The Stovebolt Site Forums The Shop Area Engine Shop Carter afb carb

Ok here goe's.I've been working at a local marina since march.Suddenly I get a customer complaing about a poor running motor.He say's that our shop just put in the motor 2 seasons ago, well i checked the records, and yes we did.The port motor actually has been replaced 7 times in 10 years,keeps blowing up!Did all the basics, comp, timing.vacume-getting low along with compression.I tried switching carbs, then port ran great and starboard ran crappy!Upon comparing carbs, I see the port carb is missing the upper air valve on the secondary.{the weighted plates}I truely belive this is causing a very lean condition and burning up the engine every couple years.Am I correct?

What are the carburetor identification numbers stamped on the edges of the mounting flanges?

Some carbs have the airvalve, some do not.

Generally, both engines would have identical carbs.

Jon.

Good move, swapping carbs! Are the carbs the AFB model with weighted air valves, or AVS (air valve secondary) with the spring-loaded valve at the top of the secondary air horn?

I'd suggest comparing part numbers, then try to find an exploded view of the carb that matches the number. It sounds like you've found the problem!

In my experience, marine engine builders have a habit of doing some really oddball things where carbs, cams, and compression ratios are concerned, particularly Mercruiser.
Jerry

carbking,I don't have the carb numbers on me, but the other carb does have the airvalve.Also I did read that some have it and some don't,but they should be the same on both engins.The owner did say that when motor was first put in this time it ran better than before but keeps getting worse,I think because compression is going away-less vacume to pull fuel in.I just spoke to marrysville mairne and they will send another carb with the airvalve in it.Hopefully this fixes it before it blow's completely as he will want his money back from a couple previous motors.geeze I hope this put's a feather in my cap!
Yeah hotrod,these are the afb carter's weighted air valve.I've worked on quite a few mercruisers,always messing with your head on stuff! I do like the quadrajets though,simple to set up and work right.{some people hate them}
This one I'm working on is a chrysler big block though.
Back to mercruiser, I've got one that the owner swapped intake, edelbrock, swapped carb, holly 750,installed electronic ign,from an old car.
now I can't seem to get it through his head that the motor was probbally wore out to begin with and all he did was multiply his troubles......I think I'll just pull out my hair=throw my arms in the air-and crack a beer!!! lol
Thank's guy's for the reply's

Grub - Without the numbers, difficult to do more than guess, but a little background information:

Carter used the AFB on most larger marine applications from about 1960 to 1967. These (mostly) have the weighted airvalve which is BELOW the secondary venturi, but above the secondary throttle plates.

In 1968, the AVS was used on some of the smaller V-8 marine engines, typically from 307 to 350 CID, while the AFB was retained on the larger V-8 engines. The AVS has the spring-loaded airvalve at the top (many call it a second choke valve).

Chris-craft used a number of different AVS units. These did not work out well (mostly, the AVS did not work well on any application), and by 1973 this bad idea was history. AFB's were again used on V-8 marine applications.

Carter offered a Coast Guard approved AFB as a "service" unit to replace the AVS, when needed.

I was always of the opinion, that a twin-engine boat should replace both, if one had problems, but not all owners agreed with me. I still believe that a twin-engined anything should have absolutely matching engines.

Assuming that the carbs in question are either original or a correct service replacement, either should (if in proper tune) run a single engine well. The fact that one does not indicates (A) either the wrong carb, or (B) a correct or service unit in need of work.

As to your comments on Q-Jets, I love the Q-Jets on passenger cars (almost as good as the Carter TQ's); but do NOT like them on trucks larger than 1/2 ton, or boats. Much prefer a square-bore carb for either of these applications.

Jon.

problem fixed! installed air valve and now goes well, thanks guy's for the good info!