Two Cycle Carb Vs. Four Cycle Carb

[Timmahhhhh]

I am building a bike with a 5 hp Tecumseh HSK 850 2-cycle engine. The snow blower carb does not offer much in the way of adjustment. Could I use a 4 cycle carb from an Hs40 or something? What is the difference? The 4 cycle carbs seem to have a lot more adjustments,such as idle speed and mixture,and a less complicated throttle plate set up. The displacement of the HSK 850 is 139cc,in case anyone is wondering. I have a pocket rocket carb for a 49cc engine,but wouldn't that be too small? Thanks for any info.

 

[paul_probus]

Since noone has spoken up, I can only relate what the Dellorto Motorcycle Carb Tuning Guide says and that is, at least for Dellorto carbs, 2 stroke carbs have fewer holes in the atomizer tube than 4 stroke carbs. The implication is that you could convert one to the other by simply replacing the atomizer tube. Of course that also means re-tuning it which could also mean re-jetting the carb.

I don't know if other carb manufacturers made the conversion as easy as Dellorto did, or, perhaps, for others it was possible to use the same carb for both with no conversion and then it could be possible that some manufacturers made completely separate carbs that could not be converted. I just don't know for sure. :shrug: You can try the carb you have and see if it will work. :shrug:

Paul

 

[KB2ROCKET]

For the most part carbs are not specific to 2 or 4 cycles

 

[Flat Head Phil]

There really isn't any difference. But the difference in the lack of adjustment for consumer product two strokes is due to 2 strokes are a lot more sensitive to fuel mixture. Too rich, and the plug will foul real quick. Too lean, and the piston will burn down and seize in the cylinder due to lean fuel mixtures elevate combustion temperatures to the moon, cause detonation, piston damage, and finally seizure. Don't use a carb from a HS 40, if anything try one from a HM 80.
Do test WOT speed runs, shut it down and take plug readings to dial it in.
Good Luck:thumbsup:

 

[Newoldstock]

Seems to me you have other options.

If you have a uniblock ( sounds like it ) with a single adjustment diaphragm carb i have good news for you.
There are better carbs based on the exact same casting with both a high and low speed adjustment for easier tuning.

See if you carb find an old Ice Auger or lawn mower and odds are you can find this carb.
The ice auger has a different head with bit more compression in some cases.

Carbs for a 2 stroke or 4 are generally not that different.
But some differences mean a 2 stroke carb in a 4 will likely be a rich running carb thats a struggle to run lean.
A 4 stroke carb on a 2 stroke is something I never tried, but i think if you mix and match enough parts you could manage to get something that works decently well.

 

[The Restore Kid]

Yes, if you have an adjustable HS body carb that would be about ideal.. The OLDER tecumseh two stroke carbs are fully adjustable.. Some are, some are not.. About like the vertical shaft motors.. For whatever dumbassed reason, they have been putting prejet primer carbs on push mowers for 40 years..

They've had one screw main fuel only adjustable two stroke carbs for 40 years also..

It seams to be (what it's on) has the biggest factor there.. Lawn mowers or snowflippers, something that just runs one speed all he time, doesn't need adjustment as much as something that throttles up and down....

Hs40 50.. H50 60.. Those are all about the rite size, Most of the Tec carbs, from like 3.5 to around 6 are just about the same CFM.. theres like a REALLY SMALL 3 horse carb, usually on the old old H25's or some of the old old sidewalk edger 3.5's..

but typically any of the newer or big iron flywheel 3.5 horse.. through like 5 to 6 horse power, are all right around the same size ventury... there's like 15 different sizes, and like 3 different sizes for the 3.5 - 6 range..

But HS40 is just about right.... AA battery sized hole... Maybe upwards of a dime... Not much over a dime though or it will be too big, and you will just have a hard time pulling fuel at idle, and acceleration will suffer... You need some pretty major upgrades if you are gonna match an 8 horse carb to 139CC motor..