1969 Briggs 3HP Model 80202 running rich- WHY?

[creia]

This one has got me stumped...
I just completed a rebuild of this engine (the same engine as in my other post with the recoil issue). The first time I start these engines up I always install a "Vacu-Jet" carb/tank assembly due to the simplicity of their design and less things that could go wrong as compared to the "Pulsa-Jet" carb/tank that this engine originally came with. The engine starts, idles, and runs absolutely perfect. So far, so good...

Then I installed the original "Pulsa-Jet" carb/tank that I had thoroughly cleaned and rebuilt (I have my system/technique down pretty good after all these years), and adjusted the valve/jet to 1 1/2 turns out which is the default setting While it STARTS fine, once you apply some throttle it sputters and puffs out black smoke every time. At full speed (3600 RPM), however, it runs fine. Drop it back to idle again and then apply throttle you get sputtering and black smoke every time. I tried adjusting the valve/jet turning it in (which helps some). Once I get it adjusted down to only about 1/3 to 1/2 turn out the sputtering and black smoke stop, but it now will not reach full 3600 rpm.

This is the first time I've ever had one of these engines displaying a RICH condition- I've had a LEAN condition a few times before and eventually been able to diagnose and correct it. The engine seems to "load up" to a richer and richer condition the longer you let it idle, which eventually makes it idle slower and very rough. I have tried changing the valve/jet and the brass screw-in seat. Also, I inspected the 2 "cat's eyes" ports under the brass seat and they do not look enlarged. I completely cleaned the carb and passageways a second time and installed another (new) diaphragm.I am out of options here that I see. Kind of a shame if this carb is a "dud" and i cannot use it as it is in beautiful (cosmetic) shape.
Does anybody have any ideas?
Michael

[bgsengine]

did you check tank to carb fitment - check for warpage (.002 was the service limit on flatness) - especially in any areas near or around the throttle lever - also might check for any hairline cracking there - sounds like you may be getting unmetered fuel sucked in past the throttle - If the engine is not loaded, your "full speed" 3600 RPM is still better known as "High Idle" - you may detect the excess fueling also when placing a load on the engine (as throttle opens more to maintain RPM under load) also look carefully at the carburetor where it mounts to tank to see if there's any signs of spots where it does not seat fully to the tank..

[creia]

Yes, I always check the "flatness" of both the top of the fuel tank and (especially) the base of these "Pulsa-Jet" carbs. On the latter nearly every one has some warpage, somed more than others, typically at the 3 "ears" where the mounting bolt holes are. (I guess that they get bent down when tightening?) I check the carb base for flatness in various directions using a feeler gauge as a small straightedge. Where needed, I sand down the high spots which is always around those 3 holes using a small Dremel tool on low speed with tiny sanding drum. Since doing this procedure, I have never had one single fuel leak at the junction of the carb to tank gasket.
Michael

[bgsengine]

Also, when this all happens is air cleaner on or off? I wonder, as some carburetors, they'd get extra fuel feeding through the air cleaner bolt hole under load, if air cleaner was not in place (or screw in the hole) - that really depends on the particular carburetor design.. also, are you sure that carburetor is specifically the build for a 3 HP? as opposed to one for a larger displacement (different jetting and orifices) as you describe its operation, seems you are getting excess fuel only as the throttle is opening wider (on acceleration or under load) so if it isn't leaking past the jets, possibly jets are just sized too large for the engine displacement.. ? (also does it have the helix insert in the intake tube after throttle? some had them, some did not.. and I think part of that design was due to fuel "puddling" in the intake until opening throttle pulled that excess fuel in.. the helix insert caused more "swirl" in the air, keeping the fuel atomized as it passed through that "long" intake pipe - without the extra swirl, fuel could be cooling/condensing causing the puddling... ) how about when engine is fully warmed up (and intake tube is "hot") - is there any difference then?

[creia]

All great questions Brian (and very analytical). Here are my answers:

1. Air cleaner (including new foam element) is on.

2. The carb appears to be original (and correct) for the engine (It still had the original Briggs copper color just like the engine and tank)

3. I installed a new Briggs OEM high speed valve/jet Part # 299060 which is used on all of the 2-5 HP engines with either a Vacu-Jet or Pulsa-Jet carb.

4. Yes the helix insert is installed. (I know from experience that they do not run as well w/o it).

5. No difference after it is warmed up - same symptoms.

This one is a real head scratcher for sure! Even working as just a home garage hack I've cleaned and rebuilt probably at least 50 Pulsa -Jet carbs and never had these kind of symptoms- there is a first for everything as they say! I'll pull the carb off tomorrow and look for any hairline cracks on the bottom that you mentioned in your earlier reply.
Thanks for the continued interest and trying to help- it is most appreciated....
Michael

[creia]

Update and question:
There were no visible cracks on the carb body that I could see allowing unmetered fuel in as per Brian's suggestion.
I DO have a follow-up thought and question, however...

This engine has different intake and exhaust valve springs. The exhaust spring is a little longer and heavier than the intake spring (They even have different Briggs part #'s). It is possible that I "MAY" have switched the springs when I reassembled the engine. Does anybody think I could be getting these "rich -like" carb symptoms if indeed I did this?
Thank you for your opinions.
Michael

[bgsengine]

Anything is possible, but yeah you do want the heavier spring on the exhaust.. might be worth a check to see if you did mix them up, and if you un-do the mix-up and it then works better, you either 1) sealed a possible air leak or 2) it was the valves... (I'd not think an air leak would cause rich conditions, but then again , i've seen weirder things...