the theory behind these skinny flat intake manifolds that hold the carburetor a bit lower than the regular one? I never noticed how much smaller the hole Liz when you hold it up to the back of the engines intake. Is a small hole causing more suction and thus the gas flowing inside faster making the engine quicker? Engine is a Tecumseh H25

 

[2strokd]

Personally, I would think the smaller passageway would cut performance by restricting flow. However it's only an hypothesis.

 

[JohnnyTillotson]

Personally, I would think the smaller passageway would cut performance by restricting flow. However it's only an hypothesis.

Yeah I agree that's possible too. There must have been a rationale for doing this though. I'm thinking they would want to squeeze more power out of an engine and not slog it down for whatever the engine was destined for.

 

[Dr. Shop Teacher]

If the engine was a fixed speed engine (edger/tiller/gray water pump), volume was not necessarily a consideration. The kart/mini bike H35s I've had had a larger intake manifold runner to allow for increased air/fuel volume across the rpm range. Most people will replace the small ID manifold with a larger one. Theoretically, a smaller diameter intake manifold will increase the speed at which the air/fuel flows but reduces the pressure of the mixture (venturi effect).

 

[jackeo21]

Another thing to point out, is that some H25's, or the ones equipped with those manifolds, had their fuel tanks right above the carburetor. Maybe this would explain the unusual length of the manifold? As the fuel tank sat right above the carb, they needed to lower the carb itself to accommodate for the fuel tank. I'm not sure what the logic was behind putting the tank on the same side as the carb, maybe they wanted a more compact unit! Or it could have been a cost cutting measure :shrug:

I think that because the manifold needed to be stretched and flattened out to accommodate for the fuel tank, they couldn't make the size of the inlet as big as the standard 3.5HP manifold as they ran out of aluminium to work with! Hence the much smaller diameter hole. I can't comprehend why they made it a two piece part however, this adds another gasket into the mix!

Will we ever get to the bottom of this? :laugh:

-Jack

 

[JohnnyTillotson]

I can't comprehend why they made it a two piece part however, this adds another gasket into the mix!

Will we ever get to the bottom of this? :laugh:

-Jack

Yeah, haha what's with that? I had one with the choke between there once. Maybe that was why..to have that option?

I just like that low slung carb look. Maybe I'll do a comparison test. True also abt the tank being on top of the carbs when this manifold was used. I think with the standard hole size being longer, the added length was too long to get the gas sucked up. So they decreased the volume of space in the intake to get the mist of fuel up into the head. I'm no rocket surgeon though....

 

[Dr. Shop Teacher]

Yeah, haha what's with that? I had one with the choke between there once. Maybe that was why..to have that option?

I just like that low slung carb look. Maybe I'll do a comparison test. True also abt the tank being on top of the carbs when this manifold was used. I think with the standard hole size being longer, the added length was too long to get the gas sucked up. So they decreased the volume of space in the intake to get the mist of fuel up into the head. I'm no rocket surgeon though....

You may be on to something....the OHH195 series has a long intake runner, and it's prone to flooding with any carb larger than one for an H35/HS40. The smaller carb venturi helps get the gas up that long tube, even then it'll still blow fuel out the carb. Drove me nuts. So, you are a Rocket Surgeon!:thumbsup:

 

[JohnnyTillotson]

Another thing to point out, is that some H25's, or the ones equipped with those manifolds, had their fuel tanks right above the carburetor. Maybe this would explain the unusual length of the manifold? As the fuel tank sat right above the carb, they needed to lower the carb itself to accommodate for the fuel tank. I'm not sure what the logic was behind putting the tank on the same side as the carb, maybe they wanted a more compact unit! Or it could have been a cost cutting measure :shrug:

I think that because the manifold needed to be stretched and flattened out to accommodate for the fuel tank, they couldn't make the size of the inlet as big as the standard 3.5HP manifold as they ran out of aluminium to work with! Hence the much smaller diameter hole. I can't comprehend why they made it a two piece part however, this adds another gasket into the mix!

Will we ever get to the bottom of this? :laugh:

-Jack

Yeah speaking of those i just picked a '68 H35 (craftsman equivalent) up for $10 at a swap meet last sunday. Came with this rather cool combo sprocket/pulley clutch. Good compression. No spark.

 

[Dr. Shop Teacher]

That must have been from an old snowblower looking at the recoil though maybe an edger.

 

[delray]

pete

That must have been from an old snowblower looking at the recoil though maybe an edger

snowblower.......:thumbsup:also sears use alot of clutch setups on there older models for example late 60's -70's
edger's never use the shield on the recoil. never seen one....:shrug:

 

[delray]

one other thing i just notes. the bracket bolted on the carb is use for snowblower. heat shield box cover bolts to it.....:thumbsup:

 

[JohnnyTillotson]

one other thing i just notes. the bracket bolted on the carb is use for snowblower. heat shield box cover bolts to it.....:thumbsup:

Yeah I had removed it already before I took a pic.

Curious clutch for sure. I was thinking maybe its for generating a battery? It spins freely with the sprocket so its not for starting for instance.

What a time I had getting the woodruff key out while taking it off. That thing was IN THERE. No manufactures name on it.

 

[delray]

wouldn't be surprise if sears told the manufacture to take there name off. so they could sell you a replacement part and other possible reasons?

 

[JohnnyTillotson]

wouldn't be surprise if sears told the manufacture to take there name off. so they could sell you a replacement part and other possible reasons?

Yeah maybe. I should look for the engine diagram under the craftsman model number and see if it has the clutch on there