Mercruiser 5.0 no holeshot/stalls

1989 Mercruiser 5.0 LX 230 4 bbl (Rochester) Alpha One. I have taken this boat out twice since I purchased it. I replaced the fuel line from tank to fuel pump and the return hose carb to pump. Replaced the fuel filter. Checked that there is no water in the fuel tank. There was none in the old fuel filter. Replaced the spark plugs and checked the spark plug wires.
When I go full throttle frkm sitting still, the motor backfires and stalls, then catches and accelerates nicely. No other issues. Any thoughts? I'm thinkin I need to rebuild the carb. Any gotchas on rebuilding a Rochester 4bbl. I did a few of em 20 years ago and dont remember them being especially difficult. Thanks!

Re: Mercruiser 5.0 no holeshot/stalls

Any thoughts? I'm thinkin I need to rebuild the carb.

Click to expand...

Ayuh,... I'm thinkin' yer Right,...

The bog at launch could be the acc. pump...

Re: Mercruiser 5.0 no holeshot/stalls

So, I rebuilt the carb. It really wasnt that dirty. The check ball for the acc pump was corroded pretty bad. I put it all back together and it wouldn't holeshot or run above 2400 rpm. So I dissassebled it and, looking at a parts diagram, realized I was missing the power piston spring. I assume it was there before I disassebled it the first time, and that it sprung itself somewhere. Bought a new one that is supposed to be specifically right for this setup, installed it and it is running better, I do have power throughtout the whole range, but really no holeshot (it backfires and falls on its face and doesnt recover unless I pull the power back). I had set the idle mixture presets at 1 1/2 turns per several sources, and found that it was dying sometimes when I put it in gear. One source said to set them at 3 turns, so I did that and set the idle rpm once is was warmed up (thermostatic choke not affecting rpm), and now it goes into gear nicer (at a better idle rpm now) and doesnt die. This has helped the holeshot some too. Still, not right though.

On a sidenote, (or could this be the problem?), every time I go out it runs the battery down and finally the engine dies and wont restart. I installed a second battery with a switch, which helps get home, but doesnt solve the running batteries dead problem. If the engine is running, and I switch from one battery to the other through the no battery position, the engine dies (shouldnt the alt provide enought power to keep the engine running?) Voltage on the gauges shows 12 and drops to bout 10 1/2 by the time it dies. Then there is jus not enough power to turn it over and start it. I took the alternator into a local electrical shop that seemed familiar with the design and they tested it and said the alternator is good. I have good continuity from the ground wire that goes to the alt to a ground on the block and from the orange positive wire that attaches to the alt to the post on the starter (there is not fuse on my setup).

Question? 1 Can a alt that is not charging, affect holeshot (Im thinking timing advance)??

Question 2 How can I test any further to see if the alt is producing power, (with a simple multimeter maybe?) with it installed on the boat?

Question 3 Can I test the two other wires that come to the alt (alt sensing and excitation) for continuity?

Moderator: Should I start a new thread on this? (I guess thats question 4 really)

Thanks for all the help!

Re: Mercruiser 5.0 no holeshot/stalls

It is going to be tough to diagnose any problems while you have a bad alternator.
Pull the alternator and take it to autozone or somewhere and have it tested. DO NOT replace with an automotive unit!
There are many little passages in the carb that need to be cleaned with a wire and compressed air. The spring on the accelerator pump must be the stiffer one NOT the automotive one that comes with the kits.

Re: Mercruiser 5.0 no holeshot/stalls

I tested the alt at an electrical shop. They were the only ones locally that knew what to do with the extra wires. It tested good with them. What is the sequence for the wires coming off the alt? There is a red one and a green one. If I labeled it correctly, the red one goes to the red wire on the harness, and the green goes to the purple. Any thoughts on this?

As far as the acc pump, i replaced it with the one that came with the kit from Mercruiser. That is, I presume, the correct part.

Re: Mercruiser 5.0 no holeshot/stalls

Well, having put a multimeter on the battery before and after starting, its clear the alt is not charging. To seal the deal, I found out the alt that was on the boatWAS an automotive one.
So, i purchased a new 65amp Mando alt and its now in. It doesnt come with instructions, but its pretty clear how to hook up the purple and red wires. I know where the ground wire goes as well. However, im not for sure where the positive (orange) wire goes. There are two posts that have plastic caps labeled P and L2. Does the orange wire go on either of these posts? If i can find a way to add a photo I will.

Re: Mercruiser 5.0 no holeshot/stalls

Switching from one battery to the other with engine running is a good recipe to induce alternator failure, unless the switch is designed to leave the load on both for that split second.

On a Rochester QJ, sometimes the (adjustable) secondary opening event is way too soon and can lean out causing backfires and bog until the engine catches up so to speak. Check the airvalve over the secondaries by pushing lightly down on it on the edge at the rear of the carb, see if it has some resistance and snaps back.

Re: Mercruiser 5.0 no holeshot/stalls

Thanks for the advice on using the battery switch while the engine is running. Not much skin off my back this last time, as the alt was an auto one and needed to be replaced. I will prob now build a guard over the switch and placard it so it doesnt happen again.

Any thoughts on the correct post to connect the orange (+) wire to?

Also, thanks for the QJ tip. I will check into that. Ive also been advised to check the condition of the cap and rotor (plugs and wires already replaced) and set the timing. All good places to look.

Again, thanks for the responses. Keep em comin!

Re: Mercruiser 5.0 no holeshot/stalls

YN2(SS) said:

1989 Mercruiser 5.0 LX 230 4 bbl (Rochester) Alpha One. I have taken this boat out twice since I purchased it. I replaced the fuel line from tank to fuel pump and the return hose carb to pump. Replaced the fuel filter. Checked that there is no water in the fuel tank. There was none in the old fuel filter. Replaced the spark plugs and checked the spark plug wires.
When I go full throttle frkm sitting still, the motor backfires and stalls, then catches and accelerates nicely. No other issues. Any thoughts? I'm thinkin I need to rebuild the carb. Any gotchas on rebuilding a Rochester 4bbl. I did a few of em 20 years ago and dont remember them being especially difficult. Thanks!

Click to expand...

Back to the original backfire problem. I had the same problem in a 72 Mercruiser with a Ford 302, Holley 4bbl - backfired on the holeshot. I took all the same steps that you did, new plugs, points & cond, new cap, & plug wires. Re-built the carb. Still backfired. A mechanic told me that the backfire was possibly caused by an open intake valve when that particular cylinder fired. Probably a sticky lifter. (???) A good solution was to pull the heads and have them re-worked. The heads have never been off before.

That sounded like a costly solution to me, so I went in search of an additive that would help sticky lifters. I settled on Sea-Foam. I dumped half a can in the crackcase, and the rest into the two or three gallons of gas I had left in the tank. After about 20 minutes at WOT, my problem disappeared. Like magic!

I swear that I am not a shill for this product, just relating my personal experience. I've heard of other stories like mine.

We put electronic ignition in it this year. HUGE DIFFERENCE!!! Starts at the tap of the key and runs like new. 40-years old this year!

Re: Mercruiser 5.0 no holeshot/stalls

tonka.boy said:

Back to the original backfire problem. I had the same problem in a 72 Mercruiser with a Ford 302, Holley 4bbl - backfired on the holeshot. I took all the same steps that you did, new plugs, points & cond, new cap, & plug wires. Re-built the carb. Still backfired. A mechanic told me that the backfire was possibly caused by an open intake valve when that particular cylinder fired. Probably a sticky lifter. (???) A good solution was to pull the heads and have them re-worked. The heads have never been off before.

That sounded like a costly solution to me, so I went in search of an additive that would help sticky lifters. I settled on Sea-Foam. I dumped half a can in the crackcase, and the rest into the two or three gallons of gas I had left in the tank. After about 20 minutes at WOT, my problem disappeared. Like magic!

I swear that I am not a shill for this product, just relating my personal experience. I've heard of other stories like mine.

We put electronic ignition in it this year. HUGE DIFFERENCE!!! Starts at the tap of the key and runs like new. 40-years old this year!

Click to expand...

Orange wire goes to the alternator, connects with the red battery wire.

Re: Mercruiser 5.0 no holeshot/stalls

Since my plan is to at least go tubing in labor day weekend, im definately gonna put a can of seafoam in the oil and try. I'll check the secondary setting on the carb as well before i leave.
As far as the wire, sorry I wasnt clearer. I need to find which post on my new Mando alt that the orange wire is bolted to. I have two posts on the alt avail, one labeled P and one labeled L2. All other wire connections are intuitive. Thanks!

Re: Mercruiser 5.0 no holeshot/stalls

Well, I installed the alternator and it is now charging and shows 14 volts while running. wiring diagrams here:

http://www.cpperformance.com/Instructions/Alternators.pdf

I have added a can of SeaFoam to the oil for good measure.

Now off to check opening of secondarys on the carb and timing and timing advance.

Re: Mercruiser 5.0 no holeshot/stalls

Ok, so I thought I would check the timing first. Given that it is a Thunderbolt IV, I presumed it would be similiar to an HEI type distributor from other GM products of that era. What I found was a distributor with an external coil. I have an engine analyzer and was hoping to attach it to get a more accurate RPM reading than from my dash but no luck. So I hooked up a standard timing light to be able to at least get a good base timing at idle, and if I have further problems, I ll invest in an advance timing light and see if the amplifier is the problem. At 700 RPM, the book calls for 8 deg BTDC on cylinder 1. When I hooked up the light, the timing was set at 1 Degree BTDC!!!! I checked my work again and came up with 1 degree again. So I loosened the bolt on the distributor, adjusted it and had my daughter watch the light while I tightened the bolt. Changing the timing increased the RPM, so I reset the RPM to 700 and rechecked the timing and it now sits at about 8 1/2 degrees. Close enough for now.

So I took her down to the lake, let it warm up and tied her off to the dock. I put it in gear to load the prop, and readjusted the RPM to 700 and rechecked the timing. Still good. Now with good timing, a clean carb, a can of seafoam in the oil, and idle mixture and RRM adjusted, it was time to leave the dock.

And what a difference! It holeshots good, runs right up to WOT of 4500, and goes flat out at 45 mph!

With one slight problem, sometimes when going from a flat idle straight to full throttle, it will begin to stutter at about 2500 RPM. That throttle lever positiion is almost exactly where the secondaries begin to open. I think the only item remaining is to set the carb adjustments for the secondary opening.

Any thoughts?