Well damnit, where else would air be getting in? That left side upcapped piece is for the fast idle circuit, per a drawing I just found. Should I cap that off too?

Should both jets be the same size, by the way?

Hi Grubeguy

Can i ask if the idle adjuster screw has a taper/point on the end, it may need to unscrew some more so its further out, some are different lengths.

Tim

Air will be getting in via the hole below lever 23 on my diagram.

Tim

Grubeguy said:

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I know next to nothing about that carb, including the inlet shown in picture 159 (I *think* that's a vacuum break, but I don't know where it would get hooked up to).
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Click to expand...

That unit below the choke cable looks to me to part of the old 'Fast Idle System' fitted to some older Fiats. It's purpose was to limit the closing of the throttle to the fast idle position when coasting in 3rd or 4th gear, with the clutch engaged, i.e. to improve emissions. The open pipe stub on this unit was connected via an electrically operated solenoid to the pipe stub (currently blocked off) on the inlet manifold below the drip tray. Afaik (!) it can be safely removed or pipes blanked off.
When fitted, there used to be a push button switch in the engine bay to press to allow the fast idle speed to be adjusted with the engine running and the car stationary.

AL.

Here's my idle screw

While I agree with all the advice/suggestions made by timmycm850, I am deeply suspicious of those who 'modify' carbs especially any 'boring out' operations, fitting oversize shafts, bushes etc.

It doesn't take very much misalignment to complete upset the relative positions of the edge of the throttle disc (butterfly) and also the progression holes. The result can you 'chasing your tail' forever or until you get another carb.....

I note, however that OP was supplied with another rebuilt? carb with the rebuilt/modified engine, and that this carb displays the same problems. Was this carb modified in the same way by the same person?

If this was my car, I'd be looking for a known-to-be-working standard carb and try to get the engine starting easily, idling reliable and working normally.

Listening to the video of the engine running, it sounds to me like the idle circuit is not delivering any fuel - the engine seems to be running on fuel from the progression holes or maybe the emulsion tube circuit. The position of the idle mixture screw is also very suspect as timmycm850 has already pointed out. I can't recall if this Fiat carb has a tapered idle mixture screw or the type that looks like a little pin on the end, but on this latter type, the pin can be broken off and completely block the idle mixture hole - does turning the screw have any effect whatsoever? Have you tried removing the screw to check if any fuel comes out or can be blown through?

My (limited) experience on removing emissions equipment on a few cars imported from the USA over here, is that removing/blanking off some of these can have unforeseen effects. If you disconnect something that bleeds extra air into the engine under certain conditions, you may have to change e.g. extra jets, e.g. air-corrector jets. If you blank off the valve that timmcm850 mentions on the throttle shaft that's supposed to draw in oil fumes above idle speed, you may have to change a few jets also.
It's not unusual to have different size idle or indeed other jets in a twin choke carb - the 2 chokes can be different sizes, or for other reasons incl. emissions reduction, economy etc.

Re:- difficulty starting. If the idle circuit is not working, then the engine has to spun over faster to get it to 'catch'. I'm not sure if a 'high-torque' starter motor spins the engine faster or just has more torque???
I well remember having difficulty in getting the 850 engines to start after head gasket replacement ( it didn't happen on any other Fiats at that time).
some oil down the bores seemed to help, but often I simply resorted to doing a tow start for the initial start - no problem after that. (doesn't apply to OP's car as he's had it running several times already).

On the few engines I worked on that had modified cams etc, again, sometimes a tow start was the quickest way to start it ( insufficient starter speed may have been to blame). Some Mechanics loosen the distributor and try rotating the distributor back and forth (thereby advancing/retarding the static ignition timing a little) to get the engine to catch. If you over retard, you may get a backfire through the carb, if you over-advance the engine may kick back against the starter (iirc). When they got the engine running this way, they would adjust the timing to give the highest idle speed, retard slightly then re-check later using a strobelight on the timing marks.

Another thought has come to me :idea:. Many Fiat engines of c. 1970's used 5 degrees btdc as their static timing figure, unlike Euro spec engines which used 10 degrees btdc. This would cause difficulty starting, so on some models e.g. 124 the distributor was fitted with 2 sets of contact breaker points, one timed to 10 degrees for starting purposes and the other 5 degrees for normal running. Remember what I said about trying some advance on the ignition timing if difficulty starting is encountered.

In summary - I reckon Op's problems are in the carb, probably the idle circuit. Trying a known good standard carb should confirm/refute this. Very important to have no air leaks also.

Al.

Grubeguy said:

Here's my idle screw

Click to expand...

That's the second type of idle mixture screw that I referred to.
Given how far you having to turn in this screw, are you sure the seating for this screw in the carb is still intact?
Normal setting iirc is about 1,1/4 - 1,1/2 turns out from lightly closed.
Maybe compare this screw, particularly it's taper tip and the seating in the carb with that of your other carb?

I reckon your idle problem is a large air leak, especially if the other carb had the same starting/idling problems.

I've heard of people spraying carb? or brake cleaner or starter spray around any possible points where air may be being drawn into the engine. If the engine speeds up, you've located the air leak. You could try around the carb base, any pipe that is disconnected, maybe around the throttle spindle?
Just be very careful if trying this, wear protective goggles, gloves etc and have a fire extinguisher to hand.

Al.

Hi Grubeguy

Good to see some other useful input here too.

ive been searching for an image to show the missing plunger

have a look at ebay item no 254210005995 $32 for complete manifold inc plunger. you can see a plunger in the hole where you have none, you are missing this part and this will cause extra air to go into the intake manifold at high vacuum ie idle. notice where the plunger is and you have a random piece of rubber in the hole. you could temporarily fill this with heavy grease as an experiment, i would only spray the carb/manifold with 3 in 1 oil and nothing more volatile, but i agree with the reason for the check.

that is a good point regarding the bored out carb, is this just the pivot for shaft bushes or is there other modified work inside. the idle jet tip does look a little short to me.

i would be focusing on the idle fuel supply channels through the carb as my suspicion is also the car is currently high idling on the main run jets.

Try adjusting the idle speed screw back until the carb is fully shut and the butterfly is fully closed and the tip of the idle speed screw is not touching. then you know the carb is closed. turn the screw in until it touches and go a further turn and a half. Visually check that the second choke of the carb is also fully closed(the one on the exhaust side)if this is open more than the tiniest amount this will ruin your vacuum.you might have to take the carb off to properly check this.

Tim

I forgot to mention

listening to the video in day time now it does sound quite loud mechanically.

yes it would make sense to recheck your timing, this can be difficult in your circumstances as the timing should be 10/11 degrees at idle which you cannot set up so you will have to set the timing to allow for the mechanical advance you would get at running speed. if you have a timing light with variable advance this will be easy just set the timing light at 40 and put the timing mark on the tdc point

So when you have the engine running at about 3.500 revs set the timing mark at 10/11 degrees plus a further 28 degrees advance, remember the engine turns anti clockwise so your timing mark should be around 40 degrees to the right at high run speed and don't forget your engine is leaning over at 15 degrees to the left as well so don't go 40 degrees from gravitational vertical.

i had best go and do some work today, i shall pop in again later

Tim

Still have not worked out the commnet at the end of the video

Tim

please see manifold image for your missing plunger, i may be able to flip the image but sorry if its upside down

Tim

Thank you all for the help, I truly appreciate it. I walked away from the almost complete resto a few years ago, out of frustration, and I feel I'm inches away from having a car that runs and idles normally. In response to the replies above:

-the carb is not modified, it was rebuilt by Pierce Manifolds. Where bushings were worn or holes enlargened by time and use, they honed out and installed slightly larger ones, to close up air leaks, etc. Nothing was modified for performance, fuel economy, etc

-the carb previous to this was rebuilt by Midwest Bayless, with the same attention to detail, and the same poor running results. Prior to THAT carb was the original one to the car, which the engine rebuilder did. All three carbs offered the same results - poor starts and bad idle

-I bought the manifold and plunger off ebay. When it comes in, I'll clean it up and install. I don't remember what kind of gasket sets these things need, so verify my understanding - gasket between engine and manifold, gasket between manifold and drain pan, and gasket between drain pan and carb, correct?

-spraying carb cleaner around to check for air leaks. I did that a while back, with no results, though I'll give it a try again after work

-I will also try resetting the idle mixture screw by screwing it fully in to gently seat it, and backing it out 1.25 to 1.5 turns. Prior to this, I will remove the idle screw again and blow through the hold to remove/release any gunk that might be there

-fuel not going through the idle circuit - How can I check to see whether this might be the case?

-timing - I'll check that also

-given that three carbs rebuilt by folks who know them all provide the same issues, I'm guessing something common to them is causing the issue. Manifold? Gaskets?

Again all, thanks for the help. i'd love to use this car finally

Hi Grubeguy

Yes 3 gaskets as described, as the car has had no real use you may find the existing gaskets to be in good order. check the connecting surfaces of all items you have there.

to check fuel flow in the idle lines you can blow air with an airline into the hole where the two .4 and .45 jets are and it should come out of a tiny hole by the butterly edge at the bottom.

please be careful not to blow petrol in your eyes,

if you carefully remove the top of the carb on a bench while its off the car you can check the small holes from the float chamber to the position of the idle jets, to get the top off the carb you will have to disconnect the choke rod( remember 11.00 O'clock angle piece) you may as well check the float level/shut off valve is set properly( see online you tube type video).

i do tend to agree with you on the same symptoms with 3 different carbs so im hoping the steel plunger has a good effect. make a mental note of all the other holes in the manifold so you can double check they are all closed off as appropriate.

fingers crossed

Tim

Oh and make sure there is no silicone used under the carb as this can block some of the tubes etc.

Tim

Hi

I would aim at an idle of about 1100/1200 revs as your rebuilt engine with have a bit more internal friction until its worn in.

Tim

No news on the manifold front - it's apparently supposed to be here in a week or so.

However, I pulled the plugs and they weren't ... well, BAD, but they're rather naughty. I've cleaned them using acetone and brake cleaner and some 600 grit sand paper, ensured the gaps were correct, and reinstalled.


I also blew air through the idle mixture hole with the screw out. Gas blew up through the right side air jet. I'm guessing this is normal?

When my electric fuel pump is running, should pumping the gas pedal send squirts of gas into the throats?

I also fiddled with the timing, having had to buy a new timing gun when the previous one gave up the ghost. Timing adjust like it should, but getting it to specs per the workshop manual is problematic. When turning the dizzy clockwise, the timing mark heads in the same direction. The sweet spot seems to be at 15 degrees as one would look at a circle, and nowhere near the 5-9 mm the manual calls for. I'm guessing that this is not normal.


I've also re-set the idle mixture to 1.25 turns from seated.

My idle won't go below about 1500 rpm without the engine stalling, and even then, it hunts up and down.

In theory, my distributor was rebuilt with the engine, but could I have a crappy distributor?


Can/should I adjust the air bleed?

Hi Grubeguy

There's no point getting stressed about your running until the manifold is fitted, in fairness you only really need to fit the steel piston/plunger and its spring.

its good that you've cleaned up and checked the idle channels so that's done ready.

now you have a new fresh timing light and the car wont idle below 1500 go back and read my previous instructions on setting the timing at 4000 revs to 40 ish degrees advance, remember engine turns anti clockwise so the timing mark on the crank should be 40 degress to the right of tdc mark

Tim

Oh and yes its good to have the squirts as you pump the throttle.

Tim

The rotating of the dissy and the moving of the timing mark makes sense to me due to he fact that you are turning the case clockwise from above which means the rotor is effectively moving anti clockwise in the case and therefore the equivalent of backwards which is what is happening with the timing mark

the US timing set up may be different from the European due to fuel and emission differences, in the UK i would set up at 10 degrees at 900 revs idle and work from there.....

You cant get 900 revs yet so you will have to set up either static or at 4000 revs per instructions given earlier.

I repeat, don't get stressed about doing too much until you have that plunger/piston fitted.



Tim

Hi Grubeguy



When your manifold/piston arrives, DO NOT fit a rubber "O" ring on the piston/plunger there is not supposed to be one there

Sit down take half an hour to read through all the previous posts and take in the info and instructions....

Any volatile leakage ie petrol / carb cleaner mop it up or let it dry before proceeding any further.

Tim