briggs needs primed until warmup

[38racing]

I have a 12 series flat head (quantum style). I actually used the carb from this engine on another similar engine and now have a carb on it scavenged from yet another engine. I put it through the USC. It would start and run at first but now I find that it starts right up after priming but then stops unless I continue to prime it. After a minute or so it will stay running. Even at that point I noticed that a little prime seemed to increase rpm slightly. I'm wondering how that could be the carb. I'm thinking that something changes on warmup. I had a similar engine that wouldn't stay running and that was due to a missing gasket and damaged intake pipe. The intake on this one was tight and does not appear damaged. I'm going to replace it and new gasket just in case . So iIm thinking maybe a valve issue. Maybe there is too much intake valve clearance and it tightens up when hot? I thought the flat heads lose clearance with age though. Any thoughts?

[Skywatcher]

Hi 38

You'll never get too much valve to tappet clearance on an L-head engine unless someone ground too much off the end of the valve stem while doing a valve job. Because the valves sit in the cylinder block directly above the lifters, the opposite is usually the case. As the valve and seat both wear, the valve sits lower and lower in the cylinder until it eventually doesn't close.

Check the screen at the bottom of the main pick-up tube, those screens will often get a coating of what looks like peach fuzz which impedes fuel flow. I don't know if an ultrasonic cleaning will remove this build-up, but a shot of compressed air across the bottom of the tube usually cleans the screen. A few drops of water in the upper chamber of the fuel tank will have the same effect. Hope this points you in the right direction,

Sky

[KE4AVB]

I am in the dark as I don't know if the engine has tank mounted or separate carburetor as I don't hardily ever these small engines any more as most walk behinds here are now are cheaper to replace than repair.

The posting of the engine model and type(spec) number usually helps but it seems that most OPs are getting to where they think we don't need these but personally I am not a know it all. And my crystal ball is out for repair as it hasn't stop laughing when I ask it "When was the current president to become understandable?". He worst than one of my ex girl friends about changing his mindset.

[bgsengine]

Yeah - what KE4 said. Quantums , when they first came out, like the Max, had float carburetors,(and many with throttle control that had a choke position) but later years they had a quantum line with pulsa-prime carburetor, so , without model-type-datecode , all we're doing is gazing into a crystal ball that tells us each something different.

[38racing]

engine is not at home so I'll post exact numbers later. It's the float bowl carb with front primer. I figured it wasn't flow to carb as that should always keep it from running as bowl would empty.

[hanz63]

Have you tried a new needle and seat? Or maybe the seat isn't all the way in. Bowl vent open? I thought all 12" engines bowl type carbs. Hmmm....

[38racing]

keep forgetting to write down the model number. I did new needle and seat already. I pulled the carb apart again but then just put it back together. I couldn't change intake pipe as one I had was different. I verified gasket was ok and bolts tight. No change. I prime for a minute or so continuously and then it runs by itself. If I shut off fuel it runs for over a minute so bowl is full and if I open fuel as it stumbles but before it actually stops then it goes back to running fine. I let it run a bit before stopping it. It should have a full bowl tomorrow so I don't understand why it won't stay running. I just got another mower in with that engine so I'll get it running ok (hopefully) then pull its carb for testing on this other one.

[38racing]

Have you tried a new needle and seat? Or maybe the seat isn't all the way in. Bowl vent open? I thought all 12" engines bowl type carbs. Hmmm....

Air venting may be it after all. A while back I had a thread about a gcv160 clone that ran poor but improved if I gently pressed the primer. That turned out to be a PO that used rtv as a gasket and was too generous. When I just tried the Briggs today I had found that the primer wants to stay in so I needed short pushes on it for 90 secs to keep it running. After it started running on its own. I put my finger over the hole in the primer and it falters. So its using the primer as an air bleed. I'll have to look up carb info when I get home and see if that's part of design or it's using it because the design vent is plugged.

[38racing]

I'm stumped. The engine is a 12h802 and the carb came from a 12h802. I put carb back through USC then sprayed all passages repeatedly with carb cleaner and compressed air. Still had to continually prime for2 minutes before it would stay running. At that point putting finger over primer bulb hole would cause to falter to point of stopping. When rpm drops with falter the governor opens the throttle but it continues to stop. After 5 minutes of running then putting finger over primer bulb hole causes a slight drop in rpm but it stays running. I'm using original o ring on the intake pipe. Can't think of anything else except checking intake valve clearance. I do have another similar mower just left at dumpster. I guess I could see if I can get it to run and then try it's carb on the problem engine.

[bgsengine]

I'm stumped. The engine is a 12h802 and the carb came from a 12h802. I put carb back through USC then sprayed all passages repeatedly with carb cleaner and compressed air. Still had to continually prime for2 minutes before it would stay running. At that point putting finger over primer bulb hole would cause to falter to point of stopping. When rpm drops with falter the governor opens the throttle but it continues to stop. After 5 minutes of running then putting finger over primer bulb hole causes a slight drop in rpm but it stays running. I'm using original o ring on the intake pipe. Can't think of anything else except checking intake valve clearance. I do have another similar mower just left at dumpster. I guess I could see if I can get it to run and then try it's carb on the problem engine.

when carb is apart do you happen to notice if the float sits level? or a bit "up" from level? (low) . If it is "up" from level, then you got a swollen float needle seat and fuel level is too low in carb bowl. (Bowl needs specific level of fuel to reach the fixed jets properly)