78 Aisin carb jetting question (1 Viewer)

[Stubby]

I’m using an A/F ratio gauge to help me get my carb jetting dialed in. I’m lean at both idle and cruising, which I expected, since I have a hotter than stock cam, and more free flowing exhaust.

Quick question on idle circuit and main jets. Does the main jet affect the amount of fuel going to the idle jet, or are they separate circuits?

I’m wondering if increasing the size of the main jet would have a corresponding richening on the idle circuit, without increasing the idle jet, or if I need to increase the size of both main and idle jets.

Of course, if the main jet is already larger and flows more than the idle jet, then I wouldn’t think there would be any change at idle unless I changed that jet too.

Also, I have two sizes larger for the primary idle jet, but don’t have any larger ones for the secondary idle jet. Do I need to increase the size for both to affect idle, or only the primary? I could always drill out the secondary to make it larger….

For reference, A/F is 14.9-15.5 steady cruising in 4th at 2300 RPM, and 15.9-17 at idle. Brand new engine with 250 miles on the rebuild, engine fully heat soaked, and I’ve confirmed that there are no intake or exhaust leaks.

Current jets are :
Primary
Idle - 80 (drilled out to 0.85)
Main - 136

Secondary
Idle - 75
Main - 171

 

[mattressking]

Separate circuits for primary and secondary jets.

We don't know what jets are in YOUR carburetor. Note that for your carb from factory you should see:

Primary - 124 or 136 (depending on Calif. or Fed Market
Secondary - 171

You can bump up to a 138 or higher towards a 147 for primary
Secondary could go up to a 180 or 200.

Note that you should also be doing plug chops to see how your engine is accepting the jet change alongside reading your wideband.

Don't go too rich, lean is mean

 

[Stubby]

Separate circuits for primary and secondary jets.

We don't know what jets are in YOUR carburetor. Note that for your carb from factory you should see:

Primary - 124 or 136 (depending on Calif. or Fed Market
Secondary - 171

You can bump up to a 138 or higher towards a 147 for primary
Secondary could go up to a 180 or 200.

Note that you should also be doing plug chops to see how your engine is accepting the jet change alongside reading your wideband.

Don't go too rich, lean is mean

Yep, definitely different circuits for primary and secondary, but what about idle to main jet?

And I was waiting for my engine to cool down so I could remove the jets when I first posted. I edited my original post to include the current jetting.

 

[Stubby]

Bueller......Bueller.......Bueller......?

Anyone other than mattressking have recommendations? I reached out to FJ40Jim, Mark A. from Mark's Offroad, and Pinhead, but haven't gotten any responses from them.

From looking at other jetting threads, it looks like the secondary idle jet should be larger than the primary, but that's not how my carb came back from Jim C. after rebuild. Is the '78 an oddball with a smaller secondary idle jet?

I went ahead and ordered some Mikuni jets a few sizes larger for the primary / secondary mains, but I'm SOL on the idle jets. Cruiser Outfitters has Keyster rebuild kits, but they don't list which jets are included in the kits. What would be the carb year with the largest venturis, and corresponding largest idle jets?

 

[Malleus]

Sounds like a research project for someone...

 

[Stubby]

Sounds like a research project for someone...

Nah, not really….just trying to get some advice from the experts, and providing as much information as possible, since that seems to be a sore spot with some folks.

 

[peesalot]

Have you tried a pm to wngrog on this board ? Georg at Valley Hybrids/Cruiser Brothers might be able to help as well. They are all on this board, patience, the answers you seek are going to come.

 

[Stubby]

Have you tried a pm to wngrog on this board ? Georg at Valley Hybrids/Cruiser Brothers might be able to help as well. They are all on this board, patience, the answers you seek are going to come.

Hadn't thought about calling Georg.....dang, total blonde move. I've spoken to him several times about other things. If I don't get any answers, I'll give him a call.

I just IM'ed wngrog....

 

[Malleus]

Nah, not really….just trying to get some advice from the experts, and providing as much information as possible, since that seems to be a sore spot with some folks.

I didn't mean you specifically, just that it would be a really good addition to the FAQs.

 

[Stubby]

I didn't mean you specifically, just that it would be a really good addition to the FAQs.

No worries.... I do have a couple of bits to add that might help with future carb questions...

Keyster Kits are still available through Sirius Consolidated. Don't use the search by make/model feature or you'll just get motorcycle stuff. Scroll all the way down to the bottom and choose the "Auto Carb Kits and Tires" link in the Products list, then scroll down to the Toyota kits - Here's a link to that page (https://www.siriusconinc.com/products/index/34)

Someone told me that Mark A. posted a chart with jets by year on here, but I searched and couldn't find it.....maybe someone else has a link to that post.

 

[77mustard40]

Here's my experience with a 76 carb that I bought and rebuilt when I switched back to a stock carb. I was struggling with idle and fuel/air mixture. I had chased every vac leak known to man, my cure for getting the right mixture and solid idle was to up size the slow jets. I used the slow jets that came with my Keyster kit and they just were not adequate, so I cleaned out the old slow jets and installed them and voila!

Also you're primary main jet seems small, IIRC I'm like 230. The secondary main looks ok.

 

[77mustard40]

In case you want to try to get something from my experience, I think I have a pretty good conversation with Jim C on the carb setting late in the thread.

Going Aisin-details, details, details

I made the commitment some time ago to ditch the aftermarket carbs and get back to stock, or as near as possible. Down the torpedos with Holley and Weber. My 77 FJ40 has never had its original carb in the years I've owned it so here is what I have pulled together: 1/76 carb - just completed a...

forum.ih8mud.com

 

[Stubby]

Here's my experience with a 76 carb that I bought and rebuilt when I switched back to a stock carb. I was struggling with idle and fuel/air mixture. I had chased every vac leak known to man, my cure for getting the right mixture and solid idle was to up size the slow jets. I used the slow jets that came with my Keyster kit and they just were not adequate, so I cleaned out the old slow jets and installed them and voila!

Also you're primary main jet seems small, IIRC I'm like 230. The secondary main looks ok.

I had a slight exhaust leak, nothing on intake, so I ditched the manifold gasket that came in the rebuild kit and put on a remflex gasket.....no leaks of any kind now.

Engine, carb, and dizzy are new, but my engine is definitely not stock (Delta Cams KC859 grind, 3FE exhaust manifolds, and some head work), so stock jetting numbers aren't correct. Instead of using vacuum / plug reading, I'm dialing it in with A/F ratio to get more accurate results.

Now that I've gotten some answers and spoken to other folks, my biggest question is if my idle jets are swapped......should the smaller idle jet be in the primary or secondary on a 78 carb?

I've ordered some larger main and idle jets, and I'll be dialing it in over the next few days, weather permitting (I only have a bikini top on my rig). This is ultimately a fun project and reminds me of my youth back in the 80's dialing in the carb on a CR125.

As Malleus mentioned, something like this really should be in the FAQ, along with a chart that lists stock idle / main jets for US/Worlds carbs by year. It would at least give a starting point for dialing things in, and help folks work out if their carb has been re-jetted.

 

[77mustard40]

Large jets (both main and slow) into the primary side. Get the biggest slow jet you can find for the primary side to help with idle, sounds fun! Good luck!

 

[Stubby]

For anyone who might find the information useful.....

Using the wideband A/F gauge made tuning the carb WAY easier....wish I'd installed one months ago.

I tried 4 combinations of different idle and mains, driving about 20 miles on each configuration, to get a mix of cruise, start/stop, and idling. On the plus side, I can now disconnect everything, open up the carb, swap jets, and have it back together is less than 20 minutes. LOL

For my engine here's what I found worked best.
Primary - Idle 1.0 / Main 150
Secondary - Idle 1.0 / Main 180
Idle mixture screw is sitting about 2.5-3 turns out from closed.

A/F at idle is 12.9 - 13.5
Cruise on flat road 4th gear (2300 RPM) - 13.4
WOT - 12.4

I can now get it to idle down to 700 RPM....previously, anything below 850 would result is sputtering / popping and shook like crazy. It idles very nice now that the fuel is right, and stays very stable, even after idling for extended periods. It's also way more responsive when accelerating.

I'm going to leave things as is for now, but with the cruise being a touch lean and WOT being a touch rich, I can probably go to a 152.5 primary main and back to the original 171 secondary main. Need to order a couple more Mikuni jets from Jets-R-Us though....the ones I have now jump from 150-156, and the 156 was too rich at cruise RPM.

Anyway, thought I would share with everyone....