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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 29836
Experience:  Service manager and technician for all brands of engines with 40 + years of experience.
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Why do I have to prime my & S pulsejet 5hp engine to get it

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Why do I have to prime my B & S pulsejet 5hp engine to get it to start. New points and spark plug. Once running its fine.

The engine has carburetor mounted on top of the fuel tank. Either the tank mounting gasket is leaking, or the diaphragm for the fule pump section of the carburetor is defective, have you rebuilt the carburetor with new diaphragm? See basic link below.

Customer: replied 1 year ago.
Hello Curtis, The carb, (299437) is a brand new item purchased fro a B & S uk dealer so I am hoping it is the genuine article, it looks identical to the one I took off. I have checked the diaphragm and all looks ok and the gaskets to the fuel tank and manifold are both new. There is no leaking from under the carb when I eventually get the motor running, confusing what!!!

Fuel pickup was used from old carb, or did it come with the new tube and screen?

Customer: replied 1 year ago.
Both tubes are new with the new carb
Customer: replied 1 year ago.
Ive had the new carb back off the tank wondering if the pickups are dirty but no there ok and the tank is clean

Same problem with old carb?

Customer: replied 1 year ago.
Could it be the tank under the carb is buckled slightly to allow the vacuum to escape but hang on, that would give a fuel leak wouldn't it?

No, it might not show as a fuel leak. Tank has to seal.

Customer: replied 1 year ago.
Do you think this is the problem then, so what to do to remedy?
Customer: replied 1 year ago.
Or is there any thing else you consider to be the problem?

I could be and you would have to straighten the tank mounting with a file, or add an extra gasket to let it seal better. Something else that could affect the fuel being drawn into the cylinder is the valves. You may have intake valve that has no clearance or is leaking, not building vacuum in the cylinder to draw in the gas charge.

Customer: replied 1 year ago.
I've also opened up the three holes in the fuel tank cap wondering if that was the problem.

Fuel tank cap would only affect the running, not the start.

Curtis B. and other Small Engine Specialists are ready to help you
Customer: replied 1 year ago.
Ok Curtis so extra gasket to the carb/tank and head off to inspect the valves, decoke at the same time. Try both that first and let you know the outcome. How can I get back in touch with you again?

Use the link in your email to get back to here. You can check the valve clearance through the vent cover, without removing the head and valves.

Customer: replied 1 year ago.
Thanks for you input Curtis lets hope you've cured the annoying problem.

Thank you.

Customer: replied 1 year ago.
Hi Curtis, its Keith again with the same problem. I received the new carb gaskets and installed the same using two gaskets under the carb. Also applied a spot of non setting gasket sealant to make doubley sure. whilst the carb was off the engine I tried to look for the valve seats through the inlet port but was not successful with this.Rebuilt the whole thing and replaced the points with a b&s electronic coil. Same problem as before engine will not start even when keeping the choke out and the throttle control on start, but it will start if I prime the carb. I then went back to installing the old carb without success. I know what you said about the seating on the inlet valve but surely to draw fuel from the carb into the cylinder the valve has to be open doesn't it so the action of the piston pulls the fuel on the inlet stroke. Look forward to hearing you scratch your head as I have been. Cheers Keith.

This is the flat head engine with the valves in the block. I wonder if the valves may be too tight and the engine does not have enough vacuum to pull the fuel charge through the carb , to the cylinder. Have you ever checked the valves or the compression?

Customer: replied 1 year ago.
Hi Curtis, thanks for the promt reply. I have not checked the valve clearances as yetr and there does seem to be very good compresseion when trying to start the engine so I guess that's the next step, attach a compression gauge and hope fo r the best. Thanks mate will try this next.

OK> Let me know.