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I have a B&S Quantum XTS 50 serial 12F802-3112-B1

It won't pull start on gas.
Replaced the primer bulb and checked it works.
The air cleaner assembly MAY be warped and I will replace that next, I understand that can affect starting but should not affect running.
(edit: the primer bulb does not prime the engine - it does puff air, and the gallery to the bowl is clear but no fule bubbles up the jet out the nozzle, so I am pretty sure the fault here for not starting must be a warped cleaner assembly (thanks to StarTech below) ).

It starts well (1 or most 2 pulls) on a squirt of ether and runs strongly while it has ether, responds to throttle, does not hunt, then stops when ether is used up.

Spark is good.
Compression is OK.
Fuel to carb, fuel in bowl.
Does not seem to be sucking fuel across the jet.

I replaced the carb with an aftermarket one - same problem, same symptoms.
I cleaned the old carb and ensured all jets and tubes (primer) are clear and float needle works.
New gaskets on both sides of the carb.

Any suggestions on what may be the issue are welcome.

Make sure the plastic intake cross tube isn't broke or cracked.

When you checked the primer, did you verify that the carb nozzle has a little fuel pumping out of it as you push the primer? As you mention a warp air cleaner base would cause this not to happen. And you don't have replace the base if you just look at the gasket impression and make up wedge gasket to applied extra pressure in the area of non contact. Don't not over tighten the filter base screws. They are torqued to 40 in/lbs otherwise you can warp the base.

Also note these filter bases can warp over time on their own from aging and heat.

ILENGINE said:

Make sure the plastic intake cross tube isn't broke or cracked. View attachment 70104

Click to expand...

Thank you for your reply. As far as I can see my engine does not have one of these, the carb attaches direct to the inlet on the head. There is a short tube protruding from the head, a rubber 'O ring' gasket goes around this (new one used) and the carb bolts over this. Is this an option for mounting the carb to the front of the engine?

StarTech said:

When you checked the primer, did you verify that the carb nozzle has a little fuel pumping out of it as you push the primer? As you mention a warp air cleaner base would cause this not to happen. And you don't have replace the base if you just look at the gasket impression and make up wedge gasket to applied extra pressure in the area of non contact. Don't not over tighten the filter base screws. They are torqued to 40 in/lbs otherwise you can warp the base.

Also note these filter bases can warp over time on their own from aging and heat.

Click to expand...

Thanks, yes I did test that, I used a new gasket as well, but I suspect the assembly is warped which is why it does not prime - I will replace that assembly next. I believe that explains why it won't start, but not explain why it won't keep running on gas. Thanks for the tips on the install too.