Thank You for Your Question XXXXXXXX,
With the symptoms you indicate and the black sparkplugs I would say that you are most likely still with carburetor issues....
But before we just dive right in, I would take a timing light and prove that the timing is advancing with the rpm increase of the engine...With that out of the way, I would proceed to go back into the carburetor.... These engines typically had Holley carburetors, and these are notoriously hard to rebuild, and don't tolerate any mistakes in overhaul, like using wrong gaskets in the kit, float level a little off, choke miss adjusted, ect..
I did check timing with a timing gun and it is advancing nicely. The carb was a Rochester 2BBL and fairly simply to rebuild. At least I thought so.
My cylinders all have 150 to 170 compression except 1 which has 110. I am kind of concerned about this, but the motor should still run better than it is. I would have just expected a rougher idle with this 1 cyclinder being a little lower that the rest. The idle is actually is not too bad.
Sorry for the delay, we have been getting rain non-stop in Florida for 3 days and the Shop is Flooding, Glad I work on Boats......
The 110 on the one cylinder is of "Grave" concern... Stop everything and do the compression test again, should the compression be confirmed, we will have to look into this. Please don't get me wrong, I am not just going to tell you to take the head off and inspect the issue, shit, that would be a "knee jerk" reaction that we both are attempting to claim denial to....
So, lets work this problem, do the compression test again, or two more times if you like...If it fails, add about two table spoons of engine oil to the cylinder and do it again, if it still fails, remove the valve cover and check the valves. These GM engines are "Notorious" for valve and cam issues. Like, rockers cracking, rocker bolts coming out of head, rusted valves, tuliped valves, burned valves, ect....
I would expect the engine to run poorly at high RPM with a bad valve or valve train issue, as compared to idle, reason is the engine needs a lot more air to produce the added power, and things are happening a lot faster, so the pieces need to be operating correctly or the engine will have symptoms in the high RPM area....
Before I would pull the head off, We need to do a power test of the cylinders. how this works is, remove one sparkplug lead at a time and listen for the RPM and Power drop of each cylinder to determine they are all carrying a load evenly...This test can only be performed with the engine on the water and cruising at 2500 RPM. Also don't take this test litely, "IT IS DANGERIOUS", The belts on the front of the engine do not have guards, the boat is moving and rocking around, the spark plug leads do have enough electrical energy to "Kill" you if you get shocked in a bad way....Only use a plier designed for this job to remove the "Live" sparkplug leads at the distributor cap...And be aware of the fire and shock hazard....If you don't feel up to the test, Don't do it....
Marine Mechanic
21 Years Experience as Mechanic, 18 Yrs Boat Yard Owner/Mechanic, Factory Certified
I do not know what happened as I sent you a reply back with more comment but it looks like I accepted your answer and my reply was not posted.
I do not accept this answer as I have no new information and my boat still is not fixed.
I restore cars as a hobby so have already tried the things you suggested.
I redid the compression check 2 days ago and it was 110. I tried the oil in the cylinder and it went to 160 on the first crank, but then went to 120 and stayed consistent there.
My boat is in the garage now so I am not sure when I can get to it again. I can only work on it in the weekends. I would not mind your suggestion of testing on the water but my boat is not in the water and my tach also does not work now. I thought that was a little strange that my tach went out about the same time the motor started acting up.
I did check each wire with my timing gun to look for consistent spark in each cyclinder.
Cylinders 8 and 6 did look a little funny so I replaced those wires and with some other test wires and there was no difference.
I did check the coil and the primary is 5500 ohms and the secondary is 1.7 Ohms. The primary seems a little low, but I do not have the specifications for the coil.
I could pull the valve cover off as you suggested and watch / inspect the valves, rockers, etc. There are only 4 bolts holding it on.
Also, when I rebuilt the carb I did not see anything unusual with it. I did soak it and blew everything out with compressed air and put in the new parts and gaskets. Still no change in the way the engine runs. Remember that the engine started to run funny at the end of the season last year so I suspected it was the carb and thus the rebuild effort.
Thanks for the suggestions but this is getting me nowhere yet.
I do see where you corrected the premature "Accept", this was the right thing to do....
Bottom line is I can't fix your boat over the chat presented here... I can only guide you through a tried and true process of testing and elimination....When you do the items that I outlined above, I believe we will find the issue....
With this new information, it is possible that the tachometer is causing this type of issue...Being directly connected to the negative side of he coil....
The information that you gave me is all over the engine, as the tests that you have already done....
Like I said, we need to confirm and prove certain systems in the engine....With out doing this in a systematic manner, we will just become frustrated and the length of time and effort to repair will be much longer.....
An ohms test on a marine coil is worthless and we would be better served with a spark test of all the ignition components at one time.....
In a few Situations we don't see the problem in a carburetor when we overhaul it, but the engine responds to the effort ( I would not eliminate the carburetor yet , but I would begin to look elsewhere)....
Please tell me why the compression is low on the one cylinder so we can move onto the next troubleshooting step.....
Thanks for your response. Yes, I tried several things and got very frustrated. I did run the engine with the Tach completely removed from the boat and that did not make any difference so I put the tach back in and reconnected it.
Anyway, I can pull the valve cover off and take a look at that cylinder. Not sure why that 1 cyclinder is down on psi even with the oil test. Valve not closing?
I may not be able to get to this for awhile as I work a day job and we are going camping this weekend.
I would actually rather be working on my hobbies instead.
Thankss
Yes , the valve could be damaged or the cam, not allowing it open and close properly....
Let me know how it goes.....
DominickB
Dominick
I did get a chance to work on the boat again this weekend.
I tried running the motor at RPM in gear and pulled each plug wire. I was surprised that there was no change in engine RPM at all. This was true for all cylinders.
I also pulled the valve cover and all the valves are working perfectly. I do not see any issues with rocker arms, spings, pushrods, etc. In fact; I was very surprised at how clean the motor was. No gunk, no sludge, or build up at all the motor looks new, the springs are shiny like new and the oil was clean also. I have tried to keep the motor clean and fogged every winter, etc. and change the oil and filters regularly. Change plugs every 2-3 years.
I also tried another coil just for gins but no difference.
To review: I checked the points gap again, the timming is about 9 Deg BTDC, the RPM is about 575 but it fluctuates from 560 to 600 (Not real smooth). The motor does idle and accelerates to about 1/2 throttle but then runs real rough and no more RPM or power.
I believe the motor is getting too much gas or not enough air. I have to put the throttle all the way down and crank a long time for the motor to start. Like when it is flooded. All the spark plugs are black. The motor seems to run better when I fully open up the restrictor plate on the top of the carb, but this does not fix the problem.
I see I can get a new carb for about $200 which maybe the way to go, but I sure do not know how I could have messed up the rebuiid. Again it ran this way last year before I started the rebuild so there maybe something else wrong like a small crack that I did not see. Maybe a vacuum leak somewhere?
The compression in the one cyclinder is still at 110 psi.
Any other suggestions????
Sorry for the rambling, but I was just thinking about those spark plugs I changed this year. They were a different brand (NGK) and I had to open the gap from .018 to the spec of .035. Nearly double the factory setting. I just checked the internet to see if anyone else had issues with these plug differences and it was pretty unanimous, do not use the NGK plugs. I do have the right cross referenced plug. I am going to get a new set of Champion RV15YC4 to replace the NGKBR6FS. The Champion plugs always ran great in my boat (20 years worth). I have an old plug to compare and the Champion plug is longer and bigger and I bet gives a much better spark deeper in the cyclinder. Besides the NGK plugs maybe fouled with all of this black soot on them and this poor running engine. What do you think or maybe I should get some platinum or crossfire plugs. This sure can not hurt????
Thanks again and let me know what you think?