low rpm bucking

[adamb113]

My 73' 2.4 liter is bucking at low RPM's ( no or little throttle). Is just a timing issue ( not retarded enough) or something is wrong with advance mechanism or somethign else? No vac advance is hooked up right now.
Thanks,
Adam

[BK911]

MFI? If so, go through the MFI check measure and adjust manual. Do a search for MFI. Almost all the threads will reference this manual and provide a link to where it is.

[adamb113]

No it's a custom EFI system. The bucking is so hard that I really doubt it is from fuel. Could be though. I will play around with it some.

[BK911]

The CMA manual takes you through several steps before you touch the fuel system. Compression, dwell, timing, etc.. One of my cars had a low rpm bucking problem which ended up being caused by poor compression on 2 cylinders. I'd start with the manual and go from there.

What sort of EFI system? Pictures and details?

[mppickett]

Since it's a custom EFI, check your timing advance table. You can get bucking at low rpm when you go from a low advance cell to a high advance cell at idle or slightly higher speeds. If this is the case, try to make the table have a smoother transition (similar timing value).

[adamb113]

I don't have ignition computer control. It's still a distributor with mechanical advance. I'll check the compression and valve timing.
The EFI system is one I designed and built over three years.
The air box is a fibeglass shape I designed for high flow.
Throttlebody is from an 86 Saab 9000
Fuel rails are hand built stainless steel
Intake Runners are CIS runners that I machined for 18#/hr electronic injectors.
I built up a true wide band O2 sensor for close loop feedback
right now it runs off of a Haltech computer.
Whole thing can be monitored and programmed live from a laptop in the pass seat.

I'm trying to include pics of the system. Why is it so hard to do in this forum? It looks like the only way is if I have a URL location for a picture. Is this the case or can I just paste it into the text?
Yeah Yeah I know He can build a custom EFI but he can't figure out how to post a pic in a thread.

[DonE]

Then I would say its too lean at that point. Is this when the engine is at operating temp?

[adamb113]

Yes it's at operating temp. I will play with the mixture. Why does the buckign feel so violent. It's almost liek some of the combustions are to far BTDC. That would be bad, very bad! I find it hard to imagine that these strong pulses are just from missing cylinders. If it was to lean would it be detinating on compression?

Seriously How the HE double hockey sticks do you get pics in a posting without putting it at a URL and referencing it?

[BK911]

Don't use the quick reply, use the post reply button. Down near the bottom:


"click here to upload a photo"

[adamb113]

Here are some pics of the system.

[914efi]

Adam,

It sounds like you are too rich in the low rpms. Also, watch your MAP sensor, it may be fluctuating quite a bit at low rpms due to cam issues, but with T cams it should not be too bad.

What is your idle MAP and how does it change as you go through the rpm range? With the T cams, it should be reasonably progressive from idle to WOT.

I had E cams and it would idle at very little vacuum, but build vac as rpms increased, and then go back to low vac closer to WOT making it difficult to tune. Lots of fiddling to arrive at a good compromise. Are you using megasquirt? If so you should be able to tune it quite well.

[adamb113]

Hey Art,
We spoke for a while at the Porsche Tech Fest in Springfield about three years ago. Sounds like your system is doing well. Last we talked you were playign around with intake runner length/shape. I am running a Haltech. I plan to get a megasquirt soon. I have been waiting for them to incorporate the Wide Band right onto the main board.
I'm actually not sure what my cams are. I should remove the cover and measure the valve throw. I will play around more with my low end mixture.
If you ever in the Danvers area give a call.
Adam