SPL Hitachi adjustment question

[Habitat.pat]

I went through the adjustment of my carbs balancing the flows & adjusting the mixtures using both the normal lifting of the pistons & using a ColorTune. I was able to get blue readings on cylinder 1 & 3, the only ones I checked. The carbs were running rich before the adjustments but are not now. Since then I've been getting a burble from the exhaust when decelerating which I've never experience before. I get about 20MPG on 90 octane no alcohol fuel.

My engine is newly rebuilt (1500 miles), stroked to 2L, East Coast ignition, DRP 1-3/4" exhaust & a header, Dean's Performance #1 cam.

Has anyone seen this before?

Peace, Pat

[Nissanman]

I'm not sure what you are asking but as a dynamic tuning aid, you can't go past a digital EGO gauge.
I used the Innovate MTX-L unit, excellent device and makes fine tuning a breeze.





The most difficult part of fitting one for tuning, is welding in the necessary bung at the end of the header pipes.
https://www.innovatemotorsports.com/mtx ... -3-ft.html

[Bwk2000]

Pat.
Sounds like you’re now running too lean and maybe have the timing retarded. Try resetting the mixture and rebalancing the carbs at 2000 RPM, plus double check your timing.

[Gregs672000]

A leaner mixture can cause some exhaust snap or pop on deceleration. I have experienced this before, and since I have EFI I did enrichen the mixture at low load deceleration and can confirm it went away. It's not hurting anything as the engine is not under load, however you don't really know what the ratio is with a load using a color tune. You might try richening it a 1/4 turn and see what's what. A carbureted engine likes an idle richer than leaner too, and I don't know if the color tune blue flame equates to about 14.7 to 1 or not, but a carbed engine likes high 12's-low 13's. You are better and safer running a little rich vs lean. Yes, you might use a bit more fuel but you're not giving up HP or risking detonation due to a lean mix. If you're not interested in using a wideband O2 set up, I'd spend some time reading your plugs. Put in a new set and go for a typical drive, noting how the engine responds, what power is like up through the rpms to redline or where you're comfortable, note snapping exhaust or hesitation, etc. Then take it home, try to not let it idle much, and after its cooled down check all your plugs to see how they are looking overall... white (lean), brown (nice) black (rich). Then report back and we can talk about more testing if needed or desired if you like. Do check your timing... power and most fuel economy is in the ignition timing. If you have a dial back timing light you want about 36-38 degrees total by 3500rpm I believe with a stock ignition curve. 20mpg seems very low to me but it is just getting sorted.

[keith0alan]

What you are describing is normal. On overrun with the throttle closed the mixture is far too weak to burn in the cylinder. It burns with a pop in the exhaust system. This will make catalytic converters totally nuts so more modern cars have provisions to stop it from happening. A lean mixture at cruise will produce a little stumble. Watch your plugs. If 1-2 or 3-4 are getting dark/black then lean out that carb a little. If you are getting the stumble a cruise then richen both carbs a little. A couple iterations and you should be spot on. The popping in the exhaust on overrun is a happy noise on these cars.