First time carb pressure test

M

Mark H_NO

Well-Known Member
Ok, I removed the carb and bowl. I cleaned the needle and seat. I reassembled needle and float, inverted, hooked up pressure tester, and gave it 5lbs. I can see the needle slowly moving down. The needle has a spring between it and the float. If I press on the float, compressing the spring, it seems to hold the pressure. The rubber tip on the needle looks good. I don't have a good tool to buff the surface it seats on, but I did use a tiny plastic brush from a carb cleaning tool. The machine mostly runs fine, I'm trying to learn about a new tool. I hooked up a clear line full of fuel with over 1 foot of elevation over the carb, and it seems to be holding. I will check it in another 20 minutes and see. What do you think?

Thanks,
Mark
 
After about 20 minutes total, the fuel level has dropped 2 inches. So my float leaks a bit, at least with inverted carb. I don't know how hard it compresses the spring with the float in a full bowl, in comparison to inverted carb. As I said earlier, I'm not really having trouble with the mower, though I normally use the shutoff. If it was not mine, I'd be more concerned.
 
What I've found.......

Pressure testing is great. Needle must be WET as in spraying PB Blaster or similar before inverting and testing. 7psi for 30+ minutes with ZERO pressure drop. Used to do all this.

Now

Carb right side up. Long clear fuel hose ON the carb inlet. Squirt a fuel like Diesel (not as flammable as gas) into the clear hose. Fill the carb bowl and clear hose up high over carb body. Mark a line where the Diesel is. Wait 30+ minutes. Any drop is a drop = fail. This is real world testing. Don't even have to pull the carb off to do this test. No need to buy a pressure tester. Most carbs are gravity jobs.
 
5 psi is a lot for any carb. Yes they will hold but it is still a lot. We used to set the psi to 2 -3 psi on our racing cars. I would use magnification to check the needle. If the seat was replaced, was the correct tool used?
 
Thanks, I should have know the needle needs to be wet. It still failed the second test. I had 2 of these mowers, and replaced the carb on the other. I will replace this one, I don't want it contaminating the oil.
 
sgkent said:
5 psi is a lot for any carb. Yes they will hold but it is still a lot.
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All I've seen on mower carbs, 7psi for a traditional pusher/rider and no more than 10 I think it was for a radial 2 stroker carb? Someone please correct me if I'm wrong on that. I went for 7psi in my fleet. Worst thing that could happen is the needle blows off and you lose a little pressure. Don't over pressure two stroke carbs.
 
Mark H_NO said:
The seat was not replaced. How do you do it?
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Not all carbs has replaceable seats.

For the wet test as described you do it with the carb in the normal upright position, just don't fully install the carb so can check leak on both in and out sides.
 
Makes sense. If it's replaceable is it like the main jet in the center and unscrews? I don't think I've seen one like that, but I wasn't looking for it either.
 
Mark H_NO said:
Makes sense. If it's replaceable is it like the main jet in the center and unscrews? I don't think I've seen one like that, but I wasn't looking for it either.
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Those are rare, are pressed in. Some Kohler Walbro carbs has pressed in seat that can be replaced.
 
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