850 marine holley

« **on:** August 01, 2016, 12:39:05 PM »

Im thinking about getting a new carb for my 454. 049 heads. 10 to 1 comp. Msd ignition. Jacuzzi Wj. 20.5 cruiser. 280 Clay smith cam. Mercruiser center rise exhaust. Clay smith says to run an 850 carb with their cam. I have access to a marine 850, but its not a double pumper. Will i be ok with the vac sec. It ran ok with the 750 vac sec with the cam thats currently in it. No signs of detonation or running lean.

« **Reply #1 on:** August 02, 2016, 02:05:07 PM »

Cricket, cricket...

« **Reply #2 on:** August 02, 2016, 03:13:51 PM »

I wouldn't be afraid to run vacuum secondary. It won't fuel until the engine is pulling enough vacuum to open the secondaries. I'm not sure the specs on that cam? But the only place it may not pull a lot of vacuum is at an idle. In that case it doesn't matter anyway what the secondaries do. That's why they use them on standard marine applications (i.e. Hand throttle).. Just my opinion though

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« **Reply #3 on:** August 02, 2016, 03:18:59 PM »

Quote from: el mojado on August 01, 2016, 12:39:05 PM

Im thinking about getting a new carb for my 454. 049 heads. 10 to 1 comp. Msd ignition. Jacuzzi Wj. 20.5 cruiser. 280 Clay smith cam. Mercruiser center rise exhaust. Clay smith says to run an 850 carb with their cam. I have access to a marine 850, but its not a double pumper. Will i be ok with the vac sec. It ran ok with the 750 vac sec with the cam thats currently in it. No signs of detonation or running lean.

The short answer is yes.

That being said, to get everything from it will take some adjustments.

Power valve, secondary spring, jetting will all need to be tweaked.

Have a part number of the carburetor your looking at?

GT

« **Reply #4 on:** August 02, 2016, 05:45:05 PM »

Gt's short answer is right, yes it will work.

But the longer answer is you probably wont see any difference between the two. What RPM is the engine turning flat out wide open throttle. They recommend an 850 because the cam has an RPM range and it would need that carb to reach the top of its RPM range. I doubt your at that RPM or ever going to get there. Also I assume this is a closed engine compartment being a 20' cruiser. So sitting on top of your carb is probably a tiny little flame arrestor that couldn't flow an honest 750cfm if you funneled a hurricane through it. If the flame arrestor can only flow 600 CFM either carb will only flow 600 CFM.

IMO, unless you turning some fairly high RPM for a jet cruiser (over 6000 RPM), any increase you might see with an 850 and all the tuning work GT is talking about, you would also see the same results with the same tuning work to the 750 you already have. Going bigger with the carb will only help if the carb has become the limiting factor at WOT.

« **Reply #5 on:** August 02, 2016, 06:38:54 PM »

Quote from: propless on August 02, 2016, 05:45:05 PM

Gt's short answer is right, yes it will work.

But the longer answer is you probably wont see any difference between the two. What RPM is the engine turning flat out wide open throttle. They recommend an 850 because the cam has an RPM range and it would need that carb to reach the top of its RPM range. I doubt your at that RPM or ever going to get there. Also I assume this is a closed engine compartment being a 20' cruiser. So sitting on top of your carb is probably a tiny little flame arrestor that couldn't flow an honest 750cfm if you funneled a hurricane through it. If the flame arrestor can only flow 600 CFM either carb will only flow 600 CFM.

IMO, unless you turning some fairly high RPM for a jet cruiser (over 6000 RPM), any increase you might see with an 850 and all the tuning work GT is talking about, you would also see the same results with the same tuning work to the 750 you already have. Going bigger with the carb will only help if the carb has become the limiting factor at WOT.

There are current flame arrestors out there that currently flow in excess of 1000cfm.

FYI

And I agree that the two carburetors are likely not going to have too different of performance attributes.

Unless the 850 is dialed and the 750 is not.

Perhaps a better question than what rpm does it run now is where is the manifold vacuum at wide open throttle, that will actually be a great indicator of where to go from here.

JMO.

GT

« **Reply #6 on:** August 02, 2016, 07:24:39 PM »

Quote from: GT Jets on August 02, 2016, 06:38:54 PM

There are current flame arrestors out there that currently flow in excess of 1000cfm.

FYI

And I agree that the two carburetors are likely not going to have too different of performance attributes.

Unless the 850 is dialed and the 750 is not.

Perhaps a better question that what rpm does it run now is where is the manifold vacuum at wide open throttle, that will actually be a great indicator of where to go from here.

JMO.

GT

Yea, and those nice K&Ns are coast guard approved flame arrestors and they filter the air.

That's what I was getting at, if both are dialed in, probably wont be a difference.

Vacuum would be the best indicator. Would like to see vacuum readings with and without the flame arrestor too. But if this thing is turning 4600 RPM at WOT, it sure doesn't "need" an 850.

« **Reply #7 on:** August 03, 2016, 07:48:07 AM »

I am running a 4 inch tall oval K&N filter. My current 750 has been tuned by Performance Carb in Ontario. My engine is exposed. Currently, my RPMs are hitting 4900 at WOT.

After speaking with Carb shop and reading these responses, im gonna stick with what I have until I can afford a double pumper. Thanks for responding guys.

el mojado

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