help tuning the Dellortos 40s

[chicagojeff]

All,
Somehow I got it into my head to fiddle with my nicely re-tuned engine because it seemed to be running rich. Needless to say, within 15 minutes I had completely screwed up the balance between the carbs, and just about everything else related to them. So, without a manometer handy, (and because I want to get this thing driveable for the big British Car Union this weekend) I need to get a baseline level of acceptability using just my trusty screwdriver.

The ignition was fine, so what I am really struggling with is an acceptable place to start with the balance screw. My initial settings are about 3-3.5 turns out on every fuel mixture screw, .5 turn out on two of the air bleed screws, and 2.5 turns in from contact on the idle screw. that gets me to a steady but shaking and sometimes popping engine. It also doesnt sound smooth across all four cylinders (big surprise). Sounds more like two dominate.

So, the main question: Once the engine is warm, and idle is about 900 rpm, roughly how many turns in should I go once the balance screw makes contact? Seems that about 3 turns in and the car runs smoother, but then doesnt like to return to idle...Balance screw can't typically be that far in, can it?

I know, I know...it's basically impossible to do without the Carbtune.

One other question...do people have a view about setting the individual mixture screws first, then trying to balance? I suppose taking out two plugs and setting one carb at a time makes some sense... ?

thanks for any help. Jeff

[iain.hamlton]

Jeff, I am not claiming expertise, but this how i do my webers. I find I get I get a better result than with a gunson carbalancer, and dont have to disturb the airbox. On my carbs there is no idle bypass air adjustment, balance and idle air set only with the butterflies and idle mixture screws. If your carbs have an idle bypass, you would have to do this differently.

Get the engine warm
Check ignition and timing - understand the advance curve around idle speed, because too much advance will make things difficult.
Check the Thackery washers. There should be about 40 thou (1 mm) between the coils. Make sure none are broken. If you have the rubber bobbin type washer, the gap between the cups is also about 40 thou.
Check for vacuum leaks aginst the o-rings by gently leaning on the carbs then lifting the airbox. If there is a leak, the idle speed will pick up as the carbs are moved even slightly.

Step 1.
Set your idle speed as close to your idle speed as you can, then add a bit (say 1000 rpm)
pull plug lead #1 see how much the revs drop.
put the lead back
do each cylinder.

Now, if the balance is wrong, two cylinders will drop more than the other two. Adjust the balance to open the butterflies on the least dropping pair of cylinders.

Go to step 1

Once you are closer, adjust each idle mixture screw for maximum revs. You have to go very slowly, 1/8 turn at a time. E.g. turn clockwise. Do th revs rise? turn another 1/8th the same way. wait 3 secs. do the revs rise? etc. If the revs drop, do the other way.

readjust idle.

do next cylinder

when finished go to step 1 and have another crack at the balance.

You end up having to do all this several times, and with luck it will get closer. Close enough for me, but probably not as good as if a properly-equipped expert did it!

Anyway, if you are brave or desperate enough to try, good luck!

(also do a google search - there are similar procedures out there for Dellortos and Webers - both with and without manometers)

best regards, iain

[Dave_Newcastle]

Had what sounds like similar fun to you with my carbs earlier in the summer (dellorto 40s on a plus 2). Im no authority but maybe folowing will be some help.

What I found was that the balance between the two carbs is crucial and the first step is to get that right.

Back off (unscrew) the idle screw completely so that it is not contacting the levers, then you can get the balance screw somewhere close by uncrewing the balance screw until it is clear of the lever on the second carb and then hold both the carbs closed on the levers (the butterflies inside are then closed). Then screw the balance adusting screw in to contact the lever on the second carb.

The carbs will be somewhere close on balance

Then screw the idle in until you have 900 rpm at idle. If you try to tune at any higher speed the butterflies are futher open and allow air in through holes further along the barrels and downstream of the idle jets.

I had set all four needle idle screws at same - about 3 - 3.5 turns.

Having done all that I found that the back carb was doing all the work and if I pulled off the plug leads on 1 or 2 the car would not drop idle speed as noticably as if pulled off 3 or 4 plug lead- in essence running mainly on two cylinders.

So I then adjusted the balance so that the front carb was drawing more air and then reset idle. In all it took about 1/3 turn on the balance screw to get it right so that there was little difference in drop off. I found that listening to each carb through a pipe there was no noticeable change in noise at each inlet.

Then took car for run. initially car felt a bit flat with throttle off, which was rich mixture so weakened on all four idle jets equally. Then had a popping at lift off which turned out to be no 4 too weak. richened that one
only.

Set off and drove to le mans (1500 mile rtn trip) and all was fine.

The above sums up about six hours of adjusting though. No doubt it would have taken an experienced person 15 mins to do.

Dave

[chicagojeff]

Guys,
the combo of your collective wisdom has helped! I got them reasonably balanced now. One port (cyl 3) seems not to behave as the others do--very little drop when the plug is yanked. So that guy aint pulling his weight, so to speak. Nothing strange about the plug. Black, running a little rich, but not radically different than the rest. Could be that the needle is somehow not seating and it is running wide open the whole time? (It's definitely getting spark, I know it's not the ignition wire.)

Jeff

[iain.hamlton]

Jeff, one thing it might be is an inbalance between the chokes of the carb. I am not going to suggest that you twist the spindle, which you might have to eventually, but I'd leave that to someone with the right equipment and lots of experience.

In the meantime, does it respond when you adjust the mixture on that cylinder? If there isn't enough air you cant set the mixture. If there isn't enough fuel (or too much), it wont drop revs when you pull the lead off.

So try a few more revs, and check mixture and balance again.

The whole thing can be very iterative.

best regards, iain

[bill308]

Jeff,

What model DHLA do you have? The early Dellortos had no stamped in suffix following the 40 DHLA id cast into the top of one of the barrels. The early OEM version of the Dellortos (and DCOE's) did not have an air balance circuit fitted in the body. Later 40 DHLA's had a stamped suffix, C, E, H, etc. These models had an air balance circuit.

Initial setting for air balance screws, when fitted (i.e. where there is an air balance circuit), is both closed or seated. This is a relatively fine adjustment meant to compensate for minor air flow differences between cylinders fed by the same carb at idle and just above idle. These differences are relatively small, so one would need a manometer system to really detect this (manifold vacuum) unbalance with some certainty. In use, only one of the built in balance screws should be opened. The purpose of these screws is to increase air flow through the low flowing barrel to match the higher flowing barrel, as it is not possible to decrease the flow of only the higher flowing barrel. Again, only one balance screw and circuit should be opened and only if necessary. Air bypassing the butterfly via the air balance circuit is not seen by the auxiliary venturi so is not automatically compensated for by additional fuel. The other thing is that the built in air balance circuit only works at small throttle openings like at idle or just above idle. If you don't air flow or manifold vacuum measurement capability, just lightly seat and lock the built in balance screws.

The most important balance is between the carbs, and previous posts have addressed this issue. You still need a manometer system to set this with precision, but an SK Synchrometer will work reasonably well to measure and compare flow rates at idle. One can get in the ball park, so to speak, using other methods, but they are pretty much hit or miss.

If you find a cylinder(s) not responding to the mixture adjustment, it may be there is insufficient air flow through that barrel. Try increasing the flow rate slightly to the affected carb using the inter carb balance screw on the linkage and try the mixture adjustment again.

I hope this helps.

Bill

[chicagojeff]

thanks Bill...that all makes sense. I do have the interflow air balance screws...i will see if altering the flow to the apparently non functioning cylinder 3 barrel vis a vis cylinder 4 makes any difference. Both were set (before I fooled around with them) to have the front barrel on each carb slightly opened, maybe 1/4 to 1/2 a turn.

Jeff

[rgh0]

The throttle bypass air screws apart from enabling fine tuning of the air flow at idle between the barrels on a carb also enable the correct positioning of the throttle plates versus the progression holes while being able to indepently set the idle air flow and thus idle speed.

The first progression hole needs to be just upstream of the throttle plate edge at idle. If the progression hole is exposed downstream the progression circuit will come into play and screw up idle speed and mixture settings. If the reverse applies and the gap from the throttle plate to the firdt progression hole is too great you get a stumble off idle.

regards
Rohan

[bill308]

Rohan,

Thanks for your insight. Your point is well taken and I think this is a legitimate use of the air balance screws when required. I do think however that this is rare need. In most instances, the air balance circuit should be reserved for throat-to-throat balancing only.

I am keenly aware of the relationship between the throttle plate and progression holes. The 40 DCOE 18 carbs fitted to my S2 Elan had a factory defect in the location of one progression hole, it was located a little closer to the head side than intended and caused an off idle stumble. The rest of the holes were fine with the edge of the throttle plates just covering the first progression holes. The throttle plate to progression hole relationship can be judged with great accuracy on DCOE?s by removing the brass cap that covers them and viewing the edge of the plate through the progression holes. I don?t think one can do this on DHLA?s as I don?t believe they have a corresponding cover over their progression holes. I guess I?ll have to dig out the DHLA 40 E?s I recently purchased to check this detail. The rework in my case was to file a small chamfer into the edge of the throttle plate, on the head side and about half way through the thickness, that only affected this one hole. This action restored the intended relationship between the hole and the throttle plate edge. All other holes were not affected.

Maybe I?ve been lucky in that I?ve never really needed to adjust the general relationship between the throttle plates and the progression holes. They were always close enough so that a stumble, at very small throttle openings or slow initial depression of the accelerator, was never an issue. Still, if one has a very hot cam or hogged out inlet ports for racing purposes, I guess the ability to bypass some airflow, to allow closing of the throttle plates and still sustain a relatively high idle, has merit. It sure beats the hell out of drilling holes in the throttle plates, which I have seen advocated from time to time. There may be other conditions where this technique would also be beneficial.

Bill

[denicholls2]

I'll make one important assumption: That you didn't fiddle with the balance before you started tweaking a more or less properly adjusted system.

If that assumption is true, then any procedure that starts by having you tweak a setting that "wasn't broke" is well-intentioned bad advice (even though it might be a fine procedure to use from any other starting point.) You needed to fix your tweaks without breaking something new.

I'm with Bill. The balance settings are in most cases pretty much for ultra-fine tuning, or when someone has tweaked a throttle shaft. Rohan is on the money as to their importance in this case, but I suspect it doesn't fit your use case. However, if you have a musical ear and a length of rubber tubing, you can effectively set balance by equalizing the tone of the hiss when the tube is placed in front of each throat.

Shade-tree adjustment of Webers or Dells (once you've elected not to mess with balance) is per the book: Make minor incremental adjustments to each mixture screw until you get maximum RPM -- at this point, any change to any throat lowers RPM. Set the idle correctly, then sit back and bask in your accomplishment, because at this point, you're done with the fixes a screwdriver will allow.

I'm intrigued by the notion of setting idle speed up. While it does provide a bigger scale to play with, it also runs you closer to transitions where the progression holes come into play. I hadn't heard this suggested before. It is important to remember that idle mixture screws only affect the behavior of the engine while running on the idle jet (typically below 3000 RPM), not the rest of the circuits. For example, you can't make your engine run richer above 3000 RPM by playing with screws. Trying to do so will just mess up your idle mixture to match your main jet or transition mixture problems.

Personally, I find that starting with low idle RPM it is much easier to sense the effect of turning the mixture screws than at higher idle. Preference, I suppose.

To your original problem (and the one you're back to), I suspect it had nothing to do with mixture screw settings. On these carbs, the screws represent a very small range of adjustment in a sea of overall tweakability.

I would carefully clean and check the jets for the "weak" throat. If there is lean or rich condition after getting all cylinders to pull their load, I would then swap the appropriate jets and air correctors. This game is expensive unless you have a friend with a drawerful because you need four of everything, but I believe it's what most carbs that need significant tuning really need. Disregard this if you know that the system was tuned by a recognized expert with a dyno and the inlet and exhaust plumbing has not changed since that time, in that case you might want to consider new jets of the same spec for any problem throats.

Advice for the next time around: If you know you will be adjusting mixture and balance screws, start by identifying to the nearest 1/8 turn where each of them is set (these pictures are easy to draw.) This information will suggest (if there's a large difference between the settings for any cylinder) that the previous tuner had some struggles with an abnormality in your setup or not. Then you have to decide if he was dealing with a real discrepancy or was just a poor tuner.

[bill308]

denicholls2,

Thanks for your ind words.

You stated:

"I'm intrigued by the notion of setting idle speed up. While it does provide a bigger scale to play with, it also runs you closer to transitions where the progression holes come into play."

If I read you correctly, you were responding to my:

"If you find a cylinder(s) not responding to the mixture adjustment, it may be there is insufficient air flow through that barrel. Try increasing the flow rate slightly to the affected carb using the inter carb balance screw on the linkage and try the mixture adjustment again."

Let me try and clarify.

There is a minimum air flow requirement for the carb to meter fuel. Below this value fuel will not be properly metered by the carb and the cylinder will not respond to a mixture adjustment. This condition can happen if the throttle valve is fully closed, ususally not resting on the idle speed adjustment screw, as the screw has been withdrawn past the point of contact or the linkage is out of balance to the point where one carb is nearly completely shut down but the other is not. The corresponding spark plug(s) will likely be black and wet because it is not firing at idle and give the indication that it is too rich. The solution is to increase the throttle setting slightly on the affected carb, to just above the threshold, minimum flow requirement at idle. If the mixture can now be set, then one knows the flow to the affected cylinder was a little too low. We're talking about really low air flows here.

In general, idle mixture should be set with the engine at as low an idle as possible or the minimum that will be used for driving. The engine should also be up to temperature and if possible a little on the hot side. If idle is reliable under this condition, plugs should remain clean provided there is not undue oil fouling and the plugs are of a suitable heat range.

I hope I haven't got myself and others into too much trouble here but I learned this lesson while tuning my 308 with 4-weber 40DCNF's. Cylinder 8 was not firing and not responding to the mixture adjustment screw so I increased the throttle opening on the linkage balance screw a flat or 2 and viola, I could now tune the mixture for the number 8 barrel. What I'm not sure about here is whether I could have achived the same affect by opening up the idle balance screw. In the end, I was able to achieve a reliable mixture adjustment and to this day both balance screws remain closed for this carb, although the other 3-carbs have one balance screw open varying amounts.

What I can't over emphesize enough is the necessity of a really good balance for normal driving. Both my S2 Elan and my 308 are very well balanced. I use a bank of mercury manometers, one per cylinder with vacuum fittings on each manifold runner. Use of this type of system takes all the guess work out of balancing air flows at both idle and just off idle. The reward is exceptional torque when launching the car from a stationary condition with minimum application of the throttle. One also gets a view of what happens upon application of the throttle. Throttle shafts distort do to the influence of the return spring fitted to carb shafts, yes just a little, but enough to make a difference, somewhere on the order of 0.5-3.0 cm of mercury. It's the range just off idle that makes a big difference in around town driveability. I find the best way to balance is just above idle by pulling on the throttle cable and watching the columns of mercury move and stabilize. When this balance is as good as I can get it, I then allow the linkage to rest on the stops which I the adjust for idle balance. The linkage is simply not very ridgid in that it will take a set different off idle than at idle. So I set it both ways, off idle first and then using using the throttle stop screws at idle to maintain the linkage set.

Bill

[Sheldon190e]

Hi guys,

Recently installed my dellortos dhla 40s but it just wont rev up. Ive gotten the butterflies matched and air screws set for a smooth perfect idle but as i put my foot down it sounds as if its starving. Ive taken the carbs out about 3 times so far, checked the jets, seals and soaked them for a few days in fuel/carb cleaner and still no luck. What could be the problem?

thanks

[billwill]

I would suspect empty fuel bowls, perhaps your pump is not putting enough pressure to open the float valves.

[Sheldon190e]

I would suspect empty fuel bowls, perhaps your pump is not putting enough pressure to open the float valves.

Ive currently got a 7 bar fuel pump which ive attached a fuel pressure regulator to drop it down to 4 psi. The car idles, but struggles to rev up. There arent any air leaks on the intake as well.

[alan.barker]

Have you replaced the Accelerator Pump Diaphragms and made sure the pivots more nice and free. Also clean and make sure the very little Ballbearing non returns are clean.
Alan