September 12, 2009
After months and months of trying to solve a problem on my 510, I've finally traced it down. And I certainly don't feel very smart about it.
Months ago, after working on the exhaust system in the car, I noticed that the engine was running really sluggishly, and it was backfiring through both SU carbs. Generally, backfiring through the carbs is caused either by a lean fuel condition, or by bad timing.
At the time, I had been tinkering with the distributor in order to shorten the throw on the mechanical advance (applying JB-weld on the mechanical advance slot to prevent the advance mechanism from traveling too far), and thought that I might have reassembled it incorrectly, so my first thought was to swap out my distributor with a known running one. I asked around on one of the local 510 forums and had a guy swing by with one, and after installing it, there was no change in performance. It still backfired.
So, I decided to run down every possible thing I could think of that could be the problem.
Fuel hose to carburetors: Replaced with new line
Dampening oil in SU carbs: Bumped up to 30wt from 20wt
Fuel mix: ran it richer and richer until it stumbled out
Fuel filter: swapped out with a new one
Intake/exhaust gasket: ruled out unmetered air by throwing on a new gasket
Cam timing: verified - notch/grove lined up correctly
Ignition timing: verified. also, ran it so far retarded that the exhaust began to glow red.
Distributor: swapped out with known runner
Coil: current one leaked after sitting on its side; swapped out with a new one
Ruling out all the simple things, I bought a Harbor Freight leak-down tester to see if I could figure anything out that way, but the readings were all within tolerance, although cylinders 2 and 3 were both a little lower. Also, the spark plugs to those cylinders were a little wet with oil and fuel.
I thought about this for a while and figured that the rings might be worn on those cylinders (a previous compression test ruled out a head or head-gasket failure), but I couldn't figure out how fuel would sit on those two plugs, and not on plugs #1 and #4. The SU carbs are set up with one carb on cylinders #1 and #2, and the other carb on #3 and #4. For the #2 and #3 plugs to be wet, it couldn't be related to the fuel mix from either carb.
On a hunch, I switched the plugs on #2 and #3 and... voila. No more backfire, and the engine was back up to running strong.
It turns out that knowing the correct firing order when installing plug wires is useless if you don't know which way the rotor under the distributor cap spins. I assumed it went one way, but it actually ran the other, so instead of running the proper firing order (1-3-4-2), I was running it the opposite way (1-2-4-3). The consequence of this is that cylinders #2 and #3 were both firing before the intake valves closed, which is I was getting backfire through both carbs.
:: sigh ::
I'm smarter for this, but I sure feel dumb for not having figured that out at the start. Fortunately, I didn't spend a lot of money trying to trace down the problem.
So... back on the road!