Airstream Forums

Airstream Forums (
-   Mechanics Corner - Engines, Transmission & More... (
-   -   Number 8 Plug (

swebster 07-31-2004 07:00 AM

Number 8 Plug
I keep noticing that my number 8 plug gets all carboned up. (345 MH on a P30, 454 with a QJet.) I used to be running too rich on all plugs until I found that the 12V feed to the ignition was burnt by my headers and down to one or two strands. Replacement of this wire seemed to improve performance and mileage and plugs 1 - 7 all look black/brown now. But number 8 is sooty and smells like gas.

Background: New Distributor, Coil, Accell 7000 Hi Temp Wires, Bosch Platinum Plugs, Rebuilt QJet, Headmann Headers to 2.5 with crossover and free flow muflers. Number 8 is the rearmost cylinder on the passenger side.

Compression on that cylinder is 165. (it 160 - 165 on all the others as well).

I'm going to replace the plug but does anyone have some thoughts on why this one is having a problem? Is this a sign of a larger problem? Are there additional tests I can do?

IPM 07-31-2004 09:14 AM

It's the Platinum Plugs. I had the smae problem long ago with them on toyotas, volvos, rovers, and bmw's. I gave up and went back to plain reliable copper plugs for inexpensive plugs. I'll use bosche silver tips when I want to get fancy. Yea they are really silver tipped. Work great in the old bimmers. Been thinking of using them in the airstream. Last 50,000 miles in the bimmers (clean and regap). Pricey, so I shop around. $3.50 a plug is a decent price.


74Argosy24MH 07-31-2004 09:18 AM

Isn't this a fairly low mileage engine? Compression sounds good, worse case I would say is valve seals on that cylinder. Any blue smoke in the exhaust on start up? How does it run, stumbles, misses, backfires, good power? Have you checked vacuum? Should be above 15" and steady. Good spark at #8? Hook a timing light on that wire and see if it fires consistently or pull the plug and check, it should be a very hot spark. Move the wire around, sometimes they will make intermittent contact or short to ground. This is very high voltage and it will follow the path of least resistance. Run it in the dark and see if there are any sparks where it is shorting. What is the timing set at?

It sounds like you are running rich if all the insulators are black/brown, they should be lighter. You could also be using too cold of a plug, try a hotter range (higher # is hotter).

Ideally all the insulators should be the same color. That means all the manifold runners need nearly identical volume and distance from carb mount flange to cylinder. In reality the low rise manifolds get the mixture to the cylinders and there will be a difference. One shouldn't be sooty, but I would try for a leaner mixture overall (or hotter plugs if you think it is running ok) then see how #8 looked if everything else checked out.


IPM 07-31-2004 09:23 AM

They don't clear once fouled.

Wife got stuck in MO with her volvo because the platinums fouled so easily on a cold winter start. I was not there so she had a mechanic look at it. He swapped in cheap copper and all was cured. She saved the platinums for me to see. They were just a couple of months old and were black and fowled.

Copper plugs are cheap. Maybe buy one just for #8 and see what happens?

I've no idea why #8 is more sensitive. But with compression that good, and good oil pressure, I'd not worry about it if a different plug stays clear. Assuming that's a cure.


BobbyW 07-31-2004 12:00 PM

I have to agree on trying the plugs BUT don't overlook those Accell 7000 wires. My Chevy truck is the same way. I tried some heavy duty Accell racing spark plug wires in my current Silverado CK1500 one time. I actually lost gas mileage and the thing ran really rough. I put new Delco's back on it and it purred.

The truck, like yours, also has Gibson headers, Borla exhaust, Airaid K&N filter and a Hypertech chip. The wires brought it to its knees.

I used to try all the fancy plugs in my little Japanese cars. I always ended up going back to what the factory put in them to begin with. I had a Subaru that would not run right at all unless it had the exact NGK plug in it.

The rule of thumb is use Champion/AC plugs in US built engines, NGK plugs in Japanese engines and Bosch plugs in European engines.

swebster 07-31-2004 12:07 PM

Thanks all. Sounds like a switch to the old reliable Champions is in order. I'll also look at running a hotter plug.

John. It starts right up but does run a little "rough" until it warms up. I do get a little smoke from the right side on startup. Barely noticeable but there. It clears up as the engine wakes up. Once shes running it seems fine. Power, no backfires. Pretty nice.

All of the plugs used to look like this until I found the low/no volatage going into the coil. Everything else is cool now except the number 8.

I pulled the valve covers in the spring (when I was having trouble climbing hills and looking for things like broken pushrods or stuck lifters, etc). I didn't notice how oil control was setup in the 454. Do they use those little rubber boots or just rely on the valve guide tolerances for oil control? Can't remember from my 350 rebuild days. If it is the valve guide what is the worst case? A top end? Can I run like this without harmful effects?

74Argosy24MH 07-31-2004 12:43 PM

They use a rubber seal for oil control (umbrella seal). Worse case, pull the right head and have new guides installed. BBs are kind of weak here and it is pretty common. The old guides can be knurled but if I had to pull a head I would splurge and be done with it. You can run like that, as it gets progressively worse there will be a larger and larger cloud of blue (not black, that is rich mixture) smoke on startup, just watch and decide when it is time. Rough running cold is probably just the norm, it needs heat to evaporate the gas and get the choke to pull off. Unless it was really bad I wouldn't worry about that.

The higher number/hotter range is for AC plugs, don't know about Champion, you will have to check at the parts store. Hate to see you get a colder plug and more problems.


ALANSD 07-31-2004 02:51 PM

I have those Accel wires and have not noticed any fouling yet. Running ac plugs.

swebster 07-31-2004 04:22 PM

Alan - switching to these wires virtually eliminated my "burnt wire" problems and was part of my performance improvements. I'm very happy with them.

I'll look at AC's. I think that was what I had in the when I bought the coach. I have no problem spending $0.99 on a plug that will work rather than $2.49 on a plug that loads up after a few hundred miles.

59toaster 07-31-2004 06:13 PM

I bet the place your coach is parked it slightly nose up and driverside is higher then passengerside.

Your coach is also has to be cranked for a while after sitting overnight but durring the day it fires first try. ;)

Let me know if I am right and I'll tell you whats wrong. :cool:

swebster 07-31-2004 08:28 PM

No on one but yes on two. Your killing me here. Oh 454 Master

59toaster 07-31-2004 08:42 PM


Originally Posted by swebster
No on one but yes on two. Your killing me here. Oh 454 Master

float bowl is leaking on the carb. Its dripping down into the intake and running down hill into that last cylinder. Common problem.

The casting plugs loosen up as the carb age. Rochester knew about the problems so they put a rubber pad on the base plate of the carb where the casting plugs are on the float bowl section to atempt to keep them sealed.

When you rebuild a Quad it is recomended to JB Weld the casting plugs. ;)

swebster 07-31-2004 08:45 PM

Grrrrrr. When I rebuilt the carb I "blew this off". So I can just pull the carb and JBWeld them?

59toaster 07-31-2004 08:47 PM


Originally Posted by swebster
Grrrrrr. When I rebuilt the carb I "blew this off". So I can just pull the carb and JBWeld them?

Got to pull the base plate. :(

All times are GMT -6. The time now is 12:00 AM.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.