Slant Six Forum
https://www.slantsix.org/forum/

Suck back at tailpipe on #’s 4, 5, & 6.
https://www.slantsix.org/forum/viewtopic.php?t=61344		 Page 1 of 1
Author:  wjajr [ Thu Jul 06, 2017 6:50 am ]
Post subject:  Suck back at tailpipe on #’s 4, 5, & 6.
I have tried to determine which exhaust valve of rear three is sticking open, first by pulling plug wire, second by color of plug, and lastly by temperature of header pipe via hand held device pointed 2 inches along header tube from its head connection. Engine was up to operating temperature, and a few minutes of running at test RPM allowed before measurement was made. Note: engine has duel exhaust, front three exhibit no suck-back or interrupted beat at any rpm.

I took readings at four different RPM levels starting at cylinder 1 to 6 with no load on engine while in park:
800 rpm; 360 - 485 – 440 – 470 – 400 – 365 = idle rpm in gear.
1000 rpm; 448 – 555 – 502 – 524 – 470 – 430 = idle rpm in park.
2000 rpm; 400 – 520 – 472 – 630 – 590 – 600 = about 40 mph.
2500 rpm; 390 – 510 – 430 – 560 – 570 – 590 = about 50 mph.

These rpm levels represent the range engine see during normal use.
At 800 rpm tailpipe suck back is more pronounced or it appears to be when hand is placed at end of pipe more or less a constant event . 1000 rpm is the same as at 800. 2000 rpm it still causes hand to flutter with an occasional slight pop-pop sound indicating one or more of exhaust valves is not fully seating, but maybe less valve hang up events take place or they are too fast it feel. 2500 rpm about the same as 2500.

My question is would higher temperature indicate cylinder with sticking valve due to lean out mixture?
Would cooler temperature be due to misfire?
What would be normal exhaust temperature readings be generated by a properly running engine?
Compression is 9.5:1, cylinder pressures are all about 163 psi measured after five rollovers, plugs removed, carburetor wide open on warm engine, lash adjusted to achieve even cylinder pressures ranging between 0.023â€￾ to 0.029â€￾ non stock cam.

Now on second tank of fuel with Marvel Mystery oil added in hopes snake oil will save the day.
Author:  slantzilla [ Thu Jul 06, 2017 9:59 am ]
Post subject: 
A misfire can give a higher temp due to fuel igniting in the pipe.

Do a compression test and see what you have. :D
Author:  wjajr [ Fri Jul 07, 2017 1:50 pm ]
Post subject: 
Compression test accomplished:

First compression sequence of five, cylinders 1 through 6:
110 – 110 – 120 – 110 – 105 – 110
Second: 140 – 130 – 150 – 140 – 140 – 130
Third: 150 – 140 – 155 – 160 – 150 – 140
Forth: 155 – 155 – 160 – 155 - 155 – 150
Fifth: 160 – 160 – 165 – 170 – 160 - 150


#4 is a little high @ 170 psi.

#6 was slow pumping up and about 10 psi lower than average pressure of the six at 160.8 psi.

To drop # 4 back to ~160psi range should its exhaust valve lash be tightened, or intake loosened?

Number six may be the offending sticking exhaust valve. To reiterate valve not always hangs open, three or four intake events can take place where exhaust valve fully closes, and sometimes based on cadence of suck-back it could be two cylinders hanging up a valve… Hard to tell.

When suck back event occurs, engine rocks on mounts, then when valve closes a few times in a row as intended engine hardly moves on mounts; very smooth.

Next step I suspect is to revisit the lash once above method of lowering and raising pressures within two suspect cylinders is confirmed.
Author:  69val6 [ Fri Jul 07, 2017 4:31 pm ]
Post subject: 
Hi, what kind of ignition system do you run? Can you give me a rundown on your ignition parts? What's your ignition timing set at?

Next, if your exhaust valves are sticking, you would have bent one already and have low compression. I would be more suspect of weak valve springs. It's pretty easy to pull the head and have it checked at a machine shop. If you've had it rebuilt recently, it's not likely to be the case.

Set your valve lash with the engine idling. It's much more accurate than doing it shut off. You can also check to make sure that cam lobes are opening properly(not worn down). If your cam lobes are worn it won't run right either. Just make sure all your intake valves and all you exhaust valve have the same amount of lift.

I'm leaning more toward an ignition issue based off you compression tests. Also, if you still run a carb, the slants have pretty poor fuel distrubution. Cyl's #1 and 6 tend to run lean while #2 and 5 run rich. #3 and 4 will be in between. You should see this in the color of the plugs.
Author:  wjajr [ Sat Jul 08, 2017 5:42 am ]
Post subject: 
69VAL6:

Orange Box, Magnecore 8 wires not running parallel to nagate any induced voltage, NGK UR5 gaped to 0.035â€￾, Blaster 2 coil, #’s 1 & 6 a wee bit darker than other four deep tan plugs. Base timing advanced to 12 or14* (can't find current number in my notes), distributor recurved to limit mechanical advance to zero from base to 1400 rpm, and is all in at 2800 rpm. This engine likes 1000 to 1200 rpm idle in park, lopes at 800 rpm in gear rolling up and down between 700 & 900 rpm. I think the vacuum signal to Holley 8009 is constantly changing at idle due to unstable low vacuum; 3â€￾ Hg to 6â€￾ Hg. You can read volumes on this topic in my old posts from around 2008 to 20010 where I tried to get this Holley to play nice with engine, or the other way around.

Head was rebuilt 2009 oversized valves (1.70â€￾/ 1.44â€￾ Engbuild), three angle seat, ported, bowl blended, gasket matched, at 18,000 mi, now at 39,500 miles. Engine always had a lope, and at the time I was too green to detect suck-back or suspect anything other than unknown cam with overlap. I installed the valves, measured all spring’s height, but had no way to measure compressive force, and probably should have shimmed several of them or replaces all. I now suspect as you that probably I have a few weak springs. I think springs installed are red Mopar P5249847 single/damper similar to what would be found on a 340 v8. Side note: This car saw four seasons of drag racing at English town, NJ back in mid 1990’s, conversation with racer /owner said he blew up five slants before figuring out how to make one stay together; comment was he shifted at 6300 rpm. If these are the same springs, or have just a few seasons of racing it’s a good chance they have had it.

I haven’t measured lift on all cam lobes, just #1’ I & E when I put a degree wheel to it to figure out the lift & duration etc. back around 2011. Several years ago(4000 mi) I adjusted lash to achieve even cylinder pressures of around 162 psi in all six, that smoothed out engine, improved idle quality and vacuum reading, and settled the wagging vacuum gage. I now know wagging vacuum gage is pointing to sticky or burnt valve, and or weak springs.

I always set lash with warned up running engine, and adjust to catch the same spot on all lift ramps of cam. One can hear and feel engine dial-in when lash hits that sweet spot. Generally it takes a few trips up and down the valve train to get them all just right. Doing this also settles and improves idle vacuum.

This carb can be tuned to idle fairly well, and tuned to give nice top end, but not do both. I have an o2 sensor & gage on dash to monitor air fuel mix, as this engine goes past 4500 rpm, it leans out, vacuum gets up to 17-18â€￾ under steady throttle opening, and starts to pull like a 318 at WOT all the way to 6000 rpm. Most of my driving is below 3000 rpm (60 mph), vacuum always floating between 10â€￾ and 16â€￾ Hg, and carb a bit on the rich side. I have the lightest spring installed so secondaries open as soon as engine can take it. No flat spot on their opening, just a rush of power with that 4V howl singing all the way to 6000 rpm.
Author:  Dart270 [ Sun Jul 09, 2017 3:34 am ]
Post subject: 
Loosen the lash on #6 valves by 0.004" and see if it goes away. If not, then open up #4 and #5 and see if it goes away. Is this the same cam that has unknown specs? I would relash everything too and see how that helps.

Lou
Author:  wjajr [ Sun Jul 09, 2017 5:36 am ]
Post subject: 
Dart 270:
Quote:
Is this the same cam that has unknown specs?
Yup, until about 2012 when I purchased a degree wheel to decode it.

Exhust:
@0.006" = 318*
@0.050" = 236*
Lift 0.324" @ valve = 0.460"

Intake:
@ 0.006" = 315*
@ 0.050" = 235*
Lift 0.321" @ valve = 0.444"

Over lap:
@ 0.006" = 101*
@ 0.050" = 23*

Center line:
210* / 2 = 105.5*

No clue as to unnamed manufacture's lash spec. Currently I & E anywhere between 0.023" to 0.028" at last lash to even out pressure to ~162 psi. There may have been some self adjustment over the last 5000 miles due to not so tight adjustment bolts, or some unknown wear. Haven't had a chance to address lash this week... Mrs wjajr is home on vacation if you catch my drift.

Perhaps Monday when I get the place to my self once again I can check lash and make advised adjustments sans useless sorties interrupting the day, and report results.

I had intended to install a new cam, lifters, and springs this spring along with any small compression change if needed, but IH 1200 B truck project has consumed all my black-ops resources and time.
Author:  69val6 [ Sun Jul 09, 2017 8:13 am ]
Post subject: 
You've got alot going on with this one. Here's my advise in this order:

1. try a different ignition control box. I've seen them cause some pretty unusual issues. Best to borrow one from someone that's known good.

2. The valve lash should be the same on all cylinders. If you have to vary the valve lash from cylinder to cylinder to make the cylinder presssure the same, you're; going to be chasing your tail. Remember on a solid lifter cam that the more valve lash you add to your engine, the milder you will make your cam specs. So more valve lash should = higher idle vacuum. Set the valve lash the same on all 6 cylinders. If you notice your valve lash changing on any of the cylinders from your last adjustment, your cam is probably worn out.

3. If you have a bent/sticking valve, it will show up in your compression test. If a valve is bent/sticking it will throw your compression readings off by more than 10%. A weak valve spring will cause major breaking up issues at higher RPM. Sounds like it runs OK at higher rpm's so I doubt that's your issue. Now, if the valve springs weren't shimmed properly from day one and you have very low seat pressure(less than 30#) it could cause your issue. If you had this issue, you would have low open pressure too and you would be breaking up at higher RPM.

4. If none of the above help any and it was mine, I would pull the head and have it gone though, then buy a new cam.

Hop this helps.

Joel
Author:  Jase [ Sun Jul 09, 2017 8:44 am ]
Post subject: 
It seems to me that a an occasional misfire at idle would be more ignition or carb related?

Your compression #'s look ok.

And a sticking valve or burned exhaust valve would be more consistent.

Could this be a bum spark plug?
Page 1 of 1 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/