In this article I would like to talk about fuel pumps on these aircooled engines. What kind of pump to run, and the pros and cons of electrical vs mechanical.
I am personally not a big fan of electric pumps unless we have an engine that needs more than what a stock mechanical pump can provide. My reasoning is that with an electrical pump if you get in a wreck the only way to stop an electrical pump is to shut off the ignition. What if you’re knocked out? With a mechanical pump if the engine quits so does the pump. For example, lets say you run off the road for any reason and you end up rolling the car and end up on your side or upside down. The engine will quit because the carb(s) are down drafts and will not run anyway but upright. So the motor has no fuel and will quit. The mechanical pump stops too. An electric pump will keep pumping fuel. If there is a fire, the electric pump can keep feeding the fire- No bueno..
I have a lot of customers thinking that if they run dual carbs then they have to run an electric pump. So not true. The stock pump can keep up with most dual carb setups. Where you would run into problems is if you have 48 IDA’s and a drag car motor, or a turbo motor where the demands of the engine will out weigh what the mechanical pump can deliver.
So in reading this, most of you can use a stock style mechanical fuel pump. Which one should you run? On a 36hp you don’t really have a choice. They only made one pump for that motor. The 40hp motor had a round top with one bolt that held the “lid” on. It also has a threaded intake port. This pump is not a bad pump and if I am doing a bone stock 40hp then I will use them. The good thing about this pump you can take them apart. The bad thing is they have a tendency to leak around the top- Not good. Then there is the “square top” pump. That is my favorite pump, because you can take them apart and they don’t leak. The bad thing is you can’t get them anymore and I have had very little success with the rebuild kits for them. Once you rebuild them we are seeing way to much fuel pressure. Then there is the round top generator pump and the round top for alternators. This is what I use the most of now. They work just fine, but the bad thing is you can’t take them apart. So with the new fuel we have it can stick the brass valves in the pump and you just have to get a new one. The other thing with this fuel pump is that the inlet is on the top and the outlet is on the bottom. This isn’t really a big deal, but most people get the lines backwards the first time. They don’t work that way.
With these two pumps make sure you have the correct push rod. The alternator style pump takes a shorter rod then the generator style pump. How can you tell which rod to use? The alternator pump (the foot that the rod pushes against) will stick past the flange that bolts it down. That pump takes the short rod. “I have a bunch of rods and they’re all the same size. Now what?? The short rod measures 3 7/8”. If you don’t have a tape measurer handy, put it in the guide and turn the motor over the short rod will be flush with the stand when its all the way down.
Now the important part; You have to check the fuel pressure. We find a lot of pumps have way too much pressure straight out of the box. The needle and seat in the carb(s) can only handle 3 ½ pounds of pressure. I like to see 2 ½ to 3. If you’re at 3 ½ that is ok but nothing over that. To check the pressure I get a low pressure gauge and “T” into the line that goes from the pump to the carb(s). If it is too high then there is a couple of things you can do. One is add another gasket under the pump. The other is to grind down the flat side of the rod. Make sure to keep it flat when grinding. Then release the pressure on your gauge and try again to see what it is.
“I have an old pump that I took off and its been sitting for years” or “I haven’t started my car in months. Is that going to be ok?” Well, it could be. If it pumps fuel you can start your car and keep an eye on the oil level. If you find that it’s increasing, change your oil right away and replace the pump. What happens here is that the new fuel we have can eat a pinhole in the diaphragm and it will leak gas into the crank case. If you’re not sure, try and light your dipstick. If it lights, guess what? You have gas in your oil. If it doesn’t light and your oil level stays the same and it pumps gas, then run it.
Well I have dual carbs and the inlet pipe on the carbs is bigger than the stock pump, so now what do I do? We run a lot of idf dual carb setups and you’re right, the line is bigger. Here is what I do. I run 5mm line out of the pump to a plastic stepped fuel filter. I push the line onto the first step of that filter that comes out of the pump, then I use 7mm line on the other side of the filter and push it onto the second step. Another myth here is you have to have equal length line here on dual carbs. Not true. The carbs don’t care. They both have their own needles and seats, so the carbs will let the pump know when it needs more fuel. I use about a foot on the drivers side and about 4 feet on the passenger side coming out of the “T”.
“My case doesn’t have a place for the stock pump, now what?” Well then you have to go to an electrical pump. All I can say here is no matter what the part guy tells you check your fuel pressure or put a regulator on it. Even drag cars with 48 IDA’s only run 3 ½ pounds of fuel pressure.
So there you have it. In my opinion you should stick with the stock mechanical fuel pump if you can get away with it, for safety purposes especially.