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  #1   IP: 23.16.248.137
Old 08-02-2017, 11:40 PM
Marty Levenson Marty Levenson is online now
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Adjustable Main Jet and another carb Q

I have a MMI rebuilt engine that is just over three years old. It has had light use, only ethanol free gas. Been running SO perfectly! Idles nicely at 650 RPM. Haven't touched timing or adjustment since it came from Moyer. FWC (180 degrees) and electronic ignition. The only anomaly, I've just discovered, is sooty plugs.

A few days ago she almost stalled at about 800 RPM as I was about to pick up the mooring. Revved her up and completed the maneuver, but was suspicious of a problem. Took the polishing filter off, and found I could barely blow through it. I was so preoccupied with other issues this spring that I had neglected to change that filter, as well as the Raycor. Thought that might be the problem, but testing after those changes she still wanted to stall below 1000 RPM. Thought the dirty filters might have led to a dirty carb, so I swapped in my spare carb, and indeed she runs fine with my old spare. BUT to my surprise, upon opening up the carb that came with the Moyer rebuild, it is absolutely spotless inside. Like new. Could the idle jets still be dirty and plugged? Seems the only explanation, but incompatible with the state of the carb. Am I missing something?

Tangentially, I have (but not installed) an adjustable main jet (from yet another carb!). Not the mixture adjustment screw (that came installed), but the one that screws in horizontally at the rear of the float chamber. What is the advantage of installing that? How is it adjusted?

Before folks jump on me, yes, I know that I am only managing an upstream problem with dirty gas! I did drain and clean the tank five years ago and have found that diligence with filters has given me reliable power.

Many thanks, in advance.
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Old 08-02-2017, 11:50 PM
JOHN COOKSON JOHN COOKSON is offline
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How long has it been since you serviced the advance? (Under the breaker plate in the distributor)

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Old 08-03-2017, 01:19 AM
Marty Levenson Marty Levenson is online now
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Never thought of checking that on my "new" (three-year-old) engine. If that were the problem wouldn't the spare carb have the same idle issue? Or are you suggesting that can account for sooty plugs?
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Old 08-03-2017, 08:05 AM
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What is the advantage of installing that? How is it adjusted?

My understanding is that the main jet adjustment allows you to control the flow of fuel available to be mixed with the air and delivered to the cylinders. Basically does the same thing the idle mixture adjustment does but the idle adjustment controls air flow and the main adjustment controls fuel.
I have used my main jet to maximize fuel efficiency. With the engine warm and running at "cruising RPM" I adjust, in reduces flow/out increases flow, to get the best performance. I may be sacrificing some performance at other RPM but have noticed no ill effects from this method. By running on the lean side I believe it helps keep the plugs clean and limits excess fuel from passing thru and fouling the water.
There is a lock nut that has to be loosened before turning the T handle to make adjustments. The biggest problem I have had is when re-tightening the locking nut the T may move slightly. Good access is key.
Dan S/V Marian Claire
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Old 08-03-2017, 09:53 AM
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Quote:
BUT to my surprise, upon opening up the carb that came with the Moyer rebuild, it is absolutely spotless inside.
Marty-
Did you actually "split" the carb open and eyeball the jets?
Back when my carb was a couple of years old, I was surprised when I found a couple of the jets were dirty.

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Originally Posted by Marian Claire View Post
There is a lock nut that has to be loosened before turning the T handle to make adjustments...
I find keeping a light counter leverage on the T-Handle while tightening the nut is important.
As Dan says, you need good access to get back in there.
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Old 08-03-2017, 10:07 AM
JOHN COOKSON JOHN COOKSON is offline
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Originally Posted by Marty Levenson View Post
Never thought of checking that on my "new" (three-year-old) engine. If that were the problem wouldn't the spare carb have the same idle issue? Or are you suggesting that can account for sooty plugs?
Well, I don't know. Maybe the advance was sticking and not sticking. We could go on and on like this.
For me servicing the advance is a yearly maintenance item. Maybe it is time to have a look see at the advance to rule it in or out as a contributing factor? You will be surprised how much better your engine runs.

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Old 08-03-2017, 10:39 AM
Marty Levenson Marty Levenson is online now
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Thanks, John - I will check and lube that this fall.
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Old 08-03-2017, 10:45 AM
Marty Levenson Marty Levenson is online now
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jets

Thanks, Roadnsky - Yes I did fully disassemble the carb for inspection.

Can one tell by looking if jets are clogged? They LOOK new. I am planning to clean with thin wires and compressed air anyway. Is it necessary to remove the jets for cleaning? I am hesitant as I once destroyed one trying to unscrew it.
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Old 08-03-2017, 10:48 AM
Marty Levenson Marty Levenson is online now
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Adjustable main jet

Thanks, Dan - appreciate the clear description. So, to adjust, warmed up at a cruising RPM you slowly tighten the T handle until any change in sound or RPM and then back off a 1/4 turn?
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Old 08-03-2017, 11:47 AM
Ken Rockwell Ken Rockwell is offline
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Are you getting any black smoke?
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Old 08-03-2017, 11:58 AM
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Quote:
Originally Posted by Marty Levenson View Post
Thanks, Dan - appreciate the clear description. So, to adjust, warmed up at a cruising RPM you slowly tighten the T handle until any change in sound or RPM and then back off a 1/4 turn?
I adjust in and out to get the best "sound". What I am looking for is max performance at a set RPM. I determined my "cruising RPM" by taking into account my speed over ground, load on the engine using a vacuum gauge and just how the boat and A-4 sounded. This way I know that when I set my throttle to give me, 2200 RPM in my case, that I am getting good performance. I tend to run for hours at a set RPM.
Dan S/V Marian Claire
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Old 08-03-2017, 12:41 PM
Marty Levenson Marty Levenson is online now
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carb

Thanks, Ken, - no smoke.
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Old 08-03-2017, 01:05 PM
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Are the plugs dry or wet?
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Old 08-03-2017, 02:03 PM
Marty Levenson Marty Levenson is online now
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plugs

slightly sticky.
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Old 08-05-2017, 12:15 PM
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Following in Hanley's experience, I installed an oxygen sensor and a fuel:air ratio gauge, and use it to set the main jet mixture screw for a perfect 14.7:1 ratio at cruise RPM.
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Old 08-05-2017, 03:55 PM
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Quote:
Originally Posted by edwardc View Post
Following in Hanley's experience, I installed an oxygen sensor and a fuel:air ratio gauge, and use it to set the main jet mixture screw for a perfect 14.7:1 ratio at cruise RPM.
Darn you! An hour ago I didn't even know there was such a thing.
And now I sorta want one.
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Old 08-05-2017, 05:09 PM
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Hanley's O2 sensor was pretty cool
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Old 08-05-2017, 05:16 PM
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The MMI manifold makes this particularly easy to install. It has exhaust take offs at both ends (to accommodate V-drive installations), with a blank flange plate bolted over the unused port. This blank plate makes the perfect place to install the oxygen sensor because it's smooth and flat, so it's easy to drill and tap.
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Old 08-05-2017, 09:26 PM
Marty Levenson Marty Levenson is online now
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sensors

This is all news to me! So, both sensors are on the exhaust manifold? I find that confusing at first glance: I would guess the ratios change after combustion?
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Old 08-06-2017, 08:20 PM
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It's simpler than that. There's only a single oxygen sensor. By measuring the amount of oxygen left in the exhaust gases, you can directly determine the ratio of fuel and air that was burned to create that exhaust.
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  #21   IP: 23.16.248.137
Old 08-07-2017, 01:08 AM
Marty Levenson Marty Levenson is online now
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O2

cool. Thanks.
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Old 08-08-2017, 07:23 PM
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Now maybe I want one, too. I've got that exhaust flange our host sells with the 1/8" NPT threads for a backpressure sensor, but maybe it would work for an oxygen sensor? Any recommendations?
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Old 08-10-2017, 02:51 PM
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Me too!

This ebay item 222026510913 seems to be a nice gauge, analog and digital display only $20! Ships from New Jersey instead of Hong Kong.

Specs are:
Input voltage: 0 to 1V
Display range: 20% to 10%.

NGK 21002 O2 1-wire sensor is only $16 on Amazon, but they don't say what the voltage is. It looks like a spark plug size tap may be required.

Also, is the plug that closes off the unused end of an original manifold "tappable" for the sensor, or do we need to install a Moyer plate?

Jim

Last edited by Jim Booth; 08-10-2017 at 02:54 PM.
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Old 08-10-2017, 05:40 PM
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I used this O2 sensor

http://www.summitracing.com/search/?...UM-G2989&dds=1

And this gauge

http://www.summitracing.com/parts/CYB-7009/

Unfortunately, cyberdyne has discontinued it, and Summit Racing has exhausted their stock . It's too bad, as it was inexpensive and simple.
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Old 08-11-2017, 02:28 PM
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I googled around and found it appears the O2 sensors make 1V, with 0.45V at the ideal air/fuel mixture. So for adjusting the carb, all we need is the sensor and a trip to Harbor Freight some day when their digital volt meter is free with any purchase. I have several of them already. While trying to find detailed spec on the NGK sensor I saw one place selling it for $12. So the plate and tap are the biggest cost, and really not that
much for a useful tool.
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