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Jetting after installing 250 bloowers - Help!

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Old 05-01-2007, 07:04 AM
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I know a guy that runs a 250 blower with a 850 on a mostly stock 502. It's low boost, maybe 4 lbs and it runs like a stocker only about 10 miles an hour faster. Good luck, it looks like you're on the right path.
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Old 05-01-2007, 08:33 AM
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I know a guy that runs a 250 blower with a 850 on a mostly stock 502. It's low boost, maybe 4 lbs and it runs like a stocker only about 10 miles an hour faster. Good luck, it looks like you're on the right path.
Now that's encourging! These were set-up for 4 pounds of boost as the engines are still high stock compress ratio. Were they Holleys or Barry Grants? Do you happen to know what jets sizes he was running?
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Old 05-01-2007, 09:18 AM
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Ok guys the "rookie" is back with an up-date. Spoke to the guys at Barry Grant today. First, no one mentioned the fact that these 850 carbs had "blower referenced" power valves on the primaries; apparently there should be vacuum hose running from under the primary bowl to the manifold under the blower. Without it, as Im's sure most of you expreienced guys already know, the 4.5 PV will never open increasing the jet size by 6-8 jet sizes.

After considerable conversation, the Barry Grant (matine) guys believe (rather strongly) that the 850 blower carb is plenty of carb when connected properly and (very important here) it is supplied with enough gas; as Doug from Grant mentioned in a reply above, the 130 gph mechancial fuel pump is more than likely not able to provide enough fuel to the carb under substain high rpm conditions. If they had sold me the carbs, one might think they were simply defending the sale however, they didn't recommend or sell me the carsb so I'm thinking it would have been very easy for them to say "wrong carbs for the application" and quickly sell me another set of bigger carbs, if they really felt that was the problem.

The lowest that I've ever heard the pop is between 3500 -4000 rpm. At 3500, the vacuum reading is -5; at 4000 rpm, the vacuum reading was -4. If the PV opens in that range and increases available fuel (6-8 jet size bigger) perhaps we are no longer lean.

Anyway, I've added the vacuum line, have installed #98 in the primaries and #100 in the secondaries. Will drop it in the water tomorrow afternoon and let you know what happens.

Sounds like your popping accurs as soon as the secondary opens in the carb, which if what they say is true regarding the boost referenced PV, maybe that's it!

I had two problems like this in a NA boat a few years ago... Up to 3500 the boat ran perfect, over that it popped. In one instance there was a piece of metal shaving stuck in the needle/seat, in the second case the coil was bad.
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Old 05-01-2007, 11:05 AM
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They were Holleys. Not sure of the jet sizes. Doubt if he knows either, his mech. set it up.
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Old 05-01-2007, 09:55 PM
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Well, I got it in the water this evening with the boost referenced PV vacuum line hooked up; how should I say this?

A quote from the move Days of Thunder comes to mind......"Don't touch a thing Harry, she's perfect!!!!!!!"

This is the best these engines have every performed!!!
Ok, here are the facts:

- Twin 454's, 9:1 compression
- 250 Blowers from the Blower Shop set-up for max 4lbs of boost
- Single 850 Barry Grant Demon blower preped carbs with boost references 4.5 power valve with #98 jets in the proimary and #100 in the secondary.
- Initial timing set at 16 with Thunderbolt pack for anothr 14 degrees producing 30 degree total.
- 130 gph, 9 psi mechanical fuel pumps into Aeromotice regulators, 1/2 inch lines all the way back into the tanks

Pulled the nubmer 7 plug after streching her legs up to 5200 rpm, looks a bit rich but I think I'm going to leave it for now. After about 20 minutes of runing between 4000 to 5200 rpm, she never popped once!!!

Like I said, "don't touch a thing HArry, she's perfect. Hope someone finds this information usefull in the future.... believe me, it was costly getting here.

Thanks to eveyone that responded with input.... it was all helpful and I really appreciate everyone of them.

Al

PS: Even got to meet Mark from Precision Marine while at the ramp this evening. Thanks for taking the time to speak with me Mark about my set-up. Your comments put my mind at ease regarding the use of the 850's on this application. Look forward to seeing you on the Tickfaw run this weekend.
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Old 05-01-2007, 10:19 PM
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Are you sure about the timing???????

A 14* advance V6 module usually does not give 14* on advance on V8. More like 10*. Check your timing at full advance around 3000rpms. You may still have some HP to gain with a simple timing adjustment.

Glad its running good for you.
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Old 05-01-2007, 10:25 PM
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Are you sure about the timing???????

A 14* advance V6 module usually does not give 14* on advance on V8. More like 10*. Check your timing at full advance around 3000rpms. You may still have some HP to gain with a simple timing adjustment.

Glad its running good for you.
Pretty sure on the timing. Was sitting about 6" from that blower belt while they were turning 3000 rpm and we were at 30 degrees.
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Old 05-01-2007, 10:35 PM
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Hey al what pressure are you running after the regs into the carbs?
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Old 05-01-2007, 10:36 PM
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Also sorry what is teh pressure at idle and how far if any does it drop at WOT (wide open throttle)
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Old 05-01-2007, 10:39 PM
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Also sorry what is teh pressure at idle and how far if any does it drop at WOT (wide open throttle)
I have the regs set at 7.5 psi. At 4500 rpm's they were holding just over 6 psi. Running very briefly at 5000 rpm's the fuel pressure was still just over 6 psi.
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