| Author |
Message |
Don McCormick (Dandy_don)
Junior Member
Username: Dandy_don
Post Number: 76
Registered: 2-2002
| | Posted on Thursday, May 15, 2003 - 10:03 pm: | |
I got home tonite, plugged up the vacuum line coming off the #8 cylinder, fired 'er up and magically #8 was firing normally. I adjusted the mixture on the carb, readjusted the air bypass for less air and took the car out for a test drive. It ran spectacularly. For the first time since I bought it, it pulled perfectly to redline with no stumble after coming off idle. Idles at 950 rpm and no backfiring. Heck I have even lost the slight belching the car has always made after I shut it down. Many thanx to Steve Magnusson and Hans Hansen, the guys who diagnosed my problem without even being here. Now I get to work on all the other things that I have not been doing since I discovered this issue. What a relief. So I guess I learned something here: one needs 4 things (not just 3) for combustion: spark, fuel,compression, and lastly no vacuum leaks. Thanx again all Don |
Don McCormick (Dandy_don)
Junior Member
Username: Dandy_don
Post Number: 75
Registered: 2-2002
| | Posted on Wednesday, May 14, 2003 - 11:01 am: | |
Thanx, all. I will try this when I get home and I am very hopeful. I never would have thought of this possibility. Don |
Dr. Mike Adams (Drmike)
New member
Username: Drmike
Post Number: 21
Registered: 5-2002
| | Posted on Tuesday, May 13, 2003 - 11:44 pm: | |
This sounds like a vacuum leak. The idle circuit of the Webers operate on manifold vacuum and throttle position. An open vacuum port on a "throat per cylinder" setup can completly kill the idle circuit; and make the main circuit dangerously lean.
Did I mention that I hate vacuum operated accessroies ? |
Mike Procopio (Pupz308)
Member
Username: Pupz308
Post Number: 358
Registered: 10-2002
| | Posted on Tuesday, May 13, 2003 - 11:37 pm: | |
Some good thoughts here. Never would have thought the problem would have been so "common," if you will.
I'm really worried that, as has been pointed out, I've gotten "used" to 7 cylinders. Ugh!
Needless to say, I'm in over my head--the car's at the mechanic in Santa Fe. There's a really good guy that does all the engine work there--I'm on travel for two weeks--by the time I get back, I'm hoping the problem will be fixed.
When that time comes, I'll post what I find out here.
Good luck, guys, with getting back up on all 8.
--Mike
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Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 463
Registered: 5-2001
| | Posted on Tuesday, May 13, 2003 - 11:10 pm: | |
Don,
Just a thought, but if there is insufficient air flow to the cylinder in question, that cylinder will be insensitive to mixture adjustments. You might want to try setting the mixture to that cylinder at a slightly higher rpm or open the throttle shaft a little more on that carb. I had a similar problem on my 7 or 8 cylinder and when I increased the air flow a little, I was able to adjust the mixture. The increase in air flow to that cylinder was not really perceptable on my BK meter.
I personally tried using a colortune to adjust mixtures on my 308 but found it gave misleading readings in some cases. It was prone to misfire and leakage. Also, it's really difficult to get and angle to see the combustion color on the front bank cylinders. |
Steve Magnusson (91tr)
Intermediate Member
Username: 91tr
Post Number: 1775
Registered: 1-2001
| | Posted on Tuesday, May 13, 2003 - 6:46 pm: | |
Don -- I can confirm that the modification you describe is a definite "no-no" (the air flap stuff is a "closed" system, while the charcoal canister is an "open" system). |
Don McCormick (Dandy_don)
Junior Member
Username: Dandy_don
Post Number: 73
Registered: 2-2002
| | Posted on Tuesday, May 13, 2003 - 6:03 pm: | |
Hmm...Vacuum leak. That actually might be the idea I was looking for. By memory, as I am traveling now and won't be home til Thursday, the #8 cylinder does indeed have a vacuum intake pointing out to the outside of the car. Formerly it tied into (as Steve reminded me of) the flap on the air box. I have disconnected that hose and tied it into the other vacuum lines which end up over at the charcoal canister as I recall. Perhaps they cannot be combined like this and that line really needs to be closed off. I will do this as soon as I get home Thursday and let you all know. Thanx very much for the thoughts Steve and Hans as this just could be the problem as it fits with all of the evidence and troubleshooting thus far. I just hope that it is the problem. James, thank you as well for your thoughts and posts Don |
Hans E. Hansen (4re_gt4)
Intermediate Member
Username: 4re_gt4
Post Number: 1259
Registered: 4-2002
| | Posted on Tuesday, May 13, 2003 - 5:20 pm: | |
Hmmm..... Yeah, vacuum leak.
Try backing out the mixture screws even farther. If there is a vacuum leak, maybe you will eventually be able to add enough fuel via the idle circuit to make the cyl fire. This obviously isn't a cure, but maybe a diagnostic.
A bit dangerous, but you can put a hose on a propane bottle, just crack open the valve, and put the end of the hose around any place where a vacuum leak could possibly occur. Just have plenty of ventilation. The miss should disappear. (The miss will be missing? Nah.) |
James Selevan (Jselevan)
Member
Username: Jselevan
Post Number: 537
Registered: 6-2002
| | Posted on Tuesday, May 13, 2003 - 3:35 pm: | |
Don - sorry - I did not note that you had changed the plugs, and did not note that you had checked compression. I did note that you swapped carburettors, and that is why I do not believe it is a fuel thing.
What about the spark plug wire or distributor cap? Might not have sufficient spark at idle.
Not much help.
Jim S. |
Steve Magnusson (91tr)
Intermediate Member
Username: 91tr
Post Number: 1772
Registered: 1-2001
| | Posted on Tuesday, May 13, 2003 - 2:17 pm: | |
Don -- (Not that this would be easy to do on a 308's cylinder #8, but) You might try a Gunson Colortune (which is a special spark plug with an optical viewing window) to confirm/deny whether combustion is happening at idle -- just a thought...
Is there a vacuum fitting on the #8 intake manifold (for the airbox flap mechanism IIRC)? If so, you might try blocking that off to make sure it's not leaking... |
Don McCormick (Dandy_don)
Junior Member
Username: Dandy_don
Post Number: 71
Registered: 2-2002
| | Posted on Tuesday, May 13, 2003 - 1:01 pm: | |
James,
I will try replacing the spark plug again (ugh!) but I doubt that it is that as I have already had two different sets of plugs in the car, BP6ES and BP5ES same problem each time. Greg, I tend to agree with you that it is a fuel problem although I have no real proof of that. In my case the problem is not carb specific-so rebuilding the carbs is not a solution- I did just did that. The tech at Pierce Manifolds was doubtful that it was even a clogged idle circuit and my switching of the carbs confirmed that. I know that the accelerator pump jets are delivering fuel to #8 because when I rev the engine I can see the fuel dumping onto the throttle plate. I just can't see if there is truly any fuel being delivered at idle. Does anyone know of a way to test for that? I suppose that I will still try the spark route also but this seems to me to be a longshot. As far as compression goes it seems that the compression in #8 is at least as good as the other cylinders as the readings are pretty consistent. Any other thoughts? It sounds as if there are a number of fellow sufferers out there. This is probably an even more common problem as it is easy to miss if all one is used to is an engine running on 7. Mine idles pretty well actually even on 7 cylinders and I would not have noticed it unless I had adjusted the carbs myself. |
JohnR. (Rivee)
Junior Member
Username: Rivee
Post Number: 200
Registered: 1-2002
| | Posted on Tuesday, May 13, 2003 - 11:50 am: | |
I have the same problem with #1/2 idle jets. When I turn both idle mixture screws all the way in on the #1/2 carb, it makes no difference in idle smoothness or RPM. |
Greg Rodgers (Joechristmas)
Member
Username: Joechristmas
Post Number: 754
Registered: 3-2001
| | Posted on Tuesday, May 13, 2003 - 7:31 am: | |
I have about the same problem. I have been trying to find out what is wrong for sometime now. I tried resetting the floats, cleaning the carb, jets, etc., checked for vacuum leaks around the carb body and intake manifold. I tried switching out plugs without much success. Every now and then it will fire on all 8. It is a fuel problem, not a spark. You will hear the engine sound much smoother and quieter when on all 8. I plan on rebuilding the carbs soon. Mine lately has stayed on all 8 but sometimes it still does this. Let me know if you find out what the problem is. Mine is not #8 but #2 on the rear bank facing the taillights. |
James Selevan (Jselevan)
Member
Username: Jselevan
Post Number: 536
Registered: 6-2002
| | Posted on Monday, May 12, 2003 - 11:01 pm: | |
Don - Suggest:
1) Put a brand new spark plug in #8, and if that does not solve it,
2) Check compression for #8 versus others.
Pulling at RPM will not tell you about compression in one cylinder.
Spark plug in #8 might be bad, despite visible spark when the plug is out of the cylinder. Poor compression in #8 might impact contribution to idle, such that closing idle-jet did not result in drop in RPM. However, at speed, reduced compression might not be sufficient to produce noticeable misfire.
Jim S. |
Don McCormick (Dandy_don)
Junior Member
Username: Dandy_don
Post Number: 70
Registered: 2-2002
| | Posted on Monday, May 12, 2003 - 10:37 pm: | |
Yes I know. Another 308 carb running on 7 cylinders. I read Mike P's thread on the same subject but I don't know if he ever found a solution or it the problem fits mine. This one has me stumped and I would like some help
PROBLEM #8 cylinder misses at idle
EVIDENCE
1) when adjusting the mixture screw all the way in the engine does not slow or stumble as it does on all other cylinders. There is no difference in the way the engine runs at idle (very stable idle at 950-1000rpm) with the plug wire attached or disconnected to #8 cylinder
2) when I disconnect the spark plug wire the car runs (above idle) very poorly and has little power. It runs and pulls cleanly all the way to redline with the spark plug wire reattached. It only misses at idle.
The car is set up as follows
Crane XR 700 ignition modules replace the points and condensers.
Two distributors, both set at 5-6 degrees BTDC.
Both distributors advance mechanisms were set up on distributor machine by Continental Imports in Hinsdale,IL
All new plugs BP6ES, all new stock plug wires, caps and rotors in good shape. When I pull the plug in #8 I am definitely getting combustion. the plug is dry and running a bit rich due to the bigger idle jets I installed. Still have the brown color on the plug but am getting some soot and carbon with prolonged idling
Here are the troubleshooting itmes I have done
FUEL PROBLEM? Thinking I had a bad carburetor idle circuit on the 7/8 carb I switched the 7/8 carb with the 1/2 carb. The problem stayed with the #8 cylinder in that the 7/8 carb worked perfectly normally on the 1/2 cylinders and the 1/2 carb functioned just as the 7/8 carb had i.e. #7 cylinder fired fine and #8 still did not fire at idle. No high revs testing on this one though but I did not think that it mattered and did not want to do yet another carb setup only to change it back again
SPARK PROBLEM? I disconnected the spark plug wire to # 8 and inserted a spare plug and grounded it out on the valve cover. Fired normally and tried the the same with #1 to compare and it fired normally. I am certain that I have spark to #8 at idle and high revs.
COMPRESSION- Should be fine as the car runs normally at high revs (anything above idle) The idle at 950-1000 rpms is actually fairly smooth condsidering that it is running on only 7.
The only thing that I can think of that I have not tested is that somehow the #8 cylinder is not getting fuel at idle but it is somehow cylinder specific and not due to the carburetor. This sounds goofy though. Last piece of evidence is that when I switched the carbs I found that the intake valves of #1,2,7 all had carbon deposits on the valve, #8 was clean.
I welcome any and all theorys and thoughts. Thanx in advance
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