Should we have to do this after spending so much money!

[Lee Smalley]

About 12 months ago i bought a MVVS 40 cc petrol engine for my db spit it currently is being ran in in a 77inch span raven and i am in the process of giving it some stick it has now had a gallon of fuel through it and now i am getting used to the model i am throwing her around a bit and the engine is misbehaving, i shall explain

 

Take off fly around no problem, knife edge left rpm drops slightly knife edge right engine almost dies snaps inverted or upright same thing, now after sending an email to a few MVVS people i get the same answer

 

aparently the float diaphram is being adversly effected by un even air pressure and it is a common problem with Walbro carbs its possible that the MVVS suffers more from this as the carb is at the front as in a traditional engine, so the fix is as follows i have to solder or fix a vent pipe or similar and route it through to the back of the firewall and into the fus so that the air is not effected by propwash etc, now my problem is this ....why the hell should i have to do this, its not like i spent £150 quid on a converted lawnmower engine this is supposed to be a purpose designed aircraft engine to fly, they say the "walbro carb was never designed to fly" so why the hell did you fit it !! would be my answer, am i alone in thinking that we keep getting stiffed with products that we have to repair or make good so that it can do what it was ment to in the first place you dont buy a car and have to weld a breather on the engine before you can go over 50 mph very dissapointed in MVVS, they have been very helpful but i should not have to do this in the first place

[Steve W-O]

I would tend to agree.

 

It would appear that it is not fit for purpose, and they seem to have admitted that.

 

Very strange, and very disappointing.

[Lee Smalley]

yeah but what can i do about it very little me thinks, just buy my next petrol from somewhere else does any else have similar problems

[Ultymate]

Similar problems occur with all petrols fitted with the generic Walbro type carb Lee. The issue is not the engine or carb but more to do with airflow and pressure  changes within the cowl. I have a Hangar 9 Extra with a ZDZ 80 and I had to fit a pipe to the diaphragm cover as you describe through the firewall and to a 1" cube balsa box with just a couple of pinholes in it. Same engine in a H9 Sukhoi needs no pipe at all. Some carbs do come with a connection nipple  fitted to the cover. I've just fitted a new DA 150 carb to a 3W 140 and that came complete with a nipple but runs OK in my Ultimate with no extra pipe fitted, it's a bit of a lottery but has been that way for a long time 

P.S    P. Rievers who sells MVVS motors I think in the Netherlands actually recommends connecting said pipe to a pitot tube outside the cowl if said problem arises Here's a LINK 

Edited By Ultymate on 27/10/2010 16:15:18

Edited By Ultymate on 27/10/2010 16:19:49

[Stephen Grigg]

Its amazing how a model aircraft petrol  engine can be sold not suitable to fit in a model aircraft

[David Turner 5]

True enough, it's a Walbro thing. They aren't designed for aeroplanes.

 

IMO, the best place to run that little tube is inside the fuselage, where the air is comparatively stagnant and gives a very constant pressure. At least, that's what I do.

 

 

[r6dan]

Electronic injection may be a solution.If you could devise something.

Edited By r6Dan on 27/10/2010 20:27:04

[Ultymate]

It's not a Walbro issue at all the same thing happens with Tillotson and the Chinese cloned carbs the issue is the airframe and more specifically the design of the cowl and any cooling baffles. If the diaphragm sees constantly changing pressures during varying phases of flight ie left knife edge right knife edge or inverted then you'll get constantly changing mixture fed to the engine it's not rocket science and relatively easy to cure. At most it means silver soldering a pipe into the breather hole in the diaphragm cover and piping it inside the fuselage, it's usually far easier than connecting up throttle and choke linkages.

[David Turner 5]

Posted by Ultymate on 27/10/2010 20:45:26:
It's not a Walbro issue at all the same thing happens with Tillotson and the Chinese cloned carbs .

 Fair point. My language was sloppy.

 

It's a "diaphragm carb issue".

Edited By David Turner 5 on 27/10/2010 20:50:07

[Ultymate]

Here's a LINK to P Reivers talking about this problem and his external Pitot tube solution on the forum he runs on his own website.

Edited By Ultymate on 27/10/2010 21:06:20

[Lee Smalley]

Hi guys thanks for all the input i agree that its a diaphragm issue, and not just to MVVS engines as i said they have been helpful but it just staggers me that they at least don't fit a pressure nipple to the plate that covers the diaphragm so that you can if the need arises run a pipe easily to the fuselage to cure this problem, no instead we are left to solder a pipe in!! i don't own any silver soldering kit!!

and it seems from ultymate that its not just cheap engines,  ZDZ as well !!

 mybe its me, in fact im sure its me, but i can't help but feel shafted, i thought people selling things would test their kit especially where and in the environment it was going to be in (ie the Air) and more often than not in a cowl !! im staggered and peed off

 

whats the point of buying quality if you are no better off for it !! Paid 300 quid now going to have to spend more to get some soldering kit brill !!

 

 

 

[Craig Carr]

Hi Lee,

 

Had exactly the same problem as you with a graupner 26cc Petrol engine in my Superstar (think they are re-badged evolution engines).  Engine ran well in the air with the cowl off....but fit the cowl and I got constant dead sticks.... due to the pressure effect

 

A club mate soldered a small piece of pipe (1/4 inch long if that)  to the the carb per previous suggestions.  However we then used some tygon fuel tube to run back to the inside of the Fuz which made the job a lot easier.

 

It did improve things dramatically although I was still suffering an occassional dead stick (not as many as before the carb mod though) and the only thing that seemed to "properly" solve it was dremel open up the underneath of the cowl even more to reduce the pressure build up.

 

Craig 

 

Ps looks like your avatar is my superstar (Graupner???)

Edited By Craig Carr on 28/10/2010 11:33:29

[Lee Smalley]

indeed it is the graupner superstar sadly lamented i lost it due to failure of the brain, i loved it so much i have bought the seagull version (which i think is exactly the same) although the colour scheme is not as good 

 

so the problem seems widespread and yet the manufacturers do nothing about it, just had a look at the OS55 gt petrol engine and guess what a pressure nipple is fitted to the cover over the diaphragm, so it looks like at least one manufacturer can be bothered to release a product fit for purpose then !!  well done OS  again !!

[Craig Spence]

Hi Lee,

 

I havent really dabled in petrol engines yet but as I was skiming through your thread one thing came to mind "not fit for purpose" someone has mentioned it on here saying the engines are not really meant for planes.

However as I got to the end you have already nailed it and got the solution, I dont mean OS lol!, but its along the same lines. Products which are specificly based for the purpose of flying, such as OS AND Saito.

 

Although I generally steer away from OS as I think they are a bit extreme with thier prices after reading this thread I will for sure buy thier petrol engine or a Saito one when I go down this route my self.

I hope to use one for a scale plane in the future but I will have to change clubs first as my club does not alow petrol, class structure & politics I think.

 

Hope it all works out for you.

 

 

[Mike Bell]

I had a similar problem with my Zenoah 20cc engine which would be fine in level flight but would cut immediately if I attempted a roll.  I came across the pressure balancing tube fix and tried it with no real success.  Then I had a chat with Glen Fletcher (Glens Models) who advised that this effect was a rich cut.  His suggestion was to try adjusting the L needle on the carb, leaning it out 1/8th of a turn at a time until it improved.  This approach was much more successful in my case.

 

Mike

[David Turner 5]

Soft solder is fine. And a bit of brass tube.

 

Make sure the steel diaphragm cover is nice and clean and that you have plenty of heat.

 

You will, of course, have removed the steel cover from the carb before applying all that heat.

[Biggles' Elder Brother - Moderator]

I think you have a valid point Lee - there are too many "half engineered" products in the R/C model plane world.

 

I suspect it happens because they know we are bit "Handy" with the tools and so think "Oh well never mind - they can fix that" It doesn't excuse it - it is unacceptable that we effectively have to finish their job for them - especially as we pay full wack - but I think it does explain it.

 

Good luck with the soldering.

 

BEB

Edited By Biggles' Elder Brother on 28/10/2010 21:11:22

[Lee Smalley]

i think BEB has it bang on, they must think we are able to fix these things, and do you know they are right i will be able to fix it but i am peed that i have to !! 

its weird i was always led to believe that petrol was more reliable and so far my experience is not going in this direction well i will sort it this weekend post what i do and let you boys know the result, i wouldn't mind but knife edge is one of my fav manoeuvres and the sight and sound of a large 40cc raven going past low knife edge is really cool !!  

[Ultymate]

Lee you will more than likely have to retune your carb slightly after fitting the pipe 

[Lee Smalley]

Ok mate i thought that might be the case! 

[Jim Newberry - Nuviation Ltd.]

Hmmm. This problem doesn't seem to occur with electric motors! 

[Myron Beaumont]

Jim

Same could be said for a milk float or similar .

[Steve W-O]

Posted by Jim Newberry on 29/10/2010 17:29:51:
Hmmm. This problem doesn't seem to occur with electric motors! 

   

 

 

The point in question here is really the suppliers selling things that work which you have paid good money for.

 

If you look at the Wot 4 Foam-E thread, you will see similar problems, where the set up does not work properly as supplied.

 

And that is electric

[Lee Smalley]

well i sent the an original email to MVVS asking for advice and i recieved an email from

Pe Reivers

 

so after i had effected my modification i emailed the above to make my point that i should not be expected to do this after paying nearly 400 pounds and this is the reply i got ! keep in mind that i had already apologised up front if he was not the right person to direct my moan to

 

Lee,

 Nobody likes a moaner, and I am no exception.

Name me one petrol model airplane engine that does not use the Walbro. True, it was not meant to fly in the first place, but it does fly quite well.

Direct your moans to your dealer or the factory please.

See their mail address at www.mvvs.cz

 

 

 

 

to say i was staggered and very angry at his reply was a small understatement

[Ultymate]

Lee, there's no reason for the curtness of  Pe Reiver's reply (rudeness)but in a way I can see where he's coming from. The "Walbro type" carb is aimed at the chainsaw, strimmer light industrial appliance market and has been grabbed and used by the model engine manufacturers. Now if chainsaws and strimmers were intended to fly then like panty liners they'd have wings. It's simply a case of if you have this in cowl pressure issue that you have to make what is a slight modification to suit your particular airframe and installation. I can however see your argument that having spent £400 that they could at least fit a nipple to the diaphragm cover. As I've said earlier more carbs are slowly appearing from Walbro /Tillotson/ Emas with nipples fitted.

     Mr Reivers must have got out of bed where there was no carpet that morning, I've exchanged posts with him on several forums and found him most amenable as a rule