Tecumseh 8hp, hmsk80, surges

[t.ca]

The engine surges badly.
I have cleaned the carb.
Installed it with new gaskets.
Set the low and high mixture screw to 1.5 turns out.
It fires right up but still surges.
It runs with the choke partly on.
I have tried setting the low speed mixture screw and nothing happens.
As a last resort I turned the screw right in and there is no change.
Any ideas?

 

[ken2]

jet in carburetor is plugged

 

[Scrubcadet10]

get some WD40 in a trigger spray bottle, with the engine running spray the area around where the carb bolts to the engine, if the engine blows any white smoke or there is a change in its running, you've found your air leak causing a lean running condition resulting in surging.

 

[hlw49]

low speed circuit in the carb is stopped up. Take the main jet out. Take the high speed adjusting screw out. In the section where there are no threads in the center of the jet there is a tiny hole the goes in at an angle make sure it is clean.

 

[t.ca]

An update
1st thanks for the replies
I have re cleaned the carb in an ultrasonic cleaner x2
All passages and jets are clear
I checked all the mating surfaces and they are flat
But I still added a thin layer of rtv to the gaskets just to seal up any imperfections
Ran the engine several times
It still surges
Looking at it last night I got the idea that my snow blower has the same engine, just older
Swapped the carbs and intake runner
It ran perfectly
Swapped the jets from my carb to the original carb
Installed the carb
It surged just like it did before
Installed my intake runner with the original carb
It still surges
I'll try the wd40 test one more time tomorrow

 

[sgkent]

so if a known good carb runs right then your problem is in the old carb. Clean it again, inspect each port, check everything on it, replace the gaskets. Try again. That said, I have one on a 5hp Tecumseh that didn't work right. Rebuilt it maybe 3 or 4 times - not hard at all having done carb rebuilds for 50+ years successfully including many years of auto racing. Still would not run right. Finally bought a NOS replacement, put it on and it runs perfectly. The old one is sitting in a box for when I have time, or maybe I will toss it. Right now I have other challenges.

 

[bertsmobile1]

Over time the pivot hole for the throttle plate floggs out oval
Because this is after the main jet has added the fuel you end up with an air leak & lean mix
And of course as the carb body gets hotter the air leak gets worse.
As you can't get oversize jets to compensate then you either have to drill out your existing jet of fill the hole in the carb with Devcon then re-drill the hole for the throttle plate .

 

[t.ca]

Ya replacing the carb is where I landed on this to.
Thanks

 

[hlw49]

I know you cleaned the carb several times but did you take the welch plug out and replace it. Some times you can't get particles out from under the welch plug unless you take the welch plug out and clean the transition ports that are under it.

 

[Rivets]

Have you checked the governor? If it is not set properly it will cause surging. Not uncommon for those older units to need a little tweeking.