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MG MGB Technical - Blow-by and Compression

A year ago I replaced the OE carb and catalytic converter on my '77B with a Weber DGV and header system. The car has been running way too rich ever since. I have tinkered with different jet arrangements to no avail; they have no effect. The car runs smoothly, but just has no pep.

Recently I have focused my attention on the crankcase ventilation because in addition to the black smoke there is a lot of oil and unburned fuel odor. The plumbing is: fuel tank to vapor seperator to charcoal canister inlet. The original line from the center canister outlet goes to the valve cover and the tappet cover goes to the underside of the K&N filter on the Weber. Vacuum is provided to the run-on switch from the base of the Weber which is also connected to the advance unit. The third canister hose connection is currently also plumbed to the K&N filter. My manifold is the U-shaped one shown in the Brit-Tek and Moss catalogs with holes for the brake vacuum (used) and an extra (plugged).

Last night I pulled the hoses off the tappet cover and the valve cover and removed the oil filler cap. Lots of blow-back! It whistles out of the valve cover tube and the tappet cover. It blows a card sized piece of paper like one of those toys you get at carnivals. I did a compression check: dry: 150, 150, 150, and 150. Wet: 180, 175/180, 190/195, and 180 (I think the variation is due to my oiling technique). Those pressures look pretty good to me. So my questions are: where is the blow-by coming from and what is the solution?

Doug Cook
D. Cook

Doug,

Others will chime in with better advice, but it's toward the end of lunch, and I have a little time, so I'm going to throw in my two cents'.

The crankcase needs to be under a vacuum to draw out the blow-by. On my '74, it seems to me the tappet cover is plumbed to the manifold side of the carbs, so that manifold vacuum draws the fumes into the cylinders and burns them. Yours are plumbed to the air filter, so there's not much vacuum if any at all.

Your pressurization doesn't surprise me. Your compression is good, but no piston ring will seal completely. There are always some gasses blowing by. If you don't have an offsetting vacuum, then I'd expect the pressure of that blow-by to vent at the open oil filler.

Not that any of this solves your rich running problem. I don't have a Weber, but I understand that there are several parts which should be replaced to tune them - two jets, an emulsion tube and something else I can't recall. It would probably be worth the time and money to have it initially set up by someone who knows MGB's and Webers well. I understand they're good carbs, but are not a bolt-on-and-go item. They must be set up to your particular motor.


Best wishes,

Matt K.
Matt Kulka

Doug,

Do you have all the other smog stuff on the engine? The gulp valve, the air pump etc? Did you get the vacuum advance attached to a vacuum scource? The hose on the Tappett Chest should go to the carb filter, or to vacuum source on the manifold, either will work. Make sure your oil filler cap is vented. Im unsure about the hose going to the K and N as that is where I put the one from the Tappet Chest.

Now back to the doggie performance, I found that my Weber DGV was not installed and tuned correctly and was way way too rich at idle. I followed the instructions for leaning it out and I was pleasantly surprized when out on a drive test, again maybe it seems to be better. When setting the carb at idle it is a slow deliberate process. I dont know if this makes a difference at 3000 to 5000 rpm but it seemed to me like I had more full acceleration power.

I just did this the other day and didnt take the time to look at the plugs after the drive to see if they were sooty. One thing that bothers me is I only get 22 miles per gallon, far less than what everyone else seems to get with the DGV carb.

fwiw
CW
CW Strong

Humm...interesting thread. I have just condemned a new Weber DGV back to Brittek cos I could not get it to idle....waaaayyyyy too rich and leaning it out just made it stall. Also...not much power. Bob at Brittek has been a great help...but he's also sold me the whole kit & caboodle for my 77 B...Peco/Weber/Cam/Distributor/Timing gear etc.... The carb was drizzling fuel no matter what I did. On the idle circuit alone/off the idle circuit....just kept choking itself. Sooooooo...Bob is sending me a 38/38 Weber. The price difference is $60.00....so what the hell...I'll bolt that bugger on and hopefully it'll be OK.
To the original question....how is your cam? Lots of blowback could also be caused by worn cam lobes...unburnt fuel smell may indicate exhuast valves closing early off a worn out lobe??!! Check you valve spring compressions with a ruler...they should all be exactly the same...even 1/16th" is HUGE by the way...and I didn't believe it till I took my cam out...39000 miles and it was toast!
Pete
P J KELLY

Matt,
Two cents is more than I had when I started this thread and I appreciate any help, thanks. I agree that the air filter connection is a weak vacuum source, but its where most of the Webers I've seen route the crankcase vent. In this case vacuum doesn't appear to be the problem, I have a strong flow of vapor from the crankcase to the carb and a strong flow back through the charcoal canister. My neighbor, who builds Nissan racing engines for the SCCA Datsun 510's, is a Weber guru and has been helping me with my set-up.

CW,
Yes, the car was desmogged per the Lindsay Porter method as soon as it was out of warranty and while it still had the cat set-up. It was running pretty good with the OE, I just hated to work around the converter and big air cleaner. Like I mentioned to Matt above, my neighbor knows the DGV pretty well, and we have nearly every jet known to man available to us, and have used most of them trying to lean this car. You're right-on about the poor mileage and fouled plugs. I'm certain that the valves are also fouled and that's why I did the compression test when I discovered how strong the blow-by was.

Doug
D. Cook

Hi guys - could the richness at idle be caused by an incorrect float level (too high)? This could allow raw fuel to drip into the intake and make it difficult to adjust idle mixture.

Doug, that much blow by seems like a lot - much more than I see on my car. There might be even more at wide open throttle/higher cylinder pressure. But I'd also expect there to be a lot of oil blown into the filter. I suspect it would be dripping with it if there was a problem with the rings. If the car still runs, I'd focus on fixing the carb problems first, then see if I wanted to fix anything else after that. You sure wouldn't want to do something like rebuild the engine and put a questionable carb back on it - a ticket to disaster...
Mike Polan

Pete,
I was in the middle of answering your post yesterday when my DSL line went down. I think you are on track about the cam, I've suspected that the problem I'm having is not carb-related, but tied to the valves, either timing or adjustment. For several years I have had trouble keeping the tappets adjusted. It will run fine for about a week and then the clatter will be too much and I will re-adjust. No. 7 and No. 3 are usually loose, but any of them can be the culprit. I hadn't thought to measure the spring compression, so that will be done this morning just to make sure, but I suspect I'll find the condition you have described.

Doug
D. Cook

Mike,
The filter is oily, but not dripping, and the primary venturi is sooty, the secondary is clean. The float level has been adjusted so that the fuel level is just below the opening in the casting where the float hinge resides, and that is supposed to be good operating level. It is not only rich at idle but throughout the range. I believe that oil-rich vapor is being drawn into the carburetor which is spoiling the mixture.

The current thread about "Wandering valve clearances" is enlightening, and many of the symtoms described there apply to my valves. Could the cam timing be off a notch causing the valves to open too soon or not close soon enough, and still give the compression results described in my opening remarks?

Doug
D. Cook

Doug; I don't think your compression readings are good. A healthy engine will have near equal wet ot dry compression readings. A worn or incorrectly timed cam should not cause wet or dry readings to vary. It would be a good idea to run a cylinder leak down test to get a better idea of engine condition. Clifton
Clifton Gordon

Pete,
The valve compression is equal on all valves. Measurement was done with an engineer's scale reading the 20th of an inch side (I can't see anything smaller)and I could find no variation. Should there be some difference between exhaust and intake?

Clifton,
The dry compression numbers are solid across all cylinders at 150 psi and they don't bleed down. I gave them two minutes each on three tests. Also, the engine runs very well both at idle and highway speeds with no oily smoke, not even on first start-up with a cold engine. The blow-by is not as strong through the tappet cover (crankcase side) as it is through the valve cover.

Doug
D. Cook

Doug,

I blew over it in my first reply, but Clifton has a point. Good piston rings should not show a change in compression when you oil them. The amount of compression you have is enough to keep the engine running strongly, but the fact that they seal better with oil means you'll have more than average blow-by.

Matt Kulka

Doug...the valve spring compression should be the same on all valves. Valve timing might be next on the list....need to check for chain stretch. Is the tensioner out a long way? Good indicator of this is also wandering ignition timing...if the marks are jumping around a bit under the lamp, this is a sign that all is not well under the timing cover.
Pete
P J KELLY

Pete,
Good points, I'm not real warm and fuzzy about anything under the timing cover. I've had the chain off and on several times lately, trying to make sure the timing was correct for the Weber set-up, and the tensioner is maybe half way out. The way the timing marks align is the most problematic, when I align them "by the manual," using a ruler, with No. 1 in the rocking (not firing) position the marks still don't appear to be perfectly matched. I suspect that my exhaust valves are opening too soon (there is no backfire or knocking) allowing pressure to escape not only down the pipes but into the head. This would explain the incomplete fuel burn and the oily fuel odor that pervades the area when the car is running. It is quite possible that I am off a cog in the mark department.

Doug
D. Cook

Doug: One test for blow-by is to hold a sheet of paper over the oil filler hole at idle. A engine in good condition will draw the paper down, whereas excessive blow-by will push it off.
Regarding the PCV set-up on the Weber, I had a 73B many years back that I converted from dual HIF to the DGV with a Pierce manifold. I regretted doing that in the end.
However for the PCV I intially ran the hose from the front sidecover to the connection on the underside of the aircleaner and experienced increased oil use and an oily intake. This is not the proper location for a PCV.
I eventually (after various trial and error attempts at using domestic type PCV valves) installed a Smiths diaphram (aka mushroom) PCV on the Pierce manifold where it is tapped for a power brake booster vacuum source. That did the trick.
Andrew Blackley

Andrew/Doug,

Initially when setting up the DGV manifold headers etc. The shop who put it on for me ran a pvc pipe from the valve cover to the intake manifold, on the side towards the firewall. The brake boooster was in front to a tapped hole in the intake manifold. In doing this, they vacuumized the block/engine and the engine idled fast. Since doing research on the BBS and other places I reran the hose from the tappet chest to the filter as described and it seems to work ok. The engine needs a vacuum on it, how much is anyones guess and the hose to the filter seems to provide it. When I reran the hose,I took the PCV off of the hose and it is sitting on my workbench now. I cant say much about the oil consumption or oil in the filter as I dont drive the car that much, except that it runs well now and has excellent acceleration. There is more to do now but I think that the dizzy needs soem attention.

The paperwork that came with the carb said to back off the idle screw til it doesnt touch the bellcrank (it affects the progressive linkage) then adjust the volume screw on the DGV. After doing that, then adjust the idle setting. What I did was screw the the volum setting about 2 full turns clockwise till it ran rough and backed it out till it ran well again maybe half a turn. Then I adjusted the idel to 800 rpm. I checked the plugs last nite and they were nice looking light brown colour indicating a bit lean (no sooot). Before I performed the adjustment, they had black soot all over the anode and base of the plug.

Doug, I think you have two separate issues to deal with. I dont think the PVC vacuum line has much to do with your lack of performance.

ciao
CW
CW Strong

Andrew and CW,
I agree, the PCV vs air filter routing is a secondary issue. I think that once the blow-by situation is solved the air filter connection to the tappet cover will adequately ventilate the crankcase. As I mentioned above, I've been working carburetor adjustments for the best part of a year to no avail; going from very small jets to very large has little if any effect. I have a seat-of-the-pants feeling that engine breathing is my biggest problem with the rich running and lack of power. Experimenting with various plumbing arrangements is how I discovered the strong blow-by through the valve cover in the first place. Also, the car has none of the other symtoms associated with piston ring failure.

Does anyone know what the engine does when the cam is off a notch? The distribuor timing is steady at 13 degrees advance, depending on where I place the phasing on the electronic ignition.

Doug
D. Cook

Doug; Did you verify that the engine was at TDC when you checked the cam timing? If you are off one cog the engine may not run well. When setting cam timing by checking the alignment of the timing marks, the marks may not line up, but it should be less than one cog.
Did you try to get it closer by moving it to the next cog? The chain should be tight when checking the alignment ot the marks. If your exhaust valves are opening too early all other cam evnets are also early. It's always best to verify cam timing using a degree wheel and dial indicator.

As I said in my earlier post, the cam timing being off should not cause the big difference you have between wet and dry compression readings. If your compression tester is like most it has a Schrader valve in the spark plug end and is shut off when the engine is not cranking, watching it two minutes only proves the compression gauge is not leaking down, it is not an indication your engine is holding pressure.

I have no experience with the Weber DGV carb but it seems I remember from previous posts that they require a lot more distributor advance than stock settings. You may want to try it at 18 or 20 degrees and see what happens. I still think you need to find out why the difference between wet and dry compression readings are so big. That is not normal for an engine in good condition.

FWIW, Clifton
Clifton Gordon

Clifton,
Wouldn't there be other symptons of piston ring leaks such as the puff of blue smoke on starting a cold engine or leaving a trail of blue smoke if the rings where the source of the blow-by? Also, I don't think I had the blow-by before my various attempts to set-up the Weber because the overpowering fumes are a relatively recent event. I think I will take your advice about the timing and use a degree wheel to check the cam, anything to put off major surgery.

This thread is getting a little too long, so I would like to thank everyone for their comments and support, so thanks! This BBS is the best tool in my box.

Doug
D. Cook

Hi Doug, yes, it's getting long but still interesting :).

Caught up on my reading and noticed couple of points:

First, I'm not sure I understand how cam timing could cause blow by - wouldn't all the pressure go out the valves and into the exhaust or the carb? I was pretty sure only ring problems can cause blow-by, and it's been my (limited) experience that the rings must be very bad indeed to cause blue smoke problems. Usually oily smoke is caused by bad valve seals or worn valve guides.

Next, you mentioned your compression test doesn't bleed down. My tester has a one-way valve to hold the pressure, with a release button. And my cylinders tend to bleed down pretty quick once the engine stops spinning - I'm not sure how I could even get the piston under test to stop with both valves closed so I could check. Are you sure you're able to hold cylinder pressure for 2 minutes?
I have to agree with Clifton, you've got ring/cylinder wall wear - though not a catastrophic amount (compression testers aren't calibrated anyway).

I know I get an oil/fume smell if I disconnect the breather, which I run to my manifold through a valve (DCOE). I've heard the same is true if you go to your filter instead. But you could always disconnect it to test if that eliminates your suspicions about oil vapours ruining your tuning (I would expect leaning it instead).

I thought the rocker cover and tappet covers bled the same volume in the engine and would expect the same blow-by pressure - is it possible the oil separator in the tappet cover is clogged?

HTH - at least you've got lots to look at from us :).
Mike Polan

This thread was discussed between 17/10/2003 and 21/10/2003

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