Question about timing...

[sreams]

How much advance does a stock, fuel-injected M30B35 run at idle?

I'm trying to work out a slight misfire at idle on my carbed M30B35. Mine has the '72-style distributor, and I'm wondering if my timing is a bit too retarded at idle as a result.

It is my understanding that the older 2800cs distributors give more initial timing, so if this is indeed the problem, I see that as a potential solution.

Or... is it possible to limit the range of the mechanical advance in my existing distributor (again, assuming initial timing is an issue)?

What do you all think?

-Scott

 

[Mike Goble]

I have a 0231184008 (eBay steal @ $75) distributor in my carbed 3.5l motor. I replaced the plugs this morning and checked the timing. I have 36° total mechanical advance, and it's about 24° mechanical advance at idle. My idle vacuum is 19 inhg, and my VA can is all in at 14 inhg of vacuum and adds about 8° of advance at idle, making the idle timing 32° advance.

I have no stumble issues, I can floor the throttle at 1000 rpm and it accelerates away smoothly for as far as I take it.

 

[Stevehose]

And you're running manifold not ported vacuum

I have a 0231184008 (eBay steal @ $75) distributor in my carbed 3.5l motor. I replaced the plugs this morning and checked the timing. I have 36° total mechanical advance, and it's about 24° mechanical advance at idle. My idle vacuum is 19 inhg, and my VA can is all in at 14 inhg of vacuum and adds about 8° of advance at idle, making the idle timing 32° advance.

I have no stumble issues, I can floor the throttle at 1000 rpm and it accelerates away smoothly for as far as I take it.

 

[sfdon]

As Mike described, anything from 10 to 30 degrees btdc will work for idle.
Are you sure you are timing at the right mark?

 

[Mike Goble]

Being an old-fashioned SBC guy, I'm used to reading the timing from the front of the engine. I added a timing tab and marked my balancer at 0° and 36° BTDC.

 

[61porsche]

mmm

Idle- as Bill Clinton says," that depends on your definition..."

Let's say 800 rpm, no vacuum, 8-10 degrees. But you're supposed to time at 1700rpm at 22 degrees. At 3500 rpm with an early distributer your number should be 36 degrees. At 3500 with a later distributer your timing should be 32. There should be markings on the distributer so you can check the repair manual.

Now the much revered M90 is said to have the same cam as a B35 stock. Old timers say that distributer curve was all in much faster and that's why they say it "feels" different other than falling on it's face at 5200 rpm.

So here's my suggestion before all the other things. Find TDC on the damper and pointer compare that to the "ball" in the timing window. More often than not, it's off. Compensate for the difference once you have a definite starting point.

But I'm assuming you know what plugs, gaps, etc. are and everything they should be including going back over them all carefully. It wouldn't be the first time something small caused a misfire like one plug not screwed in all the way, junk parts like points, etc. or as I say, " I could've had a V8" moment.

Best of luck.

 

[sfdon]

The BMW engineers are crying in shame that they couldn't think of the obvious.
Nice work!