Budman II

iTrader: (1)

Looking for some input for baseline jetting for a 489 with AFR 265 heads, Dart single plane, 800 Holley, and Lightning headers. Vital statistics listed below:

4.280 bore, 4.25 stroke, 6.385 rods
AFR 265 oval port heads, 2.19/1.88, 108 cc chambers, around 9.25:1 CR, forged flat tops

cam: Marine Kinetics hyd roller, 226*/230*, .612/.596, 114* LSA

Intake: Dart single plane oval port - port matched to AFR 265 "roval" ports

Exhaust: Lightnings with silent choice collectors

Ign: Merc T-Bolt IV

3600 lb 24-foot Baja, single engine, 5600 RPM on the top. Will spend most of it's life between 2800-3600 RPM.

I was planning to run this engine on a dyno before installing it, but alas, budget and time constraints will curtail this. I am fully aware that ideally I should be running some type of AFR meter to dial this combo in, but time and expense constraints once again prohibit that. Also, the Lightnings have silent choice collectors, so not sure if the sensors would like that too much.

Looking for suggestions on a baseline on primary and secondary jetting to get me started. I was running 80 primary and 88 secondary in the same carb last year, but that was with GM 088 heads and a Performer Air Gap dual plane, and smaller cam. I expect the jetting to be significantly different with the additional airflow characteristics of the new heads. And yes, I am fully aware that I will probably have to go with a larger carb like the HP950 to fully maximize the potential of the new heads, but that will have to come at a later date. For now, I just want a safe, conservative baseline jetting to get me up and running so I can use the boat for the remainder of the summer.

I'll hang up and listen.

billy boats

Check the list number on the air horn of the carb . Find the stock jetting that it came with and run that . A carb is factory calibrated to draw fuel for the amount of air passing through it and give you the the correct air fuel ratio. Re jetting a carb is usually for elevation differance or extreme temp conditions. An engine draws air ( vac) the more air it draws means it will lift more fuel from the bowl it's a constant which doesn't change from modifications to the engine. The biggest misconception people make about jetting it that fuel pushes through jets and if you make more power you need to up the jetting to feed more fuel. But reality is as your engine makes more power it pulls more air through the carb the more air lifts more fuel automatically without changing jets .

MILD THUNDER

iTrader: (3)

On paper, you are correct. In real world, this rarely holds true. Plus, how do we know his carbs haven't been modified by someone in the past? What if someone opened up the high speed bleeds and with stock jetting he'll go lean and burn a piston or a valve. Go to any dyno shop, and you'll see Changing jets in order to obtain a optimum AFR is a reality. You are correct in that the airflow moving thru the carb is what pulls the fuel from the jets. BUT, the jet size still limits the amount of fuel that can be drawn. Lets say your moving 800CFM of air thru the carb with #75 Jets. And then do another pass again with 800CFM of air moving thru the carb with #80 Jets. The end result (Air to fuel ratio) will be changed. To what extent this change is needed to , is the question. While it is often said that if you have to make major jetting changes from the stock calibration, you probably have the wrong carb for the application. At the end of the day its the AFR, EGT, or plug readings that talk to us. The rest of it just doesn't matter.

Budman, I'd try 84/92 to start and make a pass and check the plugs. If your old setup worked for you from a fuel curve/supply standpoint, I'd keep that and possibly add more or less jet if needed to get mixture right. Theres really no magic number here, that's why we have AFR Meters and dyno's. Since that's not an option, read the plugs, and let them tell you what the engine needs.

Budman II

iTrader: (1)

MT, and to extend what you are saying, running a smaller than optimum carb might limit my maximum power number due to CFM limitations, but if the correct jets are installed to maintain the correct A/F ratio, then no damage will be done to the engine - correct? I may look into a bigger carb down the road, but for now this is what I have, so even if it knocks 50 HP off of what this engine COULD be making, at this point if it is moving under it's own power and staying healthy, I'll be satisfied.

I would like to look into an AF meter, but not sure if it is feasible with the exhaust setup I have. Even if I installed one at the dry part of the collector, when the diverters are closed I am afraid that enough water could revert back to the very end of the collectors to ruin them. Wish someone made some waterproof sensors!

Budman II

iTrader: (1)

It ran kind of rich with the 80's/88's, but that was essentially a completely different motor. Different heads, cam, and intake.

Black Baja

iTrader: (2)

Afr's are fine and dandy but its not gonna tell you about each cylinder. I would tune it by the plugs. Dyno's are great so you can tell everyone how much horsepower your motor makes after you add on 50 or so. But a dyno is never going to duplicate the load your boat will put on the motor and if you dyno tune then put it in the boat your gonna end up with a lean motor and have to tune it on the water anyhow. I think the 80 and 88 are going to to a good starting point and be pretty close when you get it tuned in....

Budman II

iTrader: (1)

Yep, looks like I'll spend part of the summer standing on my head changing plugs on the water, but at least I'll be on the boat!

Sure wish I could get some "old school" gasoline that actually puts some color on the plugs. I remember helping my buddy with plug reading on his motor ten years or so ago, and it was almost impossible to see anything. Surprised someone hasn't come up with some type of additive to add to gas to help with plug readings.

Black Baja

iTrader: (2)

I find Ngk are the easiest to read. And if your 24 is like mine was yes you will be standing on your head its alot of fun working on a motor that is burried in the bilge. I half think my carb on my 496 had 80 88 jets in it. The carb on it was dead on the money. With that said I don't think your gonna end up with a flat spot on your head :-)

Budman II

iTrader: (1)

I have a pretty pointy head, so it would probably do me some good.

What kind of heads were you running on your 496? Single or dual plane manifold? Trying to gather as much data as I can to make an educated guestimate.

Black Baja

iTrader: (2)

Gm heads with a days work in them. They were flowed and picked up 25 Cfm but I don't remember the numbers and God knows where the sheets are. Cam was a Scneider .610 .610 [email protected] on 114. Had some more compression. Steve Baker did the carb. When I talked to him on the phone about doing the carb he told me exactly what was going to be in the carb and it was kinda like he didn't even have to think about it. When I got the carb it had what it said it would. I bolted it on and it was absolutely perfect. I had never pulled a set of plugs that had such a nice burn on them. I couldn't believe how good they looked. I pulled them a bunch of times cause I was paranoid about melting the motor. But I never touched the carb. The boat ran 74 on GPS. It ran strong to 71 then I had to play with it to get the last couple mph out of it...