Losing Power at High Speeds
 

Hello,
I am new driving Model A's. I have been learning how to maintain and drive the car for 2 years, with the mentorship of a friend that has has a model A for longer. The car is 1930 Fordor that had been sitting for 7 yrs with bad gas and existing motor issues, and last year we were able to get it running and driving, but not reliably. We still had run and idle issues. Early this spring, I swapped the carburetor with a rebuilt carb and that seemed to solve all the run issues. The car runs, idles and drives nicely around town. I joined a local model A club and I did my first tour with the group this weekend. The car is a pleasure to drive at lower speeds, but once we hit the countryside roads and higher speeds, I wasn't able to keep up.
With the fuel-air at the recommended 1/2-1 turn from full right, and timing almost all the way down, I would have sufficient power as long as I stayed under 45 indicated mph. I believe my speedometer is as much as 5 mph fast. As soon as I would try to accelerate past 45, the engine would slow down and I would immediately start slowing down as if I was engine braking. I can let off the gas and the engine would speed back up to 45. To keep up with the group, I found I could compensate by turning the fuel-air as many as 3 turns left, and accelerate to indicated 55, but no faster. I was still finding myself slower than the group which seemed to be cruising at the posted real speed of 55.
I asked around and we believe the car runs lean at high speeds, but nobody had any ideas as to why. I also had a moderate amount of blowby which only occurred at the high speeds, club members believed it was a normal amount, and the oil does not smell of gas. The fuel has good flow thru the lines to the carb, the valve and valve filter has been replaced. The lines are copper, and the carburetor side has a cut and repair with modern hose. The carb has a small fuel leak near the fuel-air adjustment but as far as I know the needle and seat function and the float floats. I enjoyed touring the car and would like to continue but would like to be able to keep up with the group. The group recommended I ask this forum. What are my options for getting the car to drive at the higher speeds?

Re: Losing Power at High Speeds
 

A common problem it with the fuel line that goes into a zenith carb. If the ferrule is set back from the end of the tube, it lets the tube go in too far and press against the screen. This only lets a little fuel flow, and not enough when under power. Only with the zenith.

Re: Losing Power at High Speeds
 

If you’re cruising at 55 I hope you have good brakes.

Re: Losing Power at High Speeds
 

also some of the rebuilt Zenith Carbs and Rebuild kits come with undersized
jets. (mine did). My main jet was undersized, I opened it up to the proper size
with a # 63 jet drill bit. I can now cruise at 55 MPH + speeds.

I would first check out JacksonIII's recommendation.

Re: Losing Power at High Speeds
 

I know you put in a rebuilt carb, but this still feels like a carb problem... If someone in the club will let you borrow the carb off their car, I would do that first just to see if that cures the problem.

If I had to guess based on the description, I would say it sounds like a problem with the main jet. That would be consistent with the failure to supply fuel at high speed, the note that boosting flow to the cap jet alleviates the problem, and the fuel leak.

Re: Losing Power at High Speeds
 

After you fix the carburetor problem, if you have not done so already, put a higher compression head on the car, like a Snyder 5.5 or 6.1. Plus, check the gear ratio in the rear end. You could have a 4.11. Jack up one rear wheel and count how many turns of the crank shaft it take to turn the wheel 20 times. Divide by 10 and that is your gear ratio. Another option is a Mitchell overdrive but only after the higher compression head.

700RPM has a good point. You should be able to lock up the wheels. See if you can stop in 20 feet at 20 mph with no tendency to pull to one side.

Re: Losing Power at High Speeds
 

Good info above, it does sound like a fuel delivery issue.

In addition, try loosening the fuel cap to bypass the vent.

Re: Losing Power at High Speeds
 

Armchair diagnosing is always a struggle however we have some of the best diagnostic dart throwers around! :D



While my advice may come across as biased on my part, my suggestion is you take the car to a knowledgeable Model-A repair shop that has the expertise to repair the problem correctly. If you can tell us your location, we can probably point you to a professional that can repair this for you.

While it is there, you probably should consider having the vehicle given a thorough inspection to verify that it will Stop, Start, & Steer safely and reliably. It used to be that I would recommend finding someone from the local club to mentor new folks however I am finding this is not the most prudent thing to do now.


https://www.fordbarn.com/forum/attac...1&d=1626728788

Re: Losing Power at High Speeds
 

I don't want to hijack this thread...is a higher compression head necessary? I've heard they cause problems as well, like harder to start and cause more oil leaks.
I'm guessing the 30 fordor I'm driving has a 4.11 rear end, as it really screams at 55 mph. How hard is it to find higher speed replacement gears? I'm just wondering is it possible to change gears instead of dropping another four grand on an OD?

No one I know in the A club here has an OD, I guess they are happy driving 45. I am.

Re: Losing Power at High Speeds
 

What jackson said about the fuel line inlet is worth checking.

Here is an easy thing to check/fix. If the point gap in the distributor is too close, or the can on the distributor shaft is worn down the points will not open enough and you will notice it at higher RPM. Have seen this with one of our chapter members on tour. He started his car and we were looking at it. He described mush as you did. Popped the cap, and we opened up the gap. Problem resolved, and when he got home he put a new set of contact points in.

Re: Losing Power at High Speeds
 

Make sure the spring force is still strong on the point arm. If broken, the arm will float. We had one like this. I opened the points with my finger and there was very little spring force. Good luck.

Re: Losing Power at High Speeds
 

The question of "is it necessary" is really up to the driver. It increases the torque and the horsepower can push 60 at 2500 rpm with a stock car. If you put other speed items on it like a different carburetor and hotter camshaft, it goes higher. Especially carburetor because it wants more fuel flow. But the difference is quite profound even with a stock engine. What all this means is a car that can go faster in several aspects. There will be less trouble going up and down hills. It can take off from a stop a little quicker. And it runs more efficient giving it better gas mileage. Now the downside of increased compression here is that the combustion is a little hotter and it is more sensative to incorrect spark timing. Basically don't drive with the spark lever too far past the middle mark!

A higher compression head makes it a little harder to start because of the higher compression. This is not an issue if you have a good battery and starter. As for causing more oil leaks, that is a complete myth. The head has nothing to do with the seals on the oil pan.

Re: Losing Power at High Speeds
 

Les Andrews book "Model A Ford Troubleshooting & Diagnostics" is a good resource for trying to find the source of problems. It has two full pages of things to check when your car misses at high speed. Too many to list here. My only comment is that his fix for most things is to replace them, which is the correct response except where the part can be repaired.

Re: Losing Power at High Speeds
 

Man, where in the world did you hear that information?

Please don't tell me you read that on one of those Facebook Model-A pages!! :D http://www.model-a-ford.com/LOL.gif

Re: Losing Power at High Speeds
 

ditto

Re: Losing Power at High Speeds
 

Regarding changing the rear end gear ratio: That is one of the first things I did on my stock 1930 coupe in about 1960. If you do it remember to change the gears for the speedometer too. Only drawback is you will need a strong engine. The 4.11 gears were put on cars that resided in mountain areas. In my opinion, the better choice is to spring for a Mitchell overdrive. But, before you do anything, measure the gear ratio as detailed in post #6. Always good to have some data.

Re: Losing Power at High Speeds
 

I had time to work on the car today.
I replaced the carburetor fuel line, tightened the fuel air mixture where it screws in.
I tested and was able to get up to an indicated 60 mph with no signs of running out of power, and probably could have gotten faster. Fuel air was at 1/2 from right. The weather was similar to the weather over the weekend. I drove long enough to get the motor hot and nothing changed.

My brakes are adjusted and stop fast. I have never had them stay locked up when stopped from heat, and I have tried sudden stops to get a idea of how the vehicle would handle.

When I got home I had 3 moderate oil leaks, one from the pan, one dripping from the lowest point of the engine/trans and another from slightly further back on where the struts? meet in the middle. I might try an oil change to 10w40.
I guess that's why they say it never stops...

Re: Losing Power at High Speeds
 

Sounds like you fixed it. As far as the oil leaks go, tighten the pan bolts and side cover etc. Run the oil level in between the add and full. I use 15-40 oil non-synthetic. I have very little oil drips after doing the above.