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DHLA 40 setup  Rating:  Rating
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 Posted: 09-16-2012 05:23 am
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sholdowa
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I finally got around to rebuilding my carbs... well trying to as I hadn't actually checked what the carbs were, and took the PO's description as gospel. Anyway, there wasn't much in the dellorto 45 kit that I could use on a pair of DHLA 40's - pretty old ones I think as the serial numbers are 00305 and 00364.

So all I really did was to check the jets were all clean, and tidy them up a bit. One thing that was out of whack was the float level: it was set to about 10mm. I reset it to 15 mm as per the dellorto manual for the 7298.1 float - although I now read that the 907 prefers 17mm. What difference will it make - I won't know for a bit as I'm re-routing all the fuel pipes to come in from the back, not the front of the carbs so I can easily fit an 'original' air filter ( I don't know if it's original or not, I think it's a JH air filter with an Elite air box ). Please tell me if this is a bad idea - I see most come from the front.

The other thing I noticed was that the rear carb uses 55 idle jets on a 7850.3 holder, whereas the front uses a 7850.1 holder. Is this a problem??

Any info on setting these carbs up would be gratefully received. They ran ok until an ignition problem fouled the plugs up - they were NGK BKR5E, now replaced by a shiny new set of BP6ES. Plugs just had a light ( colour and volume ) brown coating on them.

Although saying that,  the car has always used a huge amount of fuel!

Cheers,

Steve

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 Posted: 09-17-2012 11:57 pm
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Jensen Healey
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Hi Steve,

The idle jets and holders control the fuel mixture up to 3000 rpm where the power jets kick in.

The 7850.3 is slightly richer than the 7850.1. This may have been an attempt to solve some issue or just a mistake. Tuning will be difficult or impossible if the carbs aren't identical.

I use the Des Hammell book, "How to Build and Powertune Weber and Dellorto Carburettors" to set-up my carbs.

Kurt

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 Posted: 09-18-2012 10:07 pm
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sholdowa
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Thanks for that Kurt. While I'm trying to source a pair of idle jet holders, I have a Pair of almost new Weber 45 DCOEs sitting on the shelf that I thought I might try.

They're jetted

main: 145
air: 155
emulsion tube F16
idle 55 F9
pump 45
venturi 36

Is that close?

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 Posted: 09-20-2012 03:33 pm
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Jensen Healey
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Sorry, I have no experience with Weber carbs.

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 Posted: 09-21-2012 05:57 pm
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Greg Fletcher
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Webers are not the hot ticket IMHO on the Lotus 907/ Jensen Healey as they do not have the progression in the mid range as the Dellortos do and the 907 needs a lot of progression. If you are running a track car, it's in the high rpm most of time, so not a big deal. If it's a street car and you do a lot of normal driving you will notice it. Jetting will help, but it will never be perfect. That said, many folks don't need perfect and only want to run Webers, so whatever. The specs below are about the best I've seen for the Webers with this application-

Idle Jet 50F9
Choke(venturi) 36
Pump Jet 45
Emulsion Tube F2
Air Correction Jet 170
Main Jet 140

Most of the midrange popping should be gone with that. Also, as with Dellortos, it's important that your fuel pump does not exceed 3-4 lbs max (it should be 2.5 lbs, like the original SU pump puts out).

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 Posted: 11-17-2012 05:01 am
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sholdowa
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I finally managed to get time to strip the carbs off and test the throttle pumps... 1 jet was working!

I've got all 4 working now, but one question. 2 of the pump jets point forwards and 2 ( the inner 2 ) point down. I've read up that the forward pointing ones are newer, but will this mix make much of a difference.

Also, I can balance the pumps before I put it back together but when should the jets kick in???

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 Posted: 11-17-2012 11:58 pm
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Jensen Healey
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The pump jets should squirt fuel immediately upon movement of the butterflies.

Of course, the jets should all be identical. Since the accelerator jets are not in use during normal carb balancing it's more difficult to figure out what's going on with each jet.

You'll need a 4 tube manometer (Motion Pro or similar) to balance the barrels on each carb and then, balance the carbs with each other. 

Kurt

 

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 Posted: 11-18-2012 12:39 am
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sholdowa
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Hi Kurt,

It runs again! Last time it was a pig to start, and after rebuilding the carbs again it just started!

I now have all 4 throttle pumps squirting on first movement of throttle,( one was blocked, and one carb the ball bearing valve had stuck open so I only had one before ), and it idles perfectly at 900rpm all 4 venturis reading 4.8 kg/h ( don't know what that is in real money ) on my cheap and cheerful manometer, and it moves. This is a huge improvement.

However, it's still awful to drive down low... over 3,500rpm it seems much better, but below it's spitting and farting and generally choosing a random 2 cylinders to run on.  I'm not sure how to tune it under load, which is where it's misbehaving.

What next? Please don't make me take the carbs off again!

Cheers,


Steve

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 Posted: 11-18-2012 05:16 pm
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Jensen Healey
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Hi Steve, This is exactly what happened to me when I installed Dellortos on my JH.

It's those idle jet holders that control running to 3000+ rpm.

The order of idle jets from lean to rich is:  .10 .5 .9 .4 .1 .3 .6 .7 .2 .8  Dumb but true.

You should probably source a set of 4 7850.7. These have 6 air intake holes for excellent atomization.

The .1 and .3 holders are similar and can be drilled to match.  The upper fuel holes are 4 x 1.25 and 4 x 1.3 respectively. Enlarge the upper holes in the 7850.1 to 1.3mm.

The lower holes are the air holes. There one 1.4mm hole on both  the .1 and the .3.

If the car still sputters and hesitates you could solder up this hole and re-drill to 1.3mm.

This is exactly the 7850.6 holder! 4 x 1.3 fuel and one 1.3 air.

Kurt

 

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 Posted: 12-07-2013 02:09 am
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sholdowa
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Hi Kurt, you have transformed my car!

She got thrown into a corner after a very exciting (hilly) ride home using handbrake only after the master cylinder failed, and it's only now that summer's arrived that I've got the energy together and am getting it back on the road and legal.

I got some new 7850.7 idle jet holders from alfa1750 on ebay ( http://www.carburatori-italia.com ) who I can't recommend highly enough... indestructable packaging! I also got a couple of forward facing acc pump jets so all 4 venturis are set up the same.

It now starts, it will rev cleanly from cold, with the extended rev range the gearbox is less of a pain to use, and swearing is almost at zero, certainly an all time low.

Looking forward to a fun summer

Thanks,


Steve

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