Progressive misfire on BN1

[ianh]

Hi Chaps,

Haven't posted for quite a while since been tied up re building an old German bi plane (man the Healey felt like a walk in the park in comparison !!)

Anyway having had the Healey (BN1)'professionally' serviced to clear misfire (timing check,new points,rotor arm, condenser) it is worse than ever.

Runs fine for about 20 mins. until gets warm/hot then progressively worse misfiring until finally stops. Applicaation of choke doesn't seem to effect misfire. After cools down for ~20 mins. starts and runs for another 10 mins. then repeat cyle until bored !!

Ignition coil seems to be very hot (too hot to hold), don't know if this is normal ?

Otherwise I would guess fuel vapourizatio, although not sure what has changed since it used to run fine. Pipe between carbs. runs below vent pipe to 100M air box should it run above to take further form hot block ?

Any ideas gratefully received

Ian

 

[Ed_K]

Hello Ian,

It sounds like it could be the coil.
Put an Ohm meter across the primary side.
It should be close to 3 ohms.
If you find it to be significantly different than 3 ohms,
[ can be out either way, closer to zero ohms {shorted) or
higher ohms ] then replace the coil.

Ed

 

[big6]

Did you change the spark plugs? The plugs can break down with heat and misfire.

 

[ianh]

Hi
Yep plugs were also renewed
Which terminals am I checking for the primary side resistance ?
Thanks
Ian

 

[Healey 100]

This sounds like a high voltage ignition problem to me.

Change the distributor cap, ignition wires, and coil. Do it one at a time if you're curious about the cause. I would change the coil first since you note that it is hot -- this is a possible indicator of internal shorts in the coil.

Many problems with high voltage are temperature sensitive.

I had a low speed miss in my BN1 for a long time. I changed just about everything to fix it, but no luck. Finally, the distributor cap (the original looked perfect) was the culprit, and it has run fine for years since.

Good luck, let us know how it turns out.

Bill

 

[glemon]

Are you sure it is ignition, fuel blackage problems often manifest themselves this way. The flow and pressure buld up the sediment at the blockage point, then with no pressure you can get gas again. Frustrating but common problem on Healeys and other old cars.

 

[Michael Oritt]

Which terminals am I checking for the primary side resistance ?
---------------------
Ian--

You are checking for resistance between the + and -, and should be done with all other wires disconnected.

 

[ianh]

Hi
Ok, so I get 3 ohms across the coil although I guess it could start to break down when warm ?
Ian

 

[Ed_K]

It is not as likely to be the cause if it is at 3 ohms. Unless you see evidence that the coil has leaked out a sticky substance, I would try some of the items that others have mentioned 1st. But the coil could still be failing only when it gets hot. If you are still using the original, I would at least swap it out if you can't find the problem after trying the other suggestions. You could temporarily borrow one from another car to see if it helps
your problem.

 

[zblu]

ianh,
make sure the coil you have is not one that requires a resistor, if so i found that a '67 mustang resisitor worked for me before i put in a coil that didnt need the resistor

 

[pan]

Although I agree that the problem might be the overheated coil, glemon might be on the money.
I had an annoying misfire that only occurred when I drove my 100 in competition. After a sustained period of acceleration the engine started to misfire, but the miss went away after a short time. The problem turned out to be the fuel delivery pipe under the car which had been damaged, and was crimped nearly shut.
The carbs were starving for fuel! After I replaced the pipe the problem was solved!

 

[ianh]

Hi Chaps
Have now replaced pretty well all of ignition system !!!including replacement distributor.
On tinkering further with the carbs it seems my BN-1 with the 100M mod kit actually has AUC 866 SU's as used on the later models as opposed to the AUC 739T which I believe the 100M should use.
The give away is mone has 4 nuts securing the float chmaber rather than 1 banjo
Can anyone thorw any light on whether this is normal or not
Thanks
Ian

 

[ianh]

On further review it looks like the AUC 866 is code for the SU-HD8 used on the BJ8 which is clearly a much later model than the AUC 739T which is guess is code for the SU-H6 on the 100M.
Sorry for all the codes probably got you as confused as me now.
Ian

 

[Dave Russell]

The original 100M carbs were H6's.(1.75" dia) They had a banjo central top bowl vent that was directed to the air box, not overboard. This to equalize the bowl pressure to the incomming air pressure. The banjo requires a special internal scalloped gasket between the carb top & the banjo to permit air vent flow.

I can see no reason to need the larger HD8 carbs (2.0" diameter) on this application. Maybe some unknown to me complications. All SU carbs that are "too large" for the engine will accomplish is that the carbs will not fully open under maximum flow requirements, you will be running on a leaner section of the metering needles than intended by the original design. I would try to obtain the correct carbs.

Regardless of the differences, a common problem is blockage of the top bowl vent by using the wrong sealing washer, not internally scalloped, between the banjo & the carb top. Very easy to do because the lower banjo gasket washer is scalloped & the upper one is not. If the vent is blocked, fuel coming into the bowl can only be forced out through the main jet & the bowl is never allowed to properly fill.

Sometimes HD8's have a short metal vent from the banjo & a long plastic extension which can melt shut. Same venting problem.

Just something to check.
D

 

[BUNDYRUM]

hello ianh,

I just looked at the Burlen Fuel Systems website and they list the AUC 866 as the HD6 carb for the 100-6 and 3000 mk1. The AUC 866T is for the pair of carbs.

You're right the AUC 739 is the 100/M carb and once again the AUC 739T is for the pair of them.

I'm a little confused when you say that it has 4 nuts securing the float chamber rather than 1 banjo. Do you mean where the float chamber attaches to the carb body.? They should be "common" screwdriver slot headed screws.

bundyrum.

 

[ianh]

Hi Chaps
Ok, so after much confusion and mixing up of numbers the outcome is we have the SU HD6 (AU866 in Burlen code). This as I understand is a later model than the H6 used on the 100M but has the same 1.75" bore. Have stripped them down and they look fine with the exception of a broken spring in the base of one float chamber. Now rebuilding with full service pack.

One float chamber lid has a slot cut in it which precludes the use of a bango for the vent line since fuel will simply vent from this notch so am planning to replace this lid and get 2 bango vent lines.

Does anyone have a diag or pic of how the vent lines return into the 100M air box ?

Sorry for a bit of wild goose chase but seem to be getting it together now

Many thanks


Ian

 

[mikespain]

As this is a problem that slowly develops with usage and recovers with non usage,if it was something broken or put together wrong or wrong part I would expect the problem to be there all the time rather than developing with usage,first idea was coil,but as this has been changed and problem persists could it be vacuum build up in the tank or a feed pipe collapsing?-just a thought

 

[Dave Russell]

Hi Chaps
Does anyone have a diag or pic of how the vent lines return into the 100M air box ?
Many thanks
Ian

Ian,
See the attached pic. The vent lines are just a short straight run from the carbs to a couple of holes in the air box. I've already mentioned the banjo sealing washers.

The lines were originally sealed to the air box with rubber grommets that were shaped sort of like "Hershey's Kisses". Any grommet that will fit & form a reasonable seal will work.

You could fill the vent slots on the one lid with a good epoxy such as "JB Weld".
D

 

[ianh]

Perfect Dave
Thanks very much
Ian

 

[jyjim]

around gas use 24 hour set JB weld, all the others will wash away. i used the quick set to smooth a intake manifold, after a year it had washed away i called texas JB weld and they said use 24 hr set.