Logo
Builders' Plan Gallery  |  Hip Pocket Web Site  |  Contact Forum Admin  |  Contact Global Moderator
December 12, 2019, 04:13:14 PM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with email, password and session length
 
Home Help Search Login Register
Pages: [1]   Go Down
Print
Author Topic: DC engine woes  (Read 590 times)
0 Members and 1 Guest are viewing this topic.
whiskers
Bronze Member
***

Kudos: 1
Offline Offline

United Kingdom United Kingdom

Posts: 52

Topic starter


Ignore
« on: February 26, 2019, 08:39:38 AM »

I have mentioned this engine in another forum on here but seeing as this is the power forum I'll post this here.

I recently acquired a DC Sabre and totally stripped/cleaned it up and re-assembled it ready to go - it was horribly gummed up inside. I have fitted an 8x4 prop and it's on a test stand but I can't make it run. The engine fires up but only runs for a few seconds. The motor has good compression and turns smoothly. It has a non-standard NVA fitted. I am using Model Technics D1000 fuel (a new can). The fuel tank is mounted more or less level with the engine.

I think the prime suspect is the needle valve assembly. First, I have fitted this with the two holes in the spraybar horizontal, eg. facing front and back - is this correct? Also, the engine is still sucking in fuel (if I choke it) when the needle is fully closed. Am I right in thinking this is flooding the engine and could be the culprit? One of the best runs I've had with it was when I disconnected the fuel line and was flicking the engine to clear excess fuel! When it runs it gets up to speed then slows and stops. I don't think it is over compressed as if I slacken the comp screw after a 'clear' run the engine will 'burp' a bit before stopping.

Perhaps it's just a bad-un but I would like to persevere with it...for now.

Any assistance would be appreciated.
Logged
Mefot
Gold Member
*****

Kudos: 7
Offline Offline

United Kingdom United Kingdom

Posts: 293




Ignore
« Reply #1 on: February 26, 2019, 09:32:00 AM »

The needle valve sounds like the problem. How is the engine stopping ? If the revs drop and it stops it's generally rich. If it speeds up and stops it's more than likely it's too lean. It might be worth checking you've put oil in your fuel if you mixed it yourself !!!  Smiley

Edit; Just realized you are using commercial fuel so that shouldn't be a problem  Cool
Logged
applehoney
Titanium Member
*******

Kudos: 279
Offline Offline

Canada Canada

Posts: 3,163




Ignore
« Reply #2 on: February 26, 2019, 09:41:13 AM »

>sucking in fuel (if I choke it) when the needle is fully closed.

Engine sounds fine; just flooding as needle too short to regulate fuel supply.   Re-solder the threaded sleeve to provide more length.

Good luck!
Logged
Squirrelnet
Gold Member
*****

Kudos: 35
Online Online

United Kingdom United Kingdom

Posts: 672




Ignore
« Reply #3 on: February 26, 2019, 10:48:39 AM »

Hi Whiskers

 A two hole spray bar should be horizontal with the holes front to back. A single hole spray the hole faces into the engine.

The needle valve should shut the fuel off fully closed so something is wrong there. Sometimes the brass outer becomes unsoldered from the steel inner needle so when you turn the valve on the screw thread the inner is not following but sliding on the brass outer, sometimes the inner steel needle turns while the brass outer is sits there held by the retaining spring.

 If that's the case you can resolder it but you'll need to work out how much needle valve needs to protrude from the brass outer so that the needle can close the jet.
Logged
TimWescott
Platinum Member
******

Kudos: 15
Offline Offline

United States United States

Posts: 1,031



Ignore
« Reply #4 on: February 26, 2019, 12:14:26 PM »

I'm giving you a third vote for the thing having needle valve problems.  If it runs out a prime nicely (starts, roars, then peters out), then everything in the engine itself is fine -- you just have a problem with the venturi or needle.  Since the needle won't close all the way, that's almost certainly the problem.

If the suggestions given about re-soldering the needle don't sound like they'll fit your "non-standard" needle setup then post a picture.
Logged
whiskers
Bronze Member
***

Kudos: 1
Offline Offline

United Kingdom United Kingdom

Posts: 52

Topic starter


Ignore
« Reply #5 on: February 26, 2019, 01:15:49 PM »

Thanks guys, definitely sounds like my NVA is the culprit. As suggested above - she roars into life then slows and stops. I can't re-solder as the 'thimble' or threaded part of the needle is alloy. I will try to carefully shorten the threaded part of the spraybar.
Logged
Squirrelnet
Gold Member
*****

Kudos: 35
Online Online

United Kingdom United Kingdom

Posts: 672




Ignore
« Reply #6 on: February 26, 2019, 01:25:05 PM »

In that case it sounds suspiciously like its the wrong needle valve in the jet tube, particularly as it was action buy. Have a go but might worth buying another NVA that works ?
Logged
OZPAF
Palladium Member
********

Kudos: 180
Offline Offline

Australia Australia

Posts: 5,217



Ignore
« Reply #7 on: February 26, 2019, 07:00:36 PM »

Alternatively if it is an allow thimble - try supporting the base of the thimble and very lightly tapping the need lower.

I agree with SN that buying a new needle valve would be best. Perhaps from PAW.

John
Logged
ffkiwi
Gold Member
*****

Kudos: 21
Offline Offline

New Zealand New Zealand

Posts: 516



Ignore
« Reply #8 on: February 27, 2019, 01:46:58 AM »

I suspect-as with other posters-that you have a mismatched NVA-this is quite possible as while all the DC NVA needles are interchangeable-in THEORY...in practice they are not as the length of needle projecting past the aluminium thimble is different in different DC engines-so I suspect you may have-for example a Dart needle fitted to a Sabre spraybar (just to pick a random example)-in other words if the needle will not shut off fuel flow even when fully screwed home-then it is not the correct needle for that particular spraybar....even though it may screw on perfectly satisfactorily. ...the thread is 5BA across the RC range 0.55-2.5cc

Someone with a lathe can make you a new needle-preferably with a brass thimble-which can be soldered to the needle after you have tested the fit...and ascertained that the needle goes fully home before finally locking it solidly into the thimble by soldering....obviously this is not possible with the standard aly one-where the needle is simply a press fit...

 ChrisM
'ffkiwi'

Logged
Robmoff
Silver Member
****

Kudos: 4
Offline Offline

United Kingdom United Kingdom

Posts: 221




Ignore
« Reply #9 on: February 27, 2019, 05:09:22 AM »

You could try a 'remote' needle valve.
You run silicone fuel tube through a large electrical 'chocolate block' type connector and control the amount of fuel by tightening or loosening the screw that would normal hold one of the wires. I had a PAW 1.49 in a combat wing which was always breaking needles when landing/crashing. It was such a good reliable starter that I could set and forget the remote needle for weeks on end.
Logged

Never underestimate the innate hostility of inanimate objects.
TimWescott
Platinum Member
******

Kudos: 15
Offline Offline

United States United States

Posts: 1,031



Ignore
« Reply #10 on: February 27, 2019, 03:32:56 PM »

You could try a 'remote' needle valve.
You run silicone fuel tube through a large electrical 'chocolate block' type connector and control the amount of fuel by tightening or loosening the screw that would normal hold one of the wires. I had a PAW 1.49 in a combat wing which was always breaking needles when landing/crashing. It was such a good reliable starter that I could set and forget the remote needle for weeks on end.

Wow.  That sounds like a great idea -- could you post a picture?
Logged
whiskers
Bronze Member
***

Kudos: 1
Offline Offline

United Kingdom United Kingdom

Posts: 52

Topic starter


Ignore
« Reply #11 on: February 27, 2019, 04:09:06 PM »

Well, I've had a go at the NVA. I gradually shortened the threaded portion of the spraybar until the thimble butted up against the locking nut. This still allowed me to suck some air through. I then very carefully shortened the thimble with a dremmel cutting tool until it was able to be screwed 'fully home'. I now have a NVA that won't suck fuel through (with engine choked) when it is fully tightened. I think that might do it...

I shall try the motor on the bench ASAP and report back...
Logged
whiskers
Bronze Member
***

Kudos: 1
Offline Offline

United Kingdom United Kingdom

Posts: 52

Topic starter


Ignore
« Reply #12 on: March 01, 2019, 05:17:24 AM »

Tried the DC Sabre with the modified NVA . There is no change! Engine will only fire-up and run briefly on a 'prime'. Pretty sure it's not over-compressed. If I back the comp' off about 1/2 turn from where it fires it just will not start at all whatever the setting of the needle or state of choke/prime. Increasing the comp slightly will produce firing and brief runs. Increasing it much further it seems to become over-compressed. It just won't keep going. I'm pretty sure the NVA is now working OK because it will not suck in fuel when the needle is fully seated and engine choked.

I think the motor may be snookered or otherwise worn-out, despite seemingly good, snappy compression. Both gaskets are in place (cylinder and back-plate) and appear effective. The front bearing has no slop at all. When I stripped the engine to clear out the gunk the piston and bore looked fine and un-scored or otherwise damaged. the contra-piston is tight. The con rod appears OK too. I'm Flumaxed!

I'm using D1000 fuel - could that be off? It was from a newly purchased, sealed can...

I have acquired another DC motor (Spitfire) - glutton for punishment!! I will swap the NVA's over when that arrives and see if that cures things...

I know it is 40 years or so since I last played with diesels but I considered myself pretty good with them back then!
Logged
LASTWOODSMAN
Platinum Member
******

Kudos: 26
Offline Offline

Canada Canada

Posts: 1,036


REAL PLANES HAD ROUND ENGINES AND TWO WINGS



Ignore
« Reply #13 on: March 01, 2019, 07:17:04 AM »

This is interesting stuff.  Could you guys please post some pictures so we can see what is being talked about?  Thanks

LASTWOODSMAN
Richard
« Last Edit: March 01, 2019, 07:37:42 AM by LASTWOODSMAN » Logged

OH, I HAVE SLIPPED THE SURLY BONDS OF EARTH ... UP, UP THE LONG DELIRIOUS BURNING BLUE ... SUNWARD I'VE CLIMBED AND JOINED THE TUMBLING MIRTH OF SUN-SPLIT CLOUDS ...
raggedflyer
Bronze Member
***

Kudos: 1
Offline Offline

United Kingdom United Kingdom

Posts: 76



Ignore
« Reply #14 on: March 01, 2019, 11:03:37 AM »

Just a thought... have you tried a different tank connected with a new piece of fuel tube?
Logged
whiskers
Bronze Member
***

Kudos: 1
Offline Offline

United Kingdom United Kingdom

Posts: 52

Topic starter


Ignore
« Reply #15 on: March 01, 2019, 12:16:47 PM »

Made a tank out of one of my dogs pain relief syringes - like the ones I used years ago on my free-flighters. Engine seemed like it wanted to go a couple of times then screamed into life and...kept running!!! Ran out a tankful and started up again after a fill and ran out another tankful, then another.

I guess I'll call that fixed then. It runs best with between 1/2 - 3/4 turn out on the needle so I guess the motor must have been well too rich even with the needle fully seated prior to my mods!!

Thanks everyone.
Logged
Squirrelnet
Gold Member
*****

Kudos: 35
Online Online

United Kingdom United Kingdom

Posts: 672




Ignore
« Reply #16 on: March 01, 2019, 12:27:50 PM »

Excellent, good result. They can be a bit fine on needle adjustment.... you just need to build a model for it now ;-)

Logged
Heikki K
Bronze Member
***

Kudos: 0
Offline Offline

Finland Finland

Posts: 10



Ignore
« Reply #17 on: March 02, 2019, 12:36:25 AM »

Great the engine was fine! Earlier engines had the problem with NVA, that they broke easily when landing vertically, or just tightening too much. Then, the spray bar was replaced with anything close to suitable. from another engine with a different width choke body. Therefore, many times you had to resolder the needle into a different depth. The soldered joint of the needle itself may had came loose, was just resoldered, so there is no guarantee, how the needle is set into it's needle barrel. In general, any needle valve can be tested by blowing to a clean length of fuel tube. RC or CL carburator, no difference. This is what I have learnt from the  big boys in Pylon Racing. The air should leak through sort of "logically" - easier or less when adjusting the needle. It's amazing how fast one can learn to adjust the needle valve very close to spot on with this method.

Moreover, I have seen many test stands equipped with some kind of pinching device, spring loaded clamp, that will close the fuel feed line, when necessary. Another thing is the ether, it evaporates so easy out of the fuel can, you wrote the can was sealed. I replenish my diesel fuel cans or bottles with pure ether, marking the fluid level to the side of the bottle, when putting fuels back to storage. This helps me to detect and pour back the vanished quantity.

What comes to the fuel tube, silicone and natural rubber will not do. They just swell and come into pieces. These glass-clear PVC and polyethene tubes become hard and start to leak as they loose flexibility. I found "Tygon" fuel tube, sold for gasoline engines in model shop or at gardening equipment repair will work fine with diesel fuel.

A mustard bottle hint; empty, soft plastic mustard bottle with conical nozzle will be great blowing air into the exhaust when attempting to start. I am a little cautious not to inhale all the chemicals I have poured into my diesel fuel mixtures.
Logged
Pages: [1]   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!