I was just looking at the price of marine fuel tanks online to see if you could cheaply and quickly test using a backup (and also because I'd like to have a small reserve tank on my boat). They're not as cheap as I'd hoped.
My motor is a 1997 50 HP, so it's a 2-cyl. Yours may be a 3-cyl. It probably doesn't really matter for this part. Others on this forum can give much more detailed responses than I can, but here's what I know.
Anyways, you mentioned taking the carbs partially apart and that it won't stay running without feathering the choke.
A few extra questions:
- Is this issue mainly at idle?
- Does it have the same issue at over 2500 RPMs?
I ask because somewhere in the 2000+ RPM range, the carburetors transition over from the idle/low speed mixture circuit to the high speed mixture circuit.
I don't know the carburetors on your year model engine, but they should have a needle and seat that's operated by a float in the float bowl. Correct operation of the needle and seat allows the float bowl to retain the correct fuel level at all times.
Other things to check:
- The carburetors need to be synched. This can be done by advancing the throttle to its max stop, then adjusting the throttle linkage to make each throttle plate stand in the fully open position.
- Perform a full link and synch on your timing and carburetors according to the instructions in the manual - When you synch your carburetors, you also need to make sure the timing is correct. If your timing is incorrect, the engine won't run properly at all RPMs, and it might be a bit cold-blooded. You'll need a timing light. This isn't the procedure in the manual, but it might include some things to think about.
- Fully charge the battery.
- Empty the fuel from the bowls and disconnect the fuel line from the engine so the engine doesn't start accidentally.
- Make sure that the timing advance roller and the throttle cam roller (if equipped) are still there and not missing any vinly covering that they might require (applicable for mine, maybe not for yours). The timing roller should be the width of the timing cut in the timing synch plate. If it's narrower, it's probably missing its skin. The throttle advance roller also needs its skin in order to advance the throttle in-synch with the timing. You'll have issues otherwise.
- Pre-adjust the timing linkage rod lengths as-needed according to the manual.
- Put the lower unit on muffs and give the water pump enough water that it doesn't shred itself while you turn the engine over, or remove the lower unit. (I haven't tried spraying silicone spray up into the water pump to pre-lubricate it. I don't know if this is a good idea)
- Loosen the screw on the throttle cam follower so the carburetors don't open while you adjust the timing.
- On my engine, there is a timing stop set screw that you may need to adjust during this procedure. Loosen the lock nut so you can easily make adjustments. You don't want the timing settings to crush the timing advance roller.
- Adjust the timing so the timing advance roller is between the arrows in the slot. You'll probably change this later.
- Turn the engine over using the starter motor at the idle position on the throttle control lever and measure the timing (The link and synch instructions tell you to disconnec the throttle control cables. Follow the instructions if you're more comfortable with them). Adjust the timing on the link and synch according to the manual. The timing follower roller may not be between the arrows on the timing follower plate after the timing adjustment is complete.
- Set the timing stop set screw and tighten the lock nut.
- Adjust the throttle cam follower so the carburetors synch with the throttle cam and tighten the set screw.
- Push the throttle control lever to the wide-open stop and adjust the throttle stop screw a bit at a time (back off on the throttle control lever while making adjustments) to make the butterflies open to 90 degrees at full throttle.
- Push the throttle lever forward all the way (make sure there is no fuel) so the engine won't start and measure the timing at WOT by turning the engine over. This will help to finalize the timing settings.
- Re-test the timing settings at closed throttle and re-adjust the throttle cam follower and throttle stop as needed.
- My carburetors' needle and seats tend to want to leak just a bit, even after a recent set of carburetor rebuilds, and so I'm more likely to get an over-rich mixture at low engine speeds. Needing to feather the choke sounds to me like you're not getting enough fuel (I'll bring up another possibility in a moment).
- The carburetor mounting gaskets need to seal properly.
- My carburetors have the idle/low speed circuit at the top of the carburetor (yours probably do too). Engine manifold vacuum draws fuel up where it gets mixed with air and dumped into the carburetor throats for idle operation. My carburetors have an idle mixture adjustment that allows me to adjust how much air gets mixed with the fuel. Turning the idle mixture addjustment screws counter-clockwise admits more air for a leaner mixture. Turning clockwise richens the mixture by allowing less air. The screws on my carbs are on the starboard side, behind the throttle linkage. If your mixture is off at low engine speeds only, it could be off due to the idle mixture.
- It might be worth completely disassembling the carburetors one at a time and cleaning all passages. I use a very thin copper wire on passages, WD-40 on plastic parts, and Berryman's B-12 on metal parts. All the passages need to be clear of varnish.
If you're in FL like me, you're probably counting the days until you can get out on the water again. If you're elsewhere, you might feel like you have more time to tinker.
- DW
www.marineengine.com
