2cycle carb without impulse line

[JonCraig]

I picked up a Poulan 3400 counter-vibe the other day for a song. A little bit of research indicates that this is a good saw,56cc's, albeit a bit "vintage", but was built quality, unlike the Poulan saws of today. Not trying to start that war. All that to say, I believe it's worth fixing. Working on it today, the gas has turned to goo, so replacing the lines and a carb kit is in order.

But as I was taking the carb off to get the model #, I noticed that there is no impulse line. And when I pull up the IPL, there isn't one listed. Which made me ask, "how could it work without an impulse line?" And my next question was, "well, then, how does the carb on my lawnmower work? It doesn't have an impulse line!

So, oh wise ones, 'splain it to me, please. Are impulse lines a newer-style? Is it dependent on displacement of the engine? (a 24HP Briggs can suck enough air on its own to cause venturi effect, whereas smaller 2-cycle engines can't?) But if that's the case, it begs the question… My 3.5 HP Briggs Kool-Bore doesn't have an impulse line, but my 3.something HP Stihl MS460 does.

Help clear the carb-cleaner fumes from my brain, please! And thanks in advance!

--JC

[KE4AVB]

Your impulse is done thru the carburetor adapter on that Walbro HD just like many other handheld 2 cycles especially string trimmers.

[bgsengine]

In addition to what KE4 said - it is not about suction power of the CC's - it is because the impulse line operates the fuel pump section of the carburetor - It needs crankcase pulses just like the vacuum operated fuel pump on your 24 HP Briggs - that is what gets gas from the tank (usually below the level of the carburetor) to the carburetor - there's the ones with a separate pulse line , and then the other, increasingly common ones work through a pulse port in the intake side of crankcase through the intake adapter/insulator/manifold and to the pulse port on the carburetor which leads to the fuel pump chamber (the small round chamber on pump side of carburetor) and operates the fuel pump diaphragm to pump fuel to the carb - That is why a crankcase , seal, or intake gasket air leak (or a blocked port due to wrong or incorrectly installed gaskets or manifold) will cause the saw to go lean and/or "run a few minutes and die, and wont restart for a few minutes" (Fuel tank pressure assuming a good tank vent will push fuel up to the carb gradually.)

[JonCraig]

Lol… I was thinking about it over lunch and said to myself, "Aha! I bet that's what that little half-moon slit in the carb adaptor is!" And low and behold there's already two replies here! What would I do without you guys?

Thanks for helping me get my head screwed on right!

--JC

[Arkie]

I had a chainsaw with sticky goo in the gas tank awhile back and after cleaning the tank, tank filter, and kitting the carb the saw would run for awhile in the woodlot and start starving for fuel.
Found that the new gas was breaking down some hard sticky goo still in bottom of gas tank and the goo sticky residue was going with the new gas/oil mix thru the in tank filter when the saw was running and vibrating being used and eventually clogging a screen in the carb. The restricted in carb filter screen was not noticeable with just a casual glance.
(had to use lacquer thinner and a long handle small brush to get the bottom of the tank clean and test the cleaning procedure by pouring the tank lacquer cleaner thru a white paper towel during the finals) Could not easily detect all the goo by looking into the tank with a flash light. Had to also let the lacquer sit and soak the gooey tank for few hours at a time.

PATIENCE required!