SB 400 w/no go

Greetings all:

I'm helping out a young friend with a 1977 Monte Carlo, 400 SB. Heads were toast (cracked between 3-5). He bought replacement heads - new stock 1.94s. In addition, we installed a performer manifold and a 1406 Edelbrock (replacing stock cast manifold and Q-Jet). All other components are stock. Valves adjusted 1 turn from zero lash. Starts great, idles fine and good to agout 2500, then goes flat with bogging sound. Timing is 8 degrees initial as per the book. We have the distributer plugged into the non-timed vacuum port and the trannie plugged into the timed port at the front of the carb. P/brakes on the rear port and PCV on the big front one. K&N filter in the stock air cleaner housing.

Enginge ran fairly strong before turinig into a steam cleaner.

Any suggestions would be most helpful.

Regards, Glenn in Oregon

Did it overheat to crack the heads?
Sounds like you are losing compression or sucking air.
Is it backfiring or blowing are back thru carb or exhaust?

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Originally posted by glennsexton
Greetings all:

I'm helping out a young friend with a 1977 Monte Carlo, 400 SB. Heads were toast (cracked between 3-5). He bought replacement heads - new stock 1.94s. In addition, we installed a performer manifold and a 1406 Edelbrock (replacing stock cast manifold and Q-Jet). All other components are stock. Valves adjusted 1 turn from zero lash. Starts great, idles fine and good to agout 2500, then goes flat with bogging sound. Timing is 8 degrees initial as per the book. We have the distributer plugged into the non-timed vacuum port and the trannie plugged into the timed port at the front of the carb. P/brakes on the rear port and PCV on the big front one. K&N filter in the stock air cleaner housing.

Enginge ran fairly strong before turinig into a steam cleaner.

Any suggestions would be most helpful.

Regards, Glenn in Oregon

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check out that Q-JET secondaries, sounds like they are opening to soon. :HMMM:

The car did overheat and cause the head damage, however, that appears to have been the extent. I have no idea of how old the engine is, but there was absolutely no ridge in the cylinders and the internals of the engine looked good so we just replaced the heads.

There is no backfire and the car idles as smooth as silk.

Temp seems to be reght at 190+/-

Thanks for the tip on the leak.. I'll put a vacuum gauge on the un-timed port and see what's happening there.

Additional suggestions would be appreciated.

Regards All,

Glenn in Oregon

Thanks for the info. I suspect that this may be a vacuum related problem, i.e., not enough somehow.

The tranie was hooked to a cast aluminum "T" fitting on the original manifold which snapped off when removing. I plugged that hole in the new (Performer) mainfold and optied to use vacuum hose to hook up the trannie at the front of the carb. Maybe I should procure another fitting to get the trannie vacuum back to the original configuration.

Regards,

Glenn in Oregon

OK wise small block guys.. here's an update.

I installed a 1/2" spacer beterrn the carb and the air cleaner to make sure there was no clearance issues with linkage. No change.

Removed the carb and resealed the gasket. No change.

I checked vacuum at the carb port and it's 25+ at idle and about 10 - 12 at wide open throttle. Still bogs and it appears that the top plates in the secondaries do not open. I still think this may be vacuum related??

Thanks a lot.

Regards,

Glenn in Oregon

sounds like timing problem if vacuum advance hooked to non timed port that means running at manifold vac, correct? means vac advanced all the way from the start as you throttle you loose vac and mechanical takes over. disconnect vacuum from dist completely and plug off, set base timing to 8adv and SLOWLY rev engine and watch timing change all the way till you hit your flat spot test drive and see if that helps:::: with vacuum still unhooked::::: if it helps try to hook timing to ported/timed vacuum

Thanks Denny. I'll give that a try and report back.

Regards,

Glenn in Oregon

Here's an update:

I checked the distributer as per Denny's suggestion and it's nice and free inside. Timing set at 8 degrees and the mechanical definately works.

I installed an adaptor(as per Edelebrock) between the 1406 carb and the Performer manifold w/new gaskets to insure a tight seal.

Re tightened the intake (it has ARP staniless fasteners) to make sure there is no vacuum leak.

I still have the problem.

While at wide open throttle, a mist of fuel comes up from the carb and it just bogs down. It's almost like there's an electronic rev limiter set to about 2500 RPM.

As long as the primary (front two) barrels are all that's used under very gentle acceleration, it seems fine.

I'm really at a loss on this one and would appreciate any suggestions...

Regards,

Glenn in Oregon

Starts fine and idles smooth..

I concur. It's like a fine mist, don't know if I could catch a picture. The strange thing is that there is 12 inches of vacuum while this is happening..

I'm stumped on tihs one. Thanks for the feedback

Regards,

Glenn in Oregon

Okay, have you tried the timed port for the vaccum advance? It sounds like you have a LOT of vacuum to me. A healthy engine usually 17 to 19, not 25. It should be real close to 0 at WOT. So something is a miss.

i'm not 100% sure but i had a 350 with the same problem, i checked the carb vacuum and timing with no luck. It turned out to be a weak spark. like i said not 100% but worth a look right?:)

almost sounds like lifter problem or wiped camshaft--spits back thru carb/fine mist---higher than normal vacuum--but i would try a different distribtor or replace the distributor pickup:HMMM:

This may sound a bit elementary but, check your firing order!
I had the same thing on a 302.
Ran fine at idle. would not come up when throttle increased.

Also, on the 1406, make sure the springs on the rods for the jets are not sticking. Two small screws at the front of the carb. Check you base setting of the jets and rods to make sure you have the correct ones, also, was the carb new or used?

Thanks a lot guys..

This one really has me stumped. The distributer/bad spark may be a culpret. When I replaced the cap and roter, the inside of the distributer was rusty and gross beyond belief. Maybe the advance machanism is too weak and allowing the spark to advance to far? I noticed that with the advance set at 8 degrees BTDC and I connect the vacuum, the timing mark goes well offf the scale (probably 16 degrees +). Acceleration brings the timing mark to about 12:00 o'clock (straignt up) which is a lot o' timing!!

I'll try a different distributer and report back.

You guys are great. I've built a lot of chevy's and this is the first one that I have not been able to nail down. I maen, it's spark, gas and air, right??

Anyway, if any of you are out my way (Portland/Hillsboro Oregon) give a shout and I'll buy a great Tillamook Cheese Burger and a beer!

Regards All,

Glenn In Oregon

Glen, re-adjust the valves with the engine warm and running and only turn it 1/2 to 3/4 turn after it stops clicking.