carb problem....looking for a 2nd opinion :)

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wizrod
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carb problem....looking for a 2nd opinion :)

Post by wizrod » Thu Dec 07, 2017 8:38 am

Hi all,

The quick summary
I have a 1600TP motor, to which I'm fitting a pair of 36IDFs. The motor starts easily & reves freely, but won't idle. It also pops back through the carbs as the motor dies. My assessment is that it is running too lean. Does this sound a reasonable diagnosis, or am I barking up the wrong tree?

The full detail
The motor:
A 1600TP with a single vac dizzy & electric fuel pump.
The carbs are a pair of Italian 36IDFs (off an AlphaSud), fitted with #52 idle jets.
The ignition timing has been set statically.

Observed behaviour:
The motor starts easily on the 1st or second crank, with a dab or 2 on the throttle linkage.
The motor will rev up (in neutral) without any apparent hesitation.
All 4 exhaust headers feel equally warm after the motor has run for a short while (indicating no non-firing cylinders).
If left alone, the motor will quickly (5secs ish) slow & stop.
If I hold the throttle linkage slightly open the same, but it takes longer to die.
I have to open the throttle a fair amount (way beyond the recommended outer limit for the idle speed screw adjustment) for the motor run without eventually stalling.
When it does die, the motor pops back through one or other carb.

My assessment:
The motor starts easily & runs (if given suitable encouragement), so valve adjustment & ignition timing are probably about right.
The motor responds to the accelerator pump and will run at higher speeds, so the accelerator and the main jet circuit are functioning.
The reluctance to run at low speed/idle indicates the problem is with the idle circuit.
That, and popping through the carbs indicate a lean condition (I think...don't have access to my copy of the Weber carb tuning manual)
I think a #52 idle jet is about right for a 1600; If so, it suggests that the problem is perhaps down to one or more air leaks.

My question (yeah, I know! At last!!)
Is my assessment valid, or have I miss interpreted something?

Thanks in advance you your sage advice.

Paul.

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sideshow
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Re: carb problem....looking for a 2nd opinion :)

Post by sideshow » Thu Dec 07, 2017 8:52 pm

Forward or back flush the idle circuit with carb cleaner or air...standard IDF procedure.

But in your case will it run at fast idle? What does turning the butterfly stops a turn or two do?
Yeah some may call it overkill, but you can't have too much overkill.

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wizrod
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Re: carb problem....looking for a 2nd opinion :)

Post by wizrod » Fri Dec 08, 2017 10:57 am

It will only run if I (manually) turn the linkage so that it runs in the 2500+rpm range....so where the idle circuit is transitioning to the main circuit. This is way beyond the range of the throttle stop/idle speed screw.

If I simply start the engine (with a couple of blips of the throttle while cranking), then leave if alone (idle speed screw set to 1 then out), it will slow and stall within a few seconds. If, once started, I hold the throttle open enough to run at say 1500 rpm, it will still slow and stall, but just take longer...maybe 10secs.

I should also says the the carbs have just been cleaned...an hour in a hot ultrasonic bath. The fuel is fresh as are the fuel lines.

That said, I will indeed blow the idle circuit through, not to mention go through the usual tests for air leeks.

66brm
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Re: carb problem....looking for a 2nd opinion :)

Post by 66brm » Sat Dec 09, 2017 12:20 am

Pull the idle jets and clean them and their circuit, check you don't have the idle air system blocked with the wrong top gasket, 52s should be a little rich in my experience with the early idfs, you do understand the lean best idle?

Slow 1200
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Re: carb problem....looking for a 2nd opinion :)

Post by Slow 1200 » Mon Dec 11, 2017 5:50 am

how many turns out in the idle mixture screws?

66brm
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Re: carb problem....looking for a 2nd opinion :)

Post by 66brm » Mon Dec 11, 2017 9:49 am

The number of turns out is slightly irrelevant as the early idfs have a finer thread than the later stuff

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sideshow
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Re: carb problem....looking for a 2nd opinion :)

Post by sideshow » Mon Dec 11, 2017 9:07 pm

The minor difference in the 36's to more common IDFs is probably not the issue (Late idle screws came in different tapers). I do agree on improper top gaskets.
Another trick that might help is to unset slightly the idle jets, this will fake a larger jet.

The BIGGEST problem is the only person that can update this thread doesn't
Yeah some may call it overkill, but you can't have too much overkill.

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wizrod
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Re: carb problem....looking for a 2nd opinion :)

Post by wizrod » Tue Dec 12, 2017 11:54 am

thanks for your replies....I've been away the last couple of days.

so...
  • will re-clean the idle jets & circuit this evening (assuming it isn't snowing). Will also try fractionally unscrewing the idle jet holders as suggested.
  • the idle air system is not blocked with the wrong top gasket - visually checked to confirm this.
  • I fully intend to tune to best lean idle...once the motor will idle at all.
  • the idle mixture screws are new & wound out 1 turn.
I'll report back as soon as I have an update. As mentioned...the weather in London is below zero right now...and I don't intend to sit outside on a cold concrete floor while its snowing...!

Thanks again :)

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sideshow
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Re: carb problem....looking for a 2nd opinion :)

Post by sideshow » Wed Dec 13, 2017 7:57 am

The first link to the wayback machine seems to work with pictures and is of that excellent pictorial of Mark's Weber 36 IDF clean up
http://www.shoptalkforums.com/viewtopic ... 8&t=129571
Yeah some may call it overkill, but you can't have too much overkill.

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sideshow
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Re: carb problem....looking for a 2nd opinion :)

Post by sideshow » Wed Dec 13, 2017 7:57 am

The first link to the wayback machine seems to work with pictures and is of that excellent pictorial of Mark's Weber 36 IDF clean up
http://www.shoptalkforums.com/viewtopic ... 8&t=129571
Yeah some may call it overkill, but you can't have too much overkill.

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wizrod
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Re: carb problem....looking for a 2nd opinion :)

Post by wizrod » Wed Dec 13, 2017 12:29 pm

Sideshow...thanks for that.

As it happens. I've done basically exactly the same as Mark described, including the hot ultrasonic bath, but omitting the parts where he plugged the spare fuel spigot & re-sized the idle air bushings.

I'm also - currently - still using all the original jets. I figured I'd at least get the motor to start & idle, so I could balance them & confirm the ignition timing, before starting to think about some more appropriate main/AC jets & ETs.

Where is where I'm at. The motor starts fine, but won't idle.

It was far too cold to tinker yesterday evening...I should have an opportunity at the weekend.

cheers

Paul

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