I've just rebuilt an eBay Coleman 424 suitcase stove. It runs well, with no yellow flame at the burner even during startup, but if I try and turn the main burner down to a low simmer I get a yellow flame coming back out of the manifold. Is this normal due to the low velocity through the jet, or does it indicate a gunky generator? I have ordered a new one but for the time being I've cleaned out the old one as best I can. This isn't due to fuel leakage past the jet threads - I've fixed that. This is only there at simmer.