Ville123

Greetings,

I am hoping to rely on your experience a bit, on what to check next. I have a 5.7 merc alpha one gen 1 on a twin engine Chris Craft Stinger 260 that idles very very rich. The engine runs great after about 1500rpm but it starts with difficulty, has trouble staying running when cold idling, stays working ok when choke opens but then dumps fuel in water behind the boat and of course the smell, the smell..., and soot building up around the transom. The engine is freshly rebuilt, new heads, rings etc, and also the carburettor is rebuilt.
The timing is set correctly at 8, but jumps around (about +6degrees and back - when checked with timing light) a bit when cold. When revving, it advances as it should and the timing jump goes away when it gets warmer.
The distributor has a new cap, sensor etc.
The timing chain was checked and OK.
The carb is rochester quadrajet.
The carb float level is checked, and OK.
The carb does not overflow in any condition.
The carb idle screws are set about 1.5 turns out and make little difference when moved.
The carb bottom wells were checked, and sealed, when rebuilt.
The carb choke system is divorced, and works as it should (opens up when warm etc.)
I have also a fuel flow meter, it shows about 20l/h when idling, the other engine is about 4-6 when idling. The fuel flow is approximately the same when cruising.

I presume this is a carb issue, but does anyone have a good tip on what to check next on the carb to narrow the problem down? I know that the quadrajet has a kind of fixed system for the idle (except the screws), but is it possible that perhaps a previous owner has destroyed something, something is plugged up etc. I know that the primary jets were not changed when the carb was rebuilt (deemed ok), but do they affect idle also that much, and could that be the problem, or something else.

The other engine and carb were rebuilt by the same person, we have discussed this a lot and he did not notice any difference or state of disrepair between them when working on them. So one engine and carb worked out great, the other is... not so great.

Im so fed up with this issue and just about to get a new carb, just thought that since it would be waste of rebuilding work time and money, to give it one last go and trying to fix it. So if anyone has a good idea then i would be very grateful

Thanks in advance,
Ville

SB

I ran into similar issue last year customers boat. Swapped carburetors and problem followed.

compedgemarine

most Q jets run with the idle screws out about 3-4 turns so it is getting fuel from some where. if you look down the carb at idle is there fuel dripping from the boosters? if so replace the float with a new one but do not use the brass ones.

Ville123

Did you ever find out why?

Ville123

Ok thanks - I will check the boosters tomorrow. Today I turned the mixture screws all the way in (closed), the engine ran happily and better than before. The fuel consumption in idle showed about double from the other engine (before was much more). So you must be right, it is getting fuel from somewhere.

Ville123

BTW, When the carbs were redone the float was replaced with a plastic one, so it is new. But nevertheless I will check.

SB

Yup, i rebuilt the carb.
new needle and seat, acc pump, float, gave good cleaning, i have tiny carb brushes, etc, too.Epoxy wells, …,
used most if a Cliffs rebuild kit. Kit was decent other than kit had wrong base gasket. Oh well.

Anyway, main thing was to swap carbs to see if prob follows. If it does, it’s the carb. If it doesn’t you don’t have to think about the carb anymore.

Ville123

Hi,

I checked the nozzles, you were right. There is fuel intermittently dripping from one of them. The other one is dry so I presume the problem is not the float. Could it be the hanger/jet issue on the primary on that side, or a plugged air hole so the vacuum is higher on that side? Any ideas.

Markus

I don’t know anything about those carbs, but all carbs I have seen have an idle circuit with an idle jet and an idle air jet.

I would check
1. That the idle jet isn’t too big or missing
2. That the idle air jet isn’t clogged

To isolate the problem, swap carbs with the other engine and see if the problem moves.

Are you based in Finland?

Ville123

[QUOTE=Markus;4877532]

Hi, I´m from Estonia, so in the neighbourhood so to say . Greetings to Sweden.

I havent worked on carbs or a quadrajet before, but I got a couple of books about them and have now read quite a lot of forums. plus fiddled around with the ones I have. As much as I understand the quadrajet has a kind of fixed idle circuit, so there are no adjustable jets for idle fuel or air, the amount is metered by fixed orifices or restrictors, and if something is wrong it starts pulling air from the main venturi, not from the idle cirquit, and because of that airflow it also pulls fuel out of the main venturi nozzle. Or it could be that the bottom butterfly is not closing, or it can be because of fuel pressure or float level. I checked the float level, which is OK, and strangely it drips from one nozzle only, so I´m leaning towards a theory that it is from air passage being plugged... or of course it can be any of these things. I believe I just have to start eliminating issues, the simplest would be fuel pressure so I will start with that. If anyone has further ideas, i would be very happy to hear them.