Trouble starting, running. Stumbling sound.

[ryno1234]

This is going to be a rather detailed sequence of events, so hang in there:

I just replaced my carburetor with a new Rochester Quadrajet. I was previously having issues with my engine refusing to start after boating for 15+ minutes. This was random, so I threw money at it and bought a new carb and new spark plugs.

I got the new carb, installed it and tuned it (to the best of my ability) using a vacuum gauge.

It started and ran great in the driveway.

I took it out today to the lake. It needed 2 prime pumps of the throttle to put gas into the intake. After that it started right up and never died while idling at the dock (something which it did not do well previously). This was a good sign.

I took it out in the 5mph zone for 15 minutes, shut it down and attempted to see if it would start up again in an attempt to see if we had progress. It started up just fine, first try.

I then took the boat out outside the 5mph zone and went a *little* faster for about 5 minutes, all was good.

Went for full throttle and as I advanced the throttle, the boat seemed to accelerate and at a certain point almost choke on itself and the acceleration would slow / stop and I'd get a backfire coming from the intake manifold.

I was unsure of what I was hearing so I tried this a time or two. I realized while doing this, I could back off the throttle as the boat seemed to choke / stall and almost "save" the stall. I began to think that this may be roughly when the secondary butterflies open. Maybe the "stall" was those opening (not sure though).

The backfire concerned me so I played around with timing while out on the water. I tweaked the distributor slightly in either direction. One direction seemed to help more than the other by eliminating the backfire, but I would still get the stall that would happen eventually. Some adjustment to the distributor *seemed* to move the stall point further up throttle position than others. I could be wrong though.

Alas, none of these changes fixed the problem entirely and some of them resulted in backfire through the intake.

Furthermore on one of these times, I heard the stall and instantly pulled the throttle back (to avoid a backfire) and turned the engine off almost in one motion.

After this, the engine had a difficult time starting, but would eventually start if I advanced the throttle and left it there while attempting a start. I'm speculating but I'm thinking this action may have flooded it.

I then tried to adjust the mixture screws wondering if I was running too rich. My understanding was that these were for idle only, but I wanted to at least see if they had any bearing on throttle positions beyond idle. I tried to lean the mixture by a half turn.

This didn't seem to help much but I could be sure at this point because I was now having difficult event starting the engine.

I played around with different timing settings and returned my mixture to where it was to start. At this point I couldn't get the boat started at all no matter what over the course of 1 1/2 hours.

We pulled the boat out and I took it home.

Once home, I hooked it up to earmuffs., and attempted to start it (without priming it). I could tell it wanted to catch, but never did until I advanced the throttle 1/2 way. At this point the engine started but sounded awful. It sounded like it was stumbling. It wasn't a smooth idle. Sounded something like a race-car at the track. It wouldn't idle for more than a few seconds if I returned the throttle to idle.

--------------

So, I have a few ideas that I have not fully fleshed out:

• I have a timing issue
• I have a mixture issue
• I have destroyed something with my 3-4 backfires.

Any ideas? Any help would be appreciated.

As you all know, the season is coming to an end and I have missed most of it working on the boat. Any help to soften this blow would be appreciated.

 

[Bt Doctur]

Backfiring is usually from a very lean mixture or improper timing

 

[ryno1234]

Backfiring is usually from a very lean mixture or improper timing

I'm not sure my understanding of the idle mixture screws is 100% correct. My understanding is that they are pertinent only for idle. Is that accurate or does this actually have bearing on the mixture during other parts of the throttle range?

 

[nola mike]

You're correct, and maybe throttle response right off idle. But backfiring is a lean condition, or over advanced timing like bt said. Imo, timing an engine by ear is a recipe for disaster. I would take any odds that your timing is way off at this point. Too far advanced can make it difficult to start. Get a timing light on it and see where you're at. How's the fuel? Fuel filters? I'd also check compression at this point, especially if the above doesn't fix the problem. Don't feel bad, last weekend was my first on the water this year.

 

[Wxguru]

Definitely time the motor with a timing light. I bought one off Amazon 2 weeks ago for $30. Best money ever spent. I too have a little bit of a stumble every now and then and was having issues with hard start after running for a bit. Found my timing to be 4º BTDC, and on my motor should be 8º (1992 5.0LX V8). I haven't made the adjustment to mine just yet as I haven't had boat at the house in the last week to do so, but plan on adjusting next weekend if I get the chance and can report back.

what year/motor are you working with?

 

[ryno1234]

I have a 1991 GM 305 (5.0).

The problem I'm having with timing at this point is that my engine won't idle. Something occurred during my outing where my engine no longer runs right. Prior to going out on the water, it started up great and would idle all day if I let it. Now I have to give it 1/2 throttle just to get it to fire up. And when it does, it sounds rough as all hell.

I returned my distributor position to its original position so I know that isn't the problem.

Wondering if I messed something up with the backfiring....

 

[Scott Danforth]

you just need to be able to crank the motor to properly set timing with a timing light. as long as your cranking RPM's are over 200 RPM, it will fire the timing light.

however the other issue you may have is bad anti-siphon valve, or other issues with your fuel system as from your description, its running lean off-idle. so the idle mixture screws are no longer involved and you are running on the primaries.

your motor wont idle any more because you started playing with the idle mixture screws to correct an off-idle stumble.

so you need to go back and re-set everything on the carb to how you pulled it out of the box.

then you need to inspect the fuel pressure of your motor when the engine is running (you need 3-6 psi)

 

[nola mike]

I have a 1991 GM 305 (5.0).

The problem I'm having with timing at this point is that my engine won't idle. Something occurred during my outing where my engine no longer runs right. Prior to going out on the water, it started up great and would idle all day if I let it. Now I have to give it 1/2 throttle just to get it to fire up. And when it does, it sounds rough as all hell.

I returned my distributor position to its original position so I know that isn't the problem.

Wondering if I messed something up with the backfiring....

How do you know the original position? Put the engine at TDC and point the rotor at your #1 plug. It should start at that position. If you messed with your plug wires, double and triple check the order. Did you check your fuel filters? Compression test to make sure nothing is FUBAR'd.

 

[ryno1234]

So, everything mentioned about lean conditions makes sense for backfiring into the intake. Especially since this happens about half-throttle or roughly when the secondaries seem to open. That's good, that leads me down a path that I can follow.

What I don't understand is why this didn't happen in my driveway prior to going out on the water? I was able to rev the engine (quickly because I didn't want to create issues with my neighbors), and I have to believe I had enough throttle in while doing that to where the secondaries would have opened.

It seems that the lean condition only occurred while under load.

Does that make sense? Why is that?

 

[nola mike]

Yes, it makes sense. Only happens under load because that's when your engine needs more fuel.

 

[ryno1234]

Isn't fuel need a function of RPM? The RPM (air suction) is what determines how much fuel gets dumped into the intake. Am I off on this?

 

[kenny nunez]

One question, the carburetor you now have is it a Marine carburetor for your engine or did you buy an automotive carburetor?

 

[ryno1234]

kenny nunez , it is a marine Rochester Quadrajet

 

[Mlevy]

It makes complete sense. Fuel is needed for to increase rpm but with no load there is nothing holding the motor back and can increase the rotation with little to no effort, with load the motor is fighting against it to continue rotating, thus needing more fuel. You need to adjust your carb so you can get it up to speed to do a plug check and tune from there.

-mitch

 

[kenny nunez]

Most of the time Qjets idle correctly with the idle mixture screws 3-5 turns out from lightly seated with a vacuum gauge. If not then something else is not right. Have you checked the contents of the spin on fuel filter by draining it in a bucket?
It is possible sometimes for an ignition problem to act like a fuel delivery problem.

 

[ryno1234]

Little update: I went to work on the boat yesterday, attempted to start it and I couldn't get it started at all and it made no noise indicating it wanted to start.

I pulled the plugs, which were all new 2 weeks ago, and they were all fouled. Covered in black carbon.

My initial thoughts are that this is because of my original carburetor that I had installed. There was an issue with it where the idle mixture screws were cracked (https://forums.iboats.com/forum/eng...ven-when-bottomed-out?p=10922952#post10922952). This caused large amounts of black smoke, etc. That would make sense for why all my spark were completely black

But, I had swapped out this carb for a new one and that new carb is the one that I tested out on the lake where the engine started first time. So I'm struggle to agree that is what caused this since it *did* work fine from the perspective of getting the boat started, but after my outing, they plugs no longer seemed to be working as they should.

I bought new plugs, installed them, boat started first try.

I'm chasing my tail a little bit here with one wondering if the park plugs being close to bad could have caused the issues outlined at the lake with the bogging, or if I should continue troubleshooting before testing again.

 

[ryno1234]

Ok, I timed the engine, 8 deg BTDC as indicated by my manual, new plugs installed as mentioned above.

Blowing black exhaust smoke.

I closed my idle mixture screws all the way and I was still blowing black smoke and the engine still ran. I don't know if it takes a while to burn off what might exist in the cylinder from a previous time or not, but if not, I'm somehow the engine is still running and seems to indicate a Rich condition.

I pulled a plug just to check and it was now black as well, but these plugs still fire just fine despite that. Perhaps the carbon will burn off when I get this running right.

Any ideas?

 

[nola mike]

Can you see any gas dribbling from the carb at idle?

 

[ryno1234]

nola mike , No, I checked this. I also looked down each barrel and couldn't see anything of interest other than what looked like very small amount condensation on the top of the primary throttle blades