OMC Tech: Crossflow Jets

**Forkin' Crazy** · 05-10-2020, 02:37 PM

Originally Posted by DieselDood

I know for sure I can get 87. Most of the time 91, occasionally 93. Here in MN it seems some of the small mom and pop stations carry 91 Non-Oxy, but I haven't seen any near where I live.

I'll crank up the top end timing, and I think I'm going to clean up the 1 3/8 carbs and throw those on and see what happens. Maybe since the Jets are the same size I can get that little extra air I need.

That might get me by for now and I can get gaskets, heads and plan a day where I can pull the powerhead, install the bubble back and adapter plate, as well as the heads.

Now to go to the garage and see if I can figure out if this has stuffers or not.

I'm still on the fence about swapping reeds. Cylinder 1 had a little push back out of the boysens the other night when I was checking spark with the carbs off. I have another set of Reeds, but they appear to just be OE OMC reeds.

Carbs are off now... should I do the reeds...or hold off and see what happens?

If you can't get premium all the time, don't get to crazy with timing, compression, or leaning the jets. They are all related. This is even made worse with a heavy boat.

If it has stock 150 ports, you are wasting your time with the big carbs in my opinion.

The bubble back and adapter won't work unless you do a LOT of machine work, plus you will need the exhaust fillers that go under the bubble.

You'll be much further ahead if you just get a big bore 2.6L block. If you can find a 2.6XP/GT block, it has the best ports, heads, carbs, etc and makes a bit more than 240hp at the flywheel.

**mrichartz** · 05-10-2020, 03:16 PM

Originally Posted by FMP

That's a left handed orifice driver! HA!

It's for the other side..

Dumb old joke..
Two guys sitting on a roof doing framing ones sitting there going thru nails and throwing some out other guy looks at him what the hell are you doing he says the heads are in the wrong side.. guy looks at him tells him they are for the other side of the house..

**FMP** · 05-10-2020, 03:30 PM

Droll.....
Very droll

**DieselDood** · 05-10-2020, 05:29 PM

Originally Posted by Forkin' Crazy

If you can't get premium all the time, don't get to crazy with timing, compression, or leaning the jets. They are all related. This is even made worse with a heavy boat.

If it has stock 150 ports, you are wasting your time with the big carbs in my opinion.

The bubble back and adapter won't work unless you do a LOT of machine work, plus you will need the exhaust fillers that go under the bubble.

You'll be much further ahead if you just get a big bore 2.6L block. If you can find a 2.6XP/GT block, it has the best ports, heads, carbs, etc and makes a bit more than 240hp at the flywheel.

I have the exhaust stuffers as well. What machining would have to be done? I thought those were a bolt in.

I'd love to do a 2.6 block, but unless someone close to me has one for sale near me cheap... I gotta make due with what I've got. This one being freshly rebuilt with Wiseco's, should run out good on a boat that is max rated for 150 anyway....

The ports I'm not sure. My borescope pictures it looks like someone worked the ports a bit.

Really, with .040 over and Boysen Reeds, I don't understand why the builder would have gone with this size jet. Porting is the only thing that makes sense... but I think he overshot it, and my guess 62's were in his toolbox ready to go, but he never got to that point.

Then again... I could be totally wrong... lol He may have had the carbs laying around and just slapped them on and didn't even know what Jets were in it...

Regardless... I can't think of anything logical that would cause this thing to not get past that 2800-3000rpm mark when under load... no mater what prop I try... and that's 19, 21, and 23. And I have... after a severe left turn gotten all 3 of them up on plane. So once I make it to 3200... it has the power...

**FMP** · 05-10-2020, 06:18 PM

105 psi, not enough ign advance, way too rich, possibly raised ex and transfer too. Never liked those four wires for the later 60° v4 which you've used.
So up the psi 40 , give it more ign 28-32°, get the low mid fuel curve proper, hang 6 copper spark leads on it with strong coils and fly.

**DieselDood** · 05-10-2020, 06:27 PM

Well.. I'm working on raising the PSI... I'll throw the timing at it shortly... and the low/mid fuel curve.. we shall see what the 3/8 carbs do.. and I'll order wires this week.

**FMP** · 05-10-2020, 07:05 PM

Probably a little difference in the two size carbs fuel delivery with the same size jet so tune from the rich end.
You won't know until the heads are off how much you can shave before getting into the ramp in the chamber. Maybe the heads have never been cut. If you need to cut more and into the ramp you might need to get that shape back on the edge with some hand work, but that's down the road.
Measure the head gasket thickness and see how low in the cylinder the piston is at TDC. All good to know.

And the carbs, mid air jets? No idea what carbs you have so read up
https://www.screamandfly.com/showthr...gh-speed-jets&

https://www.screamandfly.com/showthr...rossflow-jets&

**DieselDood** · 05-10-2020, 08:26 PM

Well I cleaned the 1 3/8 carbs and installed them.

Fired right up, but bogs and will die with a little throttle when cold. I let it idle for a few min and it came right around and was quite snappy and hit 6000 very quickly, not realizing that I had gone that high. (wont' do that again without load)

So... since it's getting dark and it's 40 degrees and falling... I'll wait till tomorrow after work and report back.

I do have to say I think it's smoother and seems cleaner at idle for sure... possibly on a throttle snap too. It's for sure more crisp.

Either way.. plug wires will be ordered this week. Tuesday when those other coils arrive, they will be installed so everything matches on that side of things.

I've been chatting back and forth with the gentleman a page or 2 ago about his 150 GT heads. I'm hoping this runs out, then I can mod those heads and just make a day of it getting things modified so it's a screamer.

**FMP** · 05-10-2020, 08:49 PM

A 19 year old driver in his C63 AMG got nabbed doing 192.5 MPH over here. Just in a bit of ****.
That's moving

**DieselDood** · 05-10-2020, 08:52 PM

LOL ya I used to work on those. They are a REALLY fun car!

**FMP** · 05-10-2020, 09:05 PM

First thing I asked the news station was if he had mentioned if it had any more in it.
I knew they had legs but that's fast. Not sure what was done to it, wondering what the aero limits are on that car. Must be getting loose.

**DieselDood** · 05-10-2020, 09:18 PM

He had something done, probably just a programmer. Last I knew... Benz, BMW and Audi all have a gentlemans agreement at 155mph in the US except their flagship cars.

I can say that at 155 they are EXTREMELY stable.... but... that's just a... umm... rumor....ya.. ya... a rumor... LOL

I can say factually... That they have most of them detuned below 45mph or they wreck trans parts. So if you dance the 46mph line with the traction control off... and Stand on the throttle you can get sideways in a hurry.

LOL
I miss playing with those cars some days. I don't miss the electrical issues they had from time to time.

**racer** · 05-11-2020, 08:34 AM

A crossflow with 145 psi compression will not be reliable on 91 octane unless it is overly rich. They will make more power with less compression and jetted correctly. Same thing goes for timing, if heads are flowing right (xp heads) and jetting is correct any more than 26 degrees is asking for trouble if you get a marginal load of fuel. Bath tub heads no more than 28.

**DieselDood** · 05-11-2020, 08:41 AM

Originally Posted by racer

A crossflow with 145 psi compression will not be reliable on 91 octane unless it is overly rich. They will make more power with less compression and jetted correctly. Same thing goes for timing, if heads are flowing right (xp heads) and jetting is correct any more than 26 degrees is asking for trouble if you get a marginal load of fuel. Bath tub heads no more than 28.

So should I just install the 89 GT heads without modification? Any idea where that might set it up to?
Or... any idea where I can find Jets these days? Will a jet for a different application work?

It seems the Jets for these are getting harder and harder to find.

**FMP** · 05-11-2020, 09:42 AM

Originally Posted by racer

A crossflow with 145 psi compression will not be reliable on 91 octane unless it is overly rich. They will make more power with less compression and jetted correctly. Same thing goes for timing, if heads are flowing right (xp heads) and jetting is correct any more than 26 degrees is asking for trouble if you get a marginal load of fuel. Bath tub heads no more than 28.

With all due respect Racer

Seen it done many times with 145 and pump 91 and timing in the 30° with xflows OMC .

They run out great and don't fail.

The GT motors even had timing specs into the low 30°, consumer motors. Not workhorse barge loading.

Perhaps it's how you finish the cuts into the chamber if it's that deep.

He's also running better pistons but safe is safe and your advice I won't dispute.

As I said before, not exactly apples to apples but my xflows at 170psi and 33-34° at 6000 jetted up just a bit , hundreds of hrs, 1200lb hull never empty load. Shell 91 no alc, 40:1 nothing added but Wisecos and NGK BUHW.