Carburetor woes.....still

[k91]

I am still having problems with my 76 Aisan on my 75 40. It all started after I had a new Thrush muffler put on. Bucked, sputtered and died under load. It ran fine before the muffler.

I rebuilt the carb and it is a lot better, it will idle fine. I can slowly rev it up to higher rpms, but if I stab it, it will backfire.

Under load, it will still studder and backfire.

I adjusted the valves on the newly rebuilt head, am going to change out the spark plugs, cap and rotor look good.

Anyone know why it is doing this? I am going crazy with this thing, thanks, Matt

 

[1911]

I'm no expert on Aisin carbs on cruisers, but on every other engine I've worked on if you open up the exhaust a lot it will lean-out your carb mixture and you need to put bigger jets in it to compensate. That's what it sounds like to me.

 

[Copenhagen1]

..............I adjusted the valves on the newly rebuilt head.....

I can't answer about the carb issue because I still have issues of my own.

After reading about this problem and imagining a brand new motor running out of oil, this always come to my mind now when I hear about a newly rebuilt head. Have you seen this?
https://forum.ih8mud.com/40-55-series-tech/280385-strange-place-oil-leak-what-do-i-do.html

Just a heads up!

 

[fireflyr]

Sounds like a jetting problem to me. Don't the Aisan carbs come with high and low altitude jets? Maybe make sure the right ones are installed. If nothing else, post the jets you are running and Mark or someone else intimate with those carbs can tell you if you are in the right ballpark. Tell us what altitude you're at too. Cheers.

 

[k91]

Good idea....I'll post the size of the jets

 

[Pin_Head]

I doubt that changing the muffler had anything to do with it.
If it ran fine before, it doesn't have the wrong jets in it unless you put them in the wrong holes.
What is the fuel level in the bowl when it sputters?
Does the fuel pump provide adequate flow?
Does the accelerator pump squirt?
Is the secondary opening too early?
Is the linkage properly installed?
Does it stall when you turn in the idle mixture screw all the way?

 

[Gusb]

Does the accelerator pump squirt?

Pin_Head - if you take a truck (2F - stock carb) where everything is fine "except" the accelerator pump squirt - what symptoms would the truck display? Would it stumble a bit on slight engine acceleration, etc.?

 

[CruiserStuff]

i thought that backfires were usually related to electrical issues....am i wrong?

 

[CruiserStuff]

"bucking, sputtering and dying under load" sounds like a plugged fuel filter to me.....

 

[k91]

I doubt that changing the muffler had anything to do with it.
If it ran fine before, it doesn't have the wrong jets in it unless you put them in the wrong holes.
What is the fuel level in the bowl when it sputters?
Does the fuel pump provide adequate flow?
Does the accelerator pump squirt?
Is the secondary opening too early?
Is the linkage properly installed?
Does it stall when you turn in the idle mixture screw all the way?

-I put the jets in the same holes, I took pics during disassembly.
-Fuel level is right at the line in the window.
-I measured the fuel pump flow using the method of turning the engine over for 15 secs, should get about 1 1/2 cups...sound right?
-The accelerator does squirt.
-Not sure how to tell if the secondary is opening to early.
-I replaced all the linkage the way it came off, via my pics.
-It does stall when the idle mixture is turned all the way in.
Does this help? Thanks, Matt

 

[k91]

Alright...first jet closest to accelerator pump is 144 and the other is so marred that I can only see a two on one side and a 3 on the other.
Also, I did change the fuel filter out with an OEM one when I put the rebuilt carb on. Matt

 

[65swb45]

Alright...first jet closest to accelerator pump is 144 and the other is so marred that I can only see a two on one side and a 3 on the other.
Matt

That would be a 2.30 secondary jet.

I will know more tomorrow.

 

[k91]

Thanks Mark!!
Just for an update on this thread....I sent my carb to Mark to have a professional rebuild it properly. It's just that simple.Matt