'65 GMC 1500 project. From the Netherlands

[Prowbar]

Another piece of the puzzle is completed regarding the liquid propane conversion.

I added the filler hose and filler attachment which are hidden behind the license plate. It runs to the tank.

From the tank, a copper line is run to supply the converter. It first has to pass through a valve, which combined with the solenoid on the tank, turns on or shuts off the propane supply. The gas supply is controlled by the electric fuel pump and electric gas shut off valve.

I've also almost completed the wiring harness for the pumps and solenoids. The wiring has a relay that is enabled when the engine is turning. This is done via an induction pulse from the coil. With a switch under the dash I can change between gasoline and propane fuel.

I've added a second choke cable to operate the mixer on top of the carburetor to switch between gas and propane. This has to be hooked up.

Top tip: the 60-63 repro choke cable is a deluxe cable with a chrome bezel. Removing the bezel doesn't give you a standard choke cable with the serrations... Unfortunately.

In this conversion, I try not to modify the existing parts. I also use whenever possible, existing holes to mount parts onto. This way these modifications can easily be reversed and it keeps everything original, which I like.

All that is left to do now is to hook up the choke cable, complete the wiring and install the mixer with air cleaner. Then my fuel mileage triples (well the cost of fuel decreases that much

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)

Last thing: notice those V6 badges on there!!! Finally had them painted and they pop! Now the whole world can know there is a big block V6 in my truck

[Prowbar]

Propane conversion is, except for a few brackets and final touches, finished! I've made a little video that shows the GMC running on both gasoline and propane. Hopefully this will make it clear. Took it for a cruise today, has good power. Next thing to do will be to visit someone that has a good exhaust gas tester, to tweak the mixer settings for the proper mixture and emissions.



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[Ed Snyder]

Originally Posted by Prowbar

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Propane conversion is, except for a few brackets and final touches, finished!

By way of comparison, I thought you might like to see photos of another propane installation. This '63 GMC was owned by the regional natural gas company, and had been converted to propane when it was new. So it never ran on gasoline.

[Prowbar]

Originally Posted by Ed Snyder

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By way of comparison, I thought you might like to see photos of another propane installation. This '63 GMC was owned by the regional natural gas company, and had been converted to propane when it was new. So it never ran on gasoline.

That is neat! And they kept it as simple as possible. Looks like they removed the WW carb and installed a carb for gaseous fuels on there. Interesting to see the converter and fuel switch on the other side. As you said, only natural gas. Did they keep the fuel tank in? And the fuel pump was removed? Cannot see that in the pictures.

The installation looks clean, and that is my goal also. I have the big oil bath air cleaner from the 478, when I get the 478 in I'll see if I can 'gut' the air cleaner and 'hide' the air filter and mixer in there. Should look a lot nicer.

[Ed Snyder]

Originally Posted by Prowbar

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That is neat! And they kept it as simple as possible. Looks like they removed the WW carb and installed a carb for gaseous fuels on there. Interesting to see the converter and fuel switch on the other side. As you said, only natural gas. Did they keep the fuel tank in? And the fuel pump was removed? Cannot see that in the pictures.

I took those photos 21 years ago when I was trying to buy the truck. The owner wouldn't sell, though. The fuel tank was still behind the seat, but there was no filler cap on it. I think the fuel pump port was blanked off.

It was a cool truck -- short wheelbase half ton with a utility box on the back and the rare big back window. Hardly any rust or dents either. Odo showed about 90K miles.

[Prowbar]

A couple of things got finished today. First of all, I bought a Buick V6 HEI to use. I used the 478 distributor for the gear, and installed this on the HEI. The 478 distributor is pretty outdated because is has no vacuum advance. But it does have the uniset points, which is a neat system.

I followed Peter Chronis' method by welding up the shaft and turning it, then drilling a new hole. Akin to the original I re-riveted the gear on the shaft.

From a piece of aluminum I made the bushing myself. I made new plug wires from a generic V8 set. I set plug gaps to .045" to start.

I had already reserved a 12 volt wire from the ignition to use for the HEI. The ignition pulse for the propane is retrieved from the tachometer port on the HEI.

Installation is pretty tight but once it is in, it is fine. The first startup it idled immediately, which is awesome. I set initial timing to 12 degrees. My engine seems to like more initial to idle properly. I have ported vacuum advance, maybe that is why.

The second modification is my conversion of the stock PCV system. I don't like tapping into 4 of the 6 ports, so I removed the valves and capped them off with 1/4" NPT plugs. I then converted a push in valve cover breather to use a oil filler cap connection. A random Gates PCV hose I found online fits perfectly and redirects the blowby gases to the IMPCO mixer PCV port.

HEI, propane and the PCV conversion should clean up emissions significantly. Tomorrow I'll have it hooked up the exhaust gas tester and I'll see if I can share the final results.

[Prowbar]

Using the exhaust gas meter we adjusted the emissions today. Before it was almost 6% CO (carbon monoxide) at idle which is extremely high. However, with propane, you don't really notice the excessive hydrocarbon smell (at least outside). Readjusted the idle and checked the emissions at 2500 rpm which was fine.

We set a goal for about 1% CO at idle, which is still a little rich for propane but makes cold starts easier. With the dual exhaust on the left-right values differ a bit but nothing to worry about. The mixer is really sensitive to changes and when we got close we left it as is.

Overall very happy with the other values such as the unburned HC which are really low for an old engine like this. I think this is due to the HEI + PCV conversion. Out of curiosity, I might revert to the stock PCV and spark plug gap and then recheck. Should have done this when the points were still installed but oh well. I have a feeling the stock PCV system will produce more unburned HC's.

[Prowbar]

Had the chance to test the GMC on a chassis dyno today.

Did two pulls:
1. Propane. 123.7 net HP (DIN norm) at 3500. 296 Nm (219 ft lbs) at 2380 rpm.
2. Gasoline. 145.5 net HP (DIN norm) at 3990. 312 Nm (230 ft lbs) at 2480 rpm.

This is a good running but worn 305E with 125 psi in all cylinders, a very worn timing chain, a 2.25-inch true dual exhaust, HEI ignition, and the stock Stromberg WW carburetor.
SM420 transmission, 4.10 geared Dana 60 on 235/85R16 tires.

As you can see, the torque curve is fairly flat, with torque peak at 2400-2500 rpm compared to the factory's 1600 rpm. We tested to around 4000 rpm.
It also lacks some torque compared to Jolly's chart.

I am assuming the 305E that is in this truck is a fairly worn engine, abused throughout the years. Combined with the wide ratio SM420 it isn't a fast truck by any stretch of the imagination. I don't mean to race the thing either but I do like some power

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The power losses with running propane are definitely noticeable and this can be seen in the results.

I intend to gain a lot more power with the 478, and then I'm planning to repeat the chassis dyno test. very curious to see how it will perform.

[Prowbar]

Time to pull out the leaky 305 to install the 478. Need to borrow some parts from the 305 first. Also, finish the frame paint in the engine bay and paint the firewall in the 503 light green color. Started today by pulling the SM420, then took it apart to change the front seal (I had changed the rear seal prior) and chamfer the ends of the shift forks to allow for a cushion of oil to be forced in between the fork and shift ring. All looks good and new oil will be added when the trans is installed.

Now onto pulling the old 305, then freshening up the engine bay. Got quite a few jobs to do. Been looking to get started for a long time, quite the feeling when you get there

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[Prowbar]

Ok, pulled out the 305 today. Loosened all the bolts, connectors etc. Everything came out easily. Pulled the engine and the original mounts were pretty stuck to the frame, so the rubber separated from the steel plate. No big deal but a shame nonetheless. I have new mounts though, thanks to Musicman70 from 67-72 Chevy forum (Funky61is that you?) There was a thread from a guy rebuilding a 305 and it was hinted that these motor mounts from Anchor (2337) would work, and 2107 and 2108 for the transmission mounts.

Lifting the engine took more work than expected because the engine hoist couldn't lift the engine high enough, so we had to use a crane on the ceiling to lift the engine higher. In the end, I have the 305 sitting on wood blocks, waiting to donate its parts for its bigger brother.

In the last picture, you can see a possible oil leak that I did not discover before: either from the distributor or from the camshaft galley. There is a freeze plug in the camshaft hole but not the block-off plate as is on my 478.

Now onto cleaning the engine bay, then painting it. Then start transferring parts from the 305 to the 478.