VW 1834 RPM increase with Carb Heat on

[jja66uk]

Hello

I have a Tipsy Nipper with a VW1834 engine and Zenith Stromberg CD150 carburettor. I've been having some problems with the engine lately as follows:

1. Sudden application of throttle causes rough running and power loss (didn't used to).

2. Engine rpm (any setting) increases with application of carb heat.

3. Engine idles well but cuts out if the throttle is pulled back after a high speed taxi run.

4. Engine runs well but rpm increase causes intermittent rough running (better with carb heat on).

These faults are recent. I've replaced the inlet manifold gaskets (thought there might be an air leak) and the carburettor diaphragm but the problem remains.

I'd welcome any suggestions from those more knowledgeable than myself (that probably means pretty much everybody).

Thanks

John

[G-NIPR]

Sounds to me like its running too lean, and therefore application of carb heat is enriching it closer to the correct mixture.
The carb heat will introduce hotter air which is less dense, normally causing a drop in RPM through richer running. Because you are starting off from a position of running too lean the enrichment effect makes the engine happier.
Lean running will be due to an air leak somewhere, assuming of course that no carburation has been messed with (such as needle changes). Try running around (on the ground) with carb heat applied and see if the other symptoms you mention improve. If so, I would think I am on the right track here.
I am sure others will shortly be along to add or modify my thinking!

Paul.
G-NIPR.

[Neil Spooner]

Hi John,

I think Paul is on the right track....definately sounds like a lean mixture.

A vacuum gauge is a really useful tool and engine condition monitor. I have one installed in my Nipper which gave me good indication that the engine was matched to the prop' during development, and also gives a much more accurate setting of cruise power. If the engine is labouring for any reason I will be using a wider throttle opening to achieve the same RPM, this will show as a low manifold vacuum.

Your problem may be due to a lean mix' due to:
Inlet Manifold leak: Get a refillable squirty bottle with a soapy water mix' in it, with the engine idling squirt the fluid around the manifold at any junction/gasket face. If there is a leak the fluid will be sucked in and will affect the engine idle momentarily. (BEWARE OF ROTATING PROP!!!)
Inlet valve leak: Either check the compressions using a leak-down test (cylinder leak test) or pull the prop' through compressions WITH THE IGNITION OFF and throttle open to see if the compressions are even. Any inlet valve leak may show as a hissing noise through the open throttle. (On a Stromberg it is necessary to lift the carb' piston to hear this)

Do check that the carb' piston moves very freely without any hangup.

Another area to check is fuel flow. With the engine at full throttle the fuel pump should deliver 1.75 times (I think, need to check that) what the engine requires. The VW typically uses 3 imp gal/hr, so with a restrictor in fuel feed to the carb' and a T piece in the fuel line check the excess fuel delivered to a calibrated container over a period long enough to provide a meaningful result. Make sure the a/c is tethered and chocked.

There is an outside chance it could be ignition, but look at the above first.

I will try to keep an eye on the Forum, but am now working away from home, so i'net access is a problem.

Good luck, give me a call if you want.
07971966286

Neil

[jja66uk]

Thanks for the advice, its turned out to be a combination of things:

1. A dirty needle valve resulted in me stripping down and cleaning the carburettor in an ultrasonic bath.

2. A major oil leak from the base of one of the cylinders resulted in me taking the heads off (again!).

3. A malfunctioning rectifier and fused connectors (Leburg ignition) had to be replaced.

4. A split spark plug conector on one of the HT leads.

I think I've fixed 1 to 3 but I'm struggling with 4. Does anybody know where I could get some HT leads made?

[G-ARBG]

John,

Try Performanceleads.co.uk or Lemark Hotwire.

David G-ARBG

[Neil Spooner]

If you can't get any let me know, I can get the parts to fabricate them to suit the Vistion coils.
If you use resistive leads make sure the plugs are not resistor also.
Neil

[jja66uk]

Thanks for all the advice from all of you who've answered my query, the engine is now running smoothly (performanceleads.co.uk, thanks David) and the oil leaks are about what you'd expect of a VW. It certainly doesn't seem to be using any oil. However this must be the most gremlin ridden aeroplane ever built as it has now lost a piece out of the trailing edge of the prop.
Neil, what's the problem with using resistive leads and resistor plugs? I get a lot of 'noise' on the radio from the alternator so I'm going to try fitting a supressor.

[Neil Spooner]

Hi John,

If you have too much resistance in the output side of the coils it tends to "reflect" the HT pulse back down the primary windings and into the output transistors of the modules, this overloads them and can cause damage......similar to operating with the plug lead not connected.

Are you sure it is ignition noise? I can sometimes hear my alternator depending on load. It may be better to move the aerial away, or even check your aerial to make sure it has clean connections, the outer screen is earthed properly, as is the ground plane if you have one.

Glad to hear you are underway....Chris Lodge will still repair his prop's if it is one of his, or will consider repairing others depending on damage.

Good luck and happy flying!!

Neil

[jja66uk]

Hello Neil

Thanks again, do you have any contact details for Chris Lodge? I'd rather send it to him for repair than try to bodge it myself.

cheers

John

[Neil Spooner]

Hi John,

Chris is on: 01245440317

Neil

[jja66uk]

Hello Neil

I contacted Chris Lodge and he's taking a look at some pictures I took of the damage. In the meantime Newton propellers have quoted me £65 do to the repairs, revarnish and balance. Do you happen to know the tightening sequence and torque settings for the prop bolts and where I might get a fresh set of bolts?

Thanks

John

[G-ARBG]

John,

Attached is extract from MK II owners handbook which may help answer to first half of your question (best discuss with your inspector).

Regarding bolts this can vary with aircraft, type of engine, depth of prop boss, imperial or metric threaded prop drivers 5/16" or 8mm etc. I have left my number on your phone if I can help further.

R.

David G-ARBG

[Neil Spooner]

Hi John,

David's download is spot on, however, I would recommend you torque to 50% of the final torque working diagonally criss-crossing the flange, then 75%, then 100% final torque. If you find you have some runnout then release all the bolts and tighten the bolt that is co-located with the tip that is too far forward first and work criss-cross as usual, it sometimes works to move the prop' 180 deg'. Obviously ensure the prop' is in the correct position for swinging first!!

Best supplier of bolts and advice on ordering use LAS: http://www.lasaero.com/site/products/ar ... d=O4JRWEL9
(There is good info' at the top of the page for ordering the bolts, but it is ok to call them and be guided)

For prop' torque you can ask Chris who will advise you, or have a look at the Sesenich Prop' installation guide here: http://www.sensenich.com/files/document ... 581692.pdf

If you have spinner I suggest you leave it off until your inspector has checked the installation. You may need to review the safety locking of the bolts. ie, do you have captive nuts in the flange, if so you will need to wire-lock. If you have nuts you will need to purchase new lock nuts or use castelated nuts and drilled bolts. Again, if you are speaking with Chris he can advise you or give me a call.

Neil