I finally got my new '77 FXE running pretty well. The only remaining issue at this time is that the revs tend to die down fairly slowly when letting off on the throttle. I investigated the possibility of a vacuum leak but there is no evidence of such ( I used the old methods of propane gas directed all around the intake connections, as well as starting fluid and WD-40--no alteration of rpm's at all with any of these three methods). The next suspect is the mechanical advance unit. I have a new one installed. A friend of mine who is pretty mechanically savvy, suggested that the spring tension may be weak in the unit and suggested either heavier springs or drilling out the weights to make them lighter. Does this make sense in light of the presenting symptom?

 

The Mech Adv simply controls when your coil fires.
"Timing advance" refers to the number of degrees before top dead center (BTDC) that the spark will ignite the air-fuel mixture in the combustion chamber during the compression stroke. Retarded timing can be defined as changing the timing so that fuel ignition happens later than the manufacturer's specified time.
It is controlled by RPM and not the other way around. I would clean the carburetor and insure the throttle valve is sliding freely.

 

Yeah,I'd first suspect a sticky throttle shaft,or sticky throttle cable,Also check the routing of the cable,maker sure there's no kinks or sharp bends.

 

Sticky advances is real real common.

 

This and the usual el-cheapo people tend to put in are fugging junk out of the package, if it wasn't a good brand like Rivera or an All ***** units with the washer under the spring clip it's going to be an issue sooner than later. Next is an small intake leak will cause high idle issues, take a can of brake clean while warm at an idle give a few shots with the little red hose on around the throttle shaft on the carb and the intakes clamps on the heads, idle goes nuts you have a leak. More common than you think.

 

If they put in Chinese shet they deserve Chinese problems.

 

It's more of a don't know any better thing I've found but I have met more than my share of deliberately cheap bastards with shovels needing work.

 

today the overall condition of the machine lets us know up front WHO we are dealing with and if we want to get involved

buy stuff on dollar amounts and bringing junk they want installed, it is a wasted time pit to get involved

 

It seems unlikely that this is an air leak issue. I did the tests and reinstalled all seals , gaskets and changed out carburetor itself--with no change in rpms at all during tests. In addition, the cable is free and smooth-operating and the throttle snaps back with no interference. So, I am left with the question of the advance unit. Do those recommendations regarding altering the weights/springs make sense?
This is my first venture into anything besides late forties and early fifties panheads. So, my knowledge of things like automatic advance weights is minimal. Re: all of the comments about cheap products--what is a good brand of advance to buy? The unit I got was from Drag Specialties

 

You seem like you want to dick with the advance unit so not much we can say is going to change that. Lightening the flyweights won't do what your thinking, just cause your advance curve to come on too soon and overall performance will be erratic as the weights will tend to bounce around instead staying fully out. Another thing you can check is did the new advance unit come with a new cam lobe on it ? If so put the OEM back in, most the cheapy units the cam lobe is junk and way off time.