MTFCA Forum

Ok, I need some expert help on this one. First the problem- car starts good on batt. switch over to mag, car runs ok. idle up and car runs ok. 2- run car on batt- go from low to high at a slow speed, car runs fine, get up to a moderate speed- low 20's to 25 switch over to mag, car runs fine. 3- start car on batt, switch over to mag, start out in low- go to high and car bucks and misses out. Car has been rewired, in car mag recharge, mag post cleaned.mag output was in specifications when idling and idles up ok on mag. It's when trying to drive and starting out on mag that I have a problem. Is the mag having a problem under load when going from low to high and is there a fix. I'm still a novice and at a loss on what my next move should be. I had a problem when starting and switching over to mag-car would run rough.Took off horn-light button cause I think something was grounding out and car now runs fine on mag at idle!! but not running smooth going from low gear to high. I get the feeling something is still grounding or shorting out but I don't know where. I know this is a long post and I've tried to explain I hope what is happing. Car is a 19191 touring. if that helps. Thanks in advance.

If you measure the output of your magneto with a Fun Projects St. Louis Magneto Tester and post the results it would go a long way toward diagnosing your problem.

That's a 150.00 item. I got to think about that for only a one time use maybe. Appreciate the response thou.

When you're shifting from low to high, the engine is running the slowest and therefore your magneto output is at its lowest. It's also the time when you need all 4 cylinders participating or you get bucking & jerking. To me, it would appear that your magneto is getting very marginal in its output, hence Ron's suggestion.

Is it possible the 3rd main thrust bearing is worn?

My first thought is you have to practice shifting. There is sort of a knack to shifting a T from low to high that if you don’t do it just right, it will “buck”. Experienced T drivers might refer to this as a “bad shift”, although that may be so far back in their driving past they might only barely remember it. Hopefully this is the only issue. Have someone very experienced drive your car and see what they think.

Adam

I was thinking the same thing. I'd only add that too-far advanced timing combined with a bad shift will exacerbate the problem as well, particularly if a high compression head is involved.

Have your coils been rebuilt and properly adjusted?

Could be as others have stated the shifting from low to high maybe the issue. You can kill or stall a T when don’t have the smooth shift. At the least it will sort of chuff chuff along. It is a trick or certain way to get the smooth transition from low to high. Some slow the engine down some and others don’t along with making the shift. For me it’s slowing the engine but others may vary.

I don't think it's my shifting. Been driving the car for a few years now, along with my other one. Thanks guys but it's not my shifting. I shift it the same way on battery or mag. but thanks for responding.

Have you tried moving the spark lever? Retard it a bit for shift down to high and then as it picks up speed advance it more. Also as you run the engine at a fast idle, move the spark up and down and you will find that it moves a few notches without changing the advance. Then when you move it one notch farther it advances or retards the spark depending on the direction you move the lever. There are usually at least 3 (what I call nodes) where the spark advances and sometimes there is a 4th one when you pull the lever all the way down. Find the spot right in the center of each node and mark it with a marker or a drop of white paint on the quadrant. This is the place where all coils should be firing together at the same amount of advance. If you are right at the edge of the node, one or two coils could be either advanced or retarded from the others. Note, if the timer is in perfect condition and the coils are tuned exactly right they will all advance at the same point on the spark lever. Anyway having said the above, find that point on the quadrant for each node and when you advance or retard the spark try to find that point and it might run better.
The other possible problem could be that the magneto is not putting out enough voltage to operate at a very slow speed but as you speed up it will work better. In this case your magnets are either too far from the coils or the magnets need to be recharged. It might work better if you rev the engine a bit more in low before you shift into high especially if going uphill when you shift.
Norm

Norm, The car runs fine on battery, it goes up to speed fine on battery. I'm not having any problem on battery. If doing what you suggest, wouldn't I have the same problem on battery when starting out. Remember, I said there was a problem when at idle switching over to mag the car started running rough. Then I removed the horn-light combo switch and car idles fine on mag, I know that sounds weird. In fact I can idle it up high on mag. If the coils are not firing right wouldn't it run rough on battery or on mag at idle The car was a magneto light car. when running on mag and turning on lights the car would run rough. I have since removed the mag bulbs. I think your right that there is some kind of draw happing but the car had a in car mag recharge. I just have a feeling that something is grounding or shorting out when starting out on mag.

Carl, I am about 1 hour west of Indy, just off of I74 about 1 mile. I have the Fun Projects St Louis mag tester and their Strobospark. Would be willing to help you out, if I can.

no matter when I go from battery to magneto I have to re-adjust my spark or my 1915 or it will will run badly. nature of the beast?. bobt

nature of maladjusted coils

cve92 wrote: ↑

Mon Jun 22, 2020 12:16 am

Norm, The car runs fine on battery, it goes up to speed fine on battery. I'm not having any problem on battery. If doing what you suggest, wouldn't I have the same problem on battery when starting out. Remember, I said there was a problem when at idle switching over to mag the car started running rough. Then I removed the horn-light combo switch and car idles fine on mag, I know that sounds weird. In fact I can idle it up high on mag. If the coils are not firing right wouldn't it run rough on battery or on mag at idle The car was a magneto light car. when running on mag and turning on lights the car would run rough. I have since removed the mag bulbs. I think your right that there is some kind of draw happing but the car had a in car mag recharge. I just have a feeling that something is grounding or shorting out when starting out on mag.

Big difference between running on battery or magneto. The magneto is a wave form output, so you have to match the peak to supply power to the coils at 16 cycles per rev. The battery supplies steady power to the coils. The spark has to be timed different between the two. If the thrust is worn too much on the rear main (magnets will be moving back and forth changing the gap) or magnets are weak, magneto output will be reduced esp at lower speeds. If the battery and lights are not in use, they have no effect on magneto output running at idle unless there is a short someplace.

Before you take anything apart, take Scott C up on his offer. Good luck, all the best, jb

When you engage the trans the force will pull the magnets away from the coils... the thrust bearing on #3 prevents this, when it wears, your mag goes weak, and you need to address the problem. I understand there are even 3rd main caps that are designed to take up that thrust...

If it idles on mag, and runs in low gear on mag, it is very hard to point to a mag/coil/rear-main clearance problem.

Until the magneto is tested with a St. Louis tester (or use the bulb "burn out test"), everything is speculation. Ron's early advice would be the ideal first step in diagnosis. $150 or not, if it saved me from an engine teardown, it would be worth it. Additionally, let folks around you know you own one, and your popularity will go up several notches.

Until the previous horn button problem is understood in it's entirety, everything is speculation.

An ECCT with the correct software will do the same test as the St Louis Magneto tester.

Yes, for only $450 (because of optional software required), to someone who is not sure about spending $150. A functional tool with good reviews, but pretty pricey for someone not doing a lot of coil work. I got the info from the website. If I'm wrong or have misapplied the combined costs, I'm certainly open to correction.

I just repaired a 1924 with the same or very similar symptoms.

They were most notable at higher speeds with either the Bat or Mag selected.

The fix was a new timer with a smoother cover and a stronger roller spring.

Hi everybody. First, Thanks for all the responses, I really appreciate it. I'm going to get together with Scott C. and figure out what's going on. We'll start going thru step by step and see where it goes. Then I'll ask some questions about my 27 coupe. Ah, the joys of model T ownership. Learning things little bit by little bit.