# Looking for inexpensive carb for trial running of Seal Major 30cc



## Basil (Sep 26, 2021)

Irvine 1750 jetstream?
I found this one but cant find any bore specs. Any advice on what bore should I be looking for for this size engine and where I could find suitable carbs.  Thanks
I'm in the UK


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## petertha (Sep 28, 2021)

Your question reminded me that I forgot to post some information I put together on carb sizing based on some searches I did of RC engines - comparing displacement to venturi opening etc. Too many life distractions right now but I'll get on that. I did this to help me size carb for 50 cc 5-cyl radial. The carb specified in plans is long since out of production with no info anyways.

Anyways, as a long time RC guy, I was shocked to see how many RC engines are no longer produced even by the big name manufacturers like OS. I did find some suitable candidates but I also ordered 2 sizes from Conley. They have a progressive range. You would have to inquire about UK shipping but he airmailed me in Canada. They are plastic body, arguably a bit rough around the edges compared to a modern Japan RC carb, but I think it will ultimately do the same job from what I can see of the needle valve assemblies. Another plus is they make various sizes with the same body diameter which might assist swapping one carb in over another. RC carbs vary all over the map in this regard. 


			Perry Pumps - Carbs
		


On my radial I machined a standardized adapter so I could readily swap carbs without involving modding the whole induction manifold. Lastly, don't judge an RC carb by its engine displacement (like if you buy a used carb or whatever) because the venturi diameter can vary dramatically by its end purpose. A piped 0.15CI race engine may have the same 6.5mm hole as a sport 0.40. RC car/buggy carbs are different again.


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## Weldsol (Sep 28, 2021)

Hi Basil
I would guess it would be the same bore as my Sealion which is 1/4" (machined from solid )
Paul


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## Vietti (Sep 28, 2021)

In my quest for small carbs I called Tower Hobbies and was informed that the small carbs and small glow engines were pretty much obsolete because in that size range, electric motors have dominated.

Brian Rupnow's Traxxis carb is a good one as is the OS 35.  Should be available in the UK?  Perry carbs are good too.

Good luck, I've never been happy making small carbs.  In fact, my hobby making philosophy has become to make the things I cannot buy and buy what is available if possible, time grows shorter!


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## SmithDoor (Sep 28, 2021)

It work on model engines.
Odd's are it will fit 0.40" to 0.60" 
They not hard to on lathe and easy to up or down scale.

Dave 



Basil said:


> Irvine 1750 jetstream?
> I found this one but cant find any bore specs. Any advice on what bore should I be looking for for this size engine and where I could find suitable carbs.  Thanks
> I'm in the UK


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## petertha (Sep 28, 2021)

As mentioned, I went through a similar exercise sizing carburetor for my 50cc 5-cylinder (methanol fueled) radial this summer. The plans specified a 2-stroke glow (Magnum XL-15A). I think Magnum was a clone of an OS but that’s ancient history now, it’s no longer produced. To complicate matters I discovered I managed to buy the wrong equivalent to this carb, years go now. Glow engine selection is shrinking dramatically.

I noticed that sport engine manufacturers sometimes use the same carb model for displacement ranges like .25 & .35 or .25 & .15 CI. Caution, orifice sizes can vary significantly on engines of similar displacement depending on many variables. A 0.20 can be anything from 4-7mm depending on race/sport/boat engine, how much (if any) needle valve assembly occurs through the throat, engine timing, tuned pipe, pressurized fuel.... etc.

Anyways, I went to the Sceptre website & did a tabulation of OS 4-stroke engines orifice sizes. My simple logic was if it worked for those engines it should work for mine. Recognize that these are all single cylinder methanol glow engines. Carbs are feeding multi-cylinders are likely flowing at slightly higher net rate but I have no calculation method to support how much to factor that. I have a feeling the multi-cylinder engines actually may have had slightly smaller carbs & if so, may be a function of idling/transition characteristics over maximum rpm. Running gasoline may well be different sizing criteria. Anyways, here are my plots for your viewing pleasure.

example Sceptre link


			OS FS-40 Surpass (2)


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## petertha (Sep 28, 2021)

I purchased my OS carb (& plugs & other goodies) from this place in Japan at significantly lower cost than Tower. Very speedy shipping (to Canada).





						RC Japan | Radio Control Aircraft | Car
					

RC Japan | Radio Control Aircraft | Car  :  - K&S FunTech Futaba Hirobo O.S. Engine YS Engine Other... Quest [SALE] Curtis Youngblood SAITO Engines OK MODEL Falcon HITEC [SALE] Scorpion KONTRONIK [SALE] SPECIAL PRICE PRODRONE [SALE] KONTRONIK [SALE] FREAKWARE [SALE] FALCON Radio Control...



					www.rcjapan.net


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## Bazzer (Sep 29, 2021)

Basil said:


> Irvine 1750 jetstream?
> I found this one but cant find any bore specs. Any advice on what bore should I be looking for for this size engine and where I could find suitable carbs.  Thanks
> I'm in the UK


Basil

In the UK we are lucky as there still companies producing model aircraft engines on a small scale.

Try Weston UK and Laser Engines, the boss of Weston UK is on holiday this week, his name is Alan Greenfield he will be able to help when he is back, also worth trying is Tony Efflander at PAW, they have RC carbs for their big diesels. Laser are in Watford but I don't know so much about them.

Regarding carb sizing, err to the small side if in any doubt as it makes things much easier to set.

There is some kind of relationship in carb size to engine CC and operating RPM, for example a 3.5cc model car engine can use a 8-9mm diameter carb at 38,000 rpm but a slow running bigger engine has to use a proportionately smaller carb, it is obvious really, just down to how much air has to go through the carb which is a relationship between capacity and rpm as said earlier.

B.


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## petertha (Sep 29, 2021)

SmithDoor said:


> They not hard to on lathe and easy to up or down scale.



Not quite sure what you mean here. Yes, the cylindrical neck of the carb that typically slides into the crankcase of a 2S engine could be turned down on a lathe. But that doesn't affect the open area of the throttle barrel venturi hole & how the engine breathes. Another issue is the needle valve spray bar assembly passes through the barrel hole at right angles. The hole could be sleeved to reduce flow but not exactly easy work. Opening the hole to larger bore is maybe do-able but typically the barrel is pretty hard steel & already matches the ID of the inlet horn. I think a slightly bigger carb you could just run at reduced opening, but its probably not ideal vs matching the carb to the application.


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## Bazzer (Sep 29, 2021)

Basil said:


> Irvine 1750 jetstream?
> I found this one but cant find any bore specs. Any advice on what bore should I be looking for for this size engine and where I could find suitable carbs.  Thanks
> I'm in the UK


Basil

Remember that on carbs like you illustrate (Irvine) the needle valve is nothing more than a flow restrictor, it or equivalent flow restrictor can be placed anywhere in the fuel line.

The Irvine carb linked below is for a 20cc 2 stroke so will be a decent bore.

B.

Carburettors – Just Engines 
Irvine Carburettor Assembly - 8900A (X-IRVCJ-8900A) - Ripmax Ltd


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## Steamchick (Sep 29, 2021)

Carbs for small hand-held strimmers and other garden tools can be bought on €&@y for les than £10... I had one - picked from a £2 tray at an auto-jumble - with a 10mm bore. I couldn't replicate it with £10 in materials!
It was a simple cylinder with venturi through it, with a curved groove to lift the needle as the cylinder rotated. Many "plastic" components, couple of levers and springs.
K2


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## Bazzer (Sep 29, 2021)

Steamchick said:


> Carbs for small hand-held strimmers and other garden tools can be bought on €&@y for les than £10... I had one - picked from a £2 tray at an auto-jumble - with a 10mm bore. I couldn't replicate it with £10 in materials!
> It was a simple cylinder with venturi through it, with a curved groove to lift the needle as the cylinder rotated. Many "plastic" components, couple of levers and springs.
> K2


Most of those carbs (Walbro style) pick up some crankcase pressure pulse to work I diaphragm pump because they are mostly two strokes.

From what I see the OP's Irvine carb would seem similar to the original, although I am not sure why he does not just make the carb as per plan.


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## Basil (Sep 29, 2021)

Thank you so much for the info everyone. I think I might have lucked out. Digging through some old model engine parts I found a carb off a Saito 30 or 40 four stroke. Bore looks slightly over 1/4" with spray bar across. Had it sitting around for twenty years but never been used.


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## Alec Ryals (Sep 29, 2021)

Vietti said:


> In my quest for small carbs I called Tower Hobbies and was informed that the small carbs and small glow engines were pretty much obsolete because in that size range, electric motors have dominated.
> 
> Brian Rupnow's Traxxis carb is a good one as is the OS 35.  Should be available in the UK?  Perry carbs are good too.
> 
> Good luck, I've never been happy making small carbs.  In fact, my hobby making philosophy has become to make the things I cannot buy and buy what is available if possible, time grows shorter!


#################Try epay  #############


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## petertha (Sep 29, 2021)

Bazzer said:


> Remember that on carbs like you illustrate (Irvine) the needle valve is nothing more than a flow restrictor, it or equivalent flow restrictor can be placed anywhere in the fuel line.



Some RC needle valves can be remote. Most of what I'd loosely call race setups lend themselves to that because they are mostly concerned with mixture at a limited (high) rpm level. Also these engines typically have fuel pressure boost from exhaust pressure or crankcase or whatever. But when idling & transition is important, carbs typically have a 2nd low speed needle valve actuated off the throttle arm & their own mixture adjustment. They operate somewhat independently but work together mechanically where the low speed NV enters the spray bar inside the venturi hole, which is delivered fuel through high speed NV. So I don't think you can just pull the main NV & reposition it without other mods.


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## Bazzer (Sep 30, 2021)

Basil said:


> Thank you so much for the info everyone. I think I might have lucked out. Digging through some old model engine parts I found a carb off a Saito 30 or 40 four stroke. Bore looks slightly over 1/4" with spray bar across. Had it sitting around for twenty years but never been used.


Looks ideal for the job, good luck.


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## Bazzer (Sep 30, 2021)

petertha said:


> Some RC needle valves can be remote. Most of what I'd loosely call race setups lend themselves to that because they are mostly concerned with mixture at a limited (high) rpm level. Also these engines typically have fuel pressure boost from exhaust pressure or crankcase or whatever. But when idling & transition is important, carbs typically have a 2nd low speed needle valve actuated off the throttle arm & their own mixture adjustment. They operate somewhat independently but work together mechanically where the low speed NV enters the spray bar inside the venturi hole, which is delivered fuel through high speed NV. So I don't think you can just pull the main NV & reposition it without other mods.


Petertha

You are quite right about second needles (idle needles) but there is no required physical relationship between the main and idle needles as the idle needles relationship and interaction is with the spray bar which is slightly or greatly downstream of the main needle.

OS and other manufacturers in some cases have separated the position of the main needle on twin needle carbs to a mount off of the crankcase backplate screws for safety reasons. 

The OS carbs you illustrate actually prove the point, in both cases the main needle could be removed completely and the hole plugged off and the fuel restrictor/needle valve moved upstream towards the fuel tank and get the same result.


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## Charles Lamont (Sep 30, 2021)

Does the Seal Major kit not come with a carb. body casting and drawing?


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## Basil (Sep 30, 2021)

Yes the kit does come with a carb Charles.  The instructions for getting it to transition cleanly from idle to WOT and the throat size adjustments required to accomplish this are very good but seems like it could be long winded. I'll get it running with this first.


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## petertha (Sep 30, 2021)

Bazzer said:


> OS and other manufacturers in some cases have separated the position of the main needle on twin needle carbs to a mount off of the crankcase backplate screws for safety reasons. The OS carbs you illustrate actually prove the point, in both cases the main needle could be removed completely and the hole plugged off and the fuel restrictor/needle valve moved upstream towards the fuel tank and get the same result.



And I also agree with you 
The point I was I was trying to make, but didn't quite express fully, was that any modifications to the air opening (the orifice in rotating barrel) would potentially also encounter dealing with the spray bar entering from the main / high speed needle valve side. So whether you somehow sleeved the hole to make it smaller opening, or somehow opened it up to make it bigger, you are going to have to deal with the restoring the original distance the low speed needle tip makes relative to end spray bar assembly. That distance is important because the low speed NV moves in & out based on the rotational position of throttle arm (within limits of mixture adjustment). The high speed / spray bar end is fixed. But this is carb specific, some have this spray bar assembly, I think like img 6146 in post #13. Others function differently & may lend itself to plugging off as you say. 

Maybe I misunderstood the OP wondering out loud if that particular carb was a suitable match and/or how to go about sizing one based on displacement & engine type. So I think there are two inter-related factors: how big is the carb air opening & how is the fuel metered relative to that opening. Typically they are sized together. You can't put a 15mm dia carb on a 2.5cc engine & expect it to work. If he has the 'correct' orifice/carb/NVA size combination, then it makes little difference if the main NV is adjacent or remote. But I don't think he ever asked about NV position being a limiting factor or consideration?


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## Bazzer (Sep 30, 2021)

petertha said:


> And I also agree with you
> The point I was I was trying to make, but didn't quite express fully, was that any modifications to the air opening (the orifice in rotating barrel) would potentially also encounter dealing with the spray bar entering from the main / high speed needle valve side. So whether you somehow sleeved the hole to make it smaller opening, or somehow opened it up to make it bigger, you are going to have to deal with the restoring the original distance the low speed needle tip makes relative to end spray bar assembly. That distance is important because the low speed NV moves in & out based on the rotational position of throttle arm (within limits of mixture adjustment). The high speed / spray bar end is fixed. But this is carb specific, some have this spray bar assembly, I think like img 6146 in post #13. Others function differently & may lend itself to plugging off as you say.
> 
> Maybe I misunderstood the OP wondering out loud if that particular carb was a suitable match and/or how to go about sizing one based on displacement & engine type. So I think there are two inter-related factors: how big is the carb air opening & how is the fuel metered relative to that opening. Typically they are sized together. You can't put a 15mm dia carb on a 2.5cc engine & expect it to work. If he has the 'correct' orifice/carb/NVA size combination, then it makes little difference if the main NV is adjacent or remote. But I don't think he ever asked about NV position being a limiting factor or consideration?


On RC carbs and in particular cars, the choke area is reduced with inserts that sit in the top of the choke well above the barrel or slide, these work well and you don't have the problems that you mention with the idle needle.


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## petertha (Sep 30, 2021)

Like so, right? That's exactly the direction I was going with my radial but chickened out in the end. Mostly because I just don't know much about RC car carbs. Maybe they are the way to go. I redesigned my manifold with a standardized adapter plate so I can try different carbs.


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