[machine shop tom]

Here is a problem I have been unable to solve. I have a gas fouling problem on 2 cyilinders (3 and 4) on my SBC. All the plugs except these two are burning clean. The plugs in these two cylinders will gas (not oil) foul within minutes of start up. I have done a compression test (twice) and all cylinders are with 5 psi. I have done a leakdown test on #4 (seems to be the worse of the two) and found no leakage past the valves. The ignition is a points-triggerd MSD-6A with the red blaser coil. I have tried two sets of wires on it and have changed just the two wires individually with no improvement. The carb (650 Holley DP) and distributor ran fine on the previous engine. Jetting the carb up or down changes nothing with the 2 bad cyls. I've tried Accel, Autolite, Champion, AC, and Bosch plugs. Same &hit.

I've boiled it down to an intake manifold leak, but I'm still iffy on that because there is no oil on the plugs. Checking the intake while running shows nothing wrong externally. The cam is a solid grind so I know the lifters aren't too tight.

Any ideas?

tom

[lluciano77]

Is it possible you have a couple worn exhaust lobes? When an intake valve isn't opening it because very obvious. Exhaust valve trouble can cover themselves up a lot easier. Maybe your not getting a good burn, or you are getting residual contamination due to the exhaust valves not opening enough.

[Pony]

Numbers 3 & 4 are on opposite banks so an intake leak isn't likely but those two cylinders fire after each other 1-8-4-3-6-5-7-2 and are next to each other in the cap. I'd check inside the cap for a carbon tracer between the posts or a possible crack and also double check that they are in the right order in the cap. Good luck.

[1BAD80]

Check the spark plug wires. I've had more than 3 set's of a popular name brand, A something? changed under warranty.
They would make the plugs foul just starting it up.
I switched to Mallory same manufacture of my ignition setup and no problem since.
The new Moroso Ultra 40's are the lowest OHM on the market and will be my next set.
LOL
George

[k-star]

Tom,

Is it a single plane intake or a performer style intake??

keith

[machine shop tom]

My replies to your queris:

Illuciano 77: The valves seem to be opening just fine. I also checked the valve lash and it, too, is OK. If the cam lobes were worn that badly, I'm sure it would show up in the lash adjustment.

Pony: I've had different 2 wire sets an 3 caps on it and nothing has changed. 3 and 4 were always wired correctly (5 and 7 got mixed up a couple of times though--pop, pop, bang!).

1BAD80: I've checked the wires with an ohmmeter, both on and off the capo. I've tried Accel wires (worked fine on the old engine) and have Moroso wires on it now. Same problem with any combination.

K-star: It's an OEM Z-28/Corvette aluminum hi-rise (dual plane). 3 and 4 share the same runner (crossing over the intake) so it may be possible one is influencing the other. Perhaps I will see something when I pull the intake off.

A couple of thoughts come to mind. If it were a carb problem, I suspect more cylinders would be affected. I have thought that maybe the distributor may be at fault, but I checked it and it seemed to be ok. I think I will try another one any way (take 5 minutes to change it) to see what happens. BTW, I have gone from the stock points with an Accel Super Coil to and MSD-5 with the red Blaster coil to the MSD-6A. The only common thing with these three is the distributor. But then again, that was fine in the old motor.

I still think it is a mechanical problem, but the compression tests and leakdown test seem to rule that out. Maybe the air in the tires is too old!

Still open for any ideas.

Thanks!

tom

Anyone want to buy a 69 Camaro?

[87442lover]

This is EFI? Replace those two injectors. scratch that, did'nt see the 69 camaro part.

You try switching out carbs to see what happens?

[4 Jaw Chuck]

Sounds like a leaking accelerator pump check valve, they have a tendency to drip fuel and foul the center cylinders when they leak because fuel gets pulled through the shooters from airflow.

Holley ball style check valves used to be famous for this, the red rubber umbrella style solves the problem.

[docvette]

[color=lightblue][size=3]Doc Here

Try Reducing the Gap on those two plugs, see if they will fire without fouling..

If so, then Hotter?(shorter gap) Plugs , lower resistance wires, maybe even an extra ground wire for the Block might help.

Doc
[/color][/size]

[trees]

Tom, when you get this sorted out, let us know. I am very curious about this malfunction.

Trees

[machine shop tom]

I'm going to put a 600 vac. sec. on it today. That should eliminate the accel pump problem and any other possible problem with the carb.

4JAW, the pump ball check valve won't cause the leakage. If that isn't seating properly, it causes a lean or delayed pump squirt. However, a leaking needle check valve (under the shooter screw) may cause the problem of which you speak. I'll look into that, also.

Trees, if I get this figured out, you'll here me from where your are!

Wish me luck!

tom

[bowtieorbust]

Maybee the air in your tire's is still winter air,or spring air, now just put summer air in them (LOL) have you thought of new plug's in 3and 4 just a thought.

[Jmark]

'Morning Tom, Mark here in Phx.

The one thing that comes to mind is the "points triggered ignition". Any chance that the points are not firing those 2 cylinders right? As in severly worn lobe/lobes or worn out bushings?

If you have a wobbly shaft, you could be getting real poor coil saturation on those 2 cylinder fires as they are next to each other in the firing order. Back in "the day" I used to see lots of trashed distributors that would have a scope pattern that looked like a big "S" on the screen. Dwell was all over the place, eventually dropping a couple of cylinders. Worth a look if you have a scope.

Mark

[BstMech]

Tom, I don't know if your manifold has a passage in front of the carb, or if this could even be your problem.

I had my PCV valve hooked into a port coming right off the runner (instead of the back of the carb) and it would foul the two plugs that runner ran to.

Another thought is to re clock the distributor so that those two cylinders fire from a different position on the cap than they do now. See if the fouling problem moves to another set of cylinders.

Sumpin's screwy ....let us know what ya' find.

[machine shop tom]

Thanks for the ideas, guys.

The thoughts on the distributor seem to make sense (although it didn't seem to have any problems on the previous engine. It was pulled out of that engine and dropped into the new one).

I'll try moving the wires one over on the cap and see if the problem follows them. That should tell me if the problem is in the dist. or the carb. I have another carb. and dist. to try when I get a clue as to which way to go. I'll make changes one at a time so I know exactly where the problem is (or isn't).

I'm not running a PCV at this time. I've also tried fresh plugs in those cylinders to no avail. (I have a couple of sets of spare plugs now).

Keep the ideas coming--we'll figure it out yet!

tom