Old school, question about HEI and timing.

So as you all know, I have my wonderful GMC motorhome that has a Olds 455 in it.

I got it back from the resto shop yesterday as she was in to fix some roof leaks. The owner of the shop has been working on the GMCs for 20 years so he knows a couple of things.

Anyway, on my way to drop it off, the rig was running fine, and all of sudden, power loss, backfiring, and stalling, dead on the side of the road. I popped the engine cover, verified I was getting fuel to the carb and then tried to restart. She fired right up. Put the air cleaner back on and off we went.....for 10 miles and it happened again. This time I called the owner of the shop and he said it was probably the ignition module breaking down under the heat. I propped open the engine cover to let the hot air come in and make me sweat while I drove. I covered the last 40 miles without incident to his shop.

Ok, so since it was there, I had him replace the module. He also put on a new cap and rotor for good measure and said he reattached a vacuum line he found disconnected. He also said he advanced the timing a bit.

I know he went too far on the timing as it Pings horribly under a heavy foot and she is now a dog off the line and accelerates too slowly (compared to before). Additionally, she seems to be running really rich as my wife was following and said she occasionally saw some black smoke from the exhaust (2 or 3 times in a 170 mile drive home) and the exhaust pipe is really sooty. Normally, I get about 250-300 miles between fill ups. Yesterday I had to stop on re-fuel on the way home. I estimate my mpg dropped from about 9mpg to about 4 or 5!

It's been awhile since I've had a carbed engine, but I don't recall advancing the timing to have such a bad effect on the performance. Obviously, I'll get my timing light out and bring it back a bit, but wondering what else to check out on her.
Also the idle sounds different, it's sounds deeper and thicker somehow.

I know he was REALLY busy, so I'm guessing he didn't give it a road test as he told me he figured, I'd be pleased with the way it was running and should feel a little quicker (typical for a bit of advance on the timing)...

John

If it wasn't 170 miles, I would probably bring it back to him. Probably be a good idea to let him know and maybe he can advise.

Sounds like, besides the timing, he adjusted the carburetor. Before the days of sophisticated machines, someone taught me how adjust the carburetor with a vacuum gauge. It worked great. Hook up vacuum gauge and adjust the adjustment screws, in or out, to get maximum vacuum. When you reach that point, turn it the opposite way just a touch.

Did you sell the Caddy John?


Tom

Yeah I figure he played with the mixture screws as well. I'm on very friendly terms with him and will consult.

John:

Two things to check:

1.  Remove the distributor cap and rotor.  Remove the module and get access under the plate (after 30 years can't remember how) and check to see if the mechanical advance springs and weights are intact (can't remember if the HEI systems had a mechanical advance) and the vacumm advance is working (mechanically).  If anything was seized in there, Jim advancing the timing may have put it over the top.

2.  Good advise about setting the idle mixture screws properly.  Old school way was to adjust the idle stop so the engine was just ticking over then adjust each mixture screw for max rpm then 1/4 turn rich (counter clockwise).  Do this for both primaries, adjusting the idle speed when needed.

When you get it running right, yank the engine out and sell it and replace it with an LS or 6.5 diesel

Couldn't make it down to Florida this year.  Only made it as far as Charleston SC.  Maybe next year.

Have not given up on a GMC yet but due to room constraints am now looking at a 35ft GMC/Nova TRS transit bus.  Series 50 Detroit Diesel and an Allison automatic.  Geared right and with a proper tune these things make great RVs.

Good luck

Bill

If the HEI ignition has a module, and computer, it may be timed electronically, and if so, advancing the base timing can actually screw up the whole timing map (especially with the beast it is moving). I would set the timing back to the stock advance. Changing the idle jetting/air will not affect the engine at speed. I would also check for a couple of swapped ignition wires. Anyone can get them mixed up once in a while.

Thanks for the info guys. I'll be taking a look at it this evening as I want to get it going well asap so we can use it.

Bill, my ultimate plan would be to move to mech injection HumVee diesel and the 1 ton front end mod, but that's a 25K proposition all said and done (way outta my budget)! Hoping to get a few years out of the old gas motor first.

Was as simple as the timing was adjusted in the wrong direction. Was fun as my buddy stood next to me and we were adjusting it while driving, lol. Once I was happy with it, I stopped and we tightened down the bolt. I didn't even put my light on it.

I will do do that, as I'm ordering new wires and plugs and new vacuum hoses, as well as cooling hoses.

John:

If you are interested, we can get you into the 6.5 conversion a whole lot cheaper than 25 big ones. The AMG blocks are now plentiful and a 300hp build can be done a couple of hundred miles north of you for about 7k.

Let me know if you are interested.

Bill

Sounds like you already have it sorted out. If it's a 4 or 5 pin ignition module distributor then it has mechanical advance. If it's a 7 pin then it doesn't.

The idle mixture setting you guys are describing is called "lean idle tip in" if you need to research the specifics.

convert2diesel wrote:

John:

If you are interested, we can get you into the 6.5 conversion a whole lot cheaper than 25 big ones.  The AMG blocks are now plentiful and a 300hp build can be done a couple of hundred miles north of you for about 7k.

Let me know if you are interested.

Bill

Duly noted and logged in the old noodle for future reference.
The 25K price also included installing a 1 ton front end.