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

[wizrod]

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.

[sideshow]

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?

[wizrod]

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]

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]

how many turns out in the idle mixture screws?

[66brm]

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

[sideshow]

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

[wizrod]

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

[sideshow]

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

[sideshow]

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

[wizrod]

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

[wizrod]

Progress update.

First off, thanks for all your suggestions...very useful.

So I had a chance to try some things put this morning. First off I recheck the idle vents. Definitely not covered. In doing so I also spotted a loose nut on one of the manifold head studs? Which I nipped up. I also spotted that I'd not blanked off the 4 vacuum ports, which are now suitably covered.

None of this had any noticeable effect on the motor.

Next tool the suggestion to undo the idle jet holders a little. A quarter turn on each made a big improvement. No more popping back through the carbs, but still needing the throttle to be held open (less, though) to stay running.

Since the motor was at least not stalling, I took the opportunity to check/adjust the ignition timing. With the ignition set to 7.5deg adv the idle improved again.

The situation now I'd that the motor will idle at about 6-700rpm with the idle stacks I'm screwed a quarter turn, and the idle speed screw 2 full turns our. I haven't touched the idle mixture needle...still 1 turn out.

Tightening even just 1 idle stack stalls the motor. Adjusting the speed screws back slows the motor, stalling before reaching 1 turn out.

So...what is my best way forward? Do I attempt to balance the carbs and tune to best lean idle with the 52 jet and assess the situation from there, OR would it be simpler to assume the 52s are too small, fit larger idle jets now (55? 57?) and then go through the best lean idle procedure?

Thanks again

Paul

[Piledriver]

set the total timing to 32btdc, check timing at idle.
If the carbs have ported vacuum for vacuum advance, use it.

Up to ~15btdc at crank/idle is ok, but the total must be around 32.
Its easy to limit the advance but lets go there later, one problem at a time.
(totally ignore what the manual says, std setting optimize emissions...the manual doesn't cover anything but bone stock engines, not Webers)

Wad up the manual timing settings and forget they exist save as a baseline.

Just cracking each jet in turn will show if there is a dead hole, but that works best if already idling kinda.
You may run the fuel screws out 4-5 turns, but that would indicate you need bigger idle jets, or have a huge vacuum leak.

An unlit propane torch is a useful tool for troubleshooting.

[wizrod]

Thank you...

So what intrigues me, in relation to Mark's experience of the strip-down/rebuild of his IDFs was that he found that the #52 Idle jets were too rich & that he needed to drop down 2 sizes.

I noticed that his IDFs were fitted with 28mm venturis, where mine have 30mm. Also, there does not seem to be any mention of the particular motor he was using - stock? mild/high performance..?

I'm guessing its reasonable that the flow characteristics of his setup might dictate smaller idle jets, compared to mine...?

Any way, I'll press on & see what kind of set-up I get with the jets I have...and review the situation from there.

cheers

Paul

[wizrod]

HI all!

Hope you all had a suitably festive festive season!

I've managed to make some good progress with my twin 36IDFs. I have fitted a set of #57 idle jets and the motor will now start & maintain a steady idle at about 750-800rpm, which is a significant improvement on the situation at the start of this thread!

I think there is still some work to be done, but I now have a driveable vehicle, which will allow me to make progress on other more pressing tasks, before I come back to more finely tune the carbs.

Many thanks to all respondants for your contributions!

cheers

Paul