4 barrel carb vs performance

[cathken]

what is the affect on performance if the secondary valves of the 4 barrel carb are not opening upon acceleration.what is the likelyhood that secondaries do not function and how can this problem be verified?

 

[joncrisler]

Re: 4 barrel carb vs performance

You would see reduced high-rpm power, slower midrange acceleration and possibly slower times from a standing start. Other than reduced power, and assuming nothing else is wrong with the carb, it should not cause any permenent damage. If the secondaries not opening also causes a lean or rich condition, this may harm the engine, but generally this does not happen.

You certainly want to get this fixed.

 

[45Auto]

Re: 4 barrel carb vs performance

what is the affect on performance if the secondary valves of the 4 barrel carb are not opening upon acceleration

Not much until you get over 2000-3000 RPM. Your engine doesn't require the additional air & fuel until it gets to mid-range and above. Above that point the acceleration will be noticeably slower.

what is the likelyhood that secondaries do not function

I would say well less than 1% (in other words, less than 1 out of 100 engine problems are due to secondaries not opening).

how can this problem be verified?

Warm up the engine (some secondaries are connected to the choke linkage so they can't open till the engine is warm). Take the cover and spark arrester off the carb so you can look straight down it. Take it out ON THE WATER and run it under load. Look at the carb and see if the secondaries are open. If you have vacuum-operated secondaries you won't get enough load on the engine to get them to open if you test it in the driveway on muffs, or on the water in neutral. The primaries are easily capable of providing enough air and fuel for the engine to reach Wide-Open-Throttle with no load on it.

There are 2 types of secondaries, vacuum-operated and mechanical. Vacuum-operated are controlled by the engine's vacuum as it requires more fuel and air. It's possible that type isn't opening if the vacuum system has a leak.

Mechanical secondaries are connected to the primaries with a mechanical linkage. It would be extremely rare to have a problem with mechanical secondaries not opening.

In addition, some types of carbs have a spring-loaded air-valve (looks like a choke plate over the secondaries).

It would help if you posted what kind of engine and carb you have or a picture.

 

[John_S]

Re: 4 barrel carb vs performance

If you have a stock 4brl carb, it is highly likely it is a vacuum secondary. If it is a Weber or Q-jet it will have mechanically linked secondaries, but the air valve controls airflow, via a spring or weight. Holley uses a active vacuum to open the secondaries themselves. As 45auto has said, you would have to test under load. You want to check them at wot, and don't expect the airvalve to be fully open. Most of the carbs are oversized for the application, and will not fully open.

 

[cathken]

Re: 4 barrel carb vs performance

thanks for all the responses.i am operating a 454 merc bravo with a weber 4 barrel.the boat gets up on plane and operates as follows;15mph 3000 rpm and a max speed of 18mph @3600rpm.there is no sense that the power is what a big block should yield.the boat bottom and drive are clean and the drive and coupler are new.i will operate at wot and check secondary valves to observe if they open.thanks.

 

[Maclin]

Re: 4 barrel carb vs performance

While you are in there make sure the throttle linkage is actually opening the carb all the way up.

 

[45Auto]

Re: 4 barrel carb vs performance

Check the fuel pressure going to the carb.

 

[Bondo]

Re: 4 barrel carb vs performance

i am operating a 454 merc bravo with a weber 4 barrel.the boat gets up on plane and operates as follows;15mph 3000 rpm and a max speed of 18mph @3600rpm.

Ayuh,... I'd start with a Compression test,...