Small Engine Experts - Engine cuts out while running

[mark198]

Hoping the experts have an easy, free solution.

stihl fs45 string trimmer, it had been struggling to run at wot. I cleaned the spark arrestor, replaced the spark plug, air filter, and fuel filter. When I started it up it ran like new.

when I was putting my tools away my young apprentice decided to replace the primer bulb. When I got back it would no longer run. I have now put the old bulb back, removed the carburetor and sprayed everything with carb cleaner.

the current situation, it will start, I can begin to give it throttle but if I go wide open it cuts out. The other thing I can’t figure out is if I give it 1/3 throttle it will hold a fast idle untill it suddenly dies. No stumble, no misfire, just a hard cut off.

hopefully one of you can help me save this motor.

thanks,
mark

 

[My Old Tools]

Have you tried adjusting the hi and low mixture screws?

 

[BillK]

So he changed the primer and now it wont run ? Did he pull the lines out of the tank or wherever they go when he changed the primer ? Maybe they are not back where they belong ?

 

[mark198]

Have you tried adjusting the hi and low mixture screws?

Thanks, I have not because I was assured they weren’t touched. I’ve closed up for the day but I’ll give it a try tomorrow.

thanks,
Mark

 

[FredWanaker]

how does the primer come off without removing the carb? Most likely either something else was disturbed or the primer is sucking a little air.

 

[theoldwizard1]

Small 2 stroke engines with diaphragm carburetors are very sensitive to air/vacuum leaks. Could be a tiny leak in one of the hoses or a gasket leak.

 

[My Old Tools]

Entirely possible the gasket to engine housing is out of place.

 

[Jswain]

Could have the lines swapped on the primer bulb possibly.

Did you just fill it with freshly mixed fuel and maybe this batch is mixed different then the last? Give low & high screw 1/4 turn to the left and see if situation improves

 

[jonesg]

if no-one touched the mixture screws I would not start twiddling with them now.

 

[nadogail]

In order to run an engine needs Compression, Fuel and Ignition. Removing one of those will cause the engine to stop running. It is extremely unlikely that you will be losing the compression on a running engine, that narrows your probable cause to either Ignition or fuel.
My best guess is that fuel, including the carburettor, will be the most probable cause.

 

[mark198]

Thanks for all the replies.

To answer some questions about design and what may have happened: the prime bulb ‘can’ be replaced without removing the carb, and that is what happened originally. In addition, there is a metering diagram, something I wasn’t familiar with, sitting under the prime bulb and comes loose when the primer is removed. It turned out this got flipped on the initial change because he was working in a very tight spot with the carburetor on the engine.

I have removed the carb, replaced the primer, and positioned the metering diagram correctly but the engine will cut off as it approaches half speed.

Did this damage the diaphragm? it Looks like I can order a replacement so I will probably try a new one this week.

 

[Hank11]

I'd put all new factory parts in the card and see if that fixes it. It prob ably will.

 

[CR888]

Check that the hoses were fitted correctly. Check that the return primer hose is delivering into the tank. If they aren't reinstalled properly u get issues.. Also see if the tank vent is working as it should so unscrew it a little to let air in.

 

[Firebrick43]

I would replace the whole carb. I (and a lot of other people) have never had good luck with rebuilding the zama carbs most of the homeowner line and prosumer line comes with.

They are very inexpensive for a carb, always had good luck with the ebay specials. Most don't have the limiters either making for easier tuning.

Have you pulled the spark arrestor screen and cleaned it?????? That is the most common problem is they get clogged and choke the engine.

 

[FredWanaker]

Your engine is starving for fuel. If you know where all the carb parts go, rebuild it. If not order a new carb. Most likely there was a problem before he removed the priming bulb. The fuel diaphragm is what usually is at fault on those. I get parts from these people for those small carbs - https://www.carbtech21.com/default.asp