MSD Ignition or Weber Carb Issue?

[Chuck]

Saturday Afternoon Status Report.

Okay. . . . Now I am really confused.

Made a minor adjustment. Started it up. Virtually no popping and no backfiring. Took it for a two mile drive through the subdivision. Accellerated smoothly to 3000 RPM (as fast as I care to go for fear of the neighbors wrath). Best it has ever run. So what happened?

Before going through the steps outlined in my post this morning, thought I would try one, simple, thing.

Every reference I have read (and I have read a bunch) indicates the idle mixture screw should be pre set between a half and one full turn out. Pat Braden say's a half turn, then add quarter turns incrementally. Red Line says if you end up with it out more than 1 1/2 turns it is too lean and if it is out only a half turn you are too rich. So that has been my starting point and frame of reference.

But Bob Tomlinson's book says something different. For IDF he says the mixture settings average 2 1/2 turns off bottom or perhaps even 3 1/2.

So just for laughs, I pre set the mixture screws out 2 full turns and the idle screw about a half turn after contact with the stop.

Fired it up. It is 40 degrees and humid today, so gave it time to warm up. Cylinder #8 was not firing, so opened the mixture up another half turn and set the idle screw out a bit slower, then it started firing. Engine was idling smoothly around 900 RPM

When just partially warmed up, there was no backfiring and very few spits. When fully warmed it continued to run as good as it has ever run.

So now what? Maybe the MSD has an intermittent issue and today was a good day. Perhaps I should leave it alone and start it tomarrow and if it runs like crap (as Ryan would say) we can assume it is indeed ignition.

But what if it keeps running reasonably well? Where do we go with tuning the carbs? Leave the idle jets alone or go bigger? Is the number turns out on the mixture screw really important as long as it runs right?

So much to learn . . . . .

 

[Gregg]

Chuck, I am joining the party late and only glanced at some of the comments so I apologize if it has been suggested. At one point you talk about buying a second MSD to install on Ryan's Porsche to use as a diagnosis. If the Porsche is running fine as is, why not just put the msd you currently own in Ryan's car and check the box that way. No additional cost to you. Just a thought, and if you are not out shoveling snow, it will give you something to do tonight.

 

[Rob]

Good suggestion Gregg.

I am suspicious of vacuum leaks under the carbs. This could cause the inconsistent idle mixture settings. If you are unscrewing the #8 2.5 turns, that means you are adding fuel, and quite a bit more than others. 2.5 turns in my opinion would indicate an idle jet too lean. We know this isn't the case.......?????

Keep in mind, if you remove the spacers you will have to shorten the accelerator pump threaded ends and possibly grind a small relief in the valve cover so they don't hit the covers.

Keep us posted.

 

[Chuck]

Rob:

As I was leafing through some old notes which reminded me of one of the reasons I put in the spacers was the clearance issue with the accelerator pumps. How did you massage your valve covers to solve that issue? Would just a hallowed out space in that thick aluminum suffice?

I wonder if it would be inappropriate to use perhaps two or three regular carb gaskets to gain a bit more height. I have a whole bunch of them for just that reason sitting on the shelf.

I have been re-reading Bob Tomlinson's book on Webers. It is the best of the bunch, as far as I am concerned. But I remain curious that he indicates 2 1/2 to 3 1/2 out on the mixture screw is right, while most every one else says only 1 or so.

But I also note that the mixture screw has nothing to do with mixture: that is determined by the idle jet. Mixture screw only controls volume, so it sounds like you want to close the throttle plate as much as possible and control the flow with the mixture screw, but balance the carbs with the idle linkage, so you end up with all or nearly all of the idle flow going thorugh the mixture screw opening. With that in mind I will do a bit more fiddling with that recalcitrant #8, and will double check the points you noted.

Thanks.

 

[Rob]

Chuck,
I set the acc. pumps to optimal settings, then trimmed off the threaded rod ends flush with lever arm. Then used a radius tipped carbide to grind a round "dimple" into the valve cover as a recess for the arm to move into.

Again, I don't think the spacers are necessarily evil, just need to use gaskets and confirm no leaks.

Correct, the screw adjusts volume for a given air flow.

 

[Gregg]

Chuck, I sent you a pm.

 

[Jac Mac]

If you suspect leaks around the carb base etc, simply try to shake/move the carbs with the motor running- if the engine note changes you have found the culprit. Also have you checked the manifold bolt torque- its had a few heat cycles now, might need a retorque ( When Cold Only ) The fact that it was much better after a full warmup at last start tends to suggest this could be problem.

 

[John Oxborough]

Chuck
thanks for yours and everyone elses input regarding your weber issues, it is a great pool of info for when I eventually fire up James' motor (KVA rebuild). When you changed the rockers the valve lash must have been set on primed/loaded lifters, is it possible that you have inadvertently lifted some valves off their seats and the lifters are not bleeding down to enable the valves to seat creating/causing backfires etc.? The engine has had many full heat cycles now, maybe your lifters are freeing off enabling all cylinders to compress and fire, this may be possible as, so far, the carb adjusting and re-jetting has given inconsistent results. Hope you get it sorted.
cheers
John

 

[Chuck]

Today was a good day.

Bought a new MSD. Made appropriate connectors in order to install it without cutting anything. Fired it up. Made no discernable difference. Put it back in the box and returned it. So the MSD is good.

But today, like yesterday, it ran like a champ (compared to a week ago). Idles nicely at 900 RPM. Only a very few back fires and minimal spitting. The only thing that really seemed to make a difference difference between a week ago and now: mixture screws are out 2 full turns and idle jets are 55 instead of 60. (Of course we also replaced the distributor cap, rotor, checked and retightend power connections, checked mag pickup spacing and wires, reconnected coil connections, changed idle jets, cleaned idle circuits, put in new plugs, tightened carb bolts to manifold, etc.)

The last start up this evening after sitting outside in the 35 degree air for three hours it started right up, idled smoothly after less than a minute, and did not spit or backfire once. Figured it was time to quit while we were ahead, so we just pulled it into the garage.

I think it is still a bit too rich. A small bit of popping when let off on the accelerator while driving. May try going to a 50 idle (now at 55).

Cylinder #8 had been a bit fussy, but after blowing out the jets with compressed air it did better. Perhaps a shot of compressed air is better than the carb cleaner I had been using to clear the idle circuit.

This has been a real education. Thanks for the good suggestions and advice!

I can see that dirt could be a real issue. The idle air and the emulsion tube take in air from the top in addition to the main venturis. It would seem that the location of the idle air intake is particuarly prone to dirt. Yet most every one runs these things without air filters. The cover screens that many put over the air intake horn won't do a bit of good for the idle air. Although real filters may not look as good as those 8 shiny horns, it seems to me we are asking for trouble with out decent filters. Filters would also save the back glass from fuel damage from spitting carbs.Am I missing something?


Found these filters at CP Performance. Seem reasonable and look decent.

CB Performance - Online Catalog

Jac: I will retorque the intake manifold. Did it once already, but will check it again.

 

[Randy V]

Nice find on the air filters!!! I looked all over their site for similar air filters for IDAs and found none..

Yes - dirt is the enemy of any carb but Webers seem to be more prone to screwing up when exposed to it..

 

[Mike Drew]

Here are the ultimate air filters for 48IDA-type Webers:

Weber Air Filters

This guy has been supplying all the Pantera shops for years.

Cost is $375 each, I think that includes the K&N filter (so you will need two of them).

Here's a photo of this setup on a Chevy intake:

 

[Rob]

Chuck,
You are totally correct in your observation. The air inlets for the idle circuits are separate from the air horns. I feel the same way about the filter situation. Those are pretty good looking ones. My only suggestion would be to get the largest that would fit. There is one other thing Webers need to function at max performance.........air flow. For example, I lost 25 HPs just by using those socks that I had. That is a huge percentage just for filters.

Sorry I didn't make comment when I read you used carb cleaner. Really don't think there would be enough pressure from the can to do the trick. Glad you thought to do it with compressed air.

What about the vacuum leak potential? Is that ruled out now?

Food for thought...... if she fired right up outside in 35 degrees with no choke, then you are still too rich. Just a thought..... :shrug:

Sounds like you're getting there. Keep the faith man....

 

[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.

Chuck, like I said, try two turns out for starters....

 

[Chuck]

Cliff.

I agree with starting the mixture two turns out. That is what Tomlinson's book says.

But Pat Braden's book says a half turn out, then add quarter turns incrementally. To get the engine to run one has to have the idle stop screw in farther. So the starting point is different.

This turned out to be a big difference in my application.

So my vote goes to the Tomlinson book, which has a lot more discussion of IDF carbs, theory, jet selection, etc.

Chuck

 

[Chuck]

Rob:

You commented:

"Food for thought...... if she fired right up outside in 35 degrees with no choke, then you are still too rich. Just a thought..... "

Good observation. I will give a smaller idle a shot just to see what happens.

Yea, I really don't think there is a vacuum leak.

Chuck

 

[Randy V]

You can use an unlit propane torch to find vacumm leaks as well - just move the torch tip around the base of the carbs - if the engine speeds up - you found a leaker.. Also - since the gas is not a great amount of volume - you would not pose a large fire hazard..

 

[Chuck]

Randy:

Good idea, had not heard before. Perhaps we should underscore the word unlit. I can see some crazy home videos . . . .

 

[EGLITOM]

You can use an unlit propane torch to find vacumm leaks as well - just move the torch tip around the base of the carbs - if the engine speeds up - you found a leaker.. Also - since the gas is not a great amount of volume - you would not pose a large fire hazard..

I´m using brake cleaner for the same purpose. It has the same effect. Of course the gas solution is great becuase it leaves no traces.

TOM

 

[Scott Calabro]

Chuck,

Here is the tune up on my 385 HP 289, its close to what you are running.

11-1 C/R
5.4 Rods
C30Z Solid lifter
12 deg. initial, 34 total @ 3000 rpm
Autolite AR3934 Plugs gapped .035
Mallory Dual Point

44 IDF 71-

36mm Choke
55 Idle
125 main
F11 E-Tube
185 Air corrector
Mixture screws 1 1/2 turns out (Summer above 80F)

It only spits a little when cold (my carbs are equipped with aux venturies but they are not in use) when hot it runs like F.I.

Best,
Scott

 

[Randy V]

I´m using brake cleaner for the same purpose. It has the same effect. Of course the gas solution is great becuase it leaves no traces.

TOM

I used to use Brake Clean as well - but - we have this thing called the EPA over here. They have mandated a change to Non-Chlorinated substances to be used in cleaners like this. The end result is an agent that produces exceptionally toxic vapors when burned.. Nice job EPA.. Also the new non-chlorinated brake clean leaves water droplettes on whatever you just cleaned...