Dr Joe Ehrlich Boost Port Engine test run

Published on 20 September 2023 at 15:05

While waiting for gearbox parts I thought it would be rather fun to have a bash at getting the Joe Ehrlich boost port engine running, from what I can gather it has been silent for the last 50 years or so. The challenge was that I had 3 cylinder heads - all different in minor ways, two barrels, numerous other bits and no idea with what belonged to what. I thought it would be a cunning plan to start with the cylinder head that appeared to have done the most running. Two barrels appeared to be much the same so I picked the one that looked the best. I only had one piston so no choice there, I fitted a new ring, again choosing the one that looked best.

The bottom ends looked the same, I had two cranks with different throws, one was missing the little end bearing so I chose the complete one and hoped for the best. When I had assembled the bottom end with new bearings and seals I placed the barrel on and had a look to see where the piston came to at top dead center. It looked about right so I was about 50 / 50 that I had a compatible set of bits together. I noted though that two head gaskets were required to get the right squish clearance - I should have paid heed to this fact, as I will get to later on.

I had measured the depth of the spark plug and chose one at random that would fit and not foul the piston, I found a nice new NGK that fitted the bill nicely. I had already checked my ignition system and knew I was getting a healthy spark. I had also set the timing according to a web site - this turned out to be a bad move. Most Motobecanes use a centrifugal clutch  that drives directly to the final drive. On the D52 it goes through a gearbox with two shafts, this means that the engine rotates the opposite way round to what I first thought so the ignition timing was initially set incorrectly. You have to set these so the points crack open at 1.5mm btdc, you set that by removing the cam from the crank and turning it until it just touches the points heel and starts to open them. It is important to set the points gap to 16-18 thou before you do this.

I borrowed a pull start cord from an old mower starter that I found in the shed, I wrapped it around the vee belt pulley so I could give it a go. despite everything looking right it didn't give a murmer. I lost count of how many times I tried, it just didn't want to know. I experimented with different numbers of head gaskets, when I had 3 on there I got a couple of pops but nothing more. I kept trying until I ran out of daylight and energy but made no significant progress.

I think it was about 2 am when I had an idea - I don't think Dr Joe would have come up with a solution that would require multiple head gaskets to make it work and I know it did work because Rex Avery broke a record on it. The answer had to lie in the height of the barrel. I nearly got out of bed to go down to the workshop but then I remembered that if I disturbed my Wife at that time of night she would most likely smash my stupid little face in so I went back to sleep instead.

At a much more sensible time of morning I had another rummage through my box of bits and found a much thicker base gasket - more of a spacer really of about 2.5mm. I reckoned that could be the answer. I took it off the barrel it was stuck to and put it on the base of my engine, followed by the barrel and head I had previously been testing. I measured the squish and felt that with a single head gasket I was going to be in the right ball park. I checked very carefully that nothing was going to hit and then did everything up for another test.

I was a bit skeptical at this point but you have to keep trying things and I thought it was worth a go. It would have been better if I had remembered to reconnect the points to the coil - that wasted a few minutes. With that rather important job done I tried again. I was sure I heard a pop from the exhaust, maybe it was just wishful thinking. I kept trying with my pull cord and yes, I was getting the occasional pop and bit of smoke coming out of the exhaust - it was trying but just wouldn't quite fire.

I then had the idea of using my drill on the magneto nut, I had to go carefully otherwise it would just undo the nut so I used the variable speed of the drill to slowly crank it up. There was no doubt it was firing now but still not running. I thought maybe the throttle needed to be open a bit more so while cranking it I turned the adjuster half a turn and there it was - running like a sewing machine, albeit one that had been owned by one careful owner and 6 dodgy ones. I tweaked the mixture, which helped but there was way too much oil - I had made sure everything was very well lubed when I put it back together and was running about 20 to one two stroke mix. The workshop soon filled with blue smoke so I came in for lunch and let it clear. My Mrs was less enthusiastic than I had hoped, her only comment being - that's nice, Dave. It's more than blooming nice, it's a proper happy day scenario!! I had been starting to doubt my skill level and really, really wanted to get this one going. It's not perfect yet, it's not picking up as cleanly as it should, I still have work to do. I have no idea what timing value Dr Joe set, I have mine set to the standard 1.5mm btdc, it may need to be a bit more than that, I shall experiment with that later. It may also need an air filter on it, it definitely ran a bit better with the exhaust on.

I now need to get on with the gearbox so I will leave you with a link to it running, it's a video I took while I was still messing about, it runs slightly better now. I will do another video when it's back in the bike and the box is hooked up.



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