Lemans 1000 problem diagnosis seeking help...SOLVED

[lazlokovacs]

Hi everyone

Lemans 1000

symptoms - bike will start and idle fine, after 5 minutes of riding, one cylinder begins missing/backfiring through the carb and eventually cuts out completely. Using the flip coke will bring cylinder back to life, for a short while.

Theory - lean condition in the problem cylinder

-Valve clearances have been checked.
-compression test with WOT reads around 150psi from cold.
-3 different carburettors have been tried!
-Fuel taps have been swapped over and back again.
-Brand new coils, 4 different coils have been tried.
-HT leads have been swapped around.
-timing (controlled by ((laboriously filed)) points) is bang on the money.
-new advance springs fitted, advance is advancing, confirmed with strobe.
-brand new spark plugs.
-when balancing carbs with vacuum gauges, the levels on the gauges do not fluctuate and are responsive to throttle stop screw and mixture screw.
-intake gasket replaced and no traces of carbon at the intake
-problem plug is sooty black all over(after running with the choke) except the ground electrode which is almost white

Number one suspect is an intake leak, (though vacuum gauges are behaving when balancing carbs) or even exhaust leak? I have visually inspected...

What is the definitive method for tracing a leak? and could an exhaust leak create a lean condition?

Any other ideas welcome!

[Triple Jim]

Have you changed the condenser?

An exhaust leak isn't the problem.

[dilligaf]

One of your coils is getting hot and cutting out.  Try swapping the coils.  :BEER:
Matt

[pressureangle]

Sooty black plug, picks up with choke, good compression; specifically diagnostic for a weak spark.
You probably have a bad connection somewhere. Use an ohmmeter across each connection in the harness (after disconnecting the battery) from the distributor through the coil on the bad side and back to ground. You should not see more than .2 volts drop at any one connection and no more than .5 volts from end to end.

[Triple Jim]

Also, as I asked in your similar thread:

quote:  What coils did you buy?  What is their primary resistance?

[Kiwi_Roy]

Have you tried swapping the leads over, I had a problem recently with carbon core leads on an EV.
Actually pull the plug caps and measure the resistance from cap to chassis, it should be very similar on both sides.

[Dick]

You can expect a sooty plug after running on choke. Try another plug and as soon as problem arises shut down and pull plug for inspection. What type fuel line delivery set-up are you running? A line splitter to single carb banjos? Direct line from petcock to each carb? Double carb banjos? Is tank venting properly? Hard to believe three carbs would be bad? Good luck.

[lazlokovacs]

thanks everyone,

forgot to mention new condensors have been fitted...

I have a set of dyna coils and standard coils, the problem is there regardless of which coil I hook up... I think I've tried 4 now!

pressureangle, could you talk me through your idea in any more detail? the connection from the distributor to each coil is just one wire for each point no? (these I have recently replaced crimped and soldered...) and the coil gets it's 12v through the engine cut-off switch...

I have already measured and there is no drop in voltage between the coil terminal and the points...

direct fuel line to each carb

[Dick]

Could you have a sticking valve, causing compression loss? Any change in tappet noise when problem starts? Just throwing out random thoughts. Good luck.

[pressureangle]

thanks everyone,

forgot to mention new condensors have been fitted...

I have a set of dyna coils and standard coils, the problem is there regardless of which coil I hook up... I think I've tried 4 now!

pressureangle, could you talk me through your idea in any more detail? the connection from the distributor to each coil is just one wire for each point no? (these I have recently replaced crimped and soldered...) and the coil gets it's 12v through the engine cut-off switch...

I have already measured and there is no drop in voltage between the coil terminal and the points...

direct fuel line to each carb

you have to check the entire circuit through the system, all the way from the battery through the points. Since you have a problem on only one side, you can probably disregard anything upstream of the point at which the left and right circuits diverge.

After reviewing a few diagrams, it appears that the coils are tied together with a short piece between them, as they are on mine. Have you replaced that connector between the coils? If not, do so. More than a few times I've found a poor connection at a crimped connector, particularly where they get hot like coil terminals.

If you want to be certain, cut it out with enough of the main lead so you can verify the resistance across both terminals and the incoming wire.

Clear?

[JoeW]

If it picks up with the choke, it's probably not spark. more likely that carb is running out of fuel. Check the inlet screens behind the banjo fitting on the carb, pull the float bowl and look for debris. If there are in line filters, make sure you can blow through them. Also try running with the gas cap off, a clogged cap vent should affect both cylinders though.

[pressureangle]

A lean carb will not blacken a plug and if the choke was doing it, you'd see it on both sides equally, if the choke activates both sides.

[Tom]

What year?  LM IV or V?

[Chuck in Indiana]

If it picks up with the choke, it's probably not spark.

Oh.. It could be. A weak spark will fire a very rich mixture, but not a proper one. Don't ask me how I know..  ;D

[Dick]

A lean carb will not blacken a plug and if the choke was doing it, you'd see it on both sides equally, if the choke activates both sides.

Good point. I was thinking individually activated plungers.

[lazlokovacs]

you have to check the entire circuit through the system, all the way from the battery through the points. Since you have a problem on only one side, you can probably disregard anything upstream of the point at which the left and right circuits diverge.

After reviewing a few diagrams, it appears that the coils are tied together with a short piece between them, as they are on mine. Have you replaced that connector between the coils? If not, do so. More than a few times I've found a poor connection at a crimped connector, particularly where they get hot like coil terminals.

If you want to be certain, cut it out with enough of the main lead so you can verify the resistance across both terminals and the incoming wire.

Clear?

yep.

as soon as I wrote the words 'kill switch', I pegged it down to the garage to check... sure enough, I'm losing a whole volt just before the kill switch, so the coils end up getting less than 12v... so I guess it'll tick over fine, but when they have to work harder, faster, the spark gets weedy if its created at all... also explains why holding the plug against the engine case and seeing a nice fat spark will only tell you so much...

I'll wire it all up properly tomorrow, better than the professional mechanic i paid to do it last time and then report on how it goes.

thanks so much for the input

[pressureangle]

Nice.

High-resistance connections kill current, and heat quickly-which worstens the resistance. What runs good cold may get worse as you ride.

Wondering about one side only, current, coil rise time and firing sequence. Interesting.

[guzziart]

Hey Lazlo, for what its worth, I installed relays to power up the coils & headlight.  I first caught wind of the voltage drop with the headlight, it was dim.  Upon further investigation, the VD was across the headlight switch.  I cleaned the switch contacts but figured it would continue to be a problem so, installed relays.  The kill switch wasn't in any better condition and found that I had a VD across that too but I don't recall it causing a noticeable problem.  Anyway, I got the headlight relays & harness from Eastern Beaver and a relay w/fused buss bar from them too for the ignition coils.  And, I'm pleased with the quality & performance of the easternbeaver.com stuff. Good luck!

[Tom]

Is the problem on an '87 LM IV SE???  Right cylinder?

[lazlokovacs]

1989 lemans 1000 not a SE...

Latest news is this,

rewired the coils, they are getting a better voltage, still missing on the right hand side, it will idle perfectly and behaves with the gauges on, once on the road, the right cylinder begins to miss, it gets worse and worse...

I just checked the voltage at the coil again, it has 0.4 voltage drop from the battery voltage (an improvement from the 1v of yesterday!) but I also noticed it drops another 0.4 when the headlights are turned on, is this normal? (bike not running)

[oldbike54]

 Have you tried a new plug on the offending side ?

  Dusty

[lazlokovacs]

Have you tried a new plug on the offending side ?

  Dusty

yes, swapped em round, tried new ones, same deal...

[oldbike54]

OK , try this . When it begins to miss , shutoff the petcock and turn of the engine at the same time . Very carefully remove the float chamber on the offending side and check the fuel level . This will tell you if it is a fuel problem . Alternately , remove the top and slide assembly , and use a broom straw poked through the main jet into the float chamber to measure the fuel level . Level MC of course .

  Dusty

[lazlokovacs]

update...

Took the heads off and had them reconditioned, put them back on and voila... Thought I'd change the coils again as well.. bike seems to be bang on the money.

Sticking valve then? Or a run of bad coils?

Thanks to everyone for their input

[Chuck in Indiana]

update...

Took the heads off and had them reconditioned, put them back on and voila... Thought I'd change the coils again as well.. bike seems to be bang on the money.

Sticking valve then? Or a run of bad coils?

Thanks to everyone for their input

Hard to tell when you do two things at once.  I'm still betting electrical.

[nunzio]

update...

Took the heads off and had them reconditioned, put them back on and voila... Thought I'd change the coils again as well.. bike seems to be bang on the money.

Sticking valve then? Or a run of bad coils?

Thanks to everyone for their input

Thanks for the update!! It helps me with my diagnostic skills to know .

+1 on "Chuck from Indiana's" comment.

I make a real effort to do only one thing at a time when either adding stuff or trying to fix something..but I'm a slow learner....Alan

[lazlokovacs]

I hear ya... normal procedure is to only change one thing at once... in this case I had the bike stripped and couldn't resist changing the coils while I was there... compression is up from 140/120 cold to around 170 both sides and it is now pulling like the proverbial schoolboy. Which may point to the valves...