Strange Weber IDF problem - loss of main circuit?

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Nut124
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Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

Hello forum, I could use some help with this strange, occasional carburation(?) problem.

This is a Fiat 124 1800 twincam with dual 44IDFs. It runs great, 99.9% of the time.

Tonite, leaving a friends house at 9pm, it would not pull past 3000rpm under load. Acted like the main ckt was disabled in at least 1, more likely two barrels. Idled perfect and I could cruise home at up to 45mph, just under 3000rpm. If I tried to step on it, it would not pull, tailpipe sound would sound harsh, lean. The AFR gauge would read normal 12.5-13.5 at this idle cruise.

This has happened one or two times before. Always runs and cruises fine on the idle/progression ckt, but will not pull above 3000. Today, I stopped and checked fuel pressure at the carbs; it was 4psi, normal. I have a very nice quality gauge there.

After cruising about 5miles on the idle circuit, I could suddenly tell it got fixed and would pull again, which it did.

What is this? Stuck float? Float valve? These are the chinese IDF copies. I got some "genuine" weber IDF floats and replaced the floats last weekend, expecting to have no more issues. The chinese floats were 1-1.5mm wider than the genuine weber floats. I thought perhaps they were interfering with the bowl.

This Wednesday, I had the car at the Gingerman road course where I ran 3 20min hot lap sessions w/o any trouble.

Is it possible to foul a plug where it runs well on the idle circuit but not under load?

The fuel pump is a Holley Mighty Mite. Is it possible for a pump like this to suddenly drop down to low capacity where it can only supply idle, but not the main ckt?

Any suggestions would be appreciated.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by modok »

if the fuel level in the bowls is low, you will have a lean spot in transition, but WOT will be mostly ok.
it is not clear from your description if this was the case or not.

yes it is possible to be an ignition problem, but not certain.
The higher the cylinder pressure, and the higher the RPM, the more power it takes to fire a plug, so am ignition problem can behave like a rev limiter in some cases.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Geoff2 »

What did the A/F gauge read during the trouble period?

Have you got air cleaners? Maybe blocked air corrector jet.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

I do have a little lean spot between the progression and the main ckts, but if I give it more gas the main circuit picks righ up normally.

In this incident, though, if I gave it more gas, the engine would not pull at all, or possibly on 2 cyls only, and sounded bad and would not accelerate. AFR would go from normal 12 something to 15-16 or so. The condition persisted for miles, but it cruised perfectly well on the idle/progr circuit at about 40mph.

About a mile from home it suddenly recovered and started to pull strong, as normal. Seemed like the recovery happened at a spot where the road is very bumpy. Not sure if it is related.

My perception is that one IDF somehow gets the float stuck in a way where only a trickle of gas is coming in and it cannot support load but can idle.

The air cleaners are oiled cloth type and seem to work well.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

It did it again! Drove to Airzoo to see a B29 take off, and land 30min later, then taxi to the back lot.

On the way back, on a highway entrance ramp, floored it. Got to about 75, then total loss of power, sensing lean. After that, for the rest of the way home, it would not pull beyond 3000rpm. Drove OK on the idle/progression ckt. Had to limp along to the next exit, then cruise home on backroads at 40mph.

Pulled over; fuel pressure OK at 4psi. Pulled main jet assys, fuel level seems ok.

Is it possible to foul plugs so they fail under load but run OK at lower speeds? These are NGK 6's.

Should I try hotter plugs?
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Re: Strange Weber IDF problem - loss of main circuit?

Post by allencr267 »

It accelerates but falls down, only running a consistent lousy missing low/medium max rpm.

Fuel pump, or supply problem/restriction before or after the pump.

The carbs act like a reservoir/accumulator, filling up when below the fuel supply threshold, giving you a temporary adequate supply when accelerating.
IDK how those pumps work at all.
Good luck.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Fusion Works »

What do you have for ignition components?
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

Fusion Works wrote: Sat Jul 24, 2021 9:38 pm What do you have for ignition components?
I have Fiat, -79 and later, pointless electronic ignition with a magnetic pickup in the distributor. I pulled the plugs. #2 was all black. 1, 3 and 4 were grey, light brown. It seems #2 may be the culprit. I have been having trouble with #2 from day one with non-responsive idle screw etc. Perhaps I should just replace the front IDF rather than trying to understand it.

The strange thing is that I ran it on a track just this past Wed for 25+ laps all out and no problems. Plus 50 miles one way there and back.

I have had to blow, clean the #2 idle circuit for inconsistent idle screw response and I have had a feeling that there is something wrong with the idle ckt in the #2 bbl. Also, the weird thing is that this problem has recovered on it's own while driving, like last nite. Just suddenly, after limping along for 5 miles, it takes throttle and starts pulling.

??
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Fusion Works »

These Chebers are questionable and seem hit or miss. Might need to pull the carb apart and go through every passage and see if there is a flaw or debris blocking the passages. A non responsive idle jet points towards an issue with the carb itself.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Geoff2 »

I think you are going rich, not lean. This happened to me. You went WOT & then had 'the problem'. I suspect the float dropped & then jammed in the down position when you were @ WOT. In my case, the float drop was excessive, causing the float to bind on the body at full drop. Weber changed the specs also, indicating there was a problem. Change the float drop to 23mm, originally it was 32.5. If not already using it, 11-12mm float level might work better than 10mm.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by epoxy124 »

I had a similar issue with 45DCOEs on a fiat engine also - the retaining bolt holding the auxillary venturi ( "booster") on one barrel had fallen out despite a retaining tab. It was not sealed enough for the airflow to pull the emulsion through and the engine was then limited ironically to the speed limit, haha. Some twisted wire through the retaining bolts instead of the dodgy factory tabs has solved the problem, for the last two years at least. The problem manifested in a very similar manner to what you describe, my AFR likewise was not showing lean during the hesitation but it sure felt like it.
Im not sure how the auxillary venturi's are retained in IDF's
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

Geoff2 wrote: Sun Jul 25, 2021 5:02 am I think you are going rich, not lean. This happened to me. You went WOT & then had 'the problem'. I suspect the float dropped & then jammed in the down position when you were @ WOT. In my case, the float drop was excessive, causing the float to bind on the body at full drop. Weber changed the specs also, indicating there was a problem. Change the float drop to 23mm, originally it was 32.5. If not already using it, 11-12mm float level might work better than 10mm.
Geoff,that makes sense. I did wonder about the drop as it is obvious from visual inspection that 32mm drop would have the float interfering with the bowl.

What explains the fact that post-incident, the engine would not pull above 3000rpm but would run OK on the idle circuit? Could an ignition module be bad in such a way that there is no spark at higher rpm? This is the Marelli electronic module.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

Geoff, I think you solved my problem! Thanks, pending further testing, but looks good.

I did set the float drop to 22-23mm and went for a long test drive. No problems so far.

I have been struggling with what appeared to be total lean-out condition after long (15+sec) WOT pulls. Usually always on a straight, smooth highway. Sometimes it would take 15-20sec to recover from it. Often I had to pull over to work on the carbs. I thought this was a jetting or fuel delivery problem, but I now beleive it was likely the float stuck in the down position, like you suggested.

The AFR gauge reads the same whether super lean or flooded, fouled plug; both read lean as the sensor reads O2, not hydrocarbons.

Here's my theory; when the float gets stuck, the carb floods, fouls the rear barrel. This is because the Fiat engine is tilted slightly to the back and the level in the rear eTube well is higher, excess fuel is drawn into the rear bbl. Eventually, the float breaks loose, but the plug is fouled. The engine will not pull on 3 cyls, but one can cruise, low speed on 3. After some lean cruising, sometimes the fouled plug dries out and starts igniting. Runs good again.

The reason it never did it on the track is likely because of the "violent" nature of the track driving; a lot of curbs, g-forces shaking the float loose.

I'm surprised there is not more evicence of bore wear from excess gas. Did a compression check and all cylinders read higher than after build, 180-190psi.
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Re: Strange Weber IDF problem - loss of main circuit?

Post by allencr267 »

Have you driven it at somewhat less then the MAX you've continuously been able to coax out of it?
Drive at less then that max, 15-20 secs, down 20-30% is all it'll take, now accelerate/WOT/whatever.
Much higher RPM till the reservoirs drained?

////
Stuck float woulda hydrolocked it.
///
IDK what the insides of pump will look like, piston/valves must intermittently stick or not travel fully.
Please take it apart.
Thanks, Good Luck.
////
Also, hows the flow differential between the front & rear throats of these China things? Much, hardly, not at all?
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Re: Strange Weber IDF problem - loss of main circuit?

Post by Nut124 »

Back to square one. I adjusted the float drop per instruction from Geoff2. Initially drove fine, but the next day lost power again on a Hwy entrance ramp. After 10sec of WOT, sudden loss of power, would not accelerate. Had to cruise home again at 45mph, 3000rpm. Pulled the plugs at home; they all looked the same, nice ly brown, almost grey.

Got a new old stock GM/Fiat ignition module to try next.
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