When I has my Stromberg engine apart recently for a rebuild, I took the inlet valves out to find them quite badly coked up (this was after 10000 mile running or so from previously clean valves).
See the attachment for a pic of the worst one from number 2 cylinder - the others had a much thinner layer of carbon.
As far as I know the valve timing was OK, and the plugs were a mid brown colour after a long run (if the engine was idled for a few miniutes then they would turn black). I have been using RON 95 unleaded with an occasional fill up of RON 99 leaded fuel. The engine has been running quite well.
This does not seem normal - any ideas as to the cause?
Yes, burnt oil, agree with Les (where ya bin Les, thought you’d gone bush ) most probably from down the valve guides. What do the valve seats look like, the face does look worn.
Best way to get this carbon crap off is to soak the valve overnight in Redex, it’ll be soft in the morning.
Final thought- have you got standard air-box or K&N type filters with the crank breather routed to cylinder 2?
looks like deposits from the sh3t low octain junk gas were forced to use -had the same deposits on the elan motor Im rebuilding —valve guides were in spec but the seats were a little ropey ----- the same crap was on the 7 motor valves when I rebuilt it -----I took it off my exhaust valves with a wire wheel----octain booster helps or add lead additive --ed
Not knowing how many miles or what the engine was used for, the carbon on the back of the valves appears normal. Twin cams have no valve guide seals, so oil is going to move between the valve stem and guide and “burn” on the back of the valve. Looks like there is plenty of margin on the valve and the valve face looks OK. If this carbon becomes a problem, some GM “top end cleaner” will take it off. Early port fuel injected engines are much worse, because some injectors would fire with the intake valves closed, baking the gas on the back of the intake valve. (the carbon would clog the intake port and make for erratic cold start/drivability problems) This is just one of the reasons we now have all the ads for special cleaning chemicals in gas, like Chevron’s “Techron”. In my opinion, octane boosters will not keep the back of the valves from carboning up. So called “cheap gas” (low octane) would not do this. Give the head a good rebuild.
over ere I havent been able to get a brown tail pipe exaust deposit since the introduction of no lead ----no real valve wear or unnesseary seat wear —just a build up of black goop that used to be brown --and after a good run a brown plug deposit that goes black after a little idle —ed
The valve guides and valves have only done 15000 miles from new - the guide play looks good, and there is no oil tracking down the exhaust valves. The engine does not smoke on start up.
Could it be petrol pooling in the inlet manifold (I have a low balance pipe)?
Also - I just remembered - The valve springs are 2mm shorter than new.
the deposits are just residue from whatever the oil company is allowed to dispose of in the gas ----regular 86 octane is the worst ----there is no problem I can see in flow loss and it all eventually goes out the tail pipe —it just look unhealthy ----ed
One thing to remember is that #2 and #3 cyclinders will always run richer than #1 and #4 with the stromberg engine. So you have to set up the carbs so #1 and # are slightly lean and #2 and #3 are slightly rich. If you set up so #1 is correct than #2 will be rich. This is due to pariling the wrong cylinders to the carbs (it should be #1 and #4 paired and #2 and #3 paired).
Are the backs of #1 and #4 much cleaner than #2 and #3? This would inidicate rich as opposed to oil down the guides.
When John McCoy did my head to his 440 lift 290 duration cams he was able to get stem seals in. Of course that ended up being a $1600 job.
I am pretty sure it is not oil down the guides - there is significantly less on the other valves, and the guide clearance is the same for all inlet valves - you can barely move the valve heads from side to side if the valve is lifted 10mm - and there was no smoke even after many minutes of idling before my latest strip down (for broken piston rings)
I made the mixture deliberately rich before, as I have also had trouble with burning exhaust valves - I have this idea that it might help, but I am far from sure. Again fresh valve springs could help here…
Gidday,
I stick with my original comment, guide to valve clearance.
If it is a fuel deposit build up then all I can say is you must have the crappist petrol known to man !! Not since the 50’s have I seen a fuel build up like that. What is the Octane rating of this Kerosene ? Is it leaded or lead free ?
If this swamp muck is unleaded then I would certainly run a GOOD Octane Booster and a Lead Replacement.
See ya,
Les.
over here a certain amount of oil based polutants with carsogens are allowed in the gas as it is envionmentaly the cleanest way to dispose of it ----the envioronazies have their way pretty much ed
I re-assembled the head with new valve springs and a damn good regrind before I posted (so sure I was that it was not guide to valve clearance), so I will just have to wait and see. I will take off the carbs after a few thousand miles to see what is happening to the inlet valves with my mirror on a stick.
Funny that the exhaust valves were clean, even though the guide clearances were larger on the exhaust side to cope with expansion. However, with all things Lotus expect the unexpected…
I just finished re doing my head also ----after the bottom end in done and the motor is back in the car I shall see what the results are this summer also —ed
Dave,
The inlet valves tend to show the problem you have and not the exhaust because:
When the inlet valves are open on the expansion stroke, there is a negative pressure sucking air/fuel into the cylinder, this will also tend to suck oil down the guides. The exhaust valve opens on a compression stroke with pressure in the cylinder.
The exhaust valves are exposed to hot gases that will tend to burn off oil deposits, inlet valves carry the cooler fuel mix.
I will lower my needles and see if it is oil/fuel or a combination. it was deliberately too rich before, for the reason above in this thread.
I am not that concerned as it all seems to disappear by the time the engine has burnt it, and there were no oil deposits (wetness) in the combustion chamber.
The stuff came off originally with a scrape from a knife, and I can probably do something from the outside with the carbs off if I have to.