B&S 6.75HP Starting Problems - Carb Issue

Engine 12F702-0616-01 (this engine is gravity fed; not the diaphragm pump style)

I was given a JD 14PZ mower that has problems starting. In the past it would be really hard to start, but now it won't start at all. The air cleaner gasket is shot so the primer bulb doesn't work, but I am able to pour gas into the carb and have it fire right up....then die shortly thereafter once all the gas I poured in is gone.

I pulled the carb and disassembled it and it looked really clean. I made sure the main jet was clear by using a small pin, and blew air through all the other ports I could see. The float and fuel valve works fine; the bowl was full when I pulled it, too.

I reassembled and nothing changed. I then applied air pressure to the primer port (A) on the carb by using a small piece of hose and my mouth. Gas came out the hole opposite of it (B). Same result: started up and then died.



Now the interesting part. I applied air pressure again, but this time plugged port B with my finger. I'm guessing this forced gas through the jet and out the main nozzle into the tube. The mower started right up and ran great...throttled up and down no problem.

Shut her down and tried to restart without success. I primed the main jet again the same way and it started up and ran fine.

So, my question is if the main jet is not plugged, then what would be causing my issue? I am missing something of how this carb operates...

I thought about buying a carb kit (I do need that one gasket), but I'm not sure it would help me since this carb is pretty simple. A kit is $10, a new carb is only $30.

 

The engine runs fine once you get it started so it sounds like the core of the carb is ok. Running at different throttle positions is a good sign that the main and idle jets are clean and working properly so you're down to getting the primer working so you can start it. As you're finding without anything on the inlet face of the carburetor the primer does not work. Get the gasket and put the primer & air cleaner assembly back on and see what happens.

If you don't want to spend the $10 for the kit try a small engine shop that may buy the gaskets in bulk so you could just buy it for a few dollars. Or, you can buy sheets of gasket material from an auto parts store and cut out the shape you need. A paper hole punch can make nice clean holes in thinner gasket material. And in a pinch I've even made gaskets out of card stock from a cereal box.

 

I think the bowl nut is the main jet on that carb, 1 hole down through the center and 1 hole through the bottom, make sure it and the bowl are clean, I had one where a small piece of debris acted as a flap when I would blow compressed air through it and it would just move out if the way, finally washed it out with carb cleaner. Have a good one. Geo

 

I don't think this will solve my problem. I have to plug hole B (in picture) for it to prime correctly, and I'm pretty sure the gasket does not do that...it only seals the air cleaner to the inlet face and seals the primer port (A) for the primer bulb.

Also, the other thing that leads me to think something else is wrong is that I cannot restart it when hot. I should not have to prime the carb every time I start it when hot, right? I have to get an air filter for it, so I'll just get a carb kit while I am at it.

Yes, you are correct...the bowl nut is the main jet. It has the (tiny) jet hole down the center and actually two inlet holes on the sides where gas enters from the bowl. I cleaned it very good, but I'll give it another go. I would think it is clean, otherwise the engine wouldn't run like it did once I got it started.

When the engine is just sitting there, is there any gas sitting on the carb side (vs bowl side) of the jet? Based on the fuel level in the bowl, it seems like there should be. But I'm not sure that is the case since me priming it my special way forces gas into the emulsion tube. Hmm, I dunno.

I'm going to disassemble and clean it again, this time with carb cleaner.

 

I took a closer look at the carb and now have a little bit better understanding. There are two holes in the chamber above the main jet plug: one of the holes is goes to the main nozzle (C), and the other hole goes to the port (B) on the inlet face which is the discharge for the primer.





So, then you apply pressure with primer bulb to port A, this pressurizes the fuel bowl and forces gas out port B on the inlet face, which is what I was seeing. If I plug port B on the inlet face and prime it, this forces gas out the nozzle (which is how I got the mower started). Why is there a need for port B? Why not just let the gas go out the nozzle to prime it? I'm guessing it is needed for the venturi effect...

I'm curious what port D does. It has a tiny hole in it, but I couldn't find anything small enough to poke into it to try and clean it.

 

You said the engine runs great once you get it started and are able to vary the throttle. Does the engine idle?

 

Yeah it runs great if I do the trick to get it started. I can vary the throttle as much as the linkage and governor will allow me to. I wouldn't say I can get it down to an idle, but every other lawn mower I've used that has a throttle couldn't either...it was just either full throttle or back off a bit for half throttle.

I could disconnect the linkage/governor and vary the butterfly manually, though, but I haven't tried it.

 

Replace the gasket between the filter housing and the carb. This is the culprit. I see part of it stuck to the carb. Sometimes the housing warps a bit and it takes 2 gaskets stacked together to make it work properly.

B is the atmospheric vent. Gas came out there because you pressurized the bowl. That is what the primer does, however, it doesn't move enough to blow gas out that hole. It instead makes a little dribble out of the center tube you marked C. That pressure doesn't go out hole B because the other end of B is under the gas level, so the air can't escape there.

 

Thanks, I'll try replacing the gasket and see if that helps. I'm not sure I see the part of the gasket stuck to the carb that you mentioned...

The new gasket may help my starting issue with priming, but what about restarting when hot? I shouldn't have to prime it start it when hot, especially immediately after shutting it off. Does the primer bulb have some sort of one-way valve in it that doesn't allow the gas to bleed back into the bowl from the center tube?

 

No, it doesn't have a check valve. Sometimes some engines need to be re-primed sometimes even when they're hot.

The long slot looking groove thing you see around the port you marked A.... That is not supposed to be on the carburetor. That is the mark left by the air filter housing on the gasket. That is the area where the gasket is still stuck to the carb. This is also the critical area where if it is torn (yours is), then it probably won't work right.

 

I made up a quick gasket out of some thin rubber I had laying around. Wallah! The primer worked and I could see gas squirting out of the center tube. Started up and ran, then I shut it off and waited a few minutes. Started right up without priming! Even waited 15 minutes and didn't need priming.

Thanks for figuring out the issue! I didn't realize that gasket was the key. I still plan on getting a carb kit just so I can get a real gasket and go through the rest of the carb. At full throttle it is hunting and surging a bit...not real bad. Could just be some bad gas.

 

I noticed that when it was running, the engine was spitting a tiny amount of gas out the carb. It wasn't a huge amount, but I could see it saturating the air filter if it ran long enough.

Am I in need of a valve adjustment?

 

It doesn't have adjustable valves. It should have a metal tab that covers the opening of the carb to keep that from wetting the filter. A tiny amount it normal at times, especially when throttling up. If the engine is surging it will do it a lot more. If it's flooding it can do the same, but this should be evident in the way it runs.

 

That sounds like the float is getting stuck.

 

I read the L head engine valves can be adjusted, but it involved grinding the end of the valve; no adjustable tappet. So not an easy adjustment.

I'll go through the carb with the kit once I get it and see how it runs after that. Just thought it was odd to see anything coming out of the carb, even if it was just a little bit...thought maybe it could be from the intake valve opening a bit too soon before TDC.

 

Yeah, not adjustable, they have to be modified in the rare case that it's needed. I don't think I ever saw one of that series engine that has run long enough to ever need this procedure performed. Even the bigger briggs L heads that have been running for 20 years rarely need this done.