Revs drop when floored...

jury · Post by **jury** » Wed May 06, 2020 4:51 am

...thats essentially what is happening.

I had been chasing a running rich issue on the ModRod for a while:

2.2E
WEBER 40 IDA (factory spec Idle jets, one size bigger main jets)
MSD

I have fitted a digital AFR meter to observe what's happening across the rev range (not 100% sure how well it works on carb cars). There used to be a slight stumble around 3000 if you floor the throttle, but the bigger main jets seemed to have fixed this. However if I floor the throttle at idle, then the revs drop and the AFR shows it running lean.

I tried this yesterday on the driveway - Engine was warm and happy at idle, floor the throttle, revs drop and then stalled. However, If I slowly roll onto the throttle then it picks up fine and smooth .

Any thoughts on what may cause this ? Air corrector jets ?

Also the car does feel like is bogs down abit at low RPM, especially coming out of a slow 2nd gear corner. Not sure if this is connected to the idle issue, or is timing related etc.

Any words of wisdom appreciated

Thanks
Chris

KS · Post by KS » Wed May 06, 2020 6:06 am

Sounds to me like the accelerator pump jets (#70/71) and/or their circuits are either blocked or maybe the accelerator pump diaphragm on one carb has failed (#29 on the diagram).

Bruce M · Post by **Bruce M** » Wed May 06, 2020 8:08 am

With wideband diagnostics, it’s worth remembering that if you have a misfire (due to any reason, such as coil failure), the AFR will show lean. This is because the oxygen isn’t being consumed. So even if the tail pipe smells pig rich (cause it isn’t burning), the AFR can be misleading.

KS · Post by KS » Wed May 06, 2020 9:50 am

Also, ref: the bogging down out of a corner, check float heights...

jury · Post by **jury** » Thu May 07, 2020 5:58 am

Thanks for the info guys, that's great.

Removed the Accelerator pump Diaphragm to check for damage, however it turns out that it has incorrect Diaphragms fitted.

The carbs have early roller activated pumps, compared to later spring activated. The Diaphragms fitted are for the later style.

Early Diaphragms are ordered and looking forward to seeing what difference they will make. Hopefully this solves afew problems in one go.

Will then check float levels, good call.

Onwards !

Cheers
Chris

Sam · Post by **Sam** » Thu May 07, 2020 6:38 am

Bruce M wrote:With wideband diagnostics, it’s worth remembering that if you have a misfire (due to any reason, such as coil failure), the AFR will show lean. This is because the oxygen isn’t being consumed. So even if the tail pipe smells pig rich (cause it isn’t burning), the AFR can be misleading.

Hopefully the new diaphragms will work, but if not the above is exactly what I first thought. Suddenly dumping a very rich mix can cause a weak spark not to have the power to ignite. 99% of fuel problems are electrical, as they say.

impmad2000 · Post by **impmad2000** » Thu May 07, 2020 6:47 am

What spark plugs are you using Chris? I had a tick over misfire and general sluggish behaviour. The plugs I was using didn’t suit the engine and were not self clearing ( running too cold) . Changed to different plug and suddenly it’s sparkly again.
But do check out the pump diaphragm first.

Phill · Post by **Phill** » Sat May 09, 2020 7:49 am

Following with interest. I have a similar issue although I don't think as pronounced as the OP. It's worth remembering that AFR well show lean momentarily when you floor it due to the large volume of air going in.
I like Tim's observation, I should take my plugs out and check them.

andytat · Post by **andytat** » Sun May 10, 2020 1:33 pm

Tim.
Out of interest, what plugs did you change to.
Sorry for the hijack Chris.

impmad2000 · Post by **impmad2000** » Sun May 10, 2020 5:36 pm

I changed from NGK BPR7ES to Bosch WR5DC.

jury · Post by **jury** » Mon May 11, 2020 10:23 pm

impmad2000 wrote:What spark plugs are you using Chris? I had a tick over misfire and general sluggish behaviour. The plugs I was using didn’t suit the engine and were not self clearing ( running too cold) . Changed to different plug and suddenly it’s sparkly again.
But do check out the pump diaphragm first.

Thanks Tim, Diaphragms ordered and should be here this week. Spark plugs are BP7ES, can't remember what they were originally. Fitted these because I was told they were hotter. Will take look at the Bosch WR5DC

Will report back

Cheers
Chris

Bruce M · Post by **Bruce M** » Tue May 12, 2020 12:04 pm

One step hotter with NGK is BP6ES
The heat number is opposite for Bosch (higher is hotter), whereas lower is hotter for NGK.
Or you could try, BP6ET if you fancy an experiment. Same but with triple ground electrodes.

Note, the “R” letter in the code, for both, is for resistor plugs.

impmad2000 · Post by **impmad2000** » Tue May 12, 2020 3:04 pm

I use resistor plugs so prevent miss triggers on the EFI.
It is probably a misnomer to think hotter plugs will help. They will help if you get pinking, but if you go hotter and the engine doesn’t generate the heat then they run the risk of not clearing any fouling ....
I have a set of BPR6ES to try when I get a chance

jury · Post by **jury** » Sat Jun 06, 2020 3:25 am

Back on track with this...

Fitted the new "early" style diaphragms.....Really excited that this would solved the problem, but turned out that these aren't the ones needed.....Diaphragms tore at first pedal pump !! Went back to the later diaphragms and fitted Bosch WR5DC plugs. It still bogs downs when flooring the accelerator at idle, though it feels like it pulls really well when flooring it from 30 mph and above.

There was abit of crap (oxidation) in the right side accelerator pump housing when I dismantled it.....cleaned it out, but now wondering if it has blocked the circuit.

Next step to check is timing.....I think

Chris

mycar · Post by **mycar** » Sat Jun 06, 2020 7:06 am

Morning Chris, It's a while since I fiddled with Weber carbs but when I had them in bits and when I was lost and didn't know where to turn I studied Performance Oriented website until everything worked again.

Tech 1 and tech 2.

http://www.performanceoriented.com/spec ... cedures-1/

Also this may help with knowing where things go.

https://www.ddk-online.com/phpBB2/viewt ... 4&p=478636&

And then, more recently with the 356, the mantra, perpetuated by wise old men on the Registry, has been 'Tune your engine' The idea that many of the things that go wrong with the powertrains of these old cars can be rectified by systematically working through a series of adjustments in order.

Adjust the valves
Attend to the ignition. Check sparkplugs, leads, dizzy cap, rotor arm and timing.
And then tune the carburettors.

I hope you get to the bottom of it, there's nothing worse than stomping on the gas and going nowhere.

DDK - Die Deutschen Klassiker