Fuel Dilution of Crankase Oil

[John D.]

Re: Fuel Dilution of Crankase Oil

this what blew up a lot of 3 X 2 427 BB because the carbs bled thru diluting the oil .

Clem,Remember the 68 BB blue convertible I had new? Well in 1970 the engine was toast for exactly what you mentioned. Nice thing was the 68 had a 50,000 mile warranty so GM replaced the block but not the heads. Timmy installed the engine and said the carbs were the cause of the failure. I hated those three twos in the old days. So I put block off plates on the end two carbs and ran the car for several years on a two barrel and the car screamed and ran perfect.
I warned Stu about hydraulic lockup. Hope our friend listens. Right before Fall Carlisle an FI customer of mine send me a pic of his 58 FI engine connecting rod. It was broken in half from lockup. After the incident he put on a siphon breaker kit. JD

[Stuart F.]

Re: Fuel Dilution of Crankase Oil

John and Clem;

Yes you have warned me and told me of the dangers of what I am dealing with here, and I greatly appreciate it. Trouble is, I don't know how I can avoid the risks and still make changes and do testing in an attempt to solve the problem. Right now I'm just letting the car sit under cover while I think and plan this thing through with the valuable help from your suggestions.

I changed the cap back to original, but tested the replacement and its' PRV seems to function correctly (air and shake test). When I get up enough nerve, I'm going to refuel with my TEL and then run temp tests at every point with the I.R. gun both before and after shutdown. I'm going to recheck my pump pressure as well. I have rebuilt my old pump and drilled the bypass hole in it as suggested, but will hold off changing that until I have enough reason to believe I am getting too much pressure to the Carb.

John; what is the siphon line all about. Not quite sure I understand. Teach this old dog a new trick!

Thanks;

Stu Fox

[Bill M.]

Re: Fuel Dilution of Crankase Oil

John and Clem;

Yes you have warned me and told me of the dangers of what I am dealing with here, and I greatly appreciate it. Trouble is, I don't know how I can avoid the risks and still make changes and do testing in an attempt to solve the problem.
Stu Fox

Would pulling the plugs before cranking eliminate the potential of hydraulic lock?

[Clem Z.]

Re: Fuel Dilution of Crankase Oil

Would pulling the plugs before cranking eliminate the potential of hydraulic lock?

i would just remove the fuel line from the carb to bleed off the pressure if you are worried about hyd lock. leave off the gas cap and drill the fuel pump valve plate and you will not have this problem

[Clem Z.]

Re: Fuel Dilution of Crankase Oil

Clem,Remember the 68 BB blue convertible I had new? Well in 1970 the engine was toast for exactly what you mentioned. Nice thing was the 68 had a 50,000 mile warranty so GM replaced the block but not the heads. Timmy installed the engine and said the carbs were the cause of the failure. I hated those three twos in the old days. So I put block off plates on the end two carbs and ran the car for several years on a two barrel and the car screamed and ran perfect.
I warned Stu about hydraulic lockup. Hope our friend listens. Right before Fall Carlisle an FI customer of mine send me a pic of his 58 FI engine connecting rod. It was broken in half from lockup. After the incident he put on a siphon breaker kit. JD

that is why there are a lot of the 3 X 2 engines that have a replacement block. drill the cap solved the problem on those 3 X 2 setup. i also recommend if you don't race your 3 X 2 setup corvette that you replace the end carbs needle/seats with a .097 instead of the .110 because the fulcrum arm on the nitrophyl floats is not a long as the brass float so you loose the mechanical advantage that holds the needle/seat closed and the smaller dia ones take less pressure from the float to hold them closed

[Cecil L.]

Re: Fuel Dilution of Crankase Oil

Stu,
Your biggest problem is the risk of hydraulic lock in the cylinder, not fuel in the oil, although too much is not good. Aircraft radial engines all used an oil dilution system for really cold starts. Fuel was pumped directly into the oil tank to reduce the viscosity for start. The fuel would then evaporate from the oil during normal operation. I'm not recommending it for auto use, but I would not be overly concerned with a little fuel in the oil as long as it's not so excessive as to reduce the viscosity to the relative state of hydraulic fluid.
WWIW

[Stuart F.]

Re: Fuel Dilution of Crankase Oil

Cecil;

Thanks for your advice. You have made me a little less worried that I will loose my engine due to the fuel contamination. I will keep a close eye on it to ensure it doesn't get too thin (diluted). I hope to rectify the cause before it is too late. I still can not understand why it is happening with two different carbs - each having been rebuilt and tested to hold pressure within 6.5 psi spec. If it were just dripping out (fuel) I wouldn't be so concerned, but it flows out of the accelerator pump nozzles on one and through the venturi tubes on the other. There is a lot of pressure in those fuel bowls.

Thanks again.

Stu Fox

[John H.]

Re: Fuel Dilution of Crankase Oil

Stu -

I know nothing about AFB's (never owned one); the Holley 3367, 3810, and some others have an idle bowl vent that opens the top of the primary float bowl to atmosphere when the linkage is in the idle position. Does an AFB have some sort of an arrangement that vents the float bowl with the engine shut down?

[Stuart F.]

Re: Fuel Dilution of Crankase Oil

John;

The AFB has no such bowl vent; however, there are 4 (full time) vent tubes that are positioned on an angle in the air flow path above each Venturi and provide air to chambers above the fuel bowls (chambers formed by top plate and plate gasket). Various top plate gaskets are used on different models, some with more open area to the bowls and some with less (insignificant).

It was suggested that one could siphon fuel out of the bowls after shut down through these vents, and use them to prime them again before startup. I have found that this would not be feasible due to the small size of the vent tubes, and the fact that they don't provide a direct path to the bowls.

Oddly, the only thing that looks like a vent valve is the Idle Compensator behind the secondaries which uses a bimetallic spring and a tapered button valve, but that is used to bleed in more idle air below the butterflys when conditions are hot enough to open it. I question its value nowadays.

It's a dilema that I'm still working to rectify, but I've slowed my pace out of fear for the consequences of the oil dilution. I've changed it twice in recent weeks because of this.

Stui Fox

[Cecil L.]

Re: Fuel Dilution of Crankase Oil

Stu,
There are vents and also an anti-percolation vent. Four inside vents are located in the air horns as you stated, one outside vent in the air horns, and four outside vents in the body. See the attached pages from AFB Manual. If you don't have the manual I can email you the rest of the pages. This manual is specifically for the early AFB used on the 348 engines into 61 but as far as I know they should be pretty much the same.

[Michael R.]

Re: Fuel Dilution of Crankase Oil

hi stu. with afbs you are overlooking the throtle shaft . they all go bad over time steel wareing on aluminun. this is a big problem and it wont matter how maney times you change needles. a good carb. shop can do this . i sent mine to the guy (i cant rember his name right now ) from st. louis. his is in all the books . he is very very good . and good with advice.

[Stuart F.]

Re: Fuel Dilution of Crankase Oil

Cecil;

I would greatly appreciate it if you could E-Mail the pages of that AFB manual to me. It is far more information than I have and certainly helps in the diagnostic/understanding process. Thanks for the offer. There are differences, as to be expected, but the main design concepts are the same.

Stu Fox

[Stuart F.]

Re: Fuel Dilution of Crankase Oil

Michael:

Several posters had mentioned concerns for shaft wear and as a result, I tested them with soapy water and air pressure (as I did with the n/s,s as installed). I also checked the floats for bouyancy (held under water). The older carb is a little suspect but still not too bad, in my experience. I'm also showing good idle vacuum with either carb - for an L-76 (always 12 to 14" hg). Thanks for the reminder though. I'll keep an eye on them.

Stu Fox

[Cecil L.]

Re: Fuel Dilution of Crankase Oil

Stu,
Email sent.

[Stuart F.]

Re: Fuel Dilution of Crankase Oil

Cecil;

As of 4:30 pm EDT I have not received the E-Mail. I'll keep looking and check my other sign on screen. If it is not there, I'll probably have to impose on you to send it again. I really would like to have that data. Thanks.

Stu Fox