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Need some carb info. I have a Monojet carb # 17057009 which I believe is from a 77-78 Chevy truck 250/292. I want to install it on a friends 261 engine. The base of the carb has 4 holes being vacuum passages. The gasket which is called for blocks off 2 of the passages. Is this normal? Thanks Dave

Are you using a gasket for the 1977-1978 application or the 261 application? I'm not a monojet expert but didn't want to see your question unanswered.

77 Rochester ME.

If someone has a 1977 Service Manual, they could look up to see if it's for a 250 or a 292. The manual should have the vacuum hose routing (I only have the 1980 manual, so I'm not sure)

I believe one of the holes in the base is a vacuum port for the EGR system. You won't need that with your 261 so it's OK to be blocked. One of the holes in the base could be providing vacuum for the thermostatic air cleaner - probably not using that so it could be blocked.

One of the holes would be for the power piston - you don't want to block that.

DaveV, As you have observed the 261 only has two holes where as the Rochester Mono jet has four. If you go to U-tube Rochester rebuild (part 1 & 2), it shows the bottom of your throttle body. it does not look like it lines up with two vents on the Rochester model "B" which is the correct carb. for the 261. However if you can measure the throttle plate mounting hole's center to center I may have a gasket that will fit that I can send you. Other wise get a piece of blue typewriter copy paper and make an impression of the bottom of the Monojet and cut your gasket, line up with 261 intake manifold less two holes. To make it easy do a pencil rubbing of intake on white paper and see if the two hole's line up with two of the four holes of the Monojet. It is 4 a.m. here so I don't want to go out to the barn workshop now but I will check back with you this morning later and see how it's going. I may even have a 250/292 throttle body that will convert the fuel bowl to the new throttle body. Check back later. Doc

Good Morning DaveV. Its a little early here. Just got inside from the barn shop and getting ready to take pictures, as per our wonderful fearless leader's instructions to me. My wife is going to help. Look forward to what I may have awkwardly described at 4 a.m. Doc.

I looked up a 77-78 truck on Rock Auto and it shows a thick gasket with 4 cut out vacuum passageways to feed these holes. I do have a variety of these gaskets and all of them cover up two of the holes. I guess I could take the carb apart to see which ones are blocked off. Thanks for the info. Dave

GUESSING this carb will need MAJOR recalibration, as this carb was designed for smog emission engines. Not saying you should not use it, just suggesting it might be more than just bolting it on and hooking it up.

The company my Dad worked for bought 3 new Chevy 6 cylinder trucks in 1977, 1978, or 1979, I don't remember exactly which year (they would alternate brands in different years, and this was Chevy's turn). Those things were so lean they were virtually impossible to drive.

Jon.

Weigh the Monojet and compare it to a Rochester B. Use whichever one is heavier as a paperweight and trash the other one!
Jerry

Jerry! That is harsh (even if you are almost 100 percent correct). Instead of weighing the two, you keep the cleaner one for the paperweight, and the dirtier one for a doorstop.

Jon.

There actually is a worse Rochester- - - - -the E2SE Varajet. That's the progressive linkage 2 barrel with an air valve in the secondary passage and a metering rod that has a habit of falling out and jamming a throttle plate open or getting imbedded in the top of a piston. I believe the early 2.8 Liter V6 used that one.
Jerry

Some integral head 250 engine came w/ varajet

thanks for the info Jon even though it's not what I wanted to hear. Maybe need to scrap this idea. Dave

When I was working to get my 261 going 16 years ago, I found a Rochester Monojet (170560130) in a local junkyard. It had FIVE vacuum ports on it. That alone made it a loser for me. Where do 5 ports connect? I set the 170 aside as an objet d'art or a monojet d'art.

I then happened to find a Monojet on EBay (70334061; 7028011 stamped on a aluminum disk). I wasn't wedded to a Monojet but the price was right. It had only 2 vacuum ports so that made it a winner for me. I blocked off one and the distributor got the other.

I've been happy with the 703 Monojet since the day I started the 261 in 2004. The fact that I now have a well-regarded Carter YF 2100S still waiting in the wings speaks to my OK with the 703 Monojet.

Wouldn't a Carter YF for an American Motors/Jeep 258 engine be a pretty good choice for a 261? They don't seem to be as pricey as one for a Ford 300 six, and the engine displacement is a lot closer to correct.
Jerry

I used to service a lot of trucks with 250-292 engines & Monojet carbs. We had real good luck with them. You can tailor the fuel mixture by adjusting the height of the metering rod.
The extra vacuum outlets can be capped off.
George

Thanks for the input. If I remember correctly, the 3/4 tons and above in the 70s did not have near the emission standards as the 1/2 tons. This carb only has 2 external vacuum ports which I would think is the better carb. Have plenty of time now to try it. Dave

Dave - you are correct (from memory, through 1977). However, hotter thermostats were being used, making the engines more efficient; and the carburetors were calibrated leaner accordingly. Smog emission is a lot more than just a smog pump.

Not saying don't try it; but would suggest absolutely no load until you do some checking with an exhaust gas analyzer. Carburetors are cheaper than engines.

Jon.

Good advice Jon. Thanks

Hey carbking, any thoughts on AMC Carter YF from a 258 ci to use on a 261?

Quick thoughts only:

The first 258 of which I have a record is 1971, which means smog emission.

I personally don't like to put smog emission carbs on non-smog emission engines for two reasons:

(1) Recalibrating can be quite difficult and/or pricey.
(2) The 1966 amendment to the Clean Air Act of 1963. The key word is "tampering". The penalty is horrific.

One can possibly get by (2) above if one obliterates ALL identification numbers on the carb (thus it could not be used in a smog emission environment). This is how the racers get by.

As a licensed shop, I have to be cognizant of, and abide by, that law.

Now, before someone posts "my state doesn't do testing before....", remember it is a Federal law. And then remember Civics 101 "the Federal Government is more powerful than the state".

I personally do not see the need, as there are plenty of Zeniths from 270 CID G.M.C.'s and 269 CID I.H.C.'s. Yes, the initial cost is more expensive. But what do you think you will pay for Carter metering rods and vacuum springs?

But regardless of the need, (2) above is the reason I will not help anyone do this for any amount of money. This is true of ALL O.E. carbs for 1968 and newer applications, including the one which started this thread.

So, the quick thoughts in this post will be my only thoughts on the subject.

Jon.

just as aside, harbor freight has a hollow punch set for like $9 with itty bitty hole punches that work really well for making gaskets for this sort of thing.

Good information, Jon, and an excellent business decision as well. One cannot risk his livelihood to put a big target on his back, even if he has the knowledge to make those modifications. The Clean Air Act of 1968 that brought about a lot of the smog regulations provides for a $5,000.00 fine and/or 5 years' jail time for EACH example of making an emission-regulated vehicle (or part) non-compliant. Back in the mid-1980's a muffler shop in Nashville was caught with 4 vehicles ready for delivery to the customer with "catalytic converter test pipes" installed. The "customers" were federal agents. The shop owner paid $20,000.00 in fines and went out of business shortly afterwards. I fought that battle in California in the 1970's, and their regulations exceed the requirements for America. I was "encouraged" to make illegal modifications by drive-in customers I didn't know, and their requests just didn't pass the sniff test. "Nope- - - -try some other shop!" I was running a dirt track race car at the time, and some of the more radical proposals that didn't make it into their regulations were to make emissions standards apply to off-road vehicles, including race cars. We did have to run mufflers at some of the tracks, but that was the result of local noise laws, not statewide requirements!
Jerry

Jerry - we used to get all of the bulletins from the California Air Research Board (C.A.R.B). Often, there were several pages of violators that were caught and significantly punished. California did, and does, have a smog problem.

California began smog emission requirements in 1966, and were always more stringent than the Federal requirements.

Several years ago, California did (either SB 42 or HB 42, I don't remember) relax the more stringent California restrictions back to the Federal level for 1966~1974 vehicles.

Unfortunately for ALL of us, the first 15 years or so of smog emission standards had absolutely NOTHING to do with smog reduction and EVERYTHING to do with job creation! (OK Jon, get off the soap box )

Jon.

I opened a one-man auto/diesel/road service shop in central California 1975 after leaving the U.S. Air Force. On a typical workday, I needed to be sure to complete a job and get paid so I'd have something to eat. The back room of the building I shared with an auto body shop served as a bedroom occasionally, when I couldn't afford rent. C.A.R.B. had to be an anagram dreamed up by some sadistic bureaucrat in Sacramento! Those regulations came very close to running me, and a lot of guys like me out of business!
Jerry