Tired Engine?

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  #1  
Old 08-15-10, 04:35 PM
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Tired Engine?

I have had a 1979 century 3000 for 15 years. It had been idling rough and got worse so that it would idle in neutral, but when the engine got hot it (water temp 150......hot meaning letting the boat idle in gear 15 minutes or so. then it would die.
I went through a litany of diagnostics, some I did, some the marine mechanic did. After all is done, it still idles smooth when "cold" ie. 15 minutes or so, then it wont idle in gear and stalls out after the engine is run in gear for a while. The mechanic said the engine is old and tired. he explained that it wasn't getting enough air sucked through the carb.

This is a 1979 305 chevy v8 with a 228 mercruiser set up.

Parts replaced:

remanufact. carb (rochester quadrajet)
fuel pump
points, plugs, condenser (twice), coil, fuel separator/filter, carb fuel filter.

timing checked, dwell, pt gap, interupter switch checked, resistor wire checked by puting 12 volt to coil, carb adjusted,
compression is about 150 or better in each cyliner, plugs burn clean, and the engine uses little oil. it seems to have good power when pushed and runs well at 4200 rpm.

after all this it will idle fine a few minutes then get a little rough and if in gear, just stall. it immediately starts back up.

Question (finally) are we missing something?

any help appreciated.....Ed
 
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  #2  
Old 08-15-10, 06:23 PM
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You say the carb was adjusted. Did this include the idle mixture? An engine w/ that good of compression and all the parts that have been replaced shouldn't stall at idle unless the idle mixture screws are turned in just a tad too far. From your description of everything I don't think the engine is old and tired. Maybe the mechanic is tired of working on it!! If the idle mixture isn't the problem then quite possibly you have a vacuum leak somewhere on the intake manifold, and yes, it can be on the underside of the intake, that opens up when the engine reaches a specific temp. If it has one, has anyone tried changing the PCV valve?
JMHO,
Mike
 
  #3  
Old 08-15-10, 06:35 PM
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I thought he might be tired of my boat, but the guy was conscientous. I asked about vacuum leaks. Apparently he searched for leaks with ether around the carb area....dont know if he checked after it started acting up, but I can ask.

Also, no pcv valves, just straight hoses from each valve cover to the flame arrestor. He tried to adjust the idle mixture screws but said it didnt help. I might try that again myself tomorrow.

How would you detect a vacuum leak on the underside of the intake manifold?
 
  #4  
Old 08-16-10, 06:20 PM
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Originally Posted by 84EdH View Post

How would you detect a vacuum leak on the underside of the intake manifold?
Only way I know is to change the gaskets after you've tested for vacuum leaks everywhere else. Did he try spraying under the carb and along each side where the intake bolts to the heads while the engine is running rough? If it's running smooth you won't find the leak.
HTH,
Mike
 
  #5  
Old 08-16-10, 06:54 PM
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Mike I actually tried that today with carb cleaner, could get no sign of leak all around carb, and edge of intake manifold before and after it started acting up.

I ran the boat for about two hours today and it ran perfect until going back to the marina, its about 1 mile you have to put-put through a no wake zone. About 1/2 through this zone the engine sounded rougher, did not stall, but i feathered the gas a bit to up the rpm to about 1000. it did ok, but when i got near the dock it stalled and then it would immediately re- start then stall as I put it into gear. So the elusive "leak" or valve hanging up or whatever apparently can behave a good while before re-appearing.....frustrating!
 
  #6  
Old 08-17-10, 01:58 AM
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Are you sure the carb float is adjusted correctly? Or ... You didn't seat the needles too tight and then back off?

How 'bout the automatic choke? Are you sure it's opening all the way as it heats up? Disconnect it and prop it open when warm to check it.

Unrelated to a possible vacuum leak ... As I recall that engine doesn't use a vacuum advance. It uses a mechanical advance, correct? Is it returning when you slow down from high RPM? Easily checked by leaving the timing light connected.

Also, the old ('86) Mercruiser that I used to have wanted to be timed different than the manuf's recommended BTC. It was only a couple of degrees difference, but it made it run like a top. It was exhibiting the same signs as you're describing. With the timing set to spec the carb had to be adjusted to bring the RPM up to spec at idle in gear. Adjusting the timing increased the idle, and the carb was readjusted until it idled again at spec. (What was it -- 700 RPM?)

(Be careful running it without the flame arrestor, especially if you mess with the timing. I've burned my eyebrows on several occasions!)

I don't know about "tired". My old Merc had more than 2,000 hours on it when I sold it and it's still going strong.

Sometimes it's prudent to get a second opinion from a doctor. Maybe it's time to apply that to your mechanic. Not saying he's bad or indifferent -- just that sometimes a new pair of eyes will have a fresh perspective. Especially if you don't tell the new guy anything that could influence him before he looks at it.

I commend your diligence. A lot of guys would bail on the engine and replace it with something else. Besides, it's a fun challenge to get it right.
 
  #7  
Old 08-17-10, 04:25 PM
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Mike, thanks for the ideas. I haven't checked them out yet but I will. No, for sure didnt tighten down the needles too tight. I dont know how to adjust the float, but the reman carb is only a month old and has run perfectly at times....I am guessing if the float position was wrong, it would be all the time.

Dilegence? or Rock headed?....not sure.

Ed
 
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