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Discussion Starter #1
Hi all

I recently bought a "race ready" (I jumped too quickly before checking things properly sadly) 105 series 1750 GTV thats got a modded head and performance cams. I do not know what type of cams though.

Problem is that there is a persistent misfire, even on part or full throttle, when the revs are between 3 to 4 thousand and I am now at the end of my list of things to check and fix. I have done the following:

I have had the timing retarded and advanced with no difference.

Adjusted the float levels on both carbs which had the rear carb out by 5mm.

I have replaced the coil and plugs with cooler running plugs and replaced the coil - distributor HT lead with a new one.

I have cleaned the carb in situ.

I have checked the cam timing and it is correct.

I have also ordered a new set of carb mounts but tested if there was a manifold leak by spraying carb cleaner around the mounts and there was no discernible change in idle.

I do not know hat else to try so am hoping some of the enlightened people here can point me in the right direcrtion.
 

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Discussion Starter #3
Does it have air filters?
Checked the fuel filter?
Hi 105, thanks for the response.

Yes its running at the moment a standard air filter arrangement and I have put a new filter in.

I also removed the old fuel filter and fitted a hi flow unit but no change.
 

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Discussion Starter #5
Thanks Biscione

I have no idea what that means but I shall do some research and check it out. It sort of makes sense because I feel the car is running way too rich so looking forward to trying this out.

Many thanks
 

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Discussion Starter #6 (Edited)
Okay, so finally got round to checking the jets on Number one cylinder and found that the main jets emulsion tube is a 7772.10 and the jet on that is labelled 145.

My research suggests that a smaller emulsion tube for the 1750 motor is required, such as a 7772.6. Am I right in that? And then what about the jet size and the idle jet size?

I cannot seem to find anything about them.
 

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Posible non original Dellortos for the engine

You have Dellorto carbs ,and the emulsion tube type that you have mentioned is for later emission carbs.Check the type of your carbs (40 Dhla and another letter like C,E,F,G,L or nothing).If your sufix is nothing (only Dhla 40) or C or E you shuld use a non emission emulsion tube like .6 that you mentioned or.5 or .7 with a smaller main jet (around 135).
If you have an emission type Dellorto (F,G ,L or another) you can switch to .11 emulsion tube ,but also check your idle jets (normally 57 or 58) and pump jets (around 40 or 42).
 

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Discussion Starter #9
Thanks for that Alfacorse.

I have the DHLA f suffix so by my reading then that tube is right?

I been instructed to fit a fuel pressure regulator before messing with the jets and tubes as I have an electric fuel pump fitted. Would this sound correct? I am going to buy one but am just concerned that it will be another dead end.

Thanks for the help chaps.
 

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Yes the tube is right ,you need a pressure regulator (your model should have a standard fitted one, with Fuel filter) ,if the simptom remain after that you shold try .11 tube ,I have the same problem, during some personal testings, few years ago (1.6 engine).
Install first the pressure regulator and check for leaks the needle valves for each carb.
Float level for your type of Dellortos should be around 14,5-15 mm and 25 mm full opened.
You will find on the internet how to do this properly.
Personally I don't use cooler plugs because they foul quickly if runing to much in town.
And try to install an "original" (from Alffettas,giuliettas or 75's) Marelli or Bosch electronic ignition ,that will cure a lot of your problems (my choice is Marelli).
 

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Discussion Starter #11
Yes the tube is right ,you need a pressure regulator (your model should have a standard fitted one, with Fuel filter) ,if the simptom remain after that you shold try .11 tube ,I have the same problem, during some personal testings, few years ago (1.6 engine).
Install first the pressure regulator and check for leaks the needle valves for each carb.
Float level for your type of Dellortos should be around 14,5-15 mm and 25 mm full opened.
You will find on the internet how to do this properly.
Personally I don't use cooler plugs because they foul quickly if runing to much in town.
And try to install an "original" (from Alffettas,giuliettas or 75's) Marelli or Bosch electronic ignition ,that will cure a lot of your problems (my choice is Marelli).
Thanks for your help Alfacorse.

I found that the pump jet activator was not working so I fixed that over the weekend so that it now gets the full movement through the stroke and that has greatly improved throttle response and pickup.

I am going to have to buy a regulator though because the misfire is still there and the only way to get rid of it s to lift off the throttle somewhat, then it fires properly again so I am sure its over fuelling currently. My car is a race prepped one AC, so the original filer and regulator are removed so I must get an aftermarket one. And being a racing car its either full throttle or off throttle pretty much all the time.
 
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