'96 Yamaha 25HP 2stroke - Backfiring/lean sneeze?

I just bought a boat that has a 25hp Yamaha. it's a pull start and starts super easy but no matter what rpm it has a slight backfire. It never dies but the "cough" doesn't get better as rpm's increase. So far i've changed the spark plugs, and cleaned both carbs. I even tried seeing if i had an issue with one of the ignition coils. I can pull the plug wire off the top cylinder and it dies. I can pull the plug wire on the bottom cylinder and it keeps running on one cylinder no problem. I switched the coils and it's that same way. So i think i've narrowed the problem down to the bottom cylinder but i don't know where to go from here. I haven't tried adjusting the idle screws yet, but i might give that a try tomorrow since it sounds more like a lean sneeze. The only other thing i can think of is the CDI unit. What are the chances that the CDI unit is starting to go out? Thanks in advance!

There are a couple of possibilities:

1) lower carb is still not clean enough yet - I'd say that this is not likely the issue, as it is happening through the full RPM range, and you already went through them. Usually lean sneezes happen on the idle side of the carb, as the orifice is much smaller. Once you get to WOT, the main jet takes over, and the orifice is much larger, with fewer blockages

2) Timing issue - It may not be a lean sneeze but more of a back fire due to a timing firing issue. Again not likely, as it seems that your top cylinder is working ok.

3) Intake Leak - This is the one I'd be looking for. Your motor is drawing in air somewhere other than the carb. Make sure the carb is mounted securely and not loose. Make sure that there are no obvious cracks on the block anywhere. With you having a problem through the full rpm range, this one is a possibility

4) CDI Unit - Have you checked your spark with a gap tester? See if you are getting a strong consistent spark on both cylinders. You might be having an issue prior to the coil. You'll have to find a shop manual to diagnose whether the CDI Unit is functioning properly.

Good luck!

I visually checked for cracks and didn't see any. I'll recheck the gaskets on the carb as well. How would I test for hairline cracks I can't see? Would I see a change in compression or anything like that? I'll get a gap tester this afternoon and check the spark. Hopefully i can get it narrowed down a little better.

One older trick I've heard, though I haven't tried it and would be a little hesitant, is to take a small propane torch and open it just slightly. then you point it at and move it around the engine block and components while running, focusing on the intake side. If you get a slight uptick in the engine speed, then look more closely in the area that the propane torch is.

Again, I'm hesitant to even mention it, as I would be afraid of any potential stray sparks from your ignition causing an issue, but I've talked with a few old-timers who've done it looking for vacuum leaks.

Do some research on your own first and make sure you're in a well ventilated area and have an extinguisher available.

AT YOUR OWN RISK.

Here are some additional Methods:

https://www.crankshaftcoalition.com/wiki/Finding_vacuum_leaks

Good luck.

The infamous "mercury sneeze."

Either too lean or too much throttle plate opening combined with retarded timing, or a combination of all 3. Or carb sync is off a little bit. Or the pilot jet is partially clogged in one or both carbs. Also double check-some of them had 2 different main jets, one was slightly larger than the other and can aggravate a sneeze. I don't remember what timeframe they were doing the split main jet sizes...might have been 06-later.

Air leaks are not very common on these. Most everything uses O-rings instead of gaskets, but it is possible that a carb mounting O-ring can be missing.

Another thing to consider is the cowling. Once the engine is running close to right, you can adjust the idle mixtures slightly lean. Once the cowling is back on, it will richen up just a shade due to air flow restriction of the cowling air intake. The carbs on these motors are a bit sensitive, but it is nonetheless a great little motor!!

Timing at idle should be 7° ATDC +/- 2°. Pickup timing should be 0°. Max timing should be 25° BTDC +/- 2°. If you are real careful with fuel quality and ONLY run 93 octane, you can get away with up to about 28°. Helps a little bit. Check the timing-it has a big effect. No telling if anyone's monkey'd with it in the past.

Also on the carb sync, it's easy and doesn't require any special tools. Loosen the idle speed screw on the bottom carb until it no longer contacts the arm. Then in the middle of the throttle link is a screw. Loosen the screw and look down inside the throat of each carb. They should both be totally closed. If they are, tighten the screw. If not, you can move each throttle individually a little until they're both shut completely. This also makes a big difference in idle and low speed running quality.

Mixture screws, if they haven't been tampered with, should be your last adjustment. It's easy to start cranking on 'em but if they're close to 1 3/4 turns out already, leave them until everything else is done. I like to get the idle speed about 850 RPM in neutral, then start fine tuning. You can retard the timing at idle but once you get to about 7°, it will start to sneeze a little. I've done a couple in the past that I could not get them to 7° and maintain a good idle, so a lot of times I had to advance it slightly to regain control of the idle quality.

If done properly, it should idle in neutral cleanly at 750-800 RPM, and in gear at about 650-700 RPM.

Just a compliment to the helpful posters here. Especially turbotodd.

I frequent a number of boating sites and rarely have seen such a quality and full attempt to help. My hat is off to you Guys.

. richg99

I second that rich99! Turbotodd has helped me a couple of times. Always a promped reply. I don't have a local marine tech that I can depend on, so I'm left to my own devices. Thanks alot Todd. And to all of the other experienced tinnies out there.