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Weil Mclain Ultra 399 won't run unless I choke the inlet

I have a pair of Ultra 399s and have had intermittent failures since they were installed 10 years ago and have gotten pretty good and replacing parts, but the current problem has me baffled:

One of the boilers won't light, the original problem was the inducer bearings had seized, I oiled them and got it running until a new unit was delivered.

Originally it wouldn't light with the freed up inducer, good spark, gas (no I didn't check the pressure) etc.
I partly blocked the intake or the inducer and it light right up, if I open the intake back up it goes out.

The short version:
New gas valve, new inducer, new burner but the problem persists.
If I cover about 1/2 the inlet with tape the unit works fine.

Its twin - 2 feet away connected to the same gas line, intakes and exhausts are next to each other - has no issues.

I also cleaned the fire side of the heat exhanger with the tool provided.

Any suggestions?


  • Jamie Hall
    Jamie Hall Member Posts: 23,544
    Take your pick. Too much air, or not enough gas. Without measurements... hard to say which.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,870
    Funny you should use the word choke. I used to know Eric Morotti, drummer for the band Suffocation.
    Please put the tape away, shut it off and get a pro.
    Obviously by partially taping off the venturi, you've severely altered the air/fuel ratio. The programming is looking for X blower RPM @ Y gas pressure, by measuring flame rectification. And be within WM min/max specs of CO2 and O2 readings, on Hi and Lo fire tests. That is only achievable with a combustion analyzer and tiny throttle adjustments on the gas valve.
    Anything less and there could be another form of asphyxiation.