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crank ventilation effects on mixture

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Hi again folks, and Happy New Year. First a little background. By all indications my car runs great around town...good acceleration from low RPMs, nice exhaust note, no hesitation etc. All this along with light burble on deceleration and seemingly poor fuel economy (haha) all suggest it runs slightly rich. Coolant temp stays within range (car doesn't run hot by the water temp gauge). The plugs look too clean - this is the only indication of a possible lean condition, but from what I've read this is no longer a good indicator with today's fuels...

The only issue seems to be at cruising speed on the highway (70 or so). At this speed it seems to surge. I can't seem to find the cure...yet.

First question: I am running the crank breather tube from the timing cover to both carbs. Could this be causing the surge at high RPMs under load? I am thinking of leaving the vent tube open and blocking the inlets on the SU carbs. Does anyone have any experience with this?

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jimandcassdavis Jim Davis
Swanton, OH, USA   USA
1967 MG Midget MkIII ~ For Sale ! ~
1976 MG MGB
The tubes are part of the crankcase ventilation system. Disconnecting them could cause oil leakage from the rear seal.
Check the oil in the carb dampers. If low it might cause the surge.

refisk Avatar
refisk Rick Fisk
Frankenmuth, MI, USA   USA
The crankcase ventilation system won't cause surging if the system is configured properly. Do you have a non-vented oil fill cap and the rocker arm cover with a restiction in the breather tube?

Also, what needles are you running in the carbs?

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
I believe the oil level is correct. I was thinking of just running the tube to atmosphere. Vented cap. No tube on valve cover...

Rick, I'm not sure what needles I am running. Any way to check with the carbs on the car?

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Out of curiosity, how would oil affect the surging?

Also I hope I'm using the term correctly. By surging the car seems to pulling back or hesitation at regular intervals. I feel like I really have to put my foot in it to hold speed.



Edited 1 time(s). Last edit at 2018-01-01 05:15 PM by Poundingsand.

refisk Avatar
refisk Rick Fisk
Frankenmuth, MI, USA   USA
You have to disassemble the carbs and remove the needles to see the code stamped into the needle. :-(

Based on your other posts in the thread it sounds like you're running lean when at speed. That's why I asked about the needles. Try disconnecting the tube from the timing chain cover and plugging the carb ports. Then see how it runs at speed. But you may have an oil leak at the rear of the crank, but you should know then if the crankcase ventilation system is causing the surging.

In reply to # 3654745 by Poundingsand I believe the oil level is correct. I was thinking of just running the tube to atmosphere. Vented cap. No tube on valve cover...

Rick, I'm not sure what needles I am running. Any way to check with the carbs on the car?

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Thanks Rick. What happens if I remove the dashpots while carbs are in the car? Big oil spill?

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refisk Avatar
refisk Rick Fisk
Frankenmuth, MI, USA   USA
If you remove the dashpot and the piston (with needle) together there will be no spill. The oil is on top of the piston inside the dashpot. Take the dashpot and piston to your workbench, unscrew the black cap, and dump the oil into a suitable container. Be careful not to bang the needle on anything as it might bend.

In reply to # 3654768 by Poundingsand Thanks Rick. What happens if I remove the dashpots while carbs are in the car? Big oil spill?

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Sounds good. Is the needle size stamped on top? And whats the best oil if I replace?

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Rick, after thinking about it, I have tried adjusting mixture to the richer side (and also pulling the choke with no change in cruising...it still seesaws...would this be indicative of something else or could it still somehow be jet needle (or breather) related?

littlecars Avatar
littlecars David Bassett
Nashville, TN, USA   USA
1965 Chevrolet Corvair "Ski Team Transport SOLD!"
1965 MG Midget MkII "Buffoon"
1966 MG Midget MkII "Swiss Cheese...SCRAPPED"
1970 Triumph Spitfire MkIII "Tamara's Turd... SOLD!!!"    & more
In reply to # 3654772 by refisk If you remove the dashpot and the piston (with needle) together there will be no spill. The oil is on top of the piston inside the dashpot. Take the dashpot and piston to your workbench, unscrew the black cap, and dump the oil into a suitable container. Be careful not to bang the needle on anything as it might bend.

In reply to # 3654768 by Poundingsand Thanks Rick. What happens if I remove the dashpots while carbs are in the car? Big oil spill?

I'm in this same boat, Peer. Rick, where is the needle stamped? Also I've read it is appropriate to use regular motor oil in the dashpot? I found the rear carb dashpot practically dry and the front carb had the proper amount.

B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Surging at speed is from a vacuum leak in most cases - lean fuel mixture. Check the vac unit on the distributor for leaks, and the intake manifold/carb gaskets/end plugs. Reducing the crankcase ventilation reduces a vacuum leak that dilutes your fuel mixture. Its not necessary to vent to the carbs. In fact, I usually vent to a tube that I weld into the back of the air cleaners, and let the carbs meter fuel to ALL the air coming in, instead of having the fuel mixture disrupted by air coming in behind the jets (crankcase vents). Look at a 1963 MGB air cleaner to see what I'm talking about. Only the front one in 1963, and its easy to replicate with a piece of steel tubing and a quick weld.



jeff@advanceddistributors.com

littlecars Avatar
littlecars David Bassett
Nashville, TN, USA   USA
1965 Chevrolet Corvair "Ski Team Transport SOLD!"
1965 MG Midget MkII "Buffoon"
1966 MG Midget MkII "Swiss Cheese...SCRAPPED"
1970 Triumph Spitfire MkIII "Tamara's Turd... SOLD!!!"    & more
Jeff, didn't they do this with the Midget air cleaners for a brief period in the mid 60's?

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Hey Jeff. I did the spray test around the intake and didn't find evidence of a vacuum leak. As for the distributor, it's one of yours, just received about a month ago. What's the best way to test it for vacuum leaks? (I am guessing suck on the hose and see if it holds vacuum?)

"Reducing the crankcase ventilation reduces a vacuum leak that dilutes your fuel mixture.". I'm trying to wrap my head around this sentence (it's that kind of day). Are you suggesting the "surging at speed" condition could be from venting the crank into the carbs?

On a side note, I've been reading through a LOT of related threads, and came across a few members who cured this problem by advancing their timing. Jeff, I set it to the low end of your recommendation (13 btdc), and liked the way it ran around town so left it there. Is it worth going more advanced? Might this be the magic bullet?

In reply to # 3655302 by B-racer Surging at speed is from a vacuum leak in most cases - lean fuel mixture. Check the vac unit on the distributor for leaks, and the intake manifold/carb gaskets/end plugs. Reducing the crankcase ventilation reduces a vacuum leak that dilutes your fuel mixture. Its not necessary to vent to the carbs. In fact, I usually vent to a tube that I weld into the back of the air cleaners, and let the carbs meter fuel to ALL the air coming in, instead of having the fuel mixture disrupted by air coming in behind the jets (crankcase vents). Look at a 1963 MGB air cleaner to see what I'm talking about. Only the front one in 1963, and its easy to replicate with a piece of steel tubing and a quick weld.

refisk Avatar
refisk Rick Fisk
Frankenmuth, MI, USA   USA
The needle is stamped on the big base end that goes into the piston. You have to loosen the set screw and remove the needle from the piston to see the number.

What oil to use is one of those questions with many answers. They came from the factory with light oil in them. Some people use motor oil, some use ATF, some use gear oil. The purpose of the oil is to slow the rise of the piston down when the throttle is opened. Slowing the rise of the piston causes a momentary enrichening of the fuel mixture. Works like an accelerator pump on other carbs. I use the same oil I put in the crankcase.

In reply to # 3655278 by littlecars
In reply to # 3654772 by refisk If you remove the dashpot and the piston (with needle) together there will be no spill. The oil is on top of the piston inside the dashpot. Take the dashpot and piston to your workbench, unscrew the black cap, and dump the oil into a suitable container. Be careful not to bang the needle on anything as it might bend.

In reply to # 3654768 by Poundingsand Thanks Rick. What happens if I remove the dashpots while carbs are in the car? Big oil spill?

I'm in this same boat, Peer. Rick, where is the needle stamped? Also I've read it is appropriate to use regular motor oil in the dashpot? I found the rear carb dashpot practically dry and the front carb had the proper amount.

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