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1997 5.7 OMC Cobra Bogs Down

J

Jestertattoo

New member
1997 5.7 OMC cobra. New carb, new plugs,new wires, new fuel, new fuel water seperator, new MSD HEI Distributor. Motor timed and dialed in to book specifications hooked up to hose. Water tested it and under acceleration bogs down and back fires. let off throttle and go slowly picks up speed. back off and try to give it more throttle and bogs down and back fires. and ideas?
 
Jestertattoo said:
1997 5.7 OMC cobra. New carb, new plugs,new wires, new fuel, new fuel water seperator, new MSD HEI Distributor. Motor timed and dialed in to book specifications hooked up to hose. Water tested it and under acceleration bogs down and back fires. let off throttle and go slowly picks up speed. back off and try to give it more throttle and bogs down and back fires. and ideas?
Click to expand...

OMC was out of business at the end of 1993 as it had been purchased by Volvo Penta.
If you have a 1997 Cobra package, it will be Volvo Penta.

Tell us more about your MSD HEI system and how you dialed in the ignition advance.
 
Rick,

Thank you for your reply. I found top dead center and the timing marks alingned up perfect in stalled the distributor, connected up the wires to proper firing order, connected a new digital timing light with advance. set the advance to 8 deg BTDC. adgusted distributor until proper RPM and 8 degree BTDC were achieved. shut down let motor cool. fires up good on cold start idles good. thats the process that was used.

fast forward to yesterday. the dristibutor that was installed has a Vaccum advance on it. i questioned that when it was being installed and was told that as long as it is capped off you dont need it? well i believe that is not the case and have since ordered a new distributor that is mechanical advance. i will follow same process to install and time unless i am missing something.

I did notice that in your signature you are in Portland? I am in Vancouver.
 
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Jestertattoo said:
Rick, Thank you for your reply.
You are welcome.

I found top dead center and the timing marks alingned up perfect
You mean that you did the PPS procedure and the marks were good?

in stalled the distributor, connected up the wires to proper firing order, connected a new digital timing light with advance. set the advance to 8 deg BTDC. adgusted distributor until proper RPM and 8 degree BTDC were achieved. shut down let motor cool. fires up good on cold start idles good. thats the process that was used.
Have you looked to see what the progressive advance is doing?

fast forward to yesterday. the dristibutor that was installed has a Vaccum advance on it. i questioned that when it was being installed and was told that as long as it is capped off you dont need it?
Who ever told you that it was OK is incorrect! That person should NOT be doing Marine work!
Any distributor with vacuum advance will be for an automotive application.
Not only will it NOT meet USCG regs, it will not offer the correct progressive advance and it's doubtful that it will offer the correct total advance.

well i believe that is not the case and have since ordered a new distributor that is mechanical advance. i will follow same process to install and time unless i am missing something.
Just be sure to also look at what the progressive and total are doing.

I did notice that in your signature you are in Portland? I am in Vancouver.
I am actually just east of Portland in Gresham.
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Rick,

Thanks again,

Perhaps that is where i am not connecting the dots. I do not know the difference between the progressive and total advance. i just hooked up the timing light matched the marks and rotated the distributor until it read 8 degrees and there you have it. that is the extent of my knowledge. I followed instructions but lack the total experience to know if it is right or wrong. I appreciat any and all help, the last time i took in a boat i was taken to the cleaners for lack of knowledge, i am a little gun shy about taking this one in. Thank You!!
 
Jestertattoo said:
Rick,

Thanks again,

Perhaps that is where i am not connecting the dots. I do not know the difference between the progressive and total advance. i just hooked up the timing light matched the marks and rotated the distributor until it read 8 degrees and there you have it. that is the extent of my knowledge. I followed instructions but lack the total experience to know if it is right or wrong. I appreciat any and all help, the last time i took in a boat i was taken to the cleaners for lack of knowledge, i am a little gun shy about taking this one in. Thank You!!
Click to expand...



This will apply to an engine whereby the TDC marks are accurate.
TDC (top dead center) refers to the location of the #1 piston while it is at the absolute top of it's stroke.

Since the fuel/air does not explode, it requires an ignition lead. In other words, it must undergo initial spark combustion prior to the piston reaching TDC.
This is because fuel/air undergoes a burn rather than an explosion.

At idle, the spark lead (spelled ignition advance) is known as BASE or Initial advance.

The 8* that you mention is when the spark occurs in relationship to where the piston is (in it's travel) during idle speeds.
In other words, at idle, the spark occurs at 8 crankshaft degrees prior to the piston reaching TDC.

As RPM increase, so does the need for more spark lead (spark or ignition advance).

So.... while the engine may crank and fire up on 8* advance, it requires additional spark lead as RPM increase.

Example only:

RPM ....... degrees advance
750 ............... 8*
1,200 ............ 12*
1,500 ............ 14*
2,000 ............ 18*
2,500 ............ 20*
3,000 ............ 24*
3,400 ............ 26*
3,600 ............ 26*
3,800 ............ 26*

Note that at 3,400, 3,600 and 3,800 RPM the advance has stopped increasing.
This is known as the Total Advance or TA and/or Full-In advance.

If these numbers were plotted out on graph paper, it would show what we refer to as the "advance curve", of which also includes the TA.

If you were to look at your OEM advance curve, it will show advance in the vertical columns, and RPM in the horizontal columns.
Note that it will be minus BASE advance.
This means that when calculating the numbers, the BASE advance value must be added to the value shown in each graph cell.

However, when strobing your timing marks dynamically, BASE advance can't help but to show up in what you see!


I hope that helps explain it.




 
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Rick,

That maked perfect sence!! Thank You!!

just so i have it straight... so my exact process Find TDC... look at marks 0 and 0 line up, hook up timing strobe and set at zero, fire up engine let warm up and idle, shine strobe turn distributor to dial in at 8 degrees BTDC and maintining 750 ish rpm. once that is dialed in then then rev up the rpm to 3-3500 rpm and total advance should be 26 maybe a little tweaking then re check idle.

Thanks again!!!
 
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