Aussie merit badge: Rebuild a Carburettor
When complaining on social networks about my carburettor troubles, my
cousin Rodney asked if I was going for an “Aussie merit badge” or
something, because rebuilding a carb isn’t something that most
non-mechanic types are wont to do in this day and age. Even his brother,
who is quite the gear head tends to stick to nineties EFI Holdens.
But as I responded, I am my own mechanic when I lack the funds or means to have someone else do it. I’m weird, I suppose, in that I view cars as simply a complex system, and like all systems they can eventually be understood. I also find it quite fun and interesting learning to understand them. I do not enjoy troubleshooting, however, particularly when there’s money involved. If it weren’t a car I was depending on I think I’d be having a lot more fun.
That said, it’s going quite well. I picked up a motor and headers off Gumtree, which included a carb of questionable quality (it’s a motor that’s obviously done a few KMs on LPG). I tore into it to check for gritty, gummy nastiness and when satisfied, put it on my motor. It ran okay, didn’t idle great though.
Putting into play many hours of internet research, I deduced that the vacuum operated 2-stage idle jet was no good. Put the one off my carb on it and it idles fantastic. Oh, that sweet sound of a smooth idle when you’ve been messing with the carb for weeks!
So with a working carb to fall back on, I set about figuring out why mine wouldn’t idle - after all it was the jet off mine that fixed the other one. I already had the overhaul kit for it, so I decided to tear it down and rebuild it.
It took me the better part of four hours to tear it down and clean it, then another hour or so to put it back together. I elected to put it back together with all the original rubber the first time, in case I’d messed anything up. I bolted it back on with my idle jet back in it and it went!
After a little tuning, as best I, a completely uneducated screw-twiddler could, I took it for a test run into Maryborough. Oh, how nice it runs! Coming out of Maryborough by Craigie there’s a bridge and a turn that for some reason scares all holy hell out of me. Since no one was behind me, I took it slow, coming back to about 60.
Then I stomped on it onto the straight. It picked up a lower gear and pulled… And pulled. All the way until about 105 KM/H where I lifted the foot as the only thing worse than a broken car would be doing my license on New Years Eve.
But I am very happy with it. There’s still a hint of misfire, which I believe is ignition system related. Tomorrow I’m going to tear the carb down again and put all the new rubber in it - while it’s tempting to leave well enough alone I’m also shooting for reliability so I want it in brand new condition.
But yes, terribly happy with it. I didn’t think the acceleration of a tired old inline six would bring a smile to my face, but as it’s the result of all my own work, it has.
Update: I neglected to mention a few things: First of all, the carb is a Weber 34ADM. I also used a formula for calculating carburettor sizes, which since I now have LaTex in markdown, I can store neatly here:
$CFM = \frac{cu.in * RPM * VE}{3456}$
So assuming a fairly conservative 70% VE for a 4.1 (which is ~250 cubic inches), and a redline of 6500 RPM (bit exhorbitant in my experience), you end up with:
$\frac{250 * 6500 * 0.70}{3456} = 329.14 CFM$
However this article suggests that if you don’t want a giant vacuum, the real CFM required might be a good bit higher.
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