Hello all,

Farris here. I've searched this and other forums and can't seem to come up with a definitive answer so...

Working on my '70 F100. Recipe is: 1989 Mustang 5.0 (carb converted), 1981 C4 (stock converter), 9" rear (currently 3.08 gear). The engine was internally stock until a few weeks ago when I added GT40 heads (1.94/1.54 valves), Comp 270HR cam (.533", 215° @ .050", 110° lobe separation), 1.6 roller rockers, 20° initial timing, 38° total. Now, the truck runs much stronger than before though I'm having trouble setting a respectable idle and idle mixture. I'm pretty sure the problem is the tight stock converter. Currently I have the idle set to just under 1000 rpm because anything lower and the engine dies when pulled into gear. When hot, it diesels when shut off with that idle setting.

So, my question is, will a stall converter, say 2400 or so, allow me to get the static idle down or do I have another issue? Thanks!

 

No. I don't think the converter is your problem.

Ignition timing sounds more like it. Too high initial. IMO. Set down around 12 degrees, Then Adjust carburetor to idle correctly.

If I remember correctly, my stock C4 converter stalled at about 2300 RPM.

 

Its not the converter it's your cam timing and your base ignition timing. What was the cam timing set to? The stock converter is so loose factory a new TC would be a upgrade.

 

X2 on the above. Work on getting your idle down to where it's actually running on the idle circuit. Does your engine respond to adjustment of the idle screws? If not, your idle is too high, and the throttle plate is too far open.

That said, this is a discussion about a heavier car (4000 lbs) and that cam, the culprit being the 110 LSA.

https://www.corral.net/threads/compc...th-efi.194467/

Fiddle with that timing and idle for a while.

 

What carb? Holley? Or Edelbrock?
I don't see lots of initial timing being a problem. It should like it and make it easier to adjust carb, especially with a larger then stock cam.

 

Be sure that you are setting the timing with the vacuum advance unhooked.

 

I will try 12° initial and see what happens. I set it to 20° based on a recommendation posted on another site which, it seems, isn't working for me.


Cam timing is straight up, 0°. I read that comp grinds their shelf cams 4° retarded however I haven't confirmed that.


The engine is responding to idle mixture screw adjustments. I screwed them both all the way in then back out 2 turns. From there, if I screw in more than 1/2 turn, the engine stumbles. Out 1/2 turn from the base setting gives the highest vacuum (14"). I guess I'm doing that correctly?


That's what I read on another site. In fact the quote was, "Give it all the static timing the starter can crank when hot then cap it at around 38 degrees all in." My carb is a 4150 Holley, 600 cfm, vacuum secondaries.


Come to think of it, I don't think I disconnected the vacuum line. Thanks for the advise!

 

I replied last night but it isn't showing up. Anyway, I have a holley 4150, 600 cfm, vacuum secondaries carb. It is responding to idle mixture screw adjustments. I closed both screws then back out 2 turns. More than 1/2 turn in and the engine stumbles. 1/2 turn out from the base setting yields the highest vacuum (14"). I think I'm doing that correctly? Cam timing was set at 0°. If it stops raining here I'll set the initial timing to 12° and see what happens. I didn't disconnect the vacuum line from the advance which may be part of the problem. Thanks for that advise!

I'm sure its a simple issue. This cam isn't radical by any stretch of the imagination. I'm sure there are others with a similar setup that are idling/driving just fine.

Edit: Correction, I have a Holley Street Avenger 570 cfm carb.

 

Ok. I had some time to work on my truck today. First I confirmed the initial timing at 20°. I re-set the idle mixture screws to two turns out. I confirmed the curb idle at 900 rpm. I drove a few miles to warm things up and observed it won't stay running while stopped in gear. It dies every time. I opened the idle mixture screws 1/4 turn. No change. I opened them another 1/4 turn. Still dies but now I can smell a gas stronger from the pipes. I returned home and set the timing to 12° and idle to 1000 and the idle mixture to 2 turns out. Pulled into gear the rpm drops to 550 then slowly dies. I set the idle to 1100. Pulled into gear the rpm drops to 600 and holds. I drove it and immediately noticed a reduction in power. I also noticed that the idle mixture screws have to be moved a lot more before the engine responds at the lower timing setting (1/4 turn and it stumbles at 20°/ 1 full turn to get it to stumble at 12°). So, I set the timing back to 20° and the idle to 1100. Pulled into gear the rpm drops to 600 and holds. I drove it through town. The power is back and it idles at red lights. It diesels when shut off at the 1100 rpm idle so I put it in park and turned the switch quick before the rpm's had a chance to climb which seems to stop the dieseling. I noticed a slight surge at part throttle at both timing settings which seemed to be worse at 12°. As I understand it, this is a lean condition meaning I need to go up on the main jet size. I had this problem when I first installed this carb with the factory heads and cam. The carb came with 54/65 jets. I changed to 56/67 which stopped the surge. I have a set of 58/69's I'll try. Also, I have 14-15 inches of vacuum at both timing settings. Removing and clamping the vacuum advance line has no affect on initial timing. Total timing now is 38° at around 3000 rpm. I can still smell a noticeable gas odor at idle however I read the link above and discovered this to be a common issue with 110° LSA cams. I also read about drilling the throttle plates however mine seems to be responding to idle mixture screw adjustments. I haven't removed the carb to see how much of the idle slot is exposed. It's my understanding that the throttle plates only need to be drilled if it won't respond to screw adjustments? I may have that wrong.

It's running strong and idling in traffic now. The 1100 rpm idle setting seems high to me but perhaps that's just what it takes? This is my first head/cam swap so all is new to me. I was told the engine came from a 1989 Mustang Cobra. I don't know how to confirm that. There are no part numbers on the original cam. I know it had E7 heads. I do know it has much more power now than before and I'm happy with that.

 

A 500rpm drop seems excessive. But I guess if it runs well.....

Usually mixture screws get set to 1.5 or 2 turns out.

Are you tuning with the vacuum gauge? Or just checking? You're trying to achieve the highest vacuum possible.

What is the elevation where you are?

 

The rpm drop is why I initially thought about a tight converter. My mixture screws are currently 2 turns out. I'm using a gauge and adjusted to get the highest reading at 15". According to the google machine, I'm at 761'.

I put a suncoast converter in my '00 Ram/Cummins. That beast drags the engine way down compared to the stock (very worn out) converter. That's what led me to the tight converter theory. As with most things, I'm probably wrong.

 

Does the truck want to accelerate back to 1000-1100rpm if you let off the brakes? Or does it just keep ******* along at 5-600?

 

It comes up a little but not to 1000. 6-700 maybe. I try to stop short of the light or another car so if it tries to die I can let off the brake and it recovers. Earlier on a hill I let off the brake and it stayed at the 5-600 idle without rolling back or moving forward. If it rolls forward like on flat or sloped road it idles up.

 

There was no 89 cobra, only a 93 in that generation (fox). E7 heads are not cobra (gt40) heads. Glad you got it driving OK but something is still off. A 215 @0.050 cam is not that big and should idle fine. 110 LSA is nothing special either.
Suggest compression test, reading plugs, tune carb with wideband 02. Could be a vacuum leak someplace too.

 

Indeed, a vacuum leak would explain a few things.

Spray some carb cleaner around The intake gaskets and carb gaskets. If the idle goes down, you found something