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 The Snake Pit : Stripping a Late 80's Carburettor

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edlithgow
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PostSubject: The Snake Pit : Stripping a Late 80's Carburettor   Wed Apr 06, 2016 4:44 am

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AKA The Tar Pit. Presumably 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 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.

This 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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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Sat Apr 30, 2016 1:56 pm

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 for 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.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Mon May 02, 2016 7:50 pm

Am I getting you right that it runs okay at all engine speeds above idle? And it's always symptomatic - not just in rainy/humid conditions, or when the engine is hot or cold, etc.?

You've ruled out vacuum leaks, right?
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Tue May 03, 2016 12:03 am

It ran fine two weeks ago, though the minimum idle speed was a bit high. Now it'll start but wont run.

Can't say I've eliminated vacuum leaks but I've done the blowing butane on it thing and didn't detect any.

I'm thinking I might try and make a smoke machine to have a better shot at leak detection.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Tue May 03, 2016 2:57 pm

Diagnosis will be difficult now the car doesn't run. Perhaps you could create a rich condition by obstructing the air intake with your hand. If a vacuum leak was responsible, this should improve the engine's running.

If the above doesn't work, try feeding the engine fuel directly. This will rule out your suspicion about a fuel supply issue. Sticking float or blocked-up jet are reasonable suspects. Does your float bowl have a drain plug btw? If so, see what kind of volume you get out.

Ignition stuff can be tested without the car running. Distributor cap and rotor arm can be visually inspected. HT leads usually require a resistance of less than 10k ohm per foot in length. You'll have to refer to a manual for ignition coil resistance specs. You can test the plugs visually by observing spark when the engine is cranked. Or, you could measure electrode resistance (4k-6.5k ohm range?).

Rough idling, kangarooing, starts out okay then gets worse as the car's run, degenerative over time. Head-gasket? I'll throw that in as a wild-card.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Wed May 04, 2016 6:02 am

Thanks for the response

Justwatching wrote:
Diagnosis will be difficult now the car doesn't run. Perhaps you could create a rich condition by obstructing the air intake with your hand. If a vacuum leak was responsible, this should improve the engine's running.

If the above doesn't work, try feeding the engine fuel directly. This will rule out your suspicion about a fuel supply issue.

Ahem. Neochoke revisited?

Justwatching wrote:


Sticking float or blocked-up jet are reasonable suspects. Does your float bowl have a drain plug btw? If so, see what kind of volume you get out.


Its got two drain plugs that are opposite the jets, so their main function is access for jet removal. When I did the strip they were too tight to budge (with the carb loose) and the jets squirted through OK so I just left them. I might be able to get them unstuck now the carb is anchored on the car, though I'm a bit nervous about using a lot of force on it.

Justwatching wrote:


Ignition stuff can be tested without the car running. Distributor cap and rotor arm can be visually inspected. HT leads usually require a resistance of less than 10k ohm per foot in length. You'll have to refer to a manual for ignition coil resistance specs. You can test the plugs visually by observing spark when the engine is cranked. Or, you could measure electrode resistance (4k-6.5k ohm range?).


Good point not to get fixated on the carb, but I looked into the spark side when it started acting up, and it seemed OK AFAI could tell. I HAVE just had the carb in bits, so it remains the prime suspect.


Justwatching wrote:


Rough idling, kangarooing, starts out okay then gets worse as the car's run, degenerative over time. Head-gasket? I'll throw that in as a wild-card.  

I could run a compression test, using my new starter switch (See colourful Ethnic Starter Switch post). These engines (and perhaps 3-cyl engines in general) have a bad rep for head gasket problems, though that might be because of the turbo versions.

(I don't remember checking the coolant before I took it for that long run Embarassed )

I should have said that I started it with gas. Didn't think it would NEED to be started with gas since it'd run fine before, (and this remains the puzzling feature). Just thought it'd help.

It could be that if, say, liquid butane got in the sump, it would store enough to keep it running for a while.

One disturbing possibility might be that a residue of my oh-so-clever experimental DOT3 carb cleaner has dissolved gaskets (in the carb) in the intervening 2 weeks, though I THOUGHT I'd washed it all off.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Fri May 06, 2016 11:56 am

Neochoke...

...I think restricting air flow or popping the lid off the air filter and dripping petrol in would be easier.

I understand why you're hesitant about upping the force used on the float bowl drain plugs having, but I do think it would be diagnostically useful. If the bowl is empty (or near to empty), then it would support the sticking float valve theory (or eliminate it if the bowl was full).

Maybe check the expansion tank and run a compression test if you'd like to avoid the stuck plugs for now?
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Fri May 06, 2016 4:12 pm

I think the choke isn't functional, but I think its been at least partly like that for quite a long time. Car's been slow to warm up for 2-3 years, but its never cold here so the need for a fully-functional choke is less.

Its a manual choke so you wouldn't think there was much to go wrong, but when I move the lever attached to the choke cable, the choke butterfly doesn't move. Faulty spring?

That lever is linked to another lever which is directly connected to the choke butterfly valve spindle. If I move THAT lever the choke butterfly moves a bit (but not much), and so does the choke cable lever, so there's apparently some kind of one-way ratchet thing going on.

Tried partly blocking the air intake with a notebook as a crude choke substitute, to no effect. Maybe the volume of the air cleaner is too great. I'll try blocking the carb throat (s)directly with the air cleaner off when my battery re-charges.

Tried putting butane in the air cleaner and it did TRY to start, but didn't run for long.

Then my battery flattened.

If I work the accelerator lever while looking down the carb throat I can see movement and fuel in the bottom of smaller/rearmost venturi, so I guess the accelerator pump is at least partly working. Should I be seeing this in the larger (forward) venturi as well?

I'll investigate the choke thing further, though I doubt its the whole problem, and there are lots of dire warnings against disassembly in the CD-20 engine manual e.g. "NOTE: Make sure not to disassemble the choke valve and choke shaft"

I'm probably misunderstanding it, but it doesn't look to me as if the choke COULD work, because the plates movement seems to be restricted to a small angle around the vertical.

That would imply its been mis-assembled, but I left it alone, and the previous owners had it dealer-maintained by Daihatsu.

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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Mon May 16, 2016 7:29 pm

edlithgow wrote:
I'll try blocking the carb throat (s)directly with the air cleaner off when my battery re-charges.

+1

edlithgow wrote:
Tried putting butane in the air cleaner and it did TRY to start, but didn't run for long.

So it doesn't start any more?

edlithgow wrote:
Should I be seeing this in the larger (forward) venturi as well?

I've inspected the photos of your carburetor, but don't fully understand the design. The ports that the fuel comes out of in each venturi, are they fed by a common passage?

edlithgow wrote:
I'm probably misunderstanding it, but it doesn't look to me as if the choke COULD work, because the plates movement seems to be restricted to a small angle around the vertical.

Yeah, doesn't look right to me either. I reckon the choke plate should have a good 45 degrees of movement. The car should run with the choke stuck in the vertical position though (albeit not well), so I think you're right that it's not the problem.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Tue May 17, 2016 12:21 pm

Blocking the carb throat didn't seem to do anything, but with a lot of fiddling about with butane etc I got it running after a fashion.

Then I tried using a vacuum gauge 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 some info.

I wasn't sure where to plug it in, and tried one of the vacuum advance hoses to the distributor.

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.

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There are actually two vacuum hoses to the vacuum advance unit, one from the middle of the can, one from the base. I guessed 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 gauge 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
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1400 5
1500 7
2000 15

(That http: was added automatically. I didn't put that in)

From this I'd guessed the mid vac is "ported" and the base is manifold. The * represent the oscillation of the needle. (I've actually been told its the other way around on Toyota Corolla's, which have a similar carb, so possibly these hoses are switched)

I don't know how this works. Wikipedia has

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"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 (or any, for that matter) would be retarding the ignition, and it doesn't seem to be. 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 needed a timing light to confirm (or deny) that the vacuum advance was defective. 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.

My new old-stock timing light flashed but I couldn't see the mark, which was probably advanced out of sight.  Next day I set the static timing to the mark, as outlined here

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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 20 mls 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 overun (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 essential and seeing if it'll run steadily then.

After that, next steps seem to be:-

1. Do as full tune-up as possible with my limited gear. Do /compression test/valve clearance and check electrics again (though I think this is all OK)

2. Replace all hoses and/or paint them with a sealant of some kind. PVA, Sunflower oil, RTV, Permatex or plasti-dip seem like possibilities.

3. Take the carb apart again, only this time replace all gaskets (IF I can get them) or use Permatex sealant on them if I can't

4. Do a "deep clean" on the emulsifier jet units, involving long soaks in carb cleaner, brake fluid, boiling detergent, water, and alcohol, flushing with syringe and/or running water.

These things apparently draw air into a sintered metal matrix to froth up the petrol, which is about as fragile a design as I can readily imagine.

5. Get a new fuel filter

6. Drain and flush the petrol tank

7. Buy another carburettor/give up


This Aisan (I mis-spelled it before) 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, and a Land Cruiser website is where I've found the best information on its operation.

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Must have been VERY dependent on the fuel filter in Third World operation.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Sun May 22, 2016 5:34 am

Compression test (cold, manual doesn't specify) seemed ok, though there was blowby past the conical push-on adapter which probably reduced accuracy.

Cylinder 1 170, 180, 178
Cylinder 2 148, 186, 180
Cylinder 3 164, 190, 170


The plugs looked sootier than they have in the past, which isn't surprising.

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Plugged all the hoses I could see except the one from the solenoid valve, with (mostly double-ended, if there was space) chopstick segments and silicon (?) rubber tubing, so its only getting centrifugal advance.

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Car started easily and was drivable, but it was surging a bit, though not as badly as before, and I don't know how well its expected to run in this configuration. Rush hour and getting dark so just a short trip and only in top gear a couple of times due to crowds for some sports event.

It idled with the adjustment screw turned right in, which tends to confirm the seat or needle is damaged.

Today, harder to start but drove about the same. Longer run of maybe an hour and a half / 20 k, mostly narrow country roads but some dual carriageway. Noticed that the surging went away on some steep up-grades, suggesting it might be load sensitive. No obvious deterioration during the test run, which IS an improvement, though the earlier test run was longer and at higher speeds.

Next move might be to add some static advance, then re-instate the vacuum advance, preferably with new tubing

I''ll try and establish what the other tubes do and then decide if they're worth re-instating.

If its still running badly with the core functionality carefully restored, do the other peripheral stuff (electrics, valves)

Then take it apart again.

I'm wondering if those IR thermometers are capable of giving accurate enough exhaust temperature information to be of any use in tuning. There is, however, apparently no provision for mixture adjustment (apart from at idle, which is probably broken) on this carb, so base ignition timing is the only variable I can control.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Sun May 22, 2016 11:24 pm

You'd need to improve the blow-by situation if you wanted to make any sense of the compression data. I don't think it's necessary though. From what you've described of the car's behaviour, a compression issue has been logically ruled out. That is, a lack of compression would be invariant in it's presentation. Further, it would present as misfiring not surging.

IR as a carb tuner. Not sure about that one. The apparatus might be sensitive enough, but I have doubts about the principle itself. I'm not sure what engine temp would tell you that you've achieved a stoich ratio, nor would I know how to interpret deviation from that figure (i.e. how does each degree in temp. translate to change in air-fuel mix?).

You've got a vacuum gauge, right? Why not just use that to tune the carb?
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Mon May 23, 2016 2:55 pm

The compression readings are good enough for now. They're within spec. and the lower ones are first in the series when I was getting hit in the face by blowby and was flinching. I'll try and source a screw-in adapter of the right size when I'm next in Japan.

I think exhaust gas temperature (and cylinder head temperature) tuning is/was well established in use by light aircraft pilots who manually adjust mixtures in flight.

I'd have to look up the details of the relationships, but I'd expect EGT's to rise as the mixture becomes lean.

That's fairly academic though since, short of doping the fuel with ethanol, etc, I don't believe I have any scope for mixture adjustment with this carburettor.

The only thing I can change is ignition timing. Havn't researched this much, but I think, as a generalisation, that EGT rises fairly rapidly as the ignition is retarded beyond optimal, because still-burning combustion gases are released to the exhaust. It might, therefore, be informative used in conjunction with the vacuum gauge. Ideally I'd get/make myself a gas analyser too, but that probably isn't going to happen.

I might look into importing an after-market carburettor that I could actually tune. (Say a Weber. I'd like an SU, but they aren't downdraft). This would be illegal, but if I keep away from government inspections I should get away with it.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Fri May 27, 2016 12:54 pm

edlithgow wrote:
if I keep away from government inspections I should get away with it.  

They're optional?
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Sun May 29, 2016 7:52 am

Justwatching wrote:
edlithgow wrote:
if I keep away from government inspections I should get away with it.  

They're optional?

Sort of.

If you need to change the registration, for example, on sale or purchase, you need a govt. inspection. I had to go through that a couple of years ago because I lost the reg. documents, and it was tricky to get an emissions pass. I'd bet quite a lot of money it wouldn't get one now.

For the routine 6-monthly inspection, you have the option of a licensed, privately run test centre. These are quite a lot more...er...relaxed than the govt. centres, probably because they want the repeat business.

The test fee is less than a British MOT (about 15 quid IIRC) but the inspection probably doesn't cost much to do (emissions, brakes and lights. No bodywork or suspension checks AFAIK) so its probably quite profitable.

On the recent inspection the idle was far too high, and I don't think they even pretended to check emissions, but it still passed.
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Sun May 29, 2016 7:23 pm

I see. These test centres, are they repair garages or only do inspections?
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PostSubject: Re: The Snake Pit : Stripping a Late 80's Carburettor   Mon May 30, 2016 12:55 pm

The one I go to has a garage attached, but they don't seem to do very much, and they don't look very well set up in terms of equipment and organisation, so I think they're mostly inspections.

That (and the govt. centre) are the only ones I've been to, so they may vary.
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