Re: Carb. backfiring
Join Date: Aug 2004
Location: Ocean Shores, WA.
Posts: 12,557
HOW TO: Testing Your Boats Fuel System.
Boats seem to have a lot more problems with the fuel system compared to the auto industry. Mostly due to all the "Marine Specific" parts in the system required to make the system safe in a boat. Anti-Siphon valves, special fuel lines, water separating fuel filters just to name a few.
Below is a way of testing your boats fuel system if you suspect a problem. Note, this is for testing the BOATS fuel system which includes everything up to the engines fuel pump. It assumes (I know, bad choice of words, but you have to start somewhere) the fuel pump and carburetor are working properly.
Most of the information below was just copy and pasted out of a Volvo manual. All I did was put it in one place and separate it from other information.
The Snap-On fuel gauge mentioned below is not a necessity; any good fuel pump pressure/vacuum gauge will work or even a single vacuum gauge. I have one from NAPA that works fine for the tests. They may not be super accurate, but if you have zero vacuum, you have a leak and if it's real high you have a plugged system. Use some common sense and think about the results you have on the gauge.
You also need to make up some proper fuel hoses for YOUR boat. Since you probably won't be working on very many, get the right stuff and make your job easy and safe instead of difficult and dangerous. Fittings that will fit the carb and fuel pump will be necessary. Don't even think about cutting that steel line and hose clamping a tee for the gauge to it.
A couple of problems that can be caused by a defective fuel system is.
1. Low WOT (Wide Open Throttle) rpms. (When the engine used to be fine)
2. Engine dies out when running at any rpm.
Vacuum Testing Fuel System
This test must be performed with engine under load; either running in gear on the water or in gear connected to a dynamometer.
Ensure that all fuel line connections are leak free.
1. Install Vacuum and Fuel Pressure Gauge such as Snap-on? Tools MT311JB or equivalent, and 8 in. (20,3 cm) of clear, fuel resistant vinyl hose into the fuel line at the filter fuel pump inlet fitting. (NOTE: On some engines there is a filter hard plumbed to the fuel pump, these engines should have the gauge attached to the fuel filter and not the pump itself. )
2. Start engine and allow it to reach normal operating temperature. Run engine at full throttle (NOTE: As noted above, this is done under load, not on muffs) for at least 2 minutes; observe vacuum gauge reading and check clear hose for air or vapor bubbles. Gauge reading must not exceed 4 in. of mercury (Hg) at any time, and there should be no bubbles visible in the clear hose. Compare observations to the following:
Gauge reads 1-4 in. of mercury (Hg) - air bubbles present
Supply side of fuel system has an air leak. Check for air leak on suction side of fuel system
Possible areas of problems.
1. Loose or broken hose clamps
2. Wrong type hose clamp used (Use mini clamps not standard size clamps)
3. Filter seal leaking
4. Pickup tube in fuel tank defective.
5. Loose, cracked, corroded fittings in the system (including the tank and fuel filter)
Repair or replace suspect part, then make another vacuum test to verify repair.
Gauge reading exceeds 4 in. of mercury (Hg)
Supply side of fuel system has a restriction. Check points of possible
failure as noted below.
1. Water or debris in fuel/fuel filter
2. Fuel pickup tube and screen blockage
3. Fuel tank vent blockage
4. Plugged external canister or carburetor fuel filters
5. Inoperative, restricted or incorrectly sized anti-siphon valve
Repair or replace suspect part, then make another vacuum test to verify repair.
__________________
Don S.
Why is there never enough time to do it right the first time,
But always enough time to do it again?
You can verify that your system is doing what it is supposed to using Don S test outline......
If you establish it is working correctly, then the float level would be where to look.......
How does the screen in the tank pickup look?......
