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problems with dellorto dhla c


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I bought an 02' and wasn't happy with the big ugly thing they called a stock carburater. So i bought some dhla's , got the suggested jets and venturies , clean and rebuilt them and put em in. The problem is it keeps back fireing and they're not synchronizes when the butterflies are fully closed. What should i do.

I should also probably tell you that I'm a 19 year old kid who is quite inexperienced under the hood.

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I should also probably tell you that I'm a 19 year old kid who is quite inexperienced under the hood.

This statement shows your wisdom beyond your years... :-)

1. Do you have a syncrometer, flowmeter or vacuum gauge of any kind? If not, you will need to buy or borrow one. You can get close by ear, but that takes some experience...

2. Another good purchase for learning with would be one of those nifty sparkplugs with a site glass that tell you if the cylider is running lean or rich. Otherwise, you will have to just go by ear and checking your plugs one at a time unless you run an O2 sensor, but even then it just gives you an overall picture and not individual cylinders...

First thing you should do is to make sure everything else is in tune and to spec. Adjust your valves, make sure you have good spark plugs, wires, cap, rotor, etc... If you haven't got an electronic ignition yet you should consider it. Once these basics are taken care of, then start in on the carbs. For one, if your engine is pretty stock other than the carb, you are going to have fits getting them to run as well as some other options, but it can be done. Get the flow meter on them and make sure they are syncronized at idle and around 3000 rpm's. Then what I do is take all my idle mix screws to zero and then back them off one and a half full turns just to start. Then, adjust them out one at a time until you hear the motor start to stumble. Then go in with it until the motor starts to stumble again. Take note of the difference in the amount you turned it out and then back in and set it half way between the two. do this to all four cylinders and you will get really close.

Good luck!

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You can write a book on how to set these things up, but in the end, it all boils down to very simple things.

First, make certain the floats are properly setup. Use a Vernier Caliper to measure and also be sure the carb top gasket is in place - you need to account for it's thickness. (I suspect this is a large part of your issues). Also, be sure you have no vacuum leaks on the carb or manifold - check this with a can of Butane or an unlit Propane Torch. With the engine running, pass the Butane/Propane source carefully around the carb seals, manifold gaskets and Throttle Shaft openings - if the idle increases, you have a leak. Also be certain your Fuel Pump Pressure is no more than 3.5 PSI.

You really need a tool to properly setup the carbs and balance (synchronize) them. I cannot emphasize this strongly enough. If set too Rich, you waste fuel, increase emissions and get poor driveability. If set too lean, you run hotter and risk detonation and engine damage. I suggest the Morgan CarbTune II this is getting harder to find, but is invaluable and will give years of good service. This tool is an investment that will pay for itself through better fuel economy and driveability. Trying any other methods of setting/synching the carbs is difficult and inexact - you might as well be wearing earplugs and a blindfold.

You should really set the carbs up all over again from scratch. The Dellorto uses fine needle Mixture and Idle screws, so turn them in gently all the way closed. Be gentle do not gouge the tips. If you think you may have already done this, replace the Mixture/Idle Screws - Dellortos are much more finely calibrated than Webers and require the Screw tips to be in perfect shape.

Now back the Idle Mixture Screws out 2.5-3.0 turns. If you have the CarbTune II, now is the time to connect it with it's included adapters to the vacuum ports on the carb bodies. These are on the top of the carb right next to the balancing screws. The balancing screws are inset into a small barrel.

Start the engine and run 850-900RPM - adjust idle speed screw to get a sustained idle in this range. Do not go over 900 RPM or you will be into the progression circuit.

Now, turn all 4 Balance Screws fully closed - gently, just seat them.

Next, adjust all 4 Idle Mixture Screws to give the strongest vacuum for each cylinder. This may effect the idle, so be sure to continually adjust the Idle Speed Screw to stay within the 850-900RPM range. When finished, you will likely have a different vacuum reading from each cylinder.

Now, adjust the throttle coupling (the screw which separates the arms or levers going between both carbs) so that the weakest barrel on the Front carb matches the weakest barrel on the Rear carb - this is synchronizing the carbs.

Now, open the Balance Screw of the Strong barrel on each carb and weaken the vacuum on the Strong Barrel to match the Weaker one (you can weaken the Stronger one, but you cannot strengthen the Weaker one). When finished, you should have only one Balance Screw open on each carb and all barrels should be pulling the same vacuum reading. Again, reset the Idle Speed to the 850-900RPM range. There is no need to adjust at any speed higher than idle, say 3000 RPM because you are now off the idle circuit and everything is now metered solely by the calibrated orifices (or holes) in the Jets, Air Tubes, Emulsion Tubes, etc. - These are not adjustable without replacing the parts with ones which have bigger/smaller orifices.

Rev the engine and be sure everything remains the same and returns to the same vacuum levels at idle. If so, you're finished. If not, repeat the procedure until everything matches.

It really looks way more complicated in print than it actually is. Take your time and adjust in small increments - 1/8th to 1/4 turns and allow 5-10 seconds for the engine to 'settle' into the new settings.


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