454 mercs turn over and will run at full wot, but nothing else.

I spoke too soon. Went to do a little test drive today and it crapped out on me. Worked fine getting out of the marina in and out of gear with a little bit of gas here and there. As I went to power up though she made it to 3k and then I just watched port die off. The engine will start for a second and then just shut off, prime with some extra fuel it runs for an extra second. I'm about to pull my hair out. The carb is getting a good solid spray... I feel I have eliminated the fuel from the equation... New pump, filters, rebuilt carb, good spray into carb... After messing with the fuel mixture screws and it running for close to 3 hours I am throughly confused now.
Oil pressure sending unit decided to go bad right when I throttled up under load by chance? What else is there?
 
Q-Jets are super reliable units once properly rebuilt & set-up.
Get it to a reputable carb. shop & have it set-up on a flow bench.
I know of a couple guys that do this for a living.
Let me know if you want to contact them.
I have two '71 Buick Q-Jets here, one has been on my '71 Skylark for close to 7, maybe 8 years. Bolted it on & haven't touched it since. Second is a spare should I ever need it. Both by the same builder.
 
Just an easy test, since you have 2 engines, and its about a 20 min job. Swap the 2 carbs and see if the problem migrates to the other engine. This would assure that the problem is indeed in the carb and not elsewhere.
 
Several things to look at, not sure if you have additional fuel filters and lines like I have but study the fuel lines from the tank to the filter or filters. Its possible these are collapsing internally acting like a check valve. Switching carbs like mentioned only if the other engine works with no issues is OK. Wondering if you have a fuel pressure gauge which can be used in various areas of the fuel line to determine flows. If under 4-5 lbs I would suspect having flow problems. You said you replaced the line but was this all lines from tank to carb or just to the main filter? I have seen blockage in the steel line from the pump to the carb before thus almost blocking flow altogether. These are just some ideas from past experience. Interesting to see you solution when its corrected.
 
Almost forgot this one, if the fuelpump is a arm type and is activated by the camshaft, I have had worn lobs and it never pumped fuel under load. This one is also something the fuel pressure gauge may pick up on.
 
Could be a bad coil if you now have fuel. Some will run at idle and breakdown at higher rpms after getting warm, Mike.
 
CV-23 said:
Could be a bad coil if you now have fuel. Some will run at idle and breakdown at higher rpms after getting warm, Mike.
Click to expand...


had this happen on a 1966 Mustang with a 289 when i was a teenager.....very aggrevating to diagnose.......the car would run fine for a while but once the coil heated up it would kill the spark thus killing the engine...let it cool down a while and the cycle would repeat itself....you could swap coils from one engine to the other to verify if one coil is having problems....


cliff
 
Update and not a good one. I switched the carbs around and what do you know it runs fine on the good engine, and good engine carb runs fine on problem engine. In the process of doing this though I noticed the inline filter on the carb for the problem engine was gummed up. I clean it out and figure what the hell maybe my port side tank got some ethanol in it...(I did fill the port tank up at a different pump come to think of it). To test this theory I ran port, the problem engine at higher RPMS for around 10 min. Changing a couple of hundred RPMS now and then. Between 2800-4500. This didn't clog up the inline filter....
Now for the bad news, the engine started nocking and oil pressure alarm goes off at idle now....

Next plan of attack is to buy some of that startron stuff just to eliminate the fuel tank issue if there is one. There not fiberglass tanks so thats a plus.

Then I guess I'll have to check out the valves and so on.
 
Tell me you were not running those rpms in Neutral? That is very hard on Big Blocks running with no load on them. What kind of knocking? Under load or at idle? Does not sound good. I've used Startron in my 454 since we were forced on Ethanol last year, Mike.
 
Nock is at idle and not under load. Yes I ran those RPMs without a load on her. Yes I realize now why that was a bad idea;) This being my first boat though I have come to terms with it being a learning process, even as expensive as this item could be(cold beverage helping with that statement). The nock appears to be coming from a valve cover, but I can't locate it for sure. Anyone know of a good place to find a manual for these engines. I have a feeling I'm going to be getting into the bowls of this monster and am going to need specs and measurements, other then gud en tight.
 
Using a very long screwdriver you can place the end of the handle on your ear and use the blade end as a probe. Go cylinder by cylinder on the valve cover and you will find the knocking. You may just have a bad rocker or broke a stud off or a collapsed lifter. Reving the motor and high rpms not under load is hard on the rod bearings I've been told, Mike.
 
First of all; with all the fuel problems, lets just hope the engine oil is simply fuel diluted and a few lifters are collapsed as a result of. Before you go any further pull the oil filter, cut it open and inspect it for metal debris indicating internal engine damage.

If things look good with the filter, then do your top end inspection.

If things look good with the top end, do an oil change and re asses.


fwiw... if you ever suspect a fuel supply problem in the future with a carbed engine, simply run the engine till it dies. With the engine off and flame arrester removed, pump the throttle and watch how many squirts you get from the accelerator pump.
 
CV-23 said:
Using a very long screwdriver you can place the end of the handle on your ear and use the blade end as a probe. Go cylinder by cylinder on the valve cover and you will find the knocking. You may just have a bad rocker or broke a stud off or a collapsed lifter. Reving the motor and high rpms not under load is hard on the rod bearings I've been told, Mike.
Click to expand...

you can also pick up a 'mechanics stethoscope' at Harbor Freight for around $5....i used to use an old broom handle they way Mike mentioned above and it worked well enough...the stehoscope works much better and is a very handy diagnostic tool to have around...

in my experience you normally do not hear a 'knock' from the top of the engine...when you have issues with a collapsed lifter or something else in the valvetrain normally the sound is more of a lighter 'ticking' sound vs a deep knocking sound....the knocking sounds i have experienced come from the lower section of the engine such as pistons or crankshaft...

good luck...

cliff
 
Call Rick @ 760-881-9713. He lives near Victoriaville, CA. He works midnights so you may have to leave a message. Rebuilds/tunes Q-Jets for all applications.
 
UPDATE!
I'm back from deployment....wish I could of stayed down en sur America! Anyways decided to call a mechanic out since this is beyond me. Went over the entire thing with him and he was a bit perplexed. Did a couple of tests such as peeping in through the sparkplugs, listening and feeling, testing to see if sound was changed when specific ignition wires were removed so on...still no joy. Said that the engine would at the very least have to be lifted and oil pan removed to get a good look at things. I trust the mechanics so far, but as always I like second thoughts. I got them to quote me prices today for worst case scenario and to have a rebuilt engine put in there would cost me 7k flat, 5 for engine, 2 for labor. Sound about right to you guys? Would cost me 8k for a brand new engine and install. What do you guys think? I'm going to call another mechanic manana just to get a second option. One of the damn trim tabs is leaking also so thinking about fixing this at the same time.
 
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