Missfire at full throttle

[rudolph]

My old 73 harley Shovle seems to not like full throttle, been through everything, timing, carby, new coil, plugs, airfilter.

Anyone here good with old harleys?

[Sensei]

So I take it its a Shovel Motor . Have the Valve spring been done lately ? may be getting abit weak if they are old / plus don't rule out a slightly burnt vavle or worn cams either . Check for air leaks around carby intake as well . Getting hard to find Old school Harley motor guys now adays . Mate here is pretty up with most Harleys old & new could run your trouble by him & get some feed back if you don't have any luck else where .

[Motu]

Full throttle - but at what revs? All through,low speed or max revs?

[rok-the-boat]

Likely to be an air leak at the inlet manifold. Or a dodgy points, capacitor, or ignition advance.

[rudolph]

I managed to seal up the intake manafold witch helped. and the IGN termanils were loose on the main switch. I might call it a day

[rudolph]

The stuped screws on the back of the IGN were causing most of the troble.

I am so happy now

[Crasherfromwayback]

The stuped screws on the back of the IGN were causing most of the troble.

I am so happy now

It also may well be a bit lean....

[rudolph]

The plugs look a bit on the leen side, it seems to cut out for a second if you crack the trottle wide

[Paul in NZ]

It also may well be a bit lean....

Yeah I heard those old shovels only lean a little...

You need to consider what other mods have been done over the years. Open pipes, air filters, cams, ignition etc etc I seriously doubt it's lived this long without modifications and other dubious upgrades.

Mate - Go through the basic set up of valves, plugs, ignition, filters first. Get it up to speed someplace where you won't be irritating people and you can concentrate on what it's doing. Turn on both taps and hold it wide open in one of the upper gears and when it feels like it's hitting the wall roll the throttle closed slightly and see what happens.

That will give you some idea if it's rich or lean

[rudolph]

I might go up a size in the main jet and see what happens, it has a late moddle Ki hin carb. i would like to get rid of the points IGN as well

[ganit88]

ok. to do a richness test on a shovelhead go for a short ride an when the motors warm do this.
in third gear from 60 kays roll open the throttle slowly till your under full acceleration to its pulling about 4000 rpm then pull the clutch an shut the motor down at th same time an just coast to a stop with the clutch in. pull the plugs out and check the colour on th spec chart an that will tell if your running lean on th main jet. also works if you have a missfire in the range just shut down at th missfire point an check th plugs.
if you have bright blue pipes near the head it could be lean mixture.
Acceleration missfire causes: advance spring tension
air leaks in th inlet
to large intermediate jet
crappy points ignition
what cam,carb,coil,ignition,plugs,aircleaner type an pipes are you running ?
shovel tune ups are easy just need to know what ya got....
ganit.