'65 GMC 1500 project. From the Netherlands

[Prowbar]

Over the last few days, I reassembled the engine and front end. Started her up and broke in the cam.

Modified the intake manifold as per James' instructions, thanks for the tips James.

Took her out for a test drive. This thing pulls like a freight train... Literally twice the power of the 305. And amazing throttle response. Had her up to 3200 before I blinked. Need a rev limiter badly, going to buy an MSD 8727CT when the vendor here gets it back in stock. This is a HEI rev limiter that can be set between 3000 and 9900, so ideal for this application.

Also found out that these Walker 18230 mufflers choked up the engine pretty badly. They neck down internally. Immediately ordered Walker 17613 mufflers. These have internal baffles but do not neck down. Advertised as Mega Flow. Should be quiet when cruising but loud when you step on it.

Also found out that the SM420 trans can use new bearings... Never noticed it on the 305 because of the lower power output.

The stock clutch seems to hold up though, which kind of surprised me.

Haven't even tested it on gasoline yet, it should have even more power compared to propane.

So to anyone considering rebuilding a 305... Ditch the thing and get a bigger V6... You won't regret it.

Videos to follow.

[tbucketnut]

David, can you send a short vid through say through Whats App? I would love to see and hear your engine perform. It sounds really stout. Glad you are happy with the results. Funny how the power restrictors quickly show up like you mufflers. Too bad you are not in the Pacifi Northwest, I have a work area just for rebuilding the sm420's and a rebuild kit ready to go.
Congrats,
Steve

[Prowbar]

Hi Steve,

Sure, PM me your phone number (with the country code, ie. the full number) and I'll send you (or anyone else) some videos of the 478 running. Still need to upload them to YouTube.

The SM420 bearing noise was slightly present at first but seems to have vanished now. Maybe because the new gear oil finally has found all the crevices in the transmission. Works like it should now even shifts great into 2nd gear.

I'm going to do a driving video as well, these will follow soon also.

The mufflers make the engine sound like it has a cold, really odd. Didn't have that with the 305. Hope to receive the new mufflers tomorrow so they can be installed.

Tried gasoline today, and got the idle right, about 14:1 AFR. The 73 main jets are too large it seems. The AFR gauge went to 10 when transitioning into the main circuit

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and the engine stalled when I let off the throttle. I've set the float level correctly. Probably going to 68's and start there. The power valve opens at 8" HG and the idle vacuum is 15" HG, so right on the money there.

Driving is fun but requires a careful right foot. In 4th gear I can pull from 20 mph to 50 mph in one go, it just pours out the torque like it's nothing. And it'll get a 5000-pound truck moving for sure.

[James]

Originally Posted by Prowbar

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Tried gasoline today, and got the idle right, about 14:1 AFR. The 73 main jets are too large it seems. The AFR gauge went to 10 when transitioning into the main circuit

To view some links or images in this forum your post count must be 1 or greater. You currently have 0 posts. Maybe you should introduce yourself with a new topic?

and the engine stalled when I let off the throttle. I've set the float level correctly. Probably going to 68's and start there. The power valve opens at 8" HG and the idle vacuum is 15" HG, so right on the money there.

After all of the intake work on my 305, my main jet is a 71. My truck has a lot more power.

[Prowbar]

Originally Posted by James

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After all of the intake work on my 305, my main jet is a 71. My truck has a lot more power.

Interesting. Have to do more testing and experimenting to see what causes the running rich situation.

[Prowbar]

Quick video of the engine running:

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Driving video to follow. Haven't messed with the carb jetting yet, it is running really good on propane.

[Prowbar]

Been a month, not without its fair share of GMC action...

The bearing noise in the old SM420 started to get worse, figured it was time to do a transmission rebuild. Ordered a rebuild kit and new synchro's that arrived very quickly. Removed the trans and started the rebuild process. Used the solvent tank to clean all the parts, you'd be surprised how much old oil and sludge are present in every nook and cranny.

Disassembled everything, cleaned all parts, and assembled them with regular engine assembly lube, which easily dissolved in the gear oil.

Quite a fun transmission to rebuild. The rear ball bearing of the secondary shaft was pretty stubborn to get off, so I removed the outer ring and cage and welded up the inner race. Using a bearing puller it finally sprung loose.

All the gears looked excellent, but the bearings had a lot of play but did not look as bad as I thought they did. They were still the original New Departure bearings.

Ultimately there is no need to get all the fancy tools as described in the manual, if you use your imagination you can rebuild the trans with a shop press, some snapring pliers, and a set of bearing pullers.

The new transmission sounds and shifts great. Very satisfied with the results.

As for the HEI ignition, I had 2 modules fail in a short time. First the original developed a misfire the night before. The next day it would not start. Replaced with an OEM GM module, it lasted about 2 weeks, then quit on the same note. Now on the 3rd module. Installed it today and it runs well, hope it lasts a while this time. Checked all the wiring for shorts of problems, but couldn't find any. Getting a proper 12V signal to the BAT. Tach port is fine too. The coil and magnetic pickup all check out fine regarding resistance. The only thing that I can think of is the distributor ground, which, using the original hold down, could see why it would cause trouble. I modified the hold-down with some brass pins to improve the ground signal and hold-down ability with the HEI as the base is smaller in diameter than the points distributor.

Nothing else to report other than I am tweaking the jet sizes for gasoline. 73s were much too big, I dropped them to 66s and ran about 13.2 AFR in the main circuit at cruise. Still too rich for me. Probably change them to the original 61s to try and achieve get a 14.7-15 at cruise for fuel economy.

Also installed the new Walker 17613 turbo-style mufflers. These don't have the necked-down internal piping as the old mufflers did. They sound great but are a little loud, though. Might have to quiet them down some.

Seems the engine is fully broken in now, the plugs look great and the valve lash is stable.

[Prowbar]

Here are some additional pictures of the transmission install as well as the modified distributor hold-down. The transmission install "bridge" was something I borrowed some someone who made it for his truck, which also has the SM420.

[FetchMeAPepsi]

Originally Posted by Prowbar

To view some links or images in this forum your post count must be 1 or greater. You currently have 0 posts. Maybe you should introduce yourself with a new topic?

Been a month, not without its fair share of GMC action...

The bearing noise in the old SM420 started to get worse, figured it was time to do a transmission rebuild. Ordered a rebuild kit and new synchro's that arrived very quickly. Removed the trans and started the rebuild process. Used the solvent tank to clean all the parts, you'd be surprised how much old oil and sludge are present in every nook and cranny.

Disassembled everything, cleaned all parts, and assembled them with regular engine assembly lube, which easily dissolved in the gear oil.

Quite a fun transmission to rebuild. The rear ball bearing of the secondary shaft was pretty stubborn to get off, so I removed the outer ring and cage and welded up the inner race. Using a bearing puller it finally sprung loose.

All the gears looked excellent, but the bearings had a lot of play but did not look as bad as I thought they did. They were still the original New Departure bearings.

Ultimately there is no need to get all the fancy tools as described in the manual, if you use your imagination you can rebuild the trans with a shop press, some snapring pliers, and a set of bearing pullers.

The new transmission sounds and shifts great. Very satisfied with the results.

As for the HEI ignition, I had 2 modules fail in a short time. First the original developed a misfire the night before. The next day it would not start. Replaced with an OEM GM module, it lasted about 2 weeks, then quit on the same note. Now on the 3rd module. Installed it today and it runs well, hope it lasts a while this time. Checked all the wiring for shorts of problems, but couldn't find any. Getting a proper 12V signal to the BAT. Tach port is fine too. The coil and magnetic pickup all check out fine regarding resistance. The only thing that I can think of is the distributor ground, which, using the original hold down, could see why it would cause trouble. I modified the hold-down with some brass pins to improve the ground signal and hold-down ability with the HEI as the base is smaller in diameter than the points distributor.

Nothing else to report other than I am tweaking the jet sizes for gasoline. 73s were much too big, I dropped them to 66s and ran about 13.2 AFR in the main circuit at cruise. Still too rich for me. Probably change them to the original 61s to try and achieve get a 14.7-15 at cruise for fuel economy.

Also installed the new Walker 17613 turbo-style mufflers. These don't have the necked-down internal piping as the old mufflers did. They sound great but are a little loud, though. Might have to quiet them down some.

Seems the engine is fully broken in now, the plugs look great and the valve lash is stable.

Where did you get the rebuild kit and synchros from?

[tbucketnut]

Very nice work as usual. Did you end up buying the Novac kit? They probably have the most complete kit. I bet your synchros looked ok. I have yet to find a really worn one and Ive been through about a dozen Sm rebuilds. They are quite fun to do.

You might find that the 13.2 cruise A/F will be fine. I have a wideband and have done much carb and fuel injector tuning in the past. Striving for the stoichiometric mixture for a gasoline carb engine can be challenge, and found that lean surges will rear its ugly head. I attribute this to the differences in runner intake manifolding. I'm assuming you have one oygen sensor gathering the average of at least one bank if not the entire exhaust?? With that big monster engine, if it is running good at 13.2 I might be happy with that, but I fully understand the need to tinker for sure.

Keep up the good work, enjoy your write ups.