blown power valve

[427James]

Hi guys,

I just drove the Stingray back home and after a short while it felt as though it was running uber rich. It eventually got to the stage where it wouldn't run at all, especially if the accelerator was used and struggled to start (cue black smoke out of the back when it did). Sounds to be like a slow leak on the 4160 Holley and I suspect power valve - anyone else got experience here and can you change out a power valve without carb removal?

[Gixer]

Damm, I came in here wondering why someone was posting about a motor bike in the vette forum......

....to some of us, power valve has a whole different meaning

[roscobbc]

James - Holleys are not unknown for float bowl gasket material to break down (especially if carb had dried-out with infrequent usage) - tiny bits will get stuck under float needle valve or either primaries or secondaries creating similar symptoms to the one you described. If it is a power valve you just need to remove float bowl and metering block (power valve is attached to this on primary side) - if its a budget Holley it may not have one on the secondary side.

Edited by roscobbc on Thursday 30th August 17:55

Edited by roscobbc on Thursday 30th August 17:56

[427James]

roscobbc said:

James - Holleys are not unknown for float bowl gasket material to break down (especially if carb had dried-out with infrequent usage) - tiny bits will get stuck under float needle valve or either primaries or secondaries creating similar symptoms to the one you described. If it is a power valve you just need to remove float bowl and metering block (power valve is attached to this on primary side) - if its a budget Holley it may not have one on the secondary side.

Edited by roscobbc on Thursday 30th August 17:55

Edited by roscobbc on Thursday 30th August 17:56

Thanks Ross (as always) for the helpful info. Over here in Dubai, this is almost certainly going to be a gasket breakdown.

[roscobbc]

427James said:

Thanks Ross (as always) for the helpful info. Over here in Dubai, this is almost certainly going to be a gasket breakdown.

A pleasure James - 4160 was the lower spec Holley that many oem's would use - it doesn't have a some of the adjustability of the 4150. When we used to buy any s/h yanky motor we would always replace, plug leads, plugs, points, distributor cap, oil & filter and carb re-build kit (Jiffy kit) for first service. People just didn't bother (or didn't know how to) service the cars and the carb's always suffered from dried-out gaskets and components if the car had been unused for a time. Power valve usually only 'blows' if you get a significant backfire I believe.

[427James]

roscobbc said:

A pleasure James - 4160 was the lower spec Holley that many oem's would use - it doesn't have a some of the adjustability of the 4150. When we used to buy any s/h yanky motor we would always replace, plug leads, plugs, points, distributor cap, oil & filter and carb re-build kit (Jiffy kit) for first service. People just didn't bother (or didn't know how to) service the cars and the carb's always suffered from dried-out gaskets and components if the car had been unused for a time. Power valve usually only 'blows' if you get a significant backfire I believe.

Thanks Ross. Sorted it by totally cheating and sticking a 750 DP on it. Goes like a dream, except its a bit too rich - standing behind it makes your eyes water - will have to reduce the jets a bit.

[roscobbc]

427James said:

Thanks Ross. Sorted it by totally cheating and sticking a 750 DP on it. Goes like a dream, except its a bit too rich - standing behind it makes your eyes water - will have to reduce the jets a bit.

When I had the 427 in mine I popped a 750 dp on it - give me a few days if you like and I can look through earlier notes and see what jetting I used (although your being a bit 'hotter' will differ in requirements I would think) and give you a datum point.

[427James]

roscobbc said:

When I had the 427 in mine I popped a 750 dp on it - give me a few days if you like and I can look through earlier notes and see what jetting I used (although your being a bit 'hotter' will differ in requirements I would think) and give you a datum point.

Thanks Ross, as always.

[roscobbc]

427James said:

Thanks Ross, as always.

750 cfm DP carb
Changed Primary Power Valve to 6.5hg (these are generally 10.5hg & with hipo cam will create fueling issues)
Primary Squirter : no. 34
Primary Jets : no. 75
Secondary Jets : no. 85

[427James]

roscobbc said:

750 cfm DP carb
Changed Primary Power Valve to 6.5hg (these are generally 10.5hg & with hipo cam will create fueling issues)
Primary Squirter : no. 34
Primary Jets : no. 75
Secondary Jets : no. 85

You are a star, its running an LT-1 cam with a crazy duration and is overfueling. THis little lot ought to sort it.

If I ever get back to the UK I would like to buy you a beer for all the help.

James.

[roscobbc]

Do the power valve first - IF 10.5hg (and most seem to come stock with a high no.) AND you have low vacumn (below 10hg) at idle then the valve WILL operate and dump additional fuel in to engine - go with the lowest hg no. available and it will stop it happening! (DO NOT use a blanking plug for power valve or you will need to re-jet!)

[427James]

roscobbc said:

Do the power valve first - IF 10.5hg (and most seem to come stock with a high no.) AND you have low vacumn (below 10hg) at idle then the valve WILL operate and dump additional fuel in to engine - go with the lowest hg no. available and it will stop it happening! (DO NOT use a blanking plug for power valve or you will need to re-jet!)

Cheers Ross, will do. I may even be able to do it without carb removal. Carb is bogging when it hits the secondaries so I think it may be a touch rich on the secondary circuit (float bowls are set correctly).

John's got the 68 back putting a centreforce into it - the torque was slipping the old clutch and its lifespan was extremely limited! Also noted that with closing on 600 ft/lbs the engine torques over and the fan hits the shroud (the alternator belt also rubs the top hose at full throttle) - as the only other man with a similar lump in a 68 Corvette that I know, have you ever had this problem?

[roscobbc]

427James said:

Cheers Ross, will do. I may even be able to do it without carb removal. Carb is bogging when it hits the secondaries so I think it may be a touch rich on the secondary circuit (float bowls are set correctly).

John's got the 68 back putting a centreforce into it - the torque was slipping the old clutch and its lifespan was extremely limited! Also noted that with closing on 600 ft/lbs the engine torques over and the fan hits the shroud (the alternator belt also rubs the top hose at full throttle) - as the only other man with a similar lump in a 68 Corvette that I know, have you ever had this problem?

Yes I did have this problem! - as a matter of course when installing new engine I used new engine mounts (crap ones it seemed) - the drivers side one failed 100 yards up the road! - the fan chewed the rad shroud and the air filter was rubbing against the underside of the hood. I bought a pair of 'poly' front engine mounts (not too sure but it may be have been from Real Steel) - problem solved, the car seemed much 'tighter' afterwards with an improved gear changes and clutch operation.

[427James]

roscobbc said:

Yes I did have this problem! - as a matter of course when installing new engine I used new engine mounts (crap ones it seemed) - the drivers side one failed 100 yards up the road! - the fan chewed the rad shroud and the air filter was rubbing against the underside of the hood. I bought a pair of 'poly' front engine mounts (not too sure but it may be have been from Real Steel) - problem solved, the car seemed much 'tighter' afterwards with an improved gear changes and clutch operation.

Hah!

I have been wondering how that happened! The fan shroud is in pieces and the rad hose lasts about 200 miles! The car is with John so I'll ask him to check it.

Edited by 427James on Thursday 27th September 13:41

[roscobbc]

427James said:

Hah!t used
I have been wondering how that happened! The fan shroud is in pieces and the rad hose lasts about 200 miles! The car is with John so I'll ask him to check it.

Edited by 427James on Thursday 27th September 13:41

Truth was that until I removed the original engine I was unaware that a drivers side mount had broken - they do have a safety feature that prevent them totally coming apart, so at least the engine won't climb out of the engine bay! - I had got used to the scuttle shake and clutch judder etc and thought it was part of the Covette driving experience!