Rebuilt engine misses, won't idle

[edwardc]

A few weeks ago, I purchased an MMI rebuilt short block. Since then, I've scraped, cleaned and painted all the bits I took off of my old engine, and disassembled and cleaned the carb. I just got everything fully assembled on the bench, and, following the MMI startup procedure in the manual, successfully started and ran it to check for leaks and do the re-torquing.

The problem I have now is that I can't seem to get it tuned properly. Once warmed, and the choke is fully off, it has an occasional "miss" at cruising rpms (1800 - 2400), and will not idle smoothly, stalling below 1100 rpm.

Adjusting the distributor to increase rpms makes the miss worse. Retarding it minimizes (but does not eliminate) the missing.

Adjusting the carb idle mixture screw counterclockwise to 1 1/2 turns improves operation at cruise rpms but makes idle worse. Adjusting the mixture screw clockwise makes cruise rpms miss more, but improves idle. In order to get it to idle at 800-900 rpm, I have the mixture screw to within 1/8 th turn of being fully closed!

After 1 hour of break-in running at 1800-2000 rpm, I shut it down and pulled the plugs. All 4 were black, and numbers 1, 2, and 3 were wet. They were clean new plugs when I started.

All of these symptoms seem to me to point to the mixture being too rich, but I'm frankly puzzled at this point. The carb (a late-model 5-bolt Zenith), was running my old engine silky smooth, but just to be safe, i opened it and cleaned the float bowl and all the passages. The fuel is fresh, and in a brand new 3-gal outboard tank.

I need some help from the brain trust!

[hanleyclifford]

When starting up after a rebuild I like to make certain of dwell at specification and timing exactly at TDC. The idle mixture screw should not be less than 1 turn out or something else is off.

[edwardc]

When starting up after a rebuild I like to make certain of dwell at specification

I have the Pertronix distributor with electronic ignition, so dwell is not an issue.

... and timing exactly at TDC.

I started off at TDC, but had to go many degrees after TDC to make it even remotely happy.

The idle mixture screw should not be less than 1 turn out or something else is off.

Interesting. On my previous engine, this carb's mixture was set to 1/2 turn and the engine ran like a sewing machine, with a smooth idle @ 800 rpm.

Don's directions say to start with 2 1/2 turns, and adjust as needed, which is what I did.

When it came time to reassemble the engine, I debated over whether to open the carb or not, since it is relatively new (2007) and had been running silky smooth for the last year. But I'm glad I did. The jets and passages were all clean, but the float bowl was full of an amazing amount of some gelatin-like tan sludge! It was only a matter of time 'till a chunk of it broke off and clogged the main jet.

But now it's all clean as a whistle, and I have no clue as to what's going on.

[hanleyclifford]

I am not familiar with the electronic ignition, but there must still be dwell angle which can be measured - I believe the spec is a little greater for electronic ignition. If your engine wants retarded timing then something is definitely amiss. I would review that ignition system.

[jeffgerritsen]

I wonder if the distributor was installed incorrectly. Also electronic ignition systems usually have almost a full dwell angle, so for a four cylinder the dwell angle could be as high as 90 degrees - well not completely, but dwell angle is maximized due to eliminating the point set.

[edwardc]

I wonder if the distributor was installed incorrectly.

Pretty sure I got it right. With the engine at TDC on the compression stroke on cyl #1, the rotor is pointing straight out from the engine block.

Also electronic ignition systems usually have almost a full dwell angle, so for a four cylinder the dwell angle could be as high as 90 degrees - well not completely, but dwell angle is maximized due to eliminating the point set.

That's right. The dwell angle of points determines how wide the pulse is to the coil, longer at low rpm and shorter at hi rpm. Electronic ignition has essentially a fixed-width pulse, insuring maximum pulse energy to the coil at all rpms. But this confuses dwell angle meters, sometimes looking like a variable dwell that gets larger at higher rpms.

I've also checked the inside of the distributor cap for cracks or carbon tracks and found neither.

I wonder if a bad wire from the coil to the distributor could be causing a weak spark. Might this explain the symptoms?

[hanleyclifford]

Somewhere on this forum I read a post by Don Moyer that dwell for electronic ignitions is just slightly greater than for point style ignition. In conventional systems for Delco the dwell should be around 31-34 degrees. Ideally this should be constant throughout the rpm range but in practice the dwell falls off slightly as rpms increase. One of the advantages of the electronic ignition system is the reputed steadier dwell at all rpms. The dwell should be measurable using an ordinary dwell meter. The defective secondary wire is also a strong candidate for erratic firing as rpm increases (for both ignition styles). I strongly advise the use of a strobe timing light on the A4. At 850 rpm look for 7-10 degrees BTDC. At 1700 rpm look for 17-20 degrees BTDC.

[Dave Neptune]

The miss!

Ed, your running the engine on a stand right(?), if so I wouldn't worry to much about the cruising rpm's as there is no load on the engine. It could be someting in the cap itself like a carbon trail, is it new? If so I'd look to the carb as you know the idle screw isn't close to where it should be.
You state that the plugs were wet and you did redo the carb. I would check the float level and take a really good look at the seal in between the halves of the carb around the idle well. If the seal isn't good there it will need the screw to be in more (rich) to compensate and if the fuel is bouuncing around a bit as the engine vibrates it will slosh around and temporarily seal the leak a bit as some extra fuel is sucked into the carb's idle circuit. I think it is worth a look. I have played with various float levels and found the carb a bit sensative to the level of the fuel in the bowl, especially at idle and starting cold.

Dave Neptune

[edwardc]

Dave,

Thanks for all the great suggestions and questions. To address each in turn:

Ed, your running the engine on a stand right(?), if so I wouldn't worry to much about the cruising rpm's as there is no load on the engine.

Yes, I'm on a cradle on the bench. But it's not the actual rpms that concern me, but the fact that the engine misses at that speed.

...It could be someting in the cap itself like a carbon trail, is it new? If so I'd look to the carb as you know the idle screw isn't close to where it should be. ...

Cap is about 3 years old. I checked it and its clean. No cracks or carbon trails. I also just did a quick check of the spark, and it's nice and strong, throwing about a 3/4 inch arc. So I'm going to focus on the carb.

...You state that the plugs were wet and you did redo the carb. ...

Wet and black, indicating an over-rich mixture, which is consistent with the idle mixture screw setting.

... I would check the float level ...

Not quite sure how to do that with the carb sealed up. And with it open and empty, there's no way to know exactly how high the gas is when the float rises high enough to close the needle valve.

On a related question, should the float hinge pin slide out easily, or is it press-fit in? Mine is solidly fixed and will not budge. Press-fit or corrosion? Without being able to remove the float, I can't inspect the needle valve and seat, or remove the gasket. Would a leaking needle valve force extra gas thru the main jet, or would it just overflow somewhere?

... and take a really good look at the seal in between the halves of the carb around the idle well. If the seal isn't good there it will need the screw to be in more (rich) to compensate and if the fuel is bouuncing around a bit as the engine vibrates it will slosh around and temporarily seal the leak a bit as some extra fuel is sucked into the carb's idle circuit. I think it is worth a look. ...

Sounds like a possible cause. I'll investigate it and report back.

I have played with various float levels and found the carb a bit sensative to the level of the fuel in the bowl, especially at idle and starting cold.

How do you adjust the level? Just bend the float arm?


Thanks again for all the great advice!

[ndutton]

I think you've found it.

I've had no issue with my A4 carb but on others, float level adjustment is critical to proper performance. When I rebuilt my A4 carb it took a little pushing to get the hinge pin out but not all that much. And yes, adjustment is achieved by bending the tab.

Good luck.

[Dave Neptune]

Not breaking the float!!!!!

Ed, when adjusting the float error to the high side of the setting which is a loweer fuel level. The float pin should slide in and out easily, many remanufacturing houses set the pin with a punch to hold it in place for assembly so they don't need to be as careful as necessary. Try knocking it out with a small punch or dulled nail ~carefully.

Hold the top half of the carb upside down with no gasket and the float installed, check the height with a scale (good ruller). The factory setting is 1 5/32" to the bottom of the float, which is on top and resting against the needle which is resting on the seat. Two things of import here~~1~be sure the bottom of the float is paralell with the body (this controls the closing position of the float accurately) of the carb, or as close as possible.~~2~When tweeking the tabs use a pair of needle nose and/or duckbill pliers~DO NOT PUSH, PULL OR BEND the tabs by holding the float itself only hold the brass tabs. Now you can bend the larger portion that attaches to the float for paralell adjusting and the little tab that rests on the seat for the height. This can be a bit frustrating so go slow and easy. I suggest that if anything you set the float a bit HIGH 1 3/16 ~1 5/32, these adjustments are important to control the emulsion wells that mix air into the fuel to break it up! The factory adjustment specs are 1 5/32 +/_ 1/32" which is why you need it to be as paralell as possible. I run mine at 1 7/32 and like the way it performs.

When checking the sealing area the hole gat a magnifying glass or a hood so you can see if the gasket was actually compressed which is NECESSARY for a good seal. I had to flatten both sides of my carb to get mine to seal properly. If you need to do this stay away from the area of the idle well so it becomes a bit of a high spot which will give you a good seal.

Dave Neptune

[edwardc]

Dave,

Thanks for all the great info! As always, you're a fount of information!

I hope to be digging into this tonight. I have a couple of new carb gaskets on order and they are supposed to be in today.

[sastanley]

Ed, Isn't it great that we live so close to MMI? My stuff usually shows up within 24 hours!

Good luck with your work...it is getting chilly to be working on boats!

[edwardc]

Ed, Isn't it great that we live so close to MMI? My stuff usually shows up within 24 hours!

Yes it is convenient! But it makes me spend too much!

Good luck with your work...it is getting chilly to be working on boats!

Thanks.

[edwardc]

Success!

My gaskets came in, so I tore the carb apart again. I checked the old gasket, and it looked good and compressed all around the idle well.

The first thing I noticed, now that I know what to look for, was that the float was not even close to parallel to the top half's gasket surface. And the height was WAY above 1 5/32".

The float hinge pin was firmly stuck in, apparently due to corrosion of the aluminum ears. Trying to drive it out was getting nowhere, and I was afraid I was going to damage something. I finally got it by using pliers to FIRMLY grip the center of the pin between the two float hinges, and twist it back and forth while pushing to one side. This got it about halfway out, and I was able to use a small blunted nail to drive it the rest of the way. The center of the pin was a bit munged up, but I used a small file to clean it up. I also found that one of the larger tip cleaners from my welding torch was perfect for cleaning up the hinge pin holes.

Once I had the float and gasket off, I removed the needle valve and immediately discovered that the seat orifice was almost completely filled with a hard brown substance that was impervious to the carb cleaner. One of the smaller tip cleaners took care of that. Now, with the float, valve, and pin reassembled, the float was perfectly level, and the height was 1 7/32"!

Since this was within the range Dave recommended I elected to leave it alone, and reassembled the carb with a new gasket.

With the timing at 0 deg TDC and the idle screw at 1 1/2 turns, I hit the starter, and immediately noticed the difference. I could idle! After that, it was just the usual drill of power-timing the advance, and tweaking the idle mix, which ended up arouind 3/4 turn.