Have a friend with a speedster that ran good before he changed over to model A intake, carb, exhaust. Now while at the top end you can certainly tell much more power, at idle and low end it runs like it has no power.
Has a distributor which I have double checked timing. Engine is other wise stock.
My theory, with a stock T engine with the lesser bore/stroke(lesser vacuum and volume) with an intake system designed to provide for a larger bore/stroke(vacuum and volume) the T engine cannot create enough vacuum in the intake/carb to pull enough fuel/air mix at the slower rpm's to provide for the engines needs because the intake and carb inside bores are larger than the stock T's. Make any sense?
I have yet to figure out why the top end is so good.
To answer the top end power - he lessened the number and size of the choke points on both intake and exhaust enabling better breathing.
Bottom end - carb adjustment maybe?
You didn't mention what else has been modified on the engine - higher compression head or high top pistons? Modified cam?
The T's I have seen with Model A intake and exhaust and a Z (or similar) head with an improved cam usually perform quite nicely at the full RPM spectrum.
Walt, other than a distributor, the engine is bone stock.
That may be the answer. Bigger in and out but no increase in compression or valve opening may just be more than the motor wants to have. I'd appreciate it if others could chime in on this to confirm or deny.
My experience with more in and out coupled with a cam and higher compression has been good. A "Z" or similar head with the better carb and exhaust should wake it up. Cam too if he can do it at the same time.
Popeye had a Model A Zenith carburetor on a T Zenith intake. Otherwise stock. It ran fine.
I think Tyrone's friend likely has some other problem causing the lack of low end performance. Some distributors have no advance feature, requiring manual advance and retard. This works fine if set up properly. Others have centrifugal advance only, which is always less than ideal.
Other possibilities include vacuum leaks or float level not set properly.
The T engine is only a little smaller than a Model A. The carburetor from a Model A is tiny by modern standards for a Model A. It is by no means too large for a Model T.
Tyrone, I think you have the answer already. The T engine was deliberately designed with small valves/intake compared to its bore, and acted as a sort of "governor". This prevented owners from over winding the engine and thus guaranteed longevity. As designed, all parts and performance were balanced.
Increase the intake's diameter and at low speed there would be less suction.....like increasing the diameter of a venturi, runs like a bandit at high speed and loses low speed performance.