problems starting when engine is warm

1975 kick start Harley XLCH. The bike is perfect. No problems kicking
it over. Starts relatively easily. Until, that is, when it is warm
(not warm weather but when it has been started or used).
For 5 or 6 hours after it has been started (and then shut down) I can
not get it to restart - there is no loss of compression (this is
immense) or increased compression. Occasionally it will backfire on
the 'live' kick when attempting to restart it but that is as good as it
gets.
Has anybody got any ideas as to the cause of this. Or better still any
ideas as to how this may be overcome.
Things I have tried: numerous attempts at kicking. kicking with
differing numbers of primer kicks - 1, 2, 3, 4, 5. Fuel injection and
primer kicks prior to the live kick - 1, 2, 3, 4. Taking out the spark
plugs and allowing extra air to circulate in the cylinders (I think
that the bike may have started once in the past by using this awkward
method, but it is not a constant).
Things I have not tried - bump starting the bike as this would be a
useless format for restarting as any place that I would choose to stop
and turn off the engine would have to be on a hill!!
Think about this now... my bike is constantly refuelled by use of a
fuel can in my garage as I cannot use a petrol station as it will not
restart after refuelling. And in all other respects the bike is an
absolute dreampiece.

 

On a warm engine, my compression's definitely increased
and I sometimes find my bike starts more easily with the
throttle part or all the way open.

If there's an auto-advance on the ignition, that might be
worth checking as well, to make sure it's functioning
correctly.

 

I had a 1969 XLCH that had the magneto with the left handlebar
retarding system. When the bike was new it started hard but after I
sent the magneto off and had stronger magnets and a few other tweeks
done to it it started a lot easier. That bike was basically stock.

In 1970 I bought a new XLCH after my 1969 was stolen. The main
differences I recall between the 1969 and 1970 XLCH's were the 69 had a
magneto and I am not sure what the carb was. The 1970 XLCH had a
distributor/coil ignition and a Tillotson carb. The 70 XLCH was a real
bitch to start when stock. Soon after I bought it I stroked it with a
set of 4-5/8"s inch S&S Flywheels and a set of Dytch 3-1/4" Jugs. Other
modifications to numerous to mention. With the bore & stroke changes I
now had 77 cu/inches with a 10-1/2 to 1 compression ratio. It was even
harder to start than when it was stock. If I was going to ride that
beast I had to make it easier to start. The three things I did that
solved the problem (I'm not sure which one did it) were a set of Sifton
cams with a bit more overlap of the valves . Next was getting rid of
the point/distributor ignition and going to a Bomar magneto and the
left handlebar adjustor. Finally I installed a dual carb system made by
Vern Kramer (damned good system but scarce as hen teeth). The first
dual carb system used two Tillotsons with accelerator pumps on both
carbs. These modifications not only increased the power but made it a
heckuva lot easier to start.

The first place I would start with your 1975 XLCH is updating the
ignition system. I am not sure what kind of new ignition systems are
available for the old Ironhead Sportster motors but I am sure there are
some that would be vast improvements. If a new ignition system doesn't
solve the problem look into the carb and valve timing.

You mention that the bike starts OK when cold but when it is Hot or
warmed up it is difficult to start. Two things come to my mind that
might be causing that. First Maybe the gas in the carb "boils" off when
the engine is hot and it is not refilling properly when you try to
restart it. Check that out and if that isn't the problem maybe it is
valve timing. If the cams are indexed properly maybe your method of
adjusting the valves is the problem. As an engine heats up the pushrods
expand and can change the valve opening and closing significantly. The
old style cork seals on the top & bottom of each pushrod act as
insulators and oil seals. When they are old and worn out they don't
seal or insulate very well. If too much heat gets to those tubular
pushrods they can expand enough to change your valve adjustments. Maybe
there are better pushrods available now.

Good luck finding the problem. I still have an oversized right leg from
all those years of kicking those damned kickstart Harleys.

Dennis

 

The symptoms indicate that your idle mixture is too lean for easy
starting. Richening up the idle mixture might help, except for the old
Harley problem with the rear cylinder robbing the front cylinder of
mixture because there's only one carburetor on a T- manifold.

You don't say what carburetor you have, but modern Harleys use Mikuni
constant vacuum carbs because they are much improved over the older
Tillotson, etc that came on older Harleys. For one thing, they mix air
with the idle mixture and the midrange mixture before the fuel and air
ever get into the venturi of the carb,

This makes for better atomization of the fuel and air.

The whole idea of building a 45-degree V-twin is that it is smoother
running than a single of the same displacement. Both cylinders are
fired on the same revolution.

Since the Harley firing order is a-ONE and a-TWO and a BLUB BLUB,
suction on the intake manifold occurs rather infrequently during kick
starting, the intake events occur with only 45 degrees of crankshaft
rotation between them and the flow of idle mixture barely gets started
and then the suction stops for about 630? degrees of crankshaft
rotation as I figure.

So if your engine managed to suck up some fuel out of the float bowl,
chances are, the fuel droplets condensed out of the air and igniting
the vapors occurs sporadically.

So richening up the mixture to get an easy start means that the idle
mixture is too rich the rest of the time.

If a lean mixture is going to ignite at all, chance are it's going to
ignite too early, many more degrees before top dead center than the
engineers intended, and the expanding gasses push back against the kick
starter with a lot of force.

And then there is the problem of fuel droplets that settle out of the
idle mixture. Every now and then, a burnable mixture will get to the
spark plug at the right time (or the wrong time, if you have a
dual-fire ignition system that fires a waste spark on the exhaust
stroke).

So, if you could retard your ignition a little bit and richen up the
idle mixture a tad, your hot kickstarting would be easier.

 

****************************************************

I'm not so sure.
He said that it started easily when it was cold (denoting a rich
enough condition), but didn't start well after it was warmed up.
This would seem to signal that the mixture was too rich for "hot
starts" (if his problem is carburetion). Witness all the "hot start"
buttons that were put on the current crop of 4-stroke mx bikes, just
for this reason.
Question: Do you have to use the choke, or "tickle" the carb before
you start it cold? If they are not needed, then you may be too rich
at the idle circuit.

If it is not carburetion causing the problem, then I would look at
valve adjustment. Valves with too little clearance is also a common
cause of this sort of problem.

Good Luck and good riding to you!


Wudsracer/Jim Cook
Smackover Racing
'06 Gas Gas DE300
'82 Husqvarna XC250
Team LAGNAF

 

OK, I should have said that his idle mixture is too lean for easy HOT
starting.

Satisfied now?

Have you ever watched a Harley rider struggling to kick start his
engine, and when it finally starts, a big cloud of black smoke comes
out of the exhaust pipe? That's due to low intake velocity and fuel
puddling in the intake tract.

Show me a motocross bike with a Sportster engine in it. Even V-twin
"adventure" motorcycles would have ONE carburetor per cylinder, and the
resulting smaller intake tract has higher inlet port velocity.

Didn't we just go through the reason why multicylinder motorcycles
don't usually have manifolds
with a single carb?

Harley Davidson is probably the only company that would still do
something like that, and they have gone to Mikuni carbs or fuel
injection on their Big Twins.

 

Sir,
I believe that you didn't correctly interpret the gist of what I so
inadequately tried to convey.

Modern 4-stroke engined MX bikes have a "hot start button", not
because of their more modern carburetors, but instead, because the
mixture that is proper for a cooler engine to start is too rich for a
warmed up engine to start easily.
The "hot start button" lets in extra air, in order to lean the
mixture.

While I am not an expert at Harley tuning, I know a bit of basic
4-stroke application and theory. My somewhat inadequate advise
follows that theory.

1.Cold engines need more fuel, so the enrichment circuit (choke,
tickler, etc) is used to enrich the mix for the engine to fire.
2. Warmed engines do not need the mixture quite as rich to start, and
indeed, will flood if too much fuel is in the mix when trying to start
them. (a tattletale of this would be if there was a bunch of black
smoke emitted from the exhaust when the engine finally did start).
Hence, one does not normally use the choke on a warmed engine, when
trying to start it.
3. The hot start button further leans the mixture when used during the
start of a "competition heated" engine.

I hope that this better explains what I was trying to convey.

If the above is incorrect, I sincerely apologize.

Good riding to you,


Wudsracer/Jim Cook
Smackover Racing
'06 Gas Gas DE300
'82 Husqvarna XC250
Team LAGNAF

 

Just a little nitpick, but they are Keihin, and the term is
"constant velocity".


Phil

 

Keihin, Mikuni, whatever...

CV has been understood to mean *both* "constant vacuum" and "constant
velocity" for decades when used to describe vacuum slide carbs.

 

Well, gosh, Gomer. If the solution was as simple as letting in a little
more *air* all the OP would have to do is twist the throttle a little
bit more and leave it there while he tromped on the kicker.

It's a HARLEY, it isn't a modern single-cylinder MX motorcycle, and it
has design eccentricities that some riders would call "character".

One of those eccentricities is being hard to start because of low
velocity in the intake passages and the fact that the rear cylinder
robs mixture from the front cylinder.

 

Depends on definition of multicylinder. I reckon I could name 50 twin
cylinder motorcycles that use/used a single carb, and have run very
successfully. I could also name two single-carb conversion kits for
twin cylinder bikes.

But yes, one carb per cylinder is more common. And I certainly can't
think of any triple or four that used just one carb.

 

Well, how about just listing all the single carburetor 45 degree
V-twins made in Milwaukee to get this thread back on track?

 

Cheers guys.

The carb on the bike is a Keihin (venturi carb) and its workings are
fine.

The points are also set fine and sparking well, as are the plugs - H-D
4R (resistor type)... the spark plugs that I have used on the bike have
never tended to have that 'rusty colour' of a perfect fuel-air mix.
Instead they have always tended to have a slight black-ish deposit, but
having said that the engine runs and sounds perfect when going and a
notable adjustment in the fuel-air mix causes the bike to run and sound
less good... so I have opted for a better running bike than
plug-indicated perfect fuel-air mix!! Surely this should not have
reaching effects into warm starting?

I use choke on start up - generally requiring half choke which is
closed after 1 to 1.5 mins of engine running.

Later this evening I shall try warm starting the bike with the throttle
very slightly open on the live kick and no primer kicks (start with the
easy options first!). I am a little wary of this method and will
probably report back with a sore head from hitting the garage roof from
kick back, or requiring a new carb from complete backfire through the
system, but its worth a try.

For anyone who knows harley kickers my method of starting it is as
such:

Turn fuel tap on

3 or 4 flicks of the throttle to fire fuel into cylinders

Fuel tap off

3 primer kicks (push kicker to just before TDC then kick hard/fast to
neutral position

'Live' kick - push kicker to just before TDC, hold it there, turn
ignition on, wait (if necessary) for oil pressure light to light up,
kick hard/fast from held position.

If bike doesn't start here I turn the ignition off and give 3 or 4
kicks through from TDC to clear the fuel and start again. This
generally works within 3 goes.

Not when the engine is warm though!!

Any further suggestions are most welcome... afriend of mine has
recently suggested trying electric ignition as opposed to points - he
reckons its a godsend on his BSA.

 

*Sigh*

 

Well, there's some quasi-useful information. It's a modern instrument,
and most carburetors do have a venturi, but, are you talking about a
diaphragm carb with a butterfly or a slide valve carb?

If you use unleaded gas, you will never see the rusty color. Spark plug
reading involves looking deep into the spark plug, for a ring of soot
down where the porcelin insulator meets the steel shell of the body.

How wide should the soot ring be? With modern 4-valve engines that have
a lot of turbulence and swirl in the combustion chamber, the ring might
be 1/16th to 1/8th of an inch wide.

If you're seeing that most of the insulator nose is covered with black
soot and just the tip is grey-ish white, all that soot gives your
ignition voltage time to leak away before it rises high enough to jump
the spark gap.

The correct way to do a plug reading is start with a set of fresh spark
plugs and go out on an open stretch of road and ride with the throttle
wide open for a few miles and pull the clutch in as you shut the
ignition switch off. You don't allow the engine to idle, you coast to a
stop and pull the spark plugs and inspect them for the soot ring
inside, as well as noting whether the spark is firing across the whole
electrode, or just in the middle,
indicating insufficient voltage.

Modern Keihin and Mikuni carburetors do not have a real choke, there is
NO choke plate.

Instead, they have a bypass stating enrichener, which is a little valve
built into the side of each carburetor. The valve controls air flow
through a passageway that goes around the butterfly or the slide. Fuel
is sucked directly out of the float bowl.

If you have your carb set up correctly, your engine should start on
full "choke", without fooling around with the throttle setting. But
that's how it works on modern 4-cylinder engine designs, Harley
Davidsons may still have problems with low intake air velocity and the
resulting pooling of gasoline.

That makes sense. You would turn a manual fuel tap off whenever you
parked the motorcycle.

I suppose you're saying this carb has an accelerator pump to help you
flood the cylinders
on startup. Have you tried starting the engine *without* turning the
throttle?

Why? If there is gasoline in the float bowl, engine vacuum will suck it
up whether the fuel tap is on or off.

Well, the wait gives the gasoline droplets time to settle out...

What are you doing with the throttle at that point?

You mean "electronic". All motorcycle ignitions have been electric for
about the last century. An electronic ignition that replaces the
ignition points will reduce maintenance and give a more consistant
spark.

 

Let's just say it could, possibly, indicate a generally rich condition that
will not help your hot starts. Deposits on plugs removed after normal
running rather than after a true "plug chop" test usually tell you more
about idle adjustment then about full throttle jetting.

We're talking about extremely small throttle openings here, sort of
1/32th inch small.

That's not me, but I have some experience with single cylinder bikes.

On my thumper, I would never ever do something like that (i.e. any
action that adds fuel) if the engine was warm. Kicking for five minutes
is no fun when I'm already half dead riding enduro very badly.

Other things I've done to make life in the woods easier is to clean the
air filter often (it really makes a difference) and use ridiculusly
expensive spark plugs (the ones with a thin center electrode made from
unobtanium).

 

Yep, it's me again... the one that started this string... the one with
the so-called problematic warm starting.

NO throttle and NO primer kicks on a warm engine... that's the one that
sorts it - starts as sweet as a nut every time!

Aint it funny how you get caught up in doing things in a set pattern.
I've been using this bike for around 3 years now (my others were push
button starts) and cos it was introduced to me with:

throttle flicks
primer kicks
live kick

That's how I've always attempted to start the bike (cold or warm - cold
it works for, we know that; warm it doesn't)... it's even been
overhauled by by Harley Davidson and some of their techs - and they
were attempting to start it the same way, no questions asked!!

Warm is just find prior to TDC, turn on ignition and kick.

I don't know when it becomes cool/evaporated enough to need priming
again (probably after about 6 hours cos that is when it used to start
via priming after a rest) but I'll cross that bridge when I come to it.

For now I'm just pleased that I put this thread up

Cheers to all who gave tips and advice

 

If you don't need to use the enrichener device either, you should screw
the idle mixture screw in 1/4 of a turn at time until you do need to
use the enrichener.

Your spark plugs will stay cleaner, and you'll get better fuel economy
around town.

 

Try a new condenser.

 

Re: problems starting when engine is warm

Group: rec.motorcycles.tech Date: Thu, Nov 30, 2006, 3:18am (EST-3)
From: (Gazzer)

It's a godsend on any bike. It's just as likely, when a motor starts
easily cold, but not when it's hot, that the problem is in the ignition
circuit. You also might want to make sure the tank is venting. Leave
the gas cap off on your next hot start attempt. Despite the supposed
downside to having a single carb on a T manifold, my '02 Sportster
starts as easily as any Honda I've owned.