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Low speed missfire
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Author:  slopez [ Tue Jul 28, 2015 7:58 pm ]
Post subject:  Low speed missfire
Hi all,
I recently rebuilt a 225 motor and have been unable to get it to run very well.
At idle the motor has a random misfire that is bad enough to shake the car a little, the misfire gets better with rpm and appears to disappear above 2500rpm when the carburetor main circuit is fully on.

Motor specs:
8.9:1 ish compression, Erson Rv15/295 RDP cam (the one with the specs posted on here), degreed and advanced about 2 degrees, new bearings, timing chain, pistons etc etc.
I did put the engine together myself. I checked No1 intake and exhaust events and they matched the cam card.

Carb:
Weber pretty much a slightly bigger version of the 32/36.
I have tried two of these as well as the carter off of a 318 that was running.

Ignition:
Hei, transformer coil, new leads, cap, rotor and wiring.
I have also tried mopar electronic and points.

I hooked a scope up to the 12v ignition feed and I can't see anything wrong, it looks stable at 14.5v and I can see the coil charge events and none of them seem to be missing. I can't check the HT lead timing directly unfortunately.

I have checked around the intake, carburetor and valve guides for leaks with propane. PCV and vacuum ports plugged to no avail.
Valve lash has been checked/adjusted about 4 times!!

To me (and the mechanic down the road) it seems like an intake leak, but for the life of me I cannot find one!
So what have I missed? Any ideas would be appreciated.
Thanks
Author:  WagonsRcool [ Tue Jul 28, 2015 9:01 pm ]
Post subject: 
First things first- have you checked engine compression on all cylinders? What are manifold vacuum readings at idle & at 1500? You might need to (carefully) unplug one cylinder at a time to see if it is cylinder specific.

If you added propane & had no improvement that would tend to rule out a vac leak (feeding propane directly down the carb will overcome most leaks, at least somewhat before it causes a rich misfire instead on other cylinders).

If you changed the ignition system that would make most ignition faults unlikely- (reversed pickup coil polarity will cause problems, but not so with points). You could have had a dist rotor to cap tower phasing problem, but again unlikely if you swapped to a points dist. Have you tested ignition coil output (kV potential?)- Most HEI coils will jump a 3/4"-1+" gap on a spark tester.

Silly question but I gotta ask it- are you sure you have the firing order correct?
Author:  slopez [ Wed Jul 29, 2015 1:44 pm ]
Post subject: 
Hi,
Engine compression warm is around 140 to 150psi across the cylinders. Vacuum at idle is ~15Hg, unsteady and jumps up to 0.5-1Hg, I can't see any pattern to it, I have compared it to vacuum gauge cheat sheets and it doesn't really match up with any of the pics or descriptions. I think vacuum at 1500rpm is about 18Hg, I will have to check that. Disabling any one cylinder at a time has no noticeable effect on how it runs.
If I feed propane directly into the carb the engine dies immediately, I was checking for leaks by blowing it around all the intake joins an listening for a stumble (actually its a mix of propane and butane (bbq gas over here)).
I havn't got a spark tester, I did bend a spark plug leg out to a couple of mm and the spark is definitely stronger than on any other system I have used.
Quote:
Silly question but I gotta ask it- are you sure you have the firing order correct?
Hehe yep firing order is good.
thanks
Author:  Dart270 [ Wed Jul 29, 2015 2:55 pm ]
Post subject: 
Seems like you have covered a lot of bases. So, you get the same conditions with a known good BBD carb and separately changing to a points ignition system (dist)? What is your lowest idle RPM? What is your ignition timing at idle (and at what RPM)? Do you have light weight advance springs in the distributor? That can cause erratic idle. Have you unplugged the vac advance at idle and capped the carb/manifold port?

Best,

Lou
Author:  SlantSixDan [ Wed Jul 29, 2015 4:17 pm ]
Post subject:  Re: Low speed missfire
Quote:
I recently rebuilt a 225 motor and have been unable to get it to run very well. At idle the motor has a random misfire that is bad enough to shake the car a little, the misfire gets better with rpm and appears to disappear above 2500rpm when the carburetor main circuit is fully on.
Your symptoms strongly remind me of a couple of different problems I've seen. Once with an intake manifold that had floor cracks so exhaust was getting into the intake tract (this could also easily be caused by a leaky/misconnected EGR valve or leaky EGR blockoff plate). You mention a couple of 2bbl carbs, so you probably started with a Super Six intake of one kind or another (iron? aluminum?); that means these possibilities are real for you.

Another time, I got a bum steer on camshaft selection and wound up with an engine that had an idle misfire that seemed for all the world like an ignition fault or vacuum leak, but pulled good and strong (really strong!) above about 2000 rpm. When I swapped in a milder cam, the idle misfire went away. So did a fair chunk of the above-2krpm pull. You might try slacking the valve lash off significantly (double it!) and see if your idle smooths out.
Author:  makapipi [ Wed Jul 29, 2015 11:07 pm ]
Post subject:  misfire at idle
another thing you could check is the spark plugs.
this may give more information if the fuel is too rich or
lean.
Author:  slopez [ Fri Jul 31, 2015 5:34 pm ]
Post subject: 
Hi all, thanks for the input so far.
Quote:
What are manifold vacuum readings at idle & at 1500?
idle 750rpm 16Hg, 1500rpm 20.5Hg
Quote:
another thing you could check is the spark plugs.
Spark plugs seem ok, at least none of them are covered in soot from being too rich.
Quote:
. So, you get the same conditions with a known good BBD carb and separately changing to a points ignition system (dist)?
I cannot remember if I had points ignition installed one of the times I put the carter on or not.
Minimum rpm is 420, I cannot go any lower without adjusting mixture.
Ignition timing is set to 10 degrees BTDC at 750rpm.
All vacuum ports have been blocked a various times with no effect.
The distributor is NOS one i got off of ebay, the initial spring is quite light and is all in by ~1250-1500rpm the heavy spring much higher, I have shortened the governor slot to reduce the throw a little.

Quote:
intake manifold that had floor cracks
I had wondered about something like this, I have extractors on so have access to the heat pocket, I have gassed it with propane and also blocked it off with some plastic and I couldn't detect any change in the engine. I do have a spare manifold I can try if all else fails.
Quote:
You might try slacking the valve lash off significantly (double it!) and see if your idle smooths out
The cam is not aggressive it is the larger of the two on here that Doc worked out with Erson. I don't have big enough feelers to quite double the lash, so I set the intake to 28 and the exhaust to 32. Increasing the intake lash increased rpm by 10 and increasing the exhaust lash increased rpm a further 5. The idle was definitely smoother, misfire was also definitely still there too unfortunately, the vacuum gauge was moving by ~0.5Hg instead of ~0.75Hg.

I took a video so I will try and put that up somewhere.

William
Author:  '67 Dart 270 [ Fri Jul 31, 2015 5:49 pm ]
Post subject:  misfire...
I had a random wire that was hidden under my number 2 plug wire and pinched by the spark plug tube. It was giving me fits with the misfire 'cause I couldn't find it. Also, check to see if one of your plug wires might not be snapping tightly into place on the plug, sometimes they loosen up and it's hard to find 'cause everything seems to be where it should be, yet there is still a misfire.

brian
Author:  slopez [ Fri Jul 31, 2015 5:53 pm ]
Post subject: 
Here's a video (hopefully)

https://youtu.be/4DcCgoKcUc4
Author:  '67 Dart 270 [ Fri Jul 31, 2015 5:57 pm ]
Post subject:  that does certainly sound like a misfire...
Did you do a spark test on all the plugs, change the plugs recently? Maybe one of the plugs is fouled or something.
b
Author:  SlantSixDan [ Fri Jul 31, 2015 9:03 pm ]
Post subject: 
Video reminds me of a situation I had in my '65 long ago. The distributor vacuum advance was receiving vacuum even at engine idle, and the resultant idle quality (lack of quality) was very similar to that experienced with a vacuum leak: a random skipping misfire because too much spark advance at idle, but it ran OK at higher speeds. Disconnecting the vacuum advance and plugging the carb port smoothed it right out until I could do the necessary parts swapping and adjustment to fix the problem.

Disconnect your vacuum advance and plug the carb port. Does anything improve?
Author:  slopez [ Fri Jul 31, 2015 9:41 pm ]
Post subject: 
Quote:
Disconnect your vacuum advance and plug the carb port. Does anything improve?
Unfortunately not, I disconnect and plug it when I do the valve adjustments.
Quote:
Did you do a spark test on all the plugs, change the plugs recently? Maybe one of the plugs is fouled or something
Havn't spark tested all of the cylinders, I have hooked the timing light up to all the leads and I cannot see any jumpiness.
None of the plugs are fouled and I have tried two complete sets of plugs.
Author:  ceej [ Fri Jul 31, 2015 11:12 pm ]
Post subject: 
Did you set the valves to Erson's specs? Loosen them up a couple thousandths.

CJ
Author:  slopez [ Sat Aug 01, 2015 1:28 am ]
Post subject: 
Quote:
Did you set the valves to Erson's specs? Loosen them up a couple thousandths.
Yep specs are intake 15 thou, exhaust 20 thou. I have been setting them to 16 and 22, I have also tried a bit looser as well.
Author:  wjajr [ Sat Aug 01, 2015 10:46 am ]
Post subject: 
I wonder if you have one or two sticky valves that are randomly slow to fully close for whatever reason.
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