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Another bogging/lugging problem
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mikedjames
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PostPosted: Tue Apr 20, 2021 11:00 am    Post subject: Re: Another bogging/lugging problem Reply with quote

The plugs should fire when the points open, the collapse of current is faster than the build up of current as the points close.

If the spark is weak, you may have a defective condenser.

Cranking by hand with points should result in the same spark as when the engine is running , certainly at low RPM.

The thing with the bogging off idle, is that the first thing that happens is the idle circuit loses flow, and the air in the throat is not moving much. So it takes a while for the air speed to pick up and start sucking fuel out of the main jet.
So the engine runs very lean for a moment. The accelerator pump makes up for this after a short delay, then the engine speeds up and the air flow starts sucking fuel out of the carburettor through the vacuum in the throat of the carb.

So if you have too large a dose of fuel when you pump the pedal, it goes lean-rich-slow-lean (not enough air flow).

If you get it right it goes lean-about right- faster -correct ..

If the dose of fuel is too small it goes lean -lean -slow -lean.

If your plugs are wet it is not managing to burn all the fuel, so there is probably too much around, or the spark is very weak and the fuel is cooling down the burn.

My progressive is as always very bad if you stamp on the gas in the first few miles, but once the cylinder heads warm up and the idle air feed from the air cleaner is up to 86F /30 C, less of the fuel sits as a puddle in the manifold when you stamp on it.
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PostPosted: Wed Apr 21, 2021 9:36 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Massive improvement...

Summary: -

I think the main problem was timing, but the accelerator cable was also misadjusted.

Long story:-

I swapped the pump mechanism from the Solex to the EMPI, but the discharge volume didn't increase, even though the lever can now move its full distance.

Strange that neither pump discharges the amount stated in the Bay Bently Manual for a 1600 dual port though. Confused (see below*)

I set the pump to the middle setting, which is the maximum discharge volume, and put everything back together.

In the course of reassembling; I adjusted the throttle cable and realised it was jamming in the barrel nut. Not sure if it was always like that. I changed the barrel nut when I rebuilt the engine. I drilled the nut out and adjusted again.

I set the cable so there was no play at the top of the pedal and the throttle is wide open with the pedal to the floor. The cable ferrule sticks through the barrel nut further than I remember it ever doing. Maybe the old barrel nut was tight too. Confused

I took it out for a run. It was definitely more sprightly in general, but still boggy while accelerating under load. I think this improvement was due to the accellerator adjustment (no play, so immediate throttle response).

With the engine warmed up; I did a tuneup. When I set it last, I set it at max advance with the vacuum plugged because most people seem to recommend doing it that way.

This time though; I did it by the book and set it to 7.5 BTDC at idle (850 rpm). It was slightly retarded from that setting when I started (closer to 5 BTDC).

I re-checked the dwell but it was fine (50, so in spec for new points and these are basically brand new).

And finally, I adjusted the carb to account for the new idle jet.

Took it back out and it was massively different. It pulls much more strongly in all gears and much better under load.

I suspect that the timing was the main issue. I don't think the problem is completely solved (it stuttered a little pulling away at a couple of roundabouts), but it's close.

Pulled the plugs when I got home and they're cleaning up nicely.

I'm going to try backing off the accelerator pump again over a few weeks until it has a negative effect. Then I'll try going back down to the 55 pilot/idle jet to see if that improves things.

-------------------------------


*The pumps have fully discharged when the throttle lever reaches the half way point anyhow. The linkage attached to the pump diaphragm reaches the limit of its travel, but the actuating arm that moves it keeps going. The actuating arm slides through the linkage attached to the pump diaphragm and a spring on the arm just compresses, keeping pressure on the diaphragm linkage. That prevents the throttle lever from stopping at the halfway point, when the pump is maxed.
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PostPosted: Thu Sep 29, 2022 10:22 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Dragging my own thread back up...

Having apparently fixed the accelerator pump issue and increased the idle jet size as per earlier posts; I've got the engine to the point where it runs strongly when it's...

a/. Still on choke, or
b/. Fully up to temperature (after around 10 mins of running around in town driving), or
c/. at higher revs

...but it still bogs and stutters accelerating from low revs while the engine is off choke but not yet fully warmed up (after the first couple of minutes until it's been running for around 10 mins). The harder I try to push it e.g. flooring the accelerator, the worse it seems to be during that period.

It's most seriously a problem if I'm pulling away at roundabouts and railway crossings. I sometimes have to slip the clutch during that period to take some of the load of the engine and keep revs up, but once it's fully up to temperature it pulls strongly.

Any ideas what to check next? Or is that just normal behaviour for a stock 1600 dual port?
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chrisflstf
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PostPosted: Thu Sep 29, 2022 11:00 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Intake manifold preheat?
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PostPosted: Thu Sep 29, 2022 1:06 pm    Post subject: Re: Another bogging/lugging problem Reply with quote

chrisflstf wrote:
Intake manifold preheat?


Haven't felt the heat risers in a while, but they were clear when I rebuilt the engine a couple of years ago and were getting nice and hot then.

Stock oil bath aircleaner with warm air pickup tube picking up heat from the left hand cylinder head, under the tins.

Would carb icing be an issue on a warm summer's day? It was also doing this even at the height of our summer (which for once was slightly more than tepid Laughing ).

I will do a visual check when I'm out in it at the weekend though.

I'll also double-check that the thermostat hasn't burst so is working correctly to warm the engine up.
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Clatter
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PostPosted: Thu Sep 29, 2022 6:13 pm    Post subject: Re: Another bogging/lugging problem Reply with quote

How do you know your advance is kicking in correctly?
It should come in right as the throttle is cracked.
Razz Way too many crappy stiff diaphragms these days.
You 100% sure it’s tip-top?
My money is on the vacuum advance.

Could be the carb isn’t giving a good signal.

Putting in bigger jets or more squirt was always a band-aid for a 009..

Squirt issues will only show at tip-in.
If you’re holding steady and it’s not responding it’s not squirt.

Try bumping the advance up a good bit just quick for testing.
If the problem goes away i win the free pint!
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PostPosted: Fri Sep 30, 2022 4:31 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Who.Me? wrote:
chrisflstf wrote:
Intake manifold preheat?


Haven't felt the heat risers in a while, but they were clear when I rebuilt the engine a couple of years ago and were getting nice and hot then.

Feel them? Wow! If they are working correctly you should be smelling the burnt skin immediately!

The temperature at the ends by the way means next to nothing, it's the temperature in the center where the "tee" is that counts. Checking that the heat riser is clear is only half of the story. Very few aftermarket exhausts today create any flow through the heat exchange tubes. They all transfer heat by conduction to the ends of the heat riser and then people think they are working but the center of the manifold stays dead cold.
Who.Me? wrote:
Would carb icing be an issue on a warm summer's day? It was also doing this even at the height of our summer (which for once was slightly more than tepid Laughing ).

Not in my part of the world! It is very rare to have carb ice in summer. Carb ice manifests itself in a different way entirely. It limits top end power and the longer it goes on the more power you lose until the car won't hold highway speeds. You have retained VW's fix for carb ice with the air cleaner pre-heat so no you do not have carb ice! The heat riser was never intended for, nor does it work for actual carb ice.

Your symptoms would be more in line with fuel pooling in the horizontal portion of the intake manifold. As the air fuel mixture turns the corner at the "tee" the fuel, being heavier, wants to drop out of suspension and make a puddle there. This causes the rest of the mixture to be lean and the engine loses power and stumbles. Once the RPMs pick up (read intake velocity increases) the puddle gets flushed out and causes the engine to be too rich and again hesitates. The sole job of the heat riser is to keep that horizontal bit hot enough to evaporate that pooling gas as it tries to collect there.

If you are seeing 90*F or more with a heat gun with the engine running, pointed directly at the aluminum on the "Tee" connection below the carb you should be good and your problem is somewhere else. It's what Clatter is talking about.
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Last edited by oprn on Fri Sep 30, 2022 4:39 am; edited 1 time in total
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PostPosted: Fri Sep 30, 2022 4:36 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Clatter wrote:
Putting in bigger jets or more squirt was always a band-aid for a 009..

Squirt issues will only show at tip-in.
If you’re holding steady and it’s not responding it’s not squirt.

^^^This right here is gold!^^^
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PostPosted: Sun Oct 09, 2022 6:46 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Oprn - I grabbed my temperature gun today and took it for a run around town. I checked how the manifold was warming up. Results below.

It took around 15-20 mins for the mainfold to reach a stable temperature. The left riser definitely gets a lot hotter than the right one.

After 7 minutes (a couple of miles), the centre section where it joins the downpipe from the carb had barely increased by a few degrees C.

After 15 mins / 5 miles the centre was 42C and it basically stayed around that temperature as the rest of the manifold got hotter. By this point, the engine bogging had largely gone away.

The manifold heat peaked after around 25 minutes and the bogging was gone.

Results...

-Cold = 15C
-2 miles (7 mins) - left riser 15C, right riser 50C
-5 miles (15 mins) - left riser 88C, right riser 62C centre section below carb 26C and on the horizontal section 42C
-8 miles (25 mins) - left riser 143C, right riser 62C, below carb 28C, horizontal section 46C
-11 miles (32 mins) - left riser 126C, right riser 57C, below carb 32C, horizontal section 48C

At the end of the run the sump plate was 57C and the heads were 118C (left) and 111C (right).

Does it sound like it's warming up right? Is it just a case that the bogging off-choke while not yet hot is due to fuel pooling in the bottom of the manifold? If so, is that normal, or if not, what should I do to get the manifold to warm more quickly?

The gasket kit that I used (NOS) had two different riser gaskets - a copper-coloured one with a small hole and a silver one with a larger hole. I think that's to encourage flow through the riser in one direction. Can't remember which side I installed it, but I'm guessing from the results it's on the right hand riser.

Edit - found some useful information on VW Resource. Apparently the exhausts with the narrow tube on the left cause gas to flow from right to left in the manifold risers (right gets hotter). I have an exhaust with the narrow tube on the right, which makes sense that gas would flow from left to right (left gets hotter).


Haven't had a chance to check the carb vacuum signal yet, but I have tested the can in the last 500 miles (I have a vacuum hand pump). It advances and holds when vacuum is applied, but I haven't checked how much vacuum the carb supplies.
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PostPosted: Fri Aug 04, 2023 2:38 am    Post subject: Solved (I think) - Another bogging/lugging problem Reply with quote

After chasing yet more ghosts; I think I've finally fixed the bogging while on choke issue and I think it was the choke pull off.

I've had the problem for years to varying degrees and with two different carbs - a rebuilt Solex 34 PICT3 with the correct version number to match the distributor model (early 70s Bosch SVDA from a T2) and an Empi copy.

I damaged some threads on the Solex ages ago, so bought the EMPI.
I managed to fix the threads earlier this year and cleaned and refitted the Solex.

I still had the bogging problem.

In chasing this down, I'd checked everything except the choke pull off, because I didn't know what that bit of the carb was, so didn't know what it was supposed to do, or how.

After stumbling across a thread on here that mentioned it, I took the choke element out and found that the pull off actuator rod was sticking in its bore.

I lubed it, and now the engine pulls strongly from a first start on the cold engine, all the way through to fully warmed up.

It seemed a weird coincidence that the EMPI unit also gave the same symptoms, so I opened that one up and found a different problem.
The pull off actuator rod moved freely in its bore, but side by side with the Solex, I could see that the slot in the actuator rod was longer than the one on the Solex carb. It looks like it wasn't opening the choke until later in the actuator's stroke. It probably wasn't opening the choke up much at all under accelleration.

I've been running the Solex again for a few weeks and it's been fine. It's a night and day difference, and for the first time, this truck feels safe to drive from cold.

I'm updating this thread in case anyone else is stumped by the same problems (severe bogging part way through the choke's cycle).

It's worth checking the choke pulloff.
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PostPosted: Fri Aug 04, 2023 6:20 am    Post subject: Re: Another bogging/lugging problem Reply with quote

Thanks for the update! So many of these problem threads never see a resolution posted. That part of the carb is something that I rarely think about never mind check.
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