Stumble, stall under throttle

[jospanner]

My van ('81 Aircooled 2.0) has been stumbling when opening up the throttle.
Previous owner had the timing advanced much further than spec (according to Haynes), so I decided to fix this because I was worried about pinking. The timing now shows 4-6 degree ATDC with the idle control unit connected, at idle.

Symptoms are: a stable idle, but stumbling and stalling when opening up the throttle, and occasional pops back up through the carburettors.

Chokes seem to work correctly (they set and then come off as the thing heats up).
Distributor vac hoses seem OK.
Turning idle speed and mix screws do nothing with the idle control connected, but do seem to work with the thing disconnected (assuming that's normal).

The engine is a replacement from 2004, has done ~35k miles. No smoke from the exhaust but occasional black specs on the ground.

Can anyone help? I'm running out of ideas here.

[JeffRoo]

Not sure this will help any, but let me share a little story.
We were on holiday in the Hebrides and I had a problem with power under load. It would get there but it struggled. Ended up looking at the vac on the dizzy, the dizzy itself, changed the plugs for spares, the fuel pump (mechanical) and even ended up changing the fuel filter on the side of the road….all to no avail. Anyway holiday over and headed back down from Scotland. On the motorway I couldn’t even overtake a lorry and hills? Forget them. Last ditch attempt was at Lockerbie services, where I sent the missus in for some sustenance while I emptied the back to get to the engine. I couldn’t drive all the way to Somerset with the issue. Last ditch attempt, I changed all the ht leads with my spares. Put it all back together, and it didn’t miss a beat all the way home. Ditched all the ht leads as I didn’t know which one was faulty and I couldn’t care by that point. 

so, maybe, just maybe, check your ht leads and see if that cures the problem. They can break down over time and cause the sort of issue you are describing…. They did it to me……

[T25Convert]

Could be fuelling related.

Two things to check to start with:

1) you don’t have any air leaks - spray carb cleaner or similar around all the various manifold joints, and also around the throttle spindles. Any change in engine speed indicates a leak that needs fixing.

2) check accelerator pumps on the carbs - when you floor the throttle a little pump should squirt neat petrol into the carb whilst the main fuel circuit catches up. Not had an air cooled t25, so not sure of exact arrangement but normally with the ducting off the top of the carb you can peer in with the help of a small mirror - with engine off you should be able to see a little jet if fuel getting squirted into the main carb orifice.

Good luck!

[Rosie n' Jim]

Is it just whilst opening the throttle it stumbles, then picks up and runs ok?

[300CE]

In addition to the testing above, I had the same kind of issue which turned out to be a failing coil. I had an crap aftermarket one in place which failed and would only work on pull away & almost full throttle, nothing in between.

To check go for a short drive and then feel the coil, if it is hot to touch its a sign of possible failure - you can also check the primary resistances here:

https://www.brickwerks.co.uk/brickwerks ... ion-coils/

[jospanner]

Hi! Thanks for the replies! Here's the latest from today's tinkerings...
 

• Timing set to 4-6 deg ATDC with idle stabiliser bypassed and both vacuum tubes connected, and idle speed at minimum (speed screw in).
  
• Both dizzy vacuum canisters work just fine, applied a vacuum to them while looking at the engine under a timing light. Not leaking, either.
  
• Fiddled around with the idle mix and speed screws: they seem to affect the engine as expected. Too much or too little mixture causes rough running, idle speed drop, etc. Roughly how far out should the mixture be? I thought I read 2.5 turns somewhere, so that's what I went with, and it seemed to give me the best result.
  
• Adding more mixture helped a bit (see vid) but the stumble is still present.
  
• HT leads are brand new and the firing order should be correct.
  
• I can pull the accelerator pump linkages out slightly against the screw, and it causes drop in idle speed, so I think they're working.
  
• Chokes are working.
  
• No obvious vacuum leaks, sprayed WD40 and easy start around the base of the carbs, around pipes, cutoff valves etc.
  
• Coil is out of spec, but not by a lot. I think it's an aftermarket one (blue label).
  

This is the result from today. It's not very obvious in this video, I'll try and get a better one, but you might hear it - it's misfiring under open throttle, but it does pick up revs.


I am getting odd readings off my tachometer, too. And yeah, the previous owner had the thing running way too much advance, almost as far as it would go. I do have an old VW Polo (mk2), I wonder if the coil from that can work...

[T25Convert]

Glad to hear carb is working as expected.

Coil sounds like it’s worth a punt. What are the connections like as well.

Did you do new plugs with the new HT leads, either way are they correctly gapped?

[jospanner]

A couple of things I found just now:
Unplugging/plugging the various electrical connectors on the carb affects RPM in ways I'd expect by lowering idle RPM as the carbs get their fuel supplies shut off - except the one for the "idle enrichment device" which apparently does nothing, neither at idle nor under acceleration. I don't know what this device does - neither Haynes nor the guide I have here really say anything about it. Disconnecting the wire makes no difference to acceleration or idle speed.
https://imgur.com/tCM8fOy

There is also a hole here in the carb, I don't know if that's intentional:
https://imgur.com/fmLfNR3
 

Glad to hear carb is working as expected.

Coil sounds like it’s worth a punt. What are the connections like as well.

Did you do new plugs with the new HT leads, either way are they correctly gapped?

Plugs are as they were, I have no idea about them, I just took one out to make sure they weren't covered in oil or burnt or something (they looked fine, honestly). I'll have a look at those tomorrow, and check the gaps.
Gonna see if I can get the Polo coil on there and see what happens. The connections on the existing one are aged, but seem to make a good connection. Weather permitting, anyway!

[tobydog]

Sounds carb related to me. I'd take them off, strip them down and check for blocked jets, water and crap in them. What state are the fuel hoses and filter in?

[T25Convert]

If that hole goes all the way through into the carb it might not be all of the answer, but it’s definitely contributing!!

Try sticking something over it and see what happens - it’s on the engine side of the carb so will be drawing in unmetered air and giving you lean conditions.

[Rosie n' Jim]

The hole should have a grub screw in it which secures the venturi in the card body. Both carbs have them. It is unlikely to leak much air. The enrichment device richens the mixture when very cold, will have no effect other than on a cold winter morning.

[jospanner]

Tried swapping the coil today, no joy. New plugs, too.

Engine accelerates great... as long as the distributor is advanced until the vacuum connection hits the airbox! That can't be right.

Would rather not take the carbs apart unless there's definitely an issue there. As I said, it idles just fine, and with the timing thrown off it actually works, but that's obviously hiding a bigger issue.

Tried blocking off the "hole" in the carb and no joy either.

Get a puff of black black smoke on acceleration too. No idea if it's normal.

[jospanner]

Sorry for double post, but I may have had a breakthrough:
Accelerator pump is giving way too much fuel. Should be 0.5-0.8ml, and it's more like 2ml.
Is the rod connecting the throttle linkage to the pump (the one with the spring on it) meant to have a kink in it?

[ajsimmo]

Have you balanced the carbs? It's the first and most important step in getting the best from a twin carb setup.
Get your airflow at 3000 revs equal first, then adjust the air at idle on each carb to get it balanced, then timing right and idle circuit (3rd carb) as lean as possible whilst maintaining a smooth idle. You're aiming for each carb to do about 1/3 of the work. Then when you disconnect the idle shut-off solenoids on each carb the idle speed should drop equally, but still keep running (just, lumpily!).
You're right about the accelerator pumps, they need to be leaned off by the sound of it, as momentary flooding could cause the symptoms you describe.

Sent from my ONEPLUS A3003 using Tapatalk

[jospanner]

I think I've mostly fixed the problem.
The carb mixture settings were completely out of adjustment:

Both carbs had their accelerator pumps set up way too rich, and one of the rods was bent. Fixed that issue.
Carbs were unbalanced, I think I've fixed that by adjusting the linkage and idle screws.
Both carbs had their mixture screws all odd - one was 3.5 turns out, the other 1.5. I reset them to 2.5 as a ballpark adjustment.

After doing this, and setting the idle "third carb" mixture to 2.5 out as well, it's running much, much better. It's not quite perfect, but I think this is as good as I can get it with the tools I have - I don't have a CO meter, after all. I'm going to try and find somewhere that knows how to work on these and then get them to properly do the mixtures.

Thanks for the advice! This has been quite a head ache.