UK '79 2.0l Twin Carb - Lost Its Top End Power

[windswept]

Hello to you guys. I read many topics on here with interest, but need some guidance.

Our Devon bus burnt a valve seat, so we had a full strip-down done by a chap who used to service these when they were new.
This included all new valves, seals, heads re-ground, full carb strip & rebuild (Solex 30s), new Empi twin-tip exhaust (not a specialist one), plugs, leads, and a brand new Volkspares mechanical pump. We replaced all of the fuel & vacuum pipes.

We wanted to have electronic ignition and popped on a Bosch dizzy, originally fitted to a T25, with mechanical and vacuum advance (for all the reasons on aircooled.net) complete with the Hall unit.

So now it starts instantly, ticks over so smoothly you could probably balance a coin on the block, and pulls like it always used to at low revs and in lower gears.

However, she doesn't have the top end grunt and hesitates or 'stumbles' as one of your members said. We used to cruise at 70+ but she doesn't like more than 60-65 now. The faltering varies in intensity as the journey progresses, but not predictably(!).

My thinking is as follows:
The new fuel pump should be delivering, the carbs were done by a keen mechanic & mixture seems fine, the valve-gear has been re-set 3 times to make totally certain and the only non-standard items we've fitted are the exhaust & the dizzy.

My questions are:
Could the T25 1.9 air-cooled engine have had a very different advance curve?
Could it simply be the mechanical advance gummed up?
Without a rev-counter, how can I investigate further?

I look forward to any suggestions.

[RSorak 71Westy]

Sounds like a restricted fuel supply to me, check the filter.

[Amskeptic]

So now it starts instantly, ticks over so smoothly you could probably balance a coin on the block, and pulls like it always used to at low revs and in lower gears.

However, she doesn't have the top end grunt and hesitates or 'stumbles' as one of your members said.

My questions are:
Could the T25 1.9 air-cooled engine have had a very different advance curve?
Could it simply be the mechanical advance gummed up?
Without a rev-counter, how can I investigate further?

You need a stoboscopic timing light if nothing else.
With hoses off and plugged, the vacuum advance should pull off the idle setting by 1,100 rpm and make a decent move towards 12-16* BTDC by 1,500-1,800 rpm, then move up to 28-30* BTDC by 3,200 rpm.
As importantly, when you add the vacuum advance hose, the advance should wing on up to 30* BTDC in the 2,400 rpm range and shoot on up to 40* BTDC when you rev the engine to 3,000 rpm and release the throttle.

Without a rev counter, you need to rely on your ear for engine speeds. Either way, that mark should be able to move easily and return fully to idle setting.

The retard hose (if you are hooked up with it) is optional depending on your engine/carburetion circumstances. It should not be hooked up to a constant manifold vacuum. It should only be plumbed to the nipple just downstream of the throttle plate. You can see clearly enough if it will be blocked by the opening throttle plate, which is what we want.

Do not waste your time with exotic diagnostic considerations until you have the timing map squared away. The Bentley manual does have vacuum curve information, do not get too excited about the zillions of little variations, just get an overall sense of the vacuum/centrifugal timing advances of the 2.0 engines. If your centrifugal advance is known to hit 28*-30* at 3,200 or so, your performance deficit cannot be attributed to the advance. Then and only then, should you visit the carburetion.

You can have lovely driveability at low speeds/low loads with a perfect idle and low speed performance, and still be gasping for fuel at higher loads/speeds if the main jets are clogged/not sufficiently sized for the 2.0 engine/float adjustment is off/ etc. Also, the power circuit on PDSIT carbs (which you did not specifically mention) depends on the correct gasket with correctly positioned holes to draw fuel up into the drilling that dumps into the main discharge port. I do not know if you have PDSIT carbs or if they have been overhauled. Please do check the plug insulator color after a few minutes of highway driving where you can pull off and yank a plug after the engine has cooled but not sat around idling.

Please keep us informed of any developments with this. You do *NOT* want the engine to run lean at speed, that is like a bloody oxyacetylene torch.
Colin

[windswept]

Hi there, Colin
Thanks for the reply, which I appreciated. I have let work and a new baby get in the way of fixing the wife's bus!

Took her (the bus!) into a Bosch man to check the advances. He suspected they'd be OK, and he was right:
Static = 7*BTDC
Idle = 6*BTDC (no vacuum retard on our CJ engine)
The timing progressed smoothly up through the rev-range, to reach
Centrifugal only (Pipe off) = 26*BTDC at high revs
Centrifugal & vacuum (Pipe on)= 38*BTDC. =D>
So that seems fine.

He also checked the emissions from both sides (EMPI twin exhaust)
Cold (ambient = 15*C or 60*F) idle = 2.8%
As the chokes come off, this should ideally drop to 1.8-2.0, but he said that was difficult to achieve, even on new buses in the '80's!
And as you noted, we don't want too lean a mixture!
However, once fully warmed up, the emissions at idle were:
6% LHS and 8% RHS (!) - i.e. very rich indeed. I don't know how badly that affects the mixture at higher revs.

He also noted a flat spot, which could be no fuel from accelerator pumps, or conversely too much from them, or indeed very over-rich generally.
(Would account for our lower mpg)

As the rebuild in December included a brand new fuel pump, a full carb strip and re-assemble using all new gaskets, o-rings and diaphragms, (by another chap who worked on these in the '80's), I am at a loss

I'd rather not re-strip the carbs, but if it is needed, I'd like to know where to start. Any guidance?

Thanks
Windswept (Paul)

[Amskeptic]

Our Devon bus burnt a valve seat,
pulls like it always used to at low revs and in lower gears.
hesitates or 'stumbles' doesn't like more than 60-65 now.

The new fuel pump should be delivering, the carbs were done by a keen mechanic & mixture seems fine,

Could the T25 1.9 air-cooled engine have had a very different advance curve? Could it simply be the mechanical advance gummed up?
Without a rev-counter, how can I investigate further?

If you are suffering from low power at high rpms and it is due to poor fuel delivery. . . . off you go to new burnt valves/recessed seats.

Fuel pump on dual carbs should be delivering 5 psi and check for volume as well.

Keen mechanic must ensure that main jets are correct, but as important, there is a power circuit that must be functional on PDSIT carbs.

Advance is easy to check based on your own intuition. Get a stroboscopic timing light on the scale/pulley mark and remove the vacuum hose to the distributor. Rev engine to "I don't want to rev it any further" and read the advance. It must be 28-30-32* range. No less and no more. Now add the vacuum hose, rev the engine and release. It should get up to 40*.

Go on the highway (motorway?) and run it to 65mph where you lose power, keep it there for 5 miles and then shut off the engine and coast to a halt. Let engine cool for a few minutes and pull a plug. White insulator? Better check fuel delivery. Remember, pressure is only half the issue, it must maintain pressure and deliver a specified volume.
Colin

[windswept]

Hi Colin

As noted in the last post, the advance curve seems pretty close, and so the main problem now appears to be a very rich running engine. With a new Volkspares mechanical pump, and awful mpg, I am not too concerned about a lean problem.

Great news is I have located a specialist only 30 miles from me which does a full re-build to Solex's. This includes re-bushing the throttle spindles (a major source of irregular running & lean spots), machining the top & bottom housings to dead flat, reworking the chokes, full 4-stage clean of the whole body, setting it all up to as-new finish, etc.

It is www.carbex.demon.co.uk for any who might want to check it out. The guy is Dave Osborne.

So, as I want to have as-new carbs to work with, I'm off to start unbolting my pair right now!! I'll update you in 3 weeks or so.

Regards
Paul

[windswept]

~Hello again

I will take her out on a dual carriageway then stop to check a plug, soon.

I was all set to send off my carbs, but then I find out from the supplier of the new 2-litre fuel pump fitted at the time of the re-build that they didn't send that, as they have been deleted & out of stock for 2 years!
They sent over a pump for the 1600 engine - totally different pump, which fits on top of the block, not on the front face!

On checking, I find the original pump in place (re-builder didn't tell me).
So now I don't have the certainty that the fuel delivery is correct. = back to square one.
(To check the pump pressure, I'll need a t-piece and gauge; according to Bentley, to check the flow rate, I need to run the engine at 3800 rpm, while collecting the fuel pumped (!) - which means a completely independent fuel source in order to run the engine!!)

If anybody can source a genuine new 2-litre pump, which fits ahead of cylinder one, I would be very grateful.
If not, I guess I'll have to fit an electric pump, plus a pressure regulator to prevent overpowering the float valves, then see how she runs, and decide whether to have the carbs re-built!
Any clues?

[Amskeptic]

On checking, I find the original pump in place (re-builder didn't tell me).
(To check the pump pressure, I'll need a t-piece and gauge; according to Bentley, to check the flow rate, I need to run the engine at 3800 rpm, while collecting the fuel pumped (!) - which means a completely independent fuel source in order to run the engine!!)

You can easily perform the pressure delivery test before the float bowls empty. The factory dual carb pressure spec allows up to 5 psi.
Colin

[windswept]

Hi Colin and all

You will recall that I have the later electronic dizzy fitted, which uses a rotor arm with a centrifugal rev limiter in the centre. Dave at Mega Bug (www.megabug.co.uk) advised a new one, as the little springs fade, which might lead to limiting at far lower revs (= the sort of problem I have?).
I fitted a new BMW rotor arm (identical, but limited to 6500, not 5400!), then I roared up the dual carriageway for 5 miles at full speed, then killed the engine, coasted to a halt & waited for the engine to cool. The feel was definitely a little smoother at high revs .
I chose the No2 plug as it's simplest - it shows no oiling, no signs of over-rich, just a tan centre electrode and near black outer body. I took pix with my phone, as it was getting dark and p*ssing with rain. Will re-study them today.

I have just read in Bentley that there is a "special coil" which MUST be used with the breakerless ignition - it even gives the VW part no!!
We were using a Bosch Blue, but will now get the correct one.
In summary, I keep finding things which need fixing before going as far as a full carb re-build or new carbs (=££).
Comments welcome.
Paul

[Amskeptic]

I have just read in Bentley that there is a "special coil" which MUST be used with the breakerless ignition
We were using a Bosch Blue
Comments welcome.
Paul

Proceed Paul. . . . . and let us know how it works out.
By the way, the breakerless ignition requires its own coil

[windswept]

Hello again, Guys.
I have now replaced the majority of the vacuum pipeworks, and most of the fuel pipie clamps (plus a few lengths of fuel pipe).
Plus a crimped & insulated new loom for the electric chokes and 3 cut-off valves.

After the supplier sent the wrong coil, I hassled him into sending the correct one! Part number 211 905 115 B or C.
Having fitted it, I zoomed off for a check . . .

Can I report a smoother & more coherent response? Yes, quite a bit smoother, and I didn't detect any missing or stuttering after 15 miles on the motorway. However, I want to be 110% sure.

Next weekend, we plan to drive it 350 miles each way for a camping trip - which will be the acid test.

I'll keep reading the forums, and keep you all informed.

Regards
Paul

[Amskeptic]

Next weekend, we plan to drive it 350 miles each way for a camping trip - which will be the acid test.
I'll keep you all informed.

May the wind be at your back and 20,000 volts at your plugs. . . .