Want to check timing on my 428. Car cranks fine cold but once it warms up it idles too high and when turning off it diesels a little. Crank back on warm engine is hard. Almost like cranking on an almost dead battery. What are the timing specs running modern 93 octane pump gas. Stock engine. Any idea why it’s acting like this?
When the engine runs on, I always suspect advanced timing, but there are other causes. Easy enough to check the timing, and verify the mech & vac adv is working properly. My CJ always liked the timing set right @ the factory specs.
Checked it. Idles right around 900-1000. Timing at 10. I just let it warm up and checked. Didn’t drive it. Sometimes after driving it and coming back putting in park the idle goes up in the 1200-1500 area. Not sure why. Shutting it off then at that high rpm it will diesel a bit.
If it idles high after driving, check float levels. I have had this happen to me before, not that far off but float level was slightly high the idle was up. Just something to check.
Could be carb base gaskets causing a vacumme leak. I had that happen on my 351 C could not get the idle down below 1200. Took my old gaskets that were paper thin and put on gaskets about 1 16 of an inch.
Would that show up only when warm after driving? I’m wondering if maybe after driving it and varying the throttle if the cable may not be letting the throttle plates completely close?
That is definitely something to check too. Might want to re-adjust your idle needles and throttle blade positions ~ set them all back to “baseline”. Might as well do that when you are checking / adjusting the float levels.
How is your vacuum advance unit? Checked the timing with it disconnected and vac lines plugged, right? (always have to ask that, LOL!)
Yes of course. Think 10 initial is pretty standard for one of these.
Both your idle speed & initial timing are too high vs. the factory spec. IIRC I had mine around 750 rpms at idle and initial at 8. When you stick it in park & the idle increases, I’d suspect a vacuum leak (as others have said).
Once you chase down the (possible) vacuum leak, I’d follow the tune up procedures outlined in the shop manual, making sure you have the right heat range spark plug & the proper gap. You can even index the plugs while you’re at it.
I don’t know how hard you are driving the car but if you are into the throttle hard enough for the secondaries to open they may not be closing all the way when letting off the throttle, causing the high idle. When mine were sticking they would close if I shut off the engine and restarted it. (There’s a whole embarrassing story that the SoCal boys know regarding this, but we won’t go into that for now.)
+1 on too much timing and possible vacuum leak. What, a 68 Cougar with a vacuum leak? Never heard of it!
I’m jealous of your 93 octane gas! Weak sauce 91 here in Kalifornia.
Think I may dial back to 8 or so. Possibly factory spec of 6. Have only driven conservatively/cruising/normal no wot.
Too afraid to stick my foot in because it might cause a catastrophic failure.
Please remind me the best way to check for vacuum leaks? Done it before but can’t remember.
First things first. What is the list number of your carb? Second, is your choke fully releasing and is your fast idle cam dropping completely off so the choke is out of the picture. 10 degrees initial timing should not be a problem. So if the fast idle cam is completely off, is your primary idle stop screw in contact with the primary throttle plate stop? If you turn the screw all the way out, does it not affect or slow down the hot idle? If not then:
You may have an a air leak somewhere. Carb to spacer plate. Spacer plate to intake manifold. Intake manifold to head.
Your secondary stop screw on the carb may be adjusted to crack the secondary open to much and/or your secondary throttle shaft may be stuck, or your secondary plate throttle rod may not be adjusted properly.
Or you may excessively worn throttle plate rods that the throttle blades attach to in the throttle plate assembly.
I help on these issues not for money but to keep our cars alive. I can rebuild your carb and rebush your throttle plate if necessary and wet test it on a known big block to ensure it works on a known FE motor. But first you need to eliminate the above thoughts as the problem. Even a leaky vacuum brake booster can cause a high idle, and that will cause dieseling. Anything above 1,000 rpm on a stock 428 can/will cause dieseling. Try to unhook and plug all vacuum sources to the manifold as you zero in on you high idle problem. Call me at (360) 377-5164 as I talk way faster than I type.
Rob
I like to get the car set at the factory specs (timing & idle) & see how the car runs. Chase down any issues preventing it running the way it should. After that, you can tune for more performance (usually by just dialing in more initial timing) if desired, making sure it runs right under all situations (high load uphill, etc).
Why do you fear catastrophic failure under WOT? Is it pinging under load (too much timing)?
A quick way to check for a vacuum leak: “smother” the carb w/your hands. if it stays running, you have a decent sized vacuum leak. Your ears can also help to track down the leak, along with visual inspection & hands on to possible sources of vacuum leaks (lines, canisters, carb base, manifold fittings). If your engine compartment is not detailed, you can spray carb cleaner at the suspected source of the leak, if the idle increases, you found it. Some guys use WD-40 for this on a detailed engine bay, but I have not had occasion to try this.
DON’T go tweaking your carb until you’re convinced it’s the problem (i.e. made sure everything else is fine 1st). Seems like most drive-ability problems are timing/vacuum related.
The carburetors on these cars - if yours is the original style - has a lever on the throttle that actuates the front bowl vent. The levers have a “step” that can hang if the throttle return spring is too weak, or if the throttle return spring bracket is mounted improperly.
Here’s how it should look:
Royce, he has a 69. It has a throttle cable setup. It is not the same as the nice looking 68 you posted. But it should give him an idea of what to look for in regards to the vent linkage sticking.
I would recommend firing it up with the hood open and air cleaner off(besure to plug any vacuum lines removed taking it off). Fire the engine up, get it to run at this high idle speed you are describing, then find the throttle on the carburetor, then push it toward the front. If it easily moves to the correct idle then you may need more return spring pressure, maybe a second spring if only 1 is present(should always have 2 anyway) or replacing them with new ones.
I am leaning to your having a vacuum leak and maybe some other tuning issues. But never leave out the possibilities of simple fixes being what you need.
I’ll throw in my 2 cents. Experience with big blocks over the years, especially if bored, tend to have this hot start slow spin. Some say the rings and pistons are too hot and are ready to expand once stopped??? I have solved it several ways. 1st I try retarding the timing until the slow spin is almost gone but still have sufficient vacuum for the brakes. 2nd I have converted the battery and starter cables over to 2 gauge and put longer bolts through the starter/bellhousing and putting nuts on the back for a tight connection. 3rd a high torque starter.
As for the rpm increase could you being running hot enough that the vacuum switch in the thermostat housing is switching the vacuum to the distributor which will increase the speed.
The '69 carburetors have the same issue and the same lever. If the return spring is too weak they have inconsistent idle speed because the throttle doesn’t always shut.
I tinkered a bit with it today. I checked the float levels since it’s easy to do. The front one was what I would call right. Just a little trickle right at the bottom of the site plug hole. The back one I thought was way to high fuel pouring out. I readjusted it, but the exhaust smelled so clean I think I went a little too lean so I’ll raise that back up a bit. None of this changed anything with the idle issue or really the current driveability.
When it’s cold or warming up it idles and seems to take fuel like it should. Since we’ve gotten into warmer weather here it’s seemed to develop a hesitation/misfire at low throttle say 1000 to 2000 rpm range when starting off but then seems to clear up. I can also feel random misses at highway speed normal driving rpm just kinda seems to randomly and quickly cut out for a split second.
Again only when it’s warm. None of this is under full throttle just normal driving. I did stick my foot in it one time a while back and it did great plenty of power didn’t feel or hear anything wrong. Haven’t heard any pinging or spark knock at all.
What typically causes this kind of behavior?
Unless the rear float bowl is empty, it will not affect you idle mixture. If empty or stuck, it will. If the missing was there with the high float level in the rear, that may have been the reason. You have lots of suggestions in this thread, and it would be great if you answered each one so we can help you.
Rob
I have heard from many car friends from across the spectrum - from Early V8s to hotrods to muscle cars - any car with a carburetor - complaining about the modern gasoline/alcohol fuel that they have foist upon us. The symptoms you describe - hesitation and misfire are ubiquitous. After you’ve tried everything else try spending a couple hundred dollars on a full tank of racing fuel that doesn’t contain alcohol and see what happens.