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Correct Carb for 260A

jeffnick

Regular Contributor
My Volvo 260A has a Rochester carb. I believe the tag on it says MCM 228. In poking around for a rebuild kit, I see this MCM 228 listed for the 305 and the MCM 280 for the 350. What's the difference - do I have a mismatch?
 
Since so many things could have been changed over the years, post a few photos of the carburetor that's on the engine now.
There is a rather unique Q-jet that was assigned to the Volvo Penta V-8's.
 
Here's some pics of my 260A carb (MCM228?). I asked an outfit that remans carbs what the difference between the 280 (350) and the 228 (305) was and they said they're the same. Why have 2 different ID's for the same carb?
 

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I don't see that as being an OEM correct Q-jet.
The Volvo Penta OEM Q-jet would look like this one.
Note the different fuel line inlet orientation. Your T stat housing may not allow for this style.

Look closely at where the throttle shaft plate would connect to this shaft.

$(KGrHqEOKiEE0j6vHY!UBN(LZCR8tg~~_35.JPG



While the fuel inlet is different on this one also....., look at the piece that connects to the throttle shaft.
(circled in red)
This piece is capable of two 33C throttle cables.


Qjet throttle cable plate.jpg

You may have a Merc or OMC Q-jet.

The throttle cable bracket is also not Volvo Penta.... perhaps a Barr aftermarket bracket.
The OEM bracket would have also supported the ignition coil... and allowed for two 33C cables (dual station boats).

Your T stat housing is OMC I believe.

Your alternator mounting is likely also OMC.
Volvo Penta, during this engine model, did not mount the alternator in this location to my knowledge.

Let's hope that you have the crankshaft sea water pump.... and not the belt driven pump!!!!!!

Not a deal breaker....... but just make sure that the fuel metering is correct for the Marine Engine application.

Since parts have been swapped around some, be sure to check the distributor's advancing specs.
See the AQ260 OEM specs for ignition timing.......... and most importantly, the TA (total advance and RPM).
These MUST meet Marine Specs.... not Automotive!
You may damage the engine if these specs are not correct.

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Maybe the black color indicates Merc or OMC for both my carb and thermostat housing? And the different tag designations are for linkage and fuel inlet? The reman guy said they're both calibrated the same.

I find the alternator quite handy where it is. :)
Yes, I do have the crank mounted raw water pump.
My manual says TA - 30° at 4200 with max rpm 40 - 4400. I'll put a light on it to check it, but I do know that the distributor is a Mallory marine model w/ mechanical advance.
 
  1. My manual says TA - 30° at 4200 with max rpm 40 - 4400. I'll put a light on it to check it,
  2. but I do know that the distributor is a Mallory marine model w/ mechanical advance.
1. Check your Volvo Penta OEM specs. Toss the Seloc or Clymers out the window.

You may want the 30* to be closer to 28* @ 3.5K rpm, or so.
The 40* @ 4.4K rpm is worthless....... the SBC Marine Ignition distributor should not advance to anywhere near 40*.

See this graph...... note that BASE advance will never be included in the vertical scale..... but will be included when you strobe the marks.
171135.jpg


2. Yes.... either a YL, YLM or YLU..... and would be the 624AV for this engine.
 
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Oops. The 40 - 4400 was actually 4000 to 4400 rpm max. Nowhere do I see more than 30* max advance. I'll check it with the light when weather permits to see what I'm actually running.
The graph is beyond my comprehension.
 
The graph is actually pretty easy to understand.... don't allow it to intimidate you.

The vertical scale shows the spark advance in crankshaft degrees.
The horizontal scale shows crankshaft RPM.

The thing you'll need to know, is that the distributor provides the progressive advance only.
Any progressive advance does not begin until after 800/1,000 rpm or so (if all is working correctly).

The BASE or Initial advance is a result of where the distributor housing is physically positioned or "set"!
BASE is always prior to any progressive advance.
IOW, at any rpm below 800/1,000, the engine sees only BASE advance.... and no progressive advance.
At above 800/1,000, the progressive begins as shown in the graph.

The vertical scale does not account for any BASE or Initial advance..... so it must be added when calculating TA.
It will not need to be calculated when checking TA dynamically, as your timing light can't help but include BASE as you strobe the timing marks.


For example, look at the RPM scale at 800 rpm.
You'll see ZERO distributor advance shown at the vertical scale.
Now add a BASE advance of 8*, and your timing light will actually see 8*
Now remove the BASE advance of 8*, and your timing light will see 0*
(on engine, this would be done by rotating the distributor housing one way or the other)

Now look at the RPM scale at 3.7k rpm.
You'll see 23* distributor advance (crankshaft degrees) shown at the vertical scale.
Now add a BASE advance of 8*, and your timing light will actually see 31* @ 3.7k rpm*

(you must raise the RPM to 3.7K while you strobe, in order to see it)
That's called dynamic timing..... or TAT (total advance timing) and is much more important than BASE for our Marine gassers.


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Thanks for the lesson...makes sense now. The graph would be useful to check the spring function in the distributor. So I'll set my light to 31* and take her up to 3.7k and I should see my flywheel mark move to TDC.
 
The graph indicates what the OEM wants and suggests for BASE and TAT.
Checking the TAT let's us know if the advancing system is functioning correctly..... and of course allows for any adjustment.

Use the digitally advancing timing light if you want to.
For me, I want to see this in REAL Time and REAL degrees, so I strobe the balancer markings in STD mode.
The balancer markings must read up to approx 35* or so... and must be accurate.

NOTE: I'd NOT recommend arbritrarily changing advancing flyweight return springs.
This is best if done by a PRO who owns a Distributor Machine and who is familiar with Marine advance curves/limits.
( this would be for the ole school conventional mechanically advancing distributor... not the EST type)

sun_620_640.jpg


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