miss in engine

[incoma]

Let me start by saying I've swapped distributors, wires and plugs. Nothing has helped the problem.

I'm missing on cyl 3, 5 - 2, 8. What would cause something like this?

[jhow66]

How did it start this?

[70c10swb]

you swaped the dizzy but did you change the cap and rotor. are you sure your wires are correct?

[incoma]

I bought a used crate motor from another board member. I'm recently installed it, and this is when I found these problems. So I don't have a history with this motor, nor is there a point in time when something was changed.

[jhow66]

Sounds like you have a 4 cylinder ignition module in your HEI-see photo.

[incoma]

Well, its happening with 2 different distributors. I've swapped the wires on different cylinders and swapped plugs. When I pull a boot off of the distributor while its running, I do see a spark jumping from the distributor to the wire boot. When I do this, the engine doesn't run any worse. I'm missing something simple.

[sneakysnake]

Pull the #1 plug, put your finger over the hole, turn the crank until it is on the compression stoke,check your timing mark on your crank, pull your distributer cap and see that the rotor is pointing #1 wire. if so then check firing order, if not then change wires to right order.
If all the wires are correct then do a compression check to make sure you dont have dead cylinders(stuck valves or a head gasket)

[gn4u2c]

yeah could be alot ,1. check wires, 2.check for a cracked plug (night time) 3.comperssion /leakdown could have a cracked head or bad gasket,4 valve cover off all rockers moving?valves moving? lastly could be losing the cam on those cylinders.(hope not) .1 at a time you will find it.

[Professor]

Just a thought, and someone else please think through the logic of this.....

The plug is supposed to fire as the piston approaches TDC on the compression stroke. IF it were to fire just BTDC on the EXHAUST stroke, you'd have fire at the plug, just on the wrong stroke. The result would likely not produce any major negative consequences, other than the appearance of a "dead" cylinder.

Most small gas engines, that operate off of a flywheel magnet-driven ignition system, fire BTDC on BOTH the compression and exhaust strokes. Since there is no air/fuel mix in the cylinder on the exhaust stroke, nothing happens.

Now, if somehow you have the spark plug wires in the wrong places AND/OR the distributor 180 degrees out of time, this may allow alternating cylinders to fire on the wrong stroke. This could produce the appearance of 4 dead cylinders.

I did not take the time to diagram this cycle out for all 8 cylinders, but just my $0.02 worth.....

[boundstaffpress]

My guess was going to be the dizzy is out 180 degrees. I had a fight like this with my chevelle one time. If I left it running on 4 for 10 seconds or so I'd get a back fire.

Could be 180 degrees plus or minus one tooth on the cam. Sometimes when you pull the distributor the oil pump moves a little allowing you to advance the timing one tooth on the camshaft.

I know your situation. Very disheartening.
Good luck.

[woodridge]

Does it continue to miss above idle? If it does and you have spark at the plug and it is wired correctly I would suspect something else. I it smoothes out above idle I would check the idle circuit in the carb. Many times I have had one that would idle rough (miss on four cyl.) then smooth immediately out when the throttle was cracked. If this is the case you might try screwing the idle screws completely out and then blow with compressed air through the holes, reinstall the screws turning all the way in then out a couple of turns and see if this make any difference. My 2 c's

[incoma]

Thanks for all of the ideas!

I have had the cap off several times, and double checked the balancer mark, and #1 cyl is at tdc. I'm not getting ANY backfiring, so I am nearly positive that the dizzy isn't 180 off.

I found my compression tester this evening, and plan on putting it on my #3 cyl to see what the compression is.

A guy at work also suggested that this is a carburator problem. If you throttle up, it does seem to run a bit better, but not a whole lot. I'm going to try to clean it out this evening also, since that's a really quick thing to do. I was tuning it the other night and if I turn in the passenger side mixture screw all the way in, the engine doesn't run any different. I'm going to see if some compressed air makes a difference.

[incoma]

question on compression test - when I hook up the gauge - do I start engine to test compression or just have someone turn it over for me while I have the gauge inserted?

[incoma]

I checked my compression and have between 125 and 130 psi on my left bank cyl. I didn't check the right bank.

I found an old chilton's in the garage and thumbed through it. In the back it suggests that my carburator is clogged or not flowing correctly. I fired up the air compressor to about 40psi and blew air through the right bank of the carb. It didn't make any improvement. I'm going to call a few local carburator shops tomorrow and see what someone would charge to rebuild this Edelbrock. Everything I've tested is pointing to it.

[71-longbed]

make sure the driver and passenger side idle mixture screws are out an equal amount of turns .. if your runnin a rochester quadrajet

[Ron 586]

Do you know if the intake was off or changed out. Start engine and spray carb. cleaner around the intake/head area. Also around the vacuum ports in intake plugged off or otherwise. Be careful not to let the mist get up around carb. inlet or you won't be able to tell if you have a problem or not. If you gain rpms (while spraying) you have a vacuum leak. From the sounds of things you have been fighting it for a while. When you find it it will be something simple, just overlooked. Good luck.

[incoma]

I finally pegged the problem....


I have an edelbrock carb with square bore configuration, and a dual plane edelbrock intake. I took the carb off and installed it facing backwards. This way if my carb wasn't spraying evenly, I would see different cylinders mis firing or in my case not firing at all. This nailed the problem. Before the swap, #2,3,5,8 weren't firing. After the swap, #1,4,6,7 aren't firing. The diagnosis - I've got a clogged idle/mixture port - specifically my right side. I've hit it with 40psi of air and that didn't help. I've sprayed countless jets of carb cleaner in the hole. Should I turn up the air pressure and try again?

[woodridge]

Sounds like you may need to dissmantle it for a thorough cleaning. Dried fuel deposits can be really tough to remove. I have had to use wire drills at times to get a passage clear.

[incoma]

That's what I'm concerned about. I thought about buying a bucket of carb cleaner to let the whole unit soak overnight, but I'm not sure that will do the trick. I think this carb needs a hot pressurized bath to get clean.

Incidently woodridge, I love the Green GMC Sierra Grande. Mine looks just like yours. Here's a pic just for kicks.

I've got 255/60/15 tires on it now and I think I'm going to raise the front up about 1.5". I want the truck to sit without a rake.

The trim and badging are going on as soon as I work out some other glitches.