Help needed for diagnosis - pictures inside

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duster360

duster360

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The engine is a 340, fresh build. Engine has a bad miss at idle. Look at the pictures and fire away with questions or answers. Click on the pictures to see larger images and more pictures. Click on the pictures to see larger images and more pictures. The number chart is from the compression test.
 
Your compression is fine! Not sure if the miss is coming from this plug, but I wonder if the intake gasket isn't leaking/pulling some oil in on that cylinder.....
 

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your comp is fine but the fouled plugs might be a valve seal gone bad leaking oil or underside of intake not sealed like mentioned above getting oil in that cylinder.
 
X 2 on oil fouled plugs. It appears that there may be a fuel distribution issue, too. What carb and intake manifold are you using? Are you running exhaust headers? What plug and heat range are you using?
 
I see 2 plugs that are questionable. Did they come out of number 4 and 7 cylinders? Also noticed oil in a couple of runners in the intake shots. Is that something to consider or were those pics taken before final assembly? Reason I bring that up is that one of the runners with oil in it is number 7. Last question. Were the heads and/or deck milled and if so how much? You may need to mill the intake side of the heads so the intake manifold will sit down far enough to seal and not suck oil out of the lifter valley...
 
Besides the above, make sure your plug wires are up to snuff? A easy check is to fire it up in the "pitch dark" and visualize the engine compartment to make sure there is no arcing going on.....especially near the headers.
 
A further inspection of the old intake gasket might reveal that it's the source of oil making it into the combustion chambers. Are those few plugs oily or are they a combination of sooty and wet? I'd say that it's fairly obvious that your miss is coming from those cylinders. - But maybe you've got a few ignition wires that aren't up to snuff and haven't been firing consistently.
 
My first impression was you have a couple of cylinders running rich. I'd start by checking the cap qnd rotor especially for those two fouled plugs if you know which cylinder they came out of. Then check those two wires also. Then I would listen to each cylinder to figure out which were missing. How is engine vacuum?
 
OldmanRick said:
Your compression is fine! Not sure if the miss is coming from this plug, but I wonder if the intake gasket isn't leaking/pulling some oil in on that cylinder.....
Click to expand...

I have been fighting intake sealing issues. Last time I pulled the intake all runners were dry on the heads and intake so I bought a set of Cometic aria I'd fiber gaskets close to the same thickness as the last set ( which were Felpro steel core ). I didn't replace the plugs though. Maybe I will pick up a new set of spark plugs and try again.
 
2 Darts said:
X 2 on oil fouled plugs. It appears that there may be a fuel distribution issue, too. What carb and intake manifold are you using? Are you running exhaust headers? What plug and heat range are you using?
Click to expand...

Carb is a Holley street avenger 670. Intake manifold is edelbrock airgap. I am using NGK plugs with heat range of 6.
 
grumpuscreature said:
I see 2 plugs that are questionable. Did they come out of number 4 and 7 cylinders? Also noticed oil in a couple of runners in the intake shots. Is that something to consider or were those pics taken before final assembly? Reason I bring that up is that one of the runners with oil in it is number 7. Last question. Were the heads and/or deck milled and if so how much? You may need to mill the intake side of the heads so the intake manifold will sit down far enough to seal and not suck oil out of the lifter valley...
Click to expand...

Dark plugs are from 6 & 7. It slipped my mind to replace the plugs before restarting. The intake picture you saw is an one from when I first built the engine. Last time I pulled the intake all runners were dry. To my knowledge neither have been milled.
 
magnumdart said:
My first impression was you have a couple of cylinders running rich. I'd start by checking the cap qnd rotor especially for those two fouled plugs if you know which cylinder they came out of. Then check those two wires also. Then I would listen to each cylinder to figure out which were missing. How is engine vacuum?
Click to expand...

Brand new distributor just installed along with cap, rotor, button and wires. I didn't check the vacuum. I do have a gauge. I am going to replace the plugs and try again and check vacuum when I do.
 
1. tell us about the distributor

vacuum advance yes/no?
stock springs / light springs ?

what do you have the advance set to?

generally you need low teens initial, all in by 4000 rpms, mid/high 30's total advance

they will run like crap at idle if too retarded. or too advanced.

2. vacuum: tell us the stuff you have hooked up.
is everything hooked up or if only some things hooked up do you have the other tits plugged off
do you have everything plugged to the carb in the correct place?
what carb is it?
 
duster360 said:
The engine is a 340, fresh build. Engine has a bad miss at idle. Look at the pictures and fire away with questions or answers.
Click to expand...

A little more info on the engine and ignition system.

Adjustable rockers? Are you running MSD?
 
moparlover said:
1. tell us about the distributor

vacuum advance yes/no?
stock springs / light springs ?

what do you have the advance set to?

generally you need low teens initial, all in by 4000 rpms, mid/high 30's total advance

they will run like crap at idle if too retarded. or too advanced.

2. vacuum: tell us the stuff you have hooked up.
is everything hooked up or if only some things hooked up do you have the other tits plugged off
do you have everything plugged to the carb in the correct place?
what carb is it?
Click to expand...

Mopar performance distributor, vacuum advance not hooked up yet. I pulled it out of the box and set it in the engine. Nothing hooked to vacuum on engine and all ports on intake and carb capped or plugged. Timing is 18 initial and 34 total. Carb is Holley street avenger 670.
 
Are your rockers adjusted properly? If you have one too tight, and a valve is hanging open slightly, that cylinder isn't going to fire properly.

What gap do you have on the plugs?

Just throwing it out there, something to check.
 
well my suggestion is to hook up the vacuum advance :D

it's there to help during idle and steady highway cruise.

it will have no effect on wide open throttle, it's inoperative then.

so use it, i'm betting it will help your idle out. if you have a really big cam and low vacuum they dont help so much however. they get weak with low vacuum so never actuate.

do you have a vacuum gauge? can you borrow one?
 
and like bruce said a plug or valve adjustment

i've seen a plug installed crooked cause problems too

plug wire sparkin on a header too. you have to check that one at night though
 
moparlover said:
well my suggestion is to hook up the vacuum advance :D

it's there to help during idle and steady highway cruise.

it will have no effect on wide open throttle, it's inoperative then.

so use it, i'm betting it will help your idle out. if you have a really big cam and low vacuum they dont help so much however. they get weak with low vacuum so never actuate.

do you have a vacuum gauge? can you borrow one?
Click to expand...

Car has not been out of the garage yet, I have vacuum advance unhooked at the moment to set timing. When I say it is idling rough, I mean it looks like it might break a motor mount it is shaking so much. I do have a vacuum gauge and will be using it after I put in a new set of plugs and possibly swap out the MSD box. The one in the car was used when I got it but has worked well. But I have a new one on the shelf I have had for several years, gues it is time to break it out.
The cam is a 268 voodoo. Should have a slight lope but nothing radical.
 
Bad Sport said:
Are your rockers adjusted properly? If you have one too tight, and a valve is hanging open slightly, that cylinder isn't going to fire properly.

What gap do you have on the plugs?

Just throwing it out there, something to check.
Click to expand...

I have probably been through the valves 3 or 4 times. Spark gap is at .035".
 
I will look into the gap settings. But I do know that won't cause it to idle erratically like it is. I had this same gap on my last engine and it ran fine.
 
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