Possibly knackered CDI unit? Circuit diag anywhere?

Being an electronicky person I have no qualms whatsoever about delving
into the tinny depths of the Unit of my 1981 Yamaha XV750SE, however,
to save me working out the circuit diag, has anybody done this already
- or know where I can find a copy?

The problem is that, once running, the beast goes fine, but on
starting (and overrun), it appears "over advanced". Elsewhere I have
heard that something in the CDI retards things under these conditions
so I would like to find out which 'bit' this is and sort it. Its
really struggling to turn over against the first firing - despite new
Yuasa batt.

A thorough electrical (and valve clearance) check has revealed no
other issues. I never had this problem when I first got the bike - any
help gratefully received.

Cheers

Chris D

 

In my experience, CDI units either work or they don't - there's no
halfway house. And as the XV750 is notorious for its self-destructing
starter motor clutch, I'd suggest the problem lies here. I'd also check
things like air intakes, carb diaphragms etc to sort out the over-run
thing. Either way, I bet it's not the CDI.

 

CDI has become a generic term, like "aspirin" and "coke". People
diagnose problems
as being a "bad CDI unit" even when the motorbike in question does not
have a *capacitor discharge ignition syetem*.

I remember the first real CDI units. They would charge up a great big
capacitor with a
DC to DC high voltage converter. I would hear the oscillator inside the
converter whining at high frequency when I turned the ignition switch
on.

I don't think your Yamaha even has a CDI, but Hondas have CDI's that
get their needed high voltage from an extra exciter coil in the stator.


If there are only three wires coming out of your alternator stator, you
don't have a CDI.

Yamahas and Suzukis use a simple transistorized ingition module with a
built in "advance curve" which is seems actually to be a built in
*retarding circuit*.

The retarding circuit would count the number of pulses from the trigger
coils and retard the ignition accordingly.

Two transistors take the place of ignition points. Power goes from the
ignition switch, through the coils, through the transistors, to ground,
and the signal from each pulser coil has to be delayed by the retarding
circuit.

Well, if your starter is struggling to start the engine when it's warm,
I would suspect a lean idle mixture, low fuel level in the float bowl,
or a malfunction in the starting enrichener system. As I have discussed
the cleaning and adjustment of constant vacuum carburetors endlessly in
this forum, you can google for such things as "transition ports" and
"EPA anti-tamper plugs".

As I said to Checkers in a previous post just a few minutes ago,
constant vacuum carbs are adjusted marginally lean at the factory and
the screws are concealed by an anti-tamper plug.

Constant vacuum carburetors are very prone to gumming up and this
causes all sorts of starting problems that can be avoided by running
carburetor cleaner through the fuel a few times a year.

It's not impossible that a previous owner has tinkered with the
ignition timing though.

Inline fours have a nasty flat spot between 5000 and 7000 RPM and
owners can somewhat overcome this by slotting the screws that secure
the pulser coil plate and advancing the timing by about 5 degrees.

I wouldn't think a Vee-Twin owner would feel he needed to do this
modification.

 

IRTA electropanicky


Nick

 

i have a cdi from one of the 78-80 triples in my pile
somewhere.

 

there are fixes for the starter. check the yahoo groups.

 

sort out the over-run

There are indeed.

 

Read the OP. It's not struggling to *start* it - it's struggling to turn
the engine over. Slightly different.

This is a common fault on these engines and, as I've said, is usually
down to the crap starter motor design on the early vee-twins. Some
dreadful Bendix thing, IIRC. And as another poster has said, there are
fixes posted on the interwebthingy.

I'd investigate the starter motor before pulling the carbs apart.

 

Yahoo groups is quite good for finding owners groups who may be able to
help.

It is unlikely to be the ignitor BTW.

If you thinks its struggling to start, take the plug caps off, insert spare
spark plugs in to the caps and ground them. If the motor still struggles to
turn, it has to be the starter as the ignitor is just firing the plugs to
the open air.

The ignitor in bikes of this vintage is quite simple ( usually ). No
throttle position sensor to worry about for one thing. It careth not a
flying ferk about throttle position and knows nowt about overrun. All it
'knows' is RPM and a preset amount of advance. Most don't have a curve,
just a series of steps.

Find the base advance from a manual and start the engine. Use a strobe lamp
to see the timing marks. Some dismantlement may be needed to find them.

Next, get someone to hold the throttle at 500 RPM above the step for the
next amount of advance, ie, manual says 6 deg BTDC@1200RPM, 12 deg
BTDC@4500RPM and gives no other values, you may only have two advance
settings. If these values match what is seen, the ignitor is OK.

My money is on an air leak twixt carb(s) and head(s).

HTH

 

Trouble with posting to as many groups as you have, you get as many
opinions, some of them utter bollocks.

These bikes are known for weak starters, as has been said, so it's best
to check the obvious first. If the starter's weak, there's a mod which
involves throwing the original away and fitting an XS750 one, though
these are getting rarer nowadays.

 

barstow is not the edge of the desert, it's 25 miles up the
desert's ass.

 

Your revelation has turned up a little too late. HST is dead.

 

I have reached the 'sorting the wheat from the chaff' stage of
desperation

Interesting ta

Chris D

 

Ta, will do

Chris D

 

I remember building one

Acknowledged. In some places in the manual it is called a TCI
(=Transistor Controlled Ignition?)

By 'number' did you mean 'frequency'? If so this is getting
interesting...

Even more interesting. So what (I suspect) we have here is a circuit
where the transistor is basically triggered - probably via a
capacitor. A simple RC combination could then be added as an
integrator to modify things at low revs (ie 0 > starting)...

If the integrator components were 'ineffective' then that would mean
that firing under normal running conditions would still be ok but not
starting. That could be it - if only for a circuit diagram...

Might be worth doing anyway as the beast stood a while before I got it
- and after I first used it.

No, this is fine. When I first looked at the spinning disc in the
timing light, I thought it was way too advanced, then I realised that
what I thought was a displaced 'F' (=Firing) mark was actually the 'T'
(=TDC) mark and that the 'F' was invisible when lit 'head on'. When
not running, putting the T where I had seen it by the timing light
puts the F exactly right.

As an additional check I put the timing light on the other (front)
cylinder and saw the TDC line in exactly same place as I had
previously seen T mark. Therefore both cylinders are apparently firing
correctly.

Thanks for all the ideas.

Chris D

 

Grimly Curmudgeon's weak starter advice aside, what about checking the
battery connections to the starter relay for corrosion? My Trident 900
suffered from a sluggish (and then dead) starter last year, and the good
folk of this NG suggested I check the relay connections. Once they were
cleaned, the starter had a whole new lease of life.

 

hst?

 

HST is not Harry S. Truman, it's Hunter S. Thompson, the "gonzo
journalist" whose style introduced himself into his stories. You can
google for "The Sausage Creature", which is about his ride on a Ducati.
It was orinially published in Cycle or Cycle World magazine. Half the
writers of letters to the editor threatened to cancel their
subscriptions, the other half wanted more HST stories.

HST's other major involvement with motorcycling was "Hell's Angels",
written around 1967 after the HA's were involved in a gang rape on a
beach in Monterey or Santa Cruz. HST also wrote a newspaper story in
defense of the HA's and how the gang rape was really just a bunch of
people partying hearty and that the victims were willing, up to a
point. That's also available, in the rec.motorcycles NG.

HST was personal friends with controversial San Francisco intellectuals
such as William S. Burroughs, who served a prison sentence for
murdering a man who made sexual advances to him, and Alan Ginsburg who
wrote the banned poem "Howl!".

The line that got "Howl!" banned involved how "the finest minds of a
generation" squealed with delight when being sodomized by the "saintly
motorcylists" who were obviously the Oakland Hell's Angels.

HST committed suicide about two years ago, in his Colorado home. His
friends promised to fulfill his wish to be shot out of a cannon by
shooting his ashes out of a cannon, but I haven't heard anything since.

 

I'm amazed that anyone can do a biog, however brief, of HST and not
mention 'Fear & Loathing'.

 

Worth doing simply as a matter of course, but i'm still sure the
reluctance to turn on the starter means the motor assembly itself is
fucked.

They *all* do that, sir.