Jobro

iTrader: (2)

Just finished my 2nd rebuild. I've had my intake ports cut as big as possible without bridging. Its an early 12A motor with the big primary ports and all 4 are really big. The ports are cut wide right to the limit where the seals fall in. The ports are cut tall as well. They are a few mm taller than the pineapple large extend for the 13BREW for example.

What idle speed are people using and what vacuum are you pulling. Its looking to me like I'm going to have to put up with 1200rpm and a fair bit of noise. Is this typical?

My old port was smaller (not taken wider at all) and less height but used to be fairly happy at 900 950rpm etc.

I've been playing with timing and it looks to me like 25BTDC timing at idle is the smoothest.

arghx

iTrader: (3)

Besides the port shape, noise is of course related to the exhaust system you choose.

In my experience with big non-bridge ports (13B motors albeit), 10 inches of vacuum and 1000+ rpm is normal.

But here's the deal. It depends how rich of a mixture you are willing to tolerate, and what you are doing for spark advance and idle air feedback. If you have fixed ignition timing and just a throttlebody adjustment (no idle speed control feedback), you will have to have an overall higher baseline idle speed if you want the thing to run in all conditions.

j9fd3s

iTrader: (3)

like Arghx says it depends on a bunch of things. i have a 12A p port with an IDA and i played with the idle quite a bit. i've found the engine will happily run @700rpm, but the carb doesn't work very well there, so its between 900-1000.

so there are a number of things that effect idle. #1 is exhaust dilution. big port = more overlap = more exhaust in the engine at idle = brap brap. the exhaust needs to get out, but also reducing the intake vacuum helps. i drilled the holes in the throttle plates on mine, and the bigger they got the better it idles, to the point where it idles like a stock port, which is no fun at all!

i can do this with a carb, to a point, but if you're running on a MAP sensor it might not like it. maybe it'll want the zero throttle map

i kind of assume you have tried different fuel, i was going to say it should want more, but who knows, it depends. the P port gets mean when its rich

25BTDC timing sounds like a lot, mine idles better under 10BTDC, but since i've got a locked distributor at the moment, its 18BTDC. i assume you played with this too.

hope that helps! if i can get a P port to idle under 1,000rpm, there is hope....

thewird

iTrader: (8)

Just put up with it. Trying to achieve a lower idle on a motor that wants to idle higher is just going to carbon the internals and foul plugs.

thewird

Jobro

iTrader: (2)

I wasn't sure if it was in the right ball park. There is certainly some missing going on at idle as well as some mild brap.

I will try to take a movie.

arghx

iTrader: (3)

Is it missing at idle when warm, or just during cold? In my experience, retarding the timing (yes, retarding) actually reduces that light miss you get during warm up. You need to use a cold idle retard vs water temperature table. Use realtime tuning to make quick changes during warm up. You just have to feel it out and give it multiple tries.

On the flip side, more advance generally helps you start faster. So Mazda stock cranking timing is 5L 5T (degrees BTDC), but you should try 15 or 20 degrees (for both, 0 split).

bumpstart

i have a template thats a little later than most and goes out to the edge without dropping ends of the side seals and ive used it many times

should be 15-17 " Hg at around 1000 rpm with 5- 10 BTDC idle , fully warmed up

some higher overlap exhaust profiles push it to the low end of that and may like 1100 or 1150 to steady down some

BLUE TII

iTrader: (9)

When I had a big street port (started as Pineapple and then went later closing to get a scissor) my car liked to be at 1,000rpm idle, eye watering rich with timing retarded all the way on the Haltech. Basically a zero throttle map as the maps just above and below idle were very close to the same in fuel and timing which helped it run smoother.

It would still brap brap a little after pulling off the freeway but would settle down as it heat soaked a bit.

A stupid pain in the butt motor.

bumpstart

ive found some pics ( on some FC plates ) to give you an idea of the template im usually using
the 3rd pic on the stock G middle plate gives you an idea of the opening edge that is sometimes found on the earlier engines
though most 12a/ carb 13b have a lower runner and thus the closing edge is limited
in that you have to angle the port bowl top into a squeeze to avoid cutting through the castings



these engines ( without the bridge ) show 15-17" Hg at around 1000 - 1100..depending on exhaust port shape and rotor compression





an 12a/carb 13b plate with same template illustrating the extent of the squeezed floor and that this template is the absolute limit of the late close edge for early plates

j9fd3s

iTrader: (3)

me too, that light miss at a high idle does get better with less timing. the root cause is exhaust gas getting pulled into the intake stroke. there is vacuum in the intake and pressure in the exhaust....

lower the intake vacuum and it'll get better.

rx72c

iTrader: (8)

If you have done your ports right you should be able to lock -5/-20 and you will get a smooth as a kitten idle at 900rpm. Engine will pull 6-8 inches of vacuum. Will have to tune idle for 12.5-13:1 to get it nice and smooth as well.

My mild port idled under 1000rpm with no issues and that was with 1700cc primaries and a microtech.

Jobro

iTrader: (2)

That is exactly what it was like except 25BTDC settled it down the most. It doesn't matter now, I cracked the rear end plate at 518kms.

I can afford to fix it again but the fact of the matter is I had already worked out fuel was going to be very expensive on fuel as a daily driver which was the intended purpose.

Boriquaguerrero

iTrader: (18)

im on my second 12aT engine rebuild too.

bumpstart

here you go,, this is a 13b s5 type engine built with 9.4 s4 rotors and the inlet ports pictured above.. with higher overlap large exhaust ports

this is about 5 mins total idle run time till properly warmed,, on a 1st start freshen up---- and is untuned
( on LPG impco 225 carb and haltech e8 )

indicating -6.10 psi on the e8 ( which has a little + error at atmo ) this is -13 psi ( yet untuned )
and is indicating -14.5 psi on my independent gauge

( front pair 13b housings still with inserts )