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My Weber Carburetors are jetted differently
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mturner7
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PostPosted: Fri Sep 02, 2022 6:47 pm    Post subject: My Weber Carburetors are jetted differently Reply with quote

Hi Forum,

I purchased a 1974 914-4 last March, with a High performance exhaust, 2.0L engine with a big bore kit installed, the engine is a 2056cc build.
The Car runs alright but not perfect. I decide to look at the Main Jets & A/C Jets on all 4 carbs first, easy to pull and inspect.

Carbs: Weber 44 IDF 71S.

This is what I found:
1) The jets are the same sizes on cyl #'s 1 & 2
2) The jets are the same sizes on cyl #'s 3 & 4

But they are not the same as each other. This is not right!

See photo for complete configuration:


Image may have been reduced in size. Click image to view fullscreen.

Another note: Even though the jets have a number on them, like 1.55, they are also drilled out to a bigger size (I Looked at them under high magnification), the hole is off center an little bit, and they are much bigger than a 1.50 that I have in my jet kit. Not sure what size they are, because I don't have a jet feeler gauge, the round type. I suspect they might be as big as 1.70 or 1.80.

What I was thinking about doing is, purchasing main and A/C jets all the same size and replacing the drilled out ones and the small A/C ones. I decided to get 1.55's, 1.60's mains and 2.00 A/C's to test them out.
Also, I purchased the car in an area where the elevation was 3000' or higher. I Live at an elevation of 225' (sea level). The car was jetted for the higher elevation.

The Weber design guide recommends 36mm Venturi's (Car has 32mm installed), also a main jet of 1.60 and a A/C of 2.00. (look at the photo attached to see all the jet sizes)

Any thoughts?

Thx -Monte


Last edited by mturner7 on Fri Sep 09, 2022 7:11 am; edited 1 time in total
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[email protected]
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PostPosted: Sat Sep 03, 2022 8:58 am    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

44s are just too large. It will never run satisfactorily with them installed no matter what you do with the jets, you need 40s
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Starbucket
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PostPosted: Sun Sep 04, 2022 3:15 pm    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

Buy a number drill set to measure your jets. Look at your sparkplugs and see if the smaller ones are cleaner or if the bigger ones are sooty. For testing just solder up the jets and redrill with the # drills larger or smaller untill you get it right then buy that size jet. Someone probably put carbs from different motors on and didn't check the jets.
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malsteph
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PostPosted: Mon Sep 05, 2022 5:46 am    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

Hello, your 44 are fine in size for a 2056cc.
Now your venturies are way too smal and should at least be 34. The original 36 should work as well.

Now the 200 and 195 mains jets are on the very large size and should be around 160 with idling jets around 55. Otherwise the transition will not operate so well and you probably will have a lask of power around 2000-2500 revs under load.
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mturner7
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PostPosted: Tue Sep 06, 2022 11:16 am    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

Hey Malsteph,

Thanks for the 44 size comment, I've been doing some research also, and I came to the same conclusion that these carbs are fine for this size of engine.
I purchased the 36mm Venturies the day before I wrote this thread, got them today. Going to install them soon.

I also ordered the new main jets at the same time (1.55's and 1.60's) and the A/C jets (2.00). Got them with the Venturies this morning. I'm going to keep the idling jets at .55

Yea, between 1500 rpm to 2500 I have a loss of power and sometimes stumbles. It idles great around 950 rpms.

Before I do anything, I'm going to look at the spark plugs and possibly replace them and get a fresh start. It's been so hot here, can't work on the car. 108 degs yesterday and same for today.

So this is what I'm going to do as soon as the weather cools down:

1) Keep everything as is and check the air flow with my Synchrometer Air Flow meter at all carbs.
- Adjust as necessary

2) Change the spark plugs

3) Install the 1.60 Main Jets in all carbs to make them all the same.
- Install the A/C 2.00 jets at the same time to make them all the same

4) I might switch out the Venturies.

Hope I don't break it!

Monte
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mturner7
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PostPosted: Thu Sep 08, 2022 4:46 pm    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

08-Sep-2022 - UPDATE ON PROGRESS OF RE-JETTING THE WEBER CARBURETORS

1) I Replaced the Spark plugs with (BOSCH - WR5DC+ 7992 +15)
- Old ones looked nice and white except for Cyl #3, A little burnt, not too bad.

2) Replaced the main jets with 1.60 all around (Use to be an unknown size, but were labeled 1.55 drilled out to a mystery size)

3) Replace the Air Correction Jets to 2.00. After looking a little closer at the A/C Jets that were installed, the right side jets were also drilled out. That would make them huge, maybe 2.20 or bigger.

4) Did not replace the Venturies yet (Still has the 32mm installed)
- Will try the 36mm ones soon.

Test drove it and it ran great! Engine is much quieter, no more vibration on the right side.
Smooth take off, nice pulling power from 2500 rpms to 4K/5K.

Not sure what the 36mm Venturies are going to add, any Ideas out there?
It's a more involved task to replace the Venturies, but will do it some day. I will have to pull the carbs to do them and will clean all the other parts of them while they are out of the car.
Oh, and adjust the float levels too. I would like to get some miles on this configuration, then swap out the 32's for the 36's!

p.s.: I still need to look into the Idle Jets and the Pilot Jets.
From Idle to about 1.5K/2.0K rpms, I get a small hesitation and sometimes shifting from 1st to 2nd in the low RPMs.
According to the Weber Book, the Idle jets should be .60's, I'm running .55's.
Also the pilot Jets should be .50's, not sure what are installed.

Summary: Car runs much better, love making progress! Smile

Image may have been reduced in size. Click image to view fullscreen.


Last edited by mturner7 on Fri Sep 09, 2022 7:16 am; edited 1 time in total
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VWFIXER
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PostPosted: Thu Sep 08, 2022 9:33 pm    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

Make sure total resistance from coil to spark plug is within spec, very typical problem.

Measuring the jets was the best tip for the question.
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Starbucket
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PostPosted: Fri Sep 09, 2022 7:27 am    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

Have you checked that the accelerator pump jets start squirting gas AS SOON as the carb starts to open? if not you will get hesitation at low speed.
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mturner7
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PostPosted: Mon Sep 12, 2022 6:10 pm    Post subject: Re: My Weber Carburetors are jetted differently Reply with quote

Yes the accelerator pump jets start squirting gas AS SOON as the carb starts to open.

Car is running good, I have not replaced the Venturies (32's) at this time.

Can anyone tell me if replacing them with the larger ones (36's) will make that much difference? I know it will restrict the air a bit.

I drove the car over 120 miles on the weekend, freeway and side roads. Ran great. It definitely runs a lot smother, doesn't smell rich anymore.
I will pull the plugs and look at them after I have a few more miles on it.

Some day if I get around to it, replace the Venturies (Install to the 36's) and check the Idler Jets and Pilot Jets for correct size, replace as necessary.

Thanks everyone for your ideas!
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