MSD Ignition or Weber Carb Issue?

[EGLITOM]

Chuck

on motorcycles i always do my sync @2500 rpm. In my opinion the important output of a perfect synchronisation is having the same vacuum signal in each carb at the transition from idle to the midrange. So i always assure that the throttle slides create the same signal at app 2 -3 mm opening.
Usually the idle sync ( at almost closed sildes app 1mm) is ok than also.

I had some cases where i did an idle sync ( almost close slides) , but than experienced not so smooth transitions as with the "2500 rpm " method".

This can happpen due to different adjustments of the idle mixture screw( on most motorcycle carbs it is an air adjusting screw).
This air adjusting screw even if turned in or out the same are always flowing differently from one carb to the other.

By doing your snyc at a higher rpm range you take out the importance of the idle mixture screw. Once synchronised it is pretty easy to adjust the idle mixture screw by hearing to the changes of the engine.
ON a 4 cylinder bike even when you change the adjustment of this screw just on on carb the bike will rev higher or lower. So one does one carb by the other finding the position where the bike revs the highest and by turning more out ( more air leaning out) start top drop RMP, this is usualy also the adjustment where the bike turns back to idle rpm the fastest without droping down below and coming back or without taking forever to come down.

TOM

 

[Chuck]

From the picture you attached - the overhead shot of the setup, the two little hexbars coming off the center are not parallel...therefore, I would expect that they will open the throttles at different rates. Is this a "before modification" picture or an "after modification" picture?

Mike:

Does this look better? Had a bit more clearance than originally thought - enough to add some nylon washers to make the connecting arms more parrallel. Looking at the pic, perhaps another thin washer could be added, space permitting. Also added washers where the arms connect to the bell crank raising those connections a 1/16 " so they lie on the same horizontal plane.

Thanks for the observation.

Chuck

 

[Chuck]

Dean, Ian, Tom:

Thanks for the good tips. Synching the carb at 1200 - 1500 makes sense. May start out synching them at idle then do a final adjustment at higher RPM. I am guessing that the sync meter I have will only go up to about 1200 to 1500 before it is near full scale.

Weather permitting, hope to fire it up Sunday.

Dean: less than three weeks to Putnam Park!

 

[Chuck]

<B style="mso-bidi-font-weight: normal"><U><SPAN style="FONT-SIZE: 14pt"><FONT face="Times New Roman"><FONT size=4>Weber Carburators Revisted, Part II<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com

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The linkage reinstalled and throttle plates set, we moved on to the float chambers. The top of each carb was removed. The fuel was drained (using a vacuum pump for bleeding brakes) and the carb bowl dried out with compressed air. We wanted to make sure there was not a trace of dust or dirt.
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A simple tool was fabricated from a short piece of eighth inch aluminum rod to check the float levels. A notch at 10 mm and 32.5 mm made it easy to check the float levels in the manner described in several books, including Pat Braden’s Weber Carburetors. Here is were it got a bit interesting.
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One of the floats was set at 13 mm. Two were set at 9 mm. Only one was properly set at 10 mm. We made sure all four were exactly on 10 mm before reassembling them. Perhaps the 13 mm carb was running a bit lean contributing to our problems.

In the open position, none of the floats were close to the 32.5 mm spec, so we adjusted them all. I suspect this dimension is much less critical than the 10 mm adjustment.
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This little exercise further confirmed that one can’t assume the builder of this crate engine would check the float levels.
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The gas pedal was reset to align its position in relationship to the brake pedal. This had been on the 'B' list but needed to be done to make the 'heal and toe' positioning appropriate. The cable was then adjusted so when the gas pedal was fully depressed the throttle plates were just shy of fully opened.

 

[Mike Christian]

Mike:

Does this look better? Had a bit more clearance than originally thought - enough to add some nylon washers to make the connecting arms more parrallel. Looking at the pic, perhaps another thin washer could be added, space permitting. Also added washers where the arms connect to the bell crank raising those connections a 1/16 " so they lie on the same horizontal plane.

Thanks for the observation.

Chuck

It looks awesome Chuck!

 

[Randy V]

Chuck - I have to take off my hat to you...

You have no idea how many people these days - even "experienced automotive technicians" (what ever happened to mechanics?) that are absolutely **TERRIFIED** of carburetors..

GREAT WORK!!!

 

[Chuck]

Thanks Randy. Those that don't know about GT40s think I am nuts. Come to think of it, those that DO know about GT40s think the same.

 

[Chuck]

The day started out cold and rainy, so headed to the office. Then the sun came out. So came home. Decided to start up the GT and see if all the hours of work this past week would make a difference.

I am not yet ready to declare victory, but I think it is close.

Went through the initial start up proceedure, following the guidelines in Pat Braden's book. A half turn in on one idle stop screw. Mixture screws out one turn. Pump the accelerator twice. Start it up. Barely idles, running like a tractor until it starts to warm. Then start balancing the carbs. As they balance out the idle picks up a bit, settling down around 600 RPM. None of the idle stop screws are opened more than a half turn.

But two cylinders ar not firing, Numbers 2 and 4. That infrared remote thermometer is so handy! So we open up the mixture screws another turn (two total) on the offending carbs and they fire right up. Now were are running on all 8 cylinders and the RPMs are up to about 800.

A bit more balancing and the rearward throat of the four cars are setting right at "5" on the synch meter. Then check the foreward throats and all but one are within a fourth of a unit on the meter. Cyllinder #7 is at 4 on the meter, a full unit away from the others.

So we open the air adjustment screw about a turn and bring ithat carb up to five. Recheck everything. All eight cylinders are within a needle width of the same reading.

Time for a ride.

Drive about 10 miles. Accellerates VERY well. Runs steady at 2500 rpm, then 3000 rpm, then 4000 with virtually no popping. On the whole trip heard less than five pops, and they seemed random, not related to anything in particular.

We did not make much of an effort to adjust the mixture screws. My impression is the synching the carbs is most important and that the mixture screws are of lesser significance. We will fiddle with them a bit more next time.

So is the fact two mixture screws need to be out 2 turns while the others are out one a big deal? We are running a 55 idle jet. Maybe need to go up to 60.

We recheck the synch when we get back. It is spot on.

Shut down. Within a minute or two we hear the melody of boiling fuel in first one carb, then a second. Carb 7-8 (rear left) is the worst. I wonder if that is just a characteristic of the Webers. My impression it is the heat rising from inside the engine that starts the music of boiling and dripping.

All in all, it was a good day.

 

[Ian Clark]

Hi Chuck,

This is good news, congratulations. I had to "learn" the webers because nobody in the GTA including Ferrari mechanics could get the 48IDAs running properly on our 392 stroker. Now you know all the finessing details that allow these carbs to do their magic. So do all the other viewers of this thread, bravo.

I've always maintained 90% of Weber issues on V8s is mechanical, not carb related, you'd still be farting in the wind (or popping and back firing!) without getting the linkage/syncronization issues resolved along with blueprinting the carbs.

I can just imagine how big the smile was on your face.

One more trick to stop the boiling in the carbs, shut off your fuel pump(s) a block or so from home or where you plan to stop and park (making sure you won't run dry in traffic of course). This will drop the fuel levels so there's very little to percolate. Also I like to throw open the rear body to let out standing heat. Always draws a crowd which is fun too.

On restart be sure to allow time for the float bowls to refill. If the motor is still quite warm, it's best NOT to pump the throttle before cranking. That's good for a carb fire (don't ask me how I know!).

Great stuff, glad it's working out for you.

Cheers

 

[Mike Christian]

From my experience Chuck, it sounds like you are doing some things out of sequence. First is air bypass screw, mixture screw, idle screw adjustment, and carbs sych'ing at idle - all this with linkage disconnected. In that way you know that the linkage has no influence. Then you attached the linkage to a known good condition and work the linkage so that it doesn't change what you already know to be right and correct.

So is the fact two mixture screws need to be out 2 turns while the others are out one a big deal? We are running a 55 idle jet. Maybe need to go up to 60.

In general, your mixture screw positions should be closer than one turn from each other.

I wouldn't change the jets of one or two of the carbs. If you engine is popping - evidence of running lean AT IDLE, then think about changing idle jets. It doesn't sound like it is.

We did not make much of an effort to adjust the mixture screws. My impression is the synching the carbs is most important and that the mixture screws are of lesser significance. We will fiddle with them a bit more next time.

They are both equally important - think air/fuel ratio - that is what you are balancing. You are trying to match that ratio in each barrel.


Sounds like fun!

 

[Chuck]

Mike:

Maybe I dont' have the concept right.

My understanding was that the air / fuel ratio was determined by the idle jet size. Larger size, more fuel in the ratio. The mixture screw then has nothing to do with mixture: it actually determines the volume of the mixed air / fuel.

At idle all, or nearly all, of the mixture comes out the mixture screw port. The throttle plate should be as close to closed as possible, consistent with balanced carbs. So the mixture screws determine how much fuel / air gets in. But as soon as the throttle plates open, the progression ports add the pre-mixed air / fuel to the equation. Therefore air / fuel ratio is also determined by the size of the idle jet even off of idle as the progression ports are reached by the throttle plate.

So it would seem to me that the mixture screw is obviously critical at idle, but once the progression ports start kicking in just off of idle in the low RPM range, the mixture screw setting would be of less significance. It could have some affect at low RPM, but the effect would diminish as the RPM's increase.

But having said all that, the balance between the carbs is very critical. If one is pulling more or less it leads to all those nasties like popping or backfiring throughout the low and mid driving range, from idle on up. Hence the observation that balancing the carbs is perhaps more critical than 'precise' mixture screw settings at idle.

Is my analysis / logic flawed?

Chuck

 

[Chuck]

Ian:

Your tip regarding shutting off the fuel pump is noted. We have been doing that, but will let it run a bit longer to get rid of more fuel.

But since you noted that tip, I take it boiling fuel is a fairly common characteristic of these carbs? I am guessing one just puts up with it?

I have read other threads suggesting that adding a shield / turkey basket does not help. More ventilation does not help. The implication it is heat rising from inside the engine through the throttle body is the culprit.

Chuck

 

[Mike Christian]

Mike:

Is my analysis / logic flawed?

Chuck

I don't think so Chuck! I think you have got it - except I would like to make one clarification.

Here is a nice discussion of weber carb operation on a Porsche site (it referes to IDAs, but the same principles hold true for the IDFs I believe):


Weber carburetor theory


Notice at idle the throttle plates are not fully closed...air gets by them. The fuel/air through the idle jet mixes with the air past the throttle plate to make the total fuel/air getting to the combustion chamber. So, the fuel/air metered by the mixture screw affects the fuel/air mixture where it counts - in the combustion chamber. Crank it open and you can tell you are running richer - shut it down and you can tell you are leaning - you are adjusting the total Fuel/Air ratio.

This is most critical - like you say - at low idle. Once the throttle opens, the other parts take over and the mixture screw is additive.

That is my understanding anyhow.

Regards,
Mike

 

[Mike Christian]

Ian:
But since you noted that tip, I take it boiling fuel is a fairly common characteristic of these carbs? I am guessing one just puts up with it?

It sounds like it - I found this at Club Cobra:


Fuel leak question [Archive] - Club Cobra

If you read far enough down, it could be related to the fuel you are buying in this season! Interesting.

By the way - I found this thread here - in case you haven't already found this archive:

http://www.clubcobra.com/forums/archive/index.php?f-127.html

more light reading....

Mike

 

[Mike Christian]

Excerpt form teh thread that I thought was interesting:

"When a hot engine is turned off, heat builds up in the engine compartment. This causes fuel to rise in the carburetor bowl and drip down from the venturi boosters into the engine. The engine is now flooded and is hard to start.

The next is what we call “geysering” and happens on a running engine. The elevated temperature of the pressurized fuel in the line entering the carb bowl past the needle and seat causes the fuel to boil violently. The drop in pressure from the line to atmospheric in the bowl causes this. The fuel “geysers” up out of the bowl vent tube.


There are two times of the year when these problems are greatest for our carbureted engines. One is in the late fall and the other is in the early spring. These are periods where winter fuels are in use but summer temperatures can be present. This is because winter fuels boil at lower temperatures than summer fuels. The problem is worsened as the vehicle goes up in altitude, because the boiling temperature of gasoline, just like water, goes down as the altitude is increased.

Unlike injected engines, carbureted engines have the fuel at ambient pressure in the carb bowl before entering the engine. Remember, the higher the fuel pressure, the higher the boiling temperature. The fuel is under pressure up to the carb. But, once the fuel passes the needle & seat into the bowl, it is at ambient pressure. Here is where we experience violent vaporization or boiling. We even see it geyser up out of the bowl vent tube. But if we cool the bowl with a damp cool rag the problem stops immediately. The fuel bowl doesn’t even feel hot when this problem occurs. This tells us that the boiling temperature at this altitude for the fuel is just a little above ambient. So if we subject it to summer temperatures, the problem occurs. It all has to do with fuel volatility.

Volatility is a measure of the gasoline’s tendency to vaporize under a given set of conditions. Reid Vapor Pressure (RVP) is a commonly used indicator of gasoline volatility. Boiling is a violent version of vaporizing. The higher the RVP, the greater the tendency to vaporize or vapor lock.


We don’t have this problem with our race gas on the hottest days in the middle of the summer. So let’s look at RVP. Our 110-octane race gas that we blend down has a RVP of 6.6pounds per square inch (psi). Our summer unleaded 91-octane gas that blend with race gas to lower the octane of the race gas with has an RVP of about 9psi. That is if all the remaining winter gas has been used up from the gas station’s tank.

This is because the RVP changes during the year. The summer gas delivered from the refinery has the 9psi RVP from mid April to mid September. After September, it starts to rise monthly till it gets to a max in December & January of about 15psi. Then it starts coming down again till April. The higher the RVP, the lower the boiling temperature. The addition of ethanol can also lower the boiling temperature.

So why does the gas from some stations boil worse than from others? If the April gas is in the tank truck but there is still some March gas in the station tank, the gas you get will have a RVP higher than the 9psi for April through summer.

If you don’t run a fuel with a low RVP, the best thing to do if the vehicle has been running hard and is shut down for a while is to open the hood to let the hot air out of the engine compartment. If the vehicle is prone to geyser while running, like at high altitude climbing up to the Eisenhower Tunnel, a simple method, that may sound rinky-dink, but really works, is to carry a cold wet towel and put it around the carb bowls when the problem occurs. This works every time on our dyno at 7500-foot elevation. Following is a chart of RVP classes and which is in use at different times of the year. These apply for Colorado.


Class RVP Max 10% Boiling Temp F 50% Boiling Temp F
Class A 9 158 170 to 250
Class B 10 149 170 to 250
Class C 11.5 140 170 to 250
Class D 13.5 131 150 to 250
Class E 15 122 150 to 245


Ethanol raises the RVP about 1 psi.

Blend by month: Jan E Feb E-D Mar D-C April C-A May thru Sept15 A Oct B-C Nov C-D Dec D-E

A worse case would be a very warm day in January when the fuel starts to boil at 122deg or lower with ethanol.

Insulating lines and carburetor will help buy time till the problem occurs and may solve the problem in marginal cases. Replacing a mechanical fuel pump on the engine with an electrical pump close to the fuel tank can reduce the fuel line vapor lock problem. The mechanical pump acts as a heat source. Hot fuel with vapor can be pushed much easier than pulled as an up front pump does.

In a nutshell, cool the fuel or raise it’s boiling temperature.
"

 

[Scott Calabro]

Wow,

Reed Vapor Pressure.... reminds me of A&P school.....

Too bad I can't get 115/145 anymore.

Good discussion SpyderMike!

This has happened on my car too but it has never been a problem as I always turn the pumps off to burn off some fuel before engine shut down.

Also its happened in the summer with summer fuel :huh:

I have purchased Weber P/N 99005.122 heat insulator gaskets to see if this provides some relief also.

S

 

[Mike Christian]

Scott,

Sorry for the double post..I don't know about the validity of the argument, but it sounded interesting. This looks a recurring problem using the webers on the big american V8s, looking at the club cobra postings.

I wonder what the italians did to prevent this?

Cheers,
Mike

 

[Randy V]

Scott,

Sorry for the double post..I don't know about the validity of the argument, but it sounded interesting. This looks a recurring problem using the webers on the big american V8s, looking at the club cobra postings.

I wonder what the italians did to prevent this?

Cheers,
Mike

There were percolation issues (that's what they were called) for many years before there were blended fuels.. They actually had what was called anti-percolation valves on carburetors.. They used to run exhaust heat up through the intake and through the base of the carburetors to improve the cold weather running.

I also switch off the fuel pumps on my cars prior to killing the ignition for these one key reason. Percolation.

BTW - I deleted the duplicate post..

 

[Mike Christian]

BTW - I deleted the duplicate post..

Thanks!

How did the anti-percolation valve work - just a simple check valve?

Mike

 

[Randy V]

Thanks!

How did the anti-percolation valve work - just a simple check valve?

Mike

Hi Mike,

There were a number of different methods that addressed the percolation issue. It's a malady thats been around since the dawn of automotive time.

There were two different versions that I'm aware of;

1) Old time way 1 - there was a valve on top of the float bowl that was essentially a little rubber disk that just covered a hole in the float bowl. The arm that controlled the valve was connected to the throttle linkage and opened the valve when the linkage was in the idle position, closed it just off idle. This vented the float bowl to the atmosphere.

2) Old time way 2 - the valve was entirely enclosed and worked quite similarly to the old time one - except - instead of venting to the atmosphere it vented to the fuel pump's integral fuel filter chamber.

3) The EPA way - the valve was entirely enclosed and worked quite similarly to the old time way 2 - except - instead of venting to the fuel filter it vented to a Charcoal canister under the hood.

There was actually a pretty good discussion of this malady in regard to Webers (IDA's specifically - but IDF's would also share the same issues and potential resolution);
What is the Weber 40IDA Anti-Percolation Modification - Pelican Parts Technical BBS

Note that there's no 1 way to cure it - but the best way I know of without a lot of additional work is to just shut the pumps off as you're pulling into the driveway and then idling the engine until it falters.