Hello, I'm hoping the more seasoned guys here might offer thoughts on this question.
Working on a 1973 115hp V4 acquired a number of years ago, but didn't get around to messing with it til now.
The fundamental question:
Is it possible that a power pack may appear to check out OK via typical DMM & DVA tests at cranking speeds (plugs-removed; fresh battery)...but might then irregularly drop spark on one or two cylinders while running at either idle or low-to-modest running speeds?
Scenario: This is a 1973 Johnson 115 that was acquired several years ago, but remained stored & unused until now. I've brought it up to a running state and currently am working to resolve an intermittent runtime issue.
* New fresh battery fully charged
* Compression-test: is very good 125+ with variance below 10psi between cyls.
* Clean fresh (mid-grade E10)fuel & oil
* Carbs have been carefully/thoroughly cleaned & reassembled with new kits.
* Ignition coils were newly installed some 3 yrs ago; but have virtually no real runtime on them.
* Fresh impeller install
* Used plugs; appear to be in pretty good shape, cleaned & properly gapped.
Primary running symptom:
At both idle & low-to-moderate rpm ranges the engine will periodically, although irregularly, kick up/down in rpm's by approximately 25 percent (give or take). [**Unfortunately no tach was available for precise measurement but I'm pretty confident in my subjective speed estimates].
Initial testing:
* Static DMM ohm's tests -- stator,timer,coils,rectifier,powerpack -- all appear to check out OK
* DVA tests -- stator, timer, powerpack -- all double-checked and appear to be within expected ranges - OK
Lake ramp testing:
Most testing done at idling speeds although the symptom also occurred while motoring around a small cove (at about 25% throttle) during a brief launch/recovery round.
The rpm-change symptom (up/down) is persistent, but the timing between each event occurrence is irregular. This will happen somewhere within a roughly 10 to 40sec time frame; and each event lasts in duration for perhaps between 5 to 15sec as well. Again, this behavior is irregular but persistent
Interestingly, the motor has no problem idling at rather low speeds (around 250-300rpm) before jumping up to higher revs (approx 375-425rpm estimated). Moreover, the motor doesn't seem to run rough or sound ragged or loped (e.g. unbalanced) in either state.
Given that the change (up or down in rpms) occurs rather abruptly when it does happen, my suspicion is the problem is more likely ignition-related than a fuel-delivery issue. So despite the absence of any noticeable shudder or significant shake (which I'd normally expect to see or hear with ignition-miss problems), I'm currently suspecting a spark drop-out on one (or possibly 2 opposing ) cylinders...
So again, my principle question is this:
Is it possible, or known to happen, that a powerpack can seemingly check-out OK in ordinary typical DMM & DVA testing... yet when under idle or modest running conditions might still drop spark on one or more cyls in an oddly intermittent and irregular fashion? -- I mean, this just kinda' feels like an occasional ignition drop-out/pop-back-in situation to me.
Thanks very much for your thoughts.
Charlie
Working on a 1973 115hp V4 acquired a number of years ago, but didn't get around to messing with it til now.
The fundamental question:
Is it possible that a power pack may appear to check out OK via typical DMM & DVA tests at cranking speeds (plugs-removed; fresh battery)...but might then irregularly drop spark on one or two cylinders while running at either idle or low-to-modest running speeds?
Scenario: This is a 1973 Johnson 115 that was acquired several years ago, but remained stored & unused until now. I've brought it up to a running state and currently am working to resolve an intermittent runtime issue.
* New fresh battery fully charged
* Compression-test: is very good 125+ with variance below 10psi between cyls.
* Clean fresh (mid-grade E10)fuel & oil
* Carbs have been carefully/thoroughly cleaned & reassembled with new kits.
* Ignition coils were newly installed some 3 yrs ago; but have virtually no real runtime on them.
* Fresh impeller install
* Used plugs; appear to be in pretty good shape, cleaned & properly gapped.
Primary running symptom:
At both idle & low-to-moderate rpm ranges the engine will periodically, although irregularly, kick up/down in rpm's by approximately 25 percent (give or take). [**Unfortunately no tach was available for precise measurement but I'm pretty confident in my subjective speed estimates].
Initial testing:
* Static DMM ohm's tests -- stator,timer,coils,rectifier,powerpack -- all appear to check out OK
* DVA tests -- stator, timer, powerpack -- all double-checked and appear to be within expected ranges - OK
Lake ramp testing:
Most testing done at idling speeds although the symptom also occurred while motoring around a small cove (at about 25% throttle) during a brief launch/recovery round.
The rpm-change symptom (up/down) is persistent, but the timing between each event occurrence is irregular. This will happen somewhere within a roughly 10 to 40sec time frame; and each event lasts in duration for perhaps between 5 to 15sec as well. Again, this behavior is irregular but persistent
Interestingly, the motor has no problem idling at rather low speeds (around 250-300rpm) before jumping up to higher revs (approx 375-425rpm estimated). Moreover, the motor doesn't seem to run rough or sound ragged or loped (e.g. unbalanced) in either state.
Given that the change (up or down in rpms) occurs rather abruptly when it does happen, my suspicion is the problem is more likely ignition-related than a fuel-delivery issue. So despite the absence of any noticeable shudder or significant shake (which I'd normally expect to see or hear with ignition-miss problems), I'm currently suspecting a spark drop-out on one (or possibly 2 opposing ) cylinders...
So again, my principle question is this:
Is it possible, or known to happen, that a powerpack can seemingly check-out OK in ordinary typical DMM & DVA testing... yet when under idle or modest running conditions might still drop spark on one or more cyls in an oddly intermittent and irregular fashion? -- I mean, this just kinda' feels like an occasional ignition drop-out/pop-back-in situation to me.
Thanks very much for your thoughts.
Charlie
