Full carbs + spark + suction = no ignition?

[crabbie]

I've got a Merc 1500, 1976, 6 cyl inline, 3 carbs.

This motor hasn't really worked for me, yet. I bought it about two months ago, and at first I could only make 5 knots, cranking the throttle forward did nothing. THAT turned out to be the control box, I guess the cam had worn out. I picked up an 1980s-era controller on Craigslist for $50 and suddenly I had forward throttle! (And a much better key.)

For about half an hour, at least. I made 26 knots, took a rest, then found I'd lost most of the power, was down to 6 knots, and found when I got it home that the top carb was spewing fuel out the float bowl cover. Not having a clue, I took the whole carb apart, when in fact I only needed to open the bowl to unstick the float. Oh well, I learned some things. I put in a new float, put the carb back on and... NOTHING.

Right now I can pump the carbs full of fuel, the bulb gets nice and firm. The engine cranks just fine. All the plugs I've tried have blue sparks. But no ignition. Not even a sputter! I did some reading, took the carb back part last night, looked for any debris, nope, all looked good, and still nothing.

Today I did more searching and talked to some people and had some more things to try tonight:

• pull out the top two plugs, and check for the 'pulse' of air when the engine turns - yep
• cover the intake of the top carb, see if I feel any suction... didn't really *feel* any suction, but the chamber very quickly filled up with fuel, so it would seem the fuel is being sucked out fine
• same on the middle carb
• tried spraying some fuel/oil into the top cylinders, but nothing happened (though I am not confident I did it right)
• do a compression test -- my first compression test ever! -- and the numbers seem reasonable: 140, 135, 130, 135, 145, 135 (from top to bottom)

BUT! One definite clue from the test: the #2 spark plug was DRY. Not only that, the threads were still clean! (I replaced all six when I bought the motor, and the other five have oily threads). So I'm guessing the #2 cylinder hasn't seen any fuel in awhile.

This post makes me think the reeds are the problem, even though I'd never HEARD of reeds until today, so I can't say I checked them when I had the carb off.

So, questions...

1) are the reeds the likely suspects? maybe the fuel pump? (that seems a usual culprit in a lot of posts)

2) does this motor only use the first carb when it starts? The others kick in later at higher RPMs? I can't figure otherwise why the other two are lifeless now, when I haven't touched them.


Thanks!

rodney

 

[renny1]

Re: Full carbs + spark + suction = no ignition?

did you do the correct setting on the float you replaced????????

 

[Chris1956]

Re: Full carbs + spark + suction = no ignition?

Rodney, It would be unusual if the reeds were bad. That motor runs on all cylinders all the time.

I think you should buy a manual and inspect and set the carb floats to the correct spec. The old style floats were foam and broke down and sank, after a while. The new floats are hard plastic. Install a new gasket set into the carbs ($3-5/carb), and inlet needles and floats, if they are needed.

You also need to set or check the timing. There is a thread on this site about link and sync of inline mercs. Also check the spark plug leads for arcing to ground. When they get old, they sometimes coorode as well. A good price or new ones is at vintage mercs.com

BTW these motors need lots of choke to start cold. The idle mixture determines whether they will rev up at all.

 

[crabbie]

Re: Full carbs + spark + suction = no ignition?

The spring on the new float (yes, the plastic type) seemed good out of the bag, at least as good as I could measure. The gap on those two metal levers above the float (sorry, name escapes me) was already exactly 1/4". I did put in new gaskets, and was going to replace any metals parts that needed it, but they were all in excellent shape, AFAIK.

What would the float settings affect if they were off? My (limited) understanding is that they control the fuel coming into the carbs, and right now that doesn't seem to be an issue, it's the fuel *leaving* the carbs -- well, NOT leaving -- that's the problem.

Besides, since the carb is shared between two cylinders, wouldn't a carb problem affect #1 and #2 equally? #2 seems to have never seen fuel.

I'd scanned the link'n'sync guide last week, I'll take a closer look, now.

Thanks!

rodney

 

[Chris1956]

Re: Full carbs + spark + suction = no ignition?

The float height controls how the carb mixes air and fuel, indirectly. It is best to have them correct, or unusual issues may occur. The first setting is float height. The second float drop.

Float height: Turn the carb cover over. The two levers should be sit parallel to the carb cover.

Float Drop: Now right side the carb cover. The distance between the two levers should be 1/4 ".

I still do not know why the #2 plug is dry. The lower cylinders usually run a bit richer.