Can't stop the surging

[garage_hacker]

Hi there, new member here. Need some help -

I've got an MTD Tiller/cultivator with a Briggs and Stratton engine
Model 137202
Type 1134E1
Code 990421YA

I bought it as a non-runner. It had a gummed up tank and carb - it sat in the garage for years without use. I took it all apart, replaced the carburetor with a brand new aftermarket unit from Mower Parts (21A-442B129). I installed a new spark plug (B&S), I cleaned out the tank, removing all rust from the inside using electrolysis, cleaned it out with soap and water, then dried it out with heat gun, plus a generous coating of Marvel Mystery Oil to seal the metal. I then filled up the tank with fresh ethanol free fuel. I should also add that I replaced the gasket between the carb and the tank as well as the one between the carb and the cylinder intake port. All new and tight.

The machine starts on the first pull and runs great on low speed (turtle), but when I move the throttle to the max (Rabbit) it runs faster but it surges back and forth and won't ever settle down.
When I immobilize the throttle with my hand overriding the governor action then it runs well. When I release the hold it continues to surge as before. I ran it for a while, thought maybe it would settle down eventually when it warms up - but no. I checked all the linkages on it - the spring is installed as it should be on the governor lever. I even bent the bracket to tighten the spring a bit, but no change. Something isn't right - it shouldn't surge when everything is new.

I tried looking for solutions on-line - all the videos on Youtube show the cure for surging is to clean the carb jets and to put new clean fuel in it... seems my case is different. What else should I try?

P.S
Attaching a pic from before the re-build so you have an idea of the type of carb it is (pulse-jet).

 

[garage_hacker]

All the reading I've done indicates that surging is a sign of fuel starvation, but when I spray the choke cleaner into the carb it doesn't smooth out, it stalls out.

 

[tadawson]

Clean the origimal carb and give it a try. Aftermarket parts are pretty much a crapshoot, and your new carb may be defective.

 

[garage_hacker]

I thought that the replacement carb was bad too, so I had the company ship me another one - so this is actually a 2nd new carb that's on it. Unfortunately I threw away the old one. Chances of it being two bad carbs are very small. The surging diminishes a little under load. Still drives me crazy.

 

[tadawson]

If it's a generic replacement that is not necessarily correct for your engine (again, an issue at times with aftermarket), you may just fail over and over. I don't understand the reluctance to clean the original. It's cheap, quick, and pretty easy.

Bottom line is that it is going lean under load . . . . Fuel flow, carb metering, or an air leak in the intake are typical candidates.

????

 

[garage_hacker]

The old carb had a broken suction tube and I foolishly threw it away - that's why I can't put it back on. Tell me more about the air leak - where, between fuel tank and carb or carb and cylinder?

 

[tadawson]

Not certain the intake on your equipment, but anywhere the air flows between the carb and the head - many engines have a separate manifold with a gasket at the head and carb ends.

 

[garage_hacker]

Not certain the intake on your equipment, but anywhere the air flows between the carb and the head - many engines have a separate manifold with a gasket at the head and carb ends.

Yes, there is a thick gasket (new) between the carb and the cylinder head attached with two screws which are very difficult to access. I'll double check to make sure they are as tight as possible.

 

[garage_hacker]

I tightened up the intake bolts and the condition improved... I can live with it now.

 

[slomo]

The OEM Briggs carb to block gasket is like 1/8" thick or more.

If you buy a Chinese Amazon or Ebay carb, they give you a much thinner gasket.