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MMT deposits on Spark PLugs

JL66REDCPE

Well-known member
Joined
Feb 15, 2003
Messages
402
Location
South Jersey
Corvette
1966 Red Cpe 427/425
Those on this forum who know me also know that I do not get into the car at all except on very rare occasion. Anyway, I was coming home from a show on Sunday and brought the car up to 3500 rpm in first then second -- everything ok so far -- went into third and engine started missing. Hmmm -- never had this happen before. My first thought was a fouled plug, then maybe wires or dist cap -- same ole same ole. On Monday I pulled the plugs. All 8 were bone dry with no oil fouling or carbon however each plug was kind of encrusted with this reddish/orange/tannish stuff. It was on the side electrode, insulator and the bottom rim of the plug. Well, I got out my spark plug diagnosis color chart and learned that it was residue from MMT which is used as an anti-knock additive instead of lead. I cleaned the plugs, put them back, took the car out for a test and everything was fine. Since I use a 50/50 mix of leaded 112 race gas and 93 pump premium, the deposits could have been coming from one, the other, or both. I called the racing gas company and one of their engineers assured me they do not use MMT and in fact use lead in the racing gas I use. That left the 93 pump gas as the probable culprit. The engineer also told me if the pump gas was oxygenated that could also cause the deposits. I have been using Gulf 93 in the car because of convenience. The plugs were last cleaned and re-gapped last year at the start of spring.

Has anyone else run into these kind of deposits on their plugs ??
 

IH2LOSE

Well-known member
Joined
May 24, 2001
Messages
3,908
Location
We Will All Meet Again
Corvette
1966,and a 1962 thats almost complete
John

I have seen these deposit before but they dont seam like they are in a spot that would allow the car to break up/ stumble at only 3500 rpms. Did you find if this is actualy the cause? If its the same as I have seen they could just be wiped off by rubbing it on a pair of jeans

Also I know you know your car well ,I just had my 66 out this week end and its shocking how this motor comes alive above 4500 rpms its like a kitten down low and a BEAST above 4500 rpms. I am always impressed with this motor on a spirited drive
 

JL66REDCPE

Well-known member
Joined
Feb 15, 2003
Messages
402
Location
South Jersey
Corvette
1966 Red Cpe 427/425
Larry -- the deposits were encrusted/baked on. They could not be removed by rubbing them against a pair of jeans. I had to blast them in my spark plug cleaner and then also use the wire wheel on them to get them clean. It absolutely was causing the miss. I have a comp cams stick in the car. It is a hydraulic cam with roller tipped rockers, manley valves -- all that stuff. The lift is 520/520 just like the original L72 cam. The power range is 2000-6000 rpm. I pulls like heck all through the range in all gears even with the 3:36 rear. When I was younger I had a new 67 GTO 400 CI and 335 HP and 3:55 or 3:70 rear. The 427 in this car is a whole different animal. You are very correct -- It is shocking.
 

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