Carburetor fun after ages of letting the old girl cool her jets

[None_zero]

Long time no type on the internet to each other my fellow enthusiasts. I had pretty much parked the old 720 as I am not a rich man by any means and I'd come to the sad conclusion that without a new carburetor or at least a rebuild I was doomed to get 9 mpg until I went bankrupt. So I went to rock auto and looked forlornly at the $500 carb they carry which appears to be a near match. But I was never able to pull the trigger on a 500 dollar carb for a 300 dollar truck and so she sat. Recently I've had Call to drive her again despite the horrible performance and so I renewed my search and looked some places I wouldn't normally consider for auto parts... yes that's right I went to the devil to make a deal. Amazon. And there within its well light screens of every good imaginable I found a carb for my z20 engine (or at least for a z24 and I think they are the same) the price tag? $125 bucks but still I hesitated I hate to throw money away and who knows if this thing is even going to be right. But then I saw that listing at a much more palatable $77.00 and I thought well hell for that price I have to find out and so I ordered the thundermingo carb for the z24 and after receiving it and comparing it I can say it's an exact match (except for the wiring quick connect which I'll have to cut and splice) to the one I currently have (which I was previously unable to identify) so now here sit wondering do I dare take these 4 bolts loose and swap this carb with any expectation of going to work tomorrow. What are the odds I put it on and it runs like a top? Um probably looking at some intense dialing in at the least right? Any advice is welcome and as always I'll be looking for datzenmike to chime in with his near bottomless well of knowledge and insight. Watch for falling flattery 

[datzenmike]

In for a penny... in for a pound. Might as well keep going. Nothing to loose at this point.

 

You should be well above 20mpg. This should be something grossly wrong and should be obvious. A new carburetor won't fix any problems not carburetor related. 9mpg is extreme and likely more than one problem compounding. I hope you checked...

 

Both coils are firing.

Timing is at 3 degrees.

Valve lash set.

Dirty air filter.

 

 

Choke does warm up and turn off.

Carburetor is not flooding.

Float height set properly

 

[Stoffregen Motorsports]

Worst case, the new carb is a pile of junk, and you can use it for parts to rebuild your OEM carb.

[datzenmike]

90% of the time it just needs cleaning and the accelerator replaced. 10% is a a previous owner tried to rebuild it, tuned it or adjusted it.

[Stoffregen Motorsports]

Two things that can render a carb completely useless - Worn out throttle shaft bores. And corroded/calcified inner orifices.

 

Both of these problems can be fixed, but the average home mechanic doesn't posses the tools or the skill.

[None_zero]

Ok so for the check list. Yes I believe I've gone over everything in your list Mike. There have been some anomolous discoveries like the ab valve being full of fuel. But I've set and re set checked and rechecked everything else I can think of. Except for one thing which would be the fuel return (I assume there is a fuel return function on it) but given the lack of adjustment in the fuel air mix screw I'm thinking it needs a carb or a rebuild and since the carb itself is not much more expensive than a rebuild I figure why not start fresh. I still haven't had a chance to swap it in yet and see how it goes maybe after the sun comes up I will do it today. I've been working nonstop for the last month or more and have had zero down time to fool with it. I drove it for a week or so without any additional repair or tuning and it still runs fine but I hate dumping that much gas down it ... oh one more thing aimed at stoffregen, the oem carb is long gone I'm sure. The carb that's on it is unlabeled and up until recently unidentified. But when I found this thundermingo carb and compared them I believe I found the match. So at best it's updating the same garbage that's on there now with a new piece of garbage to see if that even solves the issue 

Edited November 4, 2023 by None_zero

[datzenmike]

Warm the engine and check that the choke has opened all the way.

 

Post a picture of the carburetor so we don't go down the wrong road.

[None_zero]

Well it looks like this one is a swing and a miss.  

Here is the new carb with gasket lined up on the bottom. And here is the old carb with the new gasket on it and only three holes will match up 

[None_zero]

This is old gasket next to new gasket not even sure that one would line up on the heater plate properly and I'd probably need a new gasket for this one also

 

[None_zero]

 

Top and front view side by side 

[None_zero]

I see now that the mating surface has some slight differences also but I would have thought if it fits on the z24 it fits on the z20

[None_zero]

So let's talk options. Is it even worth considering redrilling one of the holes in the mounting flange or are these too far different to get it to seat properly even if I could mount it ?

https://i.imgur.com/8iEibEY.jpg

We're talking about that much difference 

[None_zero]

I also see now that the barrels are slightly larger than the old one so the heater plate fits over the hole a bit and not around it as before. I guess this is a return and try something else huh? Or can I gut the new one to rebuild the old one? Or will these differences make that impossible 

Edited November 4, 2023 by None_zero

[None_zero]

This is what it shows compatible with a 84 z20  but this only slightly resembles what I've got and where are the choke plates ?

Edited November 4, 2023 by None_zero

[None_zero]

Ok at second look I see the rail fornthe choke plate on the left there as if it's just fully open is that what the omicron looks like? Mine had the little plastic seal for the air cleaner and all that originally 

[datzenmike]

The Z20 carburetor has a 32/34mm primary/secondary barrel diameters. The Z24 is 34/36mm.

 

The Z20 base mounting bolts are in a rectangular pattern.

The Z24 bolt pattern is a trapezoid with the two shortest lengths parallel to each other but different lengths and the long sides the same length but not parallel to each other. 

 

 

Here is a Z24 carburetor base mounted on an L series manifold. You will have to oval the holes closest the secondary as they are further apart. Holes 50mm and the studs are 44.5mm. Still needs clearance for the nuts.

 

[None_zero]

So just split the difference between them both instead of lining up three and tuning one into a shirt show. Got ya sweet that's good news my next thing was take the base of carb 1 and 2 off. Then attach the base of carb 1 to carb two for correct fitment then replace all the other parts within the housing unfortunately the air fuel mix is there and they are wildly different lengths so that died in utero. 

[None_zero]

I didn't want to have to get new gaskets either so who knows what sort of nonsense I'd have gotten up to rather than make the grueling drive 5 miles to the parts store or have them shipped directly to me... oh the agony. Much better to run the hairline back another inch and flex the impressive array of curse words at my disposal. All while concocting a fix the I'd say has a 30% chance of success at best. Lol

[datzenmike]

Switch bases? I see no reason why that won't work. Fingers X ed.

[None_zero]

I went with oval the holes because the base is where the air/fuel mix is and that is one of the things that seems off (as in doesn't behave as expected) although with the new carb on it doesn't seem to have added any adjustment there. Still sets about the same all the way closed then back out a half turn or so. Haven't had a chance to check if gas mileage has improved yet also air fuel all the way closed still doesn't make it stall out 

Edited November 14, 2023 by None_zero

[None_zero]

I wanted to go to the store earlier for something... trying to remember what it was so I can go run some gas through it...

[None_zero]

So im getting less than ideal performance. While the gas mileage does seem to have improved significantly (I haven't done an exact calculation but I'm thinking I went around 30 miles on the same gas it was using to go 17-20 miles) but the engine revs up to 2-3k rpm during warm up and occasionally drops off and dies while decelerating. Should I be looking for leaks around the base I guess?

[None_zero]

I'm curious. Can I remove the heater plate from below the carb without adverse effect. I'm not seeing the gaskets for it and I'm wondering if I even need that living in TN where the winter only lasts for 3 days 

[None_zero]

On second thought I'll just get some gasket material from advanced and cut one... if the Norse religion turns out to be right and the world dies in endless winter I don't want to be the jackwagon whose 720 won't start 

Edited November 16, 2023 by None_zero

[datzenmike]

You can run without it. My theory is that this is for the first minutes after cold start up before the ATC (air temperature control) kicks in. That's the air heated by the hot exhaust. This would really help vaporize the cold fuel droplets and this would improve unburned hydrocarbon emissions.