E28 inlet

[Ian]

Hi all - I came across a very interesting problem in the ongoing saga of getting my E28 running well. I have been having problems with the rear cylinder running too hot - spark plug white although the rest of the combustion chamber black - most confusing. However, I think part of the problem is the head that was on it. I decided to use a really good head I have on a "bits" motor and have set it all up - new guides, valves, etc. Went to fit it today and the inlet manifold would not fit - hole in the inlet port on the head too small !!

Were there different models of 600 with different inlet diameters ? My guess is I probably have a rebuilt inlet manifold with different size tubing - doesn't look like a home made one though. Any ideas ? I will have to modify the new head to suit.

[Doug]

Ian,

My F28 is buttoned up at the moment. Measuring the outside of the manifold, I get one inch. Figuring a 1/16th wall, that puts the i.d. a 7/8 inch. The original manifold would have the jacket for heating via exhaust gases.

I can not now recall if the inlet manifold projected into the port to the depth of the countersink for the packing gland, or went further and slipped into the port proper. If it stopped short, then the manifold and the port could share a common inner diameter.
Doug

[Ian]

Thanks Doug - we have decided we will just modify the new head to suit - the inlet manifold pipe does go into the head past the countersink on mine - it has one fixed flange and one moving - same as on the 2 3/4 - the manifold sure doesn't look home made.

[shrinker]

There are a few different ways that your symptoms can occur.
May I suggest that you check a few things on the engine as well before only blaming the head. A lack of compression will make a sparkplug white despite the chamber conditions. The sparkplug displays the combustion conditions in the environment around it, if its white its lean. If the rest of the chamber is black then it means that the fuel condition at ignition time is lean. This occurs because of a lack of fuel vaporization. The gaseous state of the fuel at ignition time is inadequate, by the time the burn has progressed in the rest of the chamber the fuel has all been converted to gas and burnt, thus forming carbon deposits.

The ignition energy may also be to blame for a white plug with a black chamber. If the ignition is weak insufficient energy is available to initiate carbon burn resulting in the immediate zone around the plug being predominately a hydrogen burn. Hydrogen does not create deposits, leaving the plug white. Weak ignition makes the cylinder react like its retarded, getting hot etc.

Another very important point to make sure of is there are no air leaks. A cylinder that is excessively lean will actually make a white plug and still blow black smoke out the exhaust.  Its a similar situation to a hydrogen burn, the carbon commences burning too late in the stroke and is incomplete so particles exit the exhaust.

Detonation will also make a plug white with a black chamber in a low compression engine. You cant hear it sometimes but its causes the plug to bet too hot.  Typically if the plug is overheated it will be white but with a slight gloss effect over the porcelain. The timing may be too far advanced and it effects the rear cylinder in that manner because of the air cooling stream. The front cylinder will not obtain the same temperature as the rear and may not be detonating.

Just a few things to think about, hope it goes well for you. Regards Bruce.

[Ian]

Hi - thanks Bruce - food for thought !! I think I have found part of the problem - may have had a crack in the exhaust valve seat. I now have modified the other head and it seems to be running much better - still need to take it for a ride and get things warmed up properly. The magneto is definitely sparking well and the advance is not set to early. Air leaks must be the problem but finding them is hard !

[shrinker]

You can check for air leaks with a can of 'start ya bastard'. Just spray it on joints etc while the engine is running. If it changes rpm or tune then there is your leak.
The crack in the seat might have leaked enough gas to affect the compression and the burn. The PSI during the burn will reach probably around 400 psi on these old engines so that would leak out of a crack that is opened up more when its hot and reduce the pressure sustained during the burn. That would make that cylinder blacker than the other one. The crack leaking compression wouldn't be a good thing either as there's not a lot to start with. It only takes a little change to alter the chemistry and incorrectly prepare the chamber for combustion, resulting in a white plug.