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4.2 with BBD stumbling and stalling when driving

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Well, it seems this vehicle of mine likes to missbehave in spurts...No sooner were the intake and springs taken care of, now I better figure out something it has been doing for a little while now.


Here goes: I put a '77 dodge truck BBD on the Jeep to replace a faulty Weber 32/36 (it was having secondary issues and was also the victim of some shop mischief). It is a manual choke, and takes just a moment to get it started in the morning. It idles like a champ, but the problem comes up that until it warms up, performance is terrible. When I press down on the accelerator, it acts as though the accelerator pump is not keeping up, or the intermediate circuit is fouIed. Even revving up from a stop sign might not keep it from stalling out in the middle of the instersection (I have also found that shifting from second to third, I had to shift back again, because the engine would sputter and start to die on a straightaway...).


I already fixed the pump linkage once (it was not the best rebuild in my opinion) and the intermediate circuit looks clear. Once it warms up, there seems to be far less of a problem to it. I have had it do the accelerator stumble when warm, though.  I am scratching my head here, as I love the Super Six BBD that I have, it is a reliable carburetor when it is built to spec...

Also am wondering if it is a problem I am overlooking, like timing advance. She is set at 9 deg. BTDC (Although I have reason to believe that the timing chain is stretched...I recently had to reset the timing back down from 19BTDC) and have already done a nutter bypass on the engine and have an older style distributor on hand (to make up for the advance issues).


Thanks again for any help.



Pete H.

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Pete, check the accelerator pump setting on the BBD. If you need adjustment specs for the AMC 258 versus the 225 Dodge applications, let me know.  If the accelerator pump is not the culprit, I would check the vacuum advance on the distributor.


Set the spark base timing with the vacuum advance hose disconnected and taped to prevent a vacuum leak during adjustments.  I'd set base timing to 6-8 degrees BTDC.  After setting base timing, reinstall the vacuum hose.  Check the spark advance with a timing light.  If the vacuum advance canister is connected to a ported (not manifold) vacuum source as required, you should not see a bump in spark timing until you open the throttle slightly.  That's the maximum vacuum advance stage for ported vacuum advance. 


As the throttle opens, the rpm increase will kick in the centrifugal advance.  Centrifugal advance should climb steadily to the maximum spark advance timing point.  The vacuum advance will drop off as you open the throttle wider...Do not lean over the cooling fan when checking the timing with the engine running.  It is very dangerous to rev the engine without air movement through the radiator.  Fans pull forward under these conditions, even breaking loose if parts are defective or hardware loose.


Check the spark timing.  Also look down the carburetor of the engine when cold and shut off.  Blipping the throttle should have fuel spurting into the carburetor throats—the correct accelerator pump action...


Update with what you find!—Moses

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Well, time for the update, and I am now even more puzzled as to what it could be. The accelerator pump gives two good shots, but only after the carburetor is warmed up. As soon as the carburetor/engine heats up, the problem all but disappears.... It makes me wonder if I should pull the Carb apart and check all of the specs on it (Throttle shaft bushings, etc.).  Is there a spec sheet or manual out there on that particular BBD? In a worst case scenario, where the rebuild is of a poor quality, should I try to rebuild it myself? (If I were to do that however, I think I would be buying a spare carburetor, and having one freshly rebuilt on hand. I am not one for worst case scenarios, but I do know that not all rebuilds are equal.)   

Wanting to be extra careful in diagnosis, I set the timing to 8 Deg. BTDC, as directed, and noticed my advance curve is not as strong as it ought to be. I know when I cut out the computer, that it affected timing curve, but would it limit me to just a few degrees of advance (like a limp home mode almost)? Should I probably install the older style pre-computer distributor in it to remedy this, or is it indicative of a problem elsewhere?


Oh, a brief list of what i went over in my engine bay today :

All cylinders show no less than 155lbs. and no more than 165 lbs. compression (above what I have read they are supposed to be, but in range of one another)

Motor oil is Castrol 20w50 (heavy, but I have found it works well at controlling lower rod bearing noise and keeps oil pressure in the 15 psi range when hot and idling [my Jeep had a tendency to dip to almost 0 when at idle with lighter weights])

All plugs are new, gaps are .035

Wires are new

Coil is new

Distributor is original (yikes)

Timing is now 8* BTDC;

Computer input removed via Nutter Bypass
Being extra careful on the timing, because the fan clutch has bought the farm.

All vacuum lines sound and attached to proper source/destination

Accelerator pump in carb gives good shots when warm

Idle set to 500rpm, high idle set to 1100 rpm



I hope this info helps, as I am a little overtired.



Pete H.

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Pete...Check for air leaks, engine idling cold, at the base of the carburetor and throttle shaft ends.  Take a light penetrant like WD-40 and spray gently around the base and shaft.  Also do the same at the intake manifold to head joint*, there may be a loose manifold bolt(s) or gasket gap.  Sounds like a lean condition until things expand—like the gaskets and manifold joint...If rpm picks up with the spray, you have a leak. 


Also do a fuel pump volume check...I'd do this on the carburetor side of the fuel filter and expect 16 oz. of fuel in 30 seconds at a curb idle.  If there is less fuel than this with the engine cold, but you get this kind of volume warm, I would suspect the tank ventilation and EVAP system.  Anything from a loose gas cap to hoses and canister out of whack can cause a low fuel supply.  You know already that the EVAP has been tampered with, consider improper ventilation a possible source for low fuel supply.  If you do get a low reading, try running a unique fuel supply source*, separate from the tank system, and see if volume improves.




Also, it might be useful to not use the OEM/ECU ignition retard on the system.  You want a straight distributor to ignition module setup, without computer functions.  This distributor does have a distinct centrifugal advance that will be apparent when the engine speed increases—if not countered by the computer/ECU. 


In my Jeep Owner's Bible, I provide details on the use of a Chrysler 5-pin module with this distributor, running the ignition 'key on' feed to the module and bypassing the ECU altogether.  Also, I run the distributor vacuum advance hose (ported vacuum source) through the CTO only, without impeding vacuum beyond this temperature control device...Efficient, failsafe and reliable, you get actual vacuum and centrifugal advance from the Motorcraft distributor:  centrifugal at all times and the vacuum advance after coolant warm-up.


As for the low oil pressure at idle, check the true pressure with a mechanical gauge.  Your AMC/Jeep engine era is notorious for the oil pressure sender getting clogged—very small orifice.  Cleaning or replacing the sender often "raises" the oil pressure reading.  If your normal cranking compression is any indication, this engine does not sound like a candidate for low oil pressure unless you actually hear bearing noise from the lower end of the engine, typically during start-up or a hot idle tip-in of the throttle.


I have distributor spark advance timing specs for the Motorcraft distributor and also the bench adjustments for the 225 Dodge truck version of the BBD carburetor.  Let me know if you need either...Moses

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Thanks for the information, Moses! I figured I might as well start the update with the oil weight subject. I have a noticeable lower end rod bearing rattle when I would cruise past the 1750 RPM mark. I deduced that since my compression was so high, plus the fact that I have replaced lower rod bearings on a friends jeep before, that the engine is sound and that the best idea would be to go up in oil weight until such time as I can replace those pesky lower rod bearings. The 20w50 performs admirably, and even though it fluctuates from idle to running pressure (fairly normal, yes? the oil pump circulates more oil in response to rpm?), I think that the lighter weight oil just did not have good enough viscosity. 

Now back to the BBD... I did something odd when I was driving it around cold yesterday, trying to figure it out.* It started to bog down on a stretch of fairly level ground - I was in fourth gear doing forty miles per hour- 1700 rpm and dropping (no revving, more like the reverse, the engine got very quiet, then sputtered) -- So I reached down quickly and pulled the manual choke out to 1/4 to 1/3, and EVERYTHING improved! the stumbling disappeared, The acceleration picked up, and it acted as if it was meant to be there. (I did not drive it too far like that.) Is this a helpful clue? I am really not a fan of this particular rebuild Carb. Mostly because when I asked the fellow helping me out on the Jeep (he purchased the carb for me as a gift), he said it had no rebuild tag, just the paper sticker, that the box was beat up, and that he had to connect some parts before he put it on for me...

* I don't know what made me think to try it, it was more of an, "oh, heck no, we are not doing this today!" type of experience

By the way, I am having some vacuum advance woes, since I have already disconnected the ECU. (Oh, as a heads up, if you have to deal with any parts stores, the OEM ICM has a different pigtail connector than the one that is sold at most parts stores now. Found it out because the ICM and ECU were both bad.) I already have an older style pre-computer distributor that I had bought a while back. I am also going to page though the steps about your chrysler 5 pin conversion setup.... I would keep my stock distributor?  Hopefully, that will solve another part of the puzzle.


Thank you again for all of your information!


Pete H.

P.S. I could probably put that BBD adjustment information through its paces around here.... Thank you, kindly!

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Agreed on the oil, Pete, you're in bearing survival mode from what you describe, although the rattle should also be at low speed throttle tip-in. Separate detonation/ping from lower end rattle.  The cranking compression actually sounded high, possibly carbon buildup on the pistons and combustion chambers.  I like to use a cylinder leakdown test to get a real sense for wear.  Cranking compression can be deceiving with cylinder taper; a leakdown is performed at TDC for each piston, the point with maximum cylinder taper/wear...Stick with the 20W-50, for sure, as long as you are not in a cold weather zone with starting issues.


Your manual choke test on the BBD was very telling.  The carburetor is either jetted lean or not flowing enough fuel for the incoming air.  (There is still the possibility of an air leak, which would create a lean factor.  Check for manifold and carburetor base leaks as I suggested, it will only take a few minutes.)  Your attempt to change running condition with the choke is very revealing and helpful.  Most engines do not have a manual choke provision, so that test would be impossible on the road unless you close the automatic choke down then road test.  There is a definite lean condition, eliminate possible causes as I outlined in earlier replies...


The five-pin Chrysler module is very slick.  You are simply converting your Motorcraft distributor to a real time ignition, eliminating the interference of the ECU and Ford-type Motorcraft ignition module.  The module I describe is early Chrysler electronic distributor and should be available through NAPA/Echlin still.  I actually learned this trick in the late '80s from Jacobs Electronics.  They discovered that the Motorcraft distributor can readily work with the five-pin Chrysler module and the correct wiring hook-up.  You then have true centrifugal and vacuum advance from your stock 4.2L Motorcraft distributor...Vacuum without interference requires following my earlier suggestion about a simple routing of vacuum from the carburetor ported source, through the CTO, to the distributor vacuum canister.  (If you don't care about vacuum advance elimination with a cold engine, you can route a hose directly from the carburetor ported source to the distributor vacuum canister.)  The Motorcraft distributor should work well with its stock centrifugal advance and vacuum spark timing—without interference from the ECU.


I've scanned and attached the PDF of a 1977 Dodge 225 inline six BBD carburetor bench adjustment list.  Since your carburetor has a manual choke, most information is not relevant.  Pay attention to the float level and pump setting.  If your carburetor is truly '77 in origins, it will not have a bowl vent that is adjustable...There's little left beyond fast idle settings...Check the float height and pump setting.  Make sure the needle is not sticking in the seat.


The wild card, Pete, is whether your Brand-X "rebuilt" carburetor has the right body parts and jetting for that application.  Jet sizing is not included in service information from that emissions era...I trust this limited information helps.  Based upon the manual choke test discovery, you may be bucking a low fuel supply or a low float setting in the carburetor—or air leaks that would lean out the mixture...Moses


BBD Carburetor Settings.pdf





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