Author Topic: 4 stroke string trimmers  (Read 143 times)

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4 stroke string trimmers
« on: July 24, 2015, 12:10:43 am »
I get 4 stroke string trimmers all the time at work in for repair.   I usually find what is wrong with them pretty quick.
The interesting part is the impulse that is generated for the diaphragms for the carb.   The intake has a channel going to a hole that matches the impulse for the carb.   My question is at what time does the impulse get generated through the intake and is the pulse a vacuum or a pressure type pulse.   The carbs used on these 4 strokes are just like what you would find on many 2 stroke engines.
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Offline 660magnum

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Re: 4 stroke string trimmers
« Reply #1 on: July 24, 2015, 07:18:22 am »
The ones that source the impulse from within the intake manifold have vacuum only for a impulse and there is a return spring on the impulse side of the pump diaphragm. (Like the Ryobi)

The ones without the return spring on the pump diaphragm have the impulse sourced from the crankcase.
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Re: 4 stroke string trimmers
« Reply #2 on: July 24, 2015, 03:03:40 pm »
I have nothing to add but this story.

Years ago.

Friend of mine from work gave me a 4 stroke trimmer with metal brush blade attachment, tiller, etc.  Told me wouldnt start anymore and he gave up on it.
Did my normal stuff and noticed oil goo on the plug when pulled. Drained the fuel and it was 2 stoke mix.  Put in new plug, fresh gas. Cant remember if I had go in carb or not (guessing not).  But anyhow started and retuned carb and was good to go.

Told friend at work about it and ask if he wanted his 4 stroke back and explained to him what he had done. He is in his 60's so prob just forgot 4 stroke.

Sure wish I had my Dan Wesson .22 on 357 frame I traded him one time, back now days.  :'(
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Re: 4 stroke string trimmers
« Reply #3 on: July 24, 2015, 11:52:07 pm »
The ones that source the impulse from within the intake manifold have vacuum only for a impulse and there is a return spring on the impulse side of the pump diaphragm. (Like the Ryobi)

The ones without the return spring on the pump diaphragm have the impulse sourced from the crankcase.

Thanks I did not know that the spring in there on the diaphragm was for an impulse that used a vacume.   It does make sense that it uses vacuum from the intake when the piston is traveling down on the intake stroke.
Thanks Jim you are the man.
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Re: 4 stroke string trimmers
« Reply #4 on: July 24, 2015, 11:55:16 pm »
I have nothing to add but this story.

Years ago.

Friend of mine from work gave me a 4 stroke trimmer with metal brush blade attachment, tiller, etc.  Told me wouldnt start anymore and he gave up on it.
Did my normal stuff and noticed oil goo on the plug when pulled. Drained the fuel and it was 2 stoke mix.  Put in new plug, fresh gas. Cant remember if I had go in carb or not (guessing not).  But anyhow started and retuned carb and was good to go.

Told friend at work about it and ask if he wanted his 4 stroke back and explained to him what he had done. He is in his 60's so prob just forgot 4 stroke.

Sure wish I had my Dan Wesson .22 on 357 frame I traded him one time, back now days.  :'(

Yea that Dan Wesson would be nice.   I have one with the heavy barrel in .357.   
PP 505, 475, 445.

 

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