Got to set the stage, so is a long one:

I have been chasing an misfire at idle on rear three cylinders (got duels) for as long as I have had the car, and have not been able to figure what is the cause. Lately it had become a bit more pronounced where engine has not seemed as perky.

In the last few days I rolled on 450 miles at freeway speeds 3000 to 3500 rpm with two round trips to the same place, on same road. On first return trip A/F gage began to sit on third lit “Ideal Mixture indicator lightâ€￾ closest to rich zone something that never has happened, and always read first of three to second of three rich indicators burning, and would respond to enrichment under WOT, or closed throttle coasting downhill. This trip no response by gage in rich direction only to more lean, until at idle it would slide off the lean side of scale.

I went through the drill looking for vacuum leaks first at the vacuum secondary articulating diaphragm for a rip, and loose carb to manifold matting previous problem areas, and knowing that to be lean during main jet condition and enrichment there had to be a massive sucking sound somewhere letting air into the intake.

Last step was to check torque on manifold bolts something I have not done since installing the Good Gasket 8 to 10,000 miles ago. The nuts had loosened to much less than 10 lb ft torque listed in FSM which I had installed them at. This engine equipped with Clifford headers & intake have not always played well with Felpro gasket hence the good gasket. Finding loose fitting manifolds was an ah-ha moment; retorqued nuts to 10 lb-ft, drove the second round trip a day later.

A/F gage was now responding more like it had in the past. Again on return trip A/F gage started to get lazy hanging at the ideal/rich reading (green-yellow), and where this leg of the trip was during a warmish afternoon temperature gage hung right of its normal of 12 o’clock now pointing to right vertical leg of “Mâ€￾ on gage.

At home, pulled into garage, shut her down, engine had a bit of halfhearted spin back, and then some gurgling / percolating noise from radiator and overflow tank. Let engine cool with hood open, checked coolant finding it out of sight from fin tubes… OH-oh; that’s the warmer than normal question answered, and it took 64 oz to bring back up to correct level.

I’m not one to check coolant often, as it has always stayed about the same year after year, and was ok about 1400 miles ago when first out of winter storage. No trace of leaking coolant, or oil level change so it must be burning it somehow.

Plug check showed light grayish/tan (Mrs wjajr would call it tope) on 1, 2, 4, 5, and darker brown on 3 & 6. Also found the rubber insert missing from my plug socket on #2 cooked to a "nice and hard". LOL

[img]http://i294.photobucket.com/albums/mm87/wjajr/Dart%20Engine/036_zps6cc2itpd.jpg[/img]

I can’t smell that tell-tale sweet antifreeze sent from either of exhaust pipes, nor have I seen any white vapor billowing out back.

Today I retorqued manifold nuts to 11 lb-ft, and some of the intermittent miss lessened but still there. Got engine to idle down to about 500 rpm in gear and was able to tell when #4 plug wire was pulled it was not contributing to the miss. Repeatedly pulling #5 & 6 yielded about the same results to exhaust cadence leading to those two being the problem area. When hand is placed over end of exhaust pipe it is pushed out from its lip, and then sucked back to lip during a miss. From front three there is a steady outward push of exhaust flow, no hand suck-back.

Grounding plug wire’s 5 & 6 produced a bright blue/white spark with regular frequency, so I don’t think there is an electrical problem unless the plugs themselves are junk. Other plugs have also given same miss, ruling out bad plugs.

This leaves some kind of combustion problem, or sticking valve problem, or burnt valve problem I should think.

Last compression test with same miss produced across board 163 psi with five turns of warmed up engine, and carburetor throttle plate held wide open. Cam has a lot of overlap, which I have limited by setting lash based on compression readings between 0.022â€￾ & 0.028â€￾. I did not pay much attention to intake & exhaust lash settings; just worked on one cylinder at a time until at 163 psi.

I wonder if additional futzing around, mindful of intake valve opening and closing of cylinders #5 & 6, would eliminate some of the miss. I’m not clear on which event is more critical, intake, or exhaust. In other words do I tend to more intake open time, and less exhaust open time to keep my 163 psi which seems the way to go, or the other way around?

Any thoughts welcomed

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

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Flyin' Brian

74 Duster, 225, rear-mounted blow through turbo at 12psi boost, street/strip car
"Sixy Beast"

I should add I haven't driven the car since tightening manifold to 11 lb ft.

After a little driving, hopefully with better sealing of intake, the plugs will be a bit more even in color. Engine is much more willing to idle down nicely than when at 10 lb ft setting indicating some vacuum leaking has been abated.

Someone that repairs cars for a living, a fellow Mopar nut, suggested using propane to see if idle is effected in anyway.
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Several minutes have passed since crafting last paragraph in which a short test drive took place.

Back to regularly scheduled programing:

Vacuum gage has settled a bit, still wagging zero to 5" at 600 rpm in gear, settles down by 900 rpm to a steady 10", and pulls 15 inches past 1600 rpm to 3000 rpm. Unable to test over 3000 rpm except in second gear where she gets to around 17" into the low 4000 rpm range. Still has a slight lope at in gear idle, lope smoothed out by 1000 rpm except that said miss.

I'll wait a while for some cooling to take place before pulling any plugs.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

I think you can torque exhaust manifold nuts much tighter than 10 with Dutra duals. That enemic 10 lb limit is to avoid cracking loooong stock exhaust manifold. I bet mine are torqued to at least 25 with no problems.

Sam

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Sam: Equipped with Clifford headers & short 4v intake, but point well taken.

Dissimilar metals coefficient of thermal linear expansion codgertating:


Aluminum intake will move twice as much as pair of steel headers, but where front and rear headers are divorced from each other they are able to expand in two directions. Where one piece intake is locked down at center and it expands equally in two directions. What I'm trying to say is ends of intake will move twice the distance or more in relation to header.

Now the question becomes is it better to more tightly clamp intake to some yet to be determined number, or slowly crank up the torque a foot pound at a time until some theoretical sweet spot is found.

Disclaimer; sometimes I wish I never studied this kind of stuff above long before cats were strapped onto exhaust systems, it just complicates what used to be uncomplicated from lack of knowledge. Dose this make sense to anyone.

Plug color up-date:

After a short rather low speed test drive yesterday, posted speed limit at times as high as 45 mph, plugs look to have slightly evened over the six; or it may just be wishful subjective thinking.

One observation of plugs made yesterday, but left out of description is that the darker plugs defiantly showed one side of center electrode to be lighter than 180* round to the other.

Another observation today is there were very small black specks sparsely deposited within valley's of ridges ringing ceramic end of plug which are covered by wire boot with no direct tracking between electrode plug wire attaches to, and metal base.

Swabbing of inner boot surface with carb cleaner soaked Q-Tip came out black same color of boot. Can't tell if this is just dead boot material, or makings of carbon tracking. Previously I had applied a light coat of dielectric grease to porcelain insulator to aid in boot removal, and to reduce tracking. New blue Magnacore 7.5mm or 8mm ( can't recall which) wires in use for last 12,000 miles or so.

Any rate there is an A/F mixture imbalance across all six cylinders which may be a result of differing quantities of air (vacuum leak) entering at mating of intake causing local lean condition.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

The only relevant question is whether an aluminum intake manifold's restrained attempt to move will crack it.

Railroad rail's thermal expansion is all forced into vertical movement by rgidly clamping the base to the cross ties. The old cast iron exhaust manifolds cracked if clamped too firmly. Forcing their thermal expansion in one direction exceeded the elastic limits of the material.

I think it is safe to crank down the bolts of your header/manifold combo.

Sam

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Not an expert, but running lean (intake leak) and intermittent sporadic miss always makes me think of a sticking or burnt exhaust valve. Has anyone ever tried to look at exhaust valves in a slant using a good quality borescope? Uneven color on the valve head would be the give-away. My thoughts.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)