Triumph Repair: MG Midget with 32/26 Weber, mg midget, dipstick tube

Question
QUESTION: Hello Howard,

I hope you can help.
I have a 75 Midget with a 1978 engine which I rebuilt in 2006. It ran great but it was hard to start. I felt it was due the Stromberg carb, as it did that on the old engine (Blew a connecting rod in that one).
I therefore invested in a new Weber conversion and installed last year. I was amazed at the improvement in performance but immediately noticed oil coming out of the dipstick tube. I ended up driving it with the valve cover hose open for the season because aside from that; it ran great. This year I wanted to fix the problem.

First thing I noticed were the plugs were soaked in oil. I then did a compression test (the throttle was closed). The readings were about 150, 150 , 90, 150.

In seeing those, I thought I had a head problem so I removed it and saw that number three piston was cleaner. I talked to the engine rebuild place I buy parts from he believes I am likely getting wash down and probably have the wrong jets in the Weber (who knew).

The jets that came with the 32/36 were 140 (main) and 135 (secondary). I was told by someone who knows Webers they should both be 130. I put the head back on and did a compression test again (manifolds both still off). My readings were now: 120, 120, 135, 127. I then did the Wet test and the new readings were 153, 153, 160, 155.

I put everything back together and started it up. I immediately determined I have an exhaust leak somewhere, but more discouraging, is the oil coming out of the dipstick tube when I rev the engine (It generally does not when at idle)

Am I taking that engine out again, please say it isnt so.

Thanks
Colin






ANSWER: Hi Colin,
First your compression test is no good with the throttle closed (especially with a Weber carb)
throttle MUST be wide open. (no air allowed in = not much compression)
Second, It is impossible to have a "Wet" test lower than the "Dry" test.
How did you vent the crankcase when you installed the Weber carb? Didn't the vent go to the side of the Stromberg? The Weber don't have a crankcase vent port.
When a dipstick blows oil out on any engine it is a clear indication of crankcase pressure. Either the crankcase is stopped up or you have excessive ring blow by. (the "Dry" and the "Wet" compression test checks that) However you readings of your wet and dry tests are impossible. No engine can have lower readings on a wet test than a dry test.
Retake the "Dry" test keeping the throttle wide open and at least 4 or 5 revolutions on each cylinder tested.
Then put about 4 or 5 squirts of engine oil in the plug hole and run the same compression test. This second "Wet" test is HIGHER than the first "Dry" test but it should not be more than 10% to 15% higher. If it is a lot higher, then it indicates a ring problem. You should see a "Dry" test of 125 to 170 PSI. and a "Wet" test of 10% to 15% higher with the "Wet" test.
The only other thing that can have oil leak out around the dip stick is if the engine is over filled with oil.
Let me know,
Howard

---------- FOLLOW-UP ----------

QUESTION: Hi Howard,

Actually those readings above show a higher compression when the wet test was done;
Before:
120
120
135
127

After (wet):
155
155
160
155

This test was done while both manifolds were removed, (right after I put the head back on). The very first test I did was prior to removing the head (that's when I had the throttle closed) which I got:
150
150
90
150

I did not do the wet test at that time (D'oh), only after I removed/replaced the head as stated above.

The valve cover vent did go directly into the manifold of the old Stromberg set-up, but I was told this was so the crankcase could get air in, not vent pressure out. Is that wrong?

I've read messages on this web-site where people have tapped the new manifold to allow crankcase pressure to vent directly into the combustion chamber. Is this the right thing to do here?

I should also point out that when I removed my head, I noticed I did have glazing on the cylinder walls, but I could still detect the cross-hatching done by the engine rebuild place (when I rebuilt the engine in 2006). Does this mean my rings did not seat properly? It this because the larger Weber jets washed them down too much? By the way I did replace those jets as recommended but the problem still exists obviously.

Thanks for your quick response by the way.

Colin

ANSWER: Ok, on the compression tests. No, on the venting the crankcase (valve cover pipe) into the intake manifold. BLM did that once on the older Austin engine once and it was not good, as it vacuumed the crankcase and caused more problems than it tried to correct. That system had an orifice to restrict the flow into the manifold so it could not modulate between idle (high vacuum & low piston blow by) and full throttle (low vacuum and high piston blow by) Also as the engine got older it would go into detonation drawing oil past the rings. It was just a bad idea with no method to control crankcase pressure.

All of the case vent systems try to take the crank case fumes into the combustion chamber so that they don't go into the outside air. The crankcase builds pressure due to blow by of the rings. This is normal even on a new engine.
If you vent directly into the intake manifold you will vacuum the case. If the rings are good this will not cause a problem (as BLM did once) If for example the oil filler cap came off, you then could burn a hole in a piston due to the mass vacuum leak. Also when the rings got worn the vacuum pulled excess oil into the combustion chamber and the burning oil will destroy a CAT (if the car had one) and it lowered the octane value of the fuel in the combustion chamber which in turn, caused detonation which in turn broke rings, ring lands and burned holes down the side of the pistons.

The best system all around is to vent the crank case hose into the air filter canister. Normally there is not enough vacuum formed in the air filter to draw oil past the rings if the rings are in fair condition.

I don't know what size jets you should have in your Weber. I modify engines often and the jetting of a carb when ANY modification is done to an engine is a process starting with a large main jet in both sides. Then advancing the timing to detonation and back the timing up a few degrees and do a plug reading test (if you don't have CO equipment and a dyno available) If you do have the equipment available you should see about 5% CO at full throttle / full load at peak torque. (probably about 3500 RPM on your 1500 Spitfire engine)
Plug readings are difficult to do now with unleaded gas, but it can be done.
You need a long straight road with no side roads and no traffic. A long hill is even better.
With the engine warm and with a new set of plugs (correct heat range for that engine)Install the richest jets you have and install the new plugs. make a long high throttle run. If local law is a problem you can use third gear up a long hill to keep you inside the law. after running at 3500 to 4000 RPM for a minute turn the key to off (not LOCK) and push the clutch in right away to stop the engine from spinning. Pull over and pull a couple of plugs. If your jets were rich, you should see flat black around the edge of the plug and the center insulator should be slightly dusted in black. If it is clean and white, you didn't go rich enough. When you get the black dusting of the insulator then you can drop down one size in jets and repeat the test. You must start at a discolored center insulator (rich) and work toward lean (never the other way) You will arrive at a point that the center insulator is clean or just slightly dusted. Then you can work on the lower RPM range jetting with different emulsion tubes and idle jet settings etc.
The is the best I can tell you that I have done. If you bought the carb set new, you can call the vendor and ask what jets you should use for that engine and don't forget to tell them what altitude you are at. Denver will require different jets than Miami.
That is the best I can tell you, I hope it helps,
Howard


---------- FOLLOW-UP ----------

QUESTION: Hi Howard,

I did actually vent the hose into the air breather of the Weber but still ended up with oil coming out of the dipstick. The only way I've found to prevent this so far is by leaving the hose open, but it makes quite a smell. Someone also mentioned running a hose under the car to prevent the smell (that doesn't seem like the right thing to do).

So that I'm clear on this:

So it is normal to have to this crankcase pressure build up? (My rings are not shot? )

So the person who told me the valve cover pipe is for letting air into the crankcase was wrong?

Does the incorrect Weber jets cause undo crankcase pressure due to wash-down?

Will running the wrong weight oil cause undo pressure too? (I only recently realized I was doing this)

Knowing that I have tried venting in to the air filter and I shouldn't vent into the manifold, what is my best solution to this problem?

Thanks again so being so prompt, this is driving me crazy.

Colin

Answer
When you leave the hose open to outside air it is probably violating local emission control laws and if pressure is not built up in the case with it is open to air, than it will be even less pressure in the case if you connect it to the air filter.

ALL engines build crank case pressure. (Too much pressure is bad and is tested by the Wet and Dry compression test.)

Air can't go in a vent pipe when the pressure is higher inside than outside. The pressure is ALWAYS higher inside.

No, you can't put a jet in so big as to wash down the cylinder walls. It would foul plugs and blow a lot of black smoke before that can happen.

The only way oil weight could effect blow by, is if you were running 5 weight oil. The rings would probably not seal and you could get excess blow by.

Vent to the air filter is the best way.