40:1 Pre-mix Chocking on Excess Oil; Options?

**croSSed** · 07-01-2025, 06:13 PM

Some background: I just finished breaking in what was a NIB factory replacement 2.4 175 chrome bore block. I’m running at 40:1 pre-mix because I regularly run it up to 6500ish rpm. However, if I run it for any length of time at displacement speeds when I go to put it on a plane, it chokes on the excess oil before getting on a plane. Additionally, it runs inconsistently and rough at idle/displacement speeds, and smokes A LOT. I also notice that at any speed below about 4000 rpm it runs kind of “ratty” like it’s fighting through the excess oil. Interestingly, this 175 came from the factory with the oil injection blocked off with a block off plate. I should also point out that I’m running one size bigger main jets for safety’s sake, and the timing is probably 2 deg. backed off, again- safety margin.
Contrast this with an XR2 150 that I ran the s—- out of for 15 years that had oil injection intact. It didn’t smoke much at displacement speeds, ran great at every throttle position, and I regularly ran it up to 6400 rpm. When I took it out of service to replace with this 2.4, I pulled the heads to see what things looked like. The cylinders were pristine, there was still factory crosshatch in each of the cylinders.
I’m seriously considering putting the oil injection from my XR2 block on to this 175. Since this 175 is new the plastic oil injection drive gear is new, so I would expect that it wouldn’t fail. Thoughts?
I’m also considering taking the XR2 advance module off the XR2 and installing it on this chrome bore, and then timing it like an XR2.
Thoughts on that? Thanks!

**mercrazy** · 07-01-2025, 07:12 PM

It’s not the oil, it’s flooding. I’m running double oil on a break in right now with no problem.

**LakeFever** · 07-01-2025, 07:55 PM

Is the compression tested good? 100% not the oil. I run 40:1 no issues at all and I’ve run it fat. I’m leaner out now but it never blipped a miss because of too much oil. Stumbling and bumbling id start with the basics. Clean all the carbs. Replace all the rubber hoses and try. If it’s still there do a continuity check on all the wiring harness and plug wires and verify health of all wiring then test. If it’s still there test stator and replace along with fresh or proven to run good switch boxes and test. If it’s still there after all this? Something wrong with the jetting is my guess. These motors can be run quite rich and they still run champ so something is off

**David - WI** · 07-01-2025, 09:04 PM

Guessing at the actual timing is silly.

**KIRCHNER** · 07-02-2025, 08:46 AM

i have a carb 150 looks showroom cond. never stopped chokin,smokin tried EVERYTHING. single piston os clamp, it sits in the corner.

**VkingMike** · 07-02-2025, 11:09 AM

I run 40:1 on every motor, no issues.

Hell I've had guys mix wrong and be in the 20:1 range and all it does is smoke more, but not choke out.

**croSSed** · 07-02-2025, 03:22 PM

Really appreciate all the feedback. Interestingly I had the same running issue with another 2.4 175 that was rebuilt 4 years ago. All of the bolt on parts from that failed 175 were installed on this new one. The carbs were fully gone through, sonic cleaned, and according to my research are the original ones to that motor. That motor lasted 2 years, was still running good and had good comp, but removal of the heads showed #1 cylinder scored, and all the others had scrapes from the rings, some worse than others. A complete mystery as to what caused this as I was running 40:1 premix and it never overheated, always had good water pressure. That motor was subjected to the same abuse as the XR2 was, but suffered a different end. When I was breaking this new one in I chalked up all the smoking and rough running to running 25:1 mix for break in. I’ve run at least 5 tank-fulls of 40:1 through it now and it’s running hasn’t changed.
I think I’m going to start with the easy stuff like timing. If that checks out I’ll move to the carbs. Two years ago I had a couple floats fail so I replaced all the floats, needles and seats, and checked/corrected the float height. The other thing I’m thinking if none of that takes care of it is to harvest the stator and switch boxes off my XR2 because those are known good.
Comp on this engine is good; checked it a week ago. The timing was not “guessed at;” the shop that put all this together for me did the timing and they know what they’re doing.
Any other thoughts keep them coming.

**David - WI** · 07-02-2025, 03:28 PM

Originally Posted by VkingMike

I run 40:1 on every motor, no issues.

Hell I've had guys mix wrong and be in the 20:1 range and all it does is smoke more, but not choke out.

Yes, that 20:1 ratio makes horsepower...and smoke!

**25two.stroke** · 07-02-2025, 04:03 PM

Check your timing on all 6 cylinders and make sure none of them are double-firing with a spark gap tester (look at all 6 simultaneously). Bias circuit in the switchboxes could be jacked. Oil definitely isn't the problem. And the timing module will do absolutely nothing for you if the motor is running correctly...however it can cause issues or mask issues when they are installed.

**pcrussell50** · 07-02-2025, 04:59 PM

Originally Posted by 25two.stroke

Check your timing on all 6 cylinders and make sure none of them are double-firing with a spark gap tester (look at all 6 simultaneously). Bias circuit in the switchboxes could be jacked. Oil definitely isn't the problem. And the timing module will do absolutely nothing for you if the motor is running correctly...however it can cause issues or mask issues when they are installed.

the one from Stephen’s instruments is a great one for this. Was it you, who pointed it out to me?

-Peter

**croSSed** · 07-02-2025, 05:30 PM

Thank you for this. I don’t know how to check for what you’re saying, so I might just replace the switch boxes. I have a few good OEM ones.

**David - WI** · 07-02-2025, 05:46 PM

If you have a tester like this, you hook your six (or 8) spark plug wires onto the brass rods and clip the big ground clamp onto the block You can screw the brass rods in or out to adjust the size of the spark gap. The sparks jump from the tip of the brass rods inside the clear cover to the ground rod running down the center.

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**pcrussell50** · 07-02-2025, 06:06 PM

Originally Posted by David - WI

I you have a tester like this, you hook your six (or 8) spark plug wires onto the brass rods and clip the big ground clamp onto the block You can screw the brass rods in or out to adjust the size of the spark gap. The sparks jump from the tip of the brass rods inside the clear cover to the ground rod running down the center.

that’s how the stevens instruments one works, except there is no cover. You could zap yourself, lol. And you can adjust the gap if you want to test how far it jumps. Wow. The CDI one is a good deal. Almost 50% cheaper than the Stevens one, too. :/

-Peter

**25two.stroke** · 07-02-2025, 06:47 PM

I use an old Chrysler/Force one LOL. I have seen the CDI it is badass. Open is ok as long as you use it for outboards only and not around flammable vapors. For inboard use you have to use the sealed ones.

To time every cylinder you follow the same timing procedure as #1 but do it in every other cylinder and make your own marks on the flywheel. A timing light is also helpful to see if it is double-firing. If you have all the cylinders laid out on the flywheel and you see more than the one you are timing lighting up then you know you got a problem...