Fluid changes on BBC - Old but low miles

[F76Marion]

Matt,

Believe 153 on the spark plug gap.

The wide gap was to help with emissions and it didn't do so poorly at passcar loads, but at RV loads, it will kill the module. Speaking of HEI, get a replacement module it should have heat sink grease with it. Make absolutuely certain that you get the genuine Delco part and not the CCP (Cheap Chinese Part). Always have one with you. Carrying a spare coil isn't stupid either.

My group is GMC motorhomes from 76 to 78 they all had HEI. We know all about having he modules go out. (Mine is a 73 and so is brother Matt's.) We have not "upgraded to HEI because of that.

Believe what you heard about brake lines too. Matt had one become a check valve to one rear brake stayed on. We narrowly avoided a disaster.

Frank

[Buck_Jones]

My only concern with changing transmission fluid is the stories I’ve heard about ‘changed my fluid and my transmission stopped working!”. With only 6K I don’t think you would run into that!

[donr103]

No one mentioned p/s.. that too.. change trans oil 3 times.. too much stays in trans.. I change filter once then suck it out using vac pump Through dipstick.. so easy..
Good luck and keep us posted

[sploke]

Originally Posted by EyezOpen

The engine seems to run strong, other than a squeaky belt.

Well not to be a alarmed bomb thrower here, that needs a deep dive. Those old 454s have a truly strange emission system, one being AIR pumps, when those old pumps start binding/freezing it's a squeaky belt first...then much worse.

I'd check those pumps carefully, belts off and no slight binds on the pulley. Actually first check if your local deq compliance requirements. No work no lisc plates. With that being said most state's do not require those old systems to be functional. And if they do...that's big trouble. Parts are no longer available for much of the system.

Just an update here....figured out the previous owner completely removed the A.I.R. system, and did a clean job of it. The pumps and tubing were all removed, and they were successful in getting them out of the exhaust manifolds also. Only thing left are some nice little brass plugs in the manifold runners. Unfortunately there was some hacked up vacuum tubing that I *think* I have sorted now...the cockpit HVAC selector wasn't working (the flappers actuate using vacuum) and it works now, so that's a good sign. The S/C compressor doesn't activate though, so I still need to dog into that.

Originally Posted by Daveinet

Don't forget to check the front diff. Make sure the breather on the top of the diff is not clogged.

The rear brake adjusters can be ornery. I had to constantly adjust mine manually. I finally gave up and converted to Disk brakes using a Kodiac kit for the 8K Dexter axle. Its pretty much a bolt on kit. You loose your parking brake, but the added safety of increased braking is worth the trade off. You will go from very marginal braking to stopping like a car.

You may just want to change all your hoses as well. If the rear heater hose blows, it kind of makes a mess in the back bedroom. Ask me how I know. Always travel with a full tank of water and an adapter to connect the bathroom faucet to a garden hose. You just never know when you will need to make a roadside repair.

This is the post I was looking for...I have to make a note of this upgraded kit. After getting all the fluids changed I took the bus out to give it the old "Italian tune-up" and quickly realized just how marginal the stock brakes are.

Also realized the hard way that the rear heater hoses are leaking

Good news though, I mentioned to my wife that a 502 crate motor would be a drop-in replacement and she didn't laugh me off...actually started considering the idea. Soooo that's probably in the cards for a longer term project

[Daveinet]

Yes, there are a few things wrong with drum brakes in general. One of the worst aspects is that there are two delays built into the design. The first is that the distribution valve will not allow the rear brakes to apply pressure until the front brakes apply pressure. They do this so you don't lock up the rear and skid sideways. Motorhomes are too long to skid very far sideways, so that aspect is not needed. The second issue is that drum brakes use the rolling of the vehicle to increase the shoe pressure. This also results in a very non-linear delayed response. Between those two factors, its pretty easy to add another 20 to 30 ft to your stopping distance. It turns most normal stops into panic stops. So when I did mine, I removed the proportioning valve. This also is required because the proportioning valve has a low pressure check valve that keeps 10 lbs of pressure in the line at all times. This keeps the shoes close to the drums and is not needed for disks. (actually it will wear out the disks prematurely) Mine also had a vacuum booster for the rear. I left it in. The peddle pressure was like stopping in my Mini Cooper. Very light, but very linear.



On the 502, if I had to do it again, I would have just gone with the RamJet 502. It uses a better head which has a more efficient burn than the high torque 502. Also with fuel injection, you don't loose any torque off the bottom end.



I would suggest that before dropping the 502 in it, have Carl Jantz build the diff. A full throttle downshift at 50 mph took out 3 teeth on my ring gear. I was towing a Grand Cherokee at the time on a two lane road and decided to pass someone just putting along. I was doing well over 80 when I pulled back in, but after that I started hearing noises off throttle. That got worse over time. Fortunately I was only about 30 miles from home when it happened, so I was able to nurse it home.


Its fun to dream about stuff, but dollar for dollar, the brakes were the best investment return of any of the modifications I made.

[sploke]

Originally Posted by Daveinet

Yes, there are a few things wrong with drum brakes in general. One of the worst aspects is that there are two delays built into the design. The first is that the distribution valve will not allow the rear brakes to apply pressure until the front brakes apply pressure. They do this so you don't lock up the rear and skid sideways. Motorhomes are too long to skid very far sideways, so that aspect is not needed. The second issue is that drum brakes use the rolling of the vehicle to increase the shoe pressure. This also results in a very non-linear delayed response. Between those two factors, its pretty easy to add another 20 to 30 ft to your stopping distance. It turns most normal stops into panic stops. So when I did mine, I removed the proportioning valve. This also is required because the proportioning valve has a low pressure check valve that keeps 10 lbs of pressure in the line at all times. This keeps the shoes close to the drums and is not needed for disks. (actually it will wear out the disks prematurely) Mine also had a vacuum booster for the rear. I left it in. The peddle pressure was like stopping in my Mini Cooper. Very light, but very linear.



On the 502, if I had to do it again, I would have just gone with the RamJet 502. It uses a better head which has a more efficient burn than the high torque 502. Also with fuel injection, you don't loose any torque off the bottom end.



I would suggest that before dropping the 502 in it, have Carl Jantz build the diff. A full throttle downshift at 50 mph took out 3 teeth on my ring gear. I was towing a Grand Cherokee at the time on a two lane road and decided to pass someone just putting along. I was doing well over 80 when I pulled back in, but after that I started hearing noises off throttle. That got worse over time. Fortunately I was only about 30 miles from home when it happened, so I was able to nurse it home.


Its fun to dream about stuff, but dollar for dollar, the brakes were the best investment return of any of the modifications I made.

Awesome information, thank you, especially regarding the differential. Did you do anything different with the transfer case? I haven't found much information as to the "stoutness" of that unit, but it worries me as a weak link in the drivetrain for no particular reason.

[1972VWbus]

For that old of a rig do not go with those wide spark gaps. You will over strain the ignition coil. Stick with the recommended gap for that year. Likely was .035

[F4Gary]

With all the emission crap gone, you might make sure you are using manifold vacuum for your vacuum advance and not port vacuum.

[1972VWbus]

Originally Posted by F4Gary

With all the emission crap gone, you might make sure you are using manifold vacuum for your vacuum advance and not port vacuum.

Don't recal the vacuum can specs on those early years. Were they DVDA or SVDA distributors?

[Daveinet]

Originally Posted by sploke

Awesome information, thank you, especially regarding the differential. Did you do anything different with the transfer case? I haven't found much information as to the "stoutness" of that unit, but it worries me as a weak link in the drivetrain for no particular reason.

I had the 502 for 9 years and averaged somewhere between 5 and 8K miles per year. I towed often and never hesitated to get on it when "necessary". Never had any issues with the transfercase. That being said, the chain is no longer available, so if you do break one, you need to find a creative solution. That being said, it would probably not be that difficult to adapt something. Its not like a TH400/4L85e is rare. (both have the same bolt pattern on the output)

Carl Jantz puts bearing girdles on the bearing caps to keep them from spreading under torque. When they spread, you loose mesh and break teeth. So if you keep that from happening, it becomes much stronger.

[sploke]

Originally Posted by 1972VWbus

For that old of a rig do not go with those wide spark gaps. You will over strain the ignition coil. Stick with the recommended gap for that year. Likely was .035

This is what I used, thanks!

[fatsurf]

I have owned a couple of this vintage big block. To wake them up with the least amount spend they need two things. 1, Install an Edlebrock Performer intake. 2. Change the advance springs on the HEI distributor to some springs that allow advance to be all in at ~ 2200 RPM and adjust initial to 10* (you can also swap out the weights to increase or reduce the amount advance). If you are Quadrajet knowledgeable changing the metering rods can get the mixture where it needs to be (richen it up).
Oh and headers don't forget the headers.

[Daveinet]

Something I discovered when tuning the 502 is that the intake efficiency is very non-linear. This means it would have been impossible to tune correctly with mechanical adjustments. The intake tuning abruptly became very efficient at 3000 RPM. So below that RPM, I could put about 25 degrees advanced at full throttle. When it hit 3000 RPM, I had to take it back to about 10 degrees advance. Basically there was a 200 RPM window, where the efficiency changed dramatically. That was something I didn't discover for about the first two years of playing around with it. I was using standard timing curves, so I lost a ton of available power in the bottom end. I was playing with the advance, and watching the knock count. I discovered the engine didn't knock in the low end. So I started playing with it. Found exactly where the intake came on the tuning. Totally changed the throttle response and sound.

With fuel injection, you are not really programing a curve, you are just filling in a table. The table has 256 different cells, so you can put any number you want in any given cell. The computer just does a look up to the table and based on RPM and MAP, just does whatever the table says. There is a steep learning curve, but after going through it, I would never go back to a carb or mechanical dizzy. You can strip off a complete fuel injection system from a bone yard for a few hundred bucks and retrofit it into just about anything. The internet has a ton of support, so there is no problem getting help.

[EyezOpen]

A 502 should is quite capable of running 32...actually 34/36 degrees at 3000. Look at GM's spec sheet, something is amiss there. Perhaps a slipped balancer is that where you took your timing read?

Some things never change..

Lean burn more timing
Rich burn less timing.....

Timing curves remain constant no matter if a hei fires the pulse or a ecu..its all the same.

https://www.chevrolet.com/content/da...ifications.pdf