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Toro Briggs Quantum surges at idle but fine when blade engaged

#1

J

JBtoro

Neighbor brought a Toro Personal Pace (Briggs 7.0 Quantum) with the blade clutch feature. When it starts with the blade not yet engaged, it surges wildly. But when the bail is pulled thus engaging the blade, it runs great (3200 RPM). Tried several known good & clean (one brand new) carbs & still happens. I even opened the bowl nut jet on one of the carbs with micro drill bits increasing the opening incrementally 3 sizes up from OEM. No success. Am puzzled by this one. Maybe the gov gear inside the engine is the problem. Anyone experienced this?


#2

ILENGINE

ILENGINE

Lets start with the actual model type and code of the briggs engine. so we can see what you are looking at. Some of the Briggs push engines have two governor springs and if they are not set correctly will cause surging. But that surge symptom you are referring to is normally due to a carb issue. But wouldn't be a bad idea to pull the engine cover and inspect the intake tube for cracks or being loose which would introduce air which would cause a lean run condition.


#3

J

JBtoro

Quantum Model 120000 675 Series. Just one gov spring. Will check the intake tube. But if in fact there is too much air getting in from the tube or elsewhere, why would the engine run great under load (blade engaged) but only surge under no load?


#4

ILENGINE

ILENGINE

The surge could be related to the different carb passages that are responsible for different speed functions.


#5

Scrubcadet10

Scrubcadet10

JBtoro said:
Quantum Model 120000 675 Series. Just one gov spring. Will check the intake tube. But if in fact there is too much air getting in from the tube or elsewhere, why would the engine run great under load (blade engaged) but only surge under no load?
Click to expand...
you can spray down the area around the intake tube, where the carb mounts etc with WD40 in a trigger sprayer, any change in engine speed or if it blows smoke will tell you where the leak is.


#6

J

JBtoro

ILENGINE said:
The surge could be related to the different carb passages that are responsible for different speed functions.
Click to expand...
Your advice is most welcome. I think you must mean passages clogged to some extent. Perhaps, but I tried 2 other used carbs pulled from working, non-surging engines plus a new carb and the problem persists. The only passages I know of are the two pictured below plus the bowl nut passages which are completely clean. Are there others?

1716170646485.png


#7

J

JBtoro

Scrubcadet10 said:
you can spray down the area around the intake tube, where the carb mounts etc with WD40 in a trigger sprayer, any change in engine speed or if it blows smoke will tell you where the leak is.
Click to expand...
Thanks, will try that in the am. But again, I must ask, if in fact there is too much air getting in from the tube or elsewhere, why would the engine run great under load (blade engaged) but only surge under no load?


#8

StarTech

StarTech

Those two air holes are 1) fuel bowl vent and 2) air bleed for fuel mixture. Neither will cause the carburetor run lean normally.

Sometimes the main jet has to be enlarged ever so slightly due oxidization. And sometimes it is simply too lean to start with. This is why I have a set of micro bits here.


#9

J

JBtoro

StarTech said:
Those two air holes are 1) fuel bowl vent and 2) air bleed for fuel mixture. Neither will cause the carburetor run lean normally.

Sometimes the main jet has to be enlarged ever so slightly due oxidization. And sometimes it is simply too lean to start with. This is why I have a set of micro bits here.
Click to expand...


#10

J

JBtoro

Thanks. Maybe you missed it in my original post, "I even opened the bowl nut jet on one of the carbs with micro drill bits increasing the opening incrementally 3 sizes up from OEM." That is, I inserted bits until I found the 1st one that would not go vertically through the OEM bowl nut jet hole. Then I enlarged said hole with that sized bit. Tested; still surging. Drilled out the next size up; still surging. Went up one more; same. Last attempt was with a 0.75 mm bit. Should I go up even more?

1716207398864.png


#11

J

JBtoro

Scrubcadet10 said:
you can spray down the area around the intake tube, where the carb mounts etc with WD40 in a trigger sprayer, any change in engine speed or if it blows smoke will tell you where the leak is.
Click to expand...
Haven't sprayed yet but I did take off the air box cover so I could temporarily block off the intake of air through the carb. Saw a video where a mower continued to run after so doing (meaning that enough air was getting in elsewhere) thus indicating and later revealing a cracked intake tube. When I did this, my mower died immediately indicating that maybe the tube is ok. Given this test, do you still recommend doing the spray test? The only reason I am hesitant is because I have to remove the recoil & shroud to access the tube for spraying which means having to start the mower with a drill.


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