Carburetor Repair Thoughts

[CBOATNAZ]

I have (2) older Evinrude engines. First, a 1978 70hp, and second a 1975 135hp. I have done carb rebuilds on both of these engines. One thing that I have not seen discussed on here is throttle shaft wear. I found out through testing that excessive wear can cause a lean condition. The 70hp had about .02-.03" wear on both shafts. A common repair is to enlarge the idle jet size a few sizes to remedy the extra air entering. The 135 had about .015-.02" wear on the throttle shafts. Being a machinist, I machined o-ring grooves into the carburetor body to seal the shaft to the housing. This repair has solved several issues. One test that I learned from a senior mechanic is to apply heavy grease around the exterior throttle shafts to "seal" them to the carb body. Also, it is my opinion that this might be an overlooked issue being the return springs are on the shaft and don't allow them to flop around during a rebuild. I am interested in hearing everyone's thoughts. Please comment!

 

[Pontoon Don]

Found your thoughts interesting. I am presently trying to discover why my 92 48spl won't operate above mid throttle. If I block the throat of the lower carb with my thumb, i can reach a sustainable WOT. Otherwise the carb runs so lean as to make the engine just die out. After reading your post I wriggled the throttle plate shaft and found what I would call excessive play - meaning I could feel and see the side to side movement. In your opinion, would airflow past the shaft be able to cause that symptom?

 

[Faztbullet]

Shaft is not the problem with your 48.. Either you don't have enough fuel pressure, high speed jet is clogged or loose, float level is to low

 

[Pontoon Don]

The key problem as I see it is too much air (lean) as opposed to not enough fuel. I have a decent WOT 4000rpm when I block the throat with my thumb. This is done on the water in gear. Seems to me that if I had low fuel pressure or a mis-set float (it's not) or a clogged jet I would not be able to get those rpms under any condition.

 

[jakedaawg]

The key problem as I see it is too much air (lean) as opposed to not enough fuel. I have a decent WOT 4000rpm when I block the throat with my thumb. This is done on the water in gear. Seems to me that if I had low fuel pressure or a mis-set float (it's not) or a clogged jet I would not be able to get those rpms under any condition.

Your wrong. Not gonna write 7 paragraphs to explain but your thinking isn't valid. No offense intended. Listen to what faztbullet said. The little carbs on the 48 are not complicated, you just missed something or have it poorly assembeled/adjusted.

 

[achris]

I have repaired a few outboards with overheated pistons caused by throttle shaft bore wear. As the OP states, this allows 'unmetered' air into the engine, leaning the mixture. Opening up the mixture screws 'cures' the problem at idle and off-idle speeds, but does nothing to alleviate the problem at higher and WOT speeds. At these speeds a lean mixture overheats the pistons, and causes a 'nip' on the bores. Once that happens, the road to a piston leaving though the exhaust port is short and quick.

I have also thought of the machined 'O' ring groove, but was concerned about extra drag on the shaft... The solution I have used, successfully, is to have the shaft bores bored out and teflon sleeves inserted, then reamed to fit the shaft perfectly... So far, this solution has worked 100% of the time, and is much cheaper then replacing the carb$$$.

Chris.........