Odd Running Problem

[DE15 CAT]

Hello everyone.
Just thought Id ask, being honest have had it checked by reputable Cat 7 garage who cannot identify the fault as Its intermittent.

basically it can start pulling back, backfiring type of behaviour. sometimes after a bit of harsh acceleration, normally when warmed up, definitely if I have to sit in traffic when I get moving again. 1st. I had it on a rolling road & adjusted when first buying the car & problem occured.
(all of the following carried out by the same Cat 7 garage)I thought a fuel feed or timing problem but 7 garage has checked & said not, in the past have also had the ECU removed & put back to conventional electronic ignition, had various new batteries fitted, carbs have had new jets & all rubber part renewed, whatever is done the problem appears to go then returns within a few months.

The garage has checked many times for any loose connections, they can only 'think' it must be the car wiring being old (1997 S3)

My mechanical skills are basic ie can change parts but diagnostic skill rubbish. Any advice greatfuly received

[TeflonTurd]

What engine?

[DE15 CAT]

TeflonTurd said:

What engine?

1700 crossflow.

[TeflonTurd]

My gut feeling is a loose auxiliary venturi in the carbs. Had this on a VX years ago. There are two ways of retaining the auxiliary venturi, but essentially if loose it can rotate and intermittently cut off fuel supply on one cylinder. It only has to move a small amount to do this, and is aggravated by more aggressive driving, as the fule flow rate is greater.

Look at the diagram below and it is part 17 which gets loose held in by screw 28:






There are two auxilary venturies in each carb, one in each intake. You only need to remove the filter and the air horns to get to it, but should be able to rotate it slightly if loose.


You can see the securing screws held in by metal tabs in here:

[TeflonTurd]

Oh and if you had your car on the rollers, its likely they would try different size chokes to find the best power/torque compromise.

You have to remove the auxiliary venturis to change the chokes hence why they may be loose.

[DE15 CAT]

Thanks Tef. prob. by weekend I will get chance to look. Hope it is that simple, but if it is will knock my confidence in my regular 7 garage that they did not spot it. Will let you know.

[DE15 CAT]

Ok, just going to have a look hope Tef. is right, prob. no time to post result before I have to rush off, already late on my planned day, away for about a week report back as I will be taking the 7, although not doing high mileage a wk. or so will be a fair trial. (As I mentioned before it runs ok after a tinkering then problem returns later)

[DE15 CAT]

Well back from travels & domestics, week longer than planned but things take over. Tef. thanks for your gut feeling problem almost gone, as you look at the trumpets it was the right one of each carb, didn't turn rocked sideways when I wobbled them, screw on left carb 1 1/4 turns, right carb 3/4 turn both so dirty looked as though not adjusted by anyone for years. (God knows what the rolling road garage & my trusted Cat garage had been looking at.) Anyway lively good acceleration no probs., last few days since warmer & had some traffic to sit in, had a few pops n farts, 2 pull backs but much less than before, acceleration rapid up to 60mph then slightly slower than before I remember. If you have any other thoughts pleased to hear them (It does shake confidence in garages who had the car & taken 100s £ for no fix, but Tef. by description can solve most of it by instinct & free advice) Cheers Teflon Turd much appreciated.

[TeflonT]

Hmm,

I would check they are aligned properly if you are getting the problem still. Perhaps you have tightened them in the wrong position. Does sound like you are close to fully solving. You will see how the fuel flows through the "spokes" in the venturi to the jet. If things are not in line, the fuel flow is interrupted, causing the lack of acceleration.

Glad to be of help. On other forums, you'd have been told it was a bad earth.

[kev b]

Just a couple of ideas.

Can you rule out fuel frothing or boiling in the float bowls, especially as symptoms mostly occur after a period of idling?

Have the float jets and needles been checked for dirt and proper operation?

Are the carbs sealing against the manifold properly if you push them up and down against the spring washers or have they been tightened up solid so they resonate with the engine?

[DE15 CAT]

Just returned from another week of chores n travels, the old Cat seems ok. last wk.end before I left Tef. had another look, loosened the screws previously mentioned tried to turn venturi with my fingers they seem stuck in there no twist just previous wobble, re tightened screws & locking nuts.

Kev. had previously checked myself & had garage check jets ect. but had a quick look again no dirt or over tight carb. seating so don't think fuel foaming through vibration.

Just relieved it seems ok. Thanks for advice lads.

[Yellow 7]

Good it's gone.

I'd add these things to check:
- Jet hats need good sealing gaskets. The orginals shrink and become oval and don't seal
- Jets not loose
- Manifold to carb 'O' rings deteriated, I had once of the bolts come and the ring slip - that you know instantly and loudly. The new style moulded rings / plates cannot do this.
- Fuel pressure too great (assume you have pressure regulator) - if not, you need one
- Assume you have an electric pump, Facit pumps were very unreliable a while back. Usually they just stop but I have seen them play up causing fuel starvation and then cure themselves for a while
- Carb float valves are in good nick - nothing sticking and adjusted correctly, 1/8" drill (pretty sure it was 1/8", I have a Weber tuning guide somewhere)
- Carbs in balance - linkage in good nick and is not wandering off
- Unlikely but the butterfly spindles can wear and causes problems
- Inner venturi (22) loose - pointed screws (28) secure them
- My first 7 had plain wrong jets in it and would do what you say, but did it the time. Richer jets cured it.

I used to do a DCOE clinic at our local L7 club meet - it was usually one of the above that was causing the problems.

Edited by Yellow 7 on Thursday 13th June 20:30

Edited by Yellow 7 on Thursday 13th June 20:32

[DE15 CAT]

Thanx, Yellow 7, will keep advice for future problems as it's a concern that £100's poorer, experts had not solved it.