Author Topic: 1970 Corvette L46 7040207 issues - several  (Read 347 times)

Offline KeithinMI

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1970 Corvette L46 7040207 issues - several
« on: May 10, 2021, 08:21:27 AM »
I have a '70 7040207 which I'm having several issues with, and here there are in no particular order:

Engine data: 1970 Chevy L46 350/350, 11:1 (allegedly but in reality probably 10.25:1). Cam specs in car 0.450/0.460...222/222...114, car does have comp cams 1.52:1 roller tips so probably squeeking out just a tad more lift. 47k miles on engine. Engine has brand new double-roller timing chain with the timing marks aligned 12/6 (no advance or retard on cam). Car is a non-california, 4 speed M21 with 4.11 rear-end.


Carb came to me "rebuilt", and I did put new airhorn to body, body to throttle-plate and carb to intake gaskets on. Carb has a nitrophil float, and has a "regular" needle/seat in it (I thought the '70 7040207 used the 'push-button' needle/seat as opposed to the 'hanging' needle (on the float).

Intake to carb gasket is a "one-hole" gasket, even though the factory intake (cast iron) is a "4-hole" intake. This appears to be the 'spec'd' gasket for 1970 L46 but could be incorrect

Appears to have stock jets, primary rods, secondary rods and hangar. Had specs but left them on my kitchen counter but can give the stamps on the rods later if needed. Have not yet measured float height (but think it should be .250, or .300-ish per Lars).

Car has new timing chagin, and timing was set using the "Lars Method" (36 degrees all in at 2800 rpm). Vacuum advance for the car bypasses the 1970 solenoid and is running directly to manifold vacuum on the car. Car has about 17.5" idle vacuum and is steady.

1) Car has off-idle stumble. Looking at the a/p holes, I see that a get a fairly decent pump-shot on the right side, but a weaker shot left side. A/P linkage is on inner hole (as spec'd for 1970 corvette L46). Checked a/p and it appears to have a new skirt on it. No idea about plunger inside.

2) Car will not idle when cold. Divorced choke is attached to coil on intake manifold and have verified that choke closes when car is cold and opens when manifold heats up (and the coil pushes the rod upwards). When I try to give it gas to start the car when cold (from idle), I actually have to pump the pedal to keep the car running. After warm-up car will idle at 800 rpm but stil has off-idle stumble.

Primary throttle plates are closed at idle and from what I could see, the transfer slots appear to be properly exposed.

3) With engine warm, if you hammer it there's a definite "bog". Pretty sure that's a lean-condition until the secondaries start flowing fuel. Have not verified using the "Lars-disconnect the a/p" method.

4) Car takes forever to start if it's been sitting overnight. Lots of cranking and then pumping the pedal for it to start. Secondary well-plugs leaking? I did notice that there was some JB weld smeared over the secondary plugs. The primary plugs are untouched.

-- I can't find the adjustment for the APT, figuring as with my other 2 QJ's on my Pontiacs, that because of the ethanol fuels, I need to fatten the APT just a tad (was about 3/4 of a turn on my Pontiacs). I'm guessing the APT adjustment is behind the steel-cap in the front-center of the carb, right? Really hate to punch that out as I want (at some point) to spend some $$ and get this car NCRS-judged ready.

-- I also haven't yet tried re-setting the secondary air valve spring (1/2 to 3/4 turn correct?).

Any suggestions would be greatly appreciated.


Offline KeithinMI

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Re: 1970 Corvette L46 7040207 issues - several
« Reply #1 on: May 10, 2021, 08:40:13 AM »
additional notes:

1.  it does not appear that the idle air tubes or restrictors have been changed. They appear to be "original" brass.
2. I have not verified (yet) whether the check-ball is loose in the bore or has been "frozen" in place. Carb was dry when sent to me and it sat inside my house for probably a year before I put it on the car.
3. Primary hangar does not appear to be bent and both arms are level. Primary rods are not bent.
4. APT is not jammed. I can press down on it with the carb assembled and it pushes down and springs back up.


 

Offline Cliff Ruggles

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Re: 1970 Corvette L46 7040207 issues - several
« Reply #2 on: May 11, 2021, 03:49:52 AM »
Comp Cams roller tip rocker arms are junk, as are most aftermarket "double roller" timing chains.

I've removed more sets of those rocker arms than I can count turned blue or black from the poor fit between the rocker balls and bodies.  Plus they used "grooved" balls which allows all the oil to run straight down the rocker studs instead of getting atomized by the rockers and cooling the springs.  Did I mention they are also "heavy" which is never a good thing on that side of the lifter.

Double roller timing chains have NEVER been an upgrade from a stock Morse timing set.  The only ones that come close are the "high end" variety with billet sprockets and seamless roller imported from Germany pre-stretched chain.  Even those stretch out pretty fast and maybe good for 25,000 miles.   We don't hear about these things simply because next to no one puts more than a few hundred miles on these cars per year so you could use a rubber band for a timing set and probably be OK for 30 years or so.  As it relates I just told a good friend that very same thing as he was having some running issues with his "high end" Pontiac 455 roller cammed engine.  He told me all the symptoms and I told him that it's the Rollmaster timing chain taking a chit.  He didn't believe me at first, then removed the distributor cap, turned the engine is both directions and saw a LONG delay before the rotor started turning again. He pulled the timing cover and the chain was ready to walk right off the gears, he had logged about 20,000 street miles on that engine since installing it 10 years ago....FWIW.

Anyhow, those carburetors have the large MAB's so pretty lean right to start with.  Not uncommon at all to see hesitation/stumble/bog right off idle at light throttle with them.  You will need to access the APT and/or go to smaller primary metering rods to cure that issue.  It may need a ver slight change to the idle system as well if you really want it to be flawless. 

Do NCRS judges really look under the front of the carburetor and would know if an external APT screw was an "add-on"?  Even if they were that anal I'd get the APT working and re-cap the hole after it was correctly adjusted and flawless everyplace.

....continued

Offline Cliff Ruggles

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Re: 1970 Corvette L46 7040207 issues - several
« Reply #3 on: May 11, 2021, 03:50:08 AM »
Get one of ,my rebuild kits and install the .135" high flow N/S (no windows) assembly and accl pump.  My accl pumps have a far superior seal on them plus custom wound springs to improve fuel delivery.  The solid seat prevents drain-back and obsoleted the windowed variety by 1975.  Set the float at .250", going lower just leans it up everyplace which is going the wrong direction here.  At a minimum it will need about 5-7 percent more fuel in the "normal" driving range to be happy.  Might look at the distributor vacuum advance to see exactly how much timing it is adding.

Bogging going to WOT is likely the choke pull-off.  If it was replaced in the last 10 years those simply open WAY too fast and cause a momentary lean condition.  It may also need a secondary cam and spring.  Some builders install a new cam but very few if any put a new spring in at the same time.  Every carb I do here gets both, and a pull-off timed correctly for the application.  Not sure why someone would disconnect the accl pump to only make stumbling WORSE going quickly to full throttle?  Taking it OUT of the equation for sure will only make things worse every single time, but it woln't tell you if simply replacing it will cure the issues. 

The only way to know if the bottom plugs are leaking is to remove the JB Weld (monkey chit) and pressure test them.  JB Weld is USELESS to hold back fuel right to start with, but 999 out of 1000 carbs that show up here have it smeared over the plugs whether they are leaking or not.  I guess folks don't know that Rochester fixe the leaky bottom plug issue by 1969 when they started pressing in aluminum plugs and swaging them in place.  For the record the leak rate prior to 1969 is 100 percent, but that doesn't mean ALL of them are leaking and dabbing any sort of epoxy over them is about as good as cutting your arm off and putting a bandaide on it.

Even if the car sat for months and fuel bowl was empty it should re-fill and start quickly.  In many cases it's the accl pump needing to get wet again and start working.  I've also see loose clamps and porous rubber hose suck air instead of fuel even when they weren't leaking fuel.  A bad fuel pump will also make restarts difficult after sitting a few days as they take forever to re-fill the carb.

Post the jet and rod sizes that you found in the carb and length and number of coils on the PP spring.  I stock the correct OEM PP springs for those, and anything else you would need to get it up to par for that application on this new fuel.......Cliff
« Last Edit: May 11, 2021, 05:03:43 AM by Cliff Ruggles »