carb jets--please gimme a ballpark on size (1 Viewer)

[rusty cerdito]

I still haven't pinned my carb back together because I had to reverse drill out on of the mains jets.

I've called around to a few folks, can't get a hold of Jim C. Hoping someone can give me a ballpark on what size jet should go in. I've looked at lots of threads, but there is a lot of different opinions and factors.

What I pulled out and what I know:

Secondary. One ring venturi. Slow jet (the long one?) is a 70. The main jet is the one that is crumbled so I have no clue what size it was.

Primary has 2 venturi rings. Slow jet is a 55, main is a 20.

The 2f is desmogged, so doesn't need to be deliberately rich as has been mentioned in other threads.

Carb is a 1975. 5F6 is the stamp.

Thanks for any help.

 

[thebigredrocker]

Here’s one of the many threads FJ40Jim has generously left info on.
75 2F Desmog ?

To find more tech poted by JimC

Enter FJ40Jim in “posted by”

I’d use search terms like: 75 carb, 75 cfm, single ring, 75-76, PV jet

 

[Dizzy]

Land Cruiser Carburetors, Parts, Kits & Gaskets

230

You'd probably buy the 159 because it is in stock, then drill it out. But, you are in Idaho, so a 230 is not an appropriate altitude answer. I'm in the same '75 high altitude boat, but a few steps behind.

My plan is to install an oxygen sensor to find the proper jets. High altitude jets

 

[rusty cerdito]

Here’s one of the many threads FJ40Jim has generously left info on.
75 2F Desmog ?

View attachment 1877117

To find more tech poted by JimC

Enter FJ40Jim in “posted by”

I’d use search terms like: 75 carb, 75 cfm, single ring, 75-76, PV jet

I hate to show my ignorance, never rebuilt a carb, but what is a PV jet? Is he saying to replace the stock 144 primary main jet with something like a 70-90?

 

[thebigredrocker]

This is a power valve from a 74 carb. The tea party invite is from my daughter(6)

Here’s my understanding of the PV. At high vacuum the brass slug/weight is sucked up and off the PV plunger, holding fuel back from flowing out the PV jet. When you romp the throttle, vacuum suddenly drops and the light spring(and gravity) allow the slug/weight to open up the plunger.

I’ve used superfine wetsanding paper to smooth the bore the weight slides in. I also polish the weight. Just guessing, if the weight sticks in the bore, that’s not ideal.

 

[Dizzy]

I hate to show my ignorance, never rebuilt a carb, but what is a PV jet? Is he saying to replace the stock 144 primary main jet with something like a 70-90?

My impression is that the 70 to 90 is for the power valve. The primary main jet size is still in question.

 

[OldBeachBob]

I'm following this - Boise most time spent is 3000 to 5000 Ft

 

[rusty cerdito]

spoke with jim today. thanks @FJ40Jim. He said to keep the stock jets and make sure the PV jet is within 70-90. Now I gotta source some jets.

 

[thebigredrocker]

Do you just need 70 thru 90 power jets?

 

[rusty cerdito]

Do you just need 70 thru 90 power jets?

no just the crumbled 230 primary main. probably have to order the 1.59 off specter and drill it.

my other jets are intact and my pv is a 70. there is a bit of screwdriver bend on one of the jets. will that affect flow? replace or use it?

 

[thebigredrocker]

Here’s some pics of a Mikuni large round main jet compared to a OEM main jet. The main jet is out of a 71-74 Aisan carb. I assume 76-76 Aisan jets are similar, just larger.

Memory is fuzzy, but I think the Mikuni thread pitch is .75 and OEM is .8. Another mudder worked in the Mikuni jet by threading it into the spare jet holder(drain plug)

Got this off ebay. I think this is a authentic Mikuni, not a Chinese knock-off. Can’t find the sellers name. It was a cycle shop

From left to right
OEM power jet
OEM main jet
Main jet from Keyster kit
Mikuni large round main jet

 

[thebigredrocker]

Not sure if Racer65 bought jets from this outfit but here’s the link: MIKUNI LARGE ROUND MAIN JETS N100.604

 

[rusty cerdito]

when you say "worked the pitch", do you mean that by doing do so in the drain plug it shapes the pitch of the mikuni and makes it fit and/or not strip threads of the carb?

 

[thebigredrocker]

My understanding is: He used the threaded drain plug just like you’d use a threading die to shape the pitch.

 

[Stumpalama]

An old-ish thread, i know; but has anyone been able to source different jet sized power valvea?

 

[Dizzy]

An old-ish thread, i know; but has anyone been able to source different jet sized power valvea?

Kinda like Jim was saying above, that it will likely always be in the closed position, aside from when the engine vacuum is really low (like not running). I don't know, but on USA carbs, it was probably better worked by the VSV, then manifold manifold vacuum routing of a desmog position. I suppose that you might take things further, and install an orifice in the vac line (weber carb jet) to bottleneck some of the pressure that you get from the manifold so that you might get a richer condition at some point.

But, no leads on different power valves, yet. It would stand to reason that non-USA carbs, and their clones (internal power valve) would have a different set-up?

 

[Stumpalama]

Kinda like Jim was saying above, that it will likely always be in the closed position, aside from when the engine vacuum is really low (like not running). I don't know, but on USA carbs, it was probably better worked by the VSV, then manifold manifold vacuum routing of a desmog position. I suppose that you might take things further, and install an orifice in the vac line (weber carb jet) to bottleneck some of the pressure that you get from the manifold so that you might get a richer condition at some point.

But, no leads on different power valves, yet. It would stand to reason that non-USA carbs, and their clones (internal power valve) would have a different set-up?

I've successfully rebuilt several carbs, but only to stock specs.
I'm interested in what is changed (primary jets, slow jets, in a 1981 and newer carb when adapting it to run on a desmogged truck.

 

[Dizzy]

@Stumpalama I don't think that jets change that much. The different jets are carb-specific. IIRC, the throat sizes change, and the different jets go with them to match.

The smog equipment isn't like a make it lean or rich kinda thing. It is spark timing, exhaust gas added to lower combustion chamber temps and reduce pumping losses, operate the power valve at different conditions, etc. I'm running headers, so the fuel is less 'atomized' on the colder intake manifold - which could be less fuel is actually burnt especially when cold, it would change your exhaust reading, but you probably wouldn't want to correct for it. I also have the 61080 - Non-USA distributor, which probably has a fresher advance curve (new parts) than the rusted springs that are probably in most original distributors. I'm running the stock coil with the stock spark gap, no need for more than what the rig came with.

However, I could see some tinkering that one might do to their tune. For me, it was a concern of high altitude, so I installed an O2 sensor to see where I'm at. I drilled out my secondary main jet because air doesn't like to go at high speeds, great volumes, and low engine vac thru the '75 USA air horn - or so I've concluded. If the 2F isn't blowing smoke, it is fine, I'm not going to fix what still works just to get the smogerator just right. Yes, there is a problem running a tired engine at 10,000 + feet, but it I'm betting on it, rather than a professional install of some aftermarket freeze plugs and a several thousand dollar hole in my account. My motor has chunks of carbon built up on top of the valves from years of running cracked valve seals, so with new valve seals, it seems to be getting better on its own.

There might be other things that you tinker with. Is there an issue that you are experiencing?

I know that the choke butterfly seems to interfere with good airflow thru the Venturi, so I run the fast-idle higher this time of year, because I don't want to use much choke without going lean upon cold acceleration (and I'd rather not warm up the engine, and yes, this isn't a problem with the same 2F and a DGEV Weber).

 

[Stumpalama]

@Stumpalama I don't think that jets change that much. The different jets are carb-specific. IIRC, the throat sizes change, and the different jets go with them to match.

The smog equipment isn't like a make it lean or rich kinda thing. It is spark timing, exhaust gas added to lower combustion chamber temps and reduce pumping losses, operate the power valve at different conditions, etc. I'm running headers, so the fuel is less 'atomized' on the colder intake manifold - which could be less fuel is actually burnt especially when cold, it would change your exhaust reading, but you probably wouldn't want to correct for it. I also have the 61080 - Non-USA distributor, which probably has a fresher advance curve (new parts) than the rusted springs that are probably in most original distributors. I'm running the stock coil with the stock spark gap, no need for more than what the rig came with.

However, I could see some tinkering that one might do to their tune. For me, it was a concern of high altitude, so I installed an O2 sensor to see where I'm at. I drilled out my secondary main jet because air doesn't like to go at high speeds, great volumes, and low engine vac thru the '75 USA air horn - or so I've concluded. If the 2F isn't blowing smoke, it is fine, I'm not going to fix what still works just to get the smogerator just right. Yes, there is a problem running a tired engine at 10,000 + feet, but it I'm betting on it, rather than a professional install of some aftermarket freeze plugs and a several thousand dollar hole in my account. My motor has chunks of carbon built up on top of the valves from years of running cracked valve seals, so with new valve seals, it seems to be getting better on its own.

There might be other things that you tinker with. Is there an issue that you are experiencing?

I know that the choke butterfly seems to interfere with good airflow thru the Venturi, so I run the fast-idle higher this time of year, because I don't want to use much choke without going lean upon cold acceleration (and I'd rather not warm up the engine, and yes, this isn't a problem with the same 2F and a DGEV Weber).

Thanks @Dizzy, for the detail in your response.
My current situation is that I will, likely, be desmogging my 1981 FJ40 soon and I will be recurring the dizzy at that time. I am pretty solid on the recurve process thanks to some other great minds on the distributor threads.
As I've had years of rebuilding carbs and them working very well for the people I've rebuilt them for, I only know how to set them up for stock specs and tune them. I haven't really messed with rejetting; only modifying the main jet selection based on excerpts from fj40jim's posts. I don't know the finer points and the systems engineering aspects of the LC carb well enough to be able to judge if how a carb is behaving is due to jetting, throttle position or something else.
I only know as much as I've read as far as desmog carb and dizzy, and they all lead to Jim C. So unless there was some magic that he does for carbs going on desmogged vs. smogged trucks, I just as soon do the work myself. I wondered if I may need to switch to smaller or larger main jets, or adjust some other base settings.