Frustrating morning

[drifter]

So I go out to tank test my latest refurbish of a 1956 Evinrude 7.5hp. Motor was stuck when I got it but patience paid off and with work it freed up and my motor appears to have little wear and is in overall in good mechanical condition. Connect the pressure tank and pump the primer and gas flows out the front of carb. What? A new kit was installed and I’ve done dozens of these carbs. Mistake somewhere says I. Disassemble and look everything over. Nothing out of place. New needle and seat from another kit – double check float – double check everything. Hook it up and gas pours out again.
Disassemble again, go over everything. Another needle, another float, another boss gasket, another body gasket. Assemble and try and gas pours out the front. Time for a beer and its only 9:00am. Disassemble again, wait a minute….what’s this? The casting where the high speed nozzle screws into has a marked slope to it where the boss gasket ("O" ring) rests. Closer look with a magnifier shows file marks. Sometime in it’s past a PO decided to "fix" something by filing this casting down and markedly crooked. I scrounged another casting and transferred parts to it (keeping the bottom casting). Tried it out and no leaks (good primer pressure) and problem solved. Engine started on very first pull and quickly dialed in. Now to my question. The upper casting I scrounged from the parts bin is probably from a 5.5hp as the opening where it mates to the intake plate is slightly smaller. The bad casting (7.5hp) measures 0.782" and the replacement I used measure 0.745". Can someone tell me what the proper size is for a 7.5hp? Like I said, the engine starts instantly and dialed in quickly. Restarts are one pull. Any idea how high speed performance may be affected?
Thanks,
Drifter

[adam1961]

I am not sure that the nozzle gasket area was the problem. It sound like the vent in your original carb was blocked. Once the fuel gets high enough in the bowl, the float seals and that is where you get the primer pressure from. The nozzle gasket is there only to make small motors respond better to the high speed needle adjustment. The fuel inside the cavity for the high speed jet is at the same level as the fuel in the bowl when the engine is not running.

If the vent is blocked, the fuel level in the bowl does not rise enough to seal the float. I hve been down this road, and it drove me nuts.

[chris-p]

I was thinking while reading your post,……his vent is blocked. Same as Adam.

[fleetwin]

Maybe we are confused….By "nozzle gasket/oring", do you mean the casting where the brass inlet needle seat mates to the carb (plus gasket)?
I’m sure I probably have a 7.5hp carb if you need it…

[drifter]

Thanks guys. I’m confused a bit. First, the area of the carb I’m referring to is the casting that accepts the threaded highspeed nozzle (p/n 303478). A "float chamber to carburetor boss gasket" (p/n 302984) slips over the nozzle. This gasket mates with the upper and lower halves of the carb. It’s the upper half mating surface that was filed and had a slope (from filing it crooked). This slope would apparently not allow the gasket to seal the upper and lower carb halves and I assumed that was causing my leaking after the float moved the needle closed. Second, you mentioned a "vent being blocked". Can you point me to the location of the vent? I went out and carefully looked over the "defective" upper casting and I’ll be jiggered if I can find any drilled or cast holes blocked. I know I’m missing or overlooking something but I don’t know where to look. Thank you all for helping me with this. I’ve been in this hobby for over 25 years and I learn something new almost every day.

[drifter]

Here is a photo of the "defective" part. Notice the center part where the nozzle screws into the casting? It’s shiny surface? That is where I thought the problem was. I have since leveled it off but have not tried it.

[frankr]

Look at the big hole where the air enters the carb (choke end). See the large hole (or holes) beside it? That hole goes in and intersects with another hole that leads down into the float bowl area. That is the vent that allows the air to escape as the fuel rises in the bowl. If the air cannot escape, the fuel won’t rise and if the fuel (and float) does not rise, it won’t shut off the inlet needle. If the fuel rather just keeps pouring in, it overflows out the main nozzle.

I don’t know if your nozzle gasket is sealing or not, but in any case that won’t cause the overflow.

[drifter]

Humble pie….yum. Well, I sure learned something today! I’ve been in this hobby for over 25 years. Probably gone through 50+ motors. Never gave a thought to those two little holes in the carburetor. Won’t make that mistake again. Sure enough, both vents were blocked. Thank you all so much for educating me. Your help sure makes this hobby more fun.
Drifter

[frankr]

Now you can help the next guy.

[Mumbles]

quote Drifter:

The bad casting (7.5hp) measures 0.782″ and the replacement I used measure 0.745″. Can someone tell me what the proper size is for a 7.5hp?

5.5 horse carb throats should measure 3/4" (.750 +/-) at the butterfly end and 7.5 carbs are a little larger at 13/16" (.812 +/_), I think. Your ’56 7.5 carb should have two humps cast into it at the butterfly end with no holes in them while a 5.5 carb will have holes in the humps for the cam follower to pass thru. The 7.5 motors use a different linkage system with the cam follower fastened to the intake manifold.

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