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Intermittent Misfire

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Griz

Griz

Jedi Hopeful
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My BJ7 has developed an intermittent misfire while cruising. It is most notable in 3rd and 4th gear, but somewhat evident in 2nd gear too. It really doesn’t show up until the car is fully warmed up. There is never any misfire while the engine is under load or accelerating through the gears. I have conventional points and condenser ignition. Plugs, wires, and coil are new within the last 500 miles. Any ideas for a “fix”?

Griz
 
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Re: Misfire

Ideas - no particular order:
--condenser
--broken ground jumper wire between movable and fixed distributor plates. Sometimes they break internally, causing intermittents.
--loose or internally broken primary wire - try jiggling it with the engine running.
--try disconnecting black/white wire from battery switch to distributor.
--float level. I had an Alfa that would 'hunt' while cruising on the freeway - a Weber specialist told me incorrect float level could do that.
 
Re: Misfire

Thanks for the ideas Steve, but I still don’t understand why the misfire never happens under acceleration.
 
Re: Misfire

Another idea - it's running lean at cruise, maybe due to throttle shaft leakage. When accelerating, the piston damper richens the system, getting rid of the miss. Would you get the miss going up a long grade at a moderate speed in an intermediate gear?

Did you try richening the carbs overall?

Is the idle irregular?

Condensers have been a problem all along. I like the yellow-wire condensers sold by Distributor Doctor, which are supposedly made to original Lucas spec.
 
Re: Misfire

What type spark plugs are u Using???
 
Re: Misfire

Hey Steve,
as you know I have been using a 1-2-3 distributor. I am having Jeff rebuild my original Lucas unit. I may switch back to original. Which condenser do you use from Distributor Doctor?

His site says: applications.
407044 fits DX/DY/DZ & DVX variants & has the bracket in the centre.
400308 fits DK4A/DK6A with the bracket at the end.
484249 fits DJ4/DJ6 /DJ4A/DJ6A.This has the long tail bracket.

Thanks,
Lin
 
Re: Misfire

Does not miss when pulling a hill, just while cruising at steady speed. One thing I have noticed since the misfire started is that the exhaust really snaps and pops now when I downshift. Used to get just a nice burbling sound under compression. I’ll have to check but I can’t recall what plugs are installed.
 
When you're cruising at steady state and it starts misfiring, try pulling the choke a small amount (enough to drop the jets a bit) - that should tell you if it's caused by a lean condition.

Dave
 
I’ll give that a try Dave, but the car is not running hot as it would if it was lean. And to answer Keoke’s question I’m running Autolite AP 65 plugs.
 
A leak in the gasket sealing the exhaust down pipe to the exhaust manifold (usually due to a loose nut) allows air in to ignite unburnt exhaust mixture causing pops rather than burbles.
 
Tim, they were a little loose. I’ll take a drive later in the evening after it cools a bit (it’s like 95 here today) to see if the exhaust pop remains.
 
When the car is accelerating the centrifugal advance causes the plate within the distributor to move, and this could be pointing to the problem: a bad wire or connection in the points set.

OTOH my experience is that leaning out the mixture causes more "backing down" or snap/crackle/pop sounds, which is not necessarily a sign of the carbs being set too-lean, just leaner than otherwise, and reading the plugs after a hot shut-down should tell the tale on that.

If it was me I'd start with the ignition.
 
Look for a spark leak. In a dark but ventilated area run the engine at cruise speed and look for arcing on distributor cap or secondary wires.
 
You did not add distributor cap or rotor to list of new parts. Check the cap for white lines on the inside as cracks. you may not know if you are getting full power from carbs as you accelerate. This is due to linkage The capacitor could be the problem as they are cheep and are used to short out spark between plug wires as not to burn points. Also the post about the flyweights under the points mounting plate could add to the problem. Moss sent out some junk rotors what color is yours. What does it do if going down hill long grade if popping when you back off popping says ( tinning is off ) My TR 6 lives at 4,500 feet and when I go down to sea level it is to lean and will go flat at high speed I add a 1/2 turn to carbs richer but that not your problem but tells you what lean does. Madflyer
 
Ok you guys have given me several things to check out. Most will have to wait until tomorrow, but I can report that pulling the choke out slightly while driving did nothing for the misfire, and there are no telltale arcs under the hood in the dark. Distributor rotor and cap are about 3 yrs old. Rotor color is rust red.
 
Check your vac advance canister. If it's leaking, it would give no additional advance while cruising. That might cause a misfire.
 
See my post on 'Problem Solved', earlier this week. Check the vacuum lines to the brake servo and the dizzy. I had a crack in mine that caused the same problem.

AJ
 
I also had an intermittent misfire issue that only appeared when the engine was fully warmed up. Turns out it was the ignition coil. Seems that an aging coil will not function properly when it gets hot. Swapped out the coil with a new one and the misfire disappeared. Not saying it's your problem, but if you have another coil handy, it might be worth the 15 minutes or so it will take to try it out.
 
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