Low RPM miss

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Model T Ford Forum: Forum 2014: Low RPM miss
Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 11:51 am:

I've been trying to eliminate a low RPM miss in my 1926 TuDor. The car starts easily, by starter or crank. High speed performance is very good. What puzzles me is that when the engine is at a slow idle AND I have the timing retarded all the way, the engine is smooth with no miss. BUT.... when the engine is at the same slow idle and the spark is advanced to one half or more, seems cylinder 3 and 4 begin to miss.

My first I thought that maybe the timer (Anderson) was shorting out as it was advanced. This is not the case. I made sure that the wires and terminals have plenty of clearance. Besides I would think that if this were the case the high speed performance would also show this miss.

I replace the timer with a new Anderson with no difference. The initial timing is very close to correct. Number 4 plug does become sooty after some running at idle, indicating that this is the cylinder. I checked the coils on a HCCT and also changed them around. I tried different spark plugs and checked the wires between the timer and coil box.

Is it possible that at a slow RPM, less than 500 rpm, is max advance to much under some cases, for that cylinder to contribute?

Any ideas?
Thanks Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Henry Petrino in Modesto, CA on Saturday, August 02, 2014 - 12:00 pm:

Does it behave like this both on mag and on battery?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Val Soupios on Saturday, August 02, 2014 - 12:08 pm:

I had that trouble with an Anderson timer. I switched to an old New Day that I had and it cleared up. I then tired a TW timer and that worked fine also and that is now what I am running on all my cars. I do not think it was the Anderson timer causing the problem per se because it worked fine on one of my other cars. One other thing to consider is that the best position of the spark lever will often differ when you switch over from battery to mag.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 12:12 pm:

Yes, the problem is there with either mag or battery. My magneto is putting out 7.5 volts at this low rpm.

Maybe I'm being to picky, but I pretty sure that last year the engine would run at full advance at idle.

Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Thode Chehalis Washington on Saturday, August 02, 2014 - 12:29 pm:

Mike,
Try switching the the coil box position that fires #4 to eliminate a coil box problem. That is, switch the timer wires and plug wires on the coil box for #3 and #4 to see if the problem changes to #3 cylinder.

Is the compression close to the same on all cylinders?

Jim


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank Harris from Long Beach & Big Bear on Saturday, August 02, 2014 - 12:43 pm:

You may have a vacuum leak. Squirt some WD40 at all of the joints while it idles and if the engine speed changes, fix the leak. Don't forget the carburetor throttle shaft and the joint where it mates to the intake manifold. A runnning engine will stand a small leak at speed but at idle it is less forgiving.

It's worth a try and a quick check my result in your finding the problem. I have fixed several T's that way.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 12:53 pm:

I have a fun projects plastic coil box and it meggers at 1000 MegOhms. That includes the plugs, the wires and the box. SO I beleive the coil box etal. is OK. I measured the compression to be
#1 50#
#2 49#
#3 50#
#4 49#

Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 01:23 pm:

According to Ron Patterson and Steve Coniff's article, Model T Ignition System, At full advance the first spark would occur at about 64.5 BTDC, battery or mag. Is it possible that at this time in the combustion cycle, and at this low rpm, could the intake valve still be open? OR, could it be that the air fuel mixture is not compressed enough either to burn or burn to develop much pressure?
Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Saturday, August 02, 2014 - 01:44 pm:

Swap the coils and maybe the plugs as suggested but I really feel that you just don't notice the miss at higher speeds. In other words it's still there when you rev it up it simply isn't as noticeable. You mention soot on #4. Some things happening there. Soot results from oil or rich running. Since it's confined to one cylinder I'd guess oil but your compression is good so not sure about that. Dumb question: are you running with a hot air pipe?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Craig Anderson, central Wisconsin on Saturday, August 02, 2014 - 01:48 pm:

Weak exhaust valve springs will cause this too.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 02:09 pm:

I've swapped plugs and coils between the used ones and other unused ones that I have. You may be correct that it is not noticed at higher RPM, yet I seem to have power. After a long run or tour at speed the spark plugs are all tan in color. A slow 4th of July parade will carbon them all up some. I don't have a hot air pipe and I'm running a Stromberg OF carb. Next time I have the valve cover off maybe I'll just change out the springs, they are pretty cheap. Stumped for now.
Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Dowd on Saturday, August 02, 2014 - 02:22 pm:

1000 megaOhms is one billion ohms. I'm not sure what the correct value should be but a billion ohms is an open circuit.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andre Valkenaers on Saturday, August 02, 2014 - 02:50 pm:

Mike,
I think your problem is not the ignition or the carburetor but the position of your spark lever.

At Idle and your spark lever all the way down the spark came much to early and you engine will knock and will not run smoothly. Just retard your spark lever till the engine run straight. From there you advance the lever as you are running faster.
Somewhere in this forum there is a schema about the way the levers should be when driving the model T.

Good luck

PS photo is me and my dad on the first run with the 1922 "woody"

Andre
Belgium


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange on Saturday, August 02, 2014 - 02:53 pm:

Here's the diagram from the T owner's manual:

diagram


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 02:58 pm:

Jim that's correct a billion ohms (a full boat load). The idea is to look for shorts. Many wooden coil box are contaminated and will short when the spark occurs. A megger is a device that places 1000 volts on a device and measures leakage current but displays the results in ohms. High ohms mean open current, just what it should be.

Andre, you may be correct, that is what I was eluding to in an early post. Just kinda thinking out load. Always curious and a little nosy.

Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Saturday, August 02, 2014 - 03:34 pm:

Have you checked the centering of the camshaft in the cover plate. There is a tool made for that purpose. It should slip over the end of the camshaft and fit within the groove where the timer fits. If that plate is not centered, it can affect your timing causing one or more cylinders to fire before others. That is not as critical with a new day timer, but with either roller or Anderson, it could be a problem.

In any engine, you will need a stronger spark when pulling hard, such as starting out from a stop or ascending a hill. If one or more plugs is weak or if the coil or coil box have a high restance short, you could have a miss at slow speed or when pulling hard, but run smoother when not pulling. Also when your engine is moving fast you might not feel the miss.

I had a similar problem with a 2006 Buick. It would miss when the engine dropped under 2,000 RPM but seem to run smoothly at higher RPM. It turned out to be a bad ignition coil.

Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 03:42 pm:

Norm, I recently replaced the large timing gear and at that time I centered the plate. I'll check it again, because it is a good idea. SOmewhere there is a good article about how not being centered changes the timing between cylinders.
Thanks for the thoughts.
Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Coiro on Saturday, August 02, 2014 - 03:53 pm:

It sounds to me like Andre is right. In a modern car, the timing of the spark is automatic and sets itself according to the engine's requirement for whatever RPM the engine is turing out at the moment.
In the case of a Model T, it's necessary for the driver to manually select, for a given RPM, that point in the power cycle where the spark should occur to optimize power and smoothness.
Unless I've misunderstood the question, it's not at all surprising (or wrong) that the engine "isn't happy" running at very low idle with the spark timing advanced for high-RPM.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Saturday, August 02, 2014 - 06:55 pm:

Another thing you didn't mention in your first post, is whether you are using a high compression head or pistons. With higher compression, the engine is more sensitive to timing. You would use the first half of the quadrant for idling and starting out, and then advance about 3/4 after it is up to speed.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Craig Anderson, central Wisconsin on Saturday, August 02, 2014 - 07:05 pm:

Mike.......if you decide to check out the valve springs there is no need to remove anything except the side cover/covers.......yet.
With the engine at idle use a large screwdriver or two to "open" a valve spring between the coils and listen for changes.
Intake valves are much more forgiving than exhaust valves.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Saturday, August 02, 2014 - 07:50 pm:

Norm, I do have a aluminum Z head on this engine, never thought that that would make a timing difference. Maybe my initial timing is a bit off. I have checked it, but nothing more that a straw in the cylinder, stopping when I think I'm at TDC. You'd think there would be a better, more accurate way.
Thanks Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ron Patterson-Nicholasville, Kentucky on Saturday, August 02, 2014 - 07:52 pm:

http://www.twcomponents.com/#!the-accurate-timing-indicator/cit


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