Help identifying Hitachi carb EA81

tambox · Post by **tambox** » Fri Nov 08, 2013 6:25 pm

David, you said you set your timing to 6 & 8 deg, but the vac hose made no difference?
If is running manifold vac for the dizzy, it should vary heaps, by connect/dis-connect of the vac hose.
If it is running ported Vac, when you open the throttle, the timing should suddenly jump heaps.
If the timing only advances slowly as you rev it faster, that will be the mechanical advance doing it job.
Suck on the vac hose (to put vac on the dizzy) and make sure your timing changes.

David D · Post by **David D** » Sun Nov 10, 2013 11:02 am

Sorry if this is a stupid question, but how do I know if it's running ported or manifold vac?

Cheers

David

littlewhiteute · Post by **littlewhiteute** » Sun Nov 10, 2013 6:06 pm

Manifold vacuum is below the throttle plate, hence "manifold vacuum".

Ported vacuum is a vacuum port on the side of the carburettor just above the throttle plate. The vacuum nipple could be anywhere on the side and communicates with said port via an internal passage within the carb.

Ported spark advance came about as part of early emissions regs to help reduce NOX, then came EGR etc.

Once above a certain throttle angle, the total advance the engine receives is the same.

The difference is the amount of advance at idle. A base of 8 BTDC with ported spark stays as 8 BTDC. With manifold vacuum, 8 BTDC becomes about 20 BTDC. That is why you disconnect and plug the vac line to the distributor while setting ignition timing. You don't have to on a ported spark engine, just good practice. A vacuum gauge on the hose will tell you which type of source you are using.

David D · Post by **David D** » Mon Nov 11, 2013 1:02 pm

Thanks for that, I'll have a look. I sucked on the dizzy and the timing changes loads so looks like the dizzy is OK unless it's manifold vac as the timing doesn't change at idle, plugged or unplugged.

David

Silverbullet · Post by **Silverbullet** » Mon Nov 11, 2013 4:43 pm

Hang on so should our mechanical advance dizzys be running on ported or manifold vac? I never knew this information and now that I think of it the engine in my wagon was running ported vac from the carby, but my ute is currently using manifold vac. Might not be related but it idles very lumpy yet no air leaks, timing spot on with dizzy vac line plugged.

And I can't remember if my current carby even has a ported vac line on it

tambox · Post by **tambox** » Mon Nov 11, 2013 5:09 pm

David, hopefully that means you have ported Vac to the dizzy.

All dizzy's (on petrol motors) have mechanical advance.
Unfortunately the Brumby, being built for so long crossed over several EPA changes.
There was also a change in 81/82 to 83/84 MY's
Find out what vac advance is correct for your motor from manuals.
It comes down to the emission control stuff on the motor, the Brumby's vary a lot, between early, middle and late.
Then, people have swapped/blocked bits, so it can be confusing.
Botton line is, what feels the best and gives the best figures is what you should have.