'68 Tri Power issues

[Pete G.]

I own a '68 427 400hp coupe. Over the past two years, I have twice had cracked cylinders due to malfunction of the tri power carbs. The carbs are correct for the car (center Holley 4055 and end carbs Holley 3659). It seems that either the jets clog or the floats malfunction that dumps too much fuel into a cylinder. All carbs both occurences had been rebuilt by an experience mechanic versed in carburetors.

Is this cylinder crack scenario something other tri power owners have experienced? It's hard for me to believe that this type of carb issue, and such the cylinder cracking, is anywhere normal when carbs act up. Up to when I first experienced the cracked cylinder, I had never heard of cylinders cracking.
As you would expect, the fix of sleeving the cylinders is really expensive and I'm not willing to repair everything just to have it happen again.

Thanks in advance for the feedback!

[Clem Z.]

Re: '68 Tri Power issues

I own a '68 427 400hp coupe. Over the past two years, I have twice had cracked cylinders due to malfunction of the tri power carbs. The carbs are correct for the car (center Holley 4055 and end carbs Holley 3659). It seems that either the jets clog or the floats malfunction that dumps too much fuel into a cylinder. All carbs both occurences had been rebuilt by an experience mechanic versed in carburetors.

Is this cylinder crack scenario something other tri power owners have experienced? It's hard for me to believe that this type of carb issue, and such the cylinder cracking, is anywhere normal when carbs act up. Up to when I first experienced the cracked cylinder, I had never heard of cylinders cracking.
As you would expect, the fix of sleeving the cylinders is really expensive and I'm not willing to repair everything just to have it happen again.

Thanks in advance for the feedback!

with the fuel tank higher than the carbs and if the carbs leaks down you can have this problem of filling the cylinders and causing your problem. i have seen it fill the oil pan. my suggestion to owners who do not race their corvettes change the needle/seat from .110 diameter to .097 as these will hold there seal better against the pressure than the .110 diameter ones. also make sure your gas tank is not pressurized after you shut off the car.

[Pete G.]

Re: '68 Tri Power issues

Thanks Clem. Was this leaking issue common back in the day? Is it more pronounced on the tri powers since with three carbs there is triple the opportunity for leakage? Just curious what is different in managing this setup today vs back in the 60's? I don't mind making adjustments (needle/seat) and precautions (release tank pressure after rides) but I'm not thinking these steps were taken when the cars first rolled out.

[Clem Z.]

Re: '68 Tri Power issues

Thanks Clem. Was this leaking issue common back in the day? Is it more pronounced on the tri powers since with three carbs there is triple the opportunity for leakage? Just curious what is different in managing this setup today vs back in the 60's? I don't mind making adjustments (needle/seat) and precautions (release tank pressure after rides) but I'm not thinking these steps were taken when the cars first rolled out.

it was a problem back then as that was went we tried different things to correct it. make sure you gas cap is a external vent to relieve the pressure in the tank. i used to drill a 1/8" dia hole in the caps to let out the pressure. i had a owner who called because his corvette would not spin over to start and i not only found gas in the cylinders but also a lot in the oil pan. this is why a lot of big block blew up as the gas thinned out the oil.

[Patrick B.]

Re: '68 Tri Power issues

Thanks Clem. Was this leaking issue common back in the day? Is it more pronounced on the tri powers since with three carbs there is triple the opportunity for leakage? Just curious what is different in managing this setup today vs back in the 60's? I don't mind making adjustments (needle/seat) and precautions (release tank pressure after rides) but I'm not thinking these steps were taken when the cars first rolled out.

I had a similar problem in 1971 shortly after I bought my first Corvette, a ratty 67 435 coupe with wheelwell flairs, 3 tail lamps,etc. It was parked on a slight hill and one or more of the carbs leaked more than a gallon of gas into the cylinders and oil pan. Fortunately, it only bent the rod when I started it. I never considered the possibility of a broken block. By the way, if you take out the spark plugs and turn over the engine to see if there is any gas in the cylinders, be sure to short the coil first (or have a hose handy to put out the fire). Don't ask me how I gained this wisdom.

[Clem Z.]

Re: '68 Tri Power issues

the problem is the end carbs have a shorter fulcrum arm,less mechanical leverage,so the float clears the jet plate and this causes the needle/seat to have problems sealing against the fuel pressure and that is why i say use .097 needle/seat. full size cars with 2X3 carb setups do not have this problem because the fuel tank is lower than the carbs and you do not get the siphon action

[Patrick N.]

Re: '68 Tri Power issues

Is this inherent with the L68 having no fuel return line? Also, I think it was recently mentioned in another thread but is it considered cheap insurance to use a vented gas cap on these cars?

Best,
Pat

[John M.]

Re: '68 Tri Power issues

I had this problem on my 69 L71. I was working on the trailing arms with the rear end of the car on jack stands. Came home the next night and my wife said the garage smelled awful. I did not turn the car over because of the intense smell. Took all the plugs out and siphoned a lot of gas out of the cylinders and then changed the oil. This incident scared the devil out of me. Gagage attached to house etc. I did replace all the carb needle and seat things but will never trust the car in this position again. Also ditto on the vented gas cap; be sure you have one as you can build up some pretty good pressure in there.

[Clem Z.]

Re: '68 Tri Power issues

if you want to install the .097 needle/seat the holley part# is 6-506

[Joe L.]

Re: '68 Tri Power issues

I had this problem on my 69 L71. I was working on the trailing arms with the rear end of the car on jack stands. Came home the next night and my wife said the garage smelled awful. I did not turn the car over because of the intense smell. Took all the plugs out and siphoned a lot of gas out of the cylinders and then changed the oil. This incident scared the devil out of me. Gagage attached to house etc. I did replace all the carb needle and seat things but will never trust the car in this position again. Also ditto on the vented gas cap; be sure you have one as you can build up some pretty good pressure in there.

John------

All 1968 Corvettes use a vented fuel cap. It relieves both pressure and vacuum. However, it does not necessarily relieve to zero pressure or vacuum.

[Joe L.]

Re: '68 Tri Power issues

I own a '68 427 400hp coupe. Over the past two years, I have twice had cracked cylinders due to malfunction of the tri power carbs. The carbs are correct for the car (center Holley 4055 and end carbs Holley 3659). It seems that either the jets clog or the floats malfunction that dumps too much fuel into a cylinder. All carbs both occurences had been rebuilt by an experience mechanic versed in carburetors.

Is this cylinder crack scenario something other tri power owners have experienced? It's hard for me to believe that this type of carb issue, and such the cylinder cracking, is anywhere normal when carbs act up. Up to when I first experienced the cracked cylinder, I had never heard of cylinders cracking.
As you would expect, the fix of sleeving the cylinders is really expensive and I'm not willing to repair everything just to have it happen again.

Thanks in advance for the feedback!

Pete------


I know this might sound like "heresy" and "sacrilegious" but what I'd recommend is getting rid of the 3X2 and converting to L-36 configuration.

[Matt L.]

Re: '68 Tri Power issues

is this problem along the lines of fuel injections cars siphoning gas into the cylinders until a anti siphoning valve was installed? would the valve work on carburated cars?

take care,
matt