What does "preliminary idle mixture" mean ?

[CaptainMak]

The manual says that my 165HP 6 cyl carb (2bbl rochester with 2 adjustment screws) is set at 1-1/4 turns preliminary idle mixture ?

Does this mean both of the bottom screws have to be seated all the way in and then backed out 1-1/4 turns for proper operation ? Just trying to figure out if the motor is running rich...... . lately it's has a pretty strong fuel odor under "the hood"...

 

[achris]

Re: What does "preliminary idle mixture" mean ?

It means that will get the mixture close enough to get the engine running. It's not a final setting, fine tuning is needed after that....

 

[CaptainMak]

Re: What does "preliminary idle mixture" mean ?

Sorry, forgot to mention the boat idle beautifully at about 550 rpm (no shake, shudder) although a bit tough to start at times......

 

[achris]

Re: What does "preliminary idle mixture" mean ?

Sorry, forgot to mention the boat idle beautifully at about 550 rpm (no shake, shudder) although a bit tough to start at times......

Those old 250 cu in engines would idle beautifully with half the pistons missing!!!

BTW, 550 is too slow (accounts for the hard starts). Bump it up to 650-700 (in forward gear)...

 

[CaptainMak]

Re: What does "preliminary idle mixture" mean ?

So is there a way to tell if it's running too rich besides trusting the good ole shnauz ?

 

[achris]

Re: What does "preliminary idle mixture" mean ?

Outlined in the manual... but...

With the timing and points gap set correctly, and the distributor known to be in good order and advancing the timing correctly, and good spark plugs....When the engine is warm and at the correct idle speed, screw the mixture screws IN slowly until the engine stumbles (too lean). Then screw them OUT until it loads up (too rich). Now set the screws in the middle of those 2 points. Check hard acceleration from idle. If the engine bogs, screw the screws out 1/8. Test again. If it still bogs, check the accelerator pump is set correctly and working as per Hoyle...

Chris.......
P.S. If you're smelling gas 'under the hood', you may have other problems...

 

[achris]

Re: What does "preliminary idle mixture" mean ?

Another way of checking the run condition of an engine is to 'read' the spark plugs. But that takes time to develop the skill and know exactly what you're looking at....

For example... Most people would look at a perfectly clean outboard plug and think the engine's perfect. Me, I see an engine sucking water ....

Chris.......

 

[CaptainMak]

Re: What does "preliminary idle mixture" mean ?

When the engine is warm and at the correct idle speed, screw the mixture screws IN slowly until the engine stumbles (too lean). Then screw them OUT until it loads up (too rich)

Thanks for the super quick reply. It's amazing what info a manual contains. I found it in the ADULTS sticky and although it's not downloadable, still very useful. Found the carb adjustment procedure (#2) I believe. Thanks again. Will try this today, and get back with results.

 

[wrestling coach]

Re: What does "preliminary idle mixture" mean ?

When setting the preliminary adjustment be very careful when screwing them in all the way. Bottom them very lightly before backing them out. Tighter IS NOT better. When setting your final adjustment you are trying to find the sweet spot between idle mixture of too lean (in) and too rich (out). The engine will let you know where that is by the way it reacts to the mixture screw adjustments at idle. The idle will stumble a bit in both directions to tell you it doesn't like it. Count your rotation/rotations (shouldn't be too much) on your mixture screws and get them between both stumbles for your smoothest idle. Remember this is the final adjustment of a newly tuned engine. Plugs, points, timing etc.

Dennis

 

[CaptainMak]

Re: What does "preliminary idle mixture" mean ?

Went out today to play around with the mixture screws. Half turn one way, the engine started losing RPM's, half turn the other way, same thing. So I just left it the way they both were. Per achris, bumped the idle up to 650 RPM. Still a pretty strong fuel odor, but my nose is kinda sensitive. Must have bear in my family tree somewhere. The exhaust smoke is not black though.

 

[achris]

Re: What does "preliminary idle mixture" mean ?

Pull the plugs and have a look at them. They should be a nice light tan in colour. If they are sooty black, it's running rich; if they are oil black, (no surprise) the engine's burning oil. If they are almost white, engine's running hot....

Chris........