axapowell

You are backwards...lower AFR=Richer. So you,are leaner at WOT than me, and really lean at 13.6 to 13.8.

Thanks anyway.

Dave

BenPerfected

Dave,
You are right I sad that backwards. I our experience, 13.6 13.8in the mid-range in a low compression NA BBC is not lean. The plugs look great with no sign of detonation. These engines are 500 ci with 9.3 CR with top HP at 6800 RPM. Our PV"s open at about 5400 RPM or about 80% of full load. Besides in the boat testing with both AFR ratio meter and knock sensors, all this has also been confirmed on the dyno.
There is nothing wrong with rich except for using more fuel and sometimes soot on the transom.
Ben

MER Performance

I see nothing wrong with 11.8-12.4 above 4k. What is the rpm that the PV opens? I have used plenty of Demon carbs in the past, with out issues, what series is the Demon? that alone will matter with the tuning of the carb. Being all the timing is in at 2500, I would consider that Std. you maybe holding the PV closed, which is leaning your primary side out. I base my PV by what the engine is doing from the transition from idle in gear bringing the boat on plane.

axapowell

Mighty Demon Marine 850. The engines are designed to run up to 5800-6000, but I try to prop no more than 5700-5800. I'll try to check the transition vacuum and change the PV if neseccary. What do you think of the spikes? I'd like to fix that first before chasing a problem that might not even be there. I'm ok with the numbers, if it weren't for the lean spikes.

Dave

blue thunder

Water drops/spray hitting the sensor makes it jump high intermittantly from my experience. Snapmorgan was right.

axapowell

I'm not going to get into a debate, maybe you are right about that happening to you, but how would you explain the engine surging at the same time the meter spikes? There is NO way I'm getting water on my O2 sensors at ANY rpm's.

Dave

blue thunder

These are just ideas axa intended to stimulate your ideation to a broader band. Mentioning them is not intended to insight debate just help you think about things you might not have thought about. It is a high probability that you are not getting water on the sensor but that doesn't mean you should not consider it. If you have any water at all in your exhaust and get any measureable leakage past an exhaust valve you will get water spray into that cylinder. Just thoughts axa, not necessary the silver bullet of conclusion most likely, just thoughts. You should take any input offered as plausible and be thankful someone took time out of thier day to try to help you out. No matter how much help thier input turns out to be.

Airpacker

I'll take a stab at this.

The meters are showing you what you are feeling. MISS FIRE!!!!!!!.

All that unburned oxygen from the miss firing cylinder is registering on your WB sensor.

Find the cause of the miss fire and your WB readings will go back to normal.

Want to prove the theory? Pull a plug wire off the other motor and look at the WB readings.

axapowell

I know that, now what's causing the mis fire? I still think its an intake manifold gasket vacuum leak. Going up tonight, taking propane to probe.

Dave

Airpacker

Hard to believe an intake leak would cause what you describe without it being very noticeable at idle in gear as well.
I would be tempted to try a set of 92's on the primary side and retest it. It might just be going into a light LEAN miss fire just before the fuel stars flowing from the secondary jets.

If you are going to use propane for leak detection, please don't.
I suggest you use non flammable brake cleaner such as CRC as your test liquid though. Handy to use in a spray can with the nozzle pipe. It will make serious white smoke from the tail pipes and stink like hell but it won't introduce an explodable fuel source into the engine bay. Propane will head down to the bottom of the bilge and, as we all know, thats not good.