Pulsajet idle circuit tuning?

[rmoore]

Short time lurker, 1st time poster.

I'm fooling around with a flat head on a Baja frame.

After some research it seems the problem I am experiencing may be common with the pulsajet. The engine I believe is a 3hp, I have replaced the carb with a known 4hp as the original was worn at the throttle shaft it acted the same as both replacements.

The carb is clean and has a new diaphragm, needle and jet. To get the high speed right the needle has to be at 3/4 turn open, but the engine won't idle well at that setting. To idle the needle has to be set closer to 1 1/2 turns. Of course there is some hesitation at throttle hit.

I've checked the valves and they are not being held open by lack of lash.

This is the 3rd carb they all acted the same.

I'm thinking I will drill out the idle passage to get the idle with the 3/4 setting.

Now finally my question... Has anyone had luck with this or am I missing something?

 

[creia]

My son and I have had dozens of engines with the Briggs Pulsa-Jet carb. They are pretty bullet proof and never once have we had 2 drill out any of the passageways or jets. It sounds like you could have an air leak somewhere which will produce a lean condition and wreck havoc when you are trying to adjust the carb to compensate for this. Any "modifications" to the engine? Are you running the stock air filter and "hot dog" muffler?
Michael

 

[rmoore]

My son and I have had dozens of engines with the Briggs Pulsa-Jet carb. They are pretty bullet proof and never once have we had 2 drill out any of the passageways or jets. It sounds like you could have an air leak somewhere which will produce a lean condition and wreck havoc when you are trying to adjust the carb to compensate for this. Any "modifications" to the engine? Are you running the stock air filter and "hot dog" muffler?
Michael

The engine is stock I believe. The muffler looks stock to me but it is not the hot dog style. The air filter is a briggs but I don't know if it is correct. The gaskets are new and the surfaces are flat. I'll try to attach a photo.

 

[MB165]

can you at least find a good middle ground at all? it doesn't have a dedicated idle circuit and that can sometimes be a pain, but they usually work pretty good. do what Michael said, check for air leaks, maybe spray some gumout or 2+2 at the intake gasket and see if it changes rpms. if its designed like the 5hp carbs, hopefully, unscrew the needle assy. and the jet itself, look and see if the holes in the carb body are open and cast correctly. with a bright light in the intake you can see the edge of the throttle valve, it should be in between the carb body holes at idle. front hole works for idle and high speed but, and the rear hole for transition and more fuel at high speed.
are these carbs your getting genuine briggs or china??

 

[rmoore]

can you at least find a good middle ground at all? it doesn't have a dedicated idle circuit and that can sometimes be a pain, but they usually work pretty good. do what Michael said, check for air leaks, maybe spray some gumout or 2+2 at the intake gasket and see if it changes rpms. if its designed like the 5hp carbs, hopefully, unscrew the needle assy. and the jet itself, look and see if the holes in the carb body are open and cast correctly. with a bright light in the intake you can see the edge of the throttle valve, it should be in between the carb body holes at idle. front hole works for idle and high speed but, and the rear hole for transition and more fuel at high speed.
are these carbs your getting genuine briggs or china??

The 2 carbs that I have tried were OEM off old engines. The 1st had a warped carb to tank surface so I thought I would try the one that is on now. I cleaned it up and checked all passages, replaced the diaphragm, seat and needle. All surfaces are flat with new gaskets.

I will check for air leaks.

Would the coil be a possible cause? The spark is blue / white when spinning by hand.

 

[rmoore]

The only air leak I could find was around the throttle shaft. I guess that might be leaning the idle out. The idle passage was clear, I set the screw a little rich at high speed and will just have to choke it to start up. Maybe the compression is low the engine doesn't seem to smoke and the cylinder is free of scars. But I didn't check the compression.

 

[MB165]

since you can get it to idle successfully id say whatever the compression/ spark is, doesn't matter. possbly the throttle shafts on all of the carbs are worn.........with wear in that area you will chase your tail crazy trying to get it right. there should be almost zero slop in that pivot. with the throttle in the idle position try and wiggle the shaft forward-backward, they wear towards the engine because of the vacuum pulling on it.

 

[rmoore]

since you can get it to idle successfully id say whatever the compression/ spark is, doesn't matter. possbly the throttle shafts on all of the carbs are worn.........with wear in that area you will chase your tail crazy trying to get it right. there should be almost zero slop in that pivot. with the throttle in the idle position try and wiggle the shaft forward-backward, they wear towards the engine because of the vacuum pulling on it.

I am sure you are correct about the shafts, they are decades old. Is there any success sealing them or is carb replacement the only out?

 

[creia]

I am sure you are correct about the shafts, they are decades old. Is there any success sealing them or is carb replacement the only out?

If the throttle SHAFT is worn (and causing the air leak) you can easily replace them. If, on the other hand, the carb CASTING HOLE is worn where the shaft rides you are screwed= new carb.
Michael

 

[Dr. Shop Teacher]

Before you go get yet another carb, see: https://www.oldminibikes.com/forum/...ical-mental-therapy-briggs-3hp-cousins-3.html starting at post 27. This shows how to completely overhaul the carb including truing mating surfaces. The carbs in these examples were 50 and 40 years old. The fuel pick ups can also contribute to the problem if these have not been changed. An hour or so is well spent--you can even use your cast offs first and swap out.

 

[rmoore]

Before you go get yet another carb, see: https://www.oldminibikes.com/forum/...ical-mental-therapy-briggs-3hp-cousins-3.html starting at post 27. This shows how to completely overhaul the carb including truing mating surfaces. The carbs in these examples were 50 and 40 years old. The fuel pick ups can also contribute to the problem if these have not been changed. An hour or so is well spent--you can even use your cast offs first and swap out.

Thanks Teach for the link and efforts to post the techniques. I have done the rebuild of both carbs I tried using those techniques and the results were what I posted above. I assume the overall health of this engine and the wear on the throttle shaft are the culprits. I'll just put up with it for now.