Knockoff Aisin Carb

[Skreddy]

Been wanting to do this for awhile and since I decided to desmog my project 60, seemed like the perfect opportunity to do a write up on the knockoff Aisin carbs. These carbs have a bad reputation but I find it’s mostly because they need a good cleaning and some minor work. These are a great option for someone missing a carb completely, or who has a Weber or something that wants to go back toward original. These are a great desmog carb because they only have 1 vacuum source. If they made one with a mechanical secondary that’d be even better!

I got this one off Amazon for about $58; it’s a Kipa brand. I’ve used 5 or 6 Partol brands in the past; they were usually around $100. Surprisingly, this one was better out of the box than any of those Partols!

My thinking was buy one of the cheapest I could find with the fewest reviews assuming it’d have the most flaws and issues to show but I was sorely disappointed. I likely could’ve ran this carb out of the box with no immediate issues.

Anyhow, bought the carb, have a Keyster kit on hand so here’s the initial pics, followed by tear down and problem areas. Hopefully able to get it back together and post that in the next couple of days.

Out of the box.

Even came with a cleaning kit and extra filter. To me, that means the manufacturer expects it to be cleaned first. Kind of like how a pound of bacon isn’t in a resealable package so clearly it’s meant to all be eaten at once?

 

[Skreddy]

Ok, tear down and problems/issues/flaws. Like I said, this one wasn’t bad but still enough to illustrate the common issues I’ve found with these.

Disconnect choke and accelerator pump linkages, then remove all the screws in the top cover and remove fuel cut solenoid. Here are the first 2 issues I’ve had on every one of these I’ve touched.
1: Screws not tight holding the carb top to body. This one was better, but no exception. 3 of the screws were barely finger tight, the rest were good.
2: The fuel cut solenoid wasn’t tight. This one was about half turn out. You could wiggle the carb and see the aluminum sealing washer flop around.
Next thing was to check float height before removing it. Gasket is in place so this one actually measures pretty good! Usually they aren’t very close.

I look pretty close at the compression marks on the gaskets to see how it’s sealing as is. This one wasn’t bad but there were definitely spots that weren’t being sealed as well. The first one of these I did, essentially the entire middle wasn’t compressing the gasket. More on that later. Hard to see but you can tell where the gasket has been squished and where it hasn’t. I check both sides and wherever it doesn’t look as squished, look at that area of the carb.

Looking at the fuel cut solenoid, the oring was covered in machining shavings that have been embedded into the oring. This was the same with accel pump plunger and the vacuum secondary rubber diaphragm. While likely not a huge deal, these chunks, or others, are going to end up migrating and blocking the smaller passages and jets in the carb, leading to poor running and starting. This is just an illustration of the poor cleaning these get after machining. Probably a quick blowout from an air gun and on to assembly.

So looking back at that upside down top plate pic above, you can see steps in the surface where it should be machined flat. (Along center line of carb going left to right). These areas of the gasket you could also see the steps and where it wasn’t compressing as much. This was the theme throughout all the gasket surfaces on this one, though not really too bad.

Continuing on, removed the carb body from the plastic spacer and the bottom and removed the rest of the linkages and the vacuum advance pot.

You can see how the center of the port on the carb doesn’t quite line up with the port for the vacuum advance pot. Not bad, but a little off. On others, I’ve found this oring also isn’t thick enough so it’s not actually even sealing anything.

 

[Skreddy]

More unflat surfaces can be seen in the top of the carb body. Screwdriver is pointing to a pretty big step in the machining.

I have a big piece of steel that’s machined flat that I wrap 220 grit sandpaper over and lightly run these parts across that in different directions. Some take a few seconds, some take minutes to make flat. Can’t put much pressure on the part or you risk sanding at an angle. As you start to sand you’ll see marks on only part of the paper. Even though the pieces may look smooth and flat, they can be warped. Looking back at the surface you’re sanding, you’ll start to see the sanding marks on the highs and the lows untouched. Just going for as little sanding as needed to make a uniform, smooth, flat surface. Here are some pieces in progress of sanding.

Carb body after sanding flat.

The first Partol I did, that plastic spacer was over 1/8” shorter on one end than the other. I ended up using one out of a spare I had. When the carb was all bolted together, you could see into the secondary barrel from the outside where the gasket should be sealing…..
From the same carb, here’s a secondary diaphragm half after sanding a bit. Before sanding this one, I could shine a light in the vacuum port and see it around the edges. Only where the screws were was touching the other half. Both halves were this bad.

Wish I had an in process pic of this carb body. Only the outside edges were touching the gasket. Took a lot to get this one flat. This is from the carb I now have 20k miles on in my daily 60 series.

Here’s the shavings in the secondary diaphragm on Kipa carb. This is after most of the loose ones dumped out when I took it apart.

Hard to see but lots of junk in accelerator pump hole.

 

[Skreddy]

Last big problems are bent linkages or improperly installed linkages.
I didn’t look close enough yet to see if any of these are installed incorrectly, but look fine at first glance. A couple bits are bent though. The secondary shaft linkages at the throttle arm kick side (not the side the vacuum diaphragm is attached to) are bent. These arms get a little “kick” from the primary throttle linkages in order to partially open the secondary so vacuum can do its thing. This one will get straightened out, then I’ll rebend it once installed to get the kick where I want it.

The nuts on both the vacuum side and the primary shaft holding the linkages on were also both barely finger tight. That could be a fun surprise when the primary nut finally spins its way off, your linkages start to fall off and there’s no spring pressure to return the primary back to idle! Offroad tractor whiskey throttle!

Other bits I’ve found that should be checked regardless of what type of carb are the curb idle screw and mixture screw. Mixture screw was 1/8 of a turn from seated. That would definitely cause some tuning issues. I think a lot of people expect these things (and the Holley Sniper) to be set pretty close out of the box to start and idle; just need a quick twist here or there and done. So the mixture was way too far in; the curb idle screw was way out, my eyeball guess is probably 2 full turns from where it would need to be to idle on its own.

More to come in the next couple days.

 

[devo]

I appreciate that extensive after market carb break down. I always wonder how many times can you rebuild an Aisin carb from 1980 before it is shot and you have to buy after market?
What size jets were supplied with his carb?

 

[RevISK]

I got a Fuji carb from City Racer and it started off great but after running it for a bit I did have to do a partial tear down and found some junk in there.

 

[RevISK]

And thank you also @Skreddy for this breakdown, super informative for carbs in general.

 

[mattressking]

The aftermarket carbs aren’t bad but they have some basic flaws, one is irritating if you don’t have spares. This is where I’ve consistently seen the venturi pickup nozzle will fall out of the aluminum casting after prolonged use which will cause starting/running issues. Aside from that and some of the casting flaws - they work fine. I usually dump the float for a new Aisan as I’ve had those absorb water and inspect the o-rings on solenoid as those have had questionable condition from new.

 

[Skreddy]

Reassembled today. Blew it all with compressed air, followed by brake clean through all the orifices and nylon brushing, then more air.
During reassembly is when I double check everything. First was the jets. There’s no numbers stamped on any of the jets so I used my feeler gauge rods to check sizes. Installed were a 160 primary and 200 secondary. (Jets are sized in millimeters with no decimal. So 2.00mm = 200 jet size). The spares under the bowl drain plugs were 125 and 205. The power valve jet was a 75 and both of the slow jets were 80’s.
Slow jets “should” be 65 and 90. Primary let’s say 145 for a desmogged 2F is a good start and secondary a 200. So the slow jets are rich and lean depending on the side and the primary is a little rich. I’m good with that because I think when desmogged, these can tolerate a little more fuel. We can lean out the slow jet a little with the mixture but the primary is stuck unless I change it. I’d prefer to start a little rich. Im actually going to run the 205 in the secondary as well.

Jets are in, now onto the rest. I didn’t mark the nozzles when I pulled them because I don’t trust they were in right to begin with. The way to tell is the primary nozzle will have a larger air bleed at the top. These were actually installed properly but that could’ve been luck; seems to be about 50/50 with these. In the pics, you can see the bottom orifice is smaller on the secondary side (right) and the air bleed is larger on primary (left second pic). You can blow in the brass tube on the bottom and can notice the primary flows much more.

Next up is set up the vacuum secondary diaphragm housing. I mentioned in an earlier post sometimes the gasket between housing and carb body isn’t thick enough to seal. I believe this is because the gasket recess in the carb body is machined too deep. Here’s the gasket from the Keyster kit in there with the edge of a razor blade showing how recessed that gasket is. That’s going to seal less than Ken Pinyan holding in a fart after a horse ride. So, I use an o ring slightly thicker than the depth of the recess. (Didn’t get a pic of the oring).

Next is the accelerator pump plunger. I ended up using the one that came in the carb. I don’t think it’s leather like stock but it’s some kind of similar-to-leather material. It seals a lot better than the rubber plunger from the Keyster kit. I put a small bit of gas in the accel pump bore and push the Keyster one down and it didn’t start squirting fuel until it was a lot farther down the bore than the original. You can see in this pic the size difference of each “cup”. Keyster is black rubber one on left.

Last topic of this post is float and inlet valve. I changed out the inlet needle and seat assembly for the Keyster one. The main reason is the bypass holes in the Keyster valve are much larger and there’s 4 instead of 2. The 2F doesn’t drink fuel to the point where this should be noticeable, but if those little holes get plugged, you’re strictly relying on how much fuel can bypass around the needle valve. Still probably plenty of flow, but I’ve got a better one so why not. Keyster assembly on right.

 

[Skreddy]

Ok, so everything is now set up and put back together so time to reinstall the linkages. Put them back how they should go which was correct out of the box on this carb. I straightened out that bent secondary kick linkage. Now, I can rotate the throttle by hand and see when the primary linkage contacts that secondary linkage. It was at least 3/4 open before it started moving the secondary. This means my secondary will only have a chance at working when I’m at 3/4 throttle or more. Naw dawg. So I’m going to bend the contact tab on the primary linkage that moves the linkage that moves the secondary linkage…. Hopefully pics show the plan.
I’m going to bend this tab so it makes contact sooner.

So now it makes contact at 1/2-5/8 throttle (I may even bend this more after some driving tests). Now we are on to problem #2 with the linkages. The primary linkage contacts the secondary closer to where I want, but the secondary isn’t opening very far. The secondary needs to open in order to pull vacuum through and let the secondary respond. If it doesn’t open far enough, it won’t create enough vacuum to overcome the pressures working against it. So I bend the secondary linkage where the primary linkage contacts it so it will open it further.

Secondary throttle blade opening before I bent that contact.

Secondary throttle blade opening after bending. May not look like much but that will make a difference. May need more but I can do it while installed.

 

[RevISK]

Great stuff.

 

[Skreddy]

Last one for getting the carb together and this is more of a general carb install so I gave it its own post.
I set the mixture screw to 1.5 turns out from lightly seated, that should get me idling well enough for warmup and tuning. (This carb is going on a fresh built engine so I want a quick start up). If you’re unaware, don’t tighten that thing down like a ham fisted gorilla. It will damage the seat and change the characteristics of the needle, possibly to the point it won’t even work.

The next setting is the curb idle or idle screw adjustment which is going to determine what rpm the engine is going to idle at. Without getting too far into details of operation, there is a transfer slot in the wall of the primary bore. Flip the carb upside down and look at the wall of the primary on the side the mixture screw is on (but look inside the bore). Open the throttle and you’ll see a slot going vertically. How much of this slot is showing with the throttle blade closed will dictate your idle rpm or whether it’ll idle at all. In ideal conditions and setup, the slot will appear as a square when the throttle blade is closed. Less than that and you won’t get enough fuel transfer to idle. More than that and you either idle high or, you idle so high that you’re actually pulling fuel from the main jet circuit, which means you’ll never tune properly because the mixture screw won’t have its effect.
Here is the idle screw backed out so throttle valve (butterfly) is all the way closed. The hole you see is the end of the mixture screw orifice.

Here is the screw turned way in to show the slot.

And here is what the slot being “square” looks like. This is a little farther open than square again, because I want to make sure I get a fast start on this rebuild.

Now I can install this carb and tune it. Once I’m ready to start it, I’ll fill the float bowl through the vents until I’m high in the sight glass. This way I won’t be cranking the engine forever trying to fill the bowl with fuel. I have a squeeze bottle with a metal tube for doing this. I’ll try to remember to get a pic of that. Likely be a week or so before I’m ready to start this one.

 

[Skreddy]

Here’s that squeeze bottle. Just used it to refill the fuel filter as well as the line from the filter to pump and pump to carb lines.

 

[Skreddy]

Was able to fire up my project 60 today with this carb. 1 pump pedal, full choke and turned the key. Got about 3 revolutions on the engine and it started! Idle was around 1300 so adjusted the choke fast idle screw to bump that up to around 1800. Then onto timing and I was about 12* advanced. Backed it so the bb was right at the lower edge of the window. Idle speed dropped to about 1500 doing this (running on 1/2 choke now). Ran it until temp gauge started climbing and choke was off. Vacuum gauge showed about 15” so adjusted mixture. In resulted in lower vacuum and less rpm. Ended up 2-1/4 turns out from lightly seated and 19” vacuum.
Never touched the curb idle setting. Point of this long post is: making that transition slot “square” makes a ton of difference. Too much slot or too little slot showing means you’re compensating for other issues like too much or too little timing, rich or lean jets, vacuum leaks.
Put 15 miles on it and my secondary kicks in right past half throttle and feels like plenty of power. Drives very similar to my other 60 that has a 5 speed and 33’s so with gearing and tire differences, this one’s coming out pretty good!

 

[mtn biker]

This is the exact carb that I got. I didn't even think of rebuilding it when I installed it at first there was some driveability issues but after some tweeking it I have gotten it to run a bit better. Actually the more I run it the more reliable it is getting although I am still wanting to go with Justin and Redline land cruisers Holly sniper setup.

 

[Trollhole]

Not a fan of the cast aluminum pot metal base carbs. The factory that made these parts did a horrible job. Wasn't a fan of the brass plugs and how they were drilled. Also they did a horrible job of honing the accelerator plunger hole. I always removed the inlet screen on the bowl needle seat. Ethanol does not like the crappy plastic they have holding the screen. I would make sure the throttle blades are aligned and then stake those screws.