I've got 99 problems...and a vacuum leak could be one

S

Scout Hokie

Member
Alright this is going to be a bit long winded but need to get the background out there. I've got a '74 Scout II with 345 and 727 trans. Also, I've installed a Holley carb (0-7448). When I bought the truck, it did not run. The po had stripped the spark plug threads on the #1 cylinder so I took the head off and had a local cylinder head machine shop install an insert into the head to accept a spark plug, so I never heard the truck run prior to installing the Holley carb. Ever since I've had the truck running, I've had a 'run-on' problem when trying to shut the truck off, sometimes last 20-30 seconds and always ending with a loud 'woosh' and white smoke gushing up through the carb. I don't drive the truck for fear that I'm running lean and thats whats causing the run on. I've been trying to diagnose the run on but I'm at whit's end. I have had trouble getting the idle speed down into the acceptable 700 rpm range. If I adjust the idle mixture screws or twist the distributor to drop the idle speed, my manifold vacuum drops substantially and the engine stumbles (I have a vacuum gauge hooked up to manifold vacuum and have the vacuum advance on the distributor disconnected and plugged). I'm at sea level and have found that the motor seems to run better right at 0* btdc. The idle mixture screws are about 1/2 turn out and that gets me to 850-900 rpm with the truck in neutral and I'm getting good manifold pressure (~18 mmhg) another issue, the motor is running hot, coolant temp climbs to 230* just under idle after 15-20 minutes of running! I wondered if I have a vacuum leak so I started spraying around the intake manifold and noticed that right in the middle of the intake manifold, I get lots of bubbles. It occurs on both sides (picture included). I wondered if I have a vacuum leak or if that portion of the intake manifold just runs hot which is boiling the fluid when I spray. This bubbling does not occur at any other location on the manifold. I've tried using seaform to clear out carbon deposits. I did get some carbon out of the exhaust when using the seafoam but otherwise, no noticeable improvement. I also decided to try colder spark plugs, I had champion plugs in (12 heat rating) and swapped them for autolite 303. I know the individual manufacturers' heat ratings don't correlate 1-to-1 but figured it was worth a shot. When inspecting the champion plugs, the ceramic looked pretty spectacular. They were all tan in color.

So whats the deal? The truck runs well if I just take it around the block, no hesitation during acceleration, no pinging. Here are some of my thoughts:

1) are the bubbles I'm seeing evidence of a vacuum leak? Causing my higher idle rpm?

2) are carbon deposits remaining ignited, causing the run on?

3) could a bum thermostat cause the engine to run hot and cause the run on?

4) could the spark plug insert be getting too hot and causing the run on? I would not expect 1 cylinder to be able to sustain run on.

Any help that the gurus can offer to help me start enjoying my Scout on the open road would be spectacular!
 

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Most times run on is caused by high throttle opening or fast idle. You need the settings that give you the highest manifold vacuum and lowest throttle opening. Very retarded timing will cause a very hot exhaust valve and increase the likelyhood of run on.
Have you tried to adjust the throttle stop screw? They should be enough to slow it down.
 
Hey Robert, thanks for the reply. Yes, I've checked the throttle stop screw and it is all the way out, meaning it is not inducing throttle opening. I will definitely double check everything and make sure nothing else is causing the throttle to stick open.

I do believe that fast idle is my problem but what I'm struggling with is whats causing it. It appears that my tweaks to timing and/or the idle mixture screws aren't getting me to low enough idle rpms. I'm also surprised to see the coolant temp get so hot after idling for only 15 minutes...
 
Robert, when you say choke is off, you mean fully open, correct? Just making sure. I've got the Holley electric choke installed and I do allow it to open fully before making timing adjustments. I will say that when I fire the truck up initially, it runs really excellent while the choke is mostly closed and will idle at ~ 600 rpm. Once the choke opens fully is when the rpms climb to 850-900 rpm.

Definitely good call on the fast idle screw. I will check that as well as double check the throttle stop screw.

One question, I read on here that not using sealer on the rocker assembly bolts could cause a vacuum leak. I did remove those when I had the cylinder head worked on and I did not use sealer when I re-installed. I May also check to see if the pcv is pulling vacuum....could that cause my problem?

I think I'll go ahead and change the thermostat as well. Cheap and easy swap.
 
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It sounds like Robert has you on the right track. Check for something holding the throttle open, perhaps the throttle cable?

Also what are you spraying on the manifold? It's bubbling in that area because that is where the exhaust cross-over is for heating the carb.

I couldn't tell from your pic, is an egr valve installed? Sometimes they will get a bit of carbon stuck in their valve seat and cause a vacuum leak to the exhaust manifold.

Basically try to get the smallest throttle opening you can @ 600 rpm with the highest vacuum reading and it should be ready to pass smog.
 
Roger that, thanks fdchappie, that explains the bubbles I'm seeing. I do not have an egr installed, I removed it and installed an egr plate. I live in sc so no smog tests, heck, we don't even have annual vehicle inspections! It May be the perfect state for owning a Scout, haha.

I completely agree with both you and Robert, I'll look closely at all of the items that could hold the throttle plates open and report back.
 
Robert...you nailed it. I checked things over and everything was as I thought, idle screw was turned all the way out, etc. Then I noticed that I really had a rather small spring for the throttle return. I pushed on the throttle lever arm on the carb and it closed a bit more. Hmmm. So I grabbed a stronger spring and sure enough, it closed more. Gave the idle screw a few turns to crack the throttle plates and I knew I found the issue when I was able to close the throttle plates via the idle screw and cut the truck off! I haven't truly found the root cause which could've been too weak of a spring or a slight bind in the throttle cable but I've got something that works. Now I've got a 650-700 rpm idle with 19 mm of hg vacuum and best of all, the truck cuts off instead of dieseling! Thanks to all for the help!
 
Good job! Simple problem but many symptoms.. The cable could be gummy or simply need cable lube. You can buy a motorcycle cable lube kit at the local mc shop. The lube will stay slippery and not gum up.

Make sure you have your timing set correctly. At the slow idle speed with the vacuum advance disconnected and plugged at the source, set your initial timing to 8* btdc
 
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