Runs on 3 cylinders at idle, but all 4 at higher rpms???

[DanLewis]

Here's a curious problem that hopefully one of you recognize and can help me solve:

At idle, cylinder #3 is not firing. I can pull the plug wire and there's no change in the way the car idles. However, at about 1500 RPM and above, everything is fine. I can cruise at 70 with plenty of power, so I know it's firing on all 4.

The plugs are not fouled, they're gapped at .025", I've tried changing the plug wires - all to no avail. The distributor is a brand new electronic distributor, so there are no points to worry about. I've removed the cap and cleaned the contacts as well as the rotor - no difference.

The only things I can think of that could cause it and haven't been tested are (1) a bad cap (although I would expect this to cause the problem at high rpms too), or (2) some sort of fuel delivery problem at low rpms - like maybe the intake valve isn't opening far enough. If I give it a little choke to make the mixture richer at idle, the problem seems to improve considerably, but of course that's not a permanent solution.

So am I right - do I probably need to check the valve adjustments? I haven't done a compression test, but thought checking the valve lash would be the more obvious first step. The engine has only about 1,500 miles on it since it was rebuilt, so I've been reluctant to assume that it's valve adjustments so early - but then who knows?

Oh - one more thing, perhaps important: The day before this began I removed the Weber DGV downdraft carb and intake manifold so that I could re-install my Weber 40DCOE sidedraft carb (and of course a different intake manifold). Would a bad seal (i.e., a vacuum leak) cause this problem?

Any insights would be appreciated!

Dan

 

[AN5Sprite]

Dan,

Odds are it's related to intake manifold swap. But it wouldn't hurt to:

Pull the #3 plug and get the piston up enough so that you can see it. Look for coolant pooling on top.
Or skip that and do a compression test.

I've experienced the dropped cylinder at idle that comes back at higher speed. In my case it was the head gasket eroded away from a coolant passage. That's why I suggest the above.

You could also try swapping the plug from #3 to say, #1 and move #1 to #3 just to rule out spark plug failure.

Just suggestions.

 

[DanLewis]

Thanks, Steve. I've got my fingers crossed that it's a vacuum leak. The water in the radiator looks normal, so I'm hoping that there's no head gasket problem. I used the old manifold gasket when I changed over to the side draft, but I've got a spare gasket, so my current plan is to replace the gasket tomorrow or Tuesday thinking that it's most likely just a vacuum leak. I'll report back what happens.

Dan

 

[nomad]

Intake ports are siamesed so I would doubt that it is intake gasket related. You can check that by spraying WD40 or something similiar around the suspected leak area. If the engine changes its speed then you likely have a leak. I would do a compression check first then if reading is low pull the valve cover to see if clearance's have tightened up. After that it could get serious.

Kurt

 

[DanLewis]

Hi Kurt,

Intake ports are siamesed so I would doubt that it is intake gasket related. You can check that by spraying WD40 or something similiar around the suspected leak area. If the engine changes its speed then you likely have a leak. I would do a compression check first then if reading is low pull the valve cover to see if clearance's have tightened up. After that it could get serious.

I hadn't thought about the siamese ports - good point (although what the reasoning suggests is a little depressing). 'Guess I'll know more when I start digging into it tonight or tomorrow - when time permits. :fat:

Thanks!
Dan

 

[dklawson]

(although what the reasoning suggests is a little depressing). 'Guess

Don't get depressed yet! I like Steve's suggestion about switching the spark plugs first. Maybe the problem will follow the plug. After that I would do the compression (or better yet, leak-down) test.

 

[DanLewis]

Hi Doug,

Don't get depressed yet! I like Steve's suggestion about switching the spark plugs first. Maybe the problem will follow the plug. After that I would do the compression (or better yet, leak-down) test.

The spark plugs were brand new last week, so I doubt they are at fault unless there was a manufacturing problem.

'Hope to know more in a day or two...

Dan

 

[AN5Sprite]

When a new/unusual problem pops up right after you've had those meat-lumps you call hands "fixing" the car, is it coincidence or is it you? Even if it not obvious at first, it's probably you.

At least that's the lesson I keep teaching myself...

 

[DanLewis]

Hi Steve,

When a new/unusual problem pops up right after you've had those meat-lumps you call hands "fixing" the car, is it coincidence or is it you? Even if it not obvious at first, it's probably you.

At least that's the lesson I keep teaching myself...

I've got my two "fat" fingers crossed, hoping that you're right! :friendly_wink: It would really be nice if it were as simple as a vacuum leak that crept in when I did the carb & manifold swap.

Dan

 

[recordsj]

Hi Doug,



The spark plugs were brand new last week, so I doubt they are at fault unless there was a manufacturing problem.

'Hope to know more in a day or two...

Dan

I have seen new spark plug wires be bad, there is always a chance of it.

 

[Healeysince59]

As others have suggested, do a compression or leak down test.

 

[nomad]

Doug is right and I should have suggested that first as well. New plugs are not always good. I had a cam going flat in the first engine I ever built [didn't know I should use new lifter's]. Engine had very few miles so the shop I took it to decided to check my new plugs. More than one of them tested bad.

Kurt.

 

[dklawson]

Thanks for the credit... but the spark plug suggestion was not mine. It was from Steve (AN5Sprite).

My advice is really just to do a lot of screening tests and only replace cheap stuff first. Few tests are as inexpensive as moving plugs around to see if the problem moves with them.

 

[Pythias]

Norry's Law: Just Because It's New, Doesn't Mean It"s Good.

 

[DanLewis]

Well, the best laid plans....

I had planned to work on the firing 3 out of 4 problem last night or tonight, but my significant other updated my "honey do" list and there just hasn't been time to get back out in the garage. :smile-new: I'm hoping to get to it tomorrow, since that's the last free time I'll have before the weekend.

Anyway, I received my order of two smaller 28 mm chokes today for the 40DCOE. I've been wanting to try to see if they will reduce the off-idle stumble I've had with the side draft. Anyway, I thought I'd install the new chokes at the same time since I'm already planning to remove the carb and manifold to install a new manifold gasket - 'still hoping (maybe unrealistically) that there might be a vacuum leak.

Now just to find the time...

Dan

 

[Jim_Gruber]

Think about solving one problem at a time before introducing another variable.IMHO of course.

 

[DanLewis]

Well, I finally had some time to work on the BE tonight. I pulled the intake manifold and indeed found a bit of damage to the gasket that had created a vacuum leak at the siamese intake port for cylinders 3 and 4. You can see it in the photo below - it's the shiny area near the top of the port where the soft paper-like gasket material is missing. I put everything back together with a new gasket, and even used a thin amount of Hylomar gasket sealer on either side of the gasket around the two intake ports. The engine is now firing on all four cylinders, even at low rpms, so apparently the vacuum leak was in fact the problem.

Dan

 

[dklawson]

Excellent! I'm glad you found the problem and thanks for posting what you found wrong.

 

[nomad]

Glad you found it.........I guessed wrong. Still can't figure why it would only misfire on the one cylinder?

Kurt.

 

[DanLewis]

Hi Kurt,

Glad you found it.........I guessed wrong. Still can't figure why it would only misfire on the one cylinder?

I agree. The way I decided that it was cylinder #3 was that while idling, I would remove the spark plug wire to see if the idle changed. I started at #1, then #2, and when I got to #3 there was no difference, which told me that #3 was not firing. I never tested #4, although I wouldn't expect it to idle at all if only cylinders 1 and 2 were firing. Weird!

I do have a bit of a different problem now, though. Although it runs smoothly again on all four at low rpms, I can't get the engine to keep running when I take my foot off the accelerator. What's odd about this is that I didn't touch any of the idle mixture or idle speed adjustments on the carb. And I don't think fixing the vacuum leak would cause this because if anything it should have made the mixture richer, not leaner (starving the engine).

Oh well, it gives me something to do this weekend. :joyous:

Dan