Bolens Diablo Rouge Fuel System



The fuel tanks are of fairly sturdy construction, but are susceptible to rust and leaks at the seams. Early tanks have mounting brackets on the side which are used with the dash of a Diablo Model 500. These brackets are not used on the Diablo 502 or 503. Tanks made later at Bolens would not have the brackets and may also have the outlet on the side of the tank rather than the typical lower right corner.

As mentioned, tanks tend to rust or leak at the seams. The best solution I can offer is to thoroughly clean the tank and install a liner within the tank. There are many manufacturers of tank lining kits. I use products from kbs-coatings. com and they have worked well. Others have used POR-15 and there are others.

Below is a video I put together to show more tank information and how I clean and coat tanks.

The fuel line I use is the 1/4″ low temperature blue polyurethane. It does not get hard quick like other hoses, but you should inspect it yearly and replace if getting hard. You can also see through this hose to know for sure the carb is pulling fuel when turning the engine over. The Tillotson Carbs used on Diablos has a fuel screen within it. However, I suggest to add a primary filter in the fuel line.


Bolens Diablos would have come standard with Tillotson HR3A or HR14A carburetors. I believe the difference between the two is only the year they were built. I’d expect to see the HR3A on the 500 which was built for 1967, and then see either the HR3A or HR14A on the 502’s and 503’s built both during the 1968 and 1969 production years. Kits for these carbs are still readily available. You can buy simple diaphragm kits or comprehensive kits that contain much of the hardware. If you have a carb that is complete, my recommendation is to buy just the diaphragm kit and use all original hardware.

One reason for using the original hardware is aftermarket hardware is often not the same as the original. This is especially true when considering the needle, seat and the spring that holds the needle into the seat which is calibrated to hold a specific tension on the needle.



 A process I use on every carb is to test and adjust the pop off pressure. This is the pressure in psi it takes to pop the spring loaded needle off the seat. By using a pump like shown above, you can pump air into the underside of the seat and see what the needle “pops” off at and what it holds at. The mechanics at Art’s Snowmobile set their carbs to pop off between 10 & 12 psi while holding pressure between 2 & 4 psi. This setting works in altitudes in the Midwest USA, but I do not know what ideal setting are for high altitude applications.

This Pop Off tester I have came from GEM Products and is part number G4464, but as of 10/28/15, I could no longer find this available at GEM.

Between the motor and the carb is the pulse line. This line sends a pulse of vacuum then pressure from the piston going in and out of the cylinder and sends it to the carb to pulse the fuel pump. Make sure this hose is good and does not leak. The blue polyurethane hose can be cut short and used as a pulse line. The vacuum nipple in the carb can be turned if needed to get this hose in place without kinking it.