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BRINGING IT ALL TOGETHER.... A's, G's & E's

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richie61

DanTheVanMan

ViewMaster

Scott

VANagain

9 posters

Straight Six mystery!!

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°1

Straight Six mystery!!

Post by VANagain Tue Jun 29, 2010 1:00 am

I hope one of you Chevy 6 cylinder owners can solve this frustrating mystery:

Driving at a very low speed, like turning a corner or starting out going up a hill, RPMs are low, then suddenly give it a lot of gas, and it coughs once (backfires, I guess), and from then on it runs like crap, like it's missing on one or two cylinders. No power. It will run this way until I drive it for several miles or shut it off for a while.

I've had three vans that have exhibited this same behavior. All had 250 engines. Two had manual tranny. My current one is a freshly rebuilt engine with electronic ignition! It did it yesterday while on a test drive.

Any ideas?! The only thing I can think of is the vacuum advance advances too much and causes a backfire, which makes a valve slam shut which collapses a lifter or two. (Can a lifter get stuck temporarily?) Then there's a cylinder or two that don't fire because the lifter(s) aren't opening their valve(s). But I am assuming my rebuilt engine has good lifters in it...

I'm hoping to hear from someone who had this same problem and found a nice, simple fix for it! Anyone?

Scott: Number of posts : 1651
Location : Anoka, MN
Age : 54
Registration date : 2008-05-20

Post n°2

Re: Straight Six mystery!!

Post by Scott Tue Jun 29, 2010 8:02 am

Don't know if would make all the difference, or any, but did you adjust your spark-plug gap for the HEI?

m1dadio wrote:I ,m with Donivan on this one. The 2nd gen HEI was dynamite and used many years. It is a good hot ignition system as it is , from what I can remember from my tune up days, I think they are stock around 43K volts, 8mm silicon jacket wires, and you will not likely need anything hotter unless your engine has higher compression (10.5:1 and up). The original points and condenser ignition was around 12K to 22K volts. The larger circle of the HEI cap is to make greater distance between the pins because of the higher voltage.

There is lots of aftermarket stuff available for making your HEI more HP. In my opinion most of it is BS, and even more of it made off shore is absolute crap. I have tuned thousands of vehicle in the HEI era using an oscilloscope, and although bigger coils and all made more power it was rarely ever needed. I have seen much cheep ass off shore parts reduce the advantages of the HEI system. Everything about the HEI ignition system is about as good as it gets for close to stock engines. The only thing I recommend for the HEI is a quality cap and rotor (with brass terminals). Fumes migrate up through the shaft of all distributors, and corrosive moisture in the "ionized, mostly ozone atmosphere inside the cap condenses on the terminals and the aluminum terminals just go away under the chemical and electrical loads. You need brass terminals and make sure the shaft and bearings are not worn out.

By the way, all you guys running the HEI ignition will get a smoother and more efficiently running engine if you set your plug gap at not less then .045". If you have a cylinder that's a little lower on compression, try widening the gap on just that cylinder about .005" more then the others, it should smoothen out the running.

My engine is a 9.3:1 compression, the stock 36K is plenty but I like to run the mixture as lean as possible (without detonation) so I have to widen the plug gap to around .050 or .060 to eliminate "lean misfire" (ESPECIALLY AT MAX rpm). So even thought my 1988 distributor is smaller diameter then your HEI I will be going with a 48K volt coil because of the wider plug gap.
http://www.summitracing.com/parts/MSD-8226/

M1D

I pulled that quote from: https://vintage-vans.forumotion.com/between-the-seats-g-f17/coil-questions-t13389.htm#50501

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°3

Re: Straight Six mystery!!

Post by VANagain Tue Jun 29, 2010 9:22 am

Thanks for the reminder. I'd forgotten that you're supposed to have a wider gap on the spark plugs with HEI. I'll check that. I never heard of "lean misfire" before.

And my auto parts guy made sure I got the cap with the brass terminals. Glad to know that matters too.

ViewMaster: Number of posts : 117
Location : Tampa, Florida
Registration date : 2008-05-26

Post n°4

Re: Straight Six mystery!!

Post by ViewMaster Tue Jun 29, 2010 9:43 am

I'm no great mechanic and haven't had a six for quite a long time but the sixes have a common problem with thier intake manifolds being so long that they can tend especially at low speed to get gas pooling up on the bottom in the middle of them. And when you step on it you just squirted even more gas with the accelerator pump and most likely the center cylinders get flooded. This is the reason for the exhaust being tied into the intake for heat to help with that. When you pull the spark plugs do you notice the center ones being darker/blacker? It seemed to be the nature of the beast. I remember I would ease on them till it reved before I could step on it. The HEIs probally help.
Now the the newer fuel injected straight sixes are sweet, smooth and strong like the ones they have in the Trailblazers. They even have counter balance shafts that made them so smooth that they had to add an electronic cut off so you can't try to start it after it's running. I thought that some of ya'll with flat faces would have tried that swap.

Scott: Number of posts : 1651
Location : Anoka, MN
Age : 54
Registration date : 2008-05-20

Post n°5

Re: Straight Six mystery!!

Post by Scott Tue Jun 29, 2010 10:13 am

What car used the newer straight six?

DanTheVanMan

DanTheVanMan: Commissioner; Number of posts : 7900
Location : Escanaba, Michigan
Age : 62
Registration date : 2008-10-08

Post n°6

Re: Straight Six mystery!!

Post by DanTheVanMan Tue Jun 29, 2010 12:19 pm

Two things: Pugs gap. dirty plugs or carb adjust ( Too rich)

_________________
DanTheVanMan
1965 Chevy G10 Sportvan Custom
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1997 Jeep TJ Sport

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Straight Six mystery!! Qr_cod10

<-<-<- Cruising is not a "Point A to Point B" thing, but an "Everything in between thing! <-<-<-

richie61: Number of posts : 445
Location : Van Nuys, CA
Age : 63
Registration date : 2010-01-04

Post n°7

Re: Straight Six mystery!!

Post by richie61 Tue Jun 29, 2010 2:09 pm

Just what I need...trying to put a computer driven modern Trailblazer engine in a 45 year old van. Although M1Dadio did something like it, it's way out of my mechanical ability...but could it be done????

Guest: Guest

Post n°8

Re: Straight Six mystery!!

Post by Guest Tue Jun 29, 2010 2:58 pm

must be a carter carb Mad

Mad

ViewMaster: Number of posts : 117
Location : Tampa, Florida
Registration date : 2008-05-26

Post n°9

Re: Straight Six mystery!!

Post by ViewMaster Wed Jun 30, 2010 8:47 am

Scott wrote:What car used the newer straight six?

You would probally need the entire truck for a swap and a lot of fabbing too. And the top looks kinda wide for the doghouse.
Here is a quote from a 4x4 forum http://colorado4x4.org/vbb/showthread.php?t=155422
LL8

Vortec 4200 LL8 engine in a 2006 Chevrolet Traiblazer.The LL8 (or Vortec 4200), is a straight-6 truck engine. It was the first Atlas engine, and was designed for GM's new SUV line. It displaces 4.2 L (4160 cc, 254 cu in)[1], with a 93 mm (3.7 in) bore and 102 mm (4 in) stroke. Engine redline is 6300 rpm. Alternator: 150 amp

LL8 Applications:

2002-2009 GMC Envoy, Envoy XL, and Envoy XUV
2002-2009 Chevrolet TrailBlazer and TrailBlazer EXT
2002-2004 Oldsmobile Bravada
2004-2007 Buick Rainier
2003-2008 Isuzu Ascender
2005-2009 Saab 9-7X

Power by year:

2002 270hp / 275ft/lbs
2003-2005 275hp / 275ft/lbs
2006-2009 291hp / 277ft/lbs (can vary slightly through the years due to the new SAE rating procedures)

The Atlas LL8 was on the Ward's 10 Best Engines list for 2002 through 2005.

donivan65: Governor; Number of posts : 12220
Location : San Diego, California
Registration date : 2008-05-12

Post n°10

Re: Straight Six mystery!!

Post by donivan65 Wed Jun 30, 2010 9:47 am

Take that spring off the needle.

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°11

Re: Straight Six mystery!!

Post by VANagain Thu Jul 01, 2010 10:29 pm

My carb is a Rochester Monojet, from a second Gen I think.

Don, do you mean that long brass needle in the bowl with the little spring on the top? The metering rod? Let's see, seems to me if I take that spring off the needle will spend more time down in the seat. Is this to limit the gas more?

I have no idea of how lean or rich it is set to. Can someone explain the procedure for setting the mixture? It's always been a mystery to me (even though Don has tried to explain it).

BTW, Don, I've got Friday off work, for an extra long Independence Day weekend! Smile

Smile

donivan65: Governor; Number of posts : 12220
Location : San Diego, California
Registration date : 2008-05-12

Post n°12

Re: Straight Six mystery!!

Post by donivan65 Thu Jul 01, 2010 10:42 pm

Nope,,,,,,the spring clip that goes on the needle that holds it to the float,,,,,,it sometimes twists and won't let the needle down which raises the fuel level up and loads up the engine,,,,,,,then it clears up and does it again,,,,,,just get rid of that clip and see if you still have the problem,,,,,,,I will be at Harrys Friday,,,,,we need to work on his van,,,,,,,,As for the mixture,,,,it goes in the middle,,,,which is to say,,,you turn it in until the engine RPMs drop,,,,,this means it is TOO LEAN,,,,,you start backing it out until the engine RPMs start going up and start dropping,,,this means TOO RICH,,,,so there are your perimeters...put it in the middle,,,,and you need to watch what the RPMs are while you are doing this,,,,,, the idle needs to be at the proper RPM,,, WHILE,,, you adjust the mixture so you are also adjusting the idle screw if the idle gets too high or low......you are setting the mixture for a certain RPM,,,,,so it needs to be right. And a vacuum gauge is a good tool,,,,,set the mixture for the highest vacuum reading,,,,,,and the smoothest idle.......

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°13

Re: Straight Six mystery!!

Post by VANagain Sat Jul 03, 2010 10:40 pm

Yesterday I re-gapped the spark plugs. They were at .040 and now are .045. It idles fine but when I took it out for a drive it was ran terribly! It almost stranded me. It got so I couldn't give it any gas or it would backfire and then miss. Up til now it had only backfired thru the carb but now I got some real loud ones thru the exhaust.

I have a different HEI distributor I might try on it. It has the external coil so I could rule out the distributor, module and coil that way.

Another clue: I have exhaust leaking out the two middle ports. I had just put on a new gasket and tightetened everything up. I recall something about exhaust leaks causing backfires because cool air gets in there and detonates. Could it be that simple?

Guest: Guest

Post n°14

Re: Straight Six mystery!!

Post by Guest Sat Jul 03, 2010 10:57 pm

how does she run with the vac adv plugged off?

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°15

Re: Straight Six mystery!!

Post by VANagain Sun Jul 04, 2010 10:13 am

I forgot to mention, most of the plugs were black and wet with gas when I took them out. And it hadn't run for several days...

donivan65: Governor; Number of posts : 12220
Location : San Diego, California
Registration date : 2008-05-12

Post n°16

Re: Straight Six mystery!!

Post by donivan65 Sun Jul 04, 2010 12:13 pm

Seems to me you did not have that problem before you overhauled the carburetor and made up bushings for those centrifugal weights in the distributor,,,,,,,,,

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°17

Re: Straight Six mystery!!

Post by VANagain Mon Jul 05, 2010 12:23 am

Hey, good thinking about those bushings I made fit on the distributor weights! You're right, it did run better before i did that. Maybe they are still too tight or otherwise throwing off the movement of those weights. That would explain some of this bad behavior. It will be easy to find out — before I try anything else I'll just take them off and take a test drive.

And yes, I suppose it's possible I made more mistakes on the carb rebuild than the ones you caught for me! A few times it died on me as if I had turned off the key. I checked and the hot wire was still on the distributor. Could the carb bowl be running empty? Maybe I will try removing that spring clip from the needle in the bowl.

kookykrispy: Number of posts : 1533
Location : Helendale, CA
Age : 50
Registration date : 2009-05-22

Post n°18

Re: Straight Six mystery!!

Post by kookykrispy Mon Jul 05, 2010 11:46 am

VANagain,

I am running the same carb you are, and rochester monojet from a 2nd gen van. When I first rebuilt the carb, I adjusted the float to the spec in the rebuild kit directions (rebuild kit was purchased at NAPA). The engine started up and ran great but after about 30 seconds or so the carb flooded itself and leaked gas from the carb. I removed the carb and found the intake full of raw fuel. I adjusted the float level lower, so less fuel would be in the bowl. Although it was adjusted below the stated spec in the rebuild kit directions, the van runs fine now. Just my personal experience and 2 cents for what its worth... good luck!

If I have the carb apart again, I will try Don's trick to remove the little spring on the needle and see if that makes it better. Monojets seem tricky, but once you get it dialed in, it is a great carburetor.

Nightmoves: Number of posts : 2214
Location : Old Hickory Tenn.
Age : 64
Registration date : 2008-11-17

Post n°19

Re: Straight Six mystery!!

Post by Nightmoves Mon Jul 05, 2010 1:17 pm

KK,I too had the same issue when I rebuilt my Mono.Never havin messed with it before,went by the data table wanting to make sure I had it right.Sheet states 9/32 & 1/4" from top on most apps..That was way too high.Fuel over flow at base & top of carb gasket.Dropped another 1/8 (precision eyeball),no problems.I did change from neo/plastic float to a new brass though.I believe neo float that was in there was incorrect to begin with.Had a rubbing issue with metering rod.

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°20

Re: Straight Six mystery!!

Post by VANagain Mon Jul 05, 2010 2:04 pm

Thanks for the tip, Kooky and Nightmoves!! I'm going to remove the top right now so and remove that spring and lower my float. I do think my carb is flooding out.

Another clue to my engine dying though is today I noticed that my tranny is not downshifting! That'll make your engine die. It's like a manual... I gotta call Bowtie Overdrive and ask about this. Plus, I need to hook up my overdrive lockup wiring. But I thought that was just to get it to go from O to D.

And on the distributor, this morning I tried removing the plastic bushings from the weights. It might have helped, not sure. I also removed that center piece between the weights for the first time and cleaned it up. It's a bit worn on the sides and the two pins that hold it are really sloppy. Not sure how much this matters, when the springs are on them. I could see worn weights causing more subtle problems but not this backfiring and stuff.

I just spoke with Don and he says I've got a bad combination: rich fuel mixture and exhaust manifold leaks. When I installed the manifolds I went by the directions that came with the gaskets, which was tighten up the hot box first, then the head bolts. Right now I'm in the middle of trying it Don's way (when will I learn...) of tightening in the opposite order.

VANagain: Number of posts : 1046
Location : San Diego, CA
Age : 62
Registration date : 2008-05-16

Post n°21

Re: Straight Six mystery!!

Post by VANagain Tue Jul 06, 2010 12:06 am

I took the top off the caburetor and pulled out the float and needle and seat. My rebuild kit came with a new needle and seat that was more slender than my old one so my spring clip wouldn't fit on it, so I didn't put them in. And the old one didn't look that bad. But now, since I'm removing that spring cilp altogether, I put in the new needle and seat. And then I adjusted the float to be about 1/8" lower.

Then I loosened up all the bolts on the exhaust manifold and this time tightened those into the head first. Then the three on the hot box. After that it sounded a lot noisier. Much less exhaust leaking out from the head but now a lot from the hotbox. So something's not square if you tighten one and the other doesn't mate up. I've got to shop for another exhaust manifold.

I did test drive it this way. This time I let it warm up for a long time, since it has a tendancy to die all the time. While it was idling I messed with the mixture screw. The exhaust smells so bad I figure it's got to be running rich. Then I took it to the end of the block and back. Better than before, no backfiring, but I never really got on it. Back at the house I could barely get it backed into the driveway. I put it in R and it dies. Over and over.

This is very disappointing after all my work, but I really doubt it is related to the rebuilt long block or tranny. It's got to be some of the old parts I've bolted onto it. Don said next weekend we could try swapping out parts with his spare carbs and distributors. I need the expert on the scene!!

whopman: Number of posts : 387
Location : Columbus, Ga
Registration date : 2010-04-01

Post n°22

Re: Straight Six mystery!!

Post by whopman Tue Jul 06, 2010 3:09 am

Vanagain ...been following this thread...don't mean to butt in ..got a question? when you start the van cold does it run ok? When it warms up it starts acting up?

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Post n°23

Re: Straight Six mystery!!

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