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220 GPU Carb set up

Jayholt

Member
I recently purchased a fresh rebuilt 220 GPU, Carb was beyond repair. I was advised to put holly 4412 500 CFM 2 barrel manual choke on it. I'm at sea level south Louisiana. It came with 73 mains, seemed to run ok, but my CHT showed very hot, would quickly climb above 550 at 2600 RPM, now have 74, and 76 jets I just picked up to see if more fuel would cool things down. Plugs look like they are a bit lean. Also verified timing, was at 28degrees before TDC, reset to 25 degrees. Any recommendations. The motor builder wants me to put vacuum assist 650 CFM 4 barrel on it, said I wasn't getting enough fuel /air to cool cylinders. Any advise would be appreciated. Looks like most folks are running the Holly 500 CFM 2 barrel from what I can see.
 

Slidin Gator

Well-known member
Jay,
You are on the right track I believe. Stick with the 2 BBL for sure, don't listen to the 650+ folks, 500 CFM is the perfect size carb for a direct drive, 470 cubic inch motor. If my motor is lean I'm looking to fatten her up with more fuel, not throw more air with a bigger carb???

I suggest you keep throwing bigger jets until it blows black and then back off a bit. I think 25 deg BTDC advance is correct but not positive, if anything the GPU would use less advance.

Search this site for posts by John Fenner with 220 gpu keywords. He will probably chime in for sure.
 

John Fenner

Well-known member
Ok, first off,,, is it single or dual mag? If single mag, bring timing back up. Freshly rebuilt with what pistons, stock dished or domed?
Many varitives are key in tuning them.
Power valve plug, #76 jets as a baseline, full run up with minimal to no idle time check plug color, if no tan on insulators step up the jet size till you get a slight tan.
Remove fuel inlet adapter from bowl, omit the scintered bronze filter and spring, reinstall adapter.
 

Jayholt

Member
Ok, first off,,, is it single or dual mag? If single mag, bring timing back up. Freshly rebuilt with what pistons, stock dished or domed?
Many varitives are key in tuning them.
Power valve plug, #76 jets as a baseline, full run up with minimal to no idle time check plug color, if no tan on insulators step up the jet size till you get a slight tan.
Remove fuel inlet adapter from bowl, omit the scintered bronze filter and spring, reinstall adapter.
Thank you guys for help! First time airboater. Just finished building my rig from scratch. Ready for testing then bowfishing.
20220521_123414.jpgSingle magneto. Was told was stock rebuild. I purchased motor from my friend who sent it to a mechanic named James in Lakeland Florida. My friend ran into a tough economic patch so I paid mechanic shop and purchased from him. I'm at 25 degrees before TDC now, 76 jetts will be in tomorrow. Brand new Holly 4412 carb. I will do run up when install new jets. How long full power run up? What CHT max is acceptable?
 

John Fenner

Well-known member
Take out a plug, safely,, look into cylinder at the piston top, if it is dished or slightly domed, if dished ramp timing up to 28 to 30° I believe single mag stock piston design was 32° but that was for simply running at 1200 rpm driving a generator or water pump, as I said, many varatives in tuning them.
P.S. throw the CHT gauge away.
 

Jayholt

Member
Ok, first off,,, is it single or dual mag? If single mag, bring timing back up. Freshly rebuilt with what pistons, stock dished or domed?
Many varitives are key in tuning them.
Power valve plug, #76 jets as a baseline, full run up with minimal to no idle time check plug color, if no tan on insulators step up the jet size till you get a slight tan.
Remove fuel inlet adapter from bowl, omit the scintered bronze filter and spring, reinstall adapter.
Thank you guys for help! First time airboater. Just finished building my rig from scratch. Ready for testing then bowfishing.
Single magneto. Was told was stock rebuild. I purchased motor from my friend who sent it to a mechanic named James in Lakeland Florida. My friend ran into a tough economic patch so I paid mechanic shop and purchased from him. I'm at 25 degrees before TDC now, 76 jetts will be in tomorrow. Brand new Holly 4412 carb. I will do run up when install new jets. How long full power run up? What CHT max is acceptable?
 

Jayholt

Member
Put the 76 jets in, has 5.0 power valve installed. It is Holly 4412 500 CFM. I tuned the idle screw adjustments, ran 23-2400 Rpm for about 15 min river run. Shut down with out idle. Pulled plug. Acts very good at all speeds. Opinions.
 

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Jayholt

Member
Color looks good
John , thank you very much! engine running very smooth, also did about 20 min extended idle to simulate bowfishing. Ran great, did not load up. Idle intake vacuum is 17.5 inches mercury. Couple of folks on different post recommended increasing power valve to 8.5 power valve. I know I'm very close with tuning. The larger jets, removing the removing the bronse filter and tuning the idle screw mixture adjustments really helped. Very runnable now. Anything else you recommend? Your opinion on current set up and if should mess with power valve. Thanks again!
 

John Fenner

Well-known member
Well, it sounds like you are getting it dialed in, my personal boat has a stroked 220, H beam moly rods, flat top Ross pistons, I replaced the holley metering plate recently with a Quick fuel 850 secondary plate with blocked power valve and #76 jets, the response is great, plus with the power valve deleted the hot starts are much better as well as the idle mixture. When tuning these things, use dielectric grease on all gaskets, no need to replace a gasket, pop bowl off, plate, whatever,,, bolt it right back up and play.
 
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