Chevy 350 crate motor problems

Topsterguy · July 14, 2012, 11:54:04 PM

I bought a "new" Chev 350/290 hp crate motor about 2 1/2 years ago and put it in my 40 about 2 years ago. It had a one year guarantee on it but I never got it in til the warranty was pretty much gone. It's ran good for the past year and a half or so, although has burned a little oil during that time. I just did a trip of about 500 miles and it went through about a liter and a half, and now it's been missing and backfiring here and there, so I pulled the plugs and they're all pretty much fouled with crusty black oil deposit. Anyone else had problems with the same motor? I've heard rumors about possible valve seal problems with them. Also while I'm thinking about it, what's the best plug to run in these? ( brand and number) Thanks!

wayne petty · July 15, 2012, 10:27:17 AM

before we tear the motor down...

are all the plugs fouled??? or just one or two...

lets go over other things on motors that can cause fouling.. and i know you have covered these.. but i just want to double check...

fuel related...

what type of carb.. and intake...

is the choke working properly.. opening fully within about 2 minutes..

the idle mixture is properly set.

the power valve not blown in the carb....

EGR valve if equipped is not leaking exhaust into the intake when the valve is closed..

pcv related.

is there a PVC valve installed in one valve cover.. a filtered breather in the other to prevent vacuum and pressure from building up in the crank case...

when you have the pvc out of the valve cover.. engine idling properly... when you block the end of the PCV valve.. does the idle speed drop 50 to 70 RPMs... showing that the PCV valve is sized correctly???

with the PCV in the valve cover... the other side open.. engine idling.. if you block the breather opening for a few seconds with your hand.. and pull it off... do you get a pop/thud as the air rushes in... you should..

with the engine idling.. after a few minutes.. pull the valve cover oil filler cap and look inside.. is there a LAKE of oil.. or just 1/2 an inch deep along the bottom...

is there much sludge.. clogged drainbacks raise the valve cover oil level over the top of the valve guides and can allow the plugs to foul... most chevy motors build since the mid 80s have ring retained valve seals instead of just an oring on the stem and a shield as earlier motors had...

exhaust related..

some applications have a heat riser valve in one exhaust pipe.. trap door where the manifold connects to the front of the exhaust.. if the trap door is stuck closed.. it will cause issues as the exhaust from one side is blocked and run through the intake passage to the other side.. causing massive amounts of heat in the carb boiling the fuel and causing rich running...

ignition related

backfiring could be caused by a missing ground strap under the HEI coil.. the folded metal tab is what i am talking about.. connects to the center wire of the 3 wires up from the module.. if that ground is missing. the voltage can build up enough on the HEI in cap coil that when it discharges it sounds like a shot gun going off under the hood..

leaking high voltage from the sides of the coil. this can cause reduced voltage output of the coil.. so it won't have a hot enough spark to properly ignite the fuel mixture..

can you OHM the spark plug wires .. set digital volt meter to 200K ohms... measure from the end to the inside cap terminal... the meter at this setting should display less than the length of the wire in inches... a 24 inch will if good will show less than 24 at this range.. probably 12 to 16..
1,000 ohms per inch max...

wires can burn out inside.. ohm test it quick and easy..

so many things can cause fouled plugs.. backfiring..

while changing the spark plugs.. do a compression test..

sorry to submerge you with info to check..

valve guides do wear... but before tearing the motor down to check that.. its important to go over the other causes..

oh.. if you plan on sand blasting the spark plugs in one of the spark plug cleaners.. please use some carb cleaner spray or brake cleaner spray with the nozzle to spray deep into the end to flush out any residual sand. the sand is harder than your cylinder walls and will fall in during operation and score your cylinder walls as the pistons and rings drag it up and down..

Topsterguy · July 15, 2012, 10:49:33 AM

WHOA! LOTS of ideas, and appreciated I'll do some more checking and get back to you! ALL of the plugs are pretty much the same - crusty black.

enjenjo · July 15, 2012, 09:28:15 PM

What do you have for a PCV system?

Topsterguy · July 15, 2012, 10:56:48 PM

Quote from: "enjenjo"What do you have for a PCV system?

PCV in the pass side cover, hose into the carb base. Breather on the other cover.

rumrumm · July 16, 2012, 03:54:04 PM

Two simple questions: Do you have a baffle in each valve cover? Are you using an aftermarket dipstick that is not reading correctly so you are overfilling the crankcase?

Topsterguy · July 23, 2012, 11:21:44 PM

Quote from: "rumrumm"Two simple questions: Do you have a baffle in each valve cover? Are you using an aftermarket dipstick that is not reading correctly so you are overfilling the crankcase?

Not sure, they're old M/T ones so I'll have to check. I filled the crankcase & filter with the right amount of oil originally then checked the dipstick and it read right so I left it. Went through about a liter of oil on a 600 mile trip recently so it's going somewhere. Been told by guys following me that they don't see it comming out the back.

wayne petty · July 24, 2012, 01:18:49 AM

all the plugs evenly fouled is probably not an engine mechanical issue...

the PCV system

or

the carb being too rich..

since you are burning oil at a rate of 600 miles per quart..

if you have the engine idling warmed up.. pull the pvc from the valve cover grommet..

if the pcv is sized correctly..
the engine idle speed should drop by 50 to 70 RPM when you BLOCK the end of the pcv valve..

if it drops more than that . the opening in the pcv is too big.

if it drops less than that.. the opening in the PCV is too small.

what kind of carb are you running.. ??

sorry to bury you with too much info above...

Topsterguy · July 24, 2012, 01:37:06 AM

I was kind of thinking that it wasn't an engine problem because they all fouled the same. I've got a holley square bore on it right now and I think that's the problem so I'm trying a Rochester Q-jet - I've got more faith in them. Will also check the PCV etc. Thanks again, will let you know what happens!

wayne petty · July 24, 2012, 09:48:58 AM

after you check the PCV.. as that is a major issue.. and improper sized PCV valves are responsible for a LOT of oil consumption.. no baffles can also cause issues..

lets look at the holley idle mixture adjustments..

if you turn them in... and please count the fractions of a turn so you can return them to where they were...

if you turn them in.. at idle.. does the engine DIE as you are shutting off the fuel... if it does not.. you may have an earlier holley and a blown power valve...

there are also along the front of the venturi's 4 tiny holes inside the air horn.. these are the idle and the high speed air bleeds... if they get dirt build up and it does happen once in a while.. you will have a carb that runs rich..

there is also a way to clean the carbon out of the cylinders .. but it involves holding the engine RPMS about 3000 on a fully warmed up motor and squirting water into the primary bores but stopping before you stall the motor.... i use usually things like a dish soap bottle with the pull up lid to dispense.. think french's mustard bottle.. warning.. this can destroy your motor.. the small amount of water gets into the chamber and turns to steam as the compression comes up and from latent heat and during combustion... blows the carbon off the combustion chamber and the tops of the pistons.. knocks its right out the exhaust port.. not a good thing to do this with a car or truck equipped with cats.. plugs them up with the ash ..

Topsterguy · July 24, 2012, 09:50:27 PM

Quote from: "wayne petty"after you check the PCV.. as that is a major issue.. and improper sized PCV valves are responsible for a LOT of oil consumption.. no baffles can also cause issues..

lets look at the holley idle mixture adjustments..

if you turn them in... and please count the fractions of a turn so you can return them to where they were...

if you turn them in.. at idle.. does the engine DIE as you are shutting off the fuel... if it does not.. you may have an earlier holley and a blown power valve...

there are also along the front of the venturi's 4 tiny holes inside the air horn.. these are the idle and the high speed air bleeds... if they get dirt build up and it does happen once in a while.. you will have a carb that runs rich..

there is also a way to clean the carbon out of the cylinders .. but it involves holding the engine RPMS about 3000 on a fully warmed up motor and squirting water into the primary bores but stopping before you stall the motor.... i use usually things like a dish soap bottle with the pull up lid to dispense.. think french's mustard bottle.. warning.. this can destroy your motor.. the small amount of water gets into the chamber and turns to steam as the compression comes up and from latent heat and during combustion... blows the carbon off the combustion chamber and the tops of the pistons.. knocks its right out the exhaust port.. not a good thing to do this with a car or truck equipped with cats.. plugs them up with the ash ..

Turns out that the grommet in the valve cover for the PCV was just a grommet - no screen / baffle etc, so it was sucking oil into the carb! So I put a proper screened / baffled one in and it seems fine.