# Smoothing/porting Tecumseh



## drmerdp (Feb 9, 2014)

Anyone mess around with smoothing out the bowl area by the valves. I'm cleaning up and doing a valve job an a LH358SA. I can't see the harm in doing it while I have the valves out.

The casting has lots of rough edges that cause turbulence on both the intake and exhaust. 

It has a fix jet carb, so if it made any improvements I might notice a lean hunt under load...next winter...


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## UNDERTAKER (Dec 30, 2013)

*This I Remember.*

* A while back my jet mechanic next door neighbor was overhaulin a Harley engine for 1 of his scooter riding buddies. and that guy wanted him to do the same on that engine. and my jet mechanic neighbor said he would lose torque on the engine by doing that. there is a reason why that stuff is there it is there to mix the fuel and air up all nice and pretty. just like a fine martini that is shaken not stirred. so you get all of that oomp out of zee engine when you need it the most.k:k:k:k:k:mg::emoticon-south-park*


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## RIT333 (Feb 6, 2014)

PS93

Great analogy !


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## drmerdp (Feb 9, 2014)

I left the rough casting as is, if nothing else, it's one less thing I need to do in my already to short of a day.


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## cranman (Jan 23, 2016)

Back in a previous life I built drag race engines...I used to port 289 Mustang engines , put huge 421 Pontiac valves in them.....screw in studs, high lift cams .....One thing I learned. Leave the porting rough....never polish. The gas/air stream needs to be really mixed well, and polishing hinders that. BTw....I still have the Quikway valve grinding machine..anyone want a five angle valve job! How the **** did I get from that to Tecumsehs??????


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## skutflut (Oct 16, 2015)

I remember in a far distant life, that a Norton motorcycle mechanic told me roughtly the same thing. 

I asked him about smoothing out the rough parts and he said, in a thick scottish accent, "Ach, they are suppose to be rough as a badger's ***hole to mix the brew"


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## UNDERTAKER (Dec 30, 2013)

RIT333 said:


> PS93
> 
> Great analogy !


*YEAHHHHHHHHHHHHH Some days I am on fire!!!!!!!!!!!!!!!!*


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## drmerdp (Feb 9, 2014)

skutflut said:


> I remember in a far distant life, that a Norton motorcycle mechanic told me roughtly the same thing.
> 
> I asked him about smoothing out the rough parts and he said, in a thick scottish accent, "Ach, they are suppose to be rough as a badger's ***hole to mix the brew"


Hah, That's funny. 

I've done quite a bit of porting, mostly on GM big blocks, Buick 455s in particular. Cleaning up the rough edges do yield flow bench results, but this is a top end improvement. There is no top end concerns on a Tecumseh.

The polishing is a big no no. The roughness actually decreases flow resistance by creating a thin air buffer which the mass of flowing air flows against with the lowest resistance. A polished surface will cause the mass of air to drag itself against the metal surface causing friction.


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## RIT333 (Feb 6, 2014)

POWERSHIFT93 said:


> *YEAHHHHHHHHHHHHH Some days I am on fire!!!!!!!!!!!!!!!!*


Yep - And other days, we wish you were ! LOL


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## jrom (Jan 10, 2014)

drmerdp said:


> ...I've done quite a bit of porting, mostly on GM big blocks, Buick 455s in particular.


Growing up in Detroit, I have fond memories of seeing, hearing and riding in – one of the first 1970 Chevelle SS 454 LS6's in the neighborhood. Weren't too many of those around.


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## snowflitesly (May 5, 2016)

Hehe, so you want to mod a Tecumseh engine huh. Groovy, but you need special tool and be really good at it.

All porting jobs, done right, will give you more flow of A/F mixture into the cylinder. After it's done, fuel delivery re-tune is needed. So a fix needle carburetor is a big no-no. 

Like everyone who replied and I are leaning on the fact that stock, porous surface is there for fuel consistent atomization until it reaches the combustion chamber. This is key for proper A/F delivery. 

Nevertheless, porting has been done before and the rough surface can be reproduced flawlessly. In the same time all those casting lips can be smoothed out, but leaving the stockish mounds (this part is the tricky part) and have a gasket match size port from intake to head. It's not all that simple, but mainly it's that. All ports are done differently and all engine react differently to different porting jobs.

On the other hand, the exhaust port, once A/F is burnt, nothing matters anymore. Get rid of all the lips, gasket match and you can mirror shine the exhaust port. 

The big ''BUT'' here, there is no exact science behind porting. It's a trial and error game. I've done it many times with the help of a shop that had a flow bench ( mainly for multiple cylinder engines, balancing between ports). Let me tell you, I've wrecked my share of cylinder heads while trying to learn this porting stuff.

The bottom line is, an internal combustion engine is nothing more than a pump. The more airflow you can get it to ''process'' (while respecting the correct A/F mixture), the more power you will get out of it. Pretty basic.

And another thing to consider, after all this said, I've worked on engines running from 200 to 1000 hp and from around 400 and up, a porting job can really show it's advantages. People asking me to port and polish their civic, I would ask them ''really?'' 

So, unless you compete in a snow removal competition that has engine class divisions, I would suggest to leave it alone. If your not satisfied with what I presume to be a 9hp engine, maybe get a bigger one.


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## snowflitesly (May 5, 2016)

Hahahah, just remembered, a buddy of mine, even more gear head mod freak than me (there is always bigger better somewhere, lol), bought a chinese made 125 cc dirt bike for his son, you know those ebay style 4 stroke ones. Twas brand new. He decided to port the intake side of the head. I can only remember his face and his jaw dropping when he took his dremel to it and 10 seconds in he went through the port. He said '' geez, I guess it was good like it was huh''. I laughed my head off while I was getting my JB weld and handing it to him. 

Aaaah, good times. 

Makes me wonder if all small engine are pretty close to max size port, so only a casting lip cleaning is the best it can be done as far as intake porting goes. hehe, food for thoughts.


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