Will engine run with incorrectly timed MFI pump?

[elflamo]

Hello all,

after one year of slowly rebuilding my engine, I'm now going through the first miles (currently driven 30). Although the engine idles nicely, there is a lot of hickups and misfiring going through the 1300-3500 rev range. Until 4500 revs the engine run smoother but low on power. I'm new to engine rebuilding so I've difficulty to assess whether this is due to the engine needing to be run in or that things are things not adjusted correctly.

Here's the question: on checking the timing of the MFI pump, I noticed that I have it timed on the stroke right after TDC on cyl no1, meaning the stroke *before* overlapping TDC. From what I understand from Check Measure Adjust, the pump should be timed during the stroke *after* overlapping TDC.

Clearly something is not correct (either my reasoning or the timing), but what puzzles me: would the car run at all if the pump was timed incorrectly as now seems to be the case?

Many thanks in (timing) advance!

[HawgRyder]

It is my understanding that the factory tried offsetting the timing by a tooth or two on the 917.
They found no significant change at the time....but I may be wrong on this.
Bob

[304065]

What you are describing is the result of a mistimed pump. By 4500 RPM there's just a cloud of fuel present before the intake valve so it doesn't make as much of a difference. At lower speeds the interval between injection events is long enough to matter.

CMA timing is easy to follow. The mark on the pump lines up with the mark on the pulley at 40 degrees after overlap TDC. "Overlap TDC" is a place well-known to engine builders, but you can find it by going to TDC on the compression stroke- that's where a whistle placed in the spark plug hole stops whistling and the Z1 mark lines up with the notch in the blower housing, then rotate the engine one more revolution until the Z1 mark lines up again. Then rotate 40 more degrees until the FE mark lines up with the notch.

You will need an M13 socket to loosen up the bolts that hold the pump to the mounting plate so you can move the pump. Using a handheld mirror and a shop light placed on the crossmember, look behind the pump, slip the belt off, rotate the pulley till the marks line up and put the belt back on.

Sounds easy? It's an all day job that requires a third joint in your arm.

[philippeF]

you set the engine on TDC cyl1 and then turn 360° more ... a little bit more after will be the FE mark
then you're right.

[confused]

Basically the FE mark is on TDC of cyl #4

So you time the pump TDC to #4 TDC

[elflamo]

Thanks for the replies. I'm aware (now) of the procedure and will correct things tomorrow.

What confuses me is that the engine runs very nicely when idle, it appears that the mistimed pump has limited effect there.

[304065]

Originally Posted by confused

Basically the FE mark is on TDC of cyl #4

So you time the pump TDC to #4 TDC

Listen, when #1 is at TDC on the compression stroke, #4 is at overlap TDC.

The FE timing point is 40 degrees PAST overlap TDC. So if you insist on timing off of #4 TDC, then you need to go 40 degrees past that to the FE mark.

I put this chart together for fun to help me with cam timing, but it shows how the otto cycle works on a flat six engine: The timing point is 40 degrees past overlap TDC #1. The intake stroke technically begins at overlap TDC, even though the intake valve is open at that point already. Once the air mass begins to move into the cylinder, WHOOF, here comes the fuel at 40 degrees in. Of course, given different cams, the timing of the intake event varies, but I never have seen instructions for altering the pump timing based on the cam setting (which is a good proxy: the bigger the cam setting, the more the intake is open at overlap.)

[confused]

Yes you need to set it to the FE mark. What I generally do it get the engine to cyl #4 and them move it back to the FE mark... so not exactly at #4 TDC.

The manual says something about get it to #1 TDC and then rotate once to x amount of degrees. I think it is simpler to understand get the engine to #4 and then over to the FE mark.

The first time I put together a MFI engine years ago I fired it up and it ran but not perfectly. I drove the car around for a day and one of my friends came over to help tune it. He asked if I timed the pump correctly and I said "how do you do that?" So yes it will run with a badly timed pump. You can get a MFI engine to idle and rev high fairly easily, its that transition from idle to full power under load that is what shows if the engine is tuned right.

[elflamo]

Hi all, I timed the pump correctly now. Took me 45 minutes of wrenching.

Big difference, transition is much smoother now. I can tell that once this engine has been run in, it will be a rapid car, great.

On to the next issue: oil leaking from the mfi pump drive shaft.... (I'll create a new thread)

[philippeF]

This is the camshaft gasket on the pulley side ...