High Idle when stopped
Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,367
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From: Virginia USA
High Idle when stopped
This just started happening after reducing my timing to 8 BTDC initial. My all in is 36 around 2800 RPM. I was chasing a pinging condition and with this timing it's gone away. I'm not running a vac can right now as my vac can puts too much timing in at cruise so I don't want to run it until I put a limiter on it.
When I'm cruising then come to a stop my idle stays around 1000 RPM then slowly drops to 700. Sometimes it drops faster than other times. Haven't had time to look closely but I think it may be a vacuum leak somewhere.
Other than that it runs great.
Any suggestions?
Thanks
Sorry - 1971 with a q-jet and ZZ4 4-speed. MSD tach drive 6AL box
When I'm cruising then come to a stop my idle stays around 1000 RPM then slowly drops to 700. Sometimes it drops faster than other times. Haven't had time to look closely but I think it may be a vacuum leak somewhere.
Other than that it runs great.
Any suggestions?
Thanks
Sorry - 1971 with a q-jet and ZZ4 4-speed. MSD tach drive 6AL box
Last edited by theandies; Sep 19, 2017 at 03:57 PM.
Pro
Joined: Sep 2017
Posts: 516
Likes: 72
From: Modesto California
This just started happening after reducing my timing to 8 BTDC initial. My all in is 36 around 2800 RPM. I was chasing a pinging condition and with this timing it's gone away. I'm not running a vac can right now as my vac can puts too much timing in at cruise so I don't want to run it until I put a limiter on it.
When I'm cruising then come to a stop my idle stays around 1000 RPM then slowly drops to 700. Sometimes it drops faster than other times. Haven't had time to look closely but I think it may be a vacuum leak somewhere.
Other than that it runs great.
Any suggestions?
Thanks
Sorry - 1971 with a q-jet and ZZ4 4-speed. MSD tach drive 6AL box
When I'm cruising then come to a stop my idle stays around 1000 RPM then slowly drops to 700. Sometimes it drops faster than other times. Haven't had time to look closely but I think it may be a vacuum leak somewhere.
Other than that it runs great.
Any suggestions?
Thanks
Sorry - 1971 with a q-jet and ZZ4 4-speed. MSD tach drive 6AL box
Your engine will run its best with 52 degrees of advance at cruising speeds and about 34 to 36 degrees of mechanical advance under a wide open throttle at 2500+ rpm. As most points-type distributors only provide around 12 to 14 degrees of mechanical advance you need to run 14 to 16 degrees of initial advance just to reach 30 degrees. Then you'd need a vacuum advance that will provide an additional 22 degrees of advance to reach the magic 52 degrees. Remember, the vacuum advance does NOT work under full power because the vacuum drops to zero.
Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,367
Likes: 1,061
From: Virginia USA
).I got to thinking about it in the shower (I do my best thinking there and on the throne). I thought maybe my distributor weights were sticking. I took the cap and rotor off and moved the weights by hand and sure enough they would not snap back in place. I had changed one of the springs to a lighter one when messing with my timing. I removed the weights and lubed the posts and put it all back together with a heavier spring. Now it springs back good.
I haven't done a test drive yet but I work tonight and will know shortly.
Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,367
Likes: 1,061
From: Virginia USA
Your engine will run its best with 52 degrees of advance at cruising speeds and about 34 to 36 degrees of mechanical advance under a wide open throttle at 2500+ rpm. As most points-type distributors only provide around 12 to 14 degrees of mechanical advance you need to run 14 to 16 degrees of initial advance just to reach 30 degrees. Then you'd need a vacuum advance that will provide an additional 22 degrees of advance to reach the magic 52 degrees. Remember, the vacuum advance does NOT work under full power because the vacuum drops to zero.
Pro
Joined: Sep 2017
Posts: 516
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From: Modesto California
Thanks, I know all that. I like an aggressive timing curve so I'm always on the hairy edge besides my ZZ4 doesn't like that much off idle cruising timing. It pings under part throttle acceleration with 52 degrees. I tune my ignition per Lar's paper and he recommends no more that 12 degrees vac advance so that puts me at 48 off idle cruise all in. My vac can puts in 16 degrees which by trial and error is just too much.
If it pings it's because of too much mechanical advance; not vacuum advance because the vacuum advance falls off IMMEDIATELY when you give it the gas. Try increasing the return spring tension first and see if that solves your problem.
Last edited by 71VetteLover; Sep 20, 2017 at 05:04 AM.
Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,367
Likes: 1,061
From: Virginia USA
Lubing my weights, changing the springs and a slight adjustment of the throttle stop screw fixed my issue. At least I hope, I adjusted the throttle stop screw when I got to work and haven't road tested it yet but did drive it around the parking lot a few times and the idle came back to 750 like normal.
My weekend starts today so I can mess around with it more. I want to play with the timing some more as since I changed the springs I need to make sure my all in timing is coming it at the right rpm.
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Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,367
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From: Virginia USA
I stated looking into vacuum leaks because even though it now returns to 750 RPM's its still slow. Sprayed carb cleaner the base of the carb and got RPM rises almost all the way around the base gasket, not the gasket between the carb and manifold but the carb base plate and the car body. Also got a RPM rise at the secondary throttle plate shaft.
My manifold vacuum at idle is 20 inches at the headlight/wiper door vac but I noticed a strange "pulsing" on the vacuum gauge and you can hear it. Is that normal? I don't remember ever hear or seeing anything like that in the past when working on buddies engines. I've never put a vacuum gauge on this engine before. It's a ZZ4 crate engine I installed in 2000. I doubt its related to my idle problem though.
Tomorrow I'm going through my timing one more time just to make sure it's right before diving into the carb.
Thanks again for you help and suggestions.
Pro
Joined: Sep 2017
Posts: 516
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From: Modesto California
If the idle speed will slow down when you manually exert pressure on the throttle lever you either need a stronger return spring or your throttle shafts are getting loose in worn bores and it's sucking in air around the primary throttle shaft. Try the stronger spring first and see if that solves your problem.
Oh, why didn't I think if this first? If you have installed lighter distributor springs it's very possible you're getting mechanical advance at idle when you don't want mechanical advance. You only want mechanical advance several hundred rpm above idle to prevent a constantly-changing idle speed.
Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,367
Likes: 1,061
From: Virginia USA
If the idle speed will slow down when you manually exert pressure on the throttle lever you either need a stronger return spring or your throttle shafts are getting loose in worn bores and it's sucking in air around the primary throttle shaft. Try the stronger spring first and see if that solves your problem.
Oh, why didn't I think if this first? If you have installed lighter distributor springs it's very possible you're getting mechanical advance at idle when you don't want mechanical advance. You only want mechanical advance several hundred rpm above idle to prevent a constantly-changing idle speed.
Oh, why didn't I think if this first? If you have installed lighter distributor springs it's very possible you're getting mechanical advance at idle when you don't want mechanical advance. You only want mechanical advance several hundred rpm above idle to prevent a constantly-changing idle speed.
I actually went to heavier springs (one size up) so that't not it.
I played with my throttle today going to work, it's damn hard to get my steel toe shoes behind it while sitting at a light. I did manage but no change.
Last weekend I sprayed carb cleaner everywhere and I did get RPM increase at the secondary throttle shaft bores and also the carb base gasket. I think I'm going to send my carb to Lars to have him work his magic. I'm waiting on my buddy to send me a q-jet I loaned him a few years ago to send it back as this is my daily driver and I can't have it down.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
Pretty rare for the gasket between the throttle plate and main body to leak but sounds like you are on to something there with the spray test. And the throttle bushings are likely "oversized" from ages of revving also. I went through this last yr. They have a kit to rebore the throttle shaft hole, insert new bushings. And there is a company that will do that for you. All said and done, it was $75-$125 for those options. So I ended up just buying a much improved and a whole lot better throttle plate for $130 on EBay.
Pro
Joined: Sep 2017
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From: Modesto California
Thanks '71,
I actually went to heavier springs (one size up) so that't not it.
I played with my throttle today going to work, it's damn hard to get my steel toe shoes behind it while sitting at a light. I did manage but no change.
Last weekend I sprayed carb cleaner everywhere and I did get RPM increase at the secondary throttle shaft bores and also the carb base gasket. I think I'm going to send my carb to Lars to have him work his magic. I'm waiting on my buddy to send me a q-jet I loaned him a few years ago to send it back as this is my daily driver and I can't have it down.
I actually went to heavier springs (one size up) so that't not it.
I played with my throttle today going to work, it's damn hard to get my steel toe shoes behind it while sitting at a light. I did manage but no change.
Last weekend I sprayed carb cleaner everywhere and I did get RPM increase at the secondary throttle shaft bores and also the carb base gasket. I think I'm going to send my carb to Lars to have him work his magic. I'm waiting on my buddy to send me a q-jet I loaned him a few years ago to send it back as this is my daily driver and I can't have it down.
As the secondaries are VERY rarely used I can't imagine your secondary shaft or shaft bore being worn out. There is a very good possibility a previous owner installed a wrong base gasket because there are many different styles for different applications. It's also very possible a previous owner over-tightened the rear mount nuts and bent the rear of the cast aluminum base enough to not seal real good. If that's the case you can "machine" the base flat by using a wide file or piece of flat bar steel backed with a course emery cloth. Another way to check for leaks is to pressurize the inside of your engine with air pressure then using soap bubbles look for any sign of leakage around your carburetor base. There's also the possibility the fumes from the carburetor cleaner (I prefer starting fluid) are getting sucked into the manifold where the vacuum modulator gets its vacuum from; the fitting screwed into the top of the #6 runner.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
As the secondaries are VERY rarely used I can't imagine your secondary shaft or shaft bore being worn out. There is a very good possibility a previous owner installed a wrong base gasket because there are many different styles for different applications. It's also very possible a previous owner over-tightened the rear mount nuts and bent the rear of the cast aluminum base enough to not seal real good. If that's the case you can "machine" the base flat by using a wide file or piece of flat bar steel backed with a course emery cloth. Another way to check for leaks is to pressurize the inside of your engine with air pressure then using soap bubbles look for any sign of leakage around your carburetor base. There's also the possibility the fumes from the carburetor cleaner (I prefer starting fluid) are getting sucked into the manifold where the vacuum modulator gets its vacuum from; the fitting screwed into the top of the #6 runner.
I prefer using Gumout spray can instead of Mr. Bubble checking for leaks.
And using starting fluid on a engine that's running might put you in the hospital.
Danger-danger. Bad advice.
Last edited by HeadsU.P.; Sep 27, 2017 at 11:04 AM.