help with Rochester Carb

[mr300z87]

Yesterday I reassembled the Rochester Quadrajet #1708-0563 which originally came on the 7.4 L that was removed from my project Sea Ray (see signature file). All was going well and I was setting the basic adjustments in preparation to sell it to the friend that bought the frozen engine from me, when I noticed a hair line crack. I picked at the crack and the hole shown in the picture was the result :facepalm:.



According to the manual that came with the rebuild kit this area of the carb houses some type of Idle compensation valve assemble for a different application as there is no mention of this in Mercruiser manual #9 or #15. I this carb now junk? Can I should I fill it with JB weld or possibly make an aluminum cover for the whole area? My running 5.7L with a Qjet is tucked away for the winter so I can't even compare them.

The replacement engine is set up for a weber carb so I was just trying to fix up the Qjet to sell and make some money to by other parts needed for the project. I do not how ever want to sell something that is junk to a stranger let alone a friend. As always any info is greatly appreciated.

Mike

 

[mr300z87]

Re: help with Rochester Carb

Not being able to leave well enough along I pulled the top back off the carb to see where this hole went and what I found in shown in the following pictures.

as seen the opening it into the gasket flange on the center section of the carb.

It will be completely blocked off by the and sealed by the top section once bolted down.

My guess would be that there would be additional machining and parts for the ^^^ mentioned Idle compensation system for non marine use. So I am thinking as is it will be of no harm or I could fill it in with JB weld and let me friend try it.

 

[NHGuy]

Re: help with Rochester Carb

Hi, Intriguing problem. I just spent some time going through my Quadrajet book and couldn't find any mention of that chamber.
You have a couple of options. You can assemble the carb and spray some carb cleaner in the hole to see if it comes out somewhere else. If it does not it might be usable as it sits.
You can fill the hole just in case it matters. Or you could try running it on your good motor to see if it is a vacuum leak. It might not do anything.
I'd JB it or marine tex it. Just cover the hole, try not to cram extra material inside the hole.

 

[NHGuy]

Re: help with Rochester Carb

There are loads of different configurations with different idle air bleeds and things. The part you are looking at is way on the other side from the primary area. Ain't any idle air in there. It's in the secondary area, so see how it acts at full throttle.

 

[LEROYDOZOIS]

Re: help with Rochester Carb

That's extremely rare... that's a casting fault and a one n a million (or few hundred thousand ) thing...

Cliff rouges book mentioned castings with porous issues in the very early castings from before 75...looks like u acquired a remaned core that just happen to have a funky fault like this.... plug that up with jb weld or something similar and be done with it...

I've stumbled upon a few qjet cores with this issue (in other areas) its enough to make someone crazy when ur trying to find a vacuum leak thinking you effed up ur qjet and its a flipping hole in the core...

I'm rebuilding a 1979 qjet right now, its a reman from a few years back... the casting is buttery smooth compared to my barely used Volvo 290's carb that's a pre 75 ish core

 

[wrench 3]

Re: help with Rochester Carb

Filling the hole would be strictly cosmetic since that whole area is redundant on your carb. The cavity was for a thermal vent that opened up a passage from that raised opening in the air horn, through that closed hole in the bottom of the cavity, to below the secondary throttle plates. Since both passages are blocked off, whatever happens in between has no effect.

 

[dan t.]

Re: help with Rochester Carb

You say the replacement motor is set up for a AFB/ Webber/Edelbrock/ POS carb, the best thing you could do is remove the adapter plate, there is a stock q-jet manifold under it , bolt on your rebuilt q-jet and go boating.

 

[mr300z87]

Re: help with Rochester Carb

Thanks for all the responses.


Wrench that is exactly what I was thinking. Come spring I might bolt it to the engine and test it. Right now it snowing like crazy! :blue:


Dan what's so bad about the Webber/Carter-AFB carb, been running a similar carb AVS on my Plymouth for years never a stitch of problems, the 426 hemis ran 2 of AFBs and we all know what they are capable of. Do GM engines like GM carbs better? Lol. Btw I do not want this to be a Chevy/Mopar/Ford or Carter/Rochester/Holly debate. I am just a Mopar guy stuck working on GM engines because I like boats but I know my boat would be faster if powered by a 440 instead of a 454.

 

[Bamaman1]

Re: help with Rochester Carb

Don't go badmouthing Chevrolet V-8 motors used in the marine industry. The engines are actually closer to industrial engines than they are to automobile/truck engines.

Mercruiser goes into the GM foundry periodically and they have engines cast to their specifications. The metal's even a little different than what GM put into their street engines. Then Mercury takes the castings back to their factories to clean up and do the machining prior to assembly.

And the engine builders at Mercruiser are second to none. They plastigauge every bearing, and have access to a bunch of different bearing sizes to assemble the engines. (They do no re-machining.)

You see some of these very high horsepower 502 cubic inch motors on offshore racing boats. They're built with the care and quality of a huge NASCAR engine. And not many engines in the world will stand up to 10 hours at full throttle--like a Mercruiser V-8.

Chrysler used be a big supplier of inboard boats. But they never had any engines that had the guts and performance of a modern big Mercruiser engine.

Your 454 is a relatively small engine by today's marine standards. But with the modifications, a different valve train and other internal marine components, they'll do a great job for you.

 

[dan t.]

Re: help with Rochester Carb

Whats wrong with Carter carbs? That could take 2 or 3 pages, but I will try to be breef, the metering isnt near as precise as a q-jet, the primary venturies are single, the q-jet is double, the secondarys have fixed jets and a counterweighted air valve, the q-jet uses metering rods and adjustable air valve, carter has 2 floats and needle valves,q-jet uses 1, and on and on. But thats just my opinion.

 

[mr300z87]

Re: help with Rochester Carb

I am not bad mouthing MerCruiser or GM, as most of the the GM engines Merc and Volvo uses are tried and true, I am just a fan of big block Mopar engines not GM (especially the 5.0L 305) and that is what makes the world go around. We could debate this all day long!!!!!!!!! The 5.7L in my Invader runs awesome and I have no complaints. I know Chrysler made alot of small and big block marine engines way back when and I had a co worker who's Penn Yan was powered by a 318.

Dan I will do some more research on this Weber/Q-jet (back in the 80 we used to call them Quadra-Bogs) debate. The guy I bought the 7.4L from who is a marine mechanic (not sure how good since I don't know him) recommended the Weber over the Q-jet. I think it is kinda funny though that Merc went to the Weber carb before completely switching to FI, Volvo used Holley carbs and now Edlebrock sells a brand new Weber style carb. What happened to the GM Q-Jet tooling? Just curious. BTW other than my 68 Plymouth I have not owed a car with a Carburetor of any kind in 25 years so my Carburetor knowledge is limited to the Carter AVS. Trying to learn.

Have a great day

 

[wrench 3]

Re: help with Rochester Carb

The only complaint I have with Quadra-jet is that the float valve is in the bottom, and if it sits idle for a week or so, the fuel drains back into the tank. Then when you go to start it you have to crank up a new batch.

 

[mr300z87]

Re: help with Rochester Carb

Wrench, What are your thoughts on the Weber carbs?

My 5.7L has a Q-jet and I have not really had to touch it. Although late last season I thought the idle was a little high so I lowered slighly with the idle screw, then while at low RPMs (no wake or docking) the exhaust smelled a bit on the rich side. Would lowering the idle and not adjusting the mixture screws cause this? I have since winterized and she is covered waiting for spring.

 

[wrench 3]

Re: help with Rochester Carb

A minor idle speed change shouldn't affect the mixture, but it wouldn't hurt to tweak the mixture screws.
Weber always made a good carb, but I'm afraid that I haven't had any experience with them in marine applications.

 

[Fastatv]

Re: help with Rochester Carb

Wrench, What are your thoughts on the Weber carbs?

My 5.7L has a Q-jet and I have not really had to touch it. Although late last season I thought the idle was a little high so I lowered slighly with the idle screw, then while at low RPMs (no wake or docking) the exhaust smelled a bit on the rich side. Would lowering the idle and not adjusting the mixture screws cause this? I have since winterized and she is covered waiting for spring.

The idle air mixtures screws on your marine app are essentially the same as an automotive Q Jet. When you fire it up the spring, ensure the flame arrester is clean, get the engine completely warmed up, and at normal idle, then adjust each mixture screw for max idle RPM, then adjust idle speed accordingly. This is best accomplished while boat is in the water, unless you have thru transom exhaust. Rick