[Che]

Hi guys, long time no AZ. I've even come back with a new name! Had a pretty tumultuous six months but that's a story for another day. Still got my old Sierra, still loving it, and still running straight LPG. It's still running great too - but in the quest to get this car perfect for my needs i'm considering another revision to my intake system.

I went ahead and installed a dedicated LPG throttle body some months back. Nothing too hardcore, just a 40mm TB from an old Hyundai... it seemed about right after doing the maths. On the plus side it's improved my 1.6's performance - no more elastic accelerator response, a massive bloom to low RPM torque, sitting on a steady highway speed it's more efficient than ever, and I can white knuckle it to 140kph if i'm feeling brave. It's greatly simplified the intake and neatened up the engine bay too.






On the negative side - jeez it can smash the fuel when I put the foot down. Even though the TB is very modest in size, if I decide to drive my Sierra like a rental i'm using about 20L/100Ks. This reduces my range from 500km+ per tank to 300... yikes! When I had the Aisin on I could drive like stink or like a grandma and i'd always be using 12-13L/100K - but after 10 refills with the Hyundai TB setup i'm yet to better 14L/100K. And from point A to point B i'm not convinced it's moving any faster. I'm also finding with all that low end torque gain it's very touchy when feathering the accelerator. It's very easy to get this car bunny hopping in first gear, though i've somewhat mastered it now. In short, as much 'percieved' performance as this install has introduced it hasn't really granted me any of my desired gains.

My main 'like' with the Hyundai TB install is that the intake system is now bulletproof - the only moving part is the TB butterfly, and near zero chance of a vacuum leak. That's a big plus for me. But ultimately i'm all about getting the most miles out of a tank of gas. As such i'm considering reverting back to my Aisin carb. But if I do there are two major issues i'd want to correct first:
(1) idle RPM had a tendency to drop really low when hot. After a long drive and getting the engine right up to temperature the RPM would drop from a happy 1000 RPM to a near stalling 400 RPM... I don't know why this was... perhaps a vacuum leak that only presented itself when hot? Something to do with the heat-driven choke?
(2) the Aisin had a very elastic relationship with the accelerator pedal. Feather the pedal with this setup and the RPMs would flounder rather than rise - you needed to give it a quick little pump of the foot to get the RPMs headed in the right direction. With my current TB setup if you drop the accelerator a millimetre the response from the engine will be a linear rise in RPM - much better.


So what do you think. Should I revert back to a sorted Aisin, or should I just stick it out with my current setup?

[want33s]

Using a TB has definite advantages.
As for fuel economy, have you considered running 2 X smaller TB's with a progressive linkage arrangement?

[Che]

No, but that sounds interesting. I'll look into it. I opted for a single TB system to stick to the K.I.S.S. rule.

I'm guessing the Aisin is a progressive link type of carb? I'm looking at it now trying to work out how I can strip it back yet maintain the dual throat system... the required linkages, holes i'll have to plug, etc.

I think i've done the wrong maths when trying to find a suitable TB. The Aisin carb has 31mm and 27mm throats, which is the equivalent of a single 41mm throat... that's kinda what I have. One thing I didn't size up when looking for a single TB was the amount of obstruction down the throat of the Aisin - it probably reduces my needed diameter for flow quite a lot. But no doubt if I installed a 30 - 35mm TB that'll affect wide open performance. Right now the setup certainly favours wide open performance than sub 4000 RPM performance which is where the bulk of my driving is done.

[Che]

Another mere ~350km from 60L and this is back on my mind.

I've had a play with the Aisin carb and stripped it right back. Not been game to do this until now as it just seemed too complicated - but after attacking it with a screwdriver and socket set there's less to it than I first assumed.

If I want to use the Aisin's throttle body (and the twin throat system), am I right in thinking it basically boils down to keeping this red follower part (secondary throat) tracking the profile on the green cam (primary throat)?

[Che]

Aisin carb is now no longer a carb. Took a hacksaw and die grinder to the fuel fuel float. Now to find some taps and fill all the holes.






Hopefully this is all worthwhile.

[JrZook]

Che wrote:

Aisin carb is now no longer a carb. Took a hacksaw and die grinder to the fuel fuel float. Now to find some JB- Weld and fill all the holes.
Hopefully this is all worthwhile.

[jonno_racing]

Jb is good stuff! Jaycar sell it locally

[Che]

Is that the grey resin stuff that smells like burning hair when you mix it? I used that on my current intake, but more as a (permanent bond) liquid gasket. Worked pretty good.

[Che]

How's this for a dodgy fix. I figure some on here might appreciate it I remembered this carby sat a little too high under the hood with the LPG mixer sitting on top. Last time I paid a shop to do the machining... this time I used a hacksaw to shave 9mm from the profile. Unconventional, but it worked out perfectly.

[atari4x4]

backyard machining 101

[jonno_racing]

Impressive! !