1/4 scale V8, first project.

[gbritnell]

I use a similar carb on my 302 engine for running while I have replicated the internal features into a miniature copy of the actual carb for my engine. The carb you are using is commonly referred to as an air bleed carb. As was already stated the main needle is adjusted for high rpm running while the air bleed screw is to reduce the vacuum signal at idle so it doesn't pull as much fuel and go rich.
While experimenting with different carbs on my 302 I ended up with this type. I'm not saying that others won't work but this type proved to be the best of the bunch. As far as venturi size, my 302 has a bore of 1.00 and a stoke of .900. My venturi size is .200 and the engine runs fine. It's easier to start with a small hole and work up then the other way around.
gbritnell

 

[Lakc]

Audio is what everyone is waiting for, I am sure.

Keith, good job, especially for a freshman effort. Take your time to get things sorted out before you let the gallery pressure you into anything.

 

[trumpy81]

I use a similar carb on my 302 engine for running while I have replicated the internal features into a miniature copy of the actual carb for my engine. The carb you are using is commonly referred to as an air bleed carb. As was already stated the main needle is adjusted for high rpm running while the air bleed screw is to reduce the vacuum signal at idle so it doesn't pull as much fuel and go rich.
While experimenting with different carbs on my 302 I ended up with this type. I'm not saying that others won't work but this type proved to be the best of the bunch. As far as venturi size, my 302 has a bore of 1.00 and a stoke of .900. My venturi size is .200 and the engine runs fine. It's easier to start with a small hole and work up then the other way around.
gbritnell

George, the carb that Keith is using is NOT an air bleed carb. Looking at the photos you will see a small screw with fibre washer on the bottom left, this screw simply holds the barrel in place, it is not an air bleed adjustment screw. You will also notice an absence of the air bleed hole in the carb body.

The only adjustment on Keith's carb is the needle valve.

 

[gbritnell]

Hi Andy,
I guess I didn't look close enough. That's probably why he's having so much trouble running the engine.
gbritnell

 

[trumpy81]

Hi Andy,
I guess I didn't look close enough. That's probably why he's having so much trouble running the engine.
gbritnell

Yeah it does explain the limited throttle response for sure. But then this is a get her running carb and is not intended for the final engine.

Keith could add an air bleed and screw to it but I think it would be a waste of time all things considered.

I love that carb of yours

 

[keith5700]

So how about this carb?
I bought it a few months ago purely for testing purposes. It's a Walbro 'type' carb according to the ebay description, but again no instructions.
I've no idea if it needs pressurised fuel feed, or what the screws do.
The bore is just under 11mm.

I tried to start the engine again yesterday but it didn't want to know. Eventually it blew the ptfe centres out of 2 of the spark plugs. Still haven't found one of them.
I'll be gone for a while now as it's time to get the oil system working, before the crank wears out.
Also it could do with the cooling system working, even crudely, as it got hot really fast when it was working.

I think the main requirement in getting these things to run is some decent fuel atomisation. I'm still hoping the single fuel injector will solve a lot of problems, if and when I get that working.
Cheers.

 

[dsage]

Keith:

Are their any model numbers on the Walborough. I have the specs (or a link) for many of them around here somewhere. I'll have to search. They either have an integral pump that requires crankcase pressure pulses ala 2-stroke (which won't work) or they are just a "standard carb" and I believe have a float bowl with needle and seat. Those should work with gravity fed fuel supply or fed with a fuel pump.

Let me know. I'll see what I can find.

Sage

 

[keith5700]

Sage, there is a number on the carb, but I suspect it isn't a genuine Walbro number, not at the price I paid.
The no. is NFI35286.

Luc, the carb is like you said, and needs some positive,(or negative?) pressure pulses to pump fuel.
Unfortunately my crankcase isn't sealed yet so that won't work.
I can rig up a pressurised fuel feed, from 1psi upwards, but I suppose the advantage of the pulse system is that when the engine stops, the fuel feed also stops, whereas a pressurised external fuel feed would just carry on and flood the engine.

I guess I need a carb with a fuel float bowl and needle. I don't mind buying one now I know the engine will run.
Luc, that big carb in post 195 is just a big worn out pile of junk really so don't want that back on.
I have a bit of time now to sort a decent carb out, as I have to make all the other stuff mentioned earlier.

Thanks everyone for your continued advice.

Made a couple of silencers yesterday. They have adjustable internals as I want to tune them to get a nice deep rumble if possible. I may put a balance pipe between the manifolds and see if that smooths things out. Never got round to trying them as the engine wouldn't start again.

 

[jixxerbill]

That is one big mountain motor, thats what we call'em down here in my neck of the woods..

 

[aonemarine]

I suddenly feel very very small....

 

[dsage]

Keith:

The carb you have certainly "looks" like a Walbro carb. The Walbro's that look like the one you have all have "W" model numbers. It's difficult to tell looking at the Walbro carbs externally which ones have the pumps although it may be that extra tube shown in Luc' s diagram is the give away. Some are listed as for 2 or 4 stroke some specifically for 4 stroke so that's all very confusing as well.
Read more about the pumps below.

Here is a link to the ones that look like yours

http://www.walbro.com/media/8260/Rotarycarbs8-16-12web.pdf

Walbro also has stand alone pulse pumps and the explaination on how they work is here:
http://www.walbro.com/media/5980/FPC pump 8-20-12.pdf
Apparently they can be made to work by connecting the pulse tube to the intake manafold on a four stroke rather than the crankcase in a 2-stroke. They mention something about a spring that needs to be added. Others tell me that connecting to the intake does not work but I think it will be less effective the more cylinders you have since the pulses will e bevened out and reduced by more cylinders.

There are service manuals here which may help identify which ones have pumps also.
http://www.walbro.com/servicemanuals.aspx

An explaination on how the pump works is here.
http://www.walbro.com/techtipsmetering

Bottom line, as Luc pointed out, you probably have a pressure pulse operated version but hey - give it a try.

Walbro sure makes a lot of nice looking carbs, but they are not cheap. Seems they start at about $60 and can go as high as a full sized carb for a car !! The prices are in the spare parts listings on the site.


My friend who is familiar with them more than I has not responded to my email yet.

Not sure if this is of any help but good luck.

Sage

 

[Lakc]

There are ways to obtain the pressure pulses that may work with that carb. Tapping into the exhaust is one of them, crank blowby is another.

 

[keith5700]

Thanks again for replies. I'm having a bit more luck getting it started, still with the small rc carb, but it won't rev over 2000 even at wide open. I think I need a rethink on the spark plugs first. I have kept the ptfe in place by crimping over the top of the plug, but it doesn't seem to work with ptfe, with it being so slippery.
I can't see the pulse system working myself, either inlet or exhaust, as the pulses are all smoothed out by there being 8 cylinders instead of one. As Sage has already hinted at. What I really want is a nice Walbro carb that can work with just, say, a 2 psi fuel pressure feed, like a normal carb.

Still googling to see if anyone has done this, but drawn a blank so far.

The engine is too quite with the exhaust by the way. Fairly deep sound, but not enough of it! The cam timing belt is louder than the exhaust.
The flywheel is also too light so I'm making another this morning.
Not all doom and gloom tho'. I'm still very happy that it runs at all!

 

[Lakc]

Far enough downstream, with a good header design, the pulses are a bit smoothed out but still present. However, you can tap off a single cylinder easily enough.
If your more prone to hide the plumbing, an eccentric or wobble plate type pump off the back of a camshaft would do the trick as well.

 

[keith5700]

Well, some mixed success. I figured out the revving problem. Just a stupid electrical fault of my own making. I was just getting the mixture right at higher rpm when it stopped suddenly. Turns out the HEI ignition module has bit the dust.
I've had it now with electronic ignition systems. I'm going to find a mini set of points and go down that route.
Presumably the HEI coil will be ok to run off points?
Anyone know of any points which are a bit smaller than the normal type, which I could pick up from Halfords in th UK?

 

[keith5700]

Oh, forgot to mention, this morning I suddenly got loads of smoke out the back. Turned out to be a head gasket ptfe ring had gone. I replaced it and carried on but this needs a different material too.

 

[dsage]

Keith:
A few items.

Other than confirming what we already figured, my one friend said he thought your carb looked like a Walbro "W" model with a pump and that the service manuals are on the Walbro site. If that helps.

I forgot all about a second friend who I believe uses a Walbro carb on his Challenger V8. I emaild hom a few minutes ago and asked him about what will and will not work re pumps and things. I'm sure he's been through it all before.
For what it's worth I have seen lots of them used on models at the shows around here.

Suggested ideas about picking off one exhaust pipe or a cam excentric might work. But by the time you arrange some sort of pressure transfer arrangement (from exhaust pipe or cam excentric), it's probably going to be the same work to use a Walbro without a pump and gravity feed or a small electric pump fuel.

A standard O-ring should work on your heads. I have them on mine. There should not be excessive heat there - at least no hotter than your head gets - water temperature . And the pressure should be well within the specs of a well chosen O-ring. In any case I think you're going to need something other than hard plastic (which I assume your PTFE is). Something that crushes more.
I'm not quite sure what I'm looking at in the pictures of your block but obviously an O-ring is going to need to be in a steel O-ring groove right in the head (or block top). Where are the PTFE seals in the picture??
Looks like black O-rings on something in the background.


Sage

 

[keith5700]

Luc, Sage, I didn't know what you meant about an O ring, but now the penny's dropped.
At the moment there is no groove in the liner, just a 15 thou' step. The white ptfe washer is 20 thou' thick, so gives 5 thou' of crush.
I never considered using an ordinary, Viton?, o ring in a full groove in the liner, but is worthy of consideration.
I'll dig out my drawings and see if I could fit one in.

 

[stevehuckss396]

Hello Keith!

If it helps any I have made gaskets from .020 teflon sheet. They have not failed me yet and the ones in Peewee have been in service for many years. Maybe you could fill the step with an aluminum washer and make an all teflon gasket.

This one is from the Demon.

 

[hammer2100]

Keith,
Your engine is awsome, hear is my Two cents on your gaskets. Cut them out of copper than anneal them. This has worked
for many high proformance engine builders for years.
Hammer