525sc dominator 1050 carb question
 

I need to know if the stock 525sc gen 6 carb (dom 1050) are already boost reference? I dont know how to determine if it is or not boost reference.

Also what is the right procedure to determine if you have the right PV on a boat engine?

I may be wrong but i thought boost reference only matters in a "blow through" application......

fixx
 

if it were boost referanced it would have a vacume port on it for a vacume hose that would go to the intake..

also see if it has 3 air bleade jets per ventury or 2 air bleeder jets..

Not boost referenced and I don't feel its needed for them. They run like an efi when properly adjusted.

The stock 6.5PV's work just fine in them. I had a bigger roller cam and was running almost 8# of boost and it idled and ran perfect.

That is for an upgrade 525sc with aluminum and b&m 250 blower and now i am in tuning of that engine and dont know if it need to have a boost reference carb. What is the right way to test if you have the right power valve?

I will take some A/f ratio # this week end see if i am ok

If you have a boost/vacuum gage on the dash, note what vacuum you run a cruise speed the select a PV.5-1 in less in vacuum. I run 5-5-5in vacuum at cruise so I run a 4.5in pv.

I also validate with my A/F meter to ensure it is not getting too lean before it opens. Thats the best way to do it.

That works ONLY if its boost referenced. The standard 525SC merc is not. Like griff said 6.5PV.

If you wanna be safe, you can put a 8.5pv. Only way to really check is either have a vaccum gauge under carb, but on top of blower. This is where the PV gets its signal from.

Boost reference the fuel psi regulator in a blow thru app.

I have the vaccum gauge that i can plug under the carb (on top of the blower) do you chek vaccum in drive at idle to determine the pv ?
What calculation +or- of the vaccum # ?

Thanks.

fixx
 

most boost gauges have a vacume part on the oposite side of the gauge..just install one in your dash to keep a eye on boost pressure..killl 2 birds with one stone..also make sure you have plenty of fuel pressure..at leaste 7 psi..

Mine is NOT boost referenced and this procedure works perfectly. My vacuum/boost signal is taken off the back of the intake under the carb, stock 525sc location. How do you check vacuum under the carb but on top of the blower? That makes no sense. I have stock 525sc top end and know my method works. You always need to check using this procedure when making any major changes like going to a 250cfm blower from a 177. To just tell him to run the stock 525sc 6.5 PV (or to even change to an 8.5) without any data of the engine running is going WAY out on a limb. Checking vacuum and a/f is imperative.

Hold on a second there big guy. You CANNOT tune your power valve based on the manifold vacuum, IF the power valve is not boost referenced. Other wise, WTF is the point of boost referencing. The entire purpose of boost referencing is so that the PV signal is based on the intake manifold vaccuum.

On a 250 blower carb plate, there is a fitting that you can install a vacuum gauge. If the pv is not boost referenced, you can install a vaccum gauge in that port, and it will show what vaccum the non referenced PV sees. The vacuum directly under the carb is what a non Referenced PV sees. Makes total sense to me. Oh, and the 420 B&M blowers have this too.

He asked what the stock PV size is in a stock 525SC, and I told him. Back in the day before boost referencing was popular, holleys supercharger carbs came with 10.5PV non referenced.

Ive ran 177's, 250's, and 420's on my engines. I think before you go telling me I don't make any sense, stop by a Border's or Barne's and Noble and get yourself a book on Carburetors.

Oh, and BTW, I ran a 1050 dominator on a 454 with a B&M 250. 6.5PV's, non referenced. By telling him he can install a 8.5 to try, it would only make it open sooner, and be fatter. It would not, cause him to lean his engine out. Its not as if I said install a 2.5PV and go boating.

Ok guy i have not test my vacum this week end but i have tested my A/F. The A/F that i read at cruise is 11.2 just on the 4 barel opening got 10,5 and WOT 11.5. I am jeting 104 4 corner and i have 6.5 pv (stock 525sc). I think that i have the right pv but litle rich on the primary side at cruise.

You need to watch the front cylinders on a single carb 250 setup. While your AFR can read within spec, youll find the front cylinders can be lean. The blower will push fuel to the rear. Dual carbs much better on this setup.

AFR is great, but fuel distribution can be a issue, so watch your plugs, ESP #1 and #2 cylinders.

No matter with that i realy apresiated every people setup idea . I am new on performance boating but i have alwase do mecanic on deferent engine, but this is my first time on BBC boat engine.

blue thunder,it makes no sence to you because you cant comprehend how a carb works.mild thunder is dead nuts on the money,a non boost referenced carb gets its vaccume signal from above the blower,not from the intake manifold.think about it,it is not that hard to understand.

Also for vacum pick up the dominator 1050 have a plug on the primary side under the carb. i think this a good place if you can't plug it on the carb plate.

You select the PV with an a/f gage. The vacuum gage is used as a reference for when you need to enrichen the engine. We are tuning an engine right? If I ran 8.5 pv in my 1050s w/177 I'd be way too rich in the mid range and wash the cylinders down. You need enrichment at the right time, not early or late with a blower motor. The OP asked just that, how to determine the correct PV size, not what comes on a stock 525sc. Why do you guys always try to start a pissing contest? It really detracts from this site.

blue thunder,its to bad that you have a problem when you are corrected,mild thunder explained some facts,and you dispute it,because you dont understand it,then i tell you he is right,and you think its a pizzing contest.mild thunder,and myself were just sharing information,and because you dont understand you get an attitude,dude,i could care less if you take good advice or not,but i will continue to put it out there for those who can understand it.i guess because you think you know everything,you dont need to listen to anyone,so,CARRY ON and have a nice day.

In order to properly tune the PV opening point, your vacuum readings need to be taken at the point at which the PV chamber is referencing vacuum. In a non boost referenced application, this would be at the base of the carburetor, above the supercharger... anything else, whether it works to a certain extent or not, could be a costly mistake. While using a wideband to monitor your tune is an excellent measure, it's not nearly as difinitive of a PV tuning measure as monitoring vacuum at the same source where the PV is referenced, and it does leave room for error.

The issue (and the entire basis behind externally referencing power valves in the first place) is that at higher supercharger speeds, low pressure (vacuum) begins to build at the base of the carburetor(s), below the throttle plates (where the PV chamber gets it's signal). Should this low pressure build to a point in excess of the power valve opening value, the power valve will close... I'm assuming no one here needs a lesson on how a closed power valve while under a load of boost typically turns out...

That being said, this is often a non-issue in a small blower marine application... smaller supercharger volume, relatively low boost levels, and relatively low engine speed keep the amount of low pressure that builds below the throttle plates to a level at which there is no danger of it ever pulling the power valve closed. Thus the reason why most applications such as the 525SC never used an externally referenced PV... it's simply not necessary.

Every roots supercharged engine I dyno is done so with a vacuum gauge attached at the base of the carburetor, above the supercharger... in a low boost environment, rarely will you see even two inches on the gauge at WOT. Bigger cube engines, big blower, higher RPM, higher supercharger speed... it can become an issue, and if PV's are even being utilized in such an application, they'd better be tuned correctly... on both ends of the spectrum... opening at the correct time during engine acceleration, and not closing back up at higher engine speed/boost level. Also, I check every PV with a hand vacuum pump and checking chamber... it's important to know that the thing opens at whatever value is stamped in the side. You'd be surprised at how many vary from their rating by as much as two inches of vacuum.

Not getting into the pissing contest here... just putting some facts out there on how it all works.

Excellent post cubic inches! I've seen your posts lately and you seem like a very sharp guy! An asset to oso for sure.

I'd go to the 4.5pv and see if you can't lean it out a little in the midrange.

very well said,i also have read many of your posts,you,r always spot on,thanks.