Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor

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Thu 7 Apr 2016 03:04
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

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AKA The Tar Pit. Presumably that's due to blowby, so maybe a compression test and/or catch can are indicated, although it might not have been done for 30 years.

Its had regular squirts of carb cleaner down the intake, but I've never been very convinced by carb cleaner.For this job I've been using brake fluid for cleaning, which (touch wood) seems to be quite a lot more effective.

I don't have a manual for this car but I have a scan of the G10 manual. This carb (AISIN Model 87773) seems to be very similar, but the photos are of the "remove tiny black dot from somewhere in the big grey fuzzy mass" school of 80's photo-impressionism.

I couldn't remove all the ancilliaries. The cross-heads just cammed-out, and I didn't want to try my hit-it-with-a-hammer impact driver on a carb casting. Perhaps this would be a job for one of the electric impact drivers Americans are so fond of (in which case I'll have to stop sneering at them)?

I couldn't (so far) remove all the jets either. The manual says "it is mandatory to employ the special carburetor screw driver set, carburetor adjusting gauge set and wire guage set" which is too bad, but I did tell myself I wouldn't try anything with ill-fitting tools.

I didn't have the right screwdriver for the needle valve seat, whch should be fairly easily addressed, but its a 40K round trip to the nearest half decent tool shop.

The other 3 jets I can't get off are shown in the photo below. The slow jet (brass thing in the curved recess in the side of the float chamber) looks like it needs a rather slim box spanner or long socket. The primary and secondary jets are in the bottom side of float chamber, and would be accessed by removing two plugs in the opposite side. I couldn't exert enough force on these to shift them.

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My options for that seem to be:-

(a) put the carb body in a vice, if I can get access to one. Bit scary, though the base seems quite strong.

(b) bolt it back on the car to anchor it. Also a bit scary, but better.

(c) Boil it, in the hope that the heat will loosen things up.

(d) forget it, since there's probably mothing wrong with them anyway

(c) would be favorite (and is my standard method with motorcycle carbs) but I don't want to boil the vacuum-pot ancilliaries which I can't get off.

I've done a couple of simple motorcycle ones (Kymco Zing 150cc Honda CG125 clone, and Yamaha RZR 133cc 2-stroke) a few times and had evolved a technique involving boiling with automatic washing powder, and then blasting the passages out with first the detergent, and then alcohol. The latter gets around the lack of compressed air, the former the fact that, in my limited experience, the sludge in motorcycle carbs was mostly a greyish mineral-looking deposit (maybe a corrosion product?) which carb cleaner didn't remove. Carb cleaner seems to be intended for tarry or gummy organic deposits, which is what this car has.

The jets I can get to so far seem pretty clean though, so while cleaning the tar out is nice, I doubt its fixing anything.

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Sat 30 Apr 2016 12:49
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

Put it back together a fortnight ago and (after a bit of fiddling with idle adjustment screws etc, took it for a 30K run, including some expressway, and it drove fine, though I couldn't get it to idle smoothly below about 1000.

This morning (having sat for two weeks) it starts up OK, and is fine fior a quick test circuit of the campus, but starts kangarooing badly in traffic.

Limp it most of the way back, coasting between kangaroos, but then it konks out completely.

It'll now start, but it won't run.

What could cause this kind of remission/relapse pattern?

My best, not particularly confident guess is dirt from the fuel line, or I suppose the float valve could be sticking.

Could'nt find anyone stocking a fuel filter so just back-flushed the old one.

Should probably have tried to order a new one.

Edited by edlithgow on 30/04/2016 at 12:53

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Sat 30 Apr 2016 13:26
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - bathtub tom

I couldn't get it to idle smoothly below about 1000.

It'll now start, but it won't run.

I'd start looking around the idle jet or progression jet.

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Mon 9 May 2016 06:13
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

I now have some clues as to what might be causing this problem.

I tried using a vacuum guage (much touted as a tuning/diagnosis aid on classic car forums, especially US ones), that I bought in Japan, to tune the idle.

So far I can't see its much better than tuning by ear, but it did "accidentally" give me so info.

I wasn't sure where to plug it in, and tried the vacuum advance hose to the distributor. I'm not sure if this gets full manifold vacuum (i.e. below the throttle) or if its "ported" (above the throttle) but it had a healthy vacuum reading, so I'd guess the former.

I noticed that the idle was steadier while doing this. Possible explanations include:-

(a) There is a vacuum leak via the vacuum advance circuit, perhaps due to a hole in its diaphragm. This will disturb the idle.

(b) As a result of (a), the vacuum advance doesn't work, so the spark happens late. This will disturb the idle too.

chevellestuff.net/tech/articles/vacuum/port_or_man...m

I'll need to get a timing light to confirm (b), but assuming this diagnosis is correct, pending repair or replacement of the distributor, I'm wondering about my scope for quick and dirty work-arounds to (maybe) get through the overdue inspection.

I can block off the vacuum advance port to eliminate the effect of the vacuum leak.

Would it be worth cautiously advancing the static timing a bit?

I THINK the emissions test is only done at idle.

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Mon 9 May 2016 07:52
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - RT

There is no vacuum "above" the throttle - that's at ambient pressure.

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Mon 9 May 2016 08:39
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

There is no vacuum "above" the throttle - that's at ambient pressure.

Think the air cleaner and venturi introduce some restriction, but the low vacuum is probably why "ported" advance designs don't/didn't work very well (see the link).

Anyway, while all contributions are of course welcome, your observation doesn't seem to have much direct bearing on my immediate problem.

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Mon 9 May 2016 09:52
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - RT
Anyway, while all contributions are of course welcome, your observation doesn't seem to have much direct bearing on my immediate problem.

OK, I'll move on to something else then.

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Mon 9 May 2016 11:47
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow
Anyway, while all contributions are of course welcome, your observation doesn't seem to have much direct bearing on my immediate problem.

OK, I'll move on to something else then.

I wish you every success in your new venture.

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Fri 13 May 2016 05:05
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

There are actually two vacuum hoses to the vacuum advance unit, one from the middle of the can, one from the base. I'm guessing one of them might be getting manifold vacuum, and the other "ported" vacuum (sometimes even more obscurely referred to as "timed" vacuum), via a port just topsides of the throttle plate, so it only sees significant vacuum when the throttle plate starts to open.

I tried the guage on both ports and recorded the vacuum reading (inches Hg) at different rpm, read off the dashboard rev counter through the windscreen via a scooter mirror tied to the steering wheel, so not terribly accurate, but I don't have a tach/dwell meter.

rpm /mid-vac /base vac

600 /2 //13***

800 /3 //12**

1000 /4 //12*

1100 //12

1200 //12.5

1400 5

1500 7

2000 15

From this I'd guess the mid vac is "ported" and the base is manifold. The * represent the oscillation of the needle.

I don't know how this works. Wikipedia has

en.wikipedia.org/wiki/Ignition_timing

"Some vacuum advance units have two vacuum connections, one at each side of the actuator membrane, connected to both manifold vacuum and ported vacuum. These units will both advance and retard the ignition timing."

Not sure why (or if) this one would be retarding the ignition. The CB-23 spec says centrifugal advance 0 degrees at 750 rpm. 10.5 degrees at 2.800 rpm, vacuum advance 0 degrees at 100 mmHg, 11 degrees at 320 rnmHg, and doesn't seem to mention retardation.

I used my brake-bleeding enema syringe to apply 12"Hg vacuum to the distributor, "calibrating" it with the vacuum gauge, as below. After holding for 30 secs, if the piston returned most of the way, I considered it to be holding vacuum.

Piston displacement (mls) /- pressure ("Hg)

10 /7

20 /12

30 /15.5

40 /18

50 /19.5

60 /21

The base vac isn't holding vacuum, but if the tube is reversed, or my brake-bleeding tube substituted, it holds.

This suggests it may be the tube, (which looks OK) rather than the advance can, that is faulty, which would be a cheap fix IF its all that's wrong.

I'll need to use the timing light to confirm (or deny) that the vacuum advance is defective. My new old-stock timing light flashes but I couldn't see the mark. Will try again tommorrow, maybe with tipex etc.

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Mon 9 May 2016 11:56
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

I spent a good portion of Sunday wandering around a distant city looking for a tool shop with poor stock control, that might still have a timing light at the back somewhere.

I had a picture of one, and was considering a poster campaign, like someone looking for a very old lost cat.

Got the standard blank incomprehension and "meio" from counter staff when I showed them the picture (I don't know the Chinese for "timing light" and there's a good chance they didn't either), but at the last place an old geezer came after me and called me back.

Long wait while he disappeared in dark recesses, then bingo.

He looked quite misty-eyed with nostalgia, but it might have been the dust.

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Mon 16 May 2016 03:49
Aisin (Japanese) Carb-equipped - The Snake Pit : Stripping a Late 80's Carburettor - edlithgow

I set the static timimg at the timing mark on the flywheel with a test light across the points. It was probably advanced out of sight before, something I might try later, once I know what the relationship between circumferential distance on the flywheel and degrees is.

With both vacuum advance hoses off and plugged, started right up, and the spark went north (ie advanced) when revved, as seen via my shiny NOS timing light, suggesting the centrifugal advance is working.

Pulling 20mls equivalent vacuum on either advance port on the distributor sent it north too, so the vacuum advance is also working.

Swapping the hoses and taping them with PTFE, it initially drove OK, but after about 15k the idle started to rise. Adjusting it down led to stalling at lights and on the overunn (which is probably a clue, if I could interpret it). Turning the idle up, it started to surge under acceleration. I could almost see the fuel gauge needle moving downward.

Best guess is I've still got a vacuum leak. I'll have to make a smoke machine. Might also try disconnecting everything that isn't absolutely essentail and seeing if it'll run then.

This Aisan (I mis-spelled it in the title) seems to be a horrible carburettor design, (though of course that doesn't cause vacuum leaks.) Admittedly the last carburettor I think I understood was a 1972 SU, but the emulsifier jets on this one seem designed to clog and are not designed to clean.

Incredible that the legendary Land Cruiser used the same design carb. Must have been VERY dependent on the fuel filter in Third World operation.

Edited by edlithgow on 16/05/2016 at 04:03

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