91 yamaha 40 rpms surge at low speed (Rodbolt?)

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brico

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1991 yamaha 40 hp 2 stroke. Previous owner said engine sat for a while. Cleaned carbs, blew out fuel lines, replace fuel filter, gas, fuel pump, plugs Etc. Cleaned a bunch of crap out of the start primer pump which made a huge difference in performance( it was all gummed up from the diaphram sitting in fuel) diaphram needs to be replaced but no visual holes or tears, just looks a bit stretched (got one on order) engine runs like a bat outta hell at WOT, but when I kick it in gear at around 1000 rpm it ramps up and down. Idle is ok seems like the idle screw is adjusted in a little much (one more turn and it's down all the way) Not sure if it electrical or fuel. What am I missing?
Any ideas
 
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99yam40

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Re: 91 yamaha 40 rpms surge at low speed (Rodbolt?)

Check the timing to see where it is at idle and opened up. My CDI was not letting the timing to be adjusted properly at idle and was retarded too far causing the idle screw to be screwed down too far to keep it running at proper idle RPM
 
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brico

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Re: 91 yamaha 40 rpms surge at low speed (Rodbolt?)

I have a clymer shop manual but it is for a newer motor. Pardon the ignorance but on the cdi for the timing there are 3 hash marks on bottom and one on top. I did a static timing adjustment but it doesn't explain to much about the needle on the cdi where it should be. I have it set to the #3 hash mark from the bottom. definetly helped with the idle adjustment needle. noticed the bottom spark plug was not brown like the others almost still white except in one little spot
 
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99yam40

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Re: 91 yamaha 40 rpms surge at low speed (Rodbolt?)

You need to use a timing light to see where it definitely is at at idle and WOT. the marks are just a rough setting, something to set it up at.

May need to look at that #3 carbs again, you do not want to be running lean
 
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brico

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Re: 91 yamaha 40 rpms surge at low speed (Rodbolt?)

alright, I have isolated the problem to Carb #3 ran it with my two fingers over the carb intake to retrict airflow and eratic behavior smoothes out like it seems to sound normal and run steady. That being said that Carb #3 spark plug was really clean am I running to lean?( I am confused, I thought retricting air flow was like choking it and dumping more fuel in the carb) so do I have a fuel restriction or what? any thoughts. When I cup my hand directly over the carb to plug it up the engine still runs but when I do that to Carbs 1 and 2 it shuts off . am i pulling air from somwhere
 
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99yam40

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Re: 91 yamaha 40 rpms surge at low speed (Rodbolt?)

pull and clean the carbs again
 
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brico

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Re: 91 yamaha 40 rpms surge at low speed (Rodbolt?)

looks like it was the prime start pump diaphram and gasket. Carbs were clean. seems to be much better although seems to jump a little bit still when cruising at low speed (maybe thats just a characteristic of a 2 stroke mounted on a small boat I don't know.
 
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