27-03-2025, 11:51 AM
Hi
That's puzzling behaviour. I'm assuming that you are testing the engine in neutral, i.e. revving up off load ? Even at quite high sustained revs the engine airflow and fuel requirements are still only a fraction of what's needed when out on the road, and the throttle will still be maybe only 5 to 10% open once the flywheel is up to speed. If it won't rev under these conditions I can think of two main possibilities:
A very restricted fuel supply such that the mixture leans out as soon as the airflow increases above idle. This will start with a lean misfire and spitting back.
A very weak ignition such that increased combustion chamber pressure as you open the throttle is enough to kill the spark. This is also usually accompanied by a progressively worsening misfire.
I can only suggest you methodically go through the ignition system (especially the points and condenser) and carburettor. Even with a rebuilt carb, its not impossible for there to be debris in the internal passages which may move along and block fine jets. A good blast with an airline and carb cleaner is recommended. Borrowing a known working Zenith might be an idea if you know a nearby Sevenner.
The 26VA has a tiny progression jet which is used to overcome a lean flat spot just above idle. The jet is under a brass cover screw on the underside. Don't try unscrewing the jet itself, you will only damage it. It can be blown through once the cover is removed.
Under these conditions, I cannot think of any detrimental effect from a bored out manifold eye. Manifold pressure is determined principally by the pressure drop across the throttle butterfly. Any pressure drop across the eye restriction would be small, and only significant when the airflow is quite high i.e. full throttle and maximum RPM simultaneously.
That's puzzling behaviour. I'm assuming that you are testing the engine in neutral, i.e. revving up off load ? Even at quite high sustained revs the engine airflow and fuel requirements are still only a fraction of what's needed when out on the road, and the throttle will still be maybe only 5 to 10% open once the flywheel is up to speed. If it won't rev under these conditions I can think of two main possibilities:
A very restricted fuel supply such that the mixture leans out as soon as the airflow increases above idle. This will start with a lean misfire and spitting back.
A very weak ignition such that increased combustion chamber pressure as you open the throttle is enough to kill the spark. This is also usually accompanied by a progressively worsening misfire.
I can only suggest you methodically go through the ignition system (especially the points and condenser) and carburettor. Even with a rebuilt carb, its not impossible for there to be debris in the internal passages which may move along and block fine jets. A good blast with an airline and carb cleaner is recommended. Borrowing a known working Zenith might be an idea if you know a nearby Sevenner.
The 26VA has a tiny progression jet which is used to overcome a lean flat spot just above idle. The jet is under a brass cover screw on the underside. Don't try unscrewing the jet itself, you will only damage it. It can be blown through once the cover is removed.
Under these conditions, I cannot think of any detrimental effect from a bored out manifold eye. Manifold pressure is determined principally by the pressure drop across the throttle butterfly. Any pressure drop across the eye restriction would be small, and only significant when the airflow is quite high i.e. full throttle and maximum RPM simultaneously.