Zenith Carb backfire after Crane installation

[Arthur Dalton]

The idea is to set the 3000 rpm timing as that is at the top end of the mechnical advance . If there is a 20 degree mechanical advance , then when at idle rpms, the car will be at 10 BTDC... but if you have manifold vac and bring this to the retard section of the vac device , when the carbs are closed , the vac retard will bring the idle timing down closer to TDC .But, as soon as you open the throttle, the reatard will collapse and you will get advance. So, this gives you are steady idle at around TDC , aout 10 degrees off idle , and full advance at 3K
I would try 30 BTDC with no vac , see where that brings the idle timing without any vac hoses, and then hook up the retard and see where that brings it closer to TDC.
The closer to TDC at idle, the better and you will not have to fool with carbs to get a 700/800 idle. The problem with having a 10 degree advance at idle is the idle will be higher than design and the faster the idle rpms, the sonner the mechanical advance starts the curve .
I believe the rpm sig can be gotten off the pos coil terminal, as the coil is now switched via the crane box , before the coil feed..
You do not nee rpm for the signal emmissions relays with the straight Crane timing set-up

[DANTRCAV]

Ryan:

I like Arthur's method better. Makes sense, I never thought that one through when I had mine. It would make the idle better without having to readjust it too.

I had no trouble hooking up my old combo tach-dwell meter to the coil with the Crane system to get RPM's. I am thinking I hooked up the tach lead to Coil(-) - but I cannot verify that as it has been just too long.

Dan

[Arthur Dalton]

Dan
The concept of just manifold vac retard at idle [ vs ported carb advance] is just a copy of the earlier Injected engines , pre emmissions controls.
They have the Bosch 051 distributor that has ONLY retard and mechanical advance. this allows for proper idle a/f mix at close to TCD and instant advance a soon as the throttle plate starts to open..no vac advance needed. Many guys who want to do away with all the later controls [ speed sensors, temp switches, dual vacs, etc] , just put the earlier distributor in and go with a normal TDC idle w/vac, 30 degree @ 3K rpm w/o vac and be done with it.
This is the same as you recommend , except we are getting the advantage of pulling the timing back to TDC at idle with engine manifold vac...
The big plus with the 051 distributors was that the vac diaphragm link to the advance/retard plate was adjustable, so you could put the timing at idle anywhere you wanted it...but TDC is close enough... the main concern is the 3K setting to be set first..
On the PRMs pick-up point , b/c the signal is interrupted before the coil [ vs after the coil on reg points ..ie , switched ground through the points] , you should be able to get that sig either side of the coil, but I think your neg would be better due to the coil voltage drop...

[rgnprof]

Arthur,

Thanks for your help - I think I am basically doing what you already suggest. I am currently only having a vacuum retard - there is no vacuum advance at any point through my rpms. Right now my timing at idle (850-900 rpm) is at 4 degrees BTDC with retard vacuum - when I disconnect the retard hose at the dizzy vacuum box, timing jumps to approx. 15 degrees BTDC.

When you say "try 30 BTDC with no vac", do you mean with the car running at 3000 rpm? If so, that's pretty much where I'm at right now and I am back to getting the off idle stumble, just can't get rid of this inital acceleration hesitation! I do not have problems with the idle, my problem has always been the off idle stumble and occasional carb backfire.

FWIW, I have Bosch Distrbutor 0 231 142 005, and at 3000 rpm - with vacuum connected - I am supposed to get 42-50 degrees advance. I am not getting this much, never have.

Again, thanks for your suggestions.

Ryan

[Arthur Dalton]

Put a vac gauge on engine and take a reading .. Adjust carbs for best run/vac and then start capping vac lines close to engine .. watch gage for difference , as I suspect vac leaks or lean carb settings..
As I posted way back, I always start by disconnecting and plugging booster, just to eliminate that source of possible internal leaks there...
What ballast resiostor did you use with the crane and which coil did you get???
.. I would also check my valve clearances..specially intakes...

[230/8]

FWIW, I suspect you may be pleasantly surprised, and maybe more satisifed, with your engine's performance if you disconnect the vacuum retard completely rather than to leave it connected so it retards the spark all the time. No vacuum to the distributor is better than retarding it all the time. Your engine needs some vacuum advance at road speeds in order for it to get reasonably good fuel economy and have good driveability. The retard feature is/was necessary for these engines to meet emissions specifications at idle speeds. That is why there is a line to retard the advance at idle and another line to the opposite side of the vacuum cannister to advance it at higher speeds.

I believe you are right to set the basic timing w/o any vacuum connected. Most cars are set this way, then the vacuum lines are attached to allow the vacuum advance mechanism to make its adjustments while the engine runs at various speeds.

Again, FWIW, my 69 has a "ported" vacuum connection on the front carb. When I run a line to the vacuum cannister from this port it gives me the vacuum advance I need at road speeds, but does not advance the spark at idle because there is no vacuum at this port at idle.

The rear carb has a similar vacuum connection which is used with the euro-style dashpot on the throttle linkage to adjust the curb idle speed when the auto trans is in gear. That way it doesn't stumble and die at idle.

230/8

[Arthur Dalton]

the reason for using the manifold retard is this allows the distributor to be turned enough to get a full mechanical advance and still be able to have a TDC at idle.. once off throttle , the mechanical advance over-rides the retard diaphragm to allow for advanced spark at higher rpms.
The correct way is to fix all valve and sensors, but to get around , either will work.. I do not believe his stumbe is timing related..

[230/8]

FWIW, every trouble-shooting guide I have ever checked has suggested that late ignition timing is a likely cause of an off-idle stumble, as well as burned points, lean idle mixture, vacuum leaks, and late valve timing. I'd suggest ruling all of them out, one by one. Usually works for me...but, I'd probably start with timing since that seems to be in the neighborhood of where this thread began way back when...

Good luck with this,

230/8

[Arthur Dalton]

Yes, any one could be the cause ..
and I personally think the car is lean, so I would look in that direction first , as the timing changes have not solved the problem.
Studder/stumble off idle and carb popping suggest fuel leanness/vac leaks. Vac leaks show up at low rpms much more than at higher ones.
Here is one of the early post that indicate vac leak or even a tight valve.
Which I have also mentioned.

<<>

.. but , one can check in any sequence one prefers and experience directs .
I have a few years on that note....

[rgnprof]

Thank you gentlemen for all of your replies. I also feel like I'm going around in circles sometimes (not being very experienced myself and learning a lot from this board). It seems like I get one thing adjusted properly and I end up with other problems and pretty soon I'm back to where I started.

So you know, I have checked most of the things listed - starting with the valves (which I recently adjusted) and the vacuum leak issue (which I can not find) and the fuel mixture/carbs (which is a big I don't know if I'm doing it right or not?). I keep getting back to the timing issue because I KNOW it's not operating properly.

I just appreciate the replies and the help offered and I have plenty to go on from here on out.

Ryan

[speedy72t]

has it ever worked right? when was the last time it worked and what changed? remember these carbs were designed with different gasoline in mind so sometiimes things just don't work right. the setup changes depending on auto or stick, maybe that's off. maybe it has the wrong size jets in it? I hate those old carbs myself. I'd rather fight with the mechanical injection that those carbs.

[dieseldiehard]

I just tuned here, a bit late but I see what the problem is, you need that signal from the ignition box under the battery to feed the RPM module. I am not sure which Crane ifgnition you have but will presume it is the XR700.
The XR700 doesn't have a tach output. If you use a CD (capacitive discharge) ignition it usually has a tach output and that can drive the RPM module. I did that on my 230.4 successfully with all the smog crap.
Since then I installed that engine in a '72 220, and am not using any of the emissions stuff that the '76 car had, so I have no module to worry about. The XR-700 is great but a real CD is powerful beyond belief and I highly recommend you get one someday (finding a place to put it can be a problem. A picture of my 220 will show how I solved that, if I can find it I'll post it (again).
If you need to keep the module and the switchover valves associated with it then I suppose buying a tach driver module would work (MSD makes one and Crane probably does too) - it interfaces the coil "points" (regardless of whether you use a optical or mechanical points) to a lower voltage capable of driving the tachometer inputs There may also be a way to use a simple dropping resistor to accomplish this, beteen the coil (+) and the module input pin. If you have the factory engine manual look for the wiring diagram, its under emissons control I believe.
Oh yes, I am using a Weber

[rgnprof]

Just thought I would provide a quick update - since I have received some great help from folks on this board...

I tried Arthur's suggsetion - leaving the retard connected and trying to set the timing at 3000 rpm first to ensure getting full mechanical advance, but that didn't work. On my distributor - at 3000 rpms I should be getting 42-50 degrees advance. If I run engine at 3000 and set timing at 42 degress, the retard vacuum will not bring the engine down anywhere near TDC. Basically the way I have been running it was right where Arthur suggested, but I just was not getting enough advance.

So, I have tried other suggestions from 230/8, Michael and Dan. I plugged the retard vacuum outlet on the vacuum box on the distributor and ran manifold vacuum directly from the front carb to the vacuum advance side of the box - set timing to 10 degrees BTDC and readjusted the idle down to 850 rpm (which was a lot easier to do for some reason - carbs seemed easier to adjust).

Results - Amazing! On the highway at 60-70 mph car runs better than ever and I got 290 miles to the last tank (20.4 mpg). By contrast, the best I have ever done with this car was MAYBE 14 mpg on the highway! I clearly have vacuum advance at cruising speeds and the car is not running labored and I am clearly not pushing on the accelerator as much. There is still a slight drag/stumble on initial acceleration (losing vacuum momentarily when I hit the gas) but I can live with it. I'm thinking that I might be able to retard the timing another 5 degrees or so to 5 degrees BTDC and it might help with this stumble.

Do I have anyhting else to be concerned with? I hear no detonation/pinging but I never expected performance to increase this much.

Again, thanks for the help.

Ryan

[230/8]

So pleased to read that you are making progress with this knotty little problem. Another suggestion...before retarding your spark, which seems contra-indicated from the symptom, I would suggest that you verify your timing. Did you set the timing at 10 degrees with the vacuum disconnected? If not, then you should have, just to ensure the vacuum advance does not influence the static timing. Once it is set w/o vacuum, reconnect the vacuum line to the advance side and check the timing again to see if it has moved. If so then your front carb carb is not like mine, it is a direct manifold vacuum source. If the timing did not change, then it is a "ported" source tied to the venturi, which is a good thing. That said, and all other things assumed to be in good order, I would begin looking at the idle fuel mixture and balance. Both of these items, if slightly off can affect your stumble.

Set your mixture on each carb at a baseline setting with the engine off...run the screws in carefuly until they stop and then back out equally about 1-1 1/2 turns so the engine will start and run. Then slowly turn each screw in or out, the same amount, but no more than 1/8 turn at a time, until you get your smoothest and fastest idle speed. Work evenly between the front and rear carbs, an 1/8 turn on one then the same on the other turning the screw in the same direction. When you are done they should both be almost at the same setting and balanced for mixture. I would guess that they will end up at approximately 1 1/2 turns since this seems to be a common goal of designers. Most cars seem to end up in this range. Maybe it is just coincidence.

Then you need to speed balance them which involves a special adjusting tool to measure air flow through the carbs. I don't recall whether you have the tool, but it is a special bonnet to fit over the carb top and also a Uni-Syn adaptor which fits the bonnet. You will adjust each carb at its bell-crank link to get the correct idle speed. You will probably need to readjust the dashpot on the rear carb to set the final in-gear idle speed if you have an automatic. Also, when you balance the carbs, this dashpot, if you have it, needs to be adjusted inward so it does not contact the linkage on the rear carb. It is then adjusted at the end when all your mixture and balance adjustements are completed. Finally, if you have not done so already, I would recommend a careful re-torquing of your intake manifold to the cylinder head to ensure it is even and tight. This will help to eliminate small vacuum leaks at this source. I probably should have suggested this earlier.

If any of this seems unclear, and if you don't have the factory manual, there is a surprisingly good description in the HAynes and Chilton manuals. They take a bit of study to understand and it helps to have the bonnet adaptor and Uni-syn and a willingness to do it and then do it again while you observe the affects of the small changes you are making.

Once done your Zeniths should be almost trouble free for you.

Good luck with this.

230/8

[Zeus]

Gentlemen - your insightful banter within this post almost, but not quite, makes me miss my trusty old 1971 250 with its dual Zeniths.

Ah, the afternoons spent twiddling and cleaning those two beasts...then trying to get the idle right...then finding a vacuum tube was loose the whole time.

Ok, I don't really miss the 250...

Good luck!