Bought the wrong carb. Will it work ?

[Chudwick]

Hey fellas, I jumped the gun and grabbed a 5.7 2 barrel ski carb when I needed a carb for a 5.0. I believe it’ll bolt up but I’m concerned about how much fuel it will put into the engine and air mixture. Any thoughts ?

 

[Bt Doctur]

idle mixture is just that, at idle. above idle fixed jets meter the fuel

 

[Scott06]

Without knowing what engine number you have and what carb you bought... a quick perusal On parts diagram show some of the 2 bbl 5.0 and 5.7 used the same main jet and power valve numbers. Venturi cluster is a different number, not sure if Venturi is different or accelerator pump nozzle holes in Venturi are. Bottom line might work ok. If it were me and it was easy to return I'd return it. If I were stuck with it I'd give it a try and read the plugs to see how it runs mixture wise

 

[Chudwick]

If it comes down to it I’ll sell it on eBay. Hopefully not though. So I just ripped my old carb off, totally seized up. Don’t know what happened but it’s shot. They are nearly identical but the one main difference is how the electric choke hooks up. The original has 2 leads in the connector and the new one only has one. I would need an adapter but with the different leads I don’t know if that’s even possible. Thoughts ?

 

[Chudwick]

It runs. Seems a little rich but I also don’t have a thermostat in it at the moment. For anybody looking for the answer on the choke issue all it took was 12 volts and no negative wire. Theres 2 ways the 12 volts is delivered. One is with the key turned to the run position and one sends it when the boat is actually running. Mine was when it’s running. Only issue it has is that I have to slowly open the throttle and once it catches I can ram it right up no problems. If I try to ram it from neutral it sounds like it’s going to stall. Has to be something with fuel air mixture I’m thinking. Any thought ?

 

[Scott06]

Did you set the idle mixture correctly, looks like an idle transition lean condition. Would try to fatten up the idle mix see if it transitions better. Is the float level correct?

 

[Bondo]

Only issue it has is that I have to slowly open the throttle and once it catches I can ram it right up no problems. If I try to ram it from neutral it sounds like it’s going to stall.

Ayuh,..... If Scotts idea don't fix it,....

Richen up the accelerator pump's stroke abit,....

 

[Chudwick]

I had the carb rebuilt and it’s running like a clock now. The reason it was stuttering from idle to throttle was that the accelerator pump was shot.

 

[achris]

... but I’m concerned about how much fuel it will put into the engine and air mixture. Any thoughts ?

Carburetors don't put fuel into the engine. The engine draws air through the carb and the design of the carb allows fuel to be drawn into the air flow as the engine requires it. As far as the carb is concerned, your 5 litre running at 3,000rpm is the same as a 5.7L running at 2,630rpm...

Most engines require the same fuel air ratio, about 14.7 parts air to each part fuel. So the running mixture isn't going to differ between carbs and engines. Where the variations come in is as the throttle position changes. In particular the accelerator pump volume. If you use a pump volume designed for a 5.7 on a 5.0, it will be slightly too much (rich) as the throttle is opened. No big deal there, shortening the pump stroke will decrease the volume if the engine floods on acceleration.

Chris........

 

[nola mike]

achris, haven't really thought about it like that, but obviously carbs aren't the same across engines . The job of the carb is to measure the air and supply enough gas to keep that mixture at 14.7 . So while a 5.7 at 3k might use the same amount of fuel (and air) as mine at 4k, not sure how the velocity of that air enters the equation. Are you saying that at steady state that everything's the same, and the only differences are when there's a change in throttle?

 

[achris]

achris, haven't really thought about it like that, but obviously carbs aren't the same across engines . The job of the carb is to measure the air and supply enough gas to keep that mixture at 14.7 . So while a 5.7 at 3k might use the same amount of fuel (and air) as mine at 4k, not sure how the velocity of that air enters the equation. Are you saying that at steady state that everything's the same, and the only differences are when there's a change in throttle?

That exactly what I'm saying.

And the carb doesn't really measure the air velocity and add fuel. The air travels though the venturi, which narrows, doing 2 things, speeding up the air, and (and this is the important one) decreasing the air pressure in the flow. It's this decrease in air pressure that draws the fuel from the fuel bowl, through the jets and into the air flow.

Understanding that, you can see that ideally a SMALL carb would be the best, and for low and middle revs, it is. The reason we have different CFM ratings on carb is for the WOT times. Obviously there is a limit to how much air can be drawn through a venturi, and that's the maximum CFM of that carb. So, we want a small CFM (and hence venturi) so the engine will idle and accelerate smoothly. And that's why dropping a 750CFM double-pumper onto a 3L engine is such a BAD idea! (Big venturi, small air demand = low air speed, which equal very poor 'sucking' power to draw the fuel through the jets and passages). So we calculate the maximum expected air demand by the engine, which is the revs and the swept volume (say 5.0L at 5,000rpm, but only every second is a firing rev) and then reduce that by the 'volumetric efficiency' (the cylinder won't completely fill with air/fuel on an induction cycle, unless it's force-induced), which is usually about 85%....

So, for our 5L (305 cubic inch) running at 5,000rpm, with a VE of 85%....
305ci = 0.1765046 cubic feet.
0.1765046 x 5,000 / 2 = 441.2615 x 85% = 375.072275 CFM

So, we can see that the maximum expected air demand by that engine running at 5,000rpm is only 375 cfm. A crab much bigger than 400cfm is pretty much wasted. Putting a 750 on those engines is why they idle like crap. Staying with the 650 is better all round. (and they put 650s on because that's pretty much the smallest 4 barrel made)...

Hope this helps. (now you know as much about carbs as I do)....

Chris..........

 

[jimmbo]

Don't forget 2bbl carbs and 4 bbl carbs have their Flow ratings done at different Pressure Drops, a 500 cfm 2 bbl flows about 378 cfm when flowed rated like a 4bbl. Another area that can be affect by engine size is at what manifold vacuum the power valve opens, a 5 liter will be under a different load than a 5.7 so airflow and the manifold will be different.

I know that Volvo used 500cfm 2bbl Holleys on both their 5 and 5,7 liter engines, some years the jetting was the same, other years different. Of course there are calibrations on a carb that can't be easily changed/adjusted, Air Bleeds, off idle transfer slots are a couple. The 500 2bbl Holley is actually a perfect size for a 5 liter spinning at 5000 rpm

MerCruiser used a GM based 2 bbl, as to what their CFM ratings were, I don't know. For 4 bbls Mercrusier used GM Quadrajets for many years, then switched to Carter/Weber carbs when GM quit producing Q-jets. Unfortunately they insisted using the Squarebore Carters on a Spreadbore manifold, and there are no good Adapters made that can make the mismatch work very well, though the one MerCrusier used was the best out there. Carb primaries are larger and get choked down to a smaller Primary hole in the intake

 

[nola mike]

So the jetting on a carb is a function of environment (altitude), and not dependent on the engine? I can slap my 2bbl mercarb on a v8 and it will run fine until you get to upper rpms?

 

[achris]

So the jetting on a carb is a function of environment (altitude), and not dependent on the engine? I can slap my 2bbl mercarb on a v8 and it will run fine until you get to upper rpms?

Pretty much....

 

[jimmbo]

idle mixture is just that, at idle. above idle fixed jets meter the fuel

The fixed jets are controlling the flow only when the main nozzles are delivering fuel. Off idle operation has the fuel flowing from the Transfer Slot and can still be the primary source of fuel depending on throttle valve position, even at rpms quite a bit higher than idle

 

[Luv2fish36]

It runs. Seems a little rich but I also don’t have a thermostat in it at the moment. For anybody looking for the answer on the choke issue all it took was 12 volts and no negative wire. Theres 2 ways the 12 volts is delivered. One is with the key turned to the run position and one sends it when the boat is actually running. Mine was when it’s running. Only issue it has is that I have to slowly open the throttle and once it catches I can ram it right up no problems. If I try to ram it from neutral it sounds like it’s going to stall. Has to be something with fuel air mixture I’m thinking. Any thought ?

Did you reuse the original base gasket when you installed the new carb? Sounds like it could be a vacuum leak.