Fuel Injection Question

[Eric V.]

The 1957 FI units 4520, 4800, 4960 all used a restrictor inserted into one of the rubber tubes connecting the choke to the main control diaphragm for the cold signal boost function, but I have never been able to find any "official" information on the size of this restrictor. My '57 parts book lists two restrictors for this application called "Restriction, signal modifier" in group 3.330, but does not list the restrictor sizes as it does for the restrictor tee used on these units. The only reference I have ever found that states the restrictor size is in the Clymer book: "Corvette V8 1955-1962 Complete Owner's Handbook". On page 325 it states the restriction size is .036" diameter for the 4800 units. Does anyone know what size Rochestor specified this restrictor to be for the 4520 and 4800/4960 FI units and any later units that used this restriction?

Eric V.

[Tom P.]

Re: Fuel Injection Question

John DeGregory has the different size restrictors available. It is quite possible that John knows the answers you seek, PLUS, it is highly likely that John will respond right away.

John----------------your turn!

[John D.]

Re: Fuel Injection Question

Tom, Thanks for giving me a turn even though I am off today. New rule for old JD. Mon-Fri = customers POC FI's. Sat and Sun I work on my own stuff.
Eric, 57 7014520's typically use a .042 signal boost restrictor.
'57-'58 7014800/7014960's typically use a .035-.036 restrictor. Although they are available in the catalogs it doesn't take a genius to make them. Just get yourself a nice piece of scrap 1/4" brakeline or even a 1/4" OD copper line. You only need a very short piece. Say 1/4" X 1/4". Then fill it up with solder and use a micro drill bit of the correct size. Or you can pay $ 12.00 for one that is made out of a solid piece of aluminum. Whatever.
This restrictor goes in the rubber hose that is connected to the "goose neck" shaped vacuum line that comes off the front of the choke housing. The one pointing toward the radiator. Mechanicals that installed new rubber hoses on old FI's typically would not pay attention and throw the hose away along with the restrictor in the center of that hose. John D.

You didn't ask but remember that the branch T fitting on the fuel meter main diaphragm cover has a restrictor in it of various sizes. The restriction points toward the firewall.

[Eric V.]

Re: Fuel Injection Question

Thanks John. I made the one for my 4800 unit out of a main jet from a Rochester 2-bbl. I always wondered if the .036" given in the Clymer book was correct for a 4800. I find it interesting that most of the Rochester info I have seen on FI as well as the FI service info in the shop manuals don't acknowledge the existence of this restrictor, or if they do, only in some (not all) schematics, with no information on exactly where it is located.

Eric V.

[Tom P.]

Re: Fuel Injection Question

And, as an added comment, just for clarification, in case someone is confused on these restrictions, the restriction in the hose coming off of the choke housing IS COMPLETELY DIFFERENT from the restriction that is in some T fittings on the main control diaphragm cover over on the fuel meter.

[Chuck S.]

Re: Fuel Injection Question

Hopefully on Monday John D. can tell us what the function of this restrictor is and what the result would be if Bubba left it out of my 4800.

[Jim L.]

Re: Fuel Injection Question

....and what the result would be if Bubba left it out of my 4800.

It's possible you'd never even notice. It all depends on how cold it is when you try to start your '57 with a 4800/4960 unit.

That restriction limits the amount of signal boost that is applied to the main diaphragm when temperatures are very cold. (the meaning of "Very cold" is determined by the adjustment of the bakelite cold enrichment cover).

If the temperature isn't "very cold", there won't be any signal boost vacuum from the cold enrichment housing and the presence or absence of that restriction won't matter.

For a 4520 unit, signal boost is the only means of achieving cold enrichment, so if the restriction is absent, you'll get too rich a mixture while the engine is cold.

How'd I do, John?

Jim

[John D.]

Re: Fuel Injection Question

Hopefully on Monday John D. can tell us what the function of this restrictor is and what the result would be if Bubba left it out of my 4800.

Chuck, Easy answer is this. The purpose of the restrictor is to cut down on the vacuum signal going to the fuel meter. Without the restrictors on a 57 unit they are a wild beast that run very rich.

The 57-4520 unit was RP's And GM's attempt at getting 283-283 so a typicaly 4520 unit that has not been massaged tends to run rich. Restrictors can tame it down although reworking the air meter is a more scientific approach to the answer.

Nothing like a 4520 on a 57 to push you up against the seat on acceleration. Althought he unit is a little crude it does run strong. On the other hand don't ask me about the 4360 unit which is Bruce's favorite.