Rough Idle/ Vacuum Leak

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Author Topic: Rough Idle/ Vacuum Leak  (Read 1913 times)

Offline RC65

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Rough Idle/ Vacuum Leak
« on: November 09, 2020, 08:48:53 AM »
I'm hoping the experts here can point me in the right direction. I've been chasing a rough idle issue on my 289W for months now with no success. I have a 600cfm Holley (recently rebuilt by a carby expert) on a Edelbrock performer RPM intake manifold. Distributor vacuum advance line is connected to ported vacuum on the carby and manifold vacuum port on the base plate of the carby is blanked off with a rubber stopper. The idling actually smooths out when I remove the stopper from the manifold vacuum port on the base plate which is exactly the opposite of what should be happening because I'm effectively creating a vacuum leak??? This makes no sense. Any advice would be appreciated. Thanks

Offline Daz 67GTA

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Re: Rough Idle/ Vacuum Leak
« Reply #1 on: November 09, 2020, 09:57:15 AM »
I'd suggest taking it to a reputable dyno place that are good with carbies. Even though it's been rebuilt, it still may not be correctly setup for your engine/cam/driveline setup.
It may be as simple as way too rich at idle or the butterfly not adjusted correctly to allow correct idle via the ports in the carby..
I'm sure there will be plenty of suggestions.. you may even hit the jackpot with a simple fix, but unless it's on the dyno and all set up correctly (Air/Fuel Ratio, Power Valve, progression, correct jets etc) to suit your engine it will never be 100%, and you'll not be getting the most out of the new bits.

PS: not sure what your setup is.. but that manifold Vac port plate is usually connected to the Rocker Cover to enable the crankcase to be ventilated via the (PCV) valve. Are you running Crankcase Ventilation?

« Last Edit: November 09, 2020, 10:01:58 AM by Daz 67GTA »
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Offline pgold

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Re: Rough Idle/ Vacuum Leak
« Reply #2 on: November 09, 2020, 11:45:05 AM »
sounds like it is to rich if it smooth's out when you open vacuum port. Is this the port just below ported vac or the one at back of carb? Have you checked float levels are correct? Is choke completely open and fast idle disengaged when engine is hot. Lots of info on u-tube on base setup. Have you spoken to the carby rebuilder ? Was this combination working previously?

Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #3 on: November 09, 2020, 12:20:35 PM »
Appreciate the advice Daz and pgold. I may well land up at a dyno place in the end. Everything is hooked up correctly as far as I can see. Large port at the back of the carb is connected to a new PCV valve in the rocker cover. The manifold vacuum port on the side of the carb is blanked off and located below the throttle bodies (as per Holley's recommendations). I only use this port when connecting a hand held vacuum gauge (showing normal readings). The ported vacuum connection above the throttle bodies is connected to the dizzy vacuum advance canister. Choke and fast idle are totally disconnected and wide open (haven't needed a choke for cold starting here in sunny QLD). I will continue trawling through YouTube but hopefully there is something straightforward that I've missed.

Cheers   

Offline GLENN 70

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Re: Rough Idle/ Vacuum Leak
« Reply #4 on: November 09, 2020, 10:14:17 PM »
How many turns out are the mixture screws ? 1 1/4 to 1 1/2  Out is about normal . Where is the vacuum connected for the brake booster ? . Try blocking off all the vacuum ports and see how it runs . What is the  timing  set at ,vacuum line blocked ? . 

Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #5 on: November 10, 2020, 08:07:48 AM »
Thanks Glenn. Idle mixture screws are 1 1/2 turns out. I have no brake booster, the port dedicated for that in the intake manifold has been blanked off and is not leaking. My base timing with the vacuum line off is set at 12 Deg BTDC. Idle RPM is approx 650 with the car in gear - auto transmission. I'm not going to open the whole ported vacuum vs manifold debate again because that's been flogged to death but after moving the distributor vacuum line from the ported vacuum on the carb to the manifold vacuum port (below the throttle plates) the idle seemed to improve. I'm going to see whether I can dial in the mixture screws further. Failing that it looks like a dyno might be the best option. Any recommendations in Brisbane area? Cheers   

Offline Daz 67GTA

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Re: Rough Idle/ Vacuum Leak
« Reply #6 on: November 10, 2020, 09:02:48 AM »
Sounds like the basics are all ok. have you followed the Holley setup guide? There's a bit in that (setting the throttle plate opening correcty is a must). This is usually setup with the carb off the car and you are measuring the position of the plate to the transition holes in the carb. Once that is set, then you do the idle/timing etc.

https://sites.google.com/site/mokmossoak/home/holley-carburetor-tuning-adjustment

Some good info in this link - if you havent read it already!
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Offline pmb0186

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Re: Rough Idle/ Vacuum Leak
« Reply #7 on: November 10, 2020, 04:37:42 PM »
Its running rich, the first thing with any carb is check is fuel pressure, then float level, throttle plate opening both primary and secondary and then adjust mixture screws. forgot power valve not ruptured.
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Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #8 on: November 10, 2020, 04:58:50 PM »
Thanks Daz and Pete - Got some more things to check on the weekend.

Cheers Rory

Offline barnett468

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Re: Rough Idle/ Vacuum Leak
« Reply #9 on: November 15, 2020, 01:26:52 PM »
I'm not going to open the whole ported vacuum vs manifold debate again because that's been flogged to death but after moving the distributor vacuum line from the ported vacuum on the carb to the manifold vacuum port (below the throttle plates) the idle seemed to improve.
1. Make sure the fuel level in the carb is around 2mm below the inspection screw hole immediately after turning the engine off, or idle if you don't mind possibly having some gas drip out in the process. You can ut some paper towels below the inspection hole. If it has a clear plastic inspection window, set the level so it is 1/4 to 1/3rd of the way up the window.

2. Disconnect the distributor vacuum and plug it off.

3. Disconnect the pcv hose and plug off the port.

4. Advance the timing until you just achieve the highest idle, which should be around 10 to 16 degrees.

5. Lower the idle to the proper rm.

6. Turn the idle mix screws in and out to see if they have any affect on the idle.

7. Never use non ported vacuum for the distributor.

Post the results.

 
« Last Edit: November 15, 2020, 01:33:31 PM by barnett468 »

Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #10 on: November 23, 2020, 12:52:16 PM »
Thanks Barnett,
Followed all the steps below. Seems to be idling relatively smooth now out of gear but in gear (auto transmission) it gets rougher. Could there be a leak with the transmission vacuum modulator? The car shifts well through the gears. Timing is now at 12 Deg BTDC, idle mixture screws 1 1/2 turns out, idle in gear around 800RPM. Cheers

Offline barnett468

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Re: Rough Idle/ Vacuum Leak
« Reply #11 on: November 24, 2020, 11:57:27 AM »
Thanks Barnett,
Followed all the steps below. Seems to be idling relatively smooth now out of gear but in gear (auto transmission) it gets rougher. Could there be a leak with the transmission vacuum modulator? The car shifts well through the gears. Timing is now at 12 Deg BTDC, idle mixture screws 1 1/2 turns out, idle in gear around 800RPM. Cheers
progress, xlnt!

Highly unlikely the modulator has a leak but you can plug it off and test it.

You can use a can of flammable brake cleaner with the nozzle and lightly spray around the carb base to see if the idle changes.

1. Does it still do this:

"The idling actually smooths out when I remove the stopper from the manifold vacuum port on the base plate".

2. Are you positive your spark plug wires are in the correct firing order for the cam you have?

3. Do you know which cam it has?

4. Does it have a big cam?

5. How many rpm does it drop when you put it in gear? It shouldn't drop more than around 150 rpm.

6. Is the distributor vacuum still disconnected?



 

« Last Edit: November 24, 2020, 12:18:46 PM by barnett468 »

Offline pmb0186

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Re: Rough Idle/ Vacuum Leak
« Reply #12 on: November 25, 2020, 03:48:38 PM »
I maybe pointing out the obvious but if there was a vacuum leak wouldn't it run worst with the vacuum hose from the manifold removed rather than better?
does anyone remember laughter

Offline GEOFF289

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Re: Rough Idle/ Vacuum Leak
« Reply #13 on: November 25, 2020, 07:17:12 PM »
I maybe pointing out the obvious but if there was a vacuum leak wouldn't it run worst with the vacuum hose from the manifold removed rather than better?

You have to plug the port when you remove the hose.

Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #14 on: November 25, 2020, 08:01:48 PM »
Pete - the weird thing is the idle actually improved when I removed the hose and basically introduced a vacuum leak :ouch: Exactly the opposite that you would expect.
I haven't had much time to troubleshoot further.... to answer Barnett:
Plug wires are in the correct firing order.
The car has been in the family for 45 years so I'm pretty sure its still has the factory original cam. Engine hasn't been rebuilt.
Idle in neutral approx 950RPM drops to around 800RPM in gear.
Distributor vacuum line still connected.
Appreciate all the advice there is definitely an improvement but still not perfect. Probably due for a rebuild anyway.   

Offline pmb0186

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Re: Rough Idle/ Vacuum Leak
« Reply #15 on: November 26, 2020, 03:27:03 PM »
Are you setting the mixture by winding out the screws 11/2 turns only or adjusting for highest revs, smoothness and or highest Vacuum?
 
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Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #16 on: November 26, 2020, 08:40:57 PM »
I've tried two of the options - highest/constant vacuum reading on the gauge, turning the screws all the way in until the engine almost stalls then 1/2 turn out. 1 1/2 turns out seems to give the best result. I can put up with the rough idle in gear for the time being. I'm considering a rebuild in the new year anyway.

Offline pmb0186

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Re: Rough Idle/ Vacuum Leak
« Reply #17 on: November 27, 2020, 09:44:58 AM »
Its been a while since I've worked on a holley so someone correct me if I wrong but, I believe holleys are backwards, to lean out screw out to enrichen screw in. So all you did is further enrichen by winding in, you need to continue out. If it make no further difference then you need to look at the secondary butterfly and how much they are open if float level and fuel pressure correct. Yes they need to be open at tad allowing a small vacuum leak exposing transition slots. Ask me how I know, I rebuilt my 4100 replated all hardware and didn't take note of how open the secondaries were before disassembly and when I put it back together I couldn't get the idle mixture right (to rich). Opened the secondaries a tat and hey presto.   
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Offline RC65

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Re: Rough Idle/ Vacuum Leak
« Reply #18 on: November 27, 2020, 11:17:11 AM »
I've just had the carb rebuilt by "old school experts" here in Brisbane so you would hope they knew what they were doing. Their shop was pretty busy with classic cars which made me feel a bit more confident. I am aware of the transition slots needing to be just exposed to activate the idle circuit in the metering block, which they are. Also the idle mixture screws definitely work the conventional way, they further out you go the richer the mixture, turn them in to lean out the mixture. I'm going to keep tinkering until I get it right.

Offline pmb0186

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Re: Rough Idle/ Vacuum Leak
« Reply #19 on: November 27, 2020, 11:29:08 AM »
what model carburettor ?
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Offline barnett468

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Re: Rough Idle/ Vacuum Leak
« Reply #20 on: December 01, 2020, 12:08:38 PM »
Its been a while since I've worked on a holley so someone correct me if I wrong but, I believe holleys are backwards, to lean out screw out to enrichen screw in.

On the majority of holley carbs, turning the idle mix screws out makes it richer.

Offline barnett468

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Re: Rough Idle/ Vacuum Leak
« Reply #21 on: December 01, 2020, 12:12:27 PM »
the idle mixture screws definitely work the conventional way, they further out you go the richer the mixture, turn them in to lean out the mixture. I'm going to keep tinkering until I get it right.

ummm...that is backwards unless you have one of the very rare holleys that works this way.

i suggest you do a compression test.

if both idle screws have a noticeable affect on the idle, the idle circuit is working.

if the idle circuit is working and the timing is fairly close and there are no air leaks, you most likely have low compression in one or more cylinders.

dropping 150 rpm when it is put in gear is acceptable and the maximum you would like to see it drop.
« Last Edit: December 01, 2020, 12:14:26 PM by barnett468 »

 

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