she blowd up!

[Mike G.]

700 miles on a new motor and it came untied. I bought a 72 lt1 form a corvette dealer in 2007. it will be a recreation of my high school car. it had just had a fresh rattle can restoration, cheap paint job and a freshly rebuilt motor. i bought this car to build as a driver. i got all the recipts for what was done to the motor so i was satisfied with what i got. i drove it for a week and pulled it into the garage and pulled it apart. i sent the body to the body shop and i went to work on the chassis. nearly 3 years later i got the car put back together. the more i drove the car the worse it ran. i drove the car down the interstate and it started to make alot of noise so i turned around and went home. by the time i got home it had lots of valve noise and what sounded like a rod knock. it would backfire through the carb and i could barely keep the car running. i pulled a valve cover and 2 valves were not moving.

it looks like whoever built the motor did not zinc coat the cam shaft before they installed it. we pulled the motor and tore it down. 3 lobes on the cam were gone. all of the metal from the cam went down to the pan and worked its way through the bearings and crank. i had to replace the crank, all the bearings, cam shaft, lifters,oil pump and rings. so much for buying a car with a fresh motor.

the motor is rebuilt and back in the car. i am now in tuning hell. i can get the car to idel good or run good. i cant seem to get it to do both. the holley carb on the car is not correct. it is a 750 dp. might have to replace that too. this car has turned in to a money pit. it will be a fun car if i can get it to act right.

[Mike R.]

Re: she blowd up!

Sorry to hear about your engine...I have been there in the past. The end result 20yrsa ago was an all-aluminium big block...

If you are having tuning problems, try to find someone with a O2 meter. It removes the ambiguity of tuning carbs and goes much faster. If you want to drive it to Idaho, we can put some chains on it and have it tuned in no time....

The O2 monitor I have is a LM-2. I records (40 minutes) O2, rpm, and vacumn. You can see everything on graphs on your computer and make sense of it all. I currently have O2 bungs welded into the exhaust but I am thinking of taking a heat riser spacer and plumbing that for the sensors so I can remove everything when not tuning the new car.

700 miles on a new motor and it came untied. I bought a 72 lt1 form a corvette dealer in 2007. it will be a recreation of my high school car. it had just had a fresh rattle can restoration, cheap paint job and a freshly rebuilt motor. i bought this car to build as a driver. i got all the recipts for what was done to the motor so i was satisfied with what i got. i drove it for a week and pulled it into the garage and pulled it apart. i sent the body to the body shop and i went to work on the chassis. nearly 3 years later i got the car put back together. the more i drove the car the worse it ran. i drove the car down the interstate and it started to make alot of noise so i turned around and went home. by the time i got home it had lots of valve noise and what sounded like a rod knock. it would backfire through the carb and i could barely keep the car running. i pulled a valve cover and 2 valves were not moving.

it looks like whoever built the motor did not zinc coat the cam shaft before they installed it. we pulled the motor and tore it down. 3 lobes on the cam were gone. all of the metal from the cam went down to the pan and worked its way through the bearings and crank. i had to replace the crank, all the bearings, cam shaft, lifters,oil pump and rings. so much for buying a car with a fresh motor.

the motor is rebuilt and back in the car. i am now in tuning hell. i can get the car to idel good or run good. i cant seem to get it to do both. the holley carb on the car is not correct. it is a 750 dp. might have to replace that too. this car has turned in to a money pit. it will be a fun car if i can get it to act right.

[Mike G.]

Re: she blowd up!

i can tell by the water pouring from my eyes that it is running way to rich and i am getting less than 5mpg. all indications are that i have it over carburated. i am going to replace the power valve today and if that does not work i will get a new carb.

[Timothy B.]

Re: she blowd up!

If that is the same carburetor that was on the motor when the cam wiped maybe that's your problem. If the oil is diluted with gasoline, camshaft wear is probably the first problem to occur.

[John H.]

Re: she blowd up!

i can tell by the water pouring from my eyes that it is running way to rich and i am getting less than 5mpg. all indications are that i have it over carburated. i am going to replace the power valve today and if that does not work i will get a new carb.

Mike -

Double-pumpers are very finicky to tune for normal street operation, especially if they have 4-corner idle; I'd consider replacing it with a more "normal" vacuum-secondary model.

[Mike R.]

Re: she blowd up!

I guess I would disagree. Four hole idle is a lot easier than adjusting the adjusting the secondary throttle plate to ldle transfer slot relationship which changes both the airflow and the mixture at the same time.

If the carb is running super rich, and was running acceptably before, there are not that many things it could be.

1) The float could be leaking (sinking) or set wrong
1a) The needle and seat could have debris and not be closing
2) The power valve could be ruptured
3) a jet could have fallen out of the metering block.

When you check the inside of your carb, look closely at the idle channel feed restrictions and see if there is debris partially blocking one of both.

The carb you have can be made to work well and I don't see a reason to change it unless you want to have the stock carb. Also check the inlet needle and seat

When I was road racing, I used Barry Grant modified double pumpers and was really happy with the results.

Check the float level first. Then the needle and seat. Then the powervalce and idle feed restrictors.

Mike

Mike -

Double-pumpers are very finicky to tune for normal street operation, especially if they have 4-corner idle; I'd consider replacing it with a more "normal" vacuum-secondary model.

[Jim T.]

Re: she blowd up!

Mike I have experienced Holley problems #1 and #2 in your post. Ruptured power valve is easily checked without taking the carb apart. Place a finger over the fuel bowl vertical vent and if the engine dies in a few seconds the power valve is ruptured.
Leaking fuel bowl float in my Holley double pumper was a surprise since the carb was not an old one. This Holley came with plastic/non metal floats. The front float leaked and took on fuel inside. It weighted down the float enough to keep the needle and seat open to continously pass fuel.
Bought new Holley brass floats and replaced fuel bowl floats.

[Mike G.]

Re: she blowd up!

this is the same carb that was on it when the motor broke. it has the brass floats. we put a rebuild kit in it. i just think its getting too much gas. i am going to put a smaller power valve on it tonight. if that does not work i will replace the carb.

[Clem Z.]

Re: she blowd up!

if it is a double pumper install some .015 dia wire in the idle fuel restrictions to lean out the idle. check how far the idle transfer slots are open and it should not be more than .020,looks like a square,.020X.020 and more than that could cause your problem. you may have to open the secondary a little more at idle. double pumpers use larger jets and smaller power valve restrictions than street carbs so jet it down and increase the PVR the same amount in area.

[Timothy B.]

Re: she blowd up!

Mike,

If the power valve is not blown, changing the power valve will only change to timing of power enrichment. The A/F mixture is controlled by the power valve channel restriction, (PVCR) hole opening size.

Double check your air bleeds to make sure they are open because they act to break the suction so no siphon of gasoline. Turn the carburetor over and set the primary throttle blades to square transfer slot exposure. Set the idle speed with the secondary blades. You should be able to trim idle clean with the four corner idle.

[Mike R.]

Re: she blowd up!

If you have put an overhaul kit in the carb, it is not unlikely that a tiny piece of the old gasktet material is now in the idle feed restriction, or main jets (Guess how I know?). Carbs have very few moving parts and generally don't wear out.

If you do buy a new carb, I would be thrilled to pay for a flat rate box to send your old one to me. I will find a good home for it on my girlfriends 1965 GTO...

this is the same carb that was on it when the motor broke. it has the brass floats. we put a rebuild kit in it. i just think its getting too much gas. i am going to put a smaller power valve on it tonight. if that does not work i will replace the carb.

[Joe C.]

Re: she blowd up!

The Holley 750 or any of it's knockoff cousins is one of the most versatile carburetors made. It is the most versatile of the Holley 4150/series. With its downleg boosters, it has stronger vacuum signal at low manifold vacuum than the 650 and 585 (stock Holley for 327 engines).

In order for the engine to be running as you describe, your jets and/or power valve would have to be GROSSLY oversized and/or the PVRC would have had to be GREATLY ENLARGED by Bubba. That said, I would look for a rebuild problem or pay a good rebuilder to do the job for you rather than shelve a great carburetor such as yours.

Double pumpers are generally jetted richer than vacuum secondary models, but, again, not so much as to create the problems you are experiencing.

If I were you, I'd check pri/sec jets: if they are larger than 72/82 then I would change to 68/78 or 70/80. I would also check for a ruptured power valve. Power valve will be either 65 or 45. You should not have a secondary power valve. With the LT1 cam, you should use a 65 power valve. Your secondary jets should ideally be numerically higher than the primaries by 10, with 8 or 12 being acceptable.

Of course, you know that the power valve does not open and pass fuel thru the PVRC while idling and cruising..............only when the engine is under a load such that the manifold vacuum drops below 4.5 in-hg or 6.5 in-hg as per your power valve number.