Dellorto Trouble
Discussion
Following a recent engine rebuild on an '84 TE, I experienced engine backfire when the engine was switched off. I returned the car to the mechanic who proceeded to rebuild the Dellortos, and he installed smaller jets while doing so. The backfire has now quit (although it did not do so immediately leaving me to question the installation of smaller jets) but my mechanic advises that the carbs are still "loading up" at idle/low speed. He is now very confused and asked that I post the issue to see if anyone had some advise. The syndrome occurs regardless of whether the engine is cold or warm. I had noticed that, prior to engine rebuild, the RPMs advanced to 1500 - 2000 on cold start and remained there for 1 - 2 minutes before droping to normal idle. The engine no longer does this but the mechanic does not feel that the RPM item is related. He is stumped so any advise would be most welcome!
Russ
Russ
russell.berry@ve said:
Following a recent engine rebuild on an '84 TE, I experienced engine backfire when the engine was switched off. I returned the car to the mechanic who proceeded to rebuild the Dellortos, and he installed smaller jets while doing so. The backfire has now quit (although it did not do so immediately leaving me to question the installation of smaller jets) but my mechanic advises that the carbs are still "loading up" at idle/low speed. He is now very confused and asked that I post the issue to see if anyone had some advise. The syndrome occurs regardless of whether the engine is cold or warm. I had noticed that, prior to engine rebuild, the RPMs advanced to 1500 - 2000 on cold start and remained there for 1 - 2 minutes before droping to normal idle. The engine no longer does this but the mechanic does not feel that the RPM item is related. He is stumped so any advise would be most welcome!
Russ
Russ,
It's not a matter of the jet size, it's a mis-adjusted idle mixture, or improper float height which is causing your problem.
On your rebuild, did you change anything from stock such as a cam or boost setting?
If anything, the idle jets on a stock turbo are too small already. Stock jets were 55's, switching to 58's makes the car better. I am running 62's in my '85 Turbo, but I also changed to the 104 cam, increased boost from 7.5PSI to 10PSI and am running an 8.5:1 CR vs. stock 7.5:1CR. I went as high as 65's, but couldn't get a good idle with these and the car Backfired because it was too rich and I couldn't dial the richness down sufficiently with the mixture screws using the 65's.
How is your mechanic tuning the carbs? Dellortos are one of the easiest carbs to tune, but they have a very particular procedure to follow which is different than webers, strombergs or SU's. I suspect your mechanic is using either a carb synch or an exhaust gas meter to tune the carbs. This is the wrong way to tune dellortos. The carb synch is waaay too inaccurate and interferes with the very airflow you want to measure. The EGM will only average the exhaust gasses once they combine in the manifold and exhaust pipe. Doing so means that errors between the carb barrels can cancel each other out so far as the meter is concerned, but one barrel could still be rich enough to cause the backfiring. Worse yet, it means that one cylinder is running lean - very bad for a turbocharged car!
Assuming that the carbs have been rebuilt or are in good shape (emphasis on good), you need a 4-pot manometer to properly tune them. Either an actual Hg manometer, or a manual one such as the CarbTuneII. Make sure the float levels are between 14mm & 14.5mm which is nearly double what a Weber float is set at.
Turn all air bleed (or air balance screws) closed and turn down all the mixture screws, taking great care not to grauch the needle screws.
Then turn out each mixture screw 3.5 turns (this will be sufficient to start the car). Start the car and let it warm to operating temp., set idle speed screw until car maintains 1000RPM.
With the manometer connected to the test port on each barrel, turn each mixture screw while observing the manometer level and the tach. Turn each screw until the idle increases and level in the manometer reaches it's maximum, then turn the screw the opposite way 1/16th to 1/8th turn (allow about 5 sec. between screw adjustments so the new metering can take place). Do this for all 4 mixture screws, all the while using the idle speed screw to maintain 1000RPM. You will now have each barrel set to it's optimum mixture, yet the levels of each barrel will not necessarilty match each other.
Next, working on a single carb (doesn't matter which), maintain 1000RPM and observe which of the two barrels reads a higher level than the other (this is the strong barrel). Open the air bleed screw of the stronger barrel until it reaches the level of the weaker one. Do this for both carbs, maintaining 1000RPM. When done, you will have only one air bleed screw per carb open and will have two pairs of equal levels, but not necessarily equal to each other.
Finally, using a 7mm wrench, adjust the carb balance screw between the two carbs so that the stronger of the two pairs of manometer levels drops to match the weaker one (the reason being that you can weaken the strong one, but cannot strengthen the weaker one).
Assuming that your carbs are in good order, no leaks, leaky soft mounts, clogged filter and proper regulator pressure, your carbs will now be perfectly balanced and the backfire should cease. If not, look for leaks in the exhaust system starting at the manifold and everything to the tailpipe. Hope this helps. Happy Motoring! ...Jim'85TE
PS Not to cast aspersions on your mechanic, but if he, or anyone, says that Dellortos are like Webers or Weber Clones, than it's a pretty good bet that they don't really understand Dellortos. Dellortos and Webers are both side-draught carburettors, but that's about where the similarity ends. Dellortos are unique and do require different methods to properly tune them. If they have gotten good results in the past using an EGM or CarbSynch, they merely stumbled into it.
>> Edited by lotusguy on Tuesday 29th June 21:18
Once again Jim, thanks for the advise. We did not change the cams, boost or set anything to specifications other than factory (except for the jets). The mechanic admits that he is not an expert on the Dellorto carbs which is why he asked me to post the question. I'll pass along your comments along with some other timing information that I tripped across that may apply (seems that lots of folks believe that ignition timing should be advanced well beyond what the factory recommends).
Best
Russ
Best
Russ
russell.berry@ve said:
Once again Jim, thanks for the advise. I'll pass along your comments along with some other timing information that I tripped across that may apply (seems that lots of folks believe that ignition timing should be advanced well beyond what the factory recommends).
Best
Russ
Russ,
The 907/10 engine loves lots of advance, about 31° is the max. You need to add the centrifugal advance (16° I believe) to the static advance for the max.
Variations between engines (tolerances, age, condition of other areas such as valves, carbs etc.), mean that you'll want to play with it a little.
My car runs best with 12° static advance (vs 1° manufac. spec.), which when added to the 16° of distributor advance totals 28°. The car runs fine up to a total of about 30°, but seems a little more smooth at 28° for my engine. Happy Motoring! ...Jim'85TE
Jim.. I have some difficulty understanding why you say the Weber and Dellorto are not similar. I have spent a lot of time tuning Webers over the years and a little time with Dellortos. From almost every point of view, that I can see, the Dellorto is fundamentally the same in form and function as the Weber with a few changes and or improvements. The Dellorto is essentially a 60s update of Eduardo Webers brilliant design.
The circuits of each carb operate essentially in the same manner as the other, and in tuning I have found similar results (once you figure out the dellorto jetting numbers).
Sure the Dellorto may be able to be more finely tuned for road use and emissions, but for most of us we dont have the equipment to make full use of that tuning feature.
What am I missing? Jan
ps: I do acknowledge the setup procedure mentioned above.
>> Edited by f1karting on Wednesday 30th June 18:09
The circuits of each carb operate essentially in the same manner as the other, and in tuning I have found similar results (once you figure out the dellorto jetting numbers).
Sure the Dellorto may be able to be more finely tuned for road use and emissions, but for most of us we dont have the equipment to make full use of that tuning feature.
What am I missing? Jan
ps: I do acknowledge the setup procedure mentioned above.
>> Edited by f1karting on Wednesday 30th June 18:09
f1karting said:
Jim.. I have some difficulty understanding why you say the Weber and Dellorto are not similar. I have spent a lot of time tuning Webers over the years and a little time with Dellortos. From almost every point of view, that I can see, the Dellorto is fundamentally the same in form and function as the Weber with a few changes and or improvements. The Dellorto is essentially a 60s update of Eduardo Webers brilliant design.
The circuits of each carb operate essentially in the same manner as the other, and in tuning I have found similar results (once you figure out the dellorto jetting numbers).
Sure the Dellorto may be able to be more finely tuned for road use and emissions, but for most of us we dont have the equipment to make full use of that tuning feature.
What am I missing? Jan
ps: I do acknowledge the setup procedure mentioned above.
>> Edited by f1karting on Wednesday 30th June 18:09
Jan,
One fundamental difference between Weber DCOE's and Dellorto DHLA's is the idle circuit and the way it operates.
Weber DCOE's were originally designed exclusively as a Race carb. As such, it was designed from the standpoint that the car would spend very little time at idle. The transition from the Idle circuit to the Mains is accomplished with a set of calibrated progression holes drilled into the carb body and taking adavantage of the famed Bernoulli effect where negative pressure is created by the airflow demanded by the engine which draws fuel through these precision drillings until sufficient negative pressure to work the Mains is achieved. A consequence of this is the somewhat infamous 2k-2.3kRPM stumble inherrent in most Weber applications.
Because of the power they produced and the ease in servicing (given the other available carbs of the time - 1920's) and re-jetting, as well as the practice of Club Racing, Sports Car owners started putting Webers on their Street Machines. The harsh idle was pretty much lived with, although a few brave souls randomly modified them for a better idle by drilling the progression holes larger. Since this practice was pretty much determined by the type of engine, it was pretty much a game of guesswork. And, once you enlarged the drillings, there was little going back.
Dellorto DHLA's on the other hand were essentially created for the Street, to fill the niche left by poor Street performing Webers. As such, the idle circuit was given much more thought and instead of transitioning through a series of progression holes, the idle circuit was made to fill a dual function. It never really cuts out, rather it adds to the Mains once they are operating. The Idle circuit provides for the transition. This is why they are easier to tune for idle. Truth be told, the DHLA isn't as good a Race Carb as the DCOE, which is why you find most tuners using Webers when prepping a Race Engine. The no cut-out idle circuit robs significant pressure from the total operation and so Webers will actually make a little more power when All-Out.
As far as any special tools for tuning Dellortos, all you need is a 4-pot Manometer, either the Hg Type or a mechanical one such as the Morgan CarbTuneII, which sells for less than $60 delivered from the UK, not that big a deal.
The Dellorto already has a 6mm Tapped Test Port cast into every barrel. You either remove the blanking plugs and screw in the adapters w/ 'O' ring each time you tune, or, as I have done, keep the adapters in place and cap them with a urethane Vacuum Blanking Cap (couple bucks at any Auto Parts Store). Vacuum pressure holds them in place. This way, I can hook up the CarbTuneII and tune the carbs in less than 5 min. even on the roadside.
I grant you, the DHLA's and DCOE's are very similar, but the differences are pretty major and fundamental. I am an excellent Dellorto tuner, but only a so-so Weber tuner, although I eventually get it right. In truth, the simplicity of the Dellorto deserves the credit. Happy Motoring! ...Jim'85TE
>> Edited by lotusguy on Wednesday 30th June 22:43
Hi Jim.. Thanks for the added insight. Interesting thought on the idle circuit.. gets me thinking..
..I imagine that a Webers' idle cicuit could also contribute to the mains delivery, similar to that of the Dellorto, after all, the progression holes are in the lower pressure airstream, and there is little to stop the fuel flow thru the idle circuit. I know that the accelerator pumps in the Weber do bleed fuel at WOT, hence they slightly enrichen the higher rpm fuel mixture. So it stands to reason that the same may occur with the idle circuit. I guess in most instances the contribution of these "other" circuits would be small, and to try and account for it would only complicate things for the average tuner. I suppose knowing their effect can be an added tuning benefit when trying to tweek between main jet or air corrector sizes.
I have found weber idle circuit tuning to be no real problem. I know that there are a few models of Weber that were not good road carbs, but I would say that most of the sixties and newer engines that had OE Webers, like Alfa and so on, used models that were quite road tunable.
I love working with the Weber (as you do the Dellorto). The Dellortos have the added benefit of being able to individually balance each barrels' airflow (as you describe).. (which is apparently used on the emissions version of the weber as well).
keep tuning... Jan
..I imagine that a Webers' idle cicuit could also contribute to the mains delivery, similar to that of the Dellorto, after all, the progression holes are in the lower pressure airstream, and there is little to stop the fuel flow thru the idle circuit. I know that the accelerator pumps in the Weber do bleed fuel at WOT, hence they slightly enrichen the higher rpm fuel mixture. So it stands to reason that the same may occur with the idle circuit. I guess in most instances the contribution of these "other" circuits would be small, and to try and account for it would only complicate things for the average tuner. I suppose knowing their effect can be an added tuning benefit when trying to tweek between main jet or air corrector sizes.
I have found weber idle circuit tuning to be no real problem. I know that there are a few models of Weber that were not good road carbs, but I would say that most of the sixties and newer engines that had OE Webers, like Alfa and so on, used models that were quite road tunable.
I love working with the Weber (as you do the Dellorto). The Dellortos have the added benefit of being able to individually balance each barrels' airflow (as you describe).. (which is apparently used on the emissions version of the weber as well).
keep tuning... Jan
Hi folks (changed my login for a more descriptive "handle"). Anyway, my mechanic has now given up on the carbs. He says that no matter what he does, the thing is still "loading up" at idle and he says he has tried everything he knows how to do, including following the generous replies that were provided in response to my post. Does anybody know of someone in the Northern Virginia area who might be able to have a look?
TKS
Russ
TKS
Russ
dr.hess said:
What's the fuel pressure? What's it supposed to be?
Dr.Hess
Hi,
You strike on a key point. The fuel pressure is a two-stage regulated affair. The fuel pressure s/b 3.0PSI off boost and the boost actuates a vacuum motor which increases the fuel pressure to 5.0PSI on boost.
You need to be sure that all vacuum connections are correct and tight or it may be possible to have the regulator on 'High' boost pressure all the time. Hope this helps... Jim'85TE
Russ.. a few suggestions...
Check to make sure that there are no problems with fuel leaking past the starter/choke circuit. Perhaps the starter circuit linkage or inner workings are hanging up and do not shut off completely. Even a small amount of leak-past the starter circuit would cause an incurable rich idle.
Check to make sure all the jets are all seated properly - dont over tighten them.
Install new needles and seats. Fuel pressure could be an influence especially if combined with worn needles and seats.
Check float levels and also make sure the floats "float" and dont fill with fuel.
If you check fuel pressure, the FPR should normally deliver about about 3-4psi at idle. If the float needles and seats are good, they should be able to handle double that before leakage would become an issue at idle. If you find the reading at idle is too high, try checking for restrictions in the fuel return line before making adjustments.
Even though this has no bearing on your idle-rich problem... On a blow-thru turbo carbed car, like we have, the FPR is designed to deliver about 3-4psi of fuel pressure at all times. The challenges with designing a blow-thru configuration, is that under boost, the plenum is subject to boost pressure, however, the float chambers are not. So, like they have done on our cars, they solved that problem by piping boost pressure to the float chambers to match the plenum, so that the carbs are operating in a pressure balanced environment. The next design challenge is the FPR, it now has to overcome the float chamber boost pressure in order to deliver fuel. What they have done to overcome that, is to run a boost signal from the plenum to the FPR, so that the FPR will "read" the boost and deliver a net of 3-4psi to the float chambers. In other words.. if you have a 10psi boost signal, the FPR will deliver 13-14psi to the float chamber giving a net of 3-4psi. Next design issue was making floats so they wouldnt collapse under boost pressures and making the carb top covers air-tight. Which seems to have been quite well sorted by Dellorto. Oh.. the fun of it all.
A bit of trivia.. Lotus actually had Dellorto modify the design of their carb specifically for the Esprit Turbo project. So, if you see a Dellorto used in any other blow-thru turbo application.. it came from Lotus' Esprit program.
hope this helps..Jan
Check to make sure that there are no problems with fuel leaking past the starter/choke circuit. Perhaps the starter circuit linkage or inner workings are hanging up and do not shut off completely. Even a small amount of leak-past the starter circuit would cause an incurable rich idle.
Check to make sure all the jets are all seated properly - dont over tighten them.
Install new needles and seats. Fuel pressure could be an influence especially if combined with worn needles and seats.
Check float levels and also make sure the floats "float" and dont fill with fuel.
If you check fuel pressure, the FPR should normally deliver about about 3-4psi at idle. If the float needles and seats are good, they should be able to handle double that before leakage would become an issue at idle. If you find the reading at idle is too high, try checking for restrictions in the fuel return line before making adjustments.
Even though this has no bearing on your idle-rich problem... On a blow-thru turbo carbed car, like we have, the FPR is designed to deliver about 3-4psi of fuel pressure at all times. The challenges with designing a blow-thru configuration, is that under boost, the plenum is subject to boost pressure, however, the float chambers are not. So, like they have done on our cars, they solved that problem by piping boost pressure to the float chambers to match the plenum, so that the carbs are operating in a pressure balanced environment. The next design challenge is the FPR, it now has to overcome the float chamber boost pressure in order to deliver fuel. What they have done to overcome that, is to run a boost signal from the plenum to the FPR, so that the FPR will "read" the boost and deliver a net of 3-4psi to the float chambers. In other words.. if you have a 10psi boost signal, the FPR will deliver 13-14psi to the float chamber giving a net of 3-4psi. Next design issue was making floats so they wouldnt collapse under boost pressures and making the carb top covers air-tight. Which seems to have been quite well sorted by Dellorto. Oh.. the fun of it all.
A bit of trivia.. Lotus actually had Dellorto modify the design of their carb specifically for the Esprit Turbo project. So, if you see a Dellorto used in any other blow-thru turbo application.. it came from Lotus' Esprit program.
hope this helps..Jan
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