It's a 650 Speed Demon with mechanical secondaries...

When it was dynoed, they (the shop) put 80/92 jets in it. Well, I discovered that after I installed the engine, that it was pig-rich. Plugs were black and the exhaust would burn your eyes. I got 3 mpg if I was lucky. I get around 12 inces of vacuum idling at 800-850, and I've changed intake gaskets twice and got the same results. I needed to lean this thing out.

Even though I know the stupid Autometer A/F single wire O2 gage I have installed isn't that accurate, I'm still kinda using it as a guide to determine what jets to use. Before, it was just bottomed out at full rich on the gage whether idling or driving.

Then I switched to 76/80's, and it still pretty much stayed full rich. so I then put in 74/78's and it's finally off the full rich, and went up a couple notches towards "stoich". However, when If I accelerate hard now, the A/F gage will shoot down to full lean for a split second, and then return. Didn't do that before.

But I noticed that the exhaust seemed to be unusually hot after I shut it down. But it seems to run even better, and I'm thinking of going to a 72/76 set of jets.

The builder told me to keep exhast temps below 1400 degrees. When he dynoed the engine, #6 cylinder reached 1389 degrees. And that was with the pig-rich jets!

When I mentioned that it was running rich to him, he said not to go down more than two sizes or it will run too "lean", and I'll burn valves possibly. So......

I obviously went further than that already.

How do I know how far to go?? I've always heard to "err to the rich side", which makes sense I guess. But is there any way to know where that is??

Also one more thing....should I try to run the same size jets on the primary AND secondaries?? Or do the secondaries need to be richer? It has 4 corner idle with "idle eeze".

Sorry this is so long, just giving details....

 

Jet sizing front-to-back varies from carb to carb. It's how they're really set up at the factory. Jets are just one of the things that carb manufacturers fiddle with when they calibrate. I'd call BG and ask them what stuff came in the carb originally, put that in as a starting point and tune from there. Unless there's something not functioning correctly inside the carb the calibration should be reasonably close with the stock jets and such- especially on a "tuner" carb like yours.

That momentary lean spot when you hit the gas could be insufficient accelerator pump shot or too small an acc. pump squirter. If you feel a slight bog while it spikes momentarily lean this is almost definitely the reason.

 

Keep leaning out the main jets till it surges at a steady cruise state, then jet back up two sizes from there.Tune the secondary jets for best MPH at the track.Search old posts on power valve selection..great info.Put in a blue #427 pump cam on the primary accelerator pump number 2 hole..the pink is just way too lazy on the Demons..trust me