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Thread: The OMC Cross Flow Thread
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11-26-2022, 05:44 PM #76
I think I set mine at 26. That was with 128 to 130 cranking compression.
Keep her fat, and she'll stay happy. I started out a few sizes rich because of my mods. It was 4 stroking and blubbering at first. But I jetted smaller one size at a time until it cleaned up, and left it alone.
ALWAYS run premium fuel and the best oil you can buy.
I wish I had a dollar for every hour I put on that old 1980 235. And both of mine are high ring motors. So there goes that myth!
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11-26-2022, 10:08 PM #77
I think that OMC changed their original timing spec from 30* down to 26* once they realized how these motors were easily damaged by crappy fuel and lugging on heavy boats. I agree that timing to 26* will give up very little performance and make it more tolerant of poor fuel and poor running conditions.
I can't agree with you on the high ring piston thing. The two 235's that I've had on my Viking were both run very hard with the tight heads for hundreds of hours each put on them, and the point of failure for them both was high ring pistons. I rebuilt motor#1 and could not reuse several other pistons in it because the top ring groove was ready to fly off. Motor #2 hasn't been disassembled yet, but that one has 2 pistons with no top rings in them. Maybe my high ring piston problems are from long runs at high rpms? Just trying to keep up with all those fools with black motors on their boats.NRA Life Member
58' Farmall Cub
Is that your mama's boat?
"I said I didn't have much use for one. Didn't say I didn't know how to use it."- Mathew Quigley
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11-26-2022, 10:26 PM #78Member
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11-26-2022, 10:44 PM #79Member
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So if I’m going to keep the internals (and everything else) of the engine stock and aim to run it at 6200 rpm, retard timing to 26 deg, then do I need to worry about replacing jets? I have no idea where to get them since they are nla, especially multiple sets of different sizes! And I run no-ethanol rec fuel which around SE MI is always 91 octane. I’d say that’s mid grade but not high octane. But I don’t want 93 octane auto fuel with 10-15% junk ethanol in it! Oh, and I use full synthetic Evinrude xd-100 oil, 50:1 in this engine.
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11-27-2022, 01:46 AM #80
I think the problem was lugging with heavy boats combined with too much compression. Detonation was the problem, not the high rings. I still have two high ring V4s, an Evinrude 140 and a Javelin 100 that have never had the heads off of them.
OMC started using looser heads AND had a revision for thicker head gaskets to patch up the "problem". I was working at an OMC dealership at the time.
You put a small enough prop on it so it was not lugging, no problem.
I have two 2.6s I built both with the tight heads. 325638 and 325639 if I remember right.
I ran this totally stock cast piston XP with high rings and never had a failure because of ring location.
I think that was a detonation issue, not a design issue.
This one is still together. 7K all day.
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11-27-2022, 02:12 AM #81
Like Forkin said earlier, the safest and best way to jet your motor would be to go up 3 or 4 jet sizes so that you are 4 stroking, then jet down one size at a time until it no longer 4 strokes. I think that everyone will agree, and yes it is that important to get this right. Jets are out there, the internet makes finding stuff a lot less difficult than it used to be.
NRA Life Member
58' Farmall Cub
Is that your mama's boat?
"I said I didn't have much use for one. Didn't say I didn't know how to use it."- Mathew Quigley
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11-27-2022, 08:11 AM #82
The synth oil is not helpful in these old two strokes. All you need is tcw3 cheap dino oil. Yep it smokes but it works best. Oil that is too slippery is not good for two strokes of these periods
Hydrostream dreamin
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11-27-2022, 11:46 AM #83
Since it was mentioned earlier. What is the physical difference between C and D jets?
I admit that I have used them interchangeably.NRA Life Member
58' Farmall Cub
Is that your mama's boat?
"I said I didn't have much use for one. Didn't say I didn't know how to use it."- Mathew Quigley
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11-27-2022, 02:14 PM #84
I had more failures due to power pack double firing causing detonation than I did from a piston failure.
Ralph Musser
27' Fountain Fever W/ 525SC
22' Rapid Craft W/ Evinrude 300 V8 January 2009 BOAT OF THE MONTH SOLD
24' Triton 240 Gold W/ Mercury 60
14' Royalcraft W/ Evinrude 75 father/daughter project
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11-27-2022, 03:37 PM #85
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11-27-2022, 06:05 PM #867000 RPM
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11-27-2022, 06:38 PM #87
hard to argue with success. I ran the living hell out of my crossflow for almost 30 years on the cheap stuff. Hmmm the common factor here aint the oil, its the tougher than snot crossflows. Could probably use motor oil and they’d survive. The synth oil thing is to keep the bearings spinning instead of sliding. Probably over thought
Hydrostream dreamin
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12-03-2022, 09:45 PM #88Member
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Not to beat this to death, but where do I get all these different sized jets from if they are NLA from OMC/BRP? If I understand right, I’ve got three two barrels, so just to try a new jet size, I need 6 new jets that are NLA?? Are these jets readily available by size via automotive channels even though these were OMC carbs?
And what do you mean by 4 stroking? Clearly I know less that you do about 2 stroke outboards!
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12-04-2022, 12:36 AM #89
I don't think these jets are available from any automotive sources. The first place you should look is here on S&F. Ask around if anyone has or knows of anyone that would part with a set of jets. I can tell you that there are not many people that are actively racing old crossflows, but they are still very popular in the mid-west, and there are still some dudes in Australia with a great deal of OMC's racing history. Maybe someone on this thread will pm you a phone number?
There is long running thread in general discussion called the OMC power tour, you could poke around in there. I remember seeing a lot of fast boats with crossflows on them in that thread. Four stroking basically when your 2 stroke engine is so rich that the cylinders are firing every other time they reach tdc instead of every time. Your motor would still have a good idle, fair midrange throttle setting, and terrible wide open throttle.NRA Life Member
58' Farmall Cub
Is that your mama's boat?
"I said I didn't have much use for one. Didn't say I didn't know how to use it."- Mathew Quigley
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12-04-2022, 09:59 AM #90Member
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ok thanks for the suggestions on the jets John. I’ll wait til I get into the engine and figure out what jets I have first and then ask guys to see who might have what.
so I will only know the engine is 4 stroking really is that it runs like **** at wot? And that’s because it’s so rich and cylinders are so full of fuel they can’t fire every time at tdc? So basically I’m probing for that condition, overfuelling and then backing off until it fires on every stroke? You basically want to feed it as much as fuel as you can (and retard timing to about 26 to 28 deg)? Just making sure I understand the idea. I find this stuff interesting!