I'm working on a '68 Monaco (to make some $$$ this time) that came stock with a 383 2bbl. It has been spiced up a bit with a Comp XE268H cam, lifters, springs and a double roller timing set. It is topped off with a stock iron 4 bbl intake and Eddy 750cfm, all breathing through shorty headers and 2 1/2" duals. It was also converted to a Mopar electronic ignition 10-12 years ago, when they were still using a "street" advance curve. It also has 2.76 highway gears.
I can't get the timing set right on this thing: I checked the cam and it is very close to straight up like Comp recommends (only a bit more than one degree advanced). When I set the initial timing to Top Dead Center like the FSM says to do with the '68 HP engines, I get 26 degrees centrifugal advance at 2500 rpm (vacuum blocked off), and 42 degrees total with vacuum. It wont idle there worth a darn, it only pulls 8" of vacuum.
When I set it to 10 degrees initial I then get 32 degrees at 2500 rpm, and 44 degrees total with vacuum. I have played around with the screw in the vacuum advance unit and it seems to make no difference--it just won't go beyond 44 degrees unless I set the initial timing at 16-18 BTDC, but then under part throttle acceleration it then pings so badly it scares me.
When the initial timing is set at 10 degrees, I get 14-15" of vacuum but it again pings too much under part throttle acceleration. I have tried 7 degrees BTDC, with minimal pinging, but it runs like a lame dog there (and way too hot at 220F).
I think since it has the dished top pistons (9 to 1 compression ratio) that there is not enough quench to keep it from pinging. I think it needs to have the distributor recurved (if I could find anyone around here that even knows what that is).
Anyone have any ideas about where to set the initial timing?
Joined: Wed Jan 25 2006, 07:03PM
Location: Milford Mi
Posts: 403
Two things if you drove the at 10 degrees BTDC with the vacuum advance hooked up you were running 44 total that is to much and could be the cause of your heating up. Try running it with out the vacuum advance hooked up at 10 degrees BTDC and just let it go to the 32 degrees. Second a 68 383 was a 9.5 or 10 to 1 motor if you have old or poor gas in it it will ping.
I've tried it with the vacuum both on and off, and the pinging was still there either way, but worse without vacuum. It's been my experience that retarded timing is usually what causes the engine to run too hot.
I usually set up Mopar big blocks at 32-35 degrees advance at 2500-2800 rpm, and 50-52 total with vacuum. This usually shows around 7-12 degrees BTDC when the idle is set back down around 800 rpm. Sometimes I've had to back them off a bit, but not too much. It's just this one just won't cooperate!
I've worked on several stock low compression 2 bbls over the years and they ran well set at 5-7 degrees BTDC initial, 45-50 degrees total (I checked and the FSM shows it has a 9:1 compression ratio). Many had the 2 bbl manifold swapped out for a 4 bbl one, but this is the first one I've worked on that has also had the cam changed from RV to Magnum style. <span class='smallblacktext'>[ Edited Wed Apr 08 2009, 06:55PM ]</span>
Joined: Sun Oct 09 2005, 05:02PM
Location: ALLEN PARK, MI.
Posts: 2007
Ok....lets get right to the MEAT of it...shall we? When any performance cam is added, here is how anyone should set the timing! Plug the advance. Run the engine up in RPM until the mechanical advance hits it's full-in point. Go a few hundread rpms over it to INSURE full advance is in. THEN...set TOTAL timing...be it 32...34...36..38 degrees. DONE!
Of course, check the vaccum advance to insure it works also when done. Do not rely on the BASE timing when any performance cam is installed IMHO! !thumb Tony P.
Joined: Sun Feb 26 2006, 08:46PM
Location: Kingston,Ontario
Posts: 5622
I agreee with Pietin440. .Once the cam has been swapped out,you cannot go by factory specs.
Recurving the dizzy is a must.I did that when I swapped the cam. I had the advance weight slots welded smaller to limit the weights travel. More initial,full advance comes in sooner,and can hammer on it with no ping.A lot of big blocks do not like a lot of timing.50 degrees would be nice but in reality most are comfy between 38 and 44 degrees depending on the engine's set up. (I have a total of 41 degrees,10.5 intial and all in at 2500 RPM vacuum is hovering around 20 inches.RV cam)
Check the number on the vacuum advance pod. For example,9.5 is the advance number times two which equals 19 degrees--for example. If you cannot adjust the pod with an allen key,swap out the pod with a different number of advance.
Higher vacuum numbers and ping tells me a lean out condition with more initial timing.
Check the sizing of the metering rods in the Eddy carb. They may not be fat enough.It can cause a lean mixture under throttle. When I went from single to dual exhaust,I had to go from .44 to .41 size. Plus it is possible the jets are not big enough.Today's Eddy carbs are usually on the lean side out of the box..Just an example and maybe some insight.
Bottom line it test and tune,test and tune. Trial and error. WRITE EVERYTHING DOWN!! Base,timing,etc.What works and does not with the combo in question..
PIETIN440 wrote ... Ok....lets get right to the MEAT of it...shall we? When any performance cam is added, here is how anyone should set the timing! Plug the advance. Run the engine up in RPM until the mechanical advance hits it's full-in point. Go a few hundread rpms over it to INSURE full advance is in. THEN...set TOTAL timing...be it 32...34...36..38 degrees. DONE!
Of course, check the vaccum advance to insure it works also when done. Do not rely on the BASE timing when any performance cam is installed IMHO! !thumb Tony P.
That's the way I've been doing it for years! It usually works well too -- looks like it's time to get wrenching on the carb to see how lean it is.
PIETIN440 wrote ... Ok....lets get right to the MEAT of it...shall we? When any performance cam is added, here is how anyone should set the timing! Plug the advance. Run the engine up in RPM until the mechanical advance hits it's full-in point. Go a few hundread rpms over it to INSURE full advance is in. THEN...set TOTAL timing...be it 32...34...36..38 degrees. DONE!
Of course, check the vaccum advance to insure it works also when done. Do not rely on the BASE timing when any performance cam is installed IMHO! !thumb Tony P.
That's the way I've been doing it for years! It usually works well too -- looks like it's time to get wrenching on the carb to see how lean it is.
I have the same cam in my 383, and it is set at 35 BTDC at 2500 rpm, 50 BTDC with vacuum, 12 BTDC initial and it doesn't ping at all, but I had to do a lot of carb tuning to get it there as it was way too lean out of the box.
Joined: Sat Sep 23 2006, 04:30AM
Location: Sharpsburg, GA
Posts: 924
The MP dist. is really a Mallory YH bulit for MP to look like a Mopar dist. and it is fully adjustable. You need to change the mechanical advance by about 10 degrees and while you are at it, put the right springs in it too. Here are their instructions for using the distributor adjustment kit they sell. I think this will get you running right. Scroll down and click on the "YH and YT advance curve kit".
They also sell some little tools or spacers for setting the mechanical advance curve to just the right amount. Without the tools you will just have to guess and use trial & error.
Joined: Thu May 01 2008, 11:15AM
Location: Chicago,IL
Posts: 2868
My experience has also been that Carterbrock carbs are too lean out of the box, especially the elec. choke ones calibrated for 'economy' or 'emissions'. First step is usually to change them over to the manual choke/power settings. You can find those on eddy's website.