'79 351w Issues - From great, to bad in 2 hrs ??

I have a '79 Ski with the 351w Commander in it. Last winter I did GT40p's, roller rockers, performer intake, and brand new QF600 carb. I ran the boat a bit at home in colorado at 8,200ft of elevation just to make sure it worked, then re-jetted back to sea level and brought it to Wisconsin for 2 weeks in August.

I got the carb dialed in within a couple hours and put another 10-15 hours on the boat over 2 weeks - and it started and ran great the whole time. It was over-revving slightly with the Acme 540, so I talked to Acme and ordered a 430. I had siphoned the tank dry before the trip and filled it with only non-e premium. Put the boat away dry when I left.

Went back to colorado for almost 2 months and just got back to Wisconsin yesterday. Pulled the boat out of storage, put the new prop on, topped it off with non-e premium and took it to the launch. It backfired once on startup, but then seemed to run pretty normally as I left the launch.   Within about 15-20 minutes I noticed a very slight stumble on acceleration, but it cruised and idled fine.   Continued driving for another hour or so and the stumble on acceleration became more and more noticeable, and it started to stumble slightly at idle. Turned around and headed back to the launch. By the time I put it on the trailer it had a slight shudder at idle that couldn't be adjusted at all with the mix screws and wasn't running right at any speed under 3k rpm.

Pulled it out, did some research, and did a visual check of everything. Crossed my fingers (wishful thinking) that somehow the prop had too much pitch and was causing the stumble. So I put the 540 back on and went to the launch. As soon as I started the boat it had a noticeable shudder at high idle and didn't want to idle below 900-1,000 rpm. Played with the mix screws, checked visually for vacuum leaks like a hose off etc. (didn't have my gauge with me) and gave up after about 10 minutes. Since the engine was warm and I was due for my fall oil change, i drained and filled the oil - noticing a slight gas smell in oil that was only 3 months and 15 hours old.

I did a bunch of reading last night and decided I needed to check my timing, choke operation, and look at the plugs. Checked a few plugs before starting it and there was a lot of variation between them but none looked terribly black. Definitely signs of rich condition on some, but others looked about perfect.

Did a quick start and let it stall, but visually the choke seemed to be operating correctly - slowly opened all the way up after I had started it (its about 55* out today).

Brought the boat home and fired it up on the trailer and it was running really rough. Had to crank the idle up to 900+ rpm to keep it running and it was definitely shaking. I could get it to clean up with throttle but it never sounded quite 'right'. Checked the timing and found it to be about 12* BTDC. Noticed (by accidental slip) that if I advanced to 16-20* it idled much smoother, but didn't keep it there for more than a few seconds. Put it back to 11/12* and tightened the distributor back down.

Shut the boat off and pulled each plug, one at a time, to find the following.
#1 - Black and sooty (much worse than before the 5-10 minutes of idling on the trailer
#2 - Brown, definitely rich but not nasty
#3 - Tan, slightly rich
#4 - Similar to #2, but actually had wet gas on it
#5 - Black, nasty
#6 - Black, nasty
#7 - Tan, slightly rich but clean
#8 - Brown, rich, but not black

I'm at a loss. Clearly seams to be getting too much fuel, but big variation between cylinders. Was a pretty quick deterioration with no physical changes to the motor. I have an almost brand new carb on there with clean fuel and a new fuel filter since day 1 (only 15-20 hours of run time on any of it). And about the timing - should it even run at ~20* BTDC?? Weird.

Only theories I can come up with from reading are:
1) Ruptured power valve from one light backfire - Seems very unlikely, and does the new QF600 even have these?
2) Gunk in the carb - doesn't seem to fit the symptoms well, and it is a brand new carb with good fuel from day one, a brand new fuel filter from day 1, etc etc.

Any thoughts/recommendations/theories? Help!

Any fuel spilling over from the primaries or secondaries if you look down with the flame arrestor off?

That would go along with theory #2. Your description of not cleaning up till higher RPM's sounds like fuel spilling over

Jonny Quick Fuel had trouble with his when metal shavings (probably an extra bonus from the factory) messed up his carburetor and it needed some cleaning.

I'd pull the bowls preferably to look at things and unscrew the needle and seat assemblies, blow everything out and try it out again.

Since you rejetted for low elevation, I'd check that your jets are in tight and everything looks as it should

The Quick Fuel has a power valve but it shouldn't have been affected by the backfire.

And.........borrow a vacuum gauge if yours isn't readily available since it could also be a vacuum leak based on those symptoms. Maybe the base gasket between the carburetor and manifold and spacer if you have a spacer which I think you do.

When you take the carburetor off, look at the gaskets first before digging into the carburetor since you're not seeing any spillover.

Do you have the flat plate about 1/8 inch thick between the Performer intake and the carburetor.

Performers and Holleys or Quick Fuels don't mate up real well without one of those plates.
The plate comes with 2 wide gaskets that give good mating with the manifold and with the carburetor.

Edit I found an old post where you mentioned having the plate and gaskets, but it's easy to check them when you take the carburetor off.

Are the jets in it now what it came with?

Some good suggestions but wouldn’t rule out a bad tank of fuel. How does it smell?

430 is most likely too much prop for that combo, I’d be surprised if the 540 wasn’t better for it. How did you determine it was “over-revving”?

GottaSki wrote: Are the jets in it now what it came with?

Yes, and as I said, ran great for 10+ hours as it is set up right now before the problems developed.

TRBenj wrote: Some good suggestions but wouldn’t rule out a bad tank of fuel. How does it smell? 430 is most likely too much prop for that combo, I’d be surprised if the 540 wasn’t better for it. How did you determine it was “over-revving”?

On the first point - the fuel is a collection of several fill-ups. The first 14 gallons were in it from the prior visit when it was running well.   I just topped it off with 6 gallons this trip.   I wouldn't think 6 gallons of questionable fuel would be enough throw off the whole tank. And it was from a big, busy BP station that sells non-e premium so I have some confidence in their fuel. So, will do the smell test tomorrow, but not real likely.

With the 540 the boat shot out of the hole and went straight to 5,300 RPM (assuming my jittery gauge is accurate). The boat was only running about 46.5-47 on the gps.

Before the boat started running poorly yesterday, i did get a speed run in with the 430. Still came out of the hole great and run up to 48.5-49 mph, with 2 people and a full tank of fuel.   Haven't tried pulling a skier but the 430 seems like the right prop otherwise.

I know this doesn't make a ton of sense, and based on others results with the 540 - if I'm spinning the 430 at proper revs I should be running in the low 50's mph easily. And the 540 should have been pushing me 49-50 with revs over 5k. Right?

Jonny Quest wrote: Could be a distributor that has a mucked-up advance mechanism. On the QF M-600 were is the fuel level in the float bowl sight glass? On my M-600 I had the float level set at the lower 1/4 of the sight glass with good results at altitude. (4,400 to 6,000 feet of elevation) Did you use a carb sealing plate on top of the Edelbrock manifold? LINK to Carb Sealing Plate Johnny QuickFuel

The front bowl is at about 1/4-1/3rd, the rear is about 1/2-2/3. I did use the adaptor plate with gaskets. Is that what you mean?   I have manifold-gasket-plate-gasket-factory riser-gasket-carb.

A rusted and sticking distributor can present in many ways.   I think the distributor allows about 22 degrees of advance, + or - 2 degrees, I did not look up the exact number.
It can stick at any position. Take your base timing you set at 11 or 12*. Add the centrifical advance number of 22* and your total timing is at 33-34*.   It can stick anywhere from idle to full advance. A sticky one can give you advance sometimes and not others or stick inbetween somewhere. Symtoms will vary depending on where it is stuck or sticking.
One of my distributors stuck once, a buddy's distributor stuck once and we have seen it happen to others on this forum. The boat is a 1979, the two I know of were a 1987 and a 1989. The advance springs can also rust out and break causing similar issues.
It is a 3 minute test to rule it out.

MourningWood wrote: Another place to check is the front and rear intake end gaskets, or sealant bead.

These gaskets or sealant beads leaking won't cause a vacuum issue, but they will let oil leak out all over the place and make a mess.

Vacuum looks to be just fine based on the gauge. It is a bit hard to get a good reading with the engine loping but it looks to be about 12-13 as close to idle as I can get it to run. On throttle it runs up.

I advanced the timing a bit. Can't get it to idle at 600 rpm so I'm setting it at about 12-13 at 750. The timing does seem to be jumping around. And on rev-up it only seems to go up to about 25*. It does ramp up in a pretty linear fashion though. Maybe MrMcD's theory is correct, or at least that is part of the issue. Definitely seems dizzy related.

Okay, now I'm being lazy and not researching - can I just pull that bolt that I loosened to turn the dizzy, pull the diz straight out, put a replacement dizzy in and clock it similar to this one, and then dial the timing with the light? Or is there more to the process?

MourningWood wrote: KENO wrote: MourningWood wrote: Another place to check is the front and rear intake end gaskets, or sealant bead. These gaskets or sealant beads leaking won't cause a vacuum issue, but they will let oil leak out all over the place and make a mess. That's right, of course... No wonder my carter YH's leak...

They were "born to leak"

How's the rebuild of the carburetors from the guy you sent them to?

While I'm asking questions, whatever happened to the 81 SN that went on a diet?    

Off Trail wrote: Vacuum looks to be just fine based on the gauge. It is a bit hard to get a good reading with the engine loping but it looks to be about 12-13 as close to idle as I can get it to run. On throttle it runs up. I advanced the timing a bit. Can't get it to idle at 600 rpm so I'm setting it at about 12-13 at 750. The timing does seem to be jumping around. And on rev-up it only seems to go up to about 25*. It does ramp up in a pretty linear fashion though. Maybe MrMcD's theory is correct, or at least that is part of the issue. Definitely seems dizzy related. Okay, now I'm being lazy and not researching - can I just pull that bolt that I loosened to turn the dizzy, pull the diz straight out, put a replacement dizzy in and clock it similar to this one, and then dial the timing with the light? Or is there more to the process?

That distributor won't reach full advance till about 4000 rpm, not sure how high you ran it up while checking.

Are both advance springs intact and the flyweights can move ?

I'd run some fine sandpaper, a piece of a paper bag or even a piece of paper lightly through the points even if you think they look good. You might be surprised

If you're gonna put another distributor in, remember that you have a reverse rotation engine and the gear is for reverse rotation. Depending what you're wanting to put in the gear might need swapping.

People have broken gears and distributors trying to get the gear off. It's a pretty tight fit on the Prestolite. No fair using a hammer on the gear

But yes......you can pull one out, clock the other one the same and be close on the timing

If you have never pulled a distributor do a little reading first.
You don't need to take the dist out to clean the weights or change the springs, it can be done in place. We fixed one parked on the beach for a buddy.
Points and condensors are a wear item and do not last forever.   If you forget your key turned to the on position while your engine is not running the points can frost over in a short amount of time.   
I am not a fan of using WD 40 inside a distributor. It will help free things up but is more of a cleaner than a lubricant and after the fact it attracts dirt and is detramental. If you use it to free things up clean it after your done. Maybe laquer thinner or something to remove residue.   As Ken said clean the parts with emery cloth or fine sand paper so the advance weights operate smoothly.
Follow the advice to check the advance springs, they control your mechanical advance.
They are under the plate your points and condensor mount to.
One buddy bought a brand new Malibu Sunsetter, it had a standard distributor with points, he kept it outside, his cover was not the best and the boat got soaked inside. The following year he could not make it run right. The moisture had rusted his advance plate and wieghts. It happens, not a bid deal to fix if that is what you face.

Thanks all. Headed out to sand/wipe the points, pull the plate to see what's under the plate, and give it one last shot.