Reduce the air valve opening

[makomark]

Related to my recent post on power pistons, I'm also looking to "economize" my WOT operations on my boat. We usually only go WOT to pop the boat up on plane and then back off once she 'pops up'. My OEM data specifies WOT rpm range of 4400. It is propped to get 4400 with minimal fuel and achives 4200 with full load.

I've noticed that the air valves open fully during WOT. My calculations, assuming a VE of 0.85 show just about 500 CFM are needed. the math indicates that the air valve shouldn't have to open fully. The only benefit I see to continue full open use is to allow the full travel of the secondary rods. My thought is to limit the opening of the air valves and then to exploit the now limited travel of the secondary rods to help fine tune the mixture, using different hangers for real fine tuning, if needed.

Has anybody tried this or something similar? Anybody see where the theory breaks down?

tnx     mark

[FORREST MILLER]

seems to me that the air valve shouldn't be fully open until the carb gets towards its CFM limit.....maybe its there at 500. Have you done any air/fuel ratio testing?? I'm not sure if Its the correct thing to do , but increasing the tension on the secondary airvalve spring will do what you want.

[Cliff Ruggles]

The problem with cfm calculations, is that they do not take into consideration specific intake manifolds and cylinder heads.  Most intakes will only flow about 89-92 percent of what the intake runners are capable of. 

There are also quite a few different heads with different flow characteristics. 

In addition, when you start limting the cfm capabilities of the carburetor, at some point the engine will start to see it as a restrictor plate.

The type of metering rod used will also have a significant impact on the A/F ratio, as they are different tapers and tip lengths.

Also keep in mind that the engine will only pull the air that it needs thru the carburetor, regardless what cfm capabilities that it has.  The factory did install several different secondary throttle shafts with different stop lengths to limit or control cfm to some degree.  We often grind most of the stop off, then drill the airhorn and install an adjustment screw, to make the airflap angle adjustable. 

If you plan on doing this, and experimenting with different hangers, make sure that the engine doesn't go lean and experience detonation.  The factory actually drilled a second set of holes in the secondary fuel nozzles on some q-jets that had reduced airflap angles, to help add additional fuel to avoid a lean condition.....Cliff

[makomark]

sstocker31: If there's an easy way to get an A/F meter installed on a 'wet' marine exhaust, please enlighten those of us in the land of ignorance. My understanding is they (O2 sensors) don't like the environment, even the heated ones. Worse yet, there is no spot to tack on a bung for the things, at least on mine. I understand there is a new emission requirement that is driving the new engines to use cat converters...I haven't seen one but can only imagine what the beast looks like with it insulation.

cliff: they still have the long tipped CL rods - factory spec'd. I've seen shorter ones, DP's (from memory), in some installs. If I can find some, I'll add that to the sheet to try in the spring. Do you offer any secondary rods, with short tips, in the leaner end of the chart?

Also, I went with P hangers (were F's) to keep the rods fully seated with the air valve closed. Between that and resetting the float from 1/4" to 11/32", we eliminated the 'weep' from the secondary acceleration ports at cruise and made another noticeable improvement in economy as well as response.The engines have the wedge plate under the carbs but still have an incline when running, back end low.

The heads are typical 'peanut' ports from the mid-80's and the intake is the factory cast iron. When the heads were rebuilt, the machinist opened the port inlets to 'match the intake'. I told him, beforehand, I wanted max torque at 3000 rpm, so not to open them much. Overall, I'd say that we haven't done anything to alter the OEM characteristics by much regarding VE.

regarding throttle linkage; we did change the secondary opening point a bit. I'd say we delayed it by about 5 degrees from the stock setting. we couldn't go any further and still let the secondary plates opened fully.
 

thanks again, for the feedback    mark

[FORREST MILLER]

My mistake, It didn't sink in about the "marine" part .......Sorry

[Cliff Ruggles]

We only stock one new secondary metering rod, it is a copy of the DA rod, .044" medium length tips, rich taper.

I can machine any tip diameter and length from the later model large factory cores, we do this all the time, they plate them....Cliff

[makomark]

Stocker - no need to apologize. I've learned a great deal over the years...sad part is I can't always get it out of the "memory bank". (That what makes those Logbooks worth the effort). Add to that how fast things change and you really appreciate they fact that people like you and Cliff are willing to share.

On the rod machining concept, I'll keep that in mind. I sure if I dig around (or beg) enough, I can find something that will work or function as a core. I trust the machining option exists for primary rods, too?