Ed Clark
Active Member
So, one of the features of a "BN" style carburetor is that it includes a built-in fuel pump. (Twin carb set-ups have one fuel pump for both carbs). When we talk about fuel pumps in general, we most often are considering how much pressure the fuel pump will develop. Of course, in any BN application we will want to check that the "pulse" side of the diaphragm will hold vacuum/pressure without leaking indicating the good health or lack thereof of the fuel pump. But, I recently had cause to wonder just how much suction the fuel pump would develop in order to "lift" the fuel to the carbs. So, I ran an experiment that you may or may not be interested in. This post details what I did.
For background I should explain that I own a pair of 1997 Speedster "project boats" that I purchased from different individuals earlier this year. Basically, I wound up with a couple of apparently sound hulls but a "dog's breakfast" collection of 717 engines with questionable parentage. "Speedster #2" has undergone two sea trials, both of which resulted in a valuable learning experience albeit I haven't gotten more than a couple hundred yards from the dock thus far. Back in the shop, the current goal is to change out the port engine with a seemingly more sound one from "inventory."
The OEM configuration for the '97 Speedsters is that each engine is fitted with a single carb (BN-40i w/accelerator pump). The spare I want to use is from a twin carb (BN-38i) application with parentage from only the gods know where. No worries, right? Hey, a 717 is a 717 is a 717, right? So the plan was and is to simply replace the twin carb set-up on the spare engine with the single carb, manifold and rotary valve cover from the OEM configuration. I can testify that this is a simple exchange in practice. BUT...
During the manifold exchange, I did notice one difference between the two engines that caused me to put my thinking cap on. If you are viewing a 717 engine from the right side and facing the carb set-up, you will notice a difference in where the crankcase pulse port fitting is located. On OEM twin carb configurations, the pulse port is on the mag cylinder (far right). This is because the mag-side carb contains the fuel pump and the location of the crankcase pulse fitting will result in the shortest and most direct path of hook-up hose. Conversely, the OEM configuration for the single carb models is just the opposite. Both the crankcase pulse fitting and the fuel pump fitting are located on the PTO cylinder (far left). Incidentally, it can be seen that crankcase has two cast bosses, one each on the mag and the PTO cylinder. The purpose for which is to drill the boss and install the fitting in the most appropriate location related to the carb configuration that the crankcase is intended for.
While it is a simple matter to extend the pulse hose from the mag-side crankcase fitting to the PTO-facing carburetor fitting to convert to the single carb configuration, it does result in about doubling the length of the pulse hose. I theorized that the pressure pulse might be diminished as a result, hence reducing the si-called suction lift capacity of the pump and resulting in fuel starvation.
If a fuel pump is gravity fed, such as one placed in the bottom of a fuel tank, one need not be concerned with suction lift. However, the BN fuel pumps are above the fuel tank so the pump must "suck" the fuel up to the carb. That's where suction lift comes in. So, I devised a scheme to test the suction lift of the carb using the longer pulse hose before going to all the trouble of sticking the engine in the boat only to find out, well, you know what I would find out in that case ....
To explain this as simply as possible, suction lift in this case is the height of a column of gasoline that the suction side of the pump will pull towards the carburetor. It may be measured many ways but I'll use the term "inches of gasoline." I found that my carb would pull 13" of gasoline. Good news (I hope) since the bottom of the fuel tank is less than 13" below the input to the carburetor.
The following are a series of poor quality pictures to show how I ascertained my alleged suction lift number. I apologize in advance for the poor quality of the photos as I was just snapping shots for my own historical work log and didn't intend to publicize the results. Before you see the photo's, you should know the following things:
1. I used the starter only to spin the engine to produce the needed pulse. I did not tach the starter speed, although from past experience I know that an engine needs to be wound up to 600 rpm before it's going to start and I assume the starter was running at least this speed or higher.
2. For this test, it does not matter how far below the fuel pump the source of fuel is. You are only trying to ascertain the height of a column of gasoline the pump will pull as measured from the surface of the fuel. Similarly, it does not matter how far the fuel pick-up tube extends into the tank below the surface of the fuel. Again, the result will be exactly the same - within reason, of course. So, in the following photos, be advised that the measurement is taken from the top of the surface in the gasoline can to the black mark you will see on the hose going to the fuel pump. I put the black mark on the hose to indicate how high the fuel rose in the hose when I was spinning the starter.
Without further ado:
The first two attached photos show the general experimental set-up. The final two photos show how the measurement was taken from the top of the fuel to the "black mark" on the hose.
For background I should explain that I own a pair of 1997 Speedster "project boats" that I purchased from different individuals earlier this year. Basically, I wound up with a couple of apparently sound hulls but a "dog's breakfast" collection of 717 engines with questionable parentage. "Speedster #2" has undergone two sea trials, both of which resulted in a valuable learning experience albeit I haven't gotten more than a couple hundred yards from the dock thus far. Back in the shop, the current goal is to change out the port engine with a seemingly more sound one from "inventory."
The OEM configuration for the '97 Speedsters is that each engine is fitted with a single carb (BN-40i w/accelerator pump). The spare I want to use is from a twin carb (BN-38i) application with parentage from only the gods know where. No worries, right? Hey, a 717 is a 717 is a 717, right? So the plan was and is to simply replace the twin carb set-up on the spare engine with the single carb, manifold and rotary valve cover from the OEM configuration. I can testify that this is a simple exchange in practice. BUT...
During the manifold exchange, I did notice one difference between the two engines that caused me to put my thinking cap on. If you are viewing a 717 engine from the right side and facing the carb set-up, you will notice a difference in where the crankcase pulse port fitting is located. On OEM twin carb configurations, the pulse port is on the mag cylinder (far right). This is because the mag-side carb contains the fuel pump and the location of the crankcase pulse fitting will result in the shortest and most direct path of hook-up hose. Conversely, the OEM configuration for the single carb models is just the opposite. Both the crankcase pulse fitting and the fuel pump fitting are located on the PTO cylinder (far left). Incidentally, it can be seen that crankcase has two cast bosses, one each on the mag and the PTO cylinder. The purpose for which is to drill the boss and install the fitting in the most appropriate location related to the carb configuration that the crankcase is intended for.
While it is a simple matter to extend the pulse hose from the mag-side crankcase fitting to the PTO-facing carburetor fitting to convert to the single carb configuration, it does result in about doubling the length of the pulse hose. I theorized that the pressure pulse might be diminished as a result, hence reducing the si-called suction lift capacity of the pump and resulting in fuel starvation.
If a fuel pump is gravity fed, such as one placed in the bottom of a fuel tank, one need not be concerned with suction lift. However, the BN fuel pumps are above the fuel tank so the pump must "suck" the fuel up to the carb. That's where suction lift comes in. So, I devised a scheme to test the suction lift of the carb using the longer pulse hose before going to all the trouble of sticking the engine in the boat only to find out, well, you know what I would find out in that case ....
To explain this as simply as possible, suction lift in this case is the height of a column of gasoline that the suction side of the pump will pull towards the carburetor. It may be measured many ways but I'll use the term "inches of gasoline." I found that my carb would pull 13" of gasoline. Good news (I hope) since the bottom of the fuel tank is less than 13" below the input to the carburetor.
The following are a series of poor quality pictures to show how I ascertained my alleged suction lift number. I apologize in advance for the poor quality of the photos as I was just snapping shots for my own historical work log and didn't intend to publicize the results. Before you see the photo's, you should know the following things:
1. I used the starter only to spin the engine to produce the needed pulse. I did not tach the starter speed, although from past experience I know that an engine needs to be wound up to 600 rpm before it's going to start and I assume the starter was running at least this speed or higher.
2. For this test, it does not matter how far below the fuel pump the source of fuel is. You are only trying to ascertain the height of a column of gasoline the pump will pull as measured from the surface of the fuel. Similarly, it does not matter how far the fuel pick-up tube extends into the tank below the surface of the fuel. Again, the result will be exactly the same - within reason, of course. So, in the following photos, be advised that the measurement is taken from the top of the surface in the gasoline can to the black mark you will see on the hose going to the fuel pump. I put the black mark on the hose to indicate how high the fuel rose in the hose when I was spinning the starter.
Without further ado:
The first two attached photos show the general experimental set-up. The final two photos show how the measurement was taken from the top of the fuel to the "black mark" on the hose.
