Idles better, but not done yet

[bmiszuk]

I had some questions about setting my idle on my 140 (see http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=011752#000003). Things are better but not perfect yet! Here's where I stand so far: dwell set to 30, idle on water is 600 (off water about 750), timing @ 6. The idle screws are giving me fits. If I turn them in the idle will go up, if I turn them out the idle will eventually go down. The book says that when I turn the screw in the idle should eventually drop, not increase!<br /><br />I tried connecting a vacuum guage to the line that goes to the choke but I got no vacuum there (should there be?). If I connect the guage to the choke inlet itself, the guage shows 2.<br /><br />What am I missing here?<br /><br />Also, I could not get much above 3200 rpm, pushing a 21 prop. The boat is a '74 16ft tri-hull. Not sure if that's related or not.

 

[rattana]

Re: Idles better, but not done yet

You want to adjust the idle mixture screws to get the highest smooth RPM. (this would also be the highest vacuum reading) Too far in or too far out will both lower the RPM. You need somewhere in between, about 1&3/4 - 2 turns out from lightly seated.<br /><br />Once you get the highest RPM or vacuum with the idle mixture screws while alternating between the two of them, adjust the idle stop screw to get the RPM down to 700-750.<br /><br />Repeat the above procedure to ensure it is still at the best RPM. Readjust the idle stop screw to 700-750 and you should be good to go.<br /><br />As for your 3200RPM max, it could be that you are over propped, or an internal problem with your engine.<br /><br />BTW, you need to find another port on your carb to get a vacuum reading, it should be nearer to 20" of vacuum.

 

[bmiszuk]

Re: Idles better, but not done yet

I couldn't find a port on the carb for vacuum, but I found a plug in the intake manifold right to behind the carb (towards the back of the boat). At that spot, the best I could get was 15" of vacuum no matter how I fiddled with the screws.<br /><br />It's a Rochester 2bbl carb, btw.<br /><br />thanks, I think I'm at least getting closer.

 

[bmiszuk]

Re: Idles better, but not done yet

^

 

[tommays]

Re: Idles better, but not done yet

one problem i have had is that if the cable barrel is not adjusted correctly the control will not pull the idle stop to the same place every time<br /><br />but you can go to tight because it will pull to much when the choke is still on<br /><br />21 in is usally to big of a prop for a 3.0 <br /><br />tommays

 

[rattana]

Re: Idles better, but not done yet

Does your vacuum readings look like any of these?<br /> http://users.bigpond.net.au/ergoff/vac2.htm

 

[bmiszuk]

Re: Idles better, but not done yet

DEFECTIVE IGNITION: if needle moves slowly between 14 and 16 at normal idle, check ignition components (plug gaps closed up/fouled,contacts not synchronized).

It only looks vaguely like this one. ^^^ Except that it's a steady 15 with occasional blips down to 14.5 when the idle hiccups a bit.<br /><br />I replaced the points, cap and rotor with parts from the auto store last year (was in a different down, had trouble and needed parts fast). So they're pretty new.

 

[rattana]

Re: Idles better, but not done yet

What color are the spark plugs? They should be a tan color. Is the firing order correct? Was any other work done on the engine?

 

[bmiszuk]

Re: Idles better, but not done yet

Plugs were new before Saturday, I have not pulled them since then. (I will do that tonight after work.) We got the boat a couple years ago, compression was okay at that time with no other obvious faults other than needing routine tune-up (plugs, wires, cap/rotor, coil, etc). I've never done internal engine work. It never idled well for me, but on a 19 prop it used to run 4000+ rpm when I opened it up. (The 19 was damaged, I put on a 21 that came with the boat until I get the 19 fixed.)<br /><br />I'm beginning to wonder if the engine is worn causing the odd idle or if some of the ignition parts are either defective (though virtually new) or wrong (though visually identical).

 

[novicetech]

Re: Idles better, but not done yet

There was a post awhile back, that someone had the same problem with low rpms and it turned out to have the wrong gears in lower unit for the engine.

 

[rattana]

Re: Idles better, but not done yet

You could do a compression test when you have the plugs out to see if it is worn.<br />DonS posted a good how to test your compression not too long ago.

 

[bmiszuk]

Re: Idles better, but not done yet

In between rain showers I was able to get a compression test. The first pass showed 125, 105, 115, 120. Then compression with oil squirted in the cylinder and throttle open (I forgot to open it first time) showed 140, 115, 135, 140.<br /><br />The plugs are Autolite 404's. They all looked the same: a slight buildup of carbon on them. I cleaned them with a wire brush and reinstalled them. The local shop sold me RV15YC4's but they don't fit because the threads aren't long enough.<br /><br />The plugs were new and only used for my recent testing plus about 4 hours of on and off tubing at the lake on Saturday.

 

[jeff13601]

Re: Idles better, but not done yet

Most people with a 120 or 140 seem to run a 14 x 19 prop. Seems you had the WOT good before with that one

 

[bmiszuk]

Re: Idles better, but not done yet

Yeah I've got to get my 19 rebuilt, one blade is quite damaged. Do you think a 17 would be too far the other way for pulling a skier?

 

[jeff13601]

Re: Idles better, but not done yet

From what I have read on here the 17 will get you out of the hole quicker but you will go over the specified WOT and lose top end