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h2oman
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I rebuilt my Dellorto's and my engine is running nearly perfectly. It starts easily cold, and runs flawlessly in the driveway. It idles beautifully after balancing it using a Morgan Carbtune.
On the road it runs super well at low throttle, but when I start to open it up, it stumbles. After the stumble, it accelerates through the gears like mad, which was very exciting since it has been 10 years since this car has run like this!. The ignition system, timing, and fuel system are all in prime condition having just restored/replaced most of those components.

I am suspecting my accelerator pump circuits as the culprits. I am looking for a way to check the functioning, and setting of the pump levers. I can see them squirting on all 4 barrels. I have searched the internet and know there is a special device to bench test the volume. Is there a way to test them on the car to ensure they are all putting out an equal amount without the magic graduated cylinders? I also am looking for initial settings of the nuts on the pump levers. I tried adjusting, and test driving, but could not really get it going as well as it used to work. Any pointers would be appreciated.

Bob Waterman
19695 - almost driveable!

subwoofer
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The 45's are large, so a stumble when stabbing the throttle from low revs is almost guaranteed (no vacuum signal to the mains, nor the idles). The 8.4:1 compression ratio does not really help either. I have found that the best way to avoid the stumble is simply to keep it on cam. Rev it before launching so you are above 3k at all times.

Works for me, anyway... :-)

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Joachim

h2oman
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Joachim -

Thanks for the prompt reply. I have noticed that once it is reved up, no problem. I know that this setup can be made to work better because it used run perfect with the same setup. This is why I believe that it is related to the work I did on the carbs rather than something I have to live with due to carb size. FYI, I have 36mm barrels with effectively a Lotus Spec9 jetting, except my pump jets are 60 which is biggish compared to what others have. Also I have flat top cosworth 10:1 forged pistons.

I understand that the stroke length determines the volume of fuel delivered and the pump jet determines the time it takes to deliver it. I really think I need to figure out if they are all working the same, and then whether I am lean or rich. I have assumed lean, but that is only a guess.

Has anyone tried to measure the output of the pumps in any way? There is a chance that I goofed something up when rebuilding these, but I want to diagnose it without tearing them both apart again. the pump stuff is not easy to access.

Thanks,
Bob

subwoofer
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Best (in reality the only) way to determine what is happening would be to attach a wideband O2 sensor in the exhaust, so you can see what it does. $250-ish for sensor, driver and gauge from diyautotune.com

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Joachim

Jensenman
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Dells (and for that matter any carb) should have an immediate 'pump shot'. This means when you look in the venturis with a light and move the throttle slowly, you should immediately see fuel from the discharge nozzle.

As a rule, if the accelerator pump is 'late' you will get a backfire along with the stumble. Since you don't mention a backfire, chances are the pumps are working normally and as mentioned venturi size is the culprit.

You can also get a stumble if the ignition advance is not coming in early enough. If you are still using the stock dizzy that came with the Strombergs and are not running the advance from 'ported vacuum' then you might bump the base timing about three degrees or so.

Greg Fletcher
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You should have zero hesitation during the transition from the idle to main jets. What are you using for idle jets? I find that pump jets closer to 45 give good performance if everything else is set up correctly.

h2oman
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Thanks for the response Greg -

My carbs were purchased new in 1991 from Dave Bean as a kit with the manifold and 2 carbs. They are labeled DHLS45 on top, and appear to have 5 progression holes. I had a Lotus/J-H specialist who did my engine rebuild back then, help with the initial setup and jetting. He used settings close to the lotus spec 5:
36mm chokes
158 mains
58 idles
60 pumps
7772.8 emulsions
230 airs
7850.9 idle emulsions
This setup ran well for about 10 years when the car began running poorly, missing and I had to change careers so it was parked until my recent rejuvenation work.

I have experimented with the pump stroke which changed the hesitation, but did not eliminate it. Others have pointed to the lean idle jet holder and suggested going to a .7 to richen it up. I live at 2000' ft elevation, which richens up my mixture somewhat anyway. The hard part is that messing with jets is somewhat trial and error, and I live out in the sticks and have limited cash. Dave Bean is up the road about an hour, so I might see if they have parts.

Bob

Greg Fletcher
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Your settings are going to put you on the rich end of the scale (more fuel is not necessarily better). As a comparison, here's what I use for a 2 liter 907 and I find this set up is as close to perfect as you can get if you're running a stock 2 liter. Great performance and good fuel economy as well.


Carb Type ...................... DHLA 45E
Choke .............................. 34 mm
Main Jet .......................... 145
Main Air Corrector Jet ... 190
Main Emulsion Tube ...... 7772.5
Idle Jet ............................. 55
Float Needle Valve ......... 170
Pump Jet .......................... 45
Starter Jet ......................... 60

If it ran fine before, you might also just have a plugged passage in the carb body. A disassemble and ultrasonic cleaning overnight will usually fix that.

Jensen Healey
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Hi Bob,

You'll need those lean idle air correctors with that idle jet.

I agree with Greg, if it worked before, it probably just needs a thorough cleaning.

Sorry I can't help with the pump adjustment.

Kurt

h2oman
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Thank you everyone for the advice. Since it is running close to perfectly, with only a slight hesitation now that I increased the accel. pump stroke, I have decided to drive it as is for now. With so many other high priority items to work on, this seems like an ok path.

The other exciting news is that I have actual headlights again, including hi-beam, instrument lights, side markers, etc! I did add relays and an inline fused line to make it extra spicy. No more off, dim, or flicker!

Now I just need a passenger floor pan, new front suspension bushings, tires, fix the leaky diff seal, fix the heater, and redo the interior plus a paint job, and I will have a car to drive. Ok, I can use it without the last 3 items, and probably will.

Bob



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