'60 270 HP idle and vacuum issues
I have a '60 270 HP. Just completing the restoration. It idles at 700 RPM with 9-10 in. Hg vacuum. I believe the vacuum is a little low for the Duntov cam, but it idles very smooth and starts easy. It will stay at these numbers after the choke opens and sitting in neutral for long period. After about 10 minutes of driving, the idle will reduce to 300-500 RPM with 3-6 in. Hg vacuum. After I turn the engine off, then immediately restart, the idle returns to 700 RPM with 9-10 in. Hg vacuum. Some other numbers, compression is 150 psi and dwell is 32 degrees. Plugs are dry, brown, black , and gray in color. The carb will on occasion back fire when accelerating. Acceleration seems a little less than expected. The carbs were rebuilt by Bob Kunz. Any inputs will be appreciated.
-
-
Re: '60 270 HP idle and vacuum issues
what are the valve lash specs? could one be slightly tight? -
Re: '60 270 HP idle and vacuum issues
I set the intakes to .012 and the exhaust to .018. I did it cold.Comment
-
Re: '60 270 HP idle and vacuum issues
In my opinion you have a linkage or throttle plate issue, whereas something is slightly being snagged and after normalization is changing.
My '59 270 hp, converted 20 years ago to hydraulic, occasionally does the same thing. After driving all day I occasionally stop at a light and my idle is well over 1000. I blip the pedal and drops back down to 700 or so. I've chased that issue for years and always tell myself I need to pull the Primary carb and tear it apart, but I just deal with it for now.
The Carter WCFB's are prone to mechanical linkage variances. Loose main lever to throttle shaft, sticky throttle plates, sticky choke fast idle linkage.....are just a few of the items to check when your engine exhibits the hi-rpm mode. My thought is everything is properly set when you're at the 400 rpm mode and sticking open a bit. i.e. You'll likely need to bring the idle up when you find it at the lower setting, then find where it sticks when at the higher RPM.
You vacuum readings are just following the rpm fluctuations so I doubt it is a camshaft lash issue. However, exactly which cam is it? Is it a original 097, or is it a replacement? I've used several of each. For example, the Crane Cams (?) and Sealed Power CS113R replacement for the original 097 includes a different spec for the valve lash. It's higher. IIRC it is somewhere near Intake 0.014", Exhaust 0.020". I have been told due to different ramp specs.
RichComment
-
Re: '60 270 HP idle and vacuum issues
My '61 270 hp dual quad car doesn't exhibit any of those problems and has slightly higher vacuum at idle.
First off .012 and .018 cold is incorrect -- those are 'hot' lash settings. You should add .002 to each of those figures for a cold setting (e.g. .014 and .020) for cast iron heads/block.
Also, as Duke will tell you and I will confirm; its VERY hard to get a consistent idle on the dual quads at less than 850 RPM; that's where my hot curb idle setting has been for 8 years. Lower than that and yes, the carbs will tend to falter and die after they 'load up'... Make sure you are adjusting the mixture for lean, best idle at highest vacuum.
As Rich said confirm your linkage is operating smoothly, NOT hitting the ignition shielding and is hooked up progressive NOT shotgun....meaning the first carb only starts to come in at 1/2 throttle on the second carb not immediately. Shotgun is bad news for street driving.
Disconnect the linkage on both carbs at the barrel connectors with the engine hot but not running and operate the carbs throttle plates by hand (slowly) several times ALL the way so that the secondaries kick in. Make sure there is no sticking and binding...sometimes the carbs' throttle plates can 'hang up' briefly on the intake manifold or carb-to-manifold gasket. Make absolutely sure your carb linkage is coming OFF the fast idle came when you are setting the hot curb idle.
If you have the correct dual point distributor I believe your dwell is supposed to be less than what you have (IIRC 29*).
Another are to check (long shot but worth checking) is the distributor centfiugal weights - make sure those move freely with no binding and aren't caked up with grease.
And finally, if none of that helps and you get your lash/dwell/idle speed set correctly. With the engine running at idle, hot and exhibiting your problem, take an unlit propane hand torch and let it blow around the base of the carbs and other likely vacuum leak areas - if the engine speeds up there's your problem. About 7 times out of 10 inconsistent idle is a vacuum leak.
These checks should keep you busy for a while!Comment
-
Re: '60 270 HP idle and vacuum issues
Where is your initial timing at? WCFB's, when rebuilt by "the master", should be assumed as being correct. I would look elsewhere. How are your throttle return springs? My experience, though long ago, found these carbs easy to work on and get super performance out of them. The only internal issue you may still have is with float settings and/or age of rebuild items such as accelerator pumps, etc.
Stu FoxComment
-
Re: '60 270 HP idle and vacuum issues
All very true - one other thing I didn't mention is that, although I generally set curb idle with the rear carb, you should have the front carb idle speed screw cranked in at least a slight bit so the front carb's primary throttle plate is never completely closed just to prevent possible sticking.Comment
-
Re: '60 270 HP idle and vacuum issues
Based on my measurements, I've never seen a difference between "hot" (idling) and cold valve clearance, which makes sense because the thermal expansion rate of cast iron is virtually the same as the steel pushrods. If the engine has an aluminum block and/or heads it's a different story.
If the engine is running hard, clearance will close up a few thou, especially on the exhaust side. That's why mechanical lifter cams need clearance ramps, and the exhaust side is often taller than the inlet side because the exhaust valve runs hotter and will grow more than the inlet valve when running hard. At idle the exhaust valve is not that hot, so stem expansion is nil to minimal.
I recommend .010/.016" cold clearance for the Duntov cam and you have a choice of two different indexing schemes to ensure the lifter is on the cam base circle. The reasons are explained in the Hinckley-Williams valve adjustment paper, which has been posted on the TDB many times.
I recommend an initial idle target speed of 900 for all mechanical lifter cams using a accurate test tach, not the in-car tach. The higher the idle speed the better the idle quality and higher the manifold vacuum, however, it's up to the owner to choose the lowest idle speed commensurate with acceptable idle quality, and that value will vary with the individual.
DukeComment
-
Re: '60 270 HP idle and vacuum issues
For good or ill, I have gone by the guidance in this Crane Cam's doc for cold valve lash for many years (attached) however I also run tighter valve lash than the .012/.018 typical specs. In fact I have sorta fallen into the exact numbers Duke suggests above (.010/.016 "hot") based on years of trial and error. I also have the initial timing at 12* BTDC.
Unfortunately, I had to discover the more advanced timing, higher idle speed and tighter lash advantages over the years before I was a member here and somebody smart could tell me about them (* SIGH *).Comment
-
Re: '60 270 HP idle and vacuum issues
Thanks for all of the input. I will get back on it next weekend. Stay tuned.Comment
Comment