# A couple of noob questions about impulse line and carb tuning



## cam76034 (Aug 12, 2009)

I've had two different projects going on recently, and one of the suggestions I was given was to check my impulse line. 

Question #1. Where exactly does the impulse line go and what does it do?

Question #2. When you talk about setting the high side needle on the carb "in the cut"...just how do you do that exactly? I have a hard enough time holding onto everything just setting the L side. Or do you have a helper with this?


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## rupedoggy (Aug 12, 2009)

OK I will try the impulse line question.
Years ago the chain saws of the day had to run with the engine nearly level because of gravity feed for the mix. Some bright fellow figured that if we pump the fuel we could run the saw in almost any position. A pump is now built into most all carburetors for this feature. Well how to run the pump? The pulse line is used. The crankcase, because of piston movement, creates alternate pressure, vacuum cycles (don't tell me there is no such thing as vacuum, science majors, this is a simplified explaination).
Anyway back to the carburetor. This vacuum/pressure is used to move a diaphram, back and forth, and one way valves insure movement of the mix into correct parts of the carburetor.
Now some saws have a pulse line and some have holes in the manifold block to serve the same fuction. Bottom line is if the holes get plugged or the line gets cracks/holes/breaks the mix is not pumped. Ergo the saw runs lean or not at all. Oh almost forgot, the pulse line looks like a fuel line most of the time, but travels to the cylinder or crankcase from the carburetor. The fuel line will travel to the fuel tank. Somebody else will tell you about the high speed adjustment.


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## Fish (Aug 12, 2009)

Kind of "terse" on the carb adjusting, eh?


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## roncoinc (Aug 12, 2009)

> cam76034 A couple of noob questions about impulse line and carb tuning
> 
> --------------------------------------------------------------------------------
> I've had two different projects going on recently, and one of the suggestions I was given was to check my impulse line.
> ...



been a member 6 1/2 yrs and havent noticed all the post's on carb tuning ??
Fish will tell you..


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## rupedoggy (Aug 12, 2009)

Fish you were just waiting for that, weren't you? LOL Look how how short the question was, that I did answer, and how long the answer was! I was afraid to tackle the long question for fear I may fall asleep writing the answer. Ha! Your turn to tell about "H" adjusting.


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## Saw Dr. (Aug 12, 2009)

I'll take it.

You need to have the saw just slightly "four stroking" or burbling in the cut. You do not have to (nor do I suggest) that you tune it while trying to run the saw. Here is what you do. Run the saw in a piece of wood, and have it tuned a bit on the rich side. Make several cuts, and after each one, lean it out about 1/8 of a turn. When you get to the point where it is running "clean" or two-stroking most of the time in the cut, but still "four stroking" a little bit, you are about perfect.


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## angelo c (Aug 12, 2009)

Ok guys it sounds like some of you are having fun with the mixture question.

I'll tack on a question to the obvious explanation. Why does the RPM increase when leaning and where exactly is Peak internal combustion pressures in relation to "4 stroking" ect. I am trying to understand this in reference to a gasoline 4 stroke terminology in that a mixture will start rich of peak, reach peak then eventually fall Lean Of peak ( on the backside of power curve) until it leans out or starves due to lack of fuel in the mixture ratio. Peak internal combustion pressures being very rich and decreasing as they lean out. 
Or should I disgregard any relationships to 4 stroke IC engines... ?

thanks. 
A


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## PonderousPiner (Aug 13, 2009)

angelo c said:


> Or should I disgregard any relationships to 4 stroke IC engines... ?


Yes, you should disregard, IMHO.

As I understand it, the "4-stroking" is essentially missing due to over-rich mixture. Not sure of the mechanism that makes it cyclic, but it's not a positive pumping action like a 4-stroke engine--the engine is dependent on the prior ignition for some of the movement of the prior charge out the exhaust, and when it misses, that doesn't happen. At the higher engine load of the cut, things smooth out.

The other issue is that when perfectly tuned, the mixture is richer than required just for combustion--the fuel also acts as a coolant for the P&C.

PPine


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## cam76034 (Aug 13, 2009)

Thanks B200Driver. I've spent over 6 years looking for that explanation!


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## Saw Dr. (Aug 13, 2009)

cam76034 said:


> Thanks B200Driver. I've spent over 6 years looking for that explanation!



Pass the word around......


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## cam76034 (Aug 13, 2009)

Seriously, I was wondering how I'm supposed to adjust the carb while cutting some test wood at the same time. Didn't want to ask the wife to help, she needs her fingers for her job.


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## wendell (Aug 16, 2009)

Did you see post 13 in this thread?

http://www.arboristsite.com/showthread.php?t=106660


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## andrethegiant70 (Aug 16, 2009)

Fish said:


> Kind of "terse" on the carb adjusting, eh?



LOL.. Fish, you are a bad boy. You have to admit, the esteemed Mr Rupley tells it like it is.


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## Tchetek (Aug 17, 2015)

Hello, 

So on my husky with out a impulse tube just a cheap grommit in the cylider/carb adapter spigot . I assume that if I make an adaptor into the impulse hole on the cylider, with a straw or tube be able to blow air into the crank, if it's plugged I have to tear off the cylinder and find the issue?????

battling an intake air leak issue, with a new top end. I need to replace my impulse gromet, and I will replace the intake rubber boot also but I can't blow air into the cylinder thought the impulse hole. I don't really want to pull the cylinder off again but now think it's plugged at the case. Used a bit of rtv on the gasket but thought I stayed away from the impulse hole. 

Thanks to all that post in these forums for the info that has gotten me this far through my first engine rebuild.


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## rupedoggy (Aug 20, 2015)

It would help to know which model saw you are messing with. Sounds a little like a 55 family. May be able to get you some quick ideas for it without all the disassembly but tell us the model. Mike


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## Tchetek (Aug 20, 2015)

rupedoggy said:


> It would help to know which model saw you are messing with. Sounds a little like a 55 family. May be able to get you some quick ideas for it without all the disassembly but tell us the model. Mike


Yes. A 55. 

I now have it running good. Pulled the cylider off and yes I did manage to plug the impulse at the base gasket. It doesn't have any holes into the crank just leaks the impulse pressure through a notch in the gasket. I wouldn't use rtv again. 

So now new impulse grommit and intake boot. (Week design). My recommendations to anyone with a husky 55 with fuel issues to start with just replacing all plastic or rubber parts between the cylider and carb. Probably any saw of this style with a lot of use leaks air somewhere here leaning them out. Saw now runs like new. 

Thanks.


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