MTFCA Forum

My Daughter and I have just returned from the Celebration of Brass festival and tour in Michigan. It was a great time and the tour had plenty of highlights. Our 1915 roadster covered about 150 miles and all was essentially trouble free but we have noticed that we have a continual annoying issue with the car. It has a small rhythmic pause in forward momentum at higher RPM and High speeds. We mainly notice it around 40+ mph and it is a small surge that is hard to explain but it has no change when the carburetor is adjusted or we run on mag or battery. It has been in the car since it has been rebuilt about 3 years and about 500 miles. it does not correspond to wheel rotation. The Mike Bender rebuilt engine is a champ and it has plenty of power and speed. Below is a list of things checked and adjusted:

Swapped out rear end from a standard rear to a rucketell, new universal joint
swapped out coils, both sets rebuilt by MTFCA coil vendors
Changed timer, however there is no miss in the engine or disruption in engine sound.
changed out plugs
checked rocky mountain brakes, not dragging
changed NH carbs, both Russ Potter rebuilds

Any suggestions are welcomed. It is a small issue as the car runs really good but I am looking to finish this off and make it as good as I can.

Some engines develop a harmonic vibration at a certain speed. If you go faster or slower it will stop. That could be the problem and if you don't like it, drive at a different speed. Another possible cause might be fuel starvation. Something in the fuel line or carburetor intake is a bit restricted and the fuel level in the carburetor drops. This can be enough to cause the engine to stop, especially when going uphill or it can just cause it to run lean. If the carburetor has a grose jet for the fuel intake, replace it with a standard needle and seat. The grose jet looks like a ball bearing and does not work well in a Model T carburetor. An in line fuel filter in addition to the original type sediment bulb can also be a problem.
Norm

That speed is a high fuel demand. Possible lean situation. Does the carb have an unmodified viaton needle seat or a groose jet? Run max speed for a distance, 50+. What happens? Does the stumble increase in frequency?
Fuel screen is clean? Drain some fuel into a clean clear jar @ sediment bowl & carb drain looking for dirt. No chance fuel line is necked down somewhere? You have a heat pipe?
Could be a bouncing timer brush also. Could be intermittent sticky valve.

Norman Kling wrote: ↑

Wed Jul 21, 2021 4:18 pm

Some engines develop a harmonic vibration at a certain speed. If you go faster or slower it will stop. That could be the problem and if you don't like it, drive at a different speed. Another possible cause might be fuel starvation. Something in the fuel line or carburetor intake is a bit restricted and the fuel level in the carburetor drops. This can be enough to cause the engine to stop, especially when going uphill or it can just cause it to run lean. If the carburetor has a grose jet for the fuel intake, replace it with a standard needle and seat. The grose jet looks like a ball bearing and does not work well in a Model T carburetor. An in line fuel filter in addition to the original type sediment bulb can also be a problem.
Norm

I had/have harmonic issues. The motor wouldnt go faster @ that point. I found retarding the spark & slowly re-advancing will get the motor past that spot. @ 5mph faster speed its gone.
Worth a try.

If you are running an NH, and traveling at that speed and experiencing "surging" which is what most people call it, and you have described it, then there is an all but certain fuel starvation going on and no amount of fiddling with the richness adjustment is going to solve it much less change it.

Russ Potter rebuilds seemingly always come with a Grose Valve which is highly restrictive, as are all other commercial valves available from standard suppliers.

If I am correct, then that can easily be resolved.

Scott_Conger wrote: ↑

Wed Jul 21, 2021 5:02 pm

If you are running an NH, and traveling at that speed and experiencing "surging" which is what most people call it, and you have described it, then there is an all but certain fuel starvation going on and no amount of fiddling with the richness adjustment is going to solve it much less change it.

Russ Potter rebuilds seemingly always come with a Grose Valve which is highly restrictive, as are all other commercial valves available from standard suppliers.

If I am correct, then that can easily be resolved.

If you are not able to modify a viaton needle & seat unit yourself, there is a guy that sells one modified for hi fuel flow in the parts section.
Thats the way to go. Nice piece of work.

Maybe the car is telling you to quit driving it so fast? On our picnic tour today ol' Clara just purred like a kitten at 29-30 mph. I know speed preferences can be as argumentative as electronic ignition, MMO and water pumps, but all I'll say is why rush the ride?

Thanks for all the replies! I am liking the fuel starvation theory. It does not have a grose jet. It has a “ rubber “ tipped needle valve.

The car likes to run and we do like to roll along and on this particular tour we were running with a Locomobile and some other higher end cars that cruise at 40+ on open roads. More back roads we absolutely enjoy the scenery and smell the roses.

Just slow down! Wanting to go fast is not the same as needing to go fast. Your T is telling you it is not comfortable at 40 MPH. If 40 MPH is the goal, you could try an A model.

Allan from down under.

jmemjr wrote: ↑

Wed Jul 21, 2021 9:21 pm

Thanks for all the replies! I am liking the fuel starvation theory. It does not have a grose jet. It has a “ rubber “ tipped needle valve.

The car likes to run and we do like to roll along and on this particular tour we were running with a Locomobile and some other higher end cars that cruise at 40+ on open roads. More back roads we absolutely enjoy the scenery and smell the roses.

The main problem with the viaton unit is the seat hole. It is much smaller than stock. @ hi demand, it restricts full & adequate fuel flow.
If I ceacll correctly its @ .093". The original ford I measured was .116". That little difference matters.

Steve Jelf wrote: ↑

Wed Jul 21, 2021 11:01 pm

I had an NH with one of those rubber tips. Theoretically it shouldn't stick, but it did. I found an original in a box of auction plunder and tried that. No more sticking. I don't know if that's your problem, but I would put in an original or a new one from Scott Conger. It can't hurt, and it might help.

A ways back, they had a sharp edged seat & needed to be deburred. They could stick & cut viaton surface.
Simple fix was to place a ball bearing in the seat & gently tap it to remove the burr.