Issues at idle

[TomG]

I've been following positron's thread on his carb rebuild hoping to find some clues, but I have an issue I need help with.

Today I was finally able to finish a long-delayed project of installing new instrumentation. I finally have a tach, which as it truns out, is both a blessing and a curse. I've always thought the engine idled high, but because she ran so darn well, I never dwelled on it (no pun intended!) Today my suspicions were confirmed. At idle in neutral, she runs at 1,400 RPM (according to the gauge). In gear, she turns 1,900 in forward (with a dirty prop) and in reverse, she turns 2,400, so I think the tach is pretty close.

I attempted to set the idle set screw on the throttle to slow her down and she did not like it one bit. She started to bog a down and I couldn't get her to idle lower than 1,100. I tried to increase the mixture on the carb, but the idle mixture screw is already fully seated (?).

So if I'm running full rich on the idle mixture and she really doesn't like the idle set screw decreased, where should I start looking? Does this sound like an idle jet that is clogged? Or an air leak? Should I just rebuild the carb?

Engine is a rebuilt MMI A-4 in 2008. Late model carb. I added the Indigo PCV kit this past spring.

At the higher idle, she runs like a champ.

I appreciate any advice!

[Loki9]

Sounds like a clogged idle jet or passageway. I'd take the carb apart and clean it, poking small copper wires through all the jets.

That was the problem on my recent rebuild, exactly the same behavior as yours.

[smosher]

The idle jet is an air bled which means you adjust of the amount of air going into the idle jet, mine is set to 1 1/2 turns from seating. Fully seated means its fully rich, turn it to start 1 1/2 turns out. I usually adjust it so it doesn't bog down when I give it power, if it bogs its too rich, could be other thinggs also, but this is where I would start

As Loki9 said you could have a clogged jet, and I had the same, wasn't until I poked small wire through the jets it went away, this is after trying carb cleaner.



Steve

[Mo]

Tom,
On my old carb that came with the engine I had (what I think was) an air leak that caused the same issue. I believe it was leaking around the the gasket /seal that connected to the throttle lever itself. I'm not quiet sure because I just cleaned up another carb that came with the boat and put it on...then all was fine.

I did try cleaning that particular carb, blew out the jets etc...it should have worked fine...but alas...same issue continued. I just put on another.

Maybe someone has some experience with "what I thought was the problem"...keep in mind that that was 4 years ago.

[hanleyclifford]

1400 rpm is not "idle". I doubt if your idle system is functioning at all. At that rpm you are running the carb "off-idle". Also, your timing could be way off, probably retarded. The 1400 rpm idle would mask this by running up the centrifugal advance. Before going any further I would advance the timing until I could slow the engine down to around 700 rpm with the idle mixtute screw 1 or 1 1/2 turns off seat. Set the timing at TDC using an automotive strobe type light (at 700 rpm). Then race the engine to 1500 rpm and confirm that total advance is around 17 degrees BTDC. Lastly, set the idle mixture screw.

[Mo]

Hanley,
Great approach....didn't think of that for this particular case.

[hanleyclifford]

Originally Posted by Maurice

Hanley,
Great approach....didn't think of that for this particular case.

Mo - I'm not certain timing is the issue but it does have to be correct before carb adjustments are made. It is possible the idle system is plugged in which case the engine would only run "off-idle". Another possibility is a vacuum leak.

[TomG]

Thanks one and all.

Hanley, how would electronic ignition figure into the timing issue?

I'm leaning (no pun intended) toward a clogged idle jet. The engine is an MMI rebuild and I seriously doubt the timing at the distributor has been changed since it was installed in 2008.

It's funny after writing down the symptoms and then re-reading my own post, it seems like a blocked idle port is the likely culprit. I'll let you all know what I find.

Thaks for letting me bounce this off you all.

[hanleyclifford]

It would seem that with it's greater dwell that electronic ignition would retard the timing if distributor position remained fixed but I am only speculating here - others may know more about this. In any case before proceeding in this case I would check timing with a light. Also before taking the carb apart I would blast every hole I could get to with carb cleaner or starting fluid - use the little red tube that comes with the can.

[Marian Claire]

And do not forget the safety glasses. Dan S/V Marian Claire

[ndutton]

When was the last time it idled properly and what if anything was done to the engine between then and the first high idle episode?

I really like Hanley's thought on timing at least as something to eliminate. Instead of checking with a timing light (can't see the front of my engine anyway so I think of other means) I'd have a look at the advance weights under the distributor plate to be sure they are free moving and functioning properly. It could be they are not and the timing was manually advanced some time ago to compensate. That could explain the poor running at lower RPM.

I'm not saying the carb isn't the problem, it might be as you suspect.

[hanleyclifford]

Could some one please post the picture of Tom Thatch's accessory drive timing pointer? A way to use a strobe light without access to the flywheel!

[ndutton]

Yep.

As I typed my post his pointer popped into my head.

[Dave Neptune]

Tom, unless you have moved the timing I doubt it is an issue with your engine, check to see if the weights are stuck and be done with the timing. I suspect an air leak or a clogged jet which would not affect the timing. You can spray a bit of "motor crack" around the carb flanges and the manifold to check for air leaks while idling (high). If the engine rev's up you have an air bleed also could be around the pcv valve. You could even plug it for the testing to eliminate it as a possibility.
If the above provides nothing back out the idle speed screw so the throttle can drop back a bit. Set the idle mix screw at 1 1/2 turns out and slowly throttle back until she begins to die and then APPLY some choke. If the choke helps you have a plugged idle passage or jet. The idle jet is tiny so poke something through it like a small wire. I do suggest that you dissassemble and blow air through the passages if you get this far. Some times you can rev the engine a bit and while opening the throttle close the choke and the additional sucking will sometimes break loose a plugged idle.
My engine will not even run with the idle at full rich (mix screw closed), when adjusting if I get to less than 3/4 turns out she starts to run so rich she stumbles and dies.

Dave Neptune

[jpian0923]

Neil, Hanley, That thatch picture is helpful if you know what it means. Care to explain? Will everyones pointer point to the same spot on the accessory drive pulley?

[Dave Neptune]

Jim, your mark will be where you mount the pointer. First you need to find TDC and then put the mark on the pulley. From there you need to mark the increments you desire for checking
Personally I think it is a waste of time, although a cool idea. Setting the timing via the "power timing" is what I got paid to do when doing power timing on a rear wheel dyno. I would only use the timing light to see where the timing wound up after setting it via power. This is the best way to set timing in a static load application like a boat, because the prop is the same load that the dyno would be. We all have differant hulls and props and the tweaking to get it right is what power timing is.
If I want to check or modify the advance curve I would use a distributor machine to get what I wanted to try and then power time it!

I'm ready for some grief on this post so have at it!

Dave Neptune

[thatch]

Dave
While power tuning the timing certainly will result in the highest power output of these engines, I have chosen to go with close to the factory recommended advance numbers. In doing so I have an engine that, despite being pretty high on the mileage scale, will start on a dime, idle smoothly at 500 rpms and produce almost zero blowby, (without the use of a pcv valve). Establishing an accurate top dead center mark on an A4 is a little difficult while using the flywheel pin, so I chose the method that you have probably used a gazillion times and just used a dial indicator down the #1 plug hole. In the case of a Catalina 30, with it's distributor side access door, using an accurate mark on the accessory drive pulley just makes sense. My timing setting?, 2 degrees btdc at idle. Another mark, 5/8" ahead of the tdc mark, gives me the 17 degree mechanical advance check position.
Tom Thatcher

[Dave Neptune]

Tom, do you have access to a distributor machine? Do you also have the "plot" of the adv curve. I am not saying that it is better to power time especially since it does require a bit of experience to develope that particular touch, it sure did for me. I do think it is far more difficult for a novice to find TDC and plot the degrees properly though. I wonder if there is any conversion numbers (timing plot) for our new fuels. Most of the fuel avalable today is designed for higher compression albeit with a lower octane rating . My guess is that the curve could be sped up a bit because of better fuel today than when the engine was designed.
Have you checked by power timing to see if it is any different than your setting at idle setting of 2 degrees? Mine too will idle all day (did so recently) at 600 however I do have blow by and low compression in two holes.
I seem to remember Tom at Indigo saying something about curving the distributor and I didn't get any numbers. However I did get his spring kit to change the advance a bit and correct for removing the drag of the points on the advance mechanism. The old dist machine I have available won't hook to an electronic ignition though.
I have your book at work and haven't explored it much yet, perhaps the curve is plotted in it.

Wish list could be a "degree wheel" sticker for the accy pulley. That would be nice.

Dave Neptune

[thatch]

Dave,
In the past, as I'm sure you have done, is to use a degree wheel and a timing light to observe the the timing through the relavent rpm range of whatever engine that we're trying to build a smooth advance curve for . In the case of our A4's there should be 0 degrees at idle (700 rpm) and 17 degrees of mechanical advance that should all be in by 1800 rpm's, according to Robert Hess's factory notes. By doing a little math concerning the
3-3/4" accessory drive pulley, I came up with 5/8" as a pretty good distance between the tdc and full advance marks. I feel that the factory timing is probably fairly conservative so I went ahead and bumped it up about 2 degrees. As far as an actual curve goes, I have not bothered to watch what is happening from idle to my target rpm of 2000 but, I do know that it's all in at cruising speed. Since I don't spend much time at half throttle, I don't see it as being too important..... My concern in setting the timing using the
"power tuning method" is that, I feel it is very difficult for the average guy to do it correctly. An engine tuned for maximum power is an engine that is on the ragged edge of detonation. I also feel that some of our "excessive blowby" cases are the result of excessive ignition advance. I knew that this conversation was far from being over.
Tom

[Dave Neptune]

Tom, I absolutely agree about experience being required to set the timing
statically under load. Good equipment like a dyno and or a good ear and tach. That is why I always set to max and then back it off a tick for an easy running smooth engine. My concern is that it would probably be more difficult for most to actually find TDC without an indicator and the skill to actually find TDC then.
I would love to see timing marks for our beasties especially for those of us that tinker. The 5/8" you refer to is around the outside diameter of the pulley I asume. Is it actually 5/8 or was that the closest fraction to your decimal figure? I think that with the modern fuels (yes even E-10) and an electronic ignition that t bit faster curve would be advantagis. One thing does suprise me though and that is a total of 17 (34) degrees. For a 3500 RPM rated engine with 6.5:1 compression that seems a bit short. My guess would be around 38~40 total, even at 2400 RPM's. I'm curious to find out if 2 or even 4degrees makes an improvement for you.
Did the max at 1800 come from Universal or Hess. For the most part I don't trust Hess's numbers on to many things that aren't copied from the manufacturer. A lot of his stuff for the A-4 is actually from the old fords regarding the carbs and plug recomendations. Yeh it is the same carb but for a different application altogether and a larger displacement.
I checked with Zenith and they have a few set ups for the same carb, they just don't have what went to what unless you have an original carb part number. My ole friend who works on A-4's says he has come across many different jetting sequences when doing the carbs. They all work just not as well as the correct one. I have seen thre in the late model carb myself.

Dave Neptune

[thatch]

Dave,
I have 9 pages of A4 info from Robert Hess which appear to be re-prints from actual universal manuals. The info includes engine numbers concerning manufacturing dates, bearing, piston, valve and tuning information that seems to all be correct as far as I can see. The 5/8" distance between my tdc and the 17 degree advance mark on the outer lip of the accessory drive is actually .556" by my math, which is actually closer to 9/16" (sorry). My attempts at establishing accurate timing marks for our A4's come from virtually all of the other gas engines that I have worked on over the years. They all have had the timing "locked down" at the factory (in the case of lawnmoyer engines) or are supplied with accurate marks, pointers and timing information. When the A4 was designed around 1950, most auto engines had similar compression ratios and timing numbers. This produced an engine that ran smoothly but that was a little bit lazy compared to todays quick accelerating, high revving motors. I have found, at least in my case, that by keeping this "lazy" state of tune I have an engine that shows almost no blowby (yes, advancing ignition timing does increase blowby) and which will idle down lower allowing me to ease into the slip a little slower. I realize that I could tune for more power but I seem to be able to reach hull speed without having to do so.
Tom

[Dave Neptune]

Thanx Tom, I hadn't tried my trig skills for a while! While sitting in the slip for a while except fopr a few whale watching excursions I will have some time. I'm going to dig out an indicator and generate a plot on the drive to see where I'm at for kicks.
I have seen 38 plus on more than a few under 8:1 load duty engines. If I remember right I set up a Ford Flathead hot rodder's dist. for 38 degrees and he wqas camed up a bit. I had to notch it out a bit and he was quite happy. I don't remember where he set it in though!!!
I'll let you know what I find out when I get it done. I'm most interested in being sure4 it is full advance at 1800.
Most of Hess's stuff is OK I just don't like the way his carb stuff accumilated.

Thanx for the info.
Dave Neptune

[thatch]

Dave,
My timing is set at 2 degrees btdc at idle at the accessory drive so when you add the 17 in the distributor, I am actually runing a total of 38 crankshaft degrees or, in other words, right about where you think I should be. The running qualities are so good at this setting, I doubt that I will be doing much more testing. As I see it now, the best thing that we could be doing is to figure out an easy way to install easilly readable marks on either the accessory drive or the flywheel. Different boats seem to present different challenges in this regard.
Tom

[roadnsky]

Gents-
FWIW, after listening to you two for most of the weekend,
I've decided to go back to a "baseline" TDC timing to get some 'numbers' to compare to my previous 'power timed' setting.
If there are enough differences to bother, I'll post 'em here.

Otherwise, I'll lay low and wait for you guys to serve-n-volley some more.

[thatch]

Jerry, Thank's for offering to help sort this thing out. If there is an area of speculation that should have been sorted out long ago it is in the area of the A4's ignition system. We have burning out coils and overloaded modules and no real timing numbers that we can trace back to the manufacture. It's amazing that these things run at all. Maybe we should all go to diesel. On second thought......
Tom