4 bolt su carbs

[kully 560]

is it normal for the idle to go higher by about 250 rpms after the car has warm up to operating conditions ? starts and idles at about 700rpms then  it goes to about 950 rpms after warm up . I am using z therapy oil in the carbs I received the oil back in the late 1990s when the carbs were rebuilt ,maybe thinner oil? the motor is a l28 with 1971 4 bolt su carbs also the car is a automatic . thanks

Edited October 15, 2017 by kully 560

[siteunseen]

Just it just start this? Could it be some type of switch that raises it when in gear for an automatic. Maybe that switch has gone bad?

What does it rev in DRIVE sitting still? Seems like there's a vacuum pull that raises the rpms on autos when you put them in gear.

@djwarner  would probably know. He has an auto and is pretty darn smart. 

[djwarner]

Nope, this is not a transmission related issue. ATs have a dual point system that retards the spark in cold temperature. However the sensor is in the passenger compartment and only affects the timing at temperatures below 40 degrees. I don't think you northerners are seeing temperatures that cold this early in the season.

For AT's the idle is set while warm and in gear. Rpms raise when moving from drive to neutral or park as the engine unloads. A 250 rpm rise in that case would be normal.

Start the cold car and after a minute of idling note the idle RPM in park with the choke off. Drive the car to let the engine achieve operating normal temperature. Park the car and note the idle RPM in park. If you are seeing an increase, the mixture is changing, the throttle setting is changing, or a leak has changed the vacuum advance.

I would suspect a vacuum leak. If the leak occurs when the engine is cold and seals as the engine warms, I would expect the cold idle RPM to be low. This is contrary to Kully's description. I could see this happening if there was a vacuum leak at one carb that caused it to go lean when cold.

If the leak develops as the engine warms, the mixture leans out. Since the idle is high when warm, it makes me wonder what conditions were when the last time the mixture and idle settings were done.

BTW I have found that these engines never idled well at the "650 rpm" in Drive as specified. This worked well for 510's but not our Z's. 

I suggest the following:

1. Check the carb mount bolts for tightness as well as the balance tube fittings and intake manifold bolts.

2. Remove the vacuum advance line from the distributor and block it with a golf tee.

3. With a warm engine in neutral, set the idle speed to 900 RPM and set the timing to 10 degree BTDC. Idle speed will be affected as you adjust the timing, so you may have to repeat this step several times.

4. With a warm engine in neutral and correct timing, adjust the carb mixtures per the manual.

5. With the car chocked and the parking brake applied, set the idle speed to 700-750 rpm.

6. With the car back in neutral,  re-install the vacuum advance line. If the idle RPM changes significantly, troubleshoot. Vacuum advance should only affect timing when letting your foot of the accelerator at high speed.

I suspect that if your problem remains after this, the symptoms will change to a lower RPM at cold idle in drive. This may require more choke usage til warmed up.

 

[kully 560]

maybe I should have fill you in more on the car ,I am running a msd ignition with a zx dissy no vacuum advance is hookup and the timming is set 17 degrees before tdc so as to see 34 degrees at full advance. plugs gap at 45 , and it has a header on it with 2 -1/4" exhaust all the way back. the carbs were just balance and the mixture is right on. starts great, runs great,  it has always done this been living with it . but I guess I would like to look further into this if it is not normal. when I start the car it idles great but 700 rpms in park, drive around with choke open up a little 1-2 miles she warms up then no choke runs great but after a good  10 minute drive the idle increases [ in park] like maybe the carb oil gets hot and the carbs are reacting different just my thoughts

Edited October 16, 2017 by kully 560

[djwarner]

Idle speed should always be measured with the car in drive with a warm engine. Idle speed in Park will always be higher. This is normal for carb'ed  engines that don't have engine speed controlled by an ECU.

17 degrees advance at what RPM? These distributors are into centrifugal advance at idle speeds and timing will shift at different RPMs.

Sorry, I don't have my manuals with me at present, if you can get the model number off the distributor, I can give you the centrifugal and vacuum advance particulars. The point timing specs listed in the FSM are measured at zero rpm (hence no centrifugal or vacuum advances) with a test light.

BTW SU carbs don't really have a choke per se, the choke lever enriches the mixture. Once the engine warms for awhile closing the choke leans the mixture.

I have a ZX dissy with an OEM ignition module and they have the equivalent of dual points as well. Though yours with an MSD ignition may have bypassed them.

[kully 560]

I bypass every thing that the stock 260 ignition had meaning the stock ignition module , the warm up switch in the thermostat housing . my msd has just a 12 switch feed to it. my distributor in the car now is a 1980 d6k8-22 which has a very high vacuum advance that is why I do not use it, so I set the timing with a advance timing light at 34 degrees at 3000 rpms with vacuum advance plug up all the time . I know it is not any thing electric that would raise the idle that is why I am looking into carbs,it is when she warms up the idle goes up.

[Captain Obvious]

I'll be the dissenting voice and say that I believe it is completely normal for the idle speed to increase as the engine warms up. All other things held constant, I think this is normal.

I have a 280, which is supposed to have a device called the AAR who's purpose is to raise the idle speed when the engine is cold. I took my AAR off, so I have nothing to raise the idle.

With no device to purposely bump up my cold idle, when I first start my car it idles a little low at 500. But then over the first ten minutes of warmup, that idle gradually creeps up to 750-800 as the engine becomes warmer and happier.

Engines do not want to idle. And cold engines really do not want to idle. Unless I'm missing something here, I don't see the problem! Seems completely normal to me.

 

[Captain Obvious]

Oh, and I forgot something... The oil weight in the carbs should have nothing to do with idle speed.

Lean spot in transition off idle? Yes. But steady state idle speed? No.

[djwarner]

Sorry the spread sheet I have for the various distributors did not have the complete specs but it still has useful information for you.

The d6k8-22 was used on the 1980 ZX for both the AT and MT. Static timing (0 RPM) is 10 degrees. Centrifugal advance is a max of 17 degrees at 2500 RPM. So as you are setting timing, set the 2500 RPM to 27 degrees BTDC and let the idle timing fall where it may.

I've checked many of the early dissy's and they all max out around 27-28 degrees without vacuum connected.

It also mentioned that the Vacuum advance may range up to an additional 30 degrees and may thus be harmful. This is a function of how your vacuum port for the dissy is set up. On Weber carbs, the port only sees vacuum when decelerating like when you suddenly close the throttle. With an AT, if you are not getting stumbling when you quickly close the throttle, I wouldn't worry about leaving it disconnected.

Again, without fuel injection, there is no idle speed compensation. Standard method is to set the idle speed with a warm engine in gear and let the idle speed in neutral fall where it may.

 

[Diseazd]

11 minutes ago, Captain Obvious said:

I'll be the dissenting voice and say that I believe it is completely normal for the idle speed to increase as the engine warms up. All other things held constant, I think this is normal.

I have a 280, which is supposed to have a device called the AAR who's purpose is to raise the idle speed when the engine is cold. I took my AAR off, so I have nothing to raise the idle.

With no device to purposely bump up my cold idle, when I first start my car it idles a little low at 500. But then over the first ten minutes of warmup, that idle gradually creeps up to 750-800 as the engine becomes warmer and happier.

Engines do not want to idle. And cold engines really do not want to idle. Unless I'm missing something here, I don't see the problem! Seems completely normal to me.

 

I agree Capt. ......all of my Z's idle at 1000 rpm when warmed up.....not so when first cold started. Modern engines seem to have a sensor to adjust for this.

[kully 560]

these cars seem to run there best with timing set between 34 -36 degrees at all in 2500- 3000 rpms that is why I have mine set to 34 degrees perfect spot.  but as captain obvious says this is normal for the rpms to increase . I was just looking to see if other forum members here have the same issue with there cars as of  doing the same thing .

[Captain Obvious]

Right. All the cars have some way to purposely hold the throttle open a little more when the engine is cold. One way or another.

On the 280's it's done with the AAR. On the 240's and 260's it's done with the cam on the choke linkage that pushes the throttle open a little bit.

If you don't employ some technique like one of those, your idle will be lower cold than warm.