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88 merc running problem HELP

ccfd40

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"Hi, I have a 1988 Sea Arrow b

"Hi, I have a 1988 Sea Arrow boat that came with a 200 h.p. GM 5.0 engine. The engine was changed to a 5.7 engine before I purchased it. It has always had this problem from the day I got it. The boat is a little doggy taking off & when you get up on plane & up to about 30 mph it starts poping back thru the carb & basicly just falls on its face. The boat has been thru 3 boat shops & no one can fix it. I purchased some service manuals & this is what I have came up with. It would not pass a cylinder leak down test so I removed the heads & had them all re-worked. The ignition module would not corectly control the timing so I removed the compleate Merc system & instaled a MSD marine system. I have done some reading on the net & found some people saying to run a heavy wire from the battery to the coil & disconect the one from the ignition switch. Also found people saying to disconect the shift assist & the tach. I tried all of the sugestions & nothing has changed? Any ideas??? Thanks, Rusty"
 
"Welcome. A more detailed desc

"Welcome. A more detailed description would be helpfull.

Firstly, why was the engine changed to a 5.7L before you bought it? Are you saying "both" this and the previous engine had this problem in the boat?

Explain what you mean by doggy taking off.

Is it actually backfiring through the carb? and when you say "falls on its face" do you mean it cuts out and stops?

If so will it re-start immediatly or after a few minutes?

Will it run and keep running normally at any speed below 30MPH? and at what RPM is 30MPH?

Could you please tell us where the fuel tank is located in this boat and if you have replaced the fuel filters."
 
Carb running to lean? Check t

Carb running to lean? Check the float level for that specific carb. number's specs. If it is on the money per specs. try dropping the shutoff point a few millimeters to provide more fuel to the bowl.
 
"Hi, thanks for the quick rep

"Hi, thanks for the quick replys.

CRAIG, From the info I got the original 5.0 engine was replaced with the 5.7 because it froze & broke the block 2 winters ago. Best I can tell, this problem started with the new engine. By doggy, I would just say it is a little slugish on take off but realy not that bad. Yes, it blows nice size fire out the top of the carb with the spark arrestor off! When you get to 30mph (about 3,000 rpm) it just starts the poping back thru the carb & just bogs way down, loosing power. You can not throttle thru it, all you can do is back off the throtle a little & it will pick back up & alow you to cruise about 28-30 mph but no faster. It never has died or had to be re-started. It runs very good up to 30 mph & 3,000 rpm. Fuel tank is under the rear seat & all filters have been changed. The screen was removed from the bottom of the fuel pickup tube & it was also checked for sucking air & tested fine.

GUY, I realy dont think the carb is running to lean but maybe I am wrong? You can look down in the carb just before it starts acting up & it looks like it is puting lots of fuel to it. The carb was just re-built & we did even raise the float level & got no change? I hope this helps. Thanks Guys"
 
"That being the case, then I w

"That being the case, then I would put a timing light on it while under way and watch it and see what it does when it starts to act up. Bet you find your problem is timing related. Did you get a close shave from the flame thrower? Use an extension mirror to look down the throat next time."
 
"As guy says your problem is l

"As guy says your problem is likely ignition related.
Usually when an engine backfires thru the carb it indicates that one or more cylinders are getting spark when the intake valve is still open on the intake stroke. Assuming the ignition wires are strung correctly, about the only way that can happen is if the spark is jumping wires, or more likely jumping posts inside the distributor.
I would look for evidence of carbon tracking or other problems in the distributor cap & rotor area. Maybe check the plug wires and replace with high quality marine grade wires if any are more than a few ohms resistance.
Rod"
 
"Rod:

Thanks for chiming in


"Rod:

Thanks for chiming in about the wire crossover and carbon tracking. I (DUHHHHH) assumed those items were new. The MSD system would definitely throw out alot more spark. Crossover shows up real good in the dark and makes for a spectacular show.

Quick story: I was 19 and troubleshooting radar transmitter HV power supply at night on Okinawa. Kept tripping breakers. Everything is enclosed behind spark screen. As I observed gauges and saw HV climbing over the limit of 50KV, the spark gap override kicked in as I turned my back to the cage. Loud lightening bang, blinding blue flash, peed my fatigues. LOL
Guy"
 
"GUY, I will try the timing li

"GUY, I will try the timing light & see what it does. I just figured that it would show weird on the light when it started running bad even if it was not timing related? It makes sence to me that for it to do this it must be firing with an intake valve open. Maybe cam or lifters? No close shave, I am used to working on race cars on alcohol...smart enuff to use a mirror!!!

ROD, it has had everything ignition replaced new & the wires are correct. I replaced the new wires that were on it when I got it with a set of good solid core wires with stainless terminals. Thanks for the input, Rusty"
 
"Guy; looks like your assumpti

"Guy; looks like your assumption was correct. Back to square one with the timing issue, or as Rusty says, maybe a sticky valve or lifter.
Rod"
 
"Rusty:

An oscilliscope w


"Rusty:

An oscilliscope would give you an excellent view of the spark pattern when it starts to act up. I am interested what the timing does when it runs bad. If it shows normal timing then you are probably right about the valve sticking.

Do you use SEA FOAM? It works in the gas or oil. If the valve is sticking due to gum or varnish, SEA FOAM should fix it.

To rule out a lean fuel condition, you could use a bottled propane torch unlit and let the flame arrestor suck it into the engine. Don't do it when it's acting up. Start the propane before it reaches the point it acts up. If it runs smooth thru the problem RPM range, back to the carb. If the timing is good and the propane doesn't solve the issue perhaps it is time to look at the valve train."
 
Guy;
First I have heard of th


Guy;
First I have heard of the propane torch idea. Have you actually tried that experiment successfully? Not saying it won't work; I have just never heard of or tried anything like that.
Rod
 
Rod:

I haven't done it


Rod:

I haven't done it yet. Only when looking for vacuum leaks. Heard about it from a local shadetree mechanic that said it works just like nitrous injection. I think it would work best using the PCV valve inlet hose if equipped. I'll have to see if it increases RPMs on my 4.3L.
Let you know.
 
You guys are great! Almost ev

You guys are great! Almost everything you listed is stuff my grandfather has recomended me trying. I am not real sure what a oscilliscope is but I will do some checking to see if I can use one. I will check to see what the timing does this weekend. I have used seafoam & other products many times with no change. At the request of my grandfather I tried the propane idea yesterday. It made it run realy bad. He said he has found problems with several vehicles in his 35 years of owning a shop by using propane. Thanks Guys.
 
"Propane enriched the fuel mix

"Propane enriched the fuel mix and it stumbled. Not a carb problem. Now you have narrowed it down to the ignition system. See if your grandfather still has his O"scope. He'll know what it is and how to use it.

Things to RECHECK:

Timing--set to new ignition system specs.?

Spark Plugs--correct no. and heat range

Plug Wires--routed properly, no crossover

Dist. Cap--correct one, use magnifying glass to check for hairline cracks and carbon tracking

Rotor--correct one, seated properly, rotor blade correct bend and tight to base

Pickup Coil--if equipped; check for minute hairline cracks at wire bends with a magnifying glass; it will fail when it gets hot

Optic Sensor--if equipped; not covered with a film of dirt or grease

Distributor--check inside base for oil seapage, fine cracks in base

Coil--substitute another known good coil

Control Module--solid wire (not corroded at crimp; replace connector and solder) connections; run test procedures and/or replace"
 
"Guy, I will check on the o-sc

"Guy, I will check on the o-scope this weekend & let you know. The timing is set to specs & i have advanced, retarded & adjusted the total advance with to be expected results. The spark plugs have been changed & are correct. Plug wires are new & I re-checked the routing. The compleate ignition system was removed & replaced with the msd system including the coil. I am starting to lean toward cam & lifters, what do you all think? Thanks, Rusty"
 
Ahhhhh! "The engine was c

Ahhhhh! "The engine was changed to a 5.7 engine before I purchased it. It has always had this problem from the day I got it."

I wonder if the previous owner installed an auto engine in the boat? Look at the freeze plugs and see if they are brass. Scrape off a little paint to see. Hope not.
 
"I wonder if you have a fuel f

"I wonder if you have a fuel filter BEFORE the fuel pump? If so, it may be starving the motor for gas when you get up to speed. (Backfires at 3,000 rpm are either crossed wires, cross firing, or a lean out.) Filters (fine filtration) should ALWAYS be placed AFTER the fuel pump. Strainers and water separators (coarse filtration) can go before the pump.

Jeff"
 
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