Carb help please
 

Wondering if I should upgrade my 750 holleys to 850s. I run a 502 with a 420 mega chiller and aluminum heads I jetted them pretty heavy the last couple years.The boat is a heavy 28 sportcat. Never seems to have that punch . Do I have enough cfms with the 750s ??? Also I run at 4000 ft

your 750s are as small as i would use,you really can,t over carb a blower motor.have you tuned the engine with a wide band o2?

No I haven't I want too don't know where to bung a hole I have lightning headers where will they not get wet?

i don,t know if you can but bungs it the lightning headers.

what are you running for boost and compression? At that altitude I would think you could run a little more boost than us peeps running at <1000.

If I recall Mild Thunder went from 750's to 850's on his 468's with 420 mb's and I'm pretty sure he made a statement that he wishes he would have went even larger 4150 1050's. He would have to verify this for 100% certainty.

For what's it's worth I ordered Lightning Headers years ago and had them install bungs in the collectors.

Absolutely. I would think at least 2 more pounds of boost, at 4000, than at sea level.

This is where a map sensor with the daytona ignitions could help, (or efi datalogging).

Absolute pressure measurment, vs gauge measurement.

What about carbs?

They are certainly on the small side. I have no experience with high altitude, but, my mind tells me, bigger carbs would help.

Roots do not draw air nearly as well as they move it. Just like its harder to breath at higher altitudes when you are used to sea level. You have less atmospheric pressure helping to fill your lungs. On the engine , there is also less atmospheric pressure, to help move air into the carbs.

Its very likely, that larger carbs alone, would net a boost psi increase for him. I know it has, without even taking in consideration, the altitude concept.

Before I bought new carbs though, or recommended them, I'd want to know 2 things.

What kind of "absolute" pressure he is getting , and what the air fuel ratio is. '

My gut tells me, that the engine may like a bit more ignition timing at higher altitude, from the lack of oxygen in the charge. The more dense the charge is, the faster it burns. The faster it burns, the later you can fire the plug. The less dense it is, the slower it burns, and therfore, you'd need to light the fire sooner.

one would need to jet down for high altitude, not up. Less oxygen, means less fuel. But who knows where we are starting from, so an "oxygen" sensor, will tell the story.

Just shooting from the hip here.

Well for spark I'm upgrading a msd box ran a very mild spark a thunder bolt stock .Ran 5 lbs boost plugs were pretty good looking ran 31 32 locked timing 4000 feet. Primary 73 jet and 81 secondary.Higher speeds Rpms just doesn't feel like it has much takes a lot of throttle and trim to get high speeds . Big rooster. Like 70 max

Not fast enough

With snowmobiles running at that altitude I'd run less jet in the carb days. With the air being less dense requiring less fuel is it possible larger carbs wouldn't be as much of a factor? I would think at that elevation they still would. Now if were boating at 30,000 ft on top of Mt Everest might be different story. But then we wouldn't be concerned with carb size as we would survival. Lol

only way we could get to work with them was about 10" from flange in # 5 primary, did work fine there..really need to see AFR's sound like its rich to from what you describe..also, we made 700+hp on a procharged blow they setup with a 750 on a 509

I hope I am saying this correctly, but wasn't it Articfriends who was testing carbs on this most recent/latest project with his one his dyno partner's modified Holley #4150 850's??? They tried several other sized carbs including a #4500 Dominator and according to Articfriends the modified Holley 850cfm carb always came out on top according to the dyno....but remember, this was a modified Holley 850cfm carb---modified by his friend who has been modifying carbs for many years.

Anyway, it might be worth some consideration....have fun.

I think its safe to say, that a "blow thru", and a N/A engine, share very little in carburetor results.

Blow thru, you can use a much smaller carb, and make big power. The compressor is BEFORE the carburetor. N/A, you need flow, but you also need fuel atomization. Roots, are totally different animals.

Either way, I am willing to bet, with proper tuning, the engine will wake up big time. 5lbs of boost at 4000 feet, isn't much at all.

What pulleys do you have on the blower setup? Are you running an intercooler? What is the static compression?

Yes I have a intercooler not for sure on pullies but I'm getting about 4-5 lbs boost

I only run about 70

I've heard that a sportcat is a caddy ride but hard to go fast . Do u think this new msd should help it. I've been trying for 3 yrs to run better.

Also Xr drive 1.50 gears 26 lab prop

Should I pulley up and run more boost

4-5 lbs boost at 4000 feet, is prob like 2-3lbs of boost at sea level.

I would get an o2 sensor installed, and get fuel dialed in. See how it runs. Id order some new pulleys, and get the boost up.

You prob have 100hp or more on the table.

I dont think adding msd is gonna do much for you.

Nothing will kill a s/c engine faster than running it lean. Make sure the fuel tune is right, before turning up the boost.

Whats the fuel system setup? Pickup, line size, filter , pump, etc ?

I'm not running lean probly a little rich plugs have a dark shade to them also back of transom has a haze on back from exhaust where do I put a 02 sensor so it won't get wet? Wiband would be so great I'll shoot u a pick of motor and exhaust tomorrow thanks buddy appreciate all your knowledge and help

2 750 holleys electric fuel pump about 7-8 pounds fuel pressure

I ran it at havasu last yr what a difference in power. WOW and I jetted up for there

What exhaust are you running ?

Mercury used to sell "high altitude " pulleys for the sc engines for this reason

Measure your pulleys you have now as precise as possible, and i could tell you what youll want to order

you cannot tell much anymore by reading plugs(I know some wont agree) again, without a wideband, you can be rich one area lean in another, only way to get any info is brand new plugs, WOT pass, shut down, ck with a reader, even then they can fool you..

I believe there lightning in gonna take some pics today and I Will post them

I believe exhaust are lightning I will take some pics today

if they are lightnings, the only place we got 02's to live was in a primary tube about 10" from flange, but like others said as well, at altitude the air is bad, you need less fuel and more boost to make power, one way to test, but really need the 02 to be safe, go out make a WOT pass record speed, lean it down say 4 sizes ft/rear go right back out and see speed, if it picks up, there's you answer..

Honestly, given the simplicity and affordability of EFI today I'm surprised to see anyone starting over from scratch would go carbs. I know they can be better for all out performance, but for those of us that hate being under the hatch, installing a couple of bungs and a couple Holley Sniper throttle bodies isn't a bad way to go. Am I wrong on this?

How much r those and how hard to do

Wow, I just looked these up. Very cost competitive with a quality carb and afr kit. I might need to do this Sniper myself. If I hadn't already installed a low pressure fuel pump for my carb it would be the same.