Author Topic: Ongoing flat spot... (Read 5456 times)

Lina · « **on:** April 24, 2012, 02:03:30 PM »

Hi all,
Well, Iâ€™ve not been posting much, but this is mostly as Iâ€™ve been out working on the car rather than on the pc

Now, Iâ€™m stuck and am in need of your expert advice!
I have a flat spot/hesitation when accelerating from tickover to moving â€“ if I continue to hold the throttle down, itâ€™ll die on me unless I back it off and bring up the revs slowly. Iâ€™ve also seen that this has been mentioned a few times on here, and Iâ€™ve done all of the below based on what Iâ€™ve read and been advised so far:

New fuel filter
New fuel hose, to keep the filter upright
Stripped and cleaned out the carb and used the carb overhaul kit from Ricambio, so all new gaskets etc
New plugs, now repeatedly cleaned
New HT leads
Have now just finished fitting electronic ignition and have had the timing set properly with a strobe
Have also had the carb adjusted so that the mixture screw can now be out two and a half turns rather than the quarter it used to tolerate!

And yet STILL, thereâ€™s that damned flat spot!

Tickoverâ€™s running beautiful now, as are the revs once youâ€™re rolling, but the acceleration is ruined by this problem grr.
My thinking is to go back to the carb again, as while the strobe was on, you could see the HT lead to no.1 was firing, so itâ€™s not an ignition problem.
I thought maybe Iâ€™d just jump in and get a newer/bigger/better carb, but have nearly fainted at some of the prices! So now Iâ€™m looking into doing what I can with the one I have, or maybe buying a refurbed one on ebay (about Â£45). But this makes me think I could look into rejetting or something on the one I have, but I *really* know diddly squat about this! Having already done the carb kit, is that not what a refurbed carb is going to offer me? Or will that have all new jets?? What exactly IS rejetting and how can it improve carburetion? (dumb question, I know!)
Iâ€™ve looked on the forum, but I canâ€™t make head nor tail of â€˜jetsâ€™ their different sizes, making my own manifold, etc!!
Can someone talk me through the basics of what I need to do or to be looking for?
I understand that the air is going faster than the petrol on acceleration, and itâ€™s not keeping up, but I donâ€™t know how to fix it! Would an electronic fuel pump solve this for me by pumping fuel out faster than the old mechanical one??

If it IS possible to get a newer/bigger/better carb/jets/whatever for a decent price (read that as approx Â£100ish!), should I? Or should I stick to the 28 IMB one, as seems to be suggested lots here, and maybe work on it. Iâ€™m sure someone will say â€˜search on the forumâ€™ but I feel I have and Iâ€™m completely lost! But if there is a thread somewhere that really covers the basics here, then yes please, send me that way!

At the end of my tether,
Lina

dajwid · « **Reply #1 on:** April 24, 2012, 03:38:29 PM »

Fuel pump ?

Lina · « **Reply #2 on:** April 24, 2012, 05:21:01 PM »

Oh no, don't tell me it's a 'classic example' of yet another 'common fault' - fuel pump?? lol ...wouldn't surprise me!

Rusty's Uncle · « **Reply #3 on:** April 24, 2012, 06:31:09 PM »

Check that you have no leaks at the base of the carb either because the carb is loose, damaged gaskets or cracked bakelite spacer. As drawing air at the base of the carb will give you those symtoms.
Also ensure that the centrifugal advance in the distributor is moving freely.
If it runs fine once up to speed then the fuel pump should be OK. The Weber 28IMB is a robust and very reliable carb and there is little to go wrong with it. Fitting another carb would probably be expensive and open a new can of worms

ciaoflyer · « **Reply #4 on:** April 24, 2012, 08:39:12 PM »

Check the vacuum advance hose from carb to dizzy as this may be damaged and is a cheap fix.

Lina · « **Reply #5 on:** April 24, 2012, 09:47:14 PM »

ciaoflyer, I'd check that if I had one! Do BIS's have them?

Rusty - I'll get on and check that during the week, although I did check it was clamped down tight a few times, but it could indeed be the bakelite thingy, which I didn't check when I stripped the carb.
Centrifugal advance is working fine, as checked with the strobe on Monday.
I have now got back from taking it out for a run and I have additional info... I took her flat out up a steep hill in 3rd and she really wasn't happy! First few seconds were okay, but then started losing power, cutting out and popping and spluttering like a smoker in the morning! So maybe you're right too dajwid, maybe the fuel pump is also at fault? I'd thought she was mostly okay at speed, but admittedly, I haven't taken her flat out for sympathy for an old engine...!
This spluttery thing is any time I try to accelerate with any more ooomph than a slow, gradual increase in speed.

As an aside, that may or may not be related, having taken her out this eve, I was reminded of another problem - when the engine's hot and I've left the car for maybe 15 mins parked up, if I go to start her up again, she starts okay, but maybe 10 seconds in or maybe 6 foot of driving, she'll just die. I then have to wait about 10 mins and try again. This has happened a few times, but one time I went back to look at the engine and the fuel filter was empty (it's one of those inline seethrough ones), so I'd presumed no fuel (I have a temperamental fuel guage...! Another problem, for another day lol).
It occurred to me this eve that could it possibly be the fuel in the float getting evaporated in that standing time? (this cutting out thing doesn't happen when the engine's been stopped only a few mins like at a petrol station).
It's definitely not caused by a blocked petrol cap vent - I need a new cap at some point as it was hit in an incident with the previous owner, so has a crack in it - there won't be any vacuum in the tank causing this!

Thanks for all your advice so far!

Lina

Hazey-n-Marc · « **Reply #6 on:** April 25, 2012, 12:09:37 AM »

BIS 's (BI?) have vacuum pipes, worth checking. Could be no2 plug lead (the one farthest from the back of the car before that argument starts again

)is fouling the lever mechanism on the vacuum unit, or the pivot can be problematical here as well (mine fell out never to be seen again

), but start with vacuum pipe checks. Ciao really knows all about vacuum pipe issues pretending to be other things (kept us all guessing for months

).

Marc

Pete126 · « **Reply #7 on:** April 25, 2012, 12:43:08 AM »

Lina's Fiat is an air-cooled and does not have a vacuum advance.

Lina Has this fault appeared after you have fitted the electronic ignition? + which one have you fitted?

Hazey-n-Marc · « **Reply #8 on:** April 25, 2012, 12:50:53 AM »

It could be a split diaphragm in the fuel pump, on a BIS this will thin the oil.

Lina · « **Reply #9 on:** April 25, 2012, 08:09:00 AM »

Morning all, thanks again for all your input here, I really appreciate it!

Marc - yep, Pete's right, I have an aircooled and we're not lucky enough to have a vacuum advance! (Although sounds not so lucky when there's problems with it?!) I'm not sure about the diaphragm on the pump, but I think once I've gone through the carb and the bakelite thingy, I'm going to go for the electronic fuel pump and just go ahead with the upgrade and see if that solves it.

Pete - no, the fault's one of the reasons I fitted the electronic ignition in the first place. I had this problem from the start, but in the weeks before I fitted the ignition, they were getting worse, and over a couple of days I started backfiring and popping all through the range. The worst day I had to drive home from work at 5mph which was as fast as she would go, banging and bouncing and limping home. The ignition has fixed that completely, but now I'm back with the initial problem. The unit's an Ultra-spark. It's working really well, tickover's so much better and the range of revs that don't have my current issue are so much smoother. I really don't think it's that.

midvillesb · « **Reply #10 on:** April 25, 2012, 08:12:16 AM »

This sounds very similar to the problems I had with my current 126 when I first got it (and the problems that Worm, the previous owner, had for a long time before).

Worm did all the things you did including rebuilding the carb etc., I did all the usual checks to see if the fuel pump was OK (ie disconnect the fuel line at the carb, stick the end of it in a jam jar and get a glamorous assistant to operate the starter while you check that petrol comes out) but was still completely stumped.

In the end I got a fully reconditioned IMB28 from a bloke in Poland off eBay, fitted it and it's run beautifully ever since. When I took the old carb apart, I reckoned that the fault was the float valve sticking shut.

Good luck!

Mike

Hazey-n-Marc · « **Reply #11 on:** April 25, 2012, 08:22:44 AM »

It could just be running very lean, when the emissions test came in, everyone with a carb and no cat panicked and turned their fuel mixture down. Some turned it back up after, whoever had our car didn't.
I am still tuning the flat spot out (winning slowly, bit like an extended siege

) does yours lose the flat spot with the choke on?

Marc

Hazey-n-Marc · « **Reply #12 on:** April 25, 2012, 09:08:46 AM »

If you install clear fuel filters between the pump and the carb, and on the return from pump to tank, as well as the send from tank to pump, you should be able to see if the pump is doing it's thing. Sounds a bit mad, but allows you to see where the fuel is getting to.

Lina · « **Reply #13 on:** April 25, 2012, 12:21:16 PM »

Mike - I hear you on that and I have been thinking about getting a refurb one on ebay. Mostly due to my lack of confidence in my own workmanship (workwomanship? lol) for the overhaul I did with the kit I got from Ricambio. Thing is, I know more than I did back then (couple of weeks ago) and I may've missed something obvious in my ignorance. I think I may go ahead and get the refurb one, it's not a huge amount of money and means I can just pop the old carb off, check the bakelite thingy and put the new carb on... tempting! It certainly can't hurt and it would be a reassurance that everything's fresh and new in the carb area (and not relying on my meagre mechanical skills!!)

Marc - my mixture screw has been right in (admittedly when the idle screw had worked itself loose!) and out to 3 turns. Seems to make no difference between 1 and 2.5 turns to the flat spot. And I tried the choke thing too, no difference either

I like your idea with the fuel filters! It does indeed sound a bit mad, but as my obsession with this flat spot is causing it's own kind of madness, I can see how easy it is to get obsessive about where the fuel is going!! lol

ciaoflyer · « **Reply #14 on:** April 25, 2012, 06:16:07 PM »

Quote from: Lina on April 24, 2012, 09:47:14 PM

ciaoflyer, I'd check that if I had one! Do BIS's have them?

Rusty - I'll get on and check that during the week, although I did check it was clamped down tight a few times, but it could indeed be the bakelite thingy, which I didn't check when I stripped the carb.
Centrifugal advance is working fine, as checked with the strobe on Monday.
I have now got back from taking it out for a run and I have additional info... I took her flat out up a steep hill in 3rd and she really wasn't happy! First few seconds were okay, but then started losing power, cutting out and popping and spluttering like a smoker in the morning! So maybe you're right too dajwid, maybe the fuel pump is also at fault? I'd thought she was mostly okay at speed, but admittedly, I haven't taken her flat out for sympathy for an old engine...!
This spluttery thing is any time I try to accelerate with any more ooomph than a slow, gradual increase in speed.

As an aside, that may or may not be related, having taken her out this eve, I was reminded of another problem - when the engine's hot and I've left the car for maybe 15 mins parked up, if I go to start her up again, she starts okay, but maybe 10 seconds in or maybe 6 foot of driving, she'll just die. I then have to wait about 10 mins and try again. This has happened a few times, but one time I went back to look at the engine and the fuel filter was empty (it's one of those inline seethrough ones), so I'd presumed no fuel (I have a temperamental fuel guage...! Another problem, for another day lol).
It occurred to me this eve that could it possibly be the fuel in the float getting evaporated in that standing time? (this cutting out thing doesn't happen when the engine's been stopped only a few mins like at a petrol station).
It's definitely not caused by a blocked petrol cap vent - I need a new cap at some point as it was hit in an incident with the previous owner, so has a crack in it - there won't be any vacuum in the tank causing this!

Thanks for all your advice so far!
Lina

Sorry, yes only the Bis.