Mercruiser 5.0 backfire up intake

[ibrw1]

If it's not a valve, push rod rocker or valve lash?
Then,,, Electronic Ignition?

 

[alldodge]

If it's not a valve, push rod rocker or valve lash?
Then,,, Electronic Ignition?

IMO if its not lean then you have a valve hanging. It's heating up and starts to hang and not fully close. This can be valve guides or weak/broken springs. If you can get a vacuum gauge on the intake see what it shows you

 

[ibrw1]

I ran 10 gallons through it testing it this Saturday, I write everything down. So regarding the valve and spring, When I have the issue, at 3200 through 3800, I slow down to 2000 RPM for 3 minutes, speed up to 3500 RPM and the problem is gone, Wait 1 to 2 minutes, its back. increase RPM to 3900 and its gone. That's the part that I don't understand...... THANKS will get the Vacuum info and the results of the Pulling each plug to narrow down to which cylinder or all cylinders which will certainly give direction. I may sneak boat out this Saturday to test. Going to the Mississippi for 4 days 17th. Curiosity is killing me. I was wondering what about the (Distributor pickup), the (Coil, Doubtful) or the electronic module mounted on the port side of the engine? LAST, I'm not going to do this, BUT, if I could change the THERMOSTAT to 143, THAT WOULD FIX IT! Having said that, has anyone ever in their life ever heard of someone changing a thermostat to band-aid a bad valve or valve spring??? THANKS FOR ALL OF THE IDEAS AND PROCEDURES.

 

[biggjimm]

It very well could be a valve hanging like alldodge suggested. Maybe carboned up a bit.

My machinist buddy suggested a leakdown test along with compression check & also putting the vacuum gauge on it. Which if it's in a valve that should show in a leakdown test.
The only thing is that crazy rpm "window". If it only acts up in that certain rpm range & in a certain temp range, it might not show in the leakdown test I suppose???
Let us know how it goes when you get down to check it out again & good luck. Jim.

 

[ibrw1]

Ok, great, everything suggested, as always will be tested. Yeah, got vacuum gauge, compression tester, timing light tach, yadda yadda. All the toys to make me dangerous! Question. You want vacuum test while in driveway or underway? Both? I have to get a leak down tester, hopefully it can be had locally. I've never done a leak down test before. Thanks again for all your help!

 

[alldodge]

Using a vacuum gauge in the drive way is fine. Seeing it under way will provide even more info. The issue you may run into is finding a place to hook it, most boats don't have vacuum ports. That's what I meant by IF statement above. Have been trying to find a pic of your intake and unable to find it

 

[biggjimm]

Mine has a vacuum gauge at the helm. I unhook it & put my shop gauge on that fitting when checking mine. Not sure how accurate the gauge on the boat is.

 

[ibrw1]

My 5.0 1999 Mercruiser has a port that normally connects to the port valve cover. Crankcase vent, I figure that should be okay for short term test. I will give results for both tests asap. I'm getting a color video camera in the morning that I can slide into the intake manifold to view and photograph the intake valve bells and seats.
I hope to have those results by Late Friday night as I intend to repeat all calibrations Saturday early morning, then leak down test, then a Sunday test run. BTW, last calibration of fuel mixture was using the vacuum gauge, turn screw in till near stall, turn screw out until highest RPM. and all done in BASE MODE, short wire, start engine, warm up to 160 F. Idle at 650. Timing at 10 before. Wife is watching me from distance. Too much time with boat. Thinks I'm cracked. Now I work on boat late at night when she's sleeping.

 

[ibrw1]

First I want to thank each and everyone of you for the knowledge and encouragement. Anyway, the issue has been proved well beyond any shadow of doubt. Number-6 has a bad intake valve. I'm taken her out for one last run, then I'm going to spend the winter with my boat in my MAN CAVE.
Lots of beer and Whine. And no, whine is spelled correctly. Wife lost her heated parking space for the 3rd winter in a row. How cold could Chicagoland get? Heh heh. Thanks Everyone!

 

[ibrw1]

Mine has a vacuum gauge at the helm. I unhook it & put my shop gauge on that fitting when checking mine. Not sure how accurate the gauge on the boat is.

Thanks lots Bigg Jimm, You were right! Valve

 

[ibrw1]

Last Questions, anyone know of a good Machine Shop in the Chicagoland area? Is now a good time to bump up to 5.7? I would need short block, heads. The remainder is all 5.0 stuff? I've rebuilt many engines in the 70s and 80s. Old school Chrysler mechanic 75-85.
I would get the 4bbl carb and intake. Where could I go to find commonality parts list? Thanks.

 

[NHGuy]

You can do it, but there are some differences. if you have a Mercarb the whole carb gets changed. If it's a Quadrajet the carburetor gets different secondary rods. But truthfully, I have been inside a qjet. It would work as it is just fine unless you run wide open. At WOT it could go a teeny bit lean.
You'd need to use the same centerbolt style of heads so you don't have to change the intake manifold. And you need to stay with the later block style which has a one piece rear main seal. Other wise you'd be on the hook for a flywheel and coupler.
You are dollars ahead by fixing what you have unless it's trashed.

 

[ibrw1]

NHGuy,
Engine is sound. Smart money says fix the 5.0, Was thinking if I was already doing open heart surgery then why stop at valve job? When I was a kid options were infinite. Reading forums goes like this, buy faster boat, or bigger engine. No real internal tweaks. My brother in law put a whipple supercharger on his boat. My neighbor has an old challenger with a 318 pushing well over 350 hp. Is there no bag of tricks? ps. I know about the 300 HP alpha one gen 2 limit.

 

[biggjimm]

It depends on what you're really after. If you just want to cruise the lake in a decent running boat at a somewhat affordable rate, I'd say do a valve job on what you have. Otherwise, if bigger, faster & better is your aim, then get yourself a bigger boat with a bigger engine, or two. Maybe I'll take your old boat off your hands. I'm not too far away from you & my wife just loves to visit the windy city. And I'm overdue for some Giordanos pizza anyways. Good luck which ever way you go. Jim.

 

[ibrw1]

I'm going to fix it, I just wanted a little more HP to blow by a few of my loud mouth brother in laws. I have 9 brother-in-laws 5 of which have boats. All faster then mine. Whats wonderful about my boat is the fact that it will fit in my 3 car garage, which is exactly what my wife hates about it. Thanks for the offer to buy it though. I love the City myself, I stay at the Holiday Inn Market, River North. Advance purchase will get you a 600 sq ft room with one heck of a view. under $200 a night. We go to the Brazilian Steak house till we get the "meat sweats". Thanks again for all your help, Popping plug wires quickly narrowed it down. I can ship a Giordanos your way if interested, let me know. Thanks again.

 

[biggjimm]

I know what you mean about showing up the loud mouth bro in laws. There are any number of mods you can do to up your power, just as in a street motor. Most of which are more beneficial in a 350 opposed to a 305 but they can still build power too.

Glad I could be of some help to you in narrowing down your problem.

Thanks for the offer to ship me a pie. They will ship them frozen to your door but luckily I just found out today that they opened a Giordanos in Indianapolis on the north side which is only one hour from me. I hope they're as good as the ones up there. Take care & good luck with your boat. Jim.

 

[Rick Stephens]

You can always stick a bigger small block in there. World Motors has a 465 hp 415 ci crate engine small blockyou could drop right in there. Have to swap to a Bravo, but pocket change compared to the $7500 engine. Probably speed you up a little.

 

[biggjimm]

You can always stick a bigger small block in there. World Motors has a 465 hp 415 ci crate engine small blockyou could drop right in there. Have to swap to a Bravo, but pocket change compared to the $7500 engine. Probably speed you up a little.

There's an option! They have awesome products & stellar customer service. Those guys know how to make big power. Especially with stroker motors. I've ran one if their stroked big blocks & several sets of heads from them. Good stuff.

 

[harleyman1975]

Fix what ya got. Ask All Dodge how much faster his boat goes with an added 100 hp. Both posts are awesome teaching posts by the way...Thanks for being so concise and seeing it through to the solution! Very interesting

 

[Rick Stephens]

Hopefully everyone knows I was kidding about a stroked blown small block. I would agree with the fix the 5L statement. If you had to purchase a new motor then the replacement might be a 5.7L or a 383, but no need here.