Weber IDF. Again

[Chuck]

We thought we had the Webers about nailed. The last several driving sessions have been good. A few spits at start up, then idles great, accelerates reasonably well, no coughing or spitting at steady speeds. Very driveable, at long last.

Today Ryan took the GT out and the Webers were back to their old tricks. Backfiring, spitting, coughing, both when driving steady and when accellerating. Ugh.

Put the flow meter on and there was a slight variation, which we adjusted, but it still ran bad. One carb was not firing so we opened the mixture scew a bit and it took off. Drove it again, and no real difference.

So I have two ideas:

1. Weather. When it last ran well the temp was around 75 to 80 and the humidity was comfortable. Today it was 95 degrees and the humidity was near 100%. Could that could have made the mixture rich across the board contributing to the bad running? If so, does one have to adjust the tune based on the weather? Surely not . . .

2. The linkage, I suspect, is an off shore cheap brand provided by the engine builder. The slightest turn of the connectors makes a noticeable difference, thus it is so easy for them to get out of adjustment perhaps just due to temperature variations from cold to hot. I am wondering if there are different brands of linkage on the market, perhaps with much finer threaded adjustments so more rotation of the connecting arms would be required. Any one have any experience with different brands of linkage? A picture of our linkage is attached.

When I have suggested "Holley" in the past Ryan has stood firm. Tonight even he seems to be giving in . . . .

 

[Bob Putnam]

I doubt whether the ambient temperature is a factor. I suspect the problem is in the ignition system (coil or distributor mostly), not the carbs but...

Check the fuel delivery (pressure and flow) to the carbs.

 

[David Briggs]

Fuel pressure is critical, 3 1/2 max, take the idle jets out and blow some air thru the internal passages of the carb, verify float levels are correct.

 

[Jim Rosenthal]

If you DO decide to get a Holley-type carburetor, get a ProForm or QuickFuels. Everything in them is replaceable/adjustable- all the orifices can be changed out, which Holley does not offer. I have one on my Kirkham Cobra and it has been terrific. (a 650) And get a manual choke. We took the electric choke off, wish I had not gotten it to begin with.

 

[Chuck]

Jets are clean, floats are exact, linkage is tight, the only change is the extreme heat and humidity.

I am wondering if we have too rich idle jets. It did run OK before the weather changed, but perhaps it was at the edge of too rich. The 20 degree temp rise may have just been enough to push it over the edge. To rich might explain why one cylinder stopped firing at idle (which quickly resumed firing when the mixture screw was adjusted).

When I pulled the plugs a week ago six of eight were black, suggesting that it was rich then. I turned in the mixture screws a bit, but perhaps a smaller idle jet would be appropriate.

I may also take a look at the linkage that Ingelese sells to see if it has finer thread adjustments on the bell crank connectors. The China made ones that came with the engine are somewhat coarse making adjustment very sensitive.

I may try something simple, just swap out the idle jets for the next smaller size.

Appreciate your thoughts and comments.

 

[Scott Calabro]

Chuck,

As always YRMV.

To me, the links in your picture just have to be the same length.

They are not used to "sync" the carbs other than making sure all four are opening and closing the butterflies equally left and right.

You are experiencing what I spoke about in the earlier thread on this subject.

As local atmospheric conditions change, so will your mixture settings.

I think this is because each clyinder is completely separated from each other, unlike a common plenum or 180 deg 4bbl setup that tends to mask this issue.

Did you install the "Jet Doctor"?

Have you utilized the by-pass screws to sync the slave barrels?

If you want, call sometime and I can get more specific on what really works on my setup.

Regards,
Scott

 

[Mike Christian]

Backfiring and coughing are signs of a lean condition, not rich, aren't they? A rich running setup smell rich, blows rich out the pipes and is just sluggish.

When accelerating, you are running off the accel pump and not the idle jets. Have you looked at the squirt being injected in the throat to see if they are all giving the same volume. You don't have to have the engine running, but while pumping hee throttle, verify about the same squirt in each barrel. It should be failry obvious of one is not up to the others.

Personally, I have never adjusted my webers (different car with dual setup) for any seasonal variations - and it gets hot (100 degrees +).

How much did you close the one barrel's mixture screw to get it to fire?


Mike

 

[Chuck]

Scott:

Thanks for the input. The linkage is exactly the same length, Indeed that was one of the issues we corrected that led to them running well - before the weather changed.

Sounds like you concur that the temp extremes may be a factor perhaps aggravated if they were on the rich side before?

Yes, I have adjusted the air screw on the 'slave' barrell. We had all eight throttles synched literally within a needle width of each other on the gauge. (Although they were off a bit when Ryan finished his ride, but we brought them back into synch, and it still ran poorly).

No, I have not added the Jet Doctor.

I am tied up the rest of the week, but before this heat wave ends I may try leaner idle jets. Can't hurt.

And Jim, thanks for the advice on the Holley type carbs. But if I went that route we would miss all those fun times standing over a hot engine adjust mixture screws!!

 

[David Sturch]

Chuck,
A little tip. I found back in the day when I played with Webers that you could save a lot of money by buying only one set of jets and a hand chuck and a numbered set of drill bits. Drill, test and do a plug cut to see if you are rich enough. Your linkage look's fine to me. The main problems I contended with were seals at the butterfly bearing and pitted low speed idle air screws. (older rebuilt units) Once they are set up you should have no problems. I would check your ignition timing also to make sure it's on.
Dave

 

[Jac Mac]

Scott:

Thanks for the input. The linkage is exactly the same length, Indeed that was one of the issues we corrected that led to them running well - before the weather changed.

Sounds like you concur that the temp extremes may be a factor perhaps aggravated if they were on the rich side before?

Yes, I have adjusted the air screw on the 'slave' barrell. We had all eight throttles synched literally within a needle width of each other on the gauge. (Although they were off a bit when Ryan finished his ride, but we brought them back into synch, and it still ran poorly).

No, I have not added the Jet Doctor.

I am tied up the rest of the week, but before this heat wave ends I may try leaner idle jets. Can't hurt.


And Jim, thanks for the advice on the Holley type carbs. But if I went that route we would miss all those fun times standing over a hot engine adjust mixture screws!!

That last sentence indicates that 'your' condition is terminal and while you may enjoy brief relapses of pleasureable motoring, long term it looks like you have a long life of trying every brand of Dr Webers magic snake oil on the market to look forward to!

 

[Al Jones]

Chuck - Is that Quickfuel 680 looking better and better? For all this trouble you could have gone with a Dynatek system and had the look but only the setup to contend with,not an ongoing situation. Yes,I know the Webers are good when right and match the 'originality' of the car, but seriously,is it much of a hassle to slap a 4bbl on there and see if that really IS the problem? You can always go back to the Webers when everything else is dialed in. Or there are the Autolite in-lines. A.J.

 

[David Sturch]

Chuck,
This is what I will be using on a DCOE intake and a brain to control them.
Dave
Products: Throttle Bodies

 

[Cliff Beer]

Chuck, nothing wrong with going up or down jet size here and there to just see what the difference is on the butt dyno. Just go up or down one size (rather than more than one) and take a few notes on each configuration. If the webers are functioning properly then there's a very limited range of jetting which will get them to run properly based upon your displacement, cams, heads/valves, etc. - maybe just +/- one jet size or other setting (etube). It's helpful to have a decent sized kit of various jets/tubes. Before installing a different set of jets, check to make sure each jet is the same size physically (people often drill jets and therefore your 55 is no longer a 55....) in addition to marked the same.

Also, take a look in the throats of your IDFs and make sure you have the same progression holes in the throat next to the butterfly. I've purchased supposedly "matched sets" of IDF/IDAs before only to find none of the progression holes match across the set and therefore that set ain't gonna run right all together....

If you just can't get the webers to run right then pop a Holley on there, get the engine humming ie. make sure there's no ignition issues, and then go back to reinstalling the webers - just my $.02 suggestion.

 

[Jim Rosenthal]

I used the ProForm 650 and an Edelbrock Performer Air-Gap on my Cobra engine. The Air-Gap was probably a waste of money- it adds riser length and according to my friend who had quite a lot of experience building race motors, in the rpm range I am using the engine the extra intake length isn't helpful. I am, though, very impressed with the quality of the ProForm carburetor- it is made much better than a Holley and all the tuning adjustments were very helpful (of course they get to sell you a lot of jets and so forth, but it WAS very easy to tune and set float levels, etc etc. Lots of fun.) All their parts are machined aluminum and they don't leak.

I love the way Webers look but I bet that of all the stuff we commonly put on GT40s the two things we have the most headaches with are Webers and cooling system problems. I am already looking around Annapolis for an IDF guru and the car isn't even painted yet.

 

[Chuck]

Would a tiny oil leak at the back of the intake manifold have anything to do with a carb vacuum leak?
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At the rear of the intake manifold, at the joint with the top of the block, is a depressed area in the block that collects a bit of oil when we go for a drive. It is not much. Never really gave it much thought. Until today. Then it occurred to me that if oil comes out, perhaps air can get in. Perhaps that could have something to do with why the Webers have been hard to tune?
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So we fired up the engine. Got it good and warm. Then squirted a bit of carb cleaner. It did not make a noticeable difference in the way it ran, but we could see what looked like air bubbles. (Can’t say whether due to pressure of vacuum). Then shut off the engine, gave it another squirt, and it did not seem to bubble. Of course it evaporated very quickly on the hot engine, so this little test is certainly not scientific and may not mean much of anything.
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Having never seen the under side of the manifold used on this set up I am hoping someone more knowledgeable than I (which is probably most of those reading this) can tell me if indeed a leak along the back joint of the intake manifold could account for a vacuum leak affecting the carb tune. I am guessing that it should have nothing to do with the carbs or if it does we have much more serious problems.
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The pictures identify the area.

 

[Ian Clark]

Hi Chuck,

Geeze I thought you had it nailed. The oil leak is a PIA but won't affect the carbs as the valley area of the block is separate from the heads and intake ports. We have seen manifolds that needed intake gaskets stacked to make up for either planed heads, decked blocks or improperly machined port face dimensions on the manifold itself.

If you simply put on a set of new gaskets, silicone seal the valley mating surfaces and bolt her down, you could be warping the manifold casting at the heads or never getting a good seal on the gaskets. Now I always check fit and port alignment with one or two sets of gaskets before final assembly.

However, not knowing this resulted in an engine being torn down and rebuilt because of smoking and high oil comsumption, everyone of course knew it had to be valve seals and or piston rings. Turned out to be poor surface contact at the lower edge of the intake manifold gaskets to open valley area. The engine was sucking oil into the intake ports through the gasket. That was a real expensive gasket set!

Your problems at elevated ambiant temperatures could be related to fuel temperature. Without a heat shield around the Webers the back pair of carbs will hit 160f at low speeds, very close to the boiling point of gasoline. Time for a heat shield my friend.

About the linkages, higher quality (ie no free play) rod ends would be nice although it is possible to set the free play so that all the lash is taken up simultaneously. This way you come up off idle smoothly, avoiding lean spits and rich backfires. It's generally safe to assume that if you're getting spitting and banging together you have a syncronization and or linkage issue.

In your previous thread about Weber woes, I brought up the point about linkages. Sometimes it's difficult to explain the nuances of the adjustment process without making it sound like an SAE paper. There are lots of ways the adjustments can be wrong and just a little wrong really upsets Mr Weber.

Regarding equal length of the cross links on the center bell crank, that is only possible and valid if the manifold was machined properly. If the center post bolt hole supporting the bellcrank is not exactly between the two banks of carbs you will not achieve or benifit from equal length cross links in this setup. The only way to know that is blueprint the manifold (prior to assembly).

That really doesn't matter if the linkages have to be somewhat different so long as ALL carbs come up off idle equally. It's the initial transition from idle to on throttle that causes most of the problems. If all throttles open equally just off idle then it won't matter above idle either so long as you don't have one bank of carbs leading the other enough to cause imbalance. This can happen and is aggrevated by mis adjusted carbs linkages at idle rest.

I hope this helps, I can identify with Jim's plight of looking for a local Weber expert, we had to get a handle on the Weber V8 setup because nobody in the GTA (Greater Toronto Area)was willing to take it on. Over 90% of Weber problems are adjustments, not the carbs themselves.

Cheers

 

[Chuck]

Ian:

Thanks for the good tips.

The wheels are off to the painter, so no driving at the moment. But while up on blocks we fired it up. Confirmed balance, adjusted mixture. Runs good throughout the RPM range. We will get it out on the road when the fresh wheel paint is dry . . . . .

Not going to worry about that tiny oil leak for now. Will pop off the intake manifold and re-seat it in the near future.

And got some ideas on shielding as well . . . .

Chuck

 

[Randy V]

Curious about the oil leak....

Could be possible that the intake gasket may have shifted during installation?

If so - you may be sucking air from the lifter valley in one or more cylinders..

Using alignment dowels (long Grade-3 bolts with heads cut-off and screwdriver slots cut in them for removal) are sometimes the answer to getting the manifold lowered into place precisely where it needs to be...

 

[David Briggs]

I always use studs on the intake, then you don't have the issue involving mis-alighnment.
Dave

 

[Chuck]

Randy, Dave:

Good points. Will likely reinstall the manifold shortly, time permitting. I am suspecting the oil leak is due to lack of sealant along the rear valley rather than a gasket issue, but then won't know for sure untill we check . . . . Thanks.