attacking a monojet

[Barry Donovan]

I hope I'm reading this right, and it's a long shot I know. But...if you are steping on the gas peddle the idle mixture screw has no effect any more.
And....I could be wrong, but your stainless fitting looks to be a little crooked....It could just be pic angle though.
I'm following this with intrest, the best that I can!

the stainless piece is now in the intake. this monojet liked metric for reasons I donlt know.
even the main jet hole has to be reamed to take the holley quarter.

the idle circuit never stops, neither does the secondary helper below the main jet.

I left the end fat, should get what I want easy..and the real needle is due on thursday with the other stuff to finish this.
I mentioned 2mm jet, but that is after measuring the first tapers end..it is a small needle that had a freakish big jet.
I can't imagine how a 250 ran with it.

I do have monojet book by ac delco in digital..
the idle circuit actually gets busyier after opening throttle, it is like the third jet to the main jet..never stops. throttle closed, it calms down, and that is the only mix we are concerned with, getting idle right.

it is easier than I may make it sound. I am using all three openings, none are to be closed. the little 2mm jet allows for that.

[Barry Donovan]

monojet cai boot ring



here is an oddity somemight remember about their monojet. See the daylight at the top, to the right of empty screw hole? that is not an error in my sheet metal, that is the top of the carb. the top screw has no rib, and over some time bends that area away from air filter plenum. Within tolerances a gasket gets it. The challenge for me is gaining a .25 inch to match subaru oems overall height, keeping the CAI boot under the hood. The flange gasket is my own on the intake, and this top end will get no gasket. Seems I gained even more than the .25 inch needed. this will all fit underhood. The boot is made of rubber, and I simply push down, and clamp.

this is the boot:

[Barry Donovan]

new jet installed, adjusted needle



I thought I was going to have a crooked seat for the needle..then I looked again at how it works in the bowl, it is not even perpendicular. So I let the jet go at the angle of the needle..this is a tight fit. No tapping. Some of it measured by eyeball. the bypass is in use to run this, so a cave in or sticking needle cannot happen.I polished the old needle with a matchbook. This is 2mm. the bypass hole is in photo as well. this gets idle and partial throttle, one wants the needle to work as you exceed the bypass setting, and be as dynamical as the needle allows. I did good adjusting. I am proud.


here is full throttle..remember the accelerator pump on a spring and check ball valve is also part of big squirt needed. this is for nominal throttle stoich, and saving some money...not to mention a stoiched sound is always exotic. This needle must be as correct as it can allow itself to be...else a hog of an engine.


in all adjustments for needle and jet, the brass round piece with arm hanging off of it can never go beyond the surface of float bowl. there is boundaries after all.. above of course is wide open throttle, surface of float bowl flush to power piston.

have fun with a monojet. one can even make their own needle, use fat jets, the taper dictates how much to gently sip into it. it is trial and error, but I have done this a few times and never been disappointed.

as for above, I reeled in the past, and calculated by memory 3 different engines with a monojet I tinkered with..this will go first try. any further adjustments can be conquered with bypass screw, the idle air mix, and if the main jet is a nuisance the little actuating rod on the thottle can be bent to gain or lose a little (hardly go there unless jet is huge). As you can see, the jet and needle are as tight as it can go..I am playing with 108ci and a 43mm throttle.



here back together. awaiting weekend, as heads are getting tapped for stronger intake bolts.

some tips from memory.. this may even bounce in first runs, be carzy for some days. baurite speckles, pressure of heat, it all has to dial in.

needle cave ins/sticking need the bypass open more..and that always comes back to needing a tighten some time later on the bypass (if it even gives a hard time).

total cost:
$49, carb delivered complete. dirty, not as in dirt..but ionization, and strangely a near caustic soap in the bowl. (fine by me)
$10 - new choke pull off. even if old one was good, that item was updated to stop sucking on the vacuum upon release.(strange error- this one had it)
$10- #79 jet from jegs, they shipped two and gave me a hat (cool). no modifying, fit right in. .079 inch = 2.0066 mm. this jet size was derived from a hot rod inline volkswagen at 2liters and ironically 43mm itb..
$10- new air idle needle, it was wrong..but did remember, if to dial in stoich, this needs its own grinding. the originals literally had marks on them for big engines and small. I made my own by eyeball, and memory.
$21 - two steel studs, a tap and bit, brass washers and grade 8 nuts.
$2.50 - the steel 8 inch brake line (gas line)

$102.50 total

I made my own flange gasket.. a tough thin one, keep the gadgets in the carb rumbling.
looking forward to a robust centered boxer..still half the noise of an inline, and twice the torque.



the rochester jet is a sad sight compared to the holley, in fact the rochester fit flat in the seat..just another oddity. the holley has a bowl.
dynamical carb to go throguh .060 to a .115, seems it can do anything.

[Barry Donovan]

paint removal



I was going to keep this fact to my blog, but..it is carb related. Hitcahi subaru had fuel so tight there is three filters. one at the tank, one to catch brass at the fuel pump (metal) and the third one was for single point fuel injection that worked into 38mpg.A large filter, extremely fine, and even rumored to be a catalyst to change fuel. (the carbs love it)

I kept the same setup for this carb, and got to thinking how sensitive even speckles get to coughing. I stripped the paint off the mini velocity stack (holds CAI boot), and kept air filter and box at the fender, with a drain.

believe it or not this chore stems from exceeding 38mpg and no way to change it. The lean conditions would even move a windshield. The monojet can go from singles to 30s, and in real freak weather, can make a change yourself. (I am thinking heat wave)

A nerdy thought I have going, is what they called the bimetal hot idle relief.. that will never work in a down direction, that must be there to release high vacuum intakes with fast throttles. Well..that is where it is mounted. I wonder what it sounds like equaling pressures on either side of a 43mm valve (I am beyond guessing this subaru will use it in the summer).
super stoich is super serious.

[Barry Donovan]

a quick question - accelerator pump

I forgot about accelerator pump.


hopefully photo is big enough.

the check ball, and spring.. (28,29,30 in photo)this dictates how much it gets if any?

it is the only adjustment I did not get to.
if it is vacuum related, then the engine should set this itself?
if vacuum not enough to hold it from mega squirting, add some washers, toughen spring...?

EDIT:
I guess I had to revive some facts..
the 1978 monojet did not get a top exit, all by itself, for the accelerator pump. it chooses the main well, like the main metering jet.

this allows for a realistic boost in the pumping, and theoretically can never over do it. As that pump is needed, the main jet is also feeding the same holes.. so boosting it, just kicks the same amount of fuel in the butt more than adding a bigger amount.

a space preoccupied can only be filled once. A smart update to their pump I presume. 1978 is headed for last generation of monojet. I was lucky to find no EGR implementation at all...yet late revisions. I found the carb I bought to be 2 years only. 1977-1979. the last monojet I used was a 1974..it shot fuel everywhere..and still managed 20s mpg.

[Barry Donovan]

dimensions


this is for showing dimensions.
I did not add in photo, the taper for jet seat on the factory needle is a 2mm and some millionths of an inch..that is why I chose just over 2mm for a main jet. good fit (refer to photos)..the way the accel pump works, the needle is not even possible to get stuck. I could downsize main jet another size if I wanted.


there is a 1 and 5/16ths venturi monojet..that is a big one. I have even found them written on 305 by factory. The most common for a true 292 that was designed to be a workhorse was the #1 spot to find the big one.

the carb in this thread is the middle one. the vega version is the smallest monojet, and missing some things..counting on vacuum to do what the bigger monojet does by physical levers... and it belongs on a briggs and stratton...very small. The hitachi carb I complain of is at about 140cfm, and seeming twice as big as the vega.

these are all survivors of the 70s fuel crunch, and it is the dark ages of accurate written..a guru had to decipher them.

anyway..if you wonder how this goes to a 108ci, the venturi is still at 1 and a 16th. I could go a tad bigger in the venturi, but not the biggest...that one is huge.

thisone is rated at 210 cfm (american numbers = under rated, japanese = twice over rated)..so, can count on 220 or more.

[Barry Donovan]

no fuel filter at the carb?



how to run without fuel filter at the carb inlet. that is a fuel injection damper on the right of photo. An expensive gadget.. I am glad I am like a junk man sometimes. I priced the subaru loyale fuel damper at 55 bucks...and found it on only one place. I kept all of mine from a wreck. it is from a strangely low pressure fuel injection (single point loyale)..maybe the carb world found these and keep the prices stupid high.

anyway, this will take the place of filter not being at carb to be the damper. even my electric pump pulses, and carbs like that..but I have wanted to try a damper on a carb + electric pump for some time. Now is a realistic time.

EDIT: I found my fuel pump is very gentle, monojet wants all of it. I did get after throttle, and then felt bowl (engine hot) it is working as engineers wanted (rochester does not want hot fuel)..so damper is scrapped, took too much. Staright shot it is.

[Barry Donovan]

up and running

working fast, no photos of what I did to tune this in (I did something to the needle and arm to stay tougher) I just new after thenight before swap, the 8 hour day I had left to go with the garage was going to keep me tinkering. These fresh rebuilt professional have a wait state, after tuning.. imagine plucking one off of ebay with an unknown chemistry inside.

doing very good actually. this one came from houston.. it had a good lift off.

thinking real tiny, it is in the 30s starting off.. not where I want to be, but interesting to prove to myself (and others) that the monojets I played with are indeed a small carb, and ready to be big as well, in time.

I had to make it so needle did not cave in to jet, that will calm down as it lubes its way micronically. I have it all within, what must be, 64ths of an inch.. and it took all day. this exactness only necessary for fresh build, or as in this carbs reasoning, it came from someplace else entirely.

I will post when it is ready for the bigger stoich (not ready yet), that should run and sound really good...as long as the original engine hangs in there (the passenger head survived a disaster)

this same carb, now that I lived it, can indeed go from 108ci all the way to 292.
this quest is actually 25 years old, I have not encountered this accomplished locally, only talked of.

[Barry Donovan]

mixture screw question 4g to 1mv

hey gurus.



I need to change out the stoich screw (the secondary screw below main jet )on the monojet..but it appears to be a fat one at .25 inch (?). I found these rochester idle screws above with springs that have an outside diameter of .295 inches. I thought that was large and wondered if the screw itself fit the monojet stoich spot at .25 inch.

I need to adjust constantly..just the past two days. pathways shrinking and growing, dialing in nicely. this must have sat a long time.
I know if a carb is good, the stoich screw on proper jet is about 1.5 turns, and so far that is holding true. As the stoich changes, the idle screw needs to go back in..then turn down idle, this will be routine for some time. fuel is just starting to move nice .the monojet is very sensitive, need to make tiny turns in coming days and weeks.


is the rochester 4g idle mix screw somewhere near .25 inch?
the thread is correct at 10-32..
photos aren't helping.

edit: the screw don't work. I have a rochester 4g as an idlemix right now. stupifd photos. gigantic out there.
I will make one out of an allen head steel, length my choosing. diamond dremel bits, in a drill gun clamped down..then approach it like a lathe..perfect round point.
the allen head screws have a grippy for hands, not just for allen wrench.

Am really loving the largeness of the one barrel. So far the back end of the car is revealing my weldings efforts, the rear left strut used to go psycho over the blow torch hitachi carb.sits up nice. The little boxer is locomotive in the cold. 700rpms can rumble something in the interior all the way in back.

on the 250, when this is right..wow. it is really fun. On the 108..I am getting to be beyond words, as there is no analogy.. it has been on its own in all combinations from day one.

The hitachi is going into the trash.. I mean ebay.

[Barry Donovan]

hack the choke resistor (the hot part)

I could not wait to dismantle the rochester choke. Never lasting long locally.
I did not get photos, but did find why they die very young.
at 25-40 bucks a pop..

I found a weakness to solder, it is a copper wire gauge about the size of the end of the metering needle (hmmm) ..it fries like a fuse. To get to that alien looking pile of weirdo was a bit of a gentle operation. I broke plastics.

I hacked a means to get it going.Soldering a glob, with broken plastics, silicones dangling, no rivets holding the bimetal spring to the broken cover...some things taken away ...I slapped it all back together with a chunked resistor disc. and it worked.

I know what to do with a brand new one now, won't make such a mess. my hack at it was so bad i did not want to share photos.

I may resort to a 56 ohm resistor, inside a video card heatsink, then put that near the bimetal.. but using their design is ok once sacrificing one. (quite weird and too small, and too large, overthunk in more than one spot).

the atomizer, believe it or not .. is electrical relation, not just good jets and meters and needles. More than for the choke opening, the resistor that gets hot keeps a carberator good. Some engines did need the baffle closed..and to me those are slobs. But hey, whatver works. (when an engine is proper, it needs no "choke closed" anytime of year)

630pm. started right up in the cold. the silicone for secondary screw is holding well. it even looks good, flushed over like the old plug.This really is the type of carb that cannot run without the resistor (choke) in the cold…and even in the spring and summer there is something huge to notice in runtime without it plugged in electrically. I am reminded of my 250ci. Still spitting things, random, much like every carb I have ever installed from a pile. It always wins. silicone is on the flange gasket, etc etc. other than the normal routine (I am looking for abnormal), this is doing good. that 43mm is a big hole to fill. The fact I threw it on in the coldest part of winter and got it going close to stoich first tries… it is a tough carb. The 2mm jet was a fantastic call. I would not have had a clue if I did not find that on the net. The next part of this is being sure I have enough fuel at top end. At the 2mm, and factory guessing for same builds..it is probably in the 80s mph. That will get tinkering in the spring or around 50s F..a couple of months.

just have to wait out. not bad. less than a week, all squared away. the bill is still $102.50

I had to turn the idle up a bit, the choke is a big one..it needs all of it. the runtime is getting helped along. I cannot describe maine in february. it can do all gadgety things known to man in for good. The rochester choke does have some strong stuff..but that weird little middle piece burns easy...and needs an operation to get at it.

[Barry Donovan]

math gas

getting nerdy here.
not sure what I am doing to come up with this.

210cfm needs 1.07 pounds of fuel. (derived from someone elses 355ci engine at 567cfm)

every 7 minutes, roughly, is a gallon at WOT.

say wot for 210cfm is 115 mph (close to a factory number). That is 1.92 miles a minute..
therefore a gallon at WOT will travel 13.44 miles.

so to figure how much I am missing...determine the death of top end (fuel run out) and compare to above..and then determine which jet to try next.
as of now..about 8mpg short of maximizing.

will figure out exacts as highway time can (have not been on a longer trip yet)

13.44 compared to 22. the jet would need to be almost as fat as the one I took out...and am not going there.

so I may be with a slow sube at the top, but low and mid is as conquered as one big source of air allows to stay pleasant clean burn..

EDIT:

woops. I took an 8cyl to four and forgot to double.

so at 26 mpg it is 115, the carb is set to dump down to low twenties.

this could be the setup for the 130mph loring goal.

ignition would be the run out before fuel.

maybe grab at an msd with advance curve.

[Ron Ward]

I know this thread is devoted to the Rochester 1GC, but what about a Carter BBD? Great sized 2-bbl for a boxer (I had one on my 1966 beetle.)

Ron

[Barry Donovan]

I know this thread is devoted to the Rochester 1GC, but what about a Carter BBD? Great sized 2-bbl for a boxer (I had one on my 1966 beetle.)

Ron

it can do alot of different carbs. I even spotted a stromberg blo through from an old bmw.

and weber 32/36 blow through (rare. they wanted 600 on ebay)

I went with a 43mm 1bbl, it just hapens to be monojet..I learned them years ago in real maine daily life. (I had to do something) Getting credit for doing well enough to be something to listen to was at a military job normally giving me shit. Never forgot it. What I did with it actually corrected some engineer problems. well worth it. They did not have to stop making them with hillbillies like me working the fix. I had my pennies going into mid 20s mpg on a 250 I-6 that could not even hold its intake up correctly.(rumored direct hit with nuclear - possibly 3 mile island)..back to now, and a pig infested subaru..

the reason for this carb on this engine now is the cams are called SPFI (single point fuel injection), I installed them, you can even have a slob of carb and the cam only accepts near like fuel injection. The draft is so close to zero, CAI changes horsepower. I like this, and it proves another point I always wanted to make.

alot more revolves around injection, because injection is errored badly. Add carb to the same ass kissing of an engine and wo. A legend that could have been.
I went as far as adding solid lifters to push boundaries even further. Very satisfying.

3psi fills the bowl, and reading into this thread, every move is a physic (not many know to mention) is conquered.

the engine may let go, 25 year , 3 headgaskets, an alaska car dealer sticker, and a maine registration.

it deserves a long nap does it not? I am doing the opposite with it.

[Barry Donovan]

fuel canister

up and running, tinkering with a few vacuum lines. reminds me of a 60s car to open the hood, but with a modern fuel vent can hooked up. The open areas allowed me to find spots on top of the engine to get a blast of cleaner. It used to be smothered in japanese insanity with a touch of california...or is that cali insanity with a touch of japanese..

the vent can is a closed bottom,one hose exit, the size of a pcv line. bimetal experiment apparently (subaru loyale- and it is one that never dies)




The error in alot of cans is no check valve for the fuel bowl vent. I simply used an old pcv valve, down at the canister. You know this is working when the valve gets warmed with physics...that is the monster I had to stop from getting back into fuel vent.

one way clean, and keeps the superlights (those are cleaners in modern fuel) in the engine.


Not being able to recycle back to engine in any way, still allows the can to work. in fact, letting what escapes fuel vent, into the line, into the canister...one way, is the winner. Every start allows it to go where they designed to go back to earth. I ponder in the evenings (my car is not far from a window), the bottom line is as if to be on a slow filter, lets it out long after shutdown..and it is never the odor of fuel of course.


The monojet was lively enough to gain a wet spot at the fuel entrance into float bowl..and never leak. This occured within a minute after check valving the fuel vent. Cleaners are in, monster is out..all with just gasoline from the station.

This one fix was unintentional in the early 90s, 74 chevelle..it occured to me, I can take a carb from a pile of manure and run it as modern..the check valve is the win, after cleaning the carb first. of course.

this also aids in several miles per gallon, stops freak fuel stoich accidents.

[Barry Donovan]

fuLL throttle


the red in front at throttle cable can be a fancy piece. I put that in to get it up and running. the back piece where white zip tie is, that is the lucky spot the sube had in it already. I simply ground a notch and dropped it in, tightened with original cable nuts. This gets full throttle. Anomoly of the old engines these came from. Not many were setup to get the whole long throttle.

I am counting on this to use oil getting through this, maybe even some coolant. I have not adjusted the accelerator pump to take on less, will count on idle screw for the time being. This is getting richer already (good news).

I smelled this seat in today. my cleaners took a flying invisible hike...it took more than 6 days.