Shovel missfire

[jules45]

Hi Guys.
A little while ago,I bought a 1983 FXRS Shovel,which required a little work to bring it up to scratch.Using the bike now in earnest,I find that if the engine is hot and I go over 50mph, it starts to missfire and explode unburned fuel in the exhausts. If I shut off the throttle to below 50mph the engine runs fine. The previous owner had removed the exhaust baffles. I'm thinking that maybe the carb needs rejetting to compensate.It has the standard Kehin carb. I have changed the coil,plug leads and plugs to no avail. The ignition system was previously converted from electronic to points.Any thoughts chaps.

[spinlondon]

You may find it is the ignition switch.
Try holding it when the back firing starts.

[panheadpete]

[quote=jules45;687 The previous owner had removed the exhaust baffles. [/quote]

Hi; if you've now got straight through drag pipes, that's most likely the problem - you need at least a little back pressure for the engine to run cleanly and it's unlikely that re-jetting will cure it, although making the mixture slightly richer throughout the range usually helps the running overall.
Assuming the timing, advance-retard etc are spot-on.
Cheers,
Pete

[locust]

Buy a proper bike instead of these 1980's jobs

try putting baffles back in, you didn't tell me they are removed, mine was the same till the engine got back pressure.

Don't go HD, custom fasteners do a pair for less then a tenner or if there 1 1/2'' I got a pair.

[jules45]

Thanks guys. Anyway Brian,I told you I couldn't afford a new one.I do have what are supposed to be the original removed baffles,but they don't seem to fit very well. Maybe it's me.

[kiwidave]

why would you want a new one when you have something as rare and nice as that? Post a pic!

Have a look at the idle mix screw when you have baffles in.....if it is still plugged in its tower, unplug and adjust....so it is set midway between stumbles in a warmed-up engine.

before then, just make sure you have no vibration-related electrical issues. i.e. coil terminal wires and battery connections tight....

[jules45]

Thanks David. Have checked/changed everything electrically apart from ignition switch.Exhausts and the balancing of is always a bit of a black art. I know a guy who is running a shovel with drag pipes and stock carb with no tweeking and it runs perfectly well throughout the rev range.Stranger still,a friend is running an 883 Evo sportster with straight through's and stock carb. No stage one or anything and it's fine. Must be as lean as anything,but hey that's Harleys for you.By the way, only Joking about the new bike. If I was running the motor company, we'd still be riding side valves with manual ignition.

[DickDastardly]

Had a similar thing on a 1980 FXS this year!

When cold it was fine stop after a few miles, and re-start it would spit back though the carb, ‘phat and phit’ from the exhaust, thought it was running lean so adjusted the carb, it was worse, made it lean, it got worse, advanced the timing it was ok for a while then it got worse, retarded it, it was ok then got worse…..we were trying everything!

It was that bad we were about to rip the heads off and check the valves, we were that desperate!

The carb (original non-CV Keihin) must have been off and on a dozen times, cleaned, float heights checked and re-checked, new exhaust gaskets, new battery, plugs, new alternator (it was shagged anyway), battery earth-lead, points, condenser, etc etc.

(BTW, this was 'electronic' ignition converted to points)

In the end we found that the condensers we’d tried were crap, basically! Then bought a ‘Blue streak’ condenser, from a local Indie!

Sorted!

Might not be the answer to your problem but there is a lot of ‘custom crap’ out there that is not up to the job, you know what I mean!

Moral to this, check the simple things first and last!!

Good luck!

[Phil]

Originally Posted by DickDastardly

In the end we found that the condensers we’d tried were crap, basically! Then bought a ‘Blue streak’ condenser, from a local Indie!

With those symptoms, just what I was thinking.

Less likely, but possible with could be plug leads.

[jules45]

Sorted guys.
Bit of tweeking to get baffles to fit. Test run up to 75 mph. and ok.Doesn't sound as good although baffle ends have been drilled by previous owner.Thanks for the advice.