720 issues

[mecwpri]

Would love to post some pics of my freebee 720 but cant figure it out, guess thats why I have a wife! anyway my 1980 720 is having issues, the truck has been sitting since 2002 and I am now putting it back on the road. So far I have soaked the carb overnight and reassembled, drained the tank and put in fresh fuel, changed the fuel filter. The prevoius owner took all the emissions off and converted to a manual choke, when everything was ready I fired it up and had a dead miss, so i ran a compression test, 120 on 3 cylinders and 115 on the other so no real problems there. New plugs seamed to fix that problem, now it runs good for about 25 miles the it starts to load up the center 2 plugs, outer 2 seem to burn clean and the fuel mileage is way off, full tank of fuel in about 100 miles. Since I have had the carb back off 2 times thinking I sucked up some debris but clean as a whistle. Set the timing and rechecked it a few times but same problem. Today another set of plugs 1 heat range hotter hopeing that might have some positive effect, also changed the plug wires the other day and same problem, the vacuum line for the distributor seems good and has an effect when disconnected to set the timing. I also ran it for a while without the air filter or the top of the air cleaner assembly thinking it might be choking off the air supply but same problem. I am thinking I have a vacuum leak, possibly in the manifold but that should cause a lean condition? Anybody have a good idea?

[flinterman2000]

It could be that the ring lands are holding the rings when the piston gets hot, so you're losing compression then.

[mecwpri]

Just did a compression test when the engine at operating temp and all cylinders lost only about 5 psi, could hardly tell the difference on the gauge!

[mecwpri]

Did some more troubleshooting, Truck seems to idle fine and only loads up when driving, so I took the top off the air cleaner and gave it some gas while watching down the carb, it seems like a lot of fuel is pouring out of the venturi at slow engine speeds, a lot more than my other carburated vehicles, there is a brass port that is in front of the venturi that faces the back of the vehicle and fuel really comes out of it when the engine is off idle, I am thinking this port might need to be plugged, possibly a plug fell out of it? It would still get fuel from the two side ports, is the venturi the same between the primary and secondary barrel? I might of gotten them swapped during the cleaning! Any body have some indepth carb knowledge?

[mecwpri]

Well making some progress, when I got the truck the carb was in pieces and in a cardboard box, when I assembled the carb I inadvertently put the secondary and much larger main jet in the primary side, now that I have them in the proper position the truck runs pretty normal with the exception of the accelerator pump not working, would have it on order but work and poor weather have kept me from repair time and parts chasing. Last tank was 14 mpg and 3 sets of plugs, thinking the economy will improve!

[flinterman2000]

Good work. Fiddling with things normally have good results. Keep us posted.