I have done everything possible to fix this off idle flat spot on my stock 1200 with 28pict carb. When I mean everything, I mean replacing carb after rebuilding it twice, replacing dizzy, using an electronic ignition, doing valves, timing etc etc ad nauseum.
Today I put my hand on the heat riser tube and noticed it is cool to the touch even after 10 minutes of warm up and idling. Have I discovered my problem?? I'm hoping so. Input would be appreciated. Thanks, Courtney
Can indeed cause flatting.
After a 1 mile drive around town (for example), the tube should be scalding hot at *both* ends.
The purpose is to create a "hot spot" dead under the carb so that raw fuel dumped in by the accel jet is vaporized immediately and can be carried off to the ports. Without the hot spot, the fuel lays in the manifold as globules. They eventually get carried along but by then their effectiveness is gone.
If the tube is clear (blow air thru it) it could be the muffler/tail pipes stopping the flow. The small diam pigtail welded to one end of the muffler *must* have a low pressure zone created inside the muffler in order for hot exh gas to flow. That zone is directly behind one of the tail pipes. The distance the pipe is inserted is critical, as is the location of the end of the pigtail. Many mufflers are slapped together without regard for this.
Speedy Jim
This is exactly the problem I am trying to solve on my '71. I cleaned the heat riser tube, which wasn't fully plugged, and have played with jets, timing, etc. Right now I am at stock jets for the 34PICT-3 (130 main, 60 idle, 80Z air-correction) and it runs pretty good with some mid-throttle flat spot especially when it's not fully warmed up. With a
65 idle jet it runs super but I it seems a little rich and I only get 20.5 MPG. So, I am restoring the original air cleaner with the heated air tube to eliminate that as a possible cause.If your heater riser tube is not hot (as Speedy Jim describes below) there is some problem with the tube and/or muffler system. Fixing this first will move you in the right direction.
-Scott S.
Speedy Jim wrote:
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