What To Do If You Have Spark But Engine Will Not Start Or Run?

By Alan Buehner

Originally printed in issue #75 of Still....Keeping Track

What do you look for when you have spark, but the engine just won't start? I had a customer in Australia e-mail me about this problem with his Sachs engine last week. This was my response:
This could be 1 out of many possible problems.

#1 - Carb - replace the missing wax (in choke chamber) with a small dab of silicone (must be flat at bottom to block small brass orifice in bottom of choke chamber). This applies to Bing 24-27mm carbs with the choke chamber on the side.

#2 - Carb - remove the idle (pilot) jet and make sure that it is not blocked up

#3 - Ignition – Weak spark is not sufficient to fire up the fuel/ air mixture. Try push starting it with the throttle closed. .
Before I installed a Power Dynamo ignition in my Jackpiner, I would have to push start it to get it going for cold engine starts.

#4 - Pipe - Make sure that the exhaust pipe is not blocked. Mice like to build nests in the pipes filling it up with bedding and shells from nuts. A blocked pipe will not allow the engine to breathe..

#5 - Carb - double check the floats in the carb. Are they level with the carb body? Are they operating the float needle correctly? Allowing gas to flow and shutting it off? Is the float needle sticking in the closed position?

#6 - Carb - double check the position of the vaporizer (brass part that surrounds the opening of the needle jet in the mouth of the carb). If it is the style that has the half moon cut out - the cut out section must be facing towards the engine. (see photo)

#7 – Flooded engine – Crank area flooded with gas, try push starting with throttle wide open.

I ran this article past Kip Kern and these are some suggestions that he added:

#8 Carb - make sure float needle doesn't have a groove worn in it from being old. If so, replace it so it will seat properly. You may also need a new seat.

#9 Petcocks - Make sure your fuel cocks are flowing properly and the seats are not cracked or plugged.

#10 Air Box - Had one guy who left a rag in the air box and it resulted in no airflow to the carb.

I ran this past Chicago Jerry and this is what he added:

#11 Carb - check that idle jet is not blocked. Running pump gas in a fiberglass gas tank can cause blockage.

#12 Ignition – out of timing from a sheared woodruff key on a Bosch or Motoplat, or spun flywheel on a PVL or PowerDynamo.

Photo showing position of the (half moon shaped) vaporizer. The shield is facing towards the air box. It acts like a wall, driving the incoming air upward pulling fuel from the needle jet. If the shield was turned 180 degrees, the engine will start and idle, however increase in air flow will drive the fuel down the needle jet, starving the engine of fuel and it will stall out.
Photo showing inside of Bing Carb with the float bowl removed. The black arrow points to sample pilot jet, resting on one of the floats. This is what it looks like. The white arrows points to where it is located in the carb. A blocked pilot jet will prevent an engine from starting.