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Discussion Starter #1
1 minute the 68 (rebuilt 289, hipo pistons) runs great then the next it stumbles. Most of the time it recovers within 10 seconds, but will stumble again within a minute or 2. Sometimes it stalls. Vacuum is good at idle. Wide open throttle will cause it to stumble, but part throttle is fine. Engine was rebuilt a few years ago, has never ran right. I was military and sent to VA/TX and then Germany. By the time I got home, the machine shop had no interest in helping me. Machine shop used a mild cam. I was away during the build, so I don't know if the parts I paid for were actually used. I requested the heads ported and a valve job (stock heads). Performance pistons and a mild cam. Machine shop claimed 300 horses. $3600 for the rebuild.

Last thing I could think of to replace would be the fuel lines. 5/16 would be too small for 300 horses, but given the shops lack of interest in my situation, I'm not feeling warm and fuzzy about anything they've claimed. I wanted another opinion before upgrading to 3/8" lines.

Replaced: ( same problem before replacing the following items )
2 barrel to 4 barrel holley carb. Chinese 4 barrel alum intake. Distributor. Fuel tank. Fuel pump. Filter from pump to carb. Plugs/wires.
 

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My first thought is manifold vacuum leak.
Second thought, carburetor choke setting/adjustment.

Then look at adjusting the idle mixture screws on the carb.
Then adjust the idle speed screw.
Then check distributor timing. Set it at 12-14 BTDC.
Then continue adjusting all those things to fine tune and get it just right.

Also, it would help to know exactly which Holley carb you have and the distributor.
 

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Discussion Starter #3
I've sprayed the hell out of the manifold. Carb cleaner didn't cause a jump in idle. It stumbled in a similar fashion with the old 2 barrel manifold. ( I'm a bit cheap since having a kid, so I had no interest in upgrading from a 2 brrl. I only did it, because I thought the 2 barrel was choking the engine. )

Carb choke has always been a bit of a mystery to me. I adjusted it for just closed with a 70 degree engine (what I read on a mustang monthly somewhere. ) I held the throttle linkage wide open and adjusted the choke idle screw until it hit 1500 rpm. Took a few times turning the stang on/off to get that right, but sometimes the car starts and revs to 2000 rpm. It does take a second of rough idle before it builds up to the 2000 rpm. Sounds like it's off to the races at initial start up sometimes. Other times it's fine.

Carb is a Holley 0-80457SA 600CFM 4bbl Factory Refurbished ALUMINUM Carb Electric Choke .
Distributor is a cheap $50 - $60 Spectre. It's been warranty replaced twice for cracks in the cap. This one is good so far.
 

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Discussion Starter #4
I've sprayed the hell out of the manifold. Carb cleaner didn't cause a jump in idle. It stumbled in a similar fashion with the old 2 barrel manifold. ( I'm a bit cheap since having a kid, so I had no interest in upgrading from a 2 brrl. I only did it, because I thought the 2 barrel was choking the engine. )

Carb choke has always been a bit of a mystery to me. I adjusted it for just closed with a 70 degree engine (what I read on a mustang monthly somewhere. ) I held the throttle linkage wide open and adjusted the choke idle screw until it hit 1500 rpm. Took a few times turning the stang on/off to get that right, but sometimes the car starts and revs to 2000 rpm. It does take a second of rough idle before it builds up to the 2000 rpm. Sounds like it's off to the races at initial start up sometimes. Other times it's fine.

Carb is a Holley 0-80457SA 600CFM 4bbl Factory Refurbished ALUMINUM Carb Electric Choke .
Distributor is a cheap $50 - $60 Spectre. It's been warranty replaced twice for cracks in the cap. This one is good so far.
Timing is set at 12*. No pings, but at 3500 rpm the timing is way passed 40* with the vac line plugged. (line is left on the distributor and plugged at the carb side. )
 

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Discussion Starter #6 (Edited)
I had a similar issue, turned out I needed to re-gap my plugs. Worth a quick check
I'll take a look! Maybe I had 2 many beers by the time I gapped them. Would higher compression require a different gap? I'm sure not sure about the compression and the machine shop has no idea what cam they used, let alone could provide an engine card
( which pistons/deck/head chamber ) lmao. Serves me right for not building the engine myself.
 

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Discussion Starter #8
Have you checked the fuel level in the carb and adjusted the float as necessary?
I'll check this as well. Holley said it was good out of the box. It was a reman. The same thing was happening with the 2 barrel before I replaced it. Funny as hell if it was the same issue.
 

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It sounds to me that the engine is loading up on fuel at idle, stumbles through it and then repeats. Check the choke, float level, idle mixture etc.
 

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Discussion Starter #10
Have you checked the fuel level in the carb and adjusted the float as necessary?
It sounds to me that the engine is loading up on fuel at idle, stumbles through it and then repeats. Check the choke, float level, idle mixture etc.


This cleared it up! Such a simple procedure to check. Took me 10 minutes. Didn't know it could cause something like this..... Thanks guys!!!! It ran great yesterday! I don't understand how the same problem followed 2 carbs on the same engine. I looked at the old carb yesterday, but couldn't find an obvious float screw. I doubt I'm going to throw the old 2 barrel/iron intake back on the car. It's running perfect right now!! It's got some great get up.

I'm pumped to drive the car!! Just gotta figure out the power steering system, but that's another thread and my last mechanical problem! A friends has a shop/lift/paint booth. Once I get the steering squared away, car is getting a redo at life. New floor from firewall to trunk, front disks, rear wheel wells, and trunk. Some good meat on some old school cragers.

Seriously guys, thank you so much. This issue at idle/driving has prevented me from moving forward with restoring this car.
 
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