lotustwincam
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| posted on 20/7/06 at 08:22 PM |
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Stumbling Dellorto DHLA's
Wondering if any of you nice people can offer any useful advice with this problem.
Engine is recently built (400 miles on it) 1660cc Xflow, Kent 244 cam, approx. 11.25:1 CR, Big Valve Head and twin 40DHLAs.
All in all running quite well tho I've yet to give it the full welly - but has transformed the car compared to the clapped out 1300.
However, there's still some work to be done. Biggest problem at the moment is a distinct stumble which can last several seconds when quickly
opening the throttle to about the 3/4 open position. If the throttle is opened smoothly the engine will accelerate fine. Jab it open to about quarter
or half way and its fine.
Ha, I thought. Just needs bigger pump jets. Currently on 35's, the consensus on and off line seems to be that it needs 40's. Now these
don’t come cheap, but in the end decided to pay the little extra for the genuine article from a well known dealer who advertises on eBay.
Got the jets today, in a little plastic grip seal bag, slightly tarnished and not a single marking on them to indicate the size or that they are
originals. The only indication of the size is hand written on the bag.
Question 1. Would you accept these jets?
Anyhow, I decided to put them in the carbs anyway. They made not one jot of difference. So now my dilemma is whether the problem is elsewhere, or are
the jets the size they’re supposed to be?
Question 2. Dellorto’s have an external rod mechanism that activates the pump jets. Should the jets me squirting fuel for the whole length of the
accelerator stroke? Mine do not, but the adjustment just seems to control when the squirting starts. Are there different spring rates available for
the rod mechanism? Having said that, I don’t think that this is the problem. Since the squirt from the jets last a while, I’m sure they’re still
supplying fuel at least at the start of the stumble phase.
Question 3. Anything else that might me causing this problem? Am I barking up the wrong tree?
Thanks
Drew
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matt_claydon
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| posted on 20/7/06 at 09:07 PM |
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Mine did the same thing on sharp throttle opening. Just tightened the nuts on the pump rods a bit so it pumped harder each time you opened the
throttle and all is well. Probably increases consumption quite a bit as every time you move your foot you pump loads more fuel in but it's loads
more fun to drive!
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lotustwincam
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| posted on 20/7/06 at 09:49 PM |
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I've tried playing with these nuts 
If I run the nuts up it just gives me a later delivery of the pump jets. This gives a huge flat spot (engine dies like trying to accelerate a cold
engine without choke) low down, as the pump jet no longer operates at small throttle openings.
However, it doesn't make any difference at the top end whatsoever.
Did you notice if your pump jets would supply fuel throughout the full throttle travel?
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Deckman001
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| posted on 20/7/06 at 10:29 PM |
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A trip to a rolling road should sort the problem, the place I went to once near Heathrow had all the jets and chokes you could ever need .Redline
tunning was the name, I beleive they have moved to the other side of the M4 now
Jason
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matt_claydon
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| posted on 20/7/06 at 10:46 PM |
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quote:
Originally posted by lotustwincam
If I run the nuts up it just gives me a later delivery of the pump jets. This gives a huge flat spot (engine dies like trying to accelerate a cold
engine without choke) low down, as the pump jet no longer operates at small throttle openings.
I'd say you need to screw them the other way then so they pump right from very small openings (= more total fuel when going from closed to open
throttle aswell). That's what I did and it sorted things out nicely.
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02GF74
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| posted on 21/7/06 at 08:23 AM |
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it does sound someting amiss with the pump.
these aren't the most precise system - but the manual describes how you test them; from what I remember, you set them up so they deliver the
quantity specified; can;t remmebr how you adjust them. they may be avaialbe in different sizes -dunno.
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lotustwincam
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| posted on 21/7/06 at 08:16 PM |
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Jason - Redline would have to move beyond the M4 and to the other side of the Irish Sea to be any good to me! I will eventually get the car rolling
roaded, but with only 400 miles on the clock it's just a tad early yet.
Matt - I've tried the nuts at both ends of the extremities, fully screwed up, fully screwed down, somewhere in the middle - no major difference
apart from the dead spot when screwed up to far.
Whats the likelyhood of this being a Rich stumble, and it's taking a while for the excess fuel to burn off?
Also, does emulsion tubes effect the sudden acceleration phase or does the fact that the engine accelerates smoothly when done gradually rule out the
other jets?
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