What are the usual failure modes on older Johnson V4 powerpacks?

[JarFisch]

I'm suspecting an intermittent powerpack spark dropout --- this despite OK DMM & DVA cranking tests..(?)

Working on a 1973 115hp V4. Recently got it running after sitting for several years, but now having variable rpms changes at idle & low-to-modest throttle speeds.

I'm happy to provide detailed info on work done & test results.. however I thought I'd get right to the main question rather than dump out a bunch of wordy details until asked or needed.

So here's really the question I'm wishing to ask:

Is it possible that an older (1973-1977 CD4) powerpack may appear to check out OK via typical DMM & DVA tests at cranking speeds (plugs-removed; fresh battery)...but might then might also irregularly drop spark on one (or more) cylinders while either at idle or low-to-mid running speeds?

Thanks,
Charlie

 

[sutor623]

First of all, I am hoping that you cleaned out the entire fuel system, with new carb kits and fuel pump diaphrams? Checked/changed all hoses etc??

It would benefit you to go through this list from top to bottom. You need to check the entire ignition at idle speed. The cranking DVA is a good test, but you need to check the DVA of the orange wires while connected to the ignition coils and spark plugs while running.

Ignition parts seem to fail after they get warm/hot. You need to check the spark condition when the issue is happening. If the motor idles well and runs like crap at cruise due to an electrical issue, you will not find the fault at idle. Make sense?

Here ya go
http://www.cdielectronics.com/support/

 

[JarFisch]

...I am hoping that you cleaned out the entire fuel system....fuel pump diaphrams? .....Checked/changed all hoses etc??

....but you need to check the DVA of the orange wires while connected to the ignition coils and spark plugs while running.

..... Ignition parts seem to fail after they get warm/hot. You need to check the spark condition when the issue is happening.....

....If the motor idles well and runs like crap at cruise due to an electrical issue, you will not find the fault at idle. Make sense?

Hey Pete,

Thanks very much for the reminders. Seems I did overlook a few things. Excellent points you've made here!

When re-doing the carbs (new kits) I *did NOT* consider the lines from the fuel-pump to carb bowls (deteriorated line-gunk from those lines clogging the idle circuits?) ~ well maybe... but she does idle really solid (in general that is). Nevertheless, that is now on the list for checking. -- AND I did check the fuel-pump screen; clean! Although I did not replace the diaphram. -- Yet another item to check on the list.

Another good point you make about checking spark condition while issue is occurring. -- So the deal is that the 'issue' didn't show up with the muffs on the motor at the house. And although the boat ramp is only 3 blocks away, when we backed her down to the water for warm-up testing... I would have had to get over waist-deep in the water myself to hook up meter/DVA to check while running... and of course I didn't do that. -- Obviously I need to get a tank/barrel setup here at the house. -- Now on the list!

And for the last point "....good idle...crappy cruise.." LoL! -- Yep! I understand that. And THAT's why I'm hoping someone can inform me of the typical failure behaviors of those powerpacks. I just wondered if those packs might be known to behave that way under 'operating' conditions despite static (or idling) conditions and the tests done at that level.

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BTW: LOVE the Starcraft Restoration Journal ;-) --- Looks like you also know a thing or two about baiting a fishhook
((AND... I'm curious ... did i notice a shuttle-bus in the background of one of your early pics (??)). -- Just so happens I've got a project bus of my own. It's been a couple years ongoing (kind of a PITA diesel ...but I'm still hoping that one of these days we'll be able to get some payback usage outta' it....

Appreciate the feedback! -- Never hurts to have someone point out your shoes are on backwards and you forgot to put on pants. -

Cheers!
charlie

 

[emdsapmgr]

Erratic idle at constant throttle can be an air leak. Possible that the butterflys on the two carbs are not in sync. If one is open more than the other, that can cause erriatic running. Pull off the airbox cover and make sure the link rod between the two carbs will completely close both seets of butterflys at dead idle. You may be able to sense plug wire firing with a timing light, (the timing lights are very sensitive to voltage) yet it still may not be enough voltage to fire a plug under compression. Once warmed up do a spark-gap test to confirm.