Triple carb install = won't start!!??

[Phred]

Benn9090

Your Comp. & flow #'s are not uncommon for a street engine with some miles on it. On the downward intake piston stroke, with the throttle plate at idle position, a vacume is created. this is what your reading with the flow gauge. If one cyl. is less effecient than another, or a slight vacume leak exsists, these numbers will vary. simply average and balance them as best you can. You can minimize the difference when setting up the carbs on the bench. With each individual carb, back the idle screw out all the way. Hold the throttle plates closed, and look through the carb bores while holding up to a light. You should not be able to see any light sneeking past the throttle plates. If you see light passing by one plate and not the other, loosen the two throttle plate retaining screws, and lightly tap on them with the back of the plastic screwdriver handle untill they both "seat" into the throttle bore. Retighten, and check with the light. Repeat if needed. This easy adjustment will assure you that both throttle plates of the carb are getting the same amount of movement (flow) when the throttle arm is moved. Now, any differential readings on the same carb will indcate an intake leak or the cylinders inability to draw air equally.

Phred

[benn9090]

Thanks Phred - I reseated the throttle valves and that helped the Syncing a little. I don't think I had a vac leak at the o-ring gaskets (I sprayed ether at the joint and the engine didn't do anything).

Jetting advice anyone???

-Sync'd with Synchrometer

-Idle mixture at 3/4 turn out

Jets are "stock", as listed in Haynes manual

Chokes 30mm

Aux venturi 4.5

Main 1.30

Emul tube F2

Air corrector 1.75

Idle jet 0.50F9

Accel pump jet 0.45

Accel pump inlet valve with exhaust orifice 0.55

Needle valve 2.00

Starts fine. Must progress with throttle VERY SLOWLY to keep from dousing the spark. If you push the gas down a little too fast it bogs and burbles. If you push faster it actually stops running for a second (but doesn't backfire). This is my first DCOEs, but it sounds like pump jets.

Plugs looked reasonable - but they have seen perhaps 45 minutes cumulative of idling, 15 minutes of cruising, and only 10-20 shifts and transitions where it burbles and bogs.

Thanks for any help,

Mark

[HS30-H]

Hi Mark,

Happy 4th of July to all you rebels over there. I have to say that I feel we really ought to have a National Holiday over here in the UK to celebrate the fact that you dumped us.............

Sorry to hear that Webers are akin to Unicorns in your neck of the woods, but actually it seems that people are frightened of these little jewels the world over............

The problem that you describe certainly sounds like an Accelerator Pump Jet problem ( too much gas being dumped in when you hit the throttle ) - but that might be due to something else. I am wondering about your Accelerator Pump Rod stroke................

Your Needle Valves also sound a little large ( not that this would necessarily affect this particular problem ) and the standard size in the rebuild kits is 1.75. Maybe you might consider changing them back in the future........

From your photo, I can see that your old brass-float "Flat-Top" ( pre-emissions type ) 40DCOE-18's are just perfect for a mildly tuned L24 / L26 or even L28. However, I hope that this is a "before" picture, and that you have rebuilt / cleaned / restored them by now! I also hope that you have re-attached the Starter Device to that centre carb - as its missing in the photo. Weber themselves even recommended the 18 series as the best choice for the L24 / L26 engine. Your jetting sounds just about optimum for a BASE LINE setting - but do remember that each individual engine is different, and a proper rolling-road dyno set up would probably change some of those jets to fine tune your particular needs. You CANNOT do this off the page and expect to be spot on. Remember that these sidedraught carbs are PRECISION measuring devices, and just a single step up or down in jet or air corrector sizes will make a very noticeable difference to your state of tune. Like I say - it needs an expert to set the final percentage. You have to figure the cost of this into the conversion. I took my last built engine ( which I thought was running pretty good ) to a dyno tester and we put it on his rollers. He ran it up and we saw a peak of 158 bhp at the rear wheels, but it sounded a little "fat". He told me to go and get a coffee and come back in half an hour. He changed the accelerator pump jets, the mains and the air correctors ( leaning the engine out through the Accelerator Pump stroke and through the transition onto the Main circuit ). Final figure was 167 bhp at the rear wheels and much improved driveability. I could not believe how the car was TRANSFORMED on the drive home; it really sounded crisp and seemed to be another engine altogether. As I say - the changes were very small indeed.............

In your particular case, I am wondering of you are trying to put the cart before the horse - so to speak. It seems, unless I misunderstood, that you have not yet got your full exhaust system hooked up to the car. If this is indeed the case, then its much too early to judge anything yet. If I misunderstood, and you have in fact now got the full system connected - then it might be worth checking your Accelerator Pump Rod stroke ( thats the little rod thats got a little hook on it and attaches to the Accelerator Pump Piston. It is pushed up by that little cam connected to the spindle, under the cover at the "back" of the body ). The recommended Pump Rod stroke is 0.394 ins. ( 10mm ) and you can measure this by taking off the top cover of the carb and actuating the throttle. You can measure the actual stroke of the rod. This Rod stroke is the thing that measures the duration and timing of the Accelerator Pump shot ( not the amount of fuel it pumps - which is governed by the size of the Accelerator Pump Jet itself ) and it makes a HUGE difference to the driveability of a carburettor conversion on a particular engine type.

If the Pump Rod stroke is any more than 10mm then I recommend that you buy new ones and change them.

All this "advice" I am giving is deeply ironic; my present car is running Mikunis! They just seem more "correct" for the Japanese-spec. theme of my car. However, I do have to say that they are not superior to a good set of early DCOE's in the performance department. The Mikunis do not have such a good "transition" as the Webers ( when the main circuit takes over from the accel. pump ) and this is probably connected to the design of the Accelerator Pump itself.

These carb postings really give you good typing practice........

I wonder how Eric B is getting on?

Happy jetting all,

Alan T.

[benn9090]

No, its true I don't have the full exhaust hooked up. I was a little afriad to turn over my 6-barrel flamethrower to the exhaust shop with no brakes. I have unspeakable brake problems, but as a brake engineer I'm hoping I can fix them without requiring assistance from Europe .

I would be happy to take my car to a dyno and have an expert such as the one you describe work on my car - unfortunately, I don't believe such a Weber expert exists for more than 1000 miles.

I did some road testing at fixed RPMs, but the results were inconclusive. I'm afraid the ultra-hot Jacobs ignition is burning the plugs clean even though I feel am richer than hell.

I should say, my needle valves could very well be what you describe (1.75 v. 2.00) - to clarify, those parts that are MARKED I checked with the manual and found them to be the same as those (hence alleviating me from having to write them down somewhere).

EricB is probably eating and socializing with family for the July 4 celeb!!!

Some pics to follow.

-Mark

[benn9090]

An example of my plugs after idling for 5 minutes. Idle mixtures at 3/4 out. Resultant action taken was 1/4 turn in on #6 (left most) and 1/8 turn on #5 (second from left).

[benn9090]

5 mins running at 70 mph/3200 rpm - plugs oriented such that darkest color faces camera.

I also took pictures from 4th gear part-throttle acceleration, which burbles and barely accelerates ... they didn't come out, but they looked exactly like Haynes manuals tell you a healthy spark plug should look - though I feel it must have been rich as hell!

-MB

[benn9090]

5 mins running at 70 mph/3200 rpm - plugs oriented such that darkest color faces camera.

I also took pictures from 4th gear part-throttle acceleration, which burbles and barely accelerates ... they didn't come out, but they looked exactly like Haynes manuals tell you a healthy spark plug should look - though I feel it must have been rich as hell!

-MB

[byunique]

I have had the 40 webers and currently run the 44 mikunis on my motor. I have had these carbs for over 10 years and have spent countless hours playing on them. Our engines are similar; I have the schneider cam, N42 head, similar displacement but a header but not much difference there. Your initial settings jet settings for the carbs look about right as far as a baseline goes. If anything, there is no reason it should bog. Generally, with these carbs the smaller chokes create a faster velocity airflow in the engine. Going to larger and larger chokes usually creates that bog. Larger choke sizes create the largest volume of air for high rpm max power, but are a detriment to off idle 0-6000 rpm response. That 30 choke should have no bog whatsoever.From your posting, I understand the problem you are having is with the popping. This has happened to me upon acceleration/deacceleration because of having a leaness in the idle circuit. This popping is coming from the carb right, as opposed to the exhaust. If it is related to the exhaust, then you have to hook your exhaust up and see how it runs. Your 3/4 turn out of the idle mixture screw sounds very suspicious. My carbs have always had 1 1/2 to 2 1/2 turns. The fact that your plugs are burnt is also a indicator of leaness The idle mixture circuit has a huge affect upon driveability, much more than I ever though. I always think of the idle circuit of having responsibility from 0-3500rpms in conjunction with the accelerator pump and main circuit. As far as accelerator jet and stroke go, if there is too much stroke or the pump is too rich, this usually causes a rich condition, but not so rich as to ever cause a bog. It is just one of those circuits which is not that sensitive to change.

I would check out the idle adjustment. Turn them out and keep on turning them out until the motor reaches the highest idle speed, once the motor starts slowing down, then turn them in until the idle picks up again and leave them there.

As far as jets go, I think you may go to 1.50main/2.00air at maximum, and depending how many rpms you want to run, you may want to go with a larger choke size down the line. But for now, you shouldn't be having major problems even with your current setup. Did you ever just blip the carbs with gas in them and watch the gas shoot out of the carbs? I know you have done a lot of work on these carbs, and perhaps something in the accel pump is out of alignment. With everything setup right, you should see a steady stream of gas shoot out of the carbs. And each barrel's stream should be very consistent.

Just for reference, here are the settings on my 44 mikunis, keeping in mind my max rpm is 7300, and I have a 2.8 motor

Main: 160 Air: 210

Idle: 57.5

Venturi: 37

Accel Pump: 50

Idle Adjust: 1.5 turns out

[benn9090]

Alan- Just measured my accelerator pump strokes at ~9.5 mm, close to the 10 mm specified for the DCOE18s.

Byunique- I *think* I read somewhere out on the Web (Redline? Carbs Unlimited?) that ~1-1/2 turns out is indeed correct for some series of Weber 40s - bit I fouhnd a spec for my 40DCOE 18s suggesting 1/2-3/4 turns as a starting point, and that is about where they idle best.

My popping is out of the exhaust and happens on overrun - but again, I have just a sawed off head pipe at this time. I have never gotten a backfire out of the carb - well, at least not since I fixed that huge dizzy advance vacuum leak!

Thanks guys - I appreciate you hanging in there with me. We'll get EricB running right too.

-MB

[Phred]

Benn9090

Hey, those carbs look great! What kind of finish did you apply to them? I used to build airplane engines too, and we used Alodine to protect bare aluminum from corrosion. Looks similar.

As far as the poping goes, that could be your distributor. Since you were fiddling around with the vac. advance, there may be something else not right in there. I'm not up on vac. adv. units as we always disable them for racing. However, I have cured some Z track cars that stumbled and popped by going to a locked breaker plate, and using only mech. adv. setting total advance to 34-36 degrees. I'm not advising you do this, I mention this only to point out theres more than one way to skin a Z.

More to the point, there are a number of sensor/gauges on the market that will take the guesswork out of what your doing. K&N among others, has a air/fuel ratio monitor, or a device called a Lamda indicator which uses oxygen sensors to indicate air/fuel ratio(ideally around 14.7:1) Or a good old Exhaust Gas Temp. gauge. (EGT) We use EGT and fuel flow to determin proper Mixture settings on an engine dyno. Hang in there. I think your about to become the local Weber expert in your area.

Phred

[benn9090]

I think I found something. It looks like my main jets have been soldered and redrilled --

[Phred]

Benn9090,

I assume you have already ordered a new set of main jets.

The makers of jets have always said never to drill jets. I used to figure they just wanted to sell more jets. But every time I run into

drilled jets, they never seem to be running right. Hope that cures your problems.

Phred