'75 40 spitting at idle

[bassboy1]

Attempting to tank-tune engine following repairs. Motor is 1975 'rude 40555. Rebuilt lower unit. Rebuilt carbs. New points, plugs and condensers. Compression even @ 120 psi. Believe I have synched carb and ignition correctly, but seloc manual is vague, contradictory and confusing. Initially started engine with all above repairs except synching, just to see what would work. It started, but would only run a few seconds without throttle intervention, and was snorty and rough. Would pop 2 or 3 times (but not through intake) then die, despite opening throttle. After trying a variety of synching combinations, things have improved (or at least changed). It was able to idle for over 1 minute without too much fuss and would accelerate from idle ok; however, when throttle was backed off suddenly, seemed to pop several times out exhaust at water stream, then died. Suspect synchronization is still primary problem, but until I can get additional technical info, I'm stuck. BTW, I have a second parts motor if any parts swapping or testing is suggested. Thanks.

 

[tashasdaddy]

Re: '75 40 spitting at idle

have you tried this,


(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

 

[bassboy1]

Re: '75 40 spitting at idle

My dad actually typed the first post, so something may have been left out. We aren't having any trouble setting the idle. We are fairly sure that the issue lies in the carb and ignition link and sync. After we adjusted the link and sync, and ran it again, we were able to tune the idle just fine with the needle valve adjustment. The problem started back when we revved it a bit, and then dropped it back down to idle. After we did that, the motor started running rougher, the exhaust starting popping, and the motor died, even though we tried to fight it with the throttle. I think we just need a little assistance in the whole link and sync method, as the manual is just about useless.

 

[tashasdaddy]

Re: '75 40 spitting at idle

wish i had a manual for it.

 

[bassboy1]

Re: '75 40 spitting at idle

Wish I did too. Anybody have any pointers? We have a couple other ideas - that maybe we are losing fuel pressure or spark after a minute or so, and dad will be bringing home a couple testers from his work to determine that, but we are still leaning toward the link and sync, as it started as lowered the throttle setting.

 

[byacey]

Re: '75 40 spitting at idle

Above all, check your points for clean and proper gap as well as the spark plug gaps.

Link and sync doesn't come into play until almost full throttle; this is to ensure opening up the butterly valve in the carb at a high degree of ignition advance for full throttle operation. With this in mind, I don't think it's a concern at this point. If you open it up 1/2 throttle momentarily and then return back to idle it should settle back to a smooth idle. If not, I would check for correct float level, good needle and seat, possible dirty carb passages and vacuum leaks. Don't overlook worn crankshaft seals, trying to set the idle on an engine with bad crank seals is a futile effort. Also check the packing on the idle needle and that the throttle butterfly isn't sticking partially open at idle.