RPM rises after warming up.

[Rill Cosby]

I wasn't sure where to post this and this is my second post in two days but this should be the "last" real big problem I'm dealing with after the rebuild of my engine. 

I honestly thought when this issue appeared that Google would have tons of information on this issue. Maybe my google-fu is getting worse but I can only find topics along the lines "hesitation after warm up, rough idle after warm up, etc." It seems like the only thing I could find similar was a guy having the same issue, but with his boat. Still gave it a read.

The issue I'm having is that no matter where I set my RPMs it will always increase about 400-500 RPMs after warming up making it difficult to have it tuned well. I'd assume that it is vacuum or carb related but I am wrong a lot when it comes to these things, as my post also proved yesterday. It also fires up right away on cold start. After the warming up and the RPMs have raised coming back to start a warm engine sometimes then it is hesitant. 

I'm running the Ztherapy carburetors, before the rebuild they sat covered indoors for about 2 years. They were cleaned before they were taken off and once put back on I had fuel issues that ended up being pump related. I still dissembled the carbs and cleaned them. I didn't see anything that looked like a problem but again, I'm no expert. Choke is also moving freely. I've contemplated the thought of a tune up kit for the carbs but I don't know if needed. 

On to vacuum, I've checked all the lines I can think of. I DO have a leaky brake booster that will hopefully be replaced this weekend but I've had a leaking brake booster way before the engine was rebuilt. The RPMs, back then, only fluctuated when braking, as it should. Before this rebuild the car ran like a top before it's catastrophic failure. Actually, it ran better than it does now which is a bit disheartening. Especially on idle and when given gas.

Checked and torqued all manifold bolts. With the rebuild came all new plugs, wires, cap and rotor. Also not sure if related but distributor holds a vacuum. Timing has been checked multiple times and seems correct. Valve lash also checked multiple times. Compression is 165-180 through all pistons. I'm unaware of the fuel pressure but it's all the same setup as I had for years. Car runs through the RPM range just fine. It's just that RPM rise when warmed up.

I guess this post is more about bouncing ideas around. You guys have a lot more experience than I do and always have a better idea of what to look for or where to start. I do all that I can before coming to post on the forum. Sometimes it's just something easy I've overlooked and I feel dumb (which I definitely prefer) or it's a PITA or head scratcher like when my transistor ignition module was going out years ago. Maybe the two things I assume that could be the culprit actually have nothing to do with the problem. Thanks for the read!

Edited May 13, 2019 by Rill Cosby

[siteunseen]

When you google always add "classiczcars.com" at the end and you'll find what you need. Our past threads will show up, you'll get your answer then. Best of luck!

[Rill Cosby]

21 minutes ago, siteunseen said:

When you google always add "classiczcars.com" at the end and you'll find what you need. Our past threads will show up, you'll get your answer then. Best of luck!

All my searches begin with or include classiczcar. Then the google correction of "did you mean: classic car". Then clicking the search instead "classiczcar." My favorite is when it doesn't offer that option. I then go to other forums. I've had this problem for months and have periodically searched this topic on this site and I'm sure there probably is a similar situation on this site somewhere but it has eluded me so far. I've only found one that was kind of close and it just ends abruptly. Everything else I find is "hesitation, rough idle, rpm drop, fluctuations" and are usually combined with an action to cause those things as braking, clutch pressing, etc. I know every forum frowns on not using the search function so I try my best to find it first. Thank you, I need more than luck!

[Patcon]

I believe, if you add "site:classiczcar.com" at the end of your search string it will only search this site

[Captain Obvious]

2 hours ago, Rill Cosby said:

no matter where I set my RPMs it will always increase about 400-500 RPMs after warming up

I DO have a leaky brake booster that will hopefully be replaced this weekend but I've had a leaking brake booster way before the engine was rebuilt. The RPMs, back then, only fluctuated when braking, as it should. Before this rebuild the car ran like a top before it's catastrophic failure. Actually, it ran better than it does now which is a bit disheartening. Especially on idle and when given gas.

Everything on the engine works better when warm. They really don't like running when cold, especially at cold idle. In fact, that's why there is usually some sort of idle speed boost method employed that can be engaged at cold idle. For the 240 and 260 Z's, that idle boost is built into the choke mechanism. And for the 280's that function is performed by the AAR.

The point is... If you are not employing the use of some sort of cold idle boosting method, or if your boosting method is not artificially raising the idle enough, then it would be perfectly normal for the idle to go up as the engine warms up.

Other thoughts? If you are having some issues when you first hit the throttle you may be running a little lean. A lean cold idle is an unhappy idle. If you're running a little lean, the idle would get better when the engine warms up. That all seems to dovetail together.

2 hours ago, Rill Cosby said:

I guess this post is more about bouncing ideas around. You guys have a lot more experience than I do and always have a better idea of what to look for or where to start. I do all that I can before coming to post on the forum.

Sounds like you feel reluctant to come to the forum for help? Don't worry about that. Bring it.  

[7tooZ]

Doesn’t seem right to set the idle speed while the engine is cold. I have always did tuning based on warm/hot engine. Living in the NW cool weather has always meant using the choke to hold idle speed during cold starts and then increasing idle speed as engine warms. Having a throttle lever helps elevate   that increasing idle during warm up. 

[Rill Cosby]

1 hour ago, Captain Obvious said:

Sounds like you feel reluctant to come to the forum for help? Don't worry about that. Bring it.  

That's what I like to hear! No offense to the guys above that responded first and second but that's kind of the reason I am reluctant to post. In there defense, you can find a discussion about almost every problem that has ever been had with these Z cars with some sort of input or solution. Hell, I've been a member since 2012 and lurking since 2011. I rarely have to post or contribute because of the insane knowledge you and other users already have here. The database is huge and the community is overall one of the nicest and most helpful I have seen. That being said, I sometimes find it counter-intuitive when told to use the search forum, especially when stated that I have exerted all my searching options. But I do get it, really. 

1 hour ago, Captain Obvious said:

Everything on the engine works better when warm. They really don't like running when cold, especially at cold idle. In fact, that's why there is usually some sort of idle speed boost method employed that can be engaged at cold idle. For the 240 and 260 Z's, that idle boost is built into the choke mechanism. And for the 280's that function is performed by the AAR.

The point is... If you are not employing the use of some sort of cold idle boosting method, or if your boosting method is not artificially raising the idle enough, then it would be perfectly normal for the idle to go up as the engine warms up.

Other thoughts? If you are having some issues when you first hit the throttle you may be running a little lean. A lean cold idle is an unhappy idle. If you're running a little lean, the idle would get better when the engine warms up. That all seems to dovetail together.

I think my 260z before the engine rebuild may have spoiled me. It also surprised others how well it ran or more specifically idled. I know it's not advised to do so but I could barely touch the key and it would crank without the choke with no hesitation in even cold weather. Of course, it would run a lot better when it warmed up but rpms were never an issue. In the warmer weather, it would crank right up to roughly 750 rpms and hold it no matter the temperature of the engine.

I know the above sounds kind of whiny but the 400 - 500 rpms that increase is way too much. Something changed somewhere whenever putting in the rebuilt engine. That doesn't narrow down much haha. But everything outside the internals of the engine have remained the same.

No issues with first hitting the throttle, no stumbling at all, on cold starts. The hesitation and then stumble does occur during startup after I restart the car once it's warmed up. Usually after some throttle it goes away but at this point RPMs are back up to around the 1200 area.

Side note, I've read hundreds of your posts at this point. Were you one of the users who got there hands on a 123ignition distributor? I guess I should use the search function.  

 

[Rill Cosby]

3 minutes ago, 7tooZ said:

Doesn’t seem right to set the idle speed while the engine is cold. I have always did tuning based on warm/hot engine. Living in the NW cool weather has always meant using the choke to hold idle speed during cold starts and then increasing idle speed as engine warms. Having a throttle lever helps elevate   that increasing idle during warm up. 

Idle speed is always set when the car is warmed up. The problem I run into is that because of that rpm jump when warmed up I can set my idle to roughly 750 rpms when warm but whenever I come back to cold start it with or without choke it won't run at all. Or it has A LOT of trouble starting.

I was still writing the post above while you were commenting that I'm so accustomed to starting the vehicle and having a steady idle right away. I don't know what magic is inside those Ztherapy carbs but the idle was solid no matter the temperature.

[7tooZ]

8 hours ago, Rill Cosby said:

Idle speed is always set when the car is warmed up. The problem I run into is that because of that rpm jump when warmed up I can set my idle to roughly 750 rpms when warm but whenever I come back to cold start it with or without choke it won't run at all. Or it has A LOT of trouble starting.

My experience is that when the warm idle speed is in the 750 range with SUs a cold start would require the use of the choke. Both my roadster and my Z have been the same. Cold start to me would be after the car has sit for a day or so I would choke to about 3/4 (depends on how you have the cables set) and may require a couple of tries of 10 seconds before it will catch.  I immediately back off the choke to get it to idle at about 1200 or so and then gradually back off the choke to maintain the idle. After driving my will start without choking during the day. I am using Ztherapy SUs.

I will let others weigh in on if that is “Normal” based on their experience. Could be another interesting discussion. Your question is making me question my “normal”.

[Mark Maras]

 Here's a visual check. Pull the air filter and watch the carb butterflies while a friend pulls the choke all the way on with no throttle. The butterflies should open a bit near full choke. That increases the idle a bit. If the butterflies don't move either the chokes are not operating fully or the cold idle needs to be adjusted. The cold idle adjustment is done by bending a linkage rod. However, I doubt that the cold idle linkage needs adjusting but it's worth checking.

[7tooZ]

It is possible to have a poorly adjusted choke cable preventing it from opening all of the way. Or causing the butterfly to stay open.

[Rill Cosby]

12 hours ago, Mark Maras said:

 Here's a visual check. Pull the air filter and watch the carb butterflies while a friend pulls the choke all the way on with no throttle. The butterflies should open a bit near full choke. That increases the idle a bit. If the butterflies don't move either the chokes are not operating fully or the cold idle needs to be adjusted. The cold idle adjustment is done by bending a linkage rod. However, I doubt that the cold idle linkage needs adjusting but it's worth checking.

This is something I didn't think of checking. I'll hopefully get some time around it tomorrow and report back!

7 hours ago, 7tooZ said:

It is possible to have a poorly adjusted choke cable preventing it from opening all of the way. Or causing the butterfly to stay open.

During this process I have the choke cables unattached temporarily. I will check around what you and Mark said and take a look at the throttle valve tomorrow for sticking or any other issues. Didn't seem to have any issues after the carb cleaning but will double check.

It would be interesting to hear about what is more "normal" for sure. Even though most of us run about roughly the same type of setups, there is a lot of variations within them and I'm sure you can only get it to run at certain level of efficiency no matter what you do, just a matter of what that level is exactly. I've had the process of what you described in other vehicles but after the mods on the Z a couple years back it purrs at the start, almost every time. It COULD start without choking if I wanted to. I've just had it that way for about 3 years, that's all I have to compare it to. Albeit, 95% percent of the time it is warm weather here and it was driven almost daily. But a couple of tries of 10 second starts, even after sitting for a couple of days. No way.

Edited May 14, 2019 by Rill Cosby