1977 Idle Issue
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
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From: Downers Grove Illinois
1977 Idle Issue
I have a 1977 with a L-48 that currently runs great with one annoying issue. over the last year the plugs, wires and distributor have been changed, as well as the carb rebuilt by LARS. After the car is fully warm and sitting idle it can drop 200-250 RPM and get rough. If I rev the engine a few times to say 1500 RPM the idle goes back up for a while. If I push back on the throttle cable that does not cause it to drop. I have disconnected the line for the EGR valve and the heat riser valve with no change. I also found if I disconnect a vacuum line when this happens causing a vacuum leak that the idle will go up some, so it doesn't seem like a vacuum leak issue. Does anyone have any ideas? The only thing that is non-standard for the L-48 is that I went to true dual exhaust with no cat(s). What is strange is that anything above idle is great. The car accelerates well (for an L-48). Thanks!
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
I don't think that is it as Lars had set that for me when he rebuilt it last year. At that time the AFR at idle was right where it should be. I am starting to wonder if I have a charcoal canister and if I do if it could be saturated with fuel. The odd thing is it will idle fine at 800 RPM in park for a while and then suddenly drop to 600 RPM and run a bit rough. In gear it can go as low as 500 RPM and you can really feel the roughness. If it was too lean would removing a vacuum line make the idle speed go up, because it does?
Drifting
Joined: Nov 2013
Posts: 1,547
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From: Scottsdale Arizona
When I got my carburetor back from Lars for my 77, he included tuning instructions. If you haven't adjusted the mixture screws I suspect that is contributing to your problems with a rough idle and maintaining the proper idle speed. I set mine using a vacuum gauge and it runs perfectly.
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
When I got my carburetor back from Lars for my 77, he included tuning instructions. If you haven't adjusted the mixture screws I suspect that is contributing to your problems with a rough idle and maintaining the proper idle speed. I set mine using a vacuum gauge and it runs perfectly.
Race Director
Joined: May 2006
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Lars set things up to run on his test engine, not yours. Start with checking the mixture screw adjustment. Gasoline also changes between summer and winter blends, if he used one type and you are using another, again the screws have to be adjusted. Before assuming lots of other issues start with this simple step,
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Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Lars set things up to run on his test engine, not yours. Start with checking the mixture screw adjustment. Gasoline also changes between summer and winter blends, if he used one type and you are using another, again the screws have to be adjusted. Before assuming lots of other issues start with this simple step,
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
I tried 2 things last night.
When the idle dropped to around 500 RPM I disconnected the charcoal canister, but saw no change.
I turned the idle mixture screws in while it was running well with a vacuum gauge connected. When I got the screws almost all the way in the car just died. I turned the screws out 3 turns and restarted the car. It idled great for about 5 minutes and I thought I had it fixed. I even tried to take a video of how abruptly the idle drops, but it wouldn't do it while I was recording. It then dropped again and I decided I was done for the day. When the idle dropped the vacuum gauge went from 19.5 to around 10.5 and RPM from 850 or so to 500 or so.
I don't understand how with the car just sitting there idling there can be such an abrupt change in idle speed and I believe it becomes very rich. Even though I have the garage doors open it smells strong and is on my clothes. So, the next time I try something I may take the car out to the carport to get better ventilation. Otherwise the car runs great.
When the idle dropped to around 500 RPM I disconnected the charcoal canister, but saw no change.
I turned the idle mixture screws in while it was running well with a vacuum gauge connected. When I got the screws almost all the way in the car just died. I turned the screws out 3 turns and restarted the car. It idled great for about 5 minutes and I thought I had it fixed. I even tried to take a video of how abruptly the idle drops, but it wouldn't do it while I was recording. It then dropped again and I decided I was done for the day. When the idle dropped the vacuum gauge went from 19.5 to around 10.5 and RPM from 850 or so to 500 or so.
I don't understand how with the car just sitting there idling there can be such an abrupt change in idle speed and I believe it becomes very rich. Even though I have the garage doors open it smells strong and is on my clothes. So, the next time I try something I may take the car out to the carport to get better ventilation. Otherwise the car runs great.
Race Director
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when you were turning the screws in were you doing one screw at a time about a 1/4 turn and then watching the vacuum gauge to see if the reading dropped? If you get a drop turn it back out the 1/4 turn and set the other screw.
If the car is an automatic, you have to do this process in gear with the wheels chocked.
If the car is an automatic, you have to do this process in gear with the wheels chocked.
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
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From: Downers Grove Illinois
when you were turning the screws in were you doing one screw at a time about a 1/4 turn and then watching the vacuum gauge to see if the reading dropped? If you get a drop turn it back out the 1/4 turn and set the other screw.
If the car is an automatic, you have to do this process in gear with the wheels chocked.
If the car is an automatic, you have to do this process in gear with the wheels chocked.
The thing I don't get is the randomness of how it can being idling great and then it drops 200-300 RPM and smells really rich. I could see if it always ran rich at idle or fluctuated slowly, but it is like someone flipped a switch from good AFR to really rich with poor idle.
I may try tonight again to get a video of it happening as it is very obvious when it happens.
Melting Slicks
Joined: Feb 2007
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From: S.E. MI
St. Jude Donor '15
I think it was the needle valve not seating good. It happened right after I rebuilt it. I took it apart and put in a different needle valve. All is good now. Look in your carb when it is acting up and see if it is dumping in a lot of gas.
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
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From: Downers Grove Illinois
Thanks, I will check out if it is dumping fuel into the carb.
Racer
Joined: May 2011
Posts: 373
Likes: 6
From: Dover Delaware
Just checking in w/my 2cents
when you were turning the screws in were you doing one screw at a time about a 1/4 turn and then watching the vacuum gauge to see if the reading dropped? If you get a drop turn it back out the 1/4 turn and set the other screw.
If the car is an automatic, you have to do this process in gear with the wheels chocked.
If the car is an automatic, you have to do this process in gear with the wheels chocked.
My 78 l-48 kinda sorta does an occasional lope in gear that does not happen at ideal in neutral....Have adjusted the mixture screws with my vac gauge and of course it does fine...butI never knew that it needed to be done IN GEAR with chocked wheels....I am hoping that you will tell me that that proceedure could be a secret trick???
One ? though..I now that you adjust one screw at a time but how far in and how many outs does it take.My steady vac is always around 20 on the gauge....
Please tell me more...Mike
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
OK, I ran it until it warmed up fully and looked into the primaries. Over a short period of time I could see like a fine mist forming on the primary throttle plates. If I rev the engine a couple of times this "mist" is removed from the plates and the idle goes back to normal for a while. What could cause such a slow/fine buildup of what I assume is fuel to appear on the throttle plates. I might assume it was water, but yesterday the humidity levels here were low and the idle issue was the same. I really thought I might see a dribble of fuel or something more obvious.
Race Director
Joined: May 2006
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I would not rule out water condensation, as fuel vaporizes it draws heat causing metal to cool and perhaps water to condense. I know you said the engine was warmed up but did the carburetor itself also feel fully warmed up? The problem with your wife stepping on the brake pedal is that it might change the idle by activating the vacuum brake booster.
Mrks78
That is not a secret trick, look in any factory service manual from that era and idle adjustments on automatics are always done in gear. There is no set rule for how many turns. With the engine off gently fully seat the screws and then turn them out 3 or 4 full turns. Start the engine and start the adjustment procedure.
Mrks78
That is not a secret trick, look in any factory service manual from that era and idle adjustments on automatics are always done in gear. There is no set rule for how many turns. With the engine off gently fully seat the screws and then turn them out 3 or 4 full turns. Start the engine and start the adjustment procedure.
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Gas filter out of carb
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
I would not rule out water condensation, as fuel vaporizes it draws heat causing metal to cool and perhaps water to condense. I know you said the engine was warmed up but did the carburetor itself also feel fully warmed up? The problem with your wife stepping on the brake pedal is that it might change the idle by activating the vacuum brake booster.
Mrks78
That is not a secret trick, look in any factory service manual from that era and idle adjustments on automatics are always done in gear. There is no set rule for how many turns. With the engine off gently fully seat the screws and then turn them out 3 or 4 full turns. Start the engine and start the adjustment procedure.
Mrks78
That is not a secret trick, look in any factory service manual from that era and idle adjustments on automatics are always done in gear. There is no set rule for how many turns. With the engine off gently fully seat the screws and then turn them out 3 or 4 full turns. Start the engine and start the adjustment procedure.