86 K20 Weak Spark

[palposity]

Hey Folks,

I need some advice. My K20 was running pretty good and all of a sudden would not start back up at the gas station after a fill up. I towed it back home with my diesel. The next morning it fired right up then died after a few seconds. I checked for spark and it seemed weak, but it was there. I put a meter on the hot wire going to the distributor and it was at 12V, but when cranking the engine it would drop to 6V or so. Is that normal? The truck will no longer start, but during cranking a cylinder or two will fire from time to time. I changed the ignition module inside the distributor, new spark plugs, I'm wondering if the coil inside the distributor cap could be drawing the current down, or would it have to be a short somewhere between the ignition switch and the distributor?

Thanks for your thoughts on the matter.

[LiveAnimals]

I would look into replacing the coil (which acts like a transformer, taking a 12 volt signal steps it up to 10,000-20,000 volts, that energy returns to the dist. and is delivered via the rotor to a particular cylinder based on timing.

[Mike C]

Yup. Most likely the coil. I would think about replacing everything. Check out Jeg's performance HEI with the polished housing. Low saturation high output coil, performance module, cap & rotor. They have it on sale right now for $100. I just bought pretty much this same one from Eastwood for my '86 K30. They ask $100 regular price but had a 15% off and free shipping that expired last day of November. Might be another coupon out there if you look, though.

[old Rusty C10]

i would replace the coil too... and be careful i bought an aftermarket one a few years back and it was defective.. turns out two others off the shelf in the same auto parts store were also bad... bought a AC Delco in another place and the vehicle is still running great

[palposity]

Hey guys thanks for the advice. I ended up having a spare coil from another truck that was known to be good. I replaced it but still can't get the engine to start. I pulled a spark plug and it had fuel on it so I'm getting gas, but not kicking over. I'm wondering if there is something goofy with the carb.

[GMC Guy]

make sure your battery is still ok, a lot of cranking can run down a battery pretty quick.

[Mike C]

The Module will sometimes do that, but not as likely as the coil. Next, replace module. Check to make sure you are getting 12 volts to the cap with the key on. Honestly, I had these same issues with my 86 K30 which is why I ponied up the $85 for the ALL NEW distributor.

[palposity]

Okay I'm really showing myself to be a rookie her, but I'm starting to think that the spark may be okay, but the carb is dumping too much gas into the cylinders. I pulled one of the plugs and it's absolutely soaked with gas. When I get home from work tonight I'm going to see if I can swap the carb with a spare that I have that worked okay before it was pulled a couple of month ago. Will advise progress.

[old Rusty C10]

if there is no spark/weak spark that will happen.
What you need to do is pull off a spark plug wire and stick a screwdriver in it. you then put it so there is a space between the shaft of the screwdriver and a ground of about a quarter inch. have someone turn the key and watch. your spark should be bright electric blue.. anything less is bad spark.this will eliminate your thoughts about the carb If the coil is new and known good there is a module in there and thats next
NOW that being said if your plugs are covered in gas you will need to clean em and re gap em or replace them as too much gas will make the truck not run.
You should also leave the plugs out for a spell so the gas in the cylinder has a chance to evacuate so it dont flood again.
From what youre describing im thinking its more of a spark problem than a carb problem but then again im not looking at your truck

[palposity]

Well, Thank God I've got some resolution to the situation. I changed the carb and cleaned up the plugs and she fired right up.
I do have a bit of a timing problem though....It seems a little too advanced so I am going to have to adjust that after work tonight as well as change the oil to drain out the 1/2 quart of gasoline that has drained into the crankcase during this process.
Thanks to all of you for your help!