MSD Ignition or Weber Carb Issue?

[Chuck]

MSD Ignition or Carb issue?
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Weber IDF carbs on a modest 302 rated at 375 HP. About five hours running time total. We have been trying to get the engine issues sorted out over the holidays.
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Started the GT a few days ago. It was backfiring and spitting a bit. Checked the exhaust manifolds with a laser temp gauge. What a surprise – one of the pipes was less than 100 degrees F. Opened up the mixture screw on that carb, and it started warming up. Then another pipe started running cool. Opened up the mixture screw on that carb a bit and it started warming. With all the pipes showing hot temps the engine ran better than it has ever run before. Drove it up and down the subdivision and it barely coughed and backfired at all. We thought we were on to something.
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Started it up yesterday. This time a different cylinder was not firing. Opened up the mixture screw but this time we could not get the exhaust manifold to warm up. The backfiring and coughing was back.
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Now it is not the same cylinder. Three different cylinders have pulled this stunt on two test runs.
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The carbs are set at approximate baseline: 1 turn out on the mixture screws and a half turn in on the idle stop screw. We are not to the fine tuning stage – for obvious reasons.
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All along I had been assuming I had a carb issue. Now I am wondering if it is an ignition issue.<o></o>
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MSD ignition. MSD distributor. Plugs are good. Pulled the plug from the offending cylinder and it looked just like one from a functioning cylinder.
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It makes no sense to me that a carb adjustment issue would move around from cylinder to cylinder from time to time.
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The coil is fine. I swapped it out for a replacement and it made no difference.
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It is almost like there was not enough energy to fire the entire system, like the MSD box was running at half strength. I am puzzled and looking for ideas. Am I missing something??

 

[Rick]

Are you sure your idle jets are not partially plugged, maybe bad fuel (if it has been setting for some time). Are the float bowls full of fuel, does the accelerator pump function? I would start by looking at the entire idle circuits, and if there is any question about the fuel, replace it. Check for moisture in distributor cap too.

 

[Cliff Beer]

Chuck, Pat Braden's "Weber Carburetors" book is a good read for really getting a good feel for how your IDFs work and are best tuned. Typically, the rough starting point for a weber mixture screw is two turns out for example. Synchronizing with a carb synchronizer is basic and essential as well. Have you set it up with a recommended base line jetting?

As to mixture setting there are a number of ways to go - I like pulling the plugs as the preferred method if I don't have a exhaust gas analyzer I can plug into individual exhaust pipes. Adjusting the mixture based upon exhaust temp is a new one to me but it seems like leaning out the mixture may raise the combustion temp (rather than the reverse).

So, net, here's what I'd do.

1. Pull the plugs and then check you have a healthy spark (in a dark garage, lay the plugs so they are grounded on the head or block and crank her over a few turns - this is a quick and dirty method).
2. If you have spark, then check the timing and make sure it's roughly in the right range for statis and dynamic through the advance curve.
3. Look for a nice chocolate brown residue on the plugs - if you see black and carbon buildup then you're running too rich, if you see white or very light brown then you're too lean on that cylinder.
4. Try a few different jets and emulsion tubes and chokes. Start with a recommended baseline setup from the folks here on the forum (or Pat's book) and then if that doesn't produce a smooth running engine then try changing one thing at a time (jet or tube or choke) moving incrementally one step at a time - one smaller jet then one larger jet, etc.
5. Check your compression and leak down just to be sure.
6. Make sure your plug leads are all good - look for sparks/arcing while running the engine in a darkened place.

If you try the above you'll identify about 80% of typical problems!

 

[Randy V]

Make certain that the HEAVY RED wire from the MSD is connected directly to the Battery's positive cable and not through a switch or relay.

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/6420_instructions.pdf

This has been the cause of many an MSD failure in the past...

 

[Mike Pass]

I would suggest that you get it on a rolling road with a competent operator. IMHO you might get it to run fairly OK by tinkering about but you will never get it set up right by this method. Compared to the cost of building an engine a rolling road session will give you the best results per £/$ that you will ever get. In many rolling road sessions I have always come away with more power and torque than I arrived with except on one single occasion where the engine had been built by Ian Carter who was the best Imp engine man ever.
The best result recently on an SBF was an improvement of 90bhp!
Cheers
Mike

 

[Chuck]

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Still not running right.<o></o>
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If I were paid for the hours spent getting engine and carburetor issues sorted out I could retire right now and spend my time getting engine and carburetor issues sorted out.
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At the start of the day the engine started up just fine, idled around 1000 RPM, revved freely, but spit from the carbs and backfired persistently at all speeds. At the end of the day the engine started up just fine, idled around 1000 RPM, revved freely, but spit from the carbs and backfired persistently at all speeds. Not much to show for a long days work.
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In between we did a lot and learned a lot, but did not resolve the problem.
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Here is what we learned:<o></o>
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1. When a timing light is hooked up to most any plug wire it flashes BUT with one noticeable difference. It skips beats. May flash 4 times, miss one, flash 6 times, miss one, flash 10 times, miss one, then flash twice and miss one. Almost as if it were having a heart attack. Checked this on several cylinders and it behaved in a similar fashion on each. Just for fun, we hooked the timing light to the 427 on the Cobra. It flashed more consistently, and seemed to flash at a higher frequency at the same RPMs (although this may be a subjective observation.)
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2. Using a laser temp sensor (as before) solely to check if a header was rising above room temperature (not for carb tuning), we again found that certain cylinders were working and others were not. One time all the headers heated up nicely, but that thrill was short lived. When we next fired it up cylinders number 1, 3 and 4 did not heat up. Another time cylinders 4 and 8 did not heat up. Yet the spark plug is firing according to the timing light. Makes no sense to me.
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3. When we pulled a plug from an offending cylinder, it was black and a tiny bit damp. Presumably black from when it was firing on a prior run, and damp from not firing on the last run.
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4. Per Pat Braden’s book, small changes in the mixture and idle stop screws should make noticeable changes in the way the engine runs. Although the idle stop screws have a noticeable affect, the mixture screws do not.
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5. We revved the engine a bit to seek if that would cause the room temperature headers to warm up. No such luck. But did notice when holding it steady at, say 2000 RPM it would back fire, but when revving it did NOT backfire. (Insufficient fuel pressure???)
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Here is what we did:<o></o>
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1. Checked the timing and reset it slightly. Idle – 14 degrees. 3000 RPM – 28 degrees. 4000 RPM – 34 degrees. Perhaps it is not getting full advance in quickly enough, but one would think that dang thing would idle without spitting and back firing regardless.
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2. Connected the MSD directly to the battery. Made no difference
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3. Pulled the rev limit plug from the MSD. Made not difference
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4. Removed the alternator from the wiring circuit, running it solely on the battery. Made no difference.
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5. Increased the fuel pressure from about 2 ½ to 3 ½ pounds. It seemed to make a difference for that run, and indeed all cylinders started warming up. But then next time we started it, three cylinders did not heat up with the same pressure. (Fuel pressure problem? Faulty pressure gauge?)
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6. Installed new spark plugs at the start of the day.
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Carb settings.<o></o>
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1. I have read the chapter in Pat Braden’s book on the IDF conversion on a small block V8. Let me rephrase that. I have committed the chapter to memory. Numerous other sources on line are also helpful, but there are variations in their approaches.
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2. Despite the issues noted, we have synched the carbs with a flow meter at idle and they are well synched. There are no linkage issues we can see.
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3. The engine idles around 1000 RPM with the mixture screws out 1 turn and the idle stop screw in about one to one and a half turns. That means the mixture screw is right about where it should be but the idle stop screw is in too far, but given the other issues that is probably not surprising.
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4. We pulled the idle jets and gave each a squirt of carb cleaner to make sure there was no debris or varnish clogging up the works. We put fresh fuel in the tank.
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5.The Technical info:
- Weber 44 IDF
- Venturi: 32. (36 was too much)
- Idle jet: 60
- Main jet: 1.35
- Ford 302, dyno at 375 HP
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I have not changed any of the jets. They are as provided by the builder and dyno tested.
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Conclusion<o></o>
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I still wonder if there is an electrical issue. Any comments on the timing light irregularities would be welcome.
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Don’t understand why there is evidence of spark but some exhaust manifolds don’t heat up.
Perhaps it is not getting enough fuel, but why would it be so intermittent from cylinder to cylinder?
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I have not checked the floats. (Yes, I know. That is in Pat Braden’s book). But one would expect a float problem to be consistent, not jump from cylinder to cylinder.
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I wonder if the fuel pressure may be too low, and perhaps the pressure gauge is giving a false reading. But the blackened plugs don’t look like an over lean condition. Yet that seems like a plausible possibility. I am using a Holley regulator and pressure gauge.
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I see no point in ‘fine tuning’ the carb adjustments until we sort out these issues. Adjusting the mixture screws does not seem to have much affect at this point.

Sorry for rambling on . . . .

 

[Steve C]

Chuck,

Good MSD box ground? Try grounding directly to the battery.

Good connection from dizzy to the MSD box?

If not the ground or dizzy connection to the box could be the MSD box (from what I'm told some can be good and some bad)...Could be an intermittent in the box electronics.

Steve

 

[Chuck]

I believe the connections are solid. The ground is to a chassis post about three inches from the box. The connections from the distributor to the box are likewise solid. The sparodic nature of it makes me think it is in the box (assuming it is not a fuel issue). Thanks for the thoughts.

 

[BigB98]

Spitting through the carbs sounds like to me either you have too much overlap in the cam (webers don't like that) or the timing is incorrect....Given that you're getting popping from the pipes and the carbs, I am highly suspecting that the box is firing on the cylinders at the wrong time.

I know that you have probably triple checked this, but all of the wires are going to the right cylinders, yes?

The other thing could be the valves are not opening/closing at the optimum times (as in adjustment needed).

 

[Al Jones]

Chuck - Try changing the cap AND rotor.The MSD box could be firing exactly when it should and every time it should.The problem is that if the spark jumps to/occurs in another wire,hence cylinder,you won't see that 1 spark occurring in the plug lead you are watching at that time.It happened and was right on time,just not where it was supposed to go.That would account for 'missed' sparks,the erratic spark pulses and the occasional backfire.Caps can go bad from carbon tracking or a hairline irregularity and rotors have been known to short to the shaft.This is probably the cheapest/easiest thing to change anyway,so try it.You can also swap wires over from your Cobra temporarily if you suspect those too.
Another thing that could be wrong is having the cam 1 tooth off.It will run and be ok at one end of the rev range but suck at the other.If there are periods when it seems fine at idle you can discount that theory.
A quick check on valve clearance would be to open each one up while idling/running until you hear the rocker just start to rattle or 'click'.Then close it down slowly until the rattle just disappears.Then tighten an additional quarter turn (slowly).That will eliminate any valves holding off their seats.You can get finicky with the exact adjustment sometime later. A.J.

 

[Jac Mac]

I wonder if Chuck finds his problem has disappeared now that he has fitted new rocker arms. IIRC the 'no name' posilocks were not even in their installed heights, could be that he had a few tight clearance settings etc.

OOP's didnt read the other thread that the problem still exists-- Oh well back to the shed!!

 

[Ivan]

sorry if you have checked this, but could it be a cam timing issue?
i only ask as you said about backfiring and carb spitting, i may be well off the ball though.

i wish you the best of luck anyway m8 :thumbsup:

 

[bob peckham]

Hi, i have read Chucks build log many times and at the start the motor was run on the dyno and made 375hp, i believe the only item that is different from the dyno run and now with the motor in the car would be the msd system. Im sure they use the same ignition setup for all there dyno runs just my 2 cents

 

[Rob]

Chuck
Please refresh my memory, was your engine dyno tuned with the webers? Have you dyno tuned it since assembled?

Your engine is similar to mine if memory serves correct. So, would seem logical they they would be fundamentally similar in carb set up. For ref. my config is as follows:
mains: 175
Idle jets: 55
Imulsion: F11
Air Correct: 180
Venturi: 40mm

With out knowing the answer to Q #1 above, it seems that with the jets you have listed your idle would be fat and main lean. If you idle is fat, you could be putting out the flame in the cylinders in question. I had this issue with mine with the 65 idles (150 mains at that time). If I could get it running well enough to push into the high RPM circuit it would clear out. With this config I was fouling at idle, popping off at (below the low to mid circuit transition ~2K rpms), and most importantly ran way lean in the high circuit (hence the clearing out the rich situation). So, I was running from super rich at idle to super lean above 3K. After a couple dyno runs and "dial ins", the cliff notes are I dropped the idles to 55, raised the main to 175 and adjusted the idle mixture screw to dial out the lean popping. Note, with the 65 idles, the idle mixture screws didn't really seem to have any affect (sounds similar to your issue).

My 0.02

 

[Bill D]

Rob
Are you running IDF or IDA Webers? The experience you described is exactly what happened to us on our first (and only) drive.

Sorry to hijack, but this is important stuff

 

[Rob]

..IDF...

 

[Chuck]

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The engine is a crate engine, dyno tested with the current carbs. Only thing that would be different is the MSD ignition box and hook up. Every thing else should be as dyno’d. That should, in theory, rule out a number of potential issues, like cams, timing chain, etc. and suggests to me an ignition problem.
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With that said, I just read the fine print in the MSD 6AL instruction manual. It states that the magnetic pick up wires should be routed away from the other wires and located near a metal surface to shield it from stray signals. I bundled that pair of wires with everything else in a nice, tight package adjacent to the coil wires (carrying a few hundred volts) and the #8 chassis bus wire (carrying several amps). So . . . . . . . .
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First plan is to reroute that pick up wire by itself to see if it was picking up a stray signal, hence the erratic firing.
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Second, MSD makes a shielded harness for the magnetic pick up wires which I plan to install: perhaps over kill, but can’t hurt. (Part number 8862).
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Third, I have ordered a new distributor cap. The old one looks fine, although it did have some nasty oxidation on the contacts which was easily cleaned away. I wanted a black cap anyway, so this is a good excuse to replace it.
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Finally, checked the clearance between the magnetic pickup inside the distributor – supposed to be .030. It looked fine.
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I am still a bit suspicious about the MSD 6AL box, but will defer that expense until the less costly options have been ruled out. Has any one had one go bad? What sort of strange things do they do before they completely quit??
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If it is not ignition, we will move on to Phase II: Carbs.
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Rob: thanks for the info on the carbs. The issues you had sounds similar. I may try changing the main and idle jets per your specs. But I am still puzzled how a carb issue would permit different cylinders to fire at different times on different runs. It seems to my uninformed mind that it should be a consistent problem if it is carbs.
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Bill: yea, we are running IDF carbs.
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So that is my plan. If I am missing any thing, let me know. Hopefully will have some answers in a week or so (frustrating how the real job interferes with the hobbies.)
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Thanks to all for wading through my rambling posts and giving good suggestions.

 

[Steve C]

Chuck,

I agree with your logic re electrics as the gremlins esp re the possibility of cross talk re the wiring but, still could be the box.

2 more things to check MSD wise (not related to your problem but, good to do/check).

You shouldn't have corroded contacts with so little miles...Chech to insure your rotor center contact is in fact touching/riding on the cap carbon button (untill I found out about the need to check this I was wearing cap center contacts and getting lots of corrosion on the plug contacts).

Also check to insure the magnetic pickup is properly phased (mine was but I have heard that not all are correct).

Steve

 

[Chuck]

Steve:

Good points. I specifically noted a shiny spot on the rotor where that black contact ball in the cap was rubbing on the rotor, so that is good.

Can't explaint the corrosion, although we have had some wild temp swings that drop dew on everything, even inside the garage. Regardless, we will replace the cap.

The phasing is right based on the wire connectors and what I have read in the MSD material.

Forget to mention that checked the resistance of the coil in the distributor. It was 600 ohms - which is right in the correct range of 400 to 1300.

Can't wait to find the time to check this all out . . . .

 

[Rob]

Chuck, your logic is all sound.
Re the box, yep...it could be bad, and I do think it would present itself in this kind of manner.
With that said, I found the idle jets were putting out the flame and causing some erratic timing light findings. So, while all the other electrical ideas are good, I'd wait on swapping out the 6A until you tune the jets. Then....see what you've got going on.
Still puzzled by the dyno tune.... Did they provide exh gas readings for you?