77 carb with pre-75 rods

[Andyman]

Hello, thank you for admitting me to this forum. I am in the process of getting a 77 c-30 modified truck to run adequately above 10,000' elevation.  The truck has a 454 with about 46K miles on it. I just read the History of the Quadrajet, because I am looking for information as to what happened in 1975 that caused the primary rods to dramatically jump in size, and there is nothing mentioned in the article. Here’s my story:
The carburetor in concern here has the serial number: 17056212 with the production date of 0977, which I believe means this is a 1977 model M4M with a divorced choke;  the carb is in great shape and looks almost like new after it was cleaned up. Here is what I found when I took it apart: Main jet .070 with .026-.031 rods. I lost the power piston spring, but I did in fact measure the length of it as being about .95 long before it got lost.  I did order a set of Edelbrock springs  and tested each one.
The problem here, is that this is supposed to be a 77 carb, but it has pre 75 metering rods. Thinking that somebody put the wrong rods in the carb, I purchased some .052 (post 75) rods and tried them with the lightest power piston spring. At this point the engine starved for gas and the power piston spring did not activate. Next I went back to the stock .031 rod and the engine flooded severely.  After 7 test runs, I now use the 3rd strongest power piston spring with either .026-.045 or .026-.039 rods with .068 main jets, and it is running good except that it tends to backfire if I give it too much gas too fast.  (The timing is currently set to 6BTDC). It runs about the same with either set of rods, and the idle mixture screws seem to have no to little effect on the RPM in either case.
As for the springs, the 2 lightest ones did not allow any action at all, and the strongest spring seems like it’s a bit too active, the 3rd strongest spring is almost as active as the strongest, but seems like the only choice I have left. I did just read that maybe the strongest spring which keeps the rods seated is the best, so should I put the strongest one back in? (since the rods are in fact seated at idle).   
My questions here are:
1.   What happened in 1975 that made such a difference in the primary rods, and can anybody explain why my 77 carb won’t run with post 75 rods? Is my carb a 77 with 75 specs, as indicated by the s/n?
2.   How can I tell if I should try bigger or smaller rods? How do I know if the current rods are adequate, and what might be causing the backfiring when I accelerate too quickly?

[Andyman]

Wow, seems like I pretty much stumped everybody. I did order Cliffs book off the internet. Yesterday I took the machine for a ride and its missing really bad coming off the idle with the .068 jets and .039 rods. It would be nice if I could get some info somewhere as to what the stock rods and jets are for that carb. I really need to figure this out I been working on the carb 2 weeks and still not running adequately. But thanks anyhow.

[429bbf]

does your carb have the front inlet? if it does there usually on pickups with 454 engine and some of them used the 1975 casting . that might be what you have which would answer your question why you have the old style metering rods.if ethan chimes in he can tell you what jets and rods were used in that carb. i dont have that info . hope this helps

[429bbf]

reread your post the carb no.17056212 translates to a 1976 carb built the 97th day of 1977. also a lot of times backfiring is blamed on the carb when its an ignition problem or from personal exp. a flat cam lobe.also went and grabbed cliffs book page 44 shows your carb 17056212 with a front inlet and he talks about it having the earlier casting design and says it was used on trucks,suburbans ,and motorhomes with big block engines. hope this helps.

[Shark Racer]

I'm fairly confident there are no M4MV carburetors (M4MC (hot air) or M4ME (electric)) only.

That part # cross references to a 4MV, which would have the early style rods.

Cars got the M4Ms in 75s, some trucks didn't get them until 79/80.

[Andyman]

Yes, the M4M designation I got off the internet, but my Haynes manual says its a M4V and it does have a forward entry fuel inlet. I don't have the manual yet. It does not backfire unless I try to accelerate, which I think indicates that its not the ignition system, and since I drove the truck 80 miles home without it misfiring, it's unlikely that the cam suddenly went bad after I did the carb work. Also, all of the wires test good with a timing light. I'm going to try the .045 rods again and see if that makes it drivable. It's pretty confusing when ALL the information available says that the rods changed dramatically in size in 1975 and there isn't any information as to why, or the possibility that some carbs didn't change over until later years. The problem is that if I need another size rod it takes a week to get one over the internet or I have to take a 1/2 day and drive to south Denver, but I don't even know what rods I might need to order yet. Thanks for all the help so far... but if there are rod sizes in the book could anybody please post what they are?

[429bbf]

andy im not trying to send you in the wrong direction. and i hope its your carb. my problem was the car(1981 corvette 19,000 miles)electronic carb ran perfect as long as you didnt smash it to the floor and activate the 4 barrel. you could drive it 200 miles ran great. but when you laided it down and really got after it you could hear it backfire through the carb. so i talked to the man and ordered a complete rebuild kit . took the carb apart it was spotless even the idle tubes were clean. i put the kit in anyway because the carbs been on for 30 years. i wasted a lot of time because i didnt check everything out in the first place and i blamed the carb.ended up being a flat lobe on the intake. fwiw

[Ethan1]

Yes, the M4M designation I got off the internet, but my Haynes manual says its a M4V and it does have a forward entry fuel inlet. I don't have the manual yet. It does not backfire unless I try to accelerate, which I think indicates that its not the ignition system, and since I drove the truck 80 miles home without it misfiring, it's unlikely that the cam suddenly went bad after I did the carb work. Also, all of the wires test good with a timing light. I'm going to try the .045 rods again and see if that makes it drivable. It's pretty confusing when ALL the information available says that the rods changed dramatically in size in 1975 and there isn't any information as to why, or the possibility that some carbs didn't change over until later years. The problem is that if I need another size rod it takes a week to get one over the internet or I have to take a 1/2 day and drive to south Denver, but I don't even know what rods I might need to order yet. Thanks for all the help so far... but if there are rod sizes in the book could anybody please post what they are?

 Just to clarify, the carb in question is a 4MV Qjet. This comes from a well-respected manual I have.

[Andyman]

SUCCESS! Next I put the stock .031 rods back in with .068 jets (.002 smaller than what was there already) and it ran like crap. It would not accelerate without backfiring and had no power. I changed all the spark plugs, and it ran a little better but not much. Today I picked up some new wires that I ordered yesterday and now its running awesome! Turns out that the spark plug wires were fine at idle, but apparently just would not transmit enough juice under strain. I can now go to work and test out how strong the hydraulics are with a load in the bed. It's an aftermarket dump bed that is bolted down with blocks of wood to the frame, which when down did not allow the stock gas tank to be filled. In the process of troubleshooting the engine, gasoline kept running out of the stock tank, which I found out was because a valve failed that was supposed to divert the return fuel back to the secondary tank. After removing the valve and rerouting the return direct back to the tank in use, I pumped the secondary tank full with gasoline from the other tank. I don't know how much gasoline is in the stock tank, but I'm expecting quite a bit more than the 30 gallons already salvaged. I can now relax knowing the truck isn't going to kill my budget!

[Andyman]

Thanks everybody and I'm looking forward to reading the book when it comes. From what I've heard the only way to know what the stock rods and jets are, is to obtain a full GM shop manual for a C-30. Is this true? Anybody got one?

[Ethan1]

 I do. For an example, the stock jets and rods that came in your current carb were: Primary Jets .071, Primary Metering Rods 32B, Secondary Holder K and Secondary Metering Rods DH.