Mercruiser 5.7 troubles. Pulling what hair I have left out

[christoner2002]

1997 Larson 206 SEI. Purchased earlier this year. Boat was originally a 5.7, with the 2bbl Mercarb carb. Engine was faltering, and stumbling on top end. I replaced the intake with one from a 1995 5.7 4bbl. I purchased the carb(Quadrajet) from a rebuilder on Ebay. Boat ran much better but has a severe bog at the point were the secondaries open. I can power through it. When I called the carb rebuilder, he told me to move the secondary metering rods up, by bending the bracket that holds them. When removing them, they are not the same side to side. It made the engine run worse. Also, if run for a long time at constant speed, engine will begin to pop through the carb, and run like hell. If engine is left to cool, it will then run fine again. Any ideas on the bog, and the problem with the carb popping? List of things we have done,

Full tune up(plugs, wires, cap rotor, all Mercruiser parts)
Timing is set(advances correctly through RPM range)
Compression test done (130-140PSI range. #8 has 90PSI)
Water/Fuel seperator checked, replaced. No water in fuel
Fuel lines are clear, clean
Thermostat removed, Secondary Metering rods raised.(made engine run worse)
I know the #8 cylinder compression is low, but I feel this is more a carb issue.. Please give me some ideas. Thank You so much in advance!

 

[JustJason]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Welcome to Iboats.

I know the #8 cylinder compression is low, but I feel this is more a carb issue

Why do you feel that way when you already know you have a problem on 1 cylinder that at the moment your choosing to ignore.

I say this all the time, but you need to troubleshoot in this order
1. mechanical
2. ignition
3. fuel

I'm not saying this is your problem... But if cylinder numero ocho is blowing half it's intake charge back through a leaking intake valve back into the intake manifold, it's going to make every other cylinder run lean, which also fits your symptoms of the lean pop your having.

Next on your list of troubleshooting after you have done your compression test is to now do a leakdown test, and find out where your loosing that 40 to 50 psi that #8 should have.

 

[JustJason]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

And oh yeah... put your thermostat back in. You'll do more harm than good without it.

 

[christoner2002]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

I chose to ignore it because the boat does not act up all the time. If it is blowing the compression into the intake, wouldn't it be a constant problem? The boat has went 3+hours of constant running, and acted fine. Even pulled skiers, and tubers. The thermostat was removed for a test, to richen the engine. It didn't work, and was replaced. My brothers old boat was down on compression on one cylinder, and we ran that boat for years, and the current owner still uses it alot. It never gave these same problems. I know there is a problem, and plan to fix it at the end of the year. But, I would like to know I am not going to Lean out the fresh motor, and hurt it just as bad, or worse. That is why I would rather have the carb right before.

 

[JustJason]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

But, I would like to know I am not going to Lean out the fresh motor, and hurt it just as bad, or worse.

And your not going to know that until you figure out the compression issue first.

 

[6meter]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

See if you can borrow a coil or just replace it. Sounds like my boat, it was the coil.

 

[John_S]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

I purchased the carb(Quadrajet) from a rebuilder on Ebay. Boat ran much better but has a severe bog at the point were the secondaries open. I can power through it. When I called the carb rebuilder, he told me to move the secondary metering rods up, by bending the bracket that holds them. When removing them, they are not the same side to side. It made the engine run worse.

I agree with the others and finding out what is causing the compression problem 1st. After that, you either need to go through the whole carb or find another. Two different 2ndary rods on a Q-jet! What, they ran out of parts? Who knows what else they did to the rest of it??? I5Auto?

 

[christoner2002]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Ok. Heads are now off. I put oil in the cylinder and it did not change the test on the compression. Valve's and piston have no marks showing any real damage(tuliping). They are now at my local machine shop for inspection. Cylinders look nice. The head gaskets look good. All pistons and valves are very black, carbony. Spark plugs also are very dark. The carb did come from I5. That scares me that you guessed that. Should the carb become a paper weight? I appreciate the help, I am just afraid that the compression problem isn't the only problem with this thing.

 

[Bondo]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

All pistons and valves are very black, carbony. Spark plugs also are very dark. The carb did come from I5.

Ayuh,... Sounds like it was runnin' extremely Rich....

 

[John_S]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

I5. That scares me that you guessed that. Should the carb become a paper weight?

The one I got from I5 ran OK, but had a similiar bog as you. Took, a bit of tuning to get most of it out. When I opened it up, it did not have the jet and rods I had discussed with him, over the phone. They did send them along with another gasket. This spring, the tuning seemed off again. Went to find my box of tuning parts, and it has disappeared. Ened up with a new Edelbrock 1409, which seems to be allot less tempermental than the Q-jet.

 

[christoner2002]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Sounds like they are a great company. They are sending me new metering rods, and a hanger. I knew I should have went with the Edelbrock. Did you install the spacer between the Edelbrock, and the manifold? I know the Weber carb(looks just like the Edelbrock)had a spacer from Mercruiser.
Don't know if this is right, but with the head off, I poured motor oil on top of the piston that has low compression. It has held there for 1 hour plus, with no loss. The cylinders have cross hatching on them, no scratches, and very little ridge. Hoping the machine shop finds something wrong in the heads tomorrow.

 

[John_S]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Did you install the spacer between the Edelbrock, and the manifold? I know the Weber carb(looks just like the Edelbrock)had a spacer from Mercruiser.

I have an Edelbrock Performer Intake which will accept a squarebore or spreadbore carb.

Sounds like you have the low profile 4brl manifold that is spreadbore only. You would have to get a different intake or the adapter to go to the 1409. Some LX's came with the same highrise dual plane as the 350 mag. Those could use both types. If you are not sure, post a picture of the top of yours.

 

[christoner2002]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

My intake does have two different sets of mounting holes for the carb. Maybe it will accept both types of carbs. I will try to snap a pic.

 

[PitchFork]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

I to got my carb from I5 and had to do lot of tuning to get the bog out although its not completely out but close enough that I'm tired of playing with it. The problem at WOT and hte popping through carb and RPM drop was due to secondary butterflies were not opening all the way it took some adjusting on the secondary spring to get it to the WOT range.

 

[christoner2002]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Ok. Got some good news today. Engine had 4 intake valves that were bad, 1 worse then the others(low compression cylinder). I will be getting the heads back Monday morning. As soon as I get it back together, I will give this I5 Quadrajet one last chance. If it doesn't work right, it will work as my second anchor. I have priced out a Edelbrock 1409 and a spacer to adapt it from spread bore manifold, to sqaure bore carb. Is there much that needs to be adjusted on the 1409, or did it work good out of the box? Thanks everyone for the help.

 

[John_S]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

You will need new fuel line, if you do not have the marine flexible type. If you had the merc throttle bracket on the q-jet, it works fine on the 1409. Idle speed and mixture needed to be adjusted. While I purchased some other size rods, jets, spring kit, for tuning, I haven't noticed anything yet to say I need to. Accel is very smooth thru the opening of the 2ndaries. Idles fine, no backfire, pop, bog, or surge. Plugs look good so far and will continue to moniter.

 

[JustJason]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Ok. Got some good news today. Engine had 4 intake valves that were bad,

I'm not saying this is your problem... But if cylinder numero ocho is blowing half it's intake charge back through a leaking intake valve back into the intake manifold

See what I mean about troubleshooting in order? Glad to hear you found the problem though.

 

[proshadetree]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

I like the old q jets. Many articles on tuning them on the net. If primary runs fine then just fool with secondary. Never ever bend the metering rod hangers. you can adjust the spring at the secondary upper butterflies to open sooner or later. The metering rod holder has a number on it. Different ones do different heights. Front metering rods can also be adjusted. These carbs size themselves to different engines as far as secondary's go not enough vacuum will not open them as far. Lots of vacuum will open them all the way. Except for the throttle shaft wearing out they are great with lots of tunability

 

[jy118lfd]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Buy this book (available at most barnes and noble book stores)

http://www.google.com/products/cata...og_result&ct=result&resnum=3&ved=0CB0Q8wIwAg#

Then go through the I5 piece of chit and see how bad they are at "rebuilding" carbs. You have a great core carb and can get it to run extremely close to fuel inj. I have one on my 5.7 and it runs from 650 rpm idle to wot with out any bogs. It is truely a great carb. You will learn so much from reading this book.

Any questions you have go to www.cliffshighperformance.com and call the guy who wrote the book. I did and we talked for hours on my carb. I ended up buying the rebuild kit from him and jets and metering rods. If you can pull the heads from your motor you can make this carb the best carb around. No carter (edelbrock) can touch the throttle response, economy, or power from a well built q-jet. the bad rap the q-jet gets is from it being so utterly precise in its fuel metering and if you dont have the carb setup right it will not run right.

 

[christoner2002]

Re: Mercruiser 5.7 troubles. Pulling what hair I have left out

Thaks for the ideas. I will have to buy that book. I have never been afraid to tear apart a engine, but carbs scare the hell outta me. I have no idea why. I have always heard the Qjet is a great carb, if tuned right. My boat is trailered, and I worry that it will be a process of unload the boat, try it out, reload, tear apart tune, unload boat, try again. If I rebuild this carb to specs of Cliff, will my tuning be able to be done in the water, and not tear apart the carb?