Blower with Carb questions
 

502 with 10-71 blower/intercooler twin 750 carbs on top. Jet size is 74/84 with 4.5 Power Valve. Set timing on stand, installed in boat and when in gear at 2800-3200 engines will flood out. PV is not boost referenced. 7 psi at the regulator. A1000's feeding aeronotive regulators....I'm no carb expert and am thinking my PV are set to low? Any help is appreciated

Just curious, and have no clue, would that motor ever see vacuum with a Roots style blower

I'm not sure, my next step is to hookup vacuum gauges above and below the blower to get an adequate reasing

Tell us more about the 502

Nothing fancy, worked 088 heads, stock cam, dry tails

Get hold of Dean Nickerson in Bristol PA. He is a blower carb guru

He's on my call list for tomorrow

explain flooding out ?

Thats a big blower for 502 displacement, what exhaust are you running

with non boost referenced carbs i use a 9.5 or 10.5 power valve.how far out are you idle mixture screws?

the answer is yes.

was this 502 origionally a 502 mag,hp500 or?

you sure its flooding out, and not leaning out? Does is sneeze/backfire thru the carbs?

I'd hold off on calling Nickerson. Best to do that when you get your tax refund in the spring. Youll need that, and a few months to get the carbs dialed in after he butchers them up.

did you reset the floats when boat was in water? cause I'll bet if ya did it on level ground ya might have a couple high ones.. also are the cabs inline or sideways mounted?

You fouling plugs at that rpm? Also what plugs are you running?

i would take that tax refund though and put towards some larger boost referenced carbs. 10-71's with 750's? Just a thought. With the 088's you may not see a lot with lot of work or aftermarket replacement.

Im betting PV size.

Nothing like having an OP disappear for a bit to answer some questions. :)

Sorry guys, wasn't getting any email alerts. For sure flooding out, set needle and seats with boat at rest in the water. 100% sure it's flooding, plugs are wet. As I accelerate to get onto plane and hold at 3k the boat runs there for 10secs or so, slowly lose rpm and then motor does. Blowers are obviously way to big for the 502's, my thought was the blower was pulling the pPV's open to soon and flooding it out.

Carbs are sideways mounted

2 1/4 turns out, idles perfectly, will run up to 4000 with no load no issues, soon as you put load on it she floods

What 10-71's you have and how fresh are they? Just curious. I would have your carbs looked at and gone through by speed shop per your application. You decide to downsize to fresh tbs 8-71's let me know. Regardless what way your u go I'd definitely have a good trustworthy carb guy check your carbs out.

makes me wonder if the ignition is up to snuff.

Good point

As far as the ignition goes it's the factory mercury ignition, new plugs and wires, set baseline at 13

they are mooneyham blowers with mooneyham intercoolers, intakes and carb adaptors

at this point i think you need to have someone with blower motor experiance take a look at it,it is tough to figure out what is going on without seeing it.

Agreed, blew a driveshaft end up and getting another one today. Will test this afternoon/tomorrow and report back

where's the fuel pressure at when flooding ?

Stays steady between 5-7

I do not see a way it could be flooding at that RPM and fuel pressure and be fine at idle, something is scewy here...think you better have someone with blower motor experiance ride with you...

yes, blower will create plenty of vacuum to carb

My thought, mind you I don't know jack about carbs, but under load, doesn't the blower start producing boost at 3k? If it does, is the air coming through the carb being sucked harder and thereby letting the power valve open and dump to much fuel? A PV with a greater opening pressure would delay the onset of fuel until the higher rpm ranges.......spitballing

Depends on how much load , as to whether or not it is in boost.

Typically, at 3000 rpm, in a 30ft boat, I would not expect it to be making boost yet. Irregardless, when the carb is NON boost referenced , the power valve is only seeing vacuum, and never boost. With a 4.5" PV, it will not open, until the vacuum drops below 4.5 inches. Going to a 10.5, will make it open sooner, and a 2.5, open later.

On a typical 750 carb, the power valve channels are not huge, like say, a dominator . Even if it was in fact opening too early, it should not be injecting enough fuel to "flood" the engine. If your carb was "boost referenced", it likely would be opening at 3k rpm on plane. Reason being, I doubt you would be seeing 5-6+ inches of vacuum there.

installing a wideband setup, would enlighten things.

Blowers really arent "wayy to big" for 502's. The 10-71 is not crazy bigger than an 871. Im running 420 B&M blowers on 468ci engines, which are close to an 871 in size. The engines run fantastic, and makes plenty of power.

If your plugs are legit wet with fuel , after a 10 second 3k rpm run, I'm thinking you have a fuel delivery issue. Lets say, you have a cheapy fuel psi gauge. It shows 7psi, but there is actually 8 or 9 psi in the setup (seen them off many times). Maybe it will idle just fine with boat at rest, and not blow the needle off its seat. Now, you get on plane, and the float becomes very sensitive, and the addtl fuel psi , simply will not regulate the fuel entering the bowl like it should. Your typical holley float, does not play well with excessive fuel pressure.

What kind of fuel pressure gauge are you using to set fuel psi ?

What regulator are you using? I've seen guys fight issues like this, when using cheap dead head style regulators with elec pumps. Fuel psi varying all over, flooding out, etc.

Were talkin about a 1071 on a basic stock 502 mag mpi long block with stock exhaust, you dont think it to big ?? Im learning the blower side of things and I know you, ICDP and Mike and others have a bunch on seat time with them.

Some have ditched the 256 blower on the 575sci, for an 871, and saw some nice speed gains. The biggest issue with too large of a blower, is inefficiency due to leakage between the rotors, at low speeds. Unless hes running it at 30% underdrive, i dont see an issue. Heck, some guys run 871s on small block chevys.

Certainly nothing that is gonna cause plugs to get soaked in raw gasoline.

All things being equal, the difference between an 871 and 1071, is less than 10% overdrive, same engine, same boost, etc. So at 5500 engine rpm, and say 1:1 ratio, the blower is turning 5500. Going to 10% under, now the blower is at 5000rpm roughly. Nothing drastic.

Now, take a 177 blower, at 2:1, and install an 871, you may go from 11,000rpm rotor speed, to maybe 5000rpm rotor speed. Now theres something worth talking about

Off topic kinda, but since we are talking blowers and boost.

Buddy of mine bought a boat with blown 572s. 1071s, dart 345s, chillers, etc. Engine expired within a few hours of ownership. Cams it had were way small for a 572, duration wise, in my opinion. Hyd roller. They made 883hp with 10psi before he took delivery of boat.

Rebuilt engines over winter. Compression was lowered from 8.5:1 to 8:1. New cams i picked for him from the lunati catalog. Very mild 629 lift .

Redynoed. Engines made 1000hp with 10psi, 950ish ft lbs. Stump pullers as it had over 900ft lbs from 4000 and up. Anyhow, in regards to the blowers. With the old pulley combo that made 10psi , and 883hp , now made 8psi, and over 900hp. He dropped 2psi, but gained power. Overdriving back up to 10psi, and then crested 1000hp. The orignal cam was choking it.

So boost is not just relative to cubic inch, as we know

it,s all about cylinder pressure,to little and power will be down,to much and detonation will kill the engine in short order.small cam&10 lbs boost won,t last long even with lower compression ratio.

for me if i have to choose between a blower that is to small or one that is to big i,l take the bigger one any day of the week,just spin it a little slower.

And plenty of lift or duration on the exhaust side to make it all work?