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MG Midget and Sprite Technical - Blowby excessive?

Hi all, I've been through the archives but not finding anything specific to this problem. I've discovered what appears to be excessive blowby. The first symptom noticed was engine oil under the carb cover, which I wiped off, drove ninety miles over 6 hours, checked again and there was the engine oil on the cover again (see pic, oil running upper right to lower left. Sorry, not the best pic but you get the idea).

Additionally I have been plagued with oil leaks (and now perhaps I know why) at the usual places, and I've just noticed the pinhole vent in the oil filler cap is blowing vapors like a geyser, some at idle, significantly more under throttle.

The crankcase vent, with PCV valve, is obviously working as evidenced by the accumulation of oil under the air filter cover.

I would guess that this problem has been around a while, probably since I've owned the car (~5,000 miles) due to the oil leaks that have always been present, and before the Weber conversion (~500 miles)

I checked compression and plugs today (all plugs out, throttle wide open, engine warm):
#1 134
#2 134
#3 133
#4 138

Plugs all looked clean and correctly colored, i.e. proper heat range for the car as currently set up. From what I've read in the archives the common causes I think I've eliminated, i.e. low compression (valves/rings) improper (blocked) venting to the intake system, so what else might be going on? I've the feeling that the vapors coming out the pinhole vent at idle that increase significantly under throttle, i.e. more pressure as revs increase, is the pertinent clue, but I can't figure it out given the compression numbers and the plugs looking proper. The car runs great, just too wet.

Stock 1979 engine with 73,000 miles
Weber 32/36 DGV
4/1 header with 2" exuast
pertronix vac. advance distributor with matching coil
oil cooler with (functioning) thermostat
emissions controls removed
head checked for torque just weeks ago (no issue)
No overheating issues ever
1/2 quart oil lost in 700 miles

Thanks, Richard





Richard 1979 1500

Richard,
Do the same compression test - same conditions, warm engine WOT - but add a teaspoonfull of oil to each cylinder just before you test each one. It will help narrow down the problem.

If your rings / bores are bad, the oil will temporarily seal and give a significant increase to the compression reading. If there is only a slight increase, then the rings are good, but the valves may be excessively worn causing raised pressures under the rocker cover.

Guy
Guy Weller

I had a similar problem with mine after a twin choke carb change, ended up adding additional breathers to the rocker cover & installed an evacusump system.
Brad
B Richards

Guy:
I did the test you suggested today:

#1 138
#2 137
#3 136
#4 143

Would anyone call this (2-3%) gain a "significant" gain? Or does this look like valves? I'm thinking valves but I really don't know.
Thanks R
Richard 1979 1500

Hi Richard,

From those figures I would say that your rings/ pistons bores are good, The increase is what one would expect for doing a "wet" test when compared to your earlier "dry" test figures. They are also all close enough to each other to suggest no problems with compression and it is unlikely that the oil problem is caused by piston blow-by.

The figures are maybe a little lower than I would like to see but I believe that the Californian 1500s were supplied with a lowered compression engines weren't they? Someone else may confirm this.

It is not conclusive, but I also suspect that your valves are sealing the combustion chambers reasonably well, but this doesn't rule out possible valve guide wear, or perhaps the rubber valve stem seals have hardened.

Given what else you have said I am not sure what else to suggest other than perhaps you are worrying unduly. 1 pint oil per 700 miles is a little high oil consumption, but not excessively bad for a 73,000 mile engine. What oil are you using?

Guy


Guy Weller

Guy:

Thanks for the info. I think that only the 1976 North American midgets had the high-compression pistons. I'm running 20-50 as per my owner's manual.

I'll re-route the crankcase venting to the intake manifold to keep all that oil from gumming up the carb. I may tap the valve cover and thread a pressure gauge onto it just to get a baseline pressure number for future reference.
Richard 1979 1500

Id say try bumping up your oil to 20/50W and see if that does anything....


also....
try running the engine 1/2 quart of oil lower then normal....we had a discusion a while back that the dip sticks might not be accurate or the wrong ones....it seemed to make a differance to some of us.

prop
Prop

hello Richard, paul dillon here Northern Indiana, having the same prob. with my 78 midget, had the head removed and new valve guides and valves installed also the head was milled 7 thousnads, never used oil till now
Paul Dillon

Paul:

Do I understand you correctly? Are you saying your oil leaks are WORSE after your head work? BTW, have you checked for a compression increase after milling the 7 thou off?
Richard 1979 1500

Hiya

If your engine is blowing by there is a good chance there may be residual oil on the bores which might give you a false compression reading. usually a head off for inspection will show soft/moist oil contaminated carbon on the pistons and chamber. I cannot think how blow by can occur up valve guides to the extent of acting like a little steam engine.The 1500 heads never had stem seals fitted and we have not come across oil burners down guides apart from one engine with excesssive oil to rockershaft via external feed.
Oil being pushed out of many places is giving you a good clue as to the cause, unless the crankcase valve is blocked.

Peter
peter burgess

Excessive blowby can be demonstrated by securing a paper towel (gas station windshield wipe is good) with a rubber band, over the filling hole atop the valve cover, (cap removed). With the engine running this paper diaphragm should lightly flutter up and down a bit. If LOTS of blowby.. the towel will move a great deal and might even soon soak with oil. A tin baffel afixed beneath the filler cap will help seperate rocker splash from the escaping gasses and reduce the fountain from your vented cap. I don't think that cap is supposed to be vented at all. Many racing sanctioning bodies require an oil capture/recovery kit. A rubber hose from the valve cover vent and into an oil can or bottle will catch much of the escaping oil and help keep the engine exterior cleaner.

Years ago.. I had a 356 Porsche engine go sour on a lengthy trip. The above roadside fix allowed me complete my trip by capturing about a quart of oil every hour of driving, I poured it back into the engine each hour. Commercial air/oil seperator units often have an automatic drain-back feature or a petcock at the bottom of the reservoir. M
Marc Judson (2- 1978's)

I said it before and I'll say it again. These little engines run best with the emission equipment hooked up I unhooked mine and the engine ran terrible with excessive blowby. Hooked everything back up and voila no more blowby and engine idled much better. You won't get anymore power nor better gas mileage by unhooking your emission equipment. Been there done that as the saying goes.
D.J. McQueen

Richard,

I had the same thing with my '79 earlier this summer after bringing it back to life after a long sleep.

The temporary trick was to vent the valve cover directly to an old brake fluid bottle via heater hose and to add a length of hose to the dipstick tube to raise the level of that opening. This resulted in lower pressure and fewer oil leaks, but did not fix the problem.

What worked for me was a dose of Marvel Mystery Oil in both the crankcase and gas tank. Within one tank the problem went away and has not come back. First time I've used a "snake oil" and I was pleasantly surprised.

Good luck.

Lee
L Fox

This thread was discussed between 27/09/2008 and 09/10/2008

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