Thread: 3.9V8 Tuning Help Needed (aka I still hate V8s - but they do sound good ;))

Originally Posted by isuzurover

I will be so happy when I can go back to working on diesels...

Bojan's 90 is up and running with the new engine, but not well.

Old engine was a 3.9V8 with 14CUX EFI - it ran really well, but was pressurising the cooling system.

New engine was taken from a county - it is a rebuilt 3.9 short motor with 3.5 heads, and was running CDs. It was running fine before we pulled it, if a little lacklustre.

We have used the dizzy, remote amp and coil from the county.

We set the dynamic timing to 6 degrees BTDC with the vacuum advance disconnected. That seemed to be what most people are running?

When we reconnected the vacuum advance, there was no change to the timing. The vacuum line is connected to the intake/plenum. When you hold the vacuum line to your ear you can hear a faint sucking noise, however you cannot feel anything when you put your finger over the line. Is all of this normal? I suspect not...

The engine starts fine and runs fine with no load with the settings as above. However when you drive it, everything is fine below about 1500ish rpm but above that it misses/surges really badly, until you really put the boot in and get over 3000ish, where it goes really well.

Any ideas???

Vacuum advance only happens above idle, hold the throttle to run 2000rpm or so, then connect/disconnect vac advance, watch the change in timing. Or open the throttle, you will feel suction from the manifold port.

Up to 10 degrees adavnce is reported to work well, depends a bit on fuel type/compression ratio.
Can't help with the funny running, unless the ignition amp is crook. Sounds a bit like like my brother's RR 87 EFI before I chucked the Lucas bits.

Does that dizzy actually advance with vacuum?
I mean - connect the vacuum hoze to the dizzy and with the cap off - suck on the hose and see if the contact plate moves. It sounds like the vacuum required at that rev range is not a happening thing. (the hose may have a split?)

theres a multitude of small things that can be just a little wrong and add up to big dramas.

are you still running the CD's or the EFI system?

if you're running the CDs you have a ruptured diaphragm in one or both carbies.

once you have the carbies set up and running correctly you can then go about trying to do the timing with the vac unit.

Ive just recently gone through it with the black thing (3.5l w cd175s) and initially when I was trying to tune the carbies I had so much difference between the carbies that it sucked the fluid out of my balance guage.

Originally Posted by Blknight.aus

are you still running the CD's or the EFI system?

Sorry - didn't make that clear - EFI

What sort of cam was in the County? carb or EFI?
While its unlikely to affect light throttle, the cam could affect idle if its a carb cam.
I can only suggest that the most likely culprit is a serious air leak, so check all the hoses are connected to the plenum, maybe with emphasis on the small hose to the Fuel pressure regulator.
Regards Philip A

yep, time for all the standard vac leak checks.

grab a can of startya bastard and with it warmed up at idle start spraying air intake joints and vac lines.

Originally Posted by matti4556

Does that dizzy actually advance with vacuum?
I mean - connect the vacuum hoze to the dizzy and with the cap off - suck on the hose and see if the contact plate moves. It sounds like the vacuum required at that rev range is not a happening thing. (the hose may have a split?)

When I suck on hose at idle I can't hear any difference. Bojan has hidden the timing light, so I can't measure it - Bojan???

After we fitted the engine we noticed the vac module was loose and tightened it. It could possibly have dropped off the pin???

Originally Posted by PhilipA

What sort of cam was in the County? carb or EFI?
While its unlikely to affect light throttle, the cam could affect idle if its a carb cam.
I can only suggest that the most likely culprit is a serious air leak, so check all the hoses are connected to the plenum, maybe with emphasis on the small hose to the Fuel pressure regulator.
Regards Philip A

I guess there is no easy way to check which cam it has??? I am pretty sure the receipts that came with the county just say "reco 3.9 short motor".

Will check for leaks...


One other issue - the vacuum module has 2 connections. The connection on the dizzy side was blanked off on the old motor, so we swapped the blanking plug over. Where does it connect on carb engines?

Don't put the blank plug on the other port, leave it off or it can stop the vac unit operating by stopping free air flow into the lower chamber.

Cheers

Andrew