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Fuel dilution 454bs

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sammi

Regular Contributor
"I bought a boat this spring w

"I bought a boat this spring with 1986 crusader 454's. They have 1200hrs, run great, burn no oil but I had an oil analysis done and they came back about 5% fuel dilution. They shpould be less than 2%. They have rochester carbs. Is that just a float adjustment? What would cause fuel dilution? I guess I will have them rebuilt this winter."
 
"It's a really expensive f

"It's a really expensive fix to a pretty small problem. Why not simply change your oil more frequently?

If it's not broke, why fix it? It's a twenty two year old motor and if it runs good, the oil pressure is good, it has no knocks or bangs, I wouldn't loose sleep over a little blow-by."
 
"Jim:

Rick's answer is

"Jim:

Rick's answer is it. I've got a pair of the same vintage engines in mine. i'd bet the source of the dilution is a lot of time at idle. my run rich at idle and the mixture screws don't have a lot of control. The 'fix' is a significant undertaking - mod to the thottle plates, on the primary side - to alter the airflow around the idle ports. a rebuild won't do this.

i haven't done my due to the time involved. Lowering the idle regains control with the screws but it results in the RPM ~ 400 which will probably due more harm than good. you can also here the damper plates springs, in gear, and that isn't good either."
 
"..."my run rich at idle a

"..."my run rich at idle and the mixture screws don't have a lot of control."

That's 'cause both of you have a "slow flooding" condition in your carbs. Quadrajets are famous for floats that absorb gasoline after a while and get too heavy to work properly. You need to replace the floats--they can be bought separately if you look around.

Note: The motor MUST stall at idle if the idle adjustments are turned all the way in. If not, the carb is flooding.

Jeff"
 
Jeff:

My floats aren't

Jeff:

My floats aren't letting the carb flood. they've been replaced with the brass ones. Believe me.

The mixture screws don't have much control due to the position of the primary throttle plates when the idle is set to spec. They are "above" the effective point for the idle ports. Its one of the drawbacks to using thosee small primaries on big CID engines.

What you windup with is a slow 'drip' from the nozzle area. The 'fix' is a simple sounding mod to provide additional air. The PIA is the process for getting the right amount of air. It is covered very well in the Doug Roe book.
 
"rochester carburetors - HPBoo

"rochester carburetors - HPBooks-014

may be out of print - if you want one, check your local library system or the amazon used book section"
 
"AS I said, I bought the boat

"AS I said, I bought the boat this spring and know little about what maintenance was done over its lifetime. I think I will put a new float in and see what happens."
 
Piece of cake! Simply drill a

Piece of cake! Simply drill a small hole in the back side of the primary throttle plates. That allows the idle speed to rise while not moving the edge of the butterfly away from the idle ports. THis is done all the time with race cars.

Jeff

PS: Start with an 1/8 inch hole and enlarge it as necessary.
 
It's the "enlarge it a

It's the "enlarge it as necessary" part that is the tedious and time consuming when done in the marine environment....I'd rather do twenty street carbs than one marine carb.
 
"So what do you suggest I do f

"So what do you suggest I do first? Make sure the float is right? Also, after getting a good look at the carbs the flame arresters were filled with dirt and "fuzzies". Could it be that I just wasn't getting enough air or would that be too easy?"
 
"Jim:

Yes, clean the flame

"Jim:

Yes, clean the flame arresters. that stuff doesn't help the airflow at all. Do it regularly during the season, too.

if it turns out your dilution is due to the 'nozzle drip' syndrome, you can do what Rick suggested or have your carbs custom modified. As I tried to convey earlier, I don't believe the mod's benefits outweigh the effort required to do it right.

If it turns out to be the float (flooding), changing it is the fix.

make sure you have a 'problem' first. get a good look around the carb, especially the gaskets at the throttle plate, and see if they are wet. You can also check the tops of the throttle plates, with the throttle closed. If they aren't wet, you not likely to be flooding. You can usually smell the unburnt fuel in the vicinity of the exhaust when they flood."
 
"Thanks guys,for your comments

"Thanks guys,for your comments. One other thing. After a week sitting at the dock all the fuel has drained(or evaporated) out of the carb.It takes about three 10 second hits of the starter to get it going. When warm, or even the next day it starts immediately. Is that a symptom of something or normal? Come to think of it I probably idle 30% of the time. Would switching to a hotter plug burn off the fuel faster?"
 
I have an older BB Chevy Crusa

I have an older BB Chevy Crusader that has the exact same symptom. If it sits for more and a couple of days I have to crank it over a few times before it starts. I have a new Edlebrook 4bl and have made sure the anti syphon valve is good and all clamps are tight. I just figured it was the nature of the beast. Any body have ideas?
 
"Jim:

unless your plugs fou

"Jim:

unless your plugs foul out really quickly I wouldn't change them, especially for the 70% of time off idle.

The evaporation phonomena is a 'feature' of what they sell, today, as gasoline.

If you spend a lot of time at idle (30% of 10 hours vs 1000 hours) you may find it worth the effort to fix the nozzle drip. Again, make sure that is your problem before you 'cure' it.

There's a lot of info on the quadrajets out on the internet. spend a rainy day researching it may be worthwhile."
 
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