[JediJD]

I feel terrible for you that the project went sour. In retrospect, I am simply at a loss for any other path you could have taken. "No idle" is serious enough to invest in the repairs, but it is SO incredibly frustrating when things you are repairing start breaking.

 

[JediJD]

Just checking back in...any luck?

 

[JediJD]

ok checked usingthe propane method, didnt come up with anything around the base of the carb, linkage shaft may be a little loose, but nothing that is pulling in air, so I just used the propane to fatten up the mixture at idle and it smoothed out and ran at lower rpms with the need for less throttle. I then revd the engine and stuffed a rag overthe top of the carb with my hand to try to clear any blockage from ports and jets. After doing this the engine ran smoothly for a minute or two then began to stall; out again. I am going to check the plugs and look for signs of a lean condition, but it seems to be to lean at idle. are the air fuel mixture screws the two in the front lower base of the carb? the bladed head screws? would these get my idle mixture in check or is there more to it? I was born and taught on fuel injection, worked on cars for ten years in a shop, but when it comes to this kind of stuff its like starting all over again....... suggestions?

Progress! Testing seems to be sending us toward a lean condition without air leaks. Mixture screws on a 4bbl are on the front. As I recall, it is two small flat blade (as opposed to Philips) screws. With the engine off, run them all the way in (remember the number of turns so you can get back to where you started if neccessary. Then run them out three full turns. Start the engine. Should be idling fine now (very rich). Run the screws in a half turn at the time until RPMs drop as noted earlier. However, be sure about fuel filters and fuel pressure. I don't recall the year of your boat, but there may be a screen filter also in the base of the carb, in addition to the inline filter.

 

[JediJD]

On a previously running engine...if you know the plugs are good, compression is right, and wires aren't arcing, then its highly likely to be an air fuel problem. For reference, pull a plug or two and look for a pattern. Whiter/more tan electrodes usually suggest a leaner operating condition and steer you toward intake/vacuum leaks. Darker/blacker electrodes signal a richer operating condition and steer more toward rich carb/secondary ignition issues. Wet tips mean that cylinder probably isn't firing at all.

Fuel can be in the carb (stopped up jets) or in the delivery system (fuel pressure, fuel filter). Air can be a vaccum leak. Check for vacuum leaks by using (most safe) a piece of rubber hose on a valved propane bottle (do not ignite the propane) and squirt propane around the suspect areas (less safe-use carb cleaner). The usual culprits are the base of the carb and the edges of the intake manifold. If engine rpm climbs, you have found your leak. For your jets...warm engine, hold throttle open by hand, rev engine to midrange, then close throttle butterflies a couple of times. You will create a lot of negative pressure in the intake manifold and if there is a piece of trash in the jet, it will sometimes suck it out. Also, check your mixture screws. Usually you run them in all the way (often opens a gunked up passage on an older carb) then bring them back four turns. Then start to run the screws in, a quarter turn at the time, until idle starts to degrade (use a digital tach and look for a 50 rpm drop). Back them off a quarter turn or, if you happen to have a vacuum gauge (a hand held vacuum pump with a gauge on it works too), hook to an intake manifold vacuum source. Look for steady vacuum. If vacuum levels are fluctuating, there are more serious issues. Adjusting mixture screws to highest vacuum (with screws turned in as much as possible while maintaining that vacuum level) is the goal.