MSD Ignition or Weber Carb Issue?

[Jim Nielsen]

Chuck,
Yes the boxes can fail and you can have a new one that is bad. I have experienced both. I used to keep a spare. On my box I could unplug it from the harness and put in a bypass plug and run without the MSD. I don't know if the distributor you have will allow that.
Good luck
Jim N

 

have had msd 6a fail also. the plug leads are "fine wire" i assume. steve

 

[Randy V]

Chuck - I wish I had an MSD-6 to send you to try out...

All I have left in my shop is an MSD-7AL2 which I will be using on my Forty. The wiring is close - if you'd like to try it let me know and we'll get something arranged..

Have you pulled the chip out yet to eliminate/disable the rev-limiter circuitry?

At an idle, you can fatten up those Webers a lot with the idle fuel mixture screws.. I am not thinking that it's a jet problem - unless - you have dirt in the carbs or air orifices / emulsion tubes - then Webers don't get finicky, they get pissed off!

 

[Rob]

Randy,
Good point! Chuck I also blew out my idle circuits. Sorry.....forgot to mention that.

Randy....the problem is I think he is already too rich. With that design, you can't lean out the mixture well if the jets are too big.

 

[Randy V]

With my engine on the dyno (48 IDAs), it was popping back through the throats until we richened the mixture in those particular cylinders.. When we got it too rich in any cylinder, the popping was still gone but the EGT dropped..
That's why I was thinking he may be a bit lean in one or more cylinders.

In other Webers I've run, if you got so much as a grain of dust in an emulsion tube, the bloody carb would have so many sputters and stutters in it between low and high speed circuit transitions, it would take an accelerator pump the size of coffee thermos to cover up the lean spots.. But those were not IDFs or IDAs..

 

[Rob]

Yep. Agree with every one of your comments.

Here's my train of thougth:
1) plugs are wet, light indicates fire may be getting put out.
2) jet settings suspect. "seems..seems" to be jetted fat at idle and lean in upper circuit.

If the plugs are wet and there is an indication that the plugs may not be firing, I'd speculate that his popping is probably more off idle than while sitting on idle. So, the first thing I would want to do is to get it to run well at idle, not foul plugs etc. Then work from there. Since it "looks" like the jets should be changed anyway (based on my -very similar- engine data point, the smaller chokes etc etc), I'd suggest doing that before other things... then tune from there. This is all suspect until we learn if the initial dyno provided gas numbers.

Re the popping, my set up popped off idle even when I was running the 65 idle jets (off idle @~ 1200 +/- rpms).

Let's see what Chuck has for info.....I think he's kinda busy at work these days.

 

[Dean Lampe]

I think it is spark related. Given the fluctuations in the timing light output I would suggest the MSD is the culprit. If the spark was extinguished by a rich condition, the timing light would still be consistent even though there would be no spark. The other thing is that the misfire moves from cylinder to cylinder. That doesn't sound like a fuel issue to me.

Just my opinion.

 

[Chuck]

Rob:

Attached are the two dyno print outs provided by T and L. This is the only data I have.

Thanks to all for the good suggestions and analysis.

Chuck

 

[Jac Mac]

MSD Systems when working correctly should virtually never let a rich condition foul a plug or produce a weak spark, so it would tend to suggest that there is an issue with the system or power supply to it. Do not forget the Earth connections, & once again, is it wired correctly-eg, have you played with the factory connections & created the 'fire off the trailing edge of the pole syndrome'?

 

[Rob]

Chuck, I'd say it's definitely something to do with air, fuel or spark. For sure....

Sorry...couldn't resist.

Jac has a good point.

Sorry, not smart enough to decipher the PDFs....the air / fuel ratios are zeros.

Chuck, if you want to eliminate the box as a possibility, feel free to buy a Crane Hi-6 and plum it in (with out cutting wires and when you're done, I'll buy it from you. If you want to do it, just send me a note first so I know not to buy one. No rush on my end, as I won't be needing it for a few months. (This is the set up that I run with MSD distributors, as I don't like the MSD boxes)

 

[Jim Rosenthal]

Chuck,
Although MSD boxes are a quality product (I have several) they are not immune to product defects. If you are certain that you have a good ground on both the engine AND the MSD box, I would try another MSD box. From your description of the problem, it does sound more related to ignition than anything else. NO carburetion system will work when there is no spark to fire the mixture, or only an intermittent spark.
You might also look in the distributor to make sure that the advance weights are not sticking; are you using an MSD distributor? They are quite well made but again everyone makes a defective one every now and again.
Good luck with this. I have been there and it is VERY frustrating. More often than not, it is a bad ground or a computerized device, like an MSD, that isn't "thinking clearly".

 

[Chuck]

Status report

Left work early and did a bit of tinkering. Now I am more puzzled than before.

1. Installed the nice new shielded MSD magnetic pick up wire. No discernable difference.

2. Per Rob's suggestion, swapped out the 60 idle jets and replaced with 55. It made a difference. We were actually able to get all 8 cylinders firing, albeit #8 was tempermental and had to get the mixture screw in just the right spot. It would idle around 900 rpm without popping or backfiring. It would blip OK, but hold it at 1500 or 2000 rpm and the popping and backfiring was relentless.

Cylinders #1 and #5 ran noticeably hotter than the others

Turn it off, start it up, and it would pop and backfire again at idle.

Did not have time to slap on the timing light to check the quality of the spark.

3. Original carb settings as dynoed by Tand L are:

Weber 44 IDF
Idle: 60
Main 135
Air 175
Venturi choke tube 32
Aux Venturi 45


4. The weights in the distributor move freely

5. Removing the rev limiter made no difference

6. The MSD is solidly grounded to the chassis. Checked with an ohm meter as well.

7. Ran carb cleaner through the idle circuit.

Conclusion:

A. It still ain't right

B. Changing the idle jet helped, but it still seems way too sensitive and unstable.

We changed the Air jet to 180 and it was the same or worse, so put it back to the 175. Have not changed the main jet (145 is the largest we have in our box of jets)

C. I still wonder if it could be the MSD box. Ryan wants to put an MSD on his 1982 911, so this may be an excuse to bite the bullet and get another. Ugh.

D. Question: Should we fiddle with jets? If 55 is better than 60, maybe we should try a 50 idle jet?

E. As Ryan so astutely observed: "what ever changes we make it runs different, but still runs like crap"

The Curve Ball

Here is another issue not mentioned before. Early on I replaced the paper intake / carb gasket with a spacer, about quarter inch, made from a plastic material to stop the carb boiling after shut down. They were not specifically made for IDFs (I think they were for IDAs), but was told they would work. The outside shape matches but the inside is not concentric with the carb bottom and intake manifold opening. The spacer has a slightely larger diameter opening. Could it just be that it is screwing up the flow?

But my memory is that the carbs were behaving the same before we put in the spacers, so never considered that a cause.

Bottom line:

A. More jet fiddling, or

B. Swap out the MSD

C. Go back to the fiber manifold / carb gaskets

(By the way Rob, what is the advantage of the Crane Hi-6 versus MSD?)

Opinions?

 

[Jac Mac]

It is highly unlikely that the main circuit has been activated @ 2000 RPM or less under no load, so confine your investigations to the idle circuit of the carbs. I would also find a plain jane 302w Distributor/cap/leads- coil-ballast, that you can 'chuck' in & jury rig to eliminate your current ign system completely ( points type from back in the 60's ). The possibility exists that at some time during the build process you might have inadvertently 'spiked' the MSD, even with a weld lead running nearby, so as Rob pointed out- try another system to tick that possibility off the list. On a relatively new motor there a host of little gremlins that an creep in as well- sticky valve guides from too much idle time is one.

 

[john]

Weber tuning is simple if you have the basics. That said, every engine is different regardless of what the baseline setting may be. From a person that has been running them for over 15 years this is what I think. You have to make sure you have No leaks in the Manifold or carbs. Fuel pressure no more than 3 1/2 lb. Floats MUST be Adjusted perfectly. Airflow with the Unisun must be perfect. Although this is a double edge sword. You can't have all the carbs perfect if there is a leak somewhere. Your MSD electronics the same what I use on my shelby . Check your timing!!! If you get allot of popping, advanced the timing by EAR!! not by the timing light. Webers Like to breath!!! Old timers that tune Webers told me this trick. And sure enough, It worked!!! Do you have My DVD on tuning Webers? The Myth about tuning webers scared me off when I was decided to purchase them for my Shelby. Education and hands on tinkering made me to be somewhat an expert..."So friends have said" But humbling time with numerous cars with webers have paid off. Frustration is normal when entering the Kingdom of Webers. However, once youe get them tuned, there is NO turnig Back to a standard carb.

Best

 

[Howard Jones]

".....first and formost, before any fine tuning can be accomplished it is absolutely imperative that the floats are set exactly to spec with a special go, no go gauge. This height is vital......

Sorce: Petersons basic carburation and fuel systems, section on Webers.

 

[Chuck]

Any good sources on float adjustment in an IDF carb? The descriptions I have seen are a bit sketchy.

 

[Rick]

Chuck you mentioned a spacer. Is it possible there is an intake leak caused by the spacers and the bases of some of the carbs?

 

[Rob]

Chuck,
Agree with JacMac, you are dealing with a low rpm circuit issue. Don't worry about the main jet yet. Although I'll bet you a frosty cold beer that when you get on the dyno you will be lean above 2K rpms. But a discussion for later. Wait..... thinking aloud here..... I can't remember.... if the main jet "has an ancillary affect" on the low circuit. If the main is far enough off, can it affect the idle? Jac???

Q: the popping at 1500 rpms.... was the engine at running temp? Mine will still do this until it warms up. These carbs don't have cold chokes, so the mixture is lean until they warm up. Any chance....????

Q: Did you blow the idle circuit in both directions? i.e. pull both the idle mixture screw out and the idle jet holder out and both both ways?

Q: Did you pull the main jet holders out and confirm the "E" tubes didn't have any crap in them?

Above comments due to the temp difference on different cylinders I would believe you still have inconsistent A/F ratios in different cylinders

​

Note: Your venturi choke tubes are smaller than mine, so your settings may not end up in the same place. i.e. air corrector etc. However, I must question the fact that you cubes are 13% higher and yet you have notably smaller venturis. I will be curious what a dyno pull graph will look like. Logic would dictate that one of the two is a more correct answer????? I will stay tuned for this.

Regarding the Idle jet "fiddle" question; are you in the 1 to 1.5 turns out range? If not, then I'd say yes, fiddle. Since your Choke tube is smaller, a smaller idle jet may be needed...../?

MSD: yep...I would agree still in question. Obviously the decision point is to get to the point that you "think" the carb tune is good enough to determine fault should then fall on the ignition. Q: If you create a spark gap from a spark plug lead to ground can you see the multiple sparks? May just be easier at this point to get one for the 911 and go with process of elim if you are going to buy one for that car anyway.

Re my preference, I had a couple MSD 6s go bad over the years..... MSD is analog vs. Crane being digital..... Crane has adjustable rev limiter in 100 rpm increments vs. the chip thing..... and the fact that I haven't "yet" had one go bad on me. Like all other preferences it's personal experience, personal "vibe" and opinion. That's about it...

Re the spacers. I don't think oversized ID would cause issue. It would create some turbulence if anything. If it was smaller ID, that would be different. I don't think it possible it could be causing an issue certainly not at low speed. However, as mentioned, I'd make certain there are no vac leaks. Did you still use the paper gaskets on each side of the spacers?

Don't forget I went through the same process, so it's not just you.

 

[Chuck]

It is Saturday, time to tinker after I leave the office.

Plan is to remove the carbs, check the float adjustment per Pat Braden's book, get rid of the spacer and go back to the standard gaskets. Figure I need to do this regardless, so might as well get it done now.

Then will try advancing the timing a bit, as suggested by John.

Then will swap out the MSD, saving the items that cost for last. Jac, it is easier for me to just swap out the MSD than bypass the set up with a conventional system.

Rob, regarding settings:

- To get the one recalcitrant cylinder to fire had to get the mixture screw more like two turns out. The others are around one turn out. I noticed that if the idle set screw was too far open and the mixture screw was in it was less likely to fire, but if the idle screw was backed out (lower RPM) and the mixture opened a bit it would fire. Still seems like the adjustments are an issue, as you and others have said.

- Yea, it was at operating temp when we were doing our adjusting

- I did pull the E tubes and gave them a squirt with carb cleaner

- I did pull the mixture screws and gave them a squirt as well. I really don't think it is dirt issue.

We did recheck and readjust the flow between the carb when all were firing and it was idling steady: we got it nailed, all carbs are flowing equal at idle.

What remains a mystery is how it could be dynoed, yet be so far out of adjustment, but then I guess they don't much care about the idle circuit on the dyno . . . .

Jac: Your comment about spiking the system makes me recall the very first start up. Due to a wiring glitch with the alternator, when the car was started the very first time it would not turn off with the ignition key. Turns out the alternator had a 'feed back' problem that kept the ignition going when the key was off. We pulled the coil plug to kill the engine. Would not think that would damage the MSD, but at this point I guess anything is possible. That is the only aberrant event I can recall.

Thanks for the suggestions and help. Hopefully will have some good news by days end.

 

[Jac Mac]

Having read your dyno sheets that may be correct- usually an operator will note the ign timing, both static adv & total used for the test, plus a few other items like fuel octane -oil temp - air pressure - etc. Most of these factors seem to be been missing on the data you have, plus the dyno doesnt use the 180° headers of your car either, so the tuning process starts all over again. It really looks like they are using the Dyno as a test bed / break-in tool & as long as it puts out a reasonable torque curve at the end they are happy.