What spark plugs are in your 912 engine?

[Gav P]

Hello Pelican 912ers...

OK so what type of spark plug do you guys have fitted in your 912s?

I've had NGK B6HS plugs fitted when my newly reconditioned solex carbs were installed. Since then, the engine has been overheating when driven for some time at high revs (ie on the motorway at 80mph+). I think the "officially" recommended plugs are what I used to have, which were Bosch W6BC (i think thats the code anyway)... and I never had any problems with the engine running hot with those.

Of course it could be to do with the carbs, but in all other ways the engine is running much better than before, eg no flatspots or stalling. Theres also nothing wrong with the cooling system - ie no missing tinware or blocked up ducting, so I'm thinking perhaps my engine just doesnt like these new plugs!

Any plug experts out there??!

Cheers for any help...

[craign]

Bosch W6BC in my 69

[craign]

I forgot this link also, they have an NGK cross reference table.

http://www.spark-plugs.co.uk/pages/technical/x-ref.htm


Craig

[Duke]

W6BC. Have you checked your timing & carb mixture? Also the oil cooler could be plugged with some debris.

[Gav P]

Cheers guys.

The oil cooler is slightly dirty, but not blocked (you can see though it all over), so this may have some impact but if its not the plugs then I'm thinking mixture is the key.

Timing is spot on, carb mixture is still something for further work I reckon. The engine ran very rich with the old carbs. Apparently the new carbs actually have larger jets than the old had.. but it runs leaner. Pretty strange. I think I may try larger jets anyway. Presumably changing the jets is the only way to alter the mixture?

[cgarr]

When you say "overheating" are you talking oil? Do you have a cht or access to one you can put on #3 or even better would be a egt which would react to carb setting very quickly. My oil hangs around 190f max with the heads around 250f If everything is very clean it could be the mixture, dont forget to take a good look at your plugs, they can say a lot. My heads always ran a tad hotter with the w6, I now have w5 in it.

Craig

[Duke]

Forgot to mention one other thing that will cause overheating-fan belt too loose!

[Morrie]

W6BC should be roughly same heat range as the B6HS. The next "cooler" NGK would be a B5HS. The next cooler Bosh would be a W7BC. See how they go opposite directions?

Hot may be too lean, which can be disasterous. Otherwise, improper cooling due to loose belt as Craig mentioned, or some junk in the oil cooler...

Just some stuff to look at...

Morrie

[cgarr]

I had to go with a colder bosch plug. I went from a 6bc to a 5bc The 5 has a shorter electrode allowing it to dissipate heat better than the 6, The 5 bosch should be the colder plug? I hope?

Craig

[Gav P]

Someone sent me an interesting web link relating to the "heat" of plugs -

http://home.clara.net/dave.moore/plugs.html

so my understanding is now that the the plug heat rating will not really affect engine running temp. Rather, the plug rating reflects how effectively the plug maintains or looses the heat given to it by the engine. Or something.

As far as my overheating is concerned, Im thinking its the fast-running mixture which needs to be looked at, eg its too lean. Fanbelt etc all OK, but will probably check that oil is flowing through the oil cooler without constrictions as well. You never know....

[Morrie]

What size mains are you running in your Solex carbs? I have been messing around alot with mine and may have some insight. When you have mains that are too lean, your car will pop and sputter at highway speed or under load. I had 122.5s in and had this problem. When I went up a step to 125s the issue went away. With 127.5s the engine seemed to run ok, but was rich and fouled plugs. Running "fat" is not a bad thing, so long as you don't have performance issues or foul plugs. Another possibility is low compression. Have you recently checked the compression, or done a leak down test?

Stay with me on this, we'll figure it out!

Morrie in Texas

[cgarr]

Hey Gav P, something I forgot about, but when I rebuilt my engine, in the front cover there is a bypass valve you can access on the bottom of the cover. When you first start the engine the oil system will bypass the cooler until the pressure equals in the system, they do this so that oil makes it to the bearings first and fast. If this valve sticks no oil will go thru the cooler. you can pull the valve out and check it real easy and they do get crudded up. There is also the same type valve on the side of the front cover which is an oil pressure releif valve.

Craig

[Gav P]

Mmm some interesting points...

Believe it or not, I havent checked the jet sizes yet. I'll take a look and see what we have. Should be able to get over to the garage tomorrow evening and check then.

Craig - so I presume you are referring to the main cover forming the end of the crankcase (towards the back of the car)? - ie where the oil pump is fitted?- I'll look into it.. perhaps the valve is mentioned in the Orange book...

[jpamental]

Gav-

Yes, they are mentioned there. You may well need a special socket to remove them. It looks like a really big flat-head screw; I found that a 'drag link socket' was big enough (and you can get leverage with the socket wrench). Hopefully you'll have an easier time finding one that fits than I did...

Jason

[Morrie]

Agreed. I could really use pointers on what tool to get to remove those plugs, as mine are stuck pretty good.

Morrie

[jpamental]

Morrie,

You can find one on the Snap-on website (search for 'drag link' I think). You need one about an inch wide, but it will most likely be too thick. Get that one and grind it a little thinner and I think you'll be in good shape. You don't have to do too much, but it's the only thing I've found that's really close.

Jason

[Gav P]

Had a look at those bypass valves last night. Actually came out pretty easily using a very large screwdriver (my socket was also too thick for the slot). The valves themselves appear to be perfectly OK, no sticking plungers or anything. I dont think they are causing the overheating.

Morrie: I checked my main jets - they are 125... which suggests to me that its unlikely to be running lean because of jetting. Is there any other adjustment apart from jet size which affects the mixture?

Cheers...

[jpamental]

Gav,

I might be stating the obvious, but how were the carbs adjusted when you had them installed? The 4 idle mixture screws (on at the base of each throat) would be the most obvious place to adjust the mix. Maybe try backing each screw out just a tad - maybe 1/8th of a turn each - and see how idle, performance and heat check out. (repeat if necessary...)

Don't want to mess with it if someone knowledgeable has already done this, but you never know.

Jason

[Morrie]

The 125s should be ok, if they were 115s or something I might be concerned. Speaking of possibilities that may seem obvious, reach around back of the fan shroud (engine off) and make sure there is no debris in the cooling fan impeller. Crap from the old sound deadening mat is notorious for getting in there....

Morrie

[cgarr]

Another obvious but important, make sure the tin is sealed well mostly that seal around the edge of the engine compartment, if its not that fan can draw a lot of hot air off the exhaust pipes and pump it right thru the cooler, after running the engine hard open up the lid and feel the fan shroud, it should be cool or even cold.

Craig