stanleyholland

any adjustments to timing required when changing a 425 two to a four new carb has electric choke

Sugar Bear

Shouldn't be; but, if it still has a switch pitch TH400 transmission behind it be sure to use a carb and linkage that keep the switch pitch working.

joe_padavano

The factory timing spec assumes a stock engine in good mechanical condition running on the gasoline available when the car was built. 4bbl engines typically got a different cam than did 2bbl engines, and the gasoline today is definitely not what it was when the car was built. Add to this the fact that it is likely that the advance mechanism in the distributor is not working as well as it was when the car was new, and you rapidly get to the point that the factory initial timing should only be a starting point. Don't be afraid to try variations to see if the car runs better that way.

Sugar Bear

^^^ Yup^^^ And before you get that extra out of the engine and enjoy the sound of the 4V, are the timing chain and gears set fresh? If not I'd replace them first.

stanleyholland

thanks for the info

droldsmorland

After you've verified the tune-up parts and the distributors' vac & centrifugal advances are in working condition like Joe said start playing around with what the engine likes. Don't get hung up on setting the timing to a "spec". Give it as much timing as it will tolerate. The fuel you're using, elevation, and driving style will have an effect.

I'm old school. I recommend tuning with a vacuum gauge hooked to a direct intake manifold source. At warm curb idle adjust the timing and air/fuel mix screws to achieve the highest reading on the vac gauge. Road test it. If it spark knocks back timing down a degree or two then repeat.

Keep in mind the advance curve in the distributor(Vacuum & Centrifugal) may need to be super-tuned to extract the last bit of performance out of the engine."Super-tunning" means you have the carb and ignition set at their optimal, Patience!

Once you think you are finished recheck the point gap(dwell) and look at where the timing is with a light. Verify you have zero vacuum leaks. A vac leak will throw all of this off course.

Start with"
12-14° base (at idle)
The vac adv should add another 8-10° (at idle)
Total all-in timing should be ~36-40° @ ~28-3000rpm

FStanley

not to rain on your parade..
BUT

simply changing to a 4V carb might not make a world of difference, some times I read performance fell off..

there's more to it then just changing one thing ...

Like Joe alluded in an previous to the cam was different.

to REALLY see a performance difference.. cam, distributor curve and timing, type of 4VCarb and jet/rod size, intake, headers, compression ratio, heads, etc etc all play into this.. if this is what you are expecting..


Let us know how it goes..

Fred