Pressure Wave Coming Back Through Intake Every 1-2 revolution?
March 9th, 2018, 1:02 AM
#1
CF Beginner
Thread Starter
Join Date: Jun 2006
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
Pressure Wave Coming Back Through Intake Every 1-2 revolution?
Good evening. I've been a Chevy guy my whole life but this is my first post on Chevy forums.
I'm building myself a custom '39 Ratrod; personally designed and hand fabricated from scratch pretty much everything but the cab (also heavily modified). I picked up a Chevy 350 (casting number 10243880) off of craigslist that was from a '98 Tahoe so I've been told. The engine has been rebuilt and had minor work done to it but the previous owner did do a carb conversion to it as well. Original Vortec heads (12558062) still with Jegs dual plane intake and HEI distributor. The block was decked and rebuilt with stock internals from what I can tell from the part numbers while inspecting the bottom end; Crank shaft 14088526 and pistons 10172839. I was told the cam was replaced but have absolutely no idea what the specs are on it. Hydraulic lifters with roller rockers. I've got a Holly 650cfm double pumper on it right now. I'm not sure if any of this has to do with my problem but I thought I should share everything I know about the motor.
Now for my problem. When I go to crank over the engine with the air filter off so I can see what the carb is doing, I would guess once every revolution or every other revolution, a rush of air comes back up through the intake. It is enough to throw liquid gasoline (like if you pumped the throttle and fuel just came out of the squirter but hasn't atomized yet) about 6-10" up and out from the carb. I haven't isolated which side it is coming out of since it is a dual plane intake.
The engine is running right now. It's not running super well, and it needs to be tuned, but it is running. When the engine is dead cold, I've started it up and let it run for 10-12 seconds then shut it down. Every exhaust tube on the header feels like it’s the exact same temperature (if we need, I can shoot an infrared thermometer to really get precise). Other notable problems would be a DISTINCT clicking sound coming from the engine while its running but I have no idea where it's coming from. I've set the valve lash/preload on the rockers, but it was my first time doing so and I very easily could have messed that process up somewhere along the way.
My theory is that I've set the valve lash incorrectly on a cylinder's intake that was in the wrong position, so air is being forced out a valve that is not fully seated. This however is not supported by the even heat distribution across all cylinders exhaust because one would be misfiring, or at a minimum not burning completely/evenly/comparably to the others.
I've never had an engine do something like this before. Any help would be greatly appreciated! Thank you very much in advance!
I'm building myself a custom '39 Ratrod; personally designed and hand fabricated from scratch pretty much everything but the cab (also heavily modified). I picked up a Chevy 350 (casting number 10243880) off of craigslist that was from a '98 Tahoe so I've been told. The engine has been rebuilt and had minor work done to it but the previous owner did do a carb conversion to it as well. Original Vortec heads (12558062) still with Jegs dual plane intake and HEI distributor. The block was decked and rebuilt with stock internals from what I can tell from the part numbers while inspecting the bottom end; Crank shaft 14088526 and pistons 10172839. I was told the cam was replaced but have absolutely no idea what the specs are on it. Hydraulic lifters with roller rockers. I've got a Holly 650cfm double pumper on it right now. I'm not sure if any of this has to do with my problem but I thought I should share everything I know about the motor.
Now for my problem. When I go to crank over the engine with the air filter off so I can see what the carb is doing, I would guess once every revolution or every other revolution, a rush of air comes back up through the intake. It is enough to throw liquid gasoline (like if you pumped the throttle and fuel just came out of the squirter but hasn't atomized yet) about 6-10" up and out from the carb. I haven't isolated which side it is coming out of since it is a dual plane intake.
The engine is running right now. It's not running super well, and it needs to be tuned, but it is running. When the engine is dead cold, I've started it up and let it run for 10-12 seconds then shut it down. Every exhaust tube on the header feels like it’s the exact same temperature (if we need, I can shoot an infrared thermometer to really get precise). Other notable problems would be a DISTINCT clicking sound coming from the engine while its running but I have no idea where it's coming from. I've set the valve lash/preload on the rockers, but it was my first time doing so and I very easily could have messed that process up somewhere along the way.
My theory is that I've set the valve lash incorrectly on a cylinder's intake that was in the wrong position, so air is being forced out a valve that is not fully seated. This however is not supported by the even heat distribution across all cylinders exhaust because one would be misfiring, or at a minimum not burning completely/evenly/comparably to the others.
I've never had an engine do something like this before. Any help would be greatly appreciated! Thank you very much in advance!
March 9th, 2018, 8:15 PM
#2
I would start with check the ignition timing. if that is ok...check the cam/crank timing marks. if thats ok, do a compression test. I've seen jumped timing chains/belts blow fuel out the throttle body and have high compression test results.
