1750 GTV weber 40doce quick throttle stumble - Alfa Romeo Bulletin Board & Forums
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post #1 of 11 (permalink) Old 09-09-2018, 06:53 PM Thread Starter
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1750 GTV weber 40doce quick throttle stumble

Hi everyone, been having a stumbling problem on my 1750 GTV with dual weber 40 doce. It's a quick thottle jab type issue where the motor will stumble and almost die. It comes off idle fine if slowly applying throttle but if giving a quick jab like pulling away on a hill it almost dies. I believe its generally running rich as the plugs are blackened (no thick deposits and not oily) but it doesn't seem to get enough fuel off idle. The idle jets are 50f6's. I read in another thread about someone going from 50f8s to 60 or 65f8s solving the bucking / stumble off idle.

How do I properly identify if I need leaner or richer idle jets? Does this sound like an issue with the idle jet or possibly the pump jet (accelerator pump) as it quick stab of throttle?

Thanks in advance and I will try to follow up with as much info as needed. Im new to carbs and don't really know what other info is needed to identify this issue im having.
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post #2 of 11 (permalink) Old 09-09-2018, 07:00 PM
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F6 is like the richest you can go on air corrector for the idle jet, and that's likely your problem. The F numbers are not sequential.

What have you done with your engine? If it's a near-stock 1750 you should be running 50F8. I've got a pretty-stock 1750 with dual DCOEs and the stock Alfa jetting with 32 venturis works pretty well. See here:

https://centerlinealfa.com/sites/cen.../weber_jet.pdf

Note that this also assumes you've got the older Webers and not the emissions models. What's the code on them?

Tom

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post #3 of 11 (permalink) Old 09-10-2018, 01:57 AM
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Last time I had those symptoms it was blocked accelerator pump jets. Fuel supply wasn't keeping up with the big gulp of air with sudden throttle openings. Lots of cranking on cold starts was a clue too.
It's certainly easy to check - can you see/hear the pumps squirting fuel into the carb throats (check all 4 squirt a similar amount, too). Just operate the throttle by hand while under the bonnet (air cleaner off, of course)? A light and mirror will help you see what's going on in the throats. Easy to extract and clean the jets too.
If not that, I bow to Gubi and others' experience on jetting.
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post #4 of 11 (permalink) Old 09-10-2018, 02:46 PM Thread Starter
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Quote:
Originally Posted by Gubi View Post
F6 is like the richest you can go on air corrector for the idle jet, and that's likely your problem. The F numbers are not sequential.

What have you done with your engine? If it's a near-stock 1750 you should be running 50F8. I've got a pretty-stock 1750 with dual DCOEs and the stock Alfa jetting with 32 venturis works pretty well.

Note that this also assumes you've got the older Webers and not the emissions models. What's the code on them?
They are 40dcoe 151's. As far as I am aware, stock except headers and stinger pipe from PO. Weirdly I found that one of the barrels had a 50f8 in it while the other 3 were 50f6's. Maybe idle jets were changed in the past?
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post #5 of 11 (permalink) Old 09-10-2018, 02:50 PM Thread Starter
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Originally Posted by Ranz View Post
Last time I had those symptoms it was blocked accelerator pump jets. Fuel supply wasn't keeping up with the big gulp of air with sudden throttle openings. Lots of cranking on cold starts was a clue too.
It's certainly easy to check - can you see/hear the pumps squirting fuel into the carb throats (check all 4 squirt a similar amount, too). Just operate the throttle by hand while under the bonnet (air cleaner off, of course)? A light and mirror will help you see what's going on in the throats. Easy to extract and clean the jets too.
If not that, I bow to Gubi and others' experience on jetting.
Yeah I have also noticed lots of cranking on cold starts since the hesitation / stumble which is why I also considered it could be the accelerator sprayers. I took the caps off the accel pump jets and pushed the throttle linkage, i can see them filling up but I can't say I hear them actually squirting fuel. I have to get a better look today with a mirror and flashlight.

If that is the case how should I go about cleaning them? Fill the bowl with carb cleaner and give a few pumps of the throttle? Thanks!
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post #6 of 11 (permalink) Old 09-10-2018, 03:01 PM
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First thing you need to do is get a set of 50F8's in it, the F6's are way too rich.
tThen check your carburettor mounts to make sure there are no air leaks( if you're on the original rubber mounts). Then check ignition timing to make sure it's not retarded.
If all is good, as has been said above, blocked accelerator pump jets is favourite, even one of the four blocked will give you those symptoms.

A new set of plugs would be a good idea if you've been running rich for a long time.

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post #7 of 11 (permalink) Old 09-10-2018, 04:07 PM
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Cleaning jets...
The jets themselves are accessed from beneath one of the screw-in caps on each carb throat.
The screw cap is visible from the top. The jets just pull up/out after removing the screw cap. They are a needle-like item with a tiny hole near the bottom, and round bit with a flat on it to locate them in the 'hole' to point them the right way.

Of course the pump mechanism or 'channels' which feeds these jets might also be stuck or blocked, respectively.

With the caps off, you should see fuel 'arrive' at the jet hole when you operate the throttle (fuel, air bubbles). With the jets in and caps on, you should see/hear fuel squirting out the hole in the bottom of the jet, which sticks down into the intakes, beyond the butterflies. Fully clear, that squirt should be steady and quite strong. That's where a light and small mirror comes in handy. The intake for the pumps is at the bottom of the float bowls.

You could probably turn up a YouTube video on finding/removing/cleaning them (Eg try searching Weber DCOE cleaning accelerator pump jets) There is plenty of great info here on the BB too.
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post #8 of 11 (permalink) Old 07-12-2019, 07:03 PM Thread Starter
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Originally Posted by Gubi View Post
F6 is like the richest you can go on air corrector for the idle jet, and that's likely your problem. The F numbers are not sequential.

What have you done with your engine? If it's a near-stock 1750 you should be running 50F8. I've got a pretty-stock 1750 with dual DCOEs and the stock Alfa jetting with 32 venturis works pretty well. See here:

https://centerlinealfa.com/sites/cen.../weber_jet.pdf

Note that this also assumes you've got the older Webers and not the emissions models. What's the code on them?
Sorry I know its been a while but I just realized I didn't mention im fairly certain the car has a euro cam / upgraded cam of some sort, thats what ive been told and it would make sense to have a more rich mixture given more air / overlap from the more aggressive than stock cam? To be honest I don't know a whole lot of what was done to the car but it was restored some years ago by a reputable company out in california. I just know that it has had issues with needing to pump the throttle a bunch on cold start since I've owned it and it has always had an issue of backfire / popping when off throttle downhill (engine braking). I did just install a new set of spark plugs (nkg iridium) and have a cap and rotor on the way as the old cap has aluminum contacts that have white corrosion and groves in them. I will follow up tomorrow once I start the car up for the firs time with new plugs and will look for more specifics on the webers on the car. If it helps at all it has a bosch blue coil and all plug wires test betweek high 6's - mid 9's khoms(so good from what I can tell?). I haven't been able to find any kind of inline fuel filter at the tank / fuel pump but it has one of those glass see through fuel filters in the engine bay. It never seems to get more than half full but flows seemingly "enough fuel" when I pulled the line going into the carbs off and into a bottle to check for flow.
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post #9 of 11 (permalink) Old 07-12-2019, 08:21 PM
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If it had a mismatch of idle jets, how can you be sure some others aren't mismatched?

I would do an inventory of the current jets.

If your ignition timing isn't set properly, or the distributor isn't advancing, that could be part of your issue. So could carbs being out of sync.
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post #10 of 11 (permalink) Old 07-16-2019, 06:27 AM Thread Starter
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Quote:
Originally Posted by slowcreek View Post
If it had a mismatch of idle jets, how can you be sure some others aren't mismatched?

I would do an inventory of the current jets.

If your ignition timing isn't set properly, or the distributor isn't advancing, that could be part of your issue. So could carbs being out of sync.
Thanks for the input slowcreek. How does one check the ignition timing? Is the only way with a timing light (I don't have one if thats the case). I would expect the mechanical advance is not in tip top shape as it takes a little while for the idle to drop back down after its been rev'd. I'm going to pick up a set of 50f8's just to try them out.

One more thing I noticed last night is that the accelerator pump jets are not really "spraying" fuel when working the throttle from in the engine bay. 1 of them kinda does (weak spray) but the other 3 either have droplets or no fuel at all. I'm gonna assume this plays a large part of why its falling on its face when the throttle opens up but revs out to redline just fine.
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post #11 of 11 (permalink) Old 07-16-2019, 06:31 AM
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Quote:
Originally Posted by AlexBassila View Post
Thanks for the input slowcreek. How does one check the ignition timing? Is the only way with a timing light (I don't have one if thats the case). I would expect the mechanical advance is not in tip top shape as it takes a little while for the idle to drop back down after its been rev'd. I'm going to pick up a set of 50f8's just to try them out.

One more thing I noticed last night is that the accelerator pump jets are not really "spraying" fuel when working the throttle from in the engine bay. 1 of them kinda does (weak spray) but the other 3 either have droplets or no fuel at all. I'm gonna assume this plays a large part of why its falling on its face when the throttle opens up but revs out to redline just fine.
Wait on the F8s. With the pump juts removed, hell all jets removed, spray carb cleaner through every passage and jet before doing anything else.

Ever have the carb tops off to see how much crud is in the bowls? Anothe opportunity to go nuts with carb cleaner. Buy 3 cans of it. brand doesn't matter.

As for timing, yes, you must have a light. But, let's go crazy with carb cleaner first.

It's Giuseppe's giubos, not Guido's guibos, on my 78 Spider and Sport Sedan
REFRESH CONNECTIONS BEFORE REPLACING COMPONENTS
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