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| Joined: | 04-16-2018 |
| Location: | Santa Barbara, California USA |
| Posts: | 22 |
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I rebuilt the engine over the course of last year, put about 1,200 miles on it and the engine ran great and pulled well throughout the rev range (2.0 engine, Dellorto DHLA45Cs, 9.5:1 CR, stock cams). The car sat for a month or more until I had time to re-adjust the valves, and I got it going again last week.
The original Smiths tach did not work (needle was physically sticking and I have a Pertronix Ignitor), so last week I also installed a universal Auto Meter tach, which worked, except I would get missing/hesitation during light load/cruise, but only at about 3000 RPM, and only before the engine had fully warmed up. The tach needle would "blip" at the same time as the miss/hesitation, which would normally point to an ignition issue. I double-checked everything ignition-wise, and re-wired the primary circuit, which I had planned to do anyway: no change. The next plan was to take the car to my shop and use the oscilloscope to properly check the ignition primary and secondary circuits.
However, as I drove the car a bit more, I noticed the hesitation/tach blips would really only happen during part-throttle and in the RPM range during which the Dellortos transition from the idle to main circuits. The engine would pull cleanly to redline once out of this range. Furthermore, the engine would occasionally pop back/spit during a long period of closed-throttle deceleration, and the tach would blip then as well.
I pulled the idle jets, and while they weren't completely plugged, they were restricted. Once cleaned out, the hesitation and "blips" were completely gone. The restricted idle jets would allow the engine to idle and run normally in the low-RPM range, but would cause a lean condition in the upper reaches of the idle circuit. Because more fuel condenses on the walls of the intake manifold and ports of a cold engine, this effect was more pronounced before the engine had completely warmed up.
I already run two fuel filters (one upstream of the fuel pump, one in the engine compartment) and an in-tank strainer on the fuel pickup, but I might add another EFI-type fuel filter as well. I cleaned out the fuel tank as well as I could before I ran the car, but CA environmental regs make it difficult to properly clean a fuel tank . . . .
Interestingly, the slight bog/hesitation that I get during brisk acceleration from a dead stop is slightly worse with de-restricted idle jets, which tells me that my 58 idle jets are probably too much for my particular engine setup. The carbs are otherwise balanced and tuned well, and my engine setup and carb jetting pretty much mirrors Lotus "Spec 5."
I am not 100% sure why the tach needle would blip during a lean hiccup, but it might have something to do with how the Auto Meter tach is constructed:
https://www.autometer.com/resources/index/faq_view/id/49
Last edited on 06-16-2019 03:53 am by CDA951