I have been through this unit trying to figure out the problem. It is a 399 engine. It usually starts good, idles find and has low rpm. Once I get to about 1/2 throttle it looses power, like it's running out of fuel. If I flip the choke on a bit or all the way or cup my hand over the breather it seems to pick up rmp again. Plugs are good, points are good, I have changed the diaphrams in the carb, the fuel is new, oil mixture is correct, fuel lines are clear, the line from the crankcase to the carb is clear and there is pressure there when the engine is running. Compression on both cylinders is 110-115. I've had the carb off a few times and double checked the diaphrams, they are in the right way. No carb intake leak that I can see.

If anyone has any ideas I would greatly appreciate it.

Thank!

Just some things to check had the same sled spent a lot of time messing with Tilly carbs.

When you cleaned carb did you take out welsh plugs and clean the small ball check valve in the high speed jet? They get gummed up and cause problems.

Did you set the little lever for the needle flush with the carb body?

Check fuel filter is it screwed on tight could be leaking

Are your fuel lines good any bubbles in the lines can mean an air leak that cuts off fuel to the carb.

Is your high speed screw set a 1 1/4 turns out from lightly seated?

Hey SB49, thanks for your suggestions and ideas. I would answer yes to all of them though. Cleaned ball under welsh plug, filter is new and tight, lever is flush with carb body, no bubbles in fuel line, h/s jet is 1.25 turns out approx. no fuel leaks that I can detect.

Does anyone know if this problem could be a leak in the crankcase, like old worn out gaskets or crankcase seals?

Thanks again!

Just some more ideas for you(I got a million of them lol)

Well did you replace needle and seat? Had a problem with aftermarket needle once my oly just wouldn't run right with the new one put the old one in and it worked great.

On my 72 Elan with new needle I had to lift lever above carb body to get rid of mid range bogg.

You can check seals by spraying carb cleaner around crank on pto side while the engine is running. If the engine changes speed you may have a seal leak. Most of the time you can see oil drooling from bottom of crank or cases if any seals are leaking.

I have a Wonderosa gasket kit with seals for a 399 if you need it I don't have my oly any more so I don't need it.

My Oly had 145psi compression after i rebuilt top end but 115psi should still be good enough for yours to run.

Did you try adjusting the high speed needle some times they need a little tweaking to get rid of boggs just don't make it to lean don't want a burned piston.

Sometimes you do need to raise the lever above the carb body to get a little bit of extra lift on the fuel inlet needle. Did you use anything like a very fine wire to run through all the tiny fuel passages under the welch plugs? I suspect one of them has a bit of blockage in it.

"Sometimes you do need to raise the lever above the carb body to get a little bit of extra lift on the fuel inlet needle."Hey thanks retiredpop, this made some noticable difference although I didn't try it with a load. I may have it too high now but it doesn't seem to be flooding more like a serge at full throttle


"Did you use anything like a very fine wire to run through all the tiny fuel passages under the welch plugs? I suspect one of them has a bit of blockage in it." Under one welch plug there is nothing that i could see, the other has a ball, seems to me everything there looked fine but i may check again, see what the lever accomplished first.

Well retiredpop it didn't work out when I rode the machine. It again started starving for fuel. I again pulled the carb and am now going to get a complete kit. The only problem is I don't have a thin wall socket to remove the seat so I will have to shop around.

The other question I have... when I first pulled the card the series of holes in the gaskets and intake block didn't line up with that on the carb and intake. Not sure if they are suppose to or not?? Any ideas on that?

Thanks,
hailfriend

You have a HR carb I believe. If you need a reference here is a link with Tillotson diagrams.

http://www.vintagesnow.com/Carburetors.html

unfortunately the link doesn't show the gaskets between the carb and intake nor does the manual i borrowed from the library :hmmm: I put a complete carb kit in today and it didn't help so a waste of $30. I did however pull the pulse line or whatever it's called off the crank case and while I open the throttle i started sucking on the line and the machine ran much better so.... now i have to figure out is the new brass filter/check valve in the tank causing problems or something else obstructing the fuel draw or is the pulsating gadget not pulsating enough and if that's it what would cause that??? Fun fun!!:wacko:

If you have the external connection for the pulse line coming from the block to the carb then you don't need the internal one that is in the phenolic spacer between the carb and the engine. In that case it does not matter if the holes in the gasket and spacer are lined up or not. Do you have matching holes in the carburetor body as well? I know on our 74 Elan 250 Deluxe that it had the HR carb and did not need the internal pulse line. I don't think there were any holes in the carburetor body. My old 72 TNT 440 had an HD carb and it used the internal pulse line so the holes all lined up in the gasket, spacer and carburetor.

Is the pulse line clear in the engine crankcase end? You should be able to blow into the engine block easily.

or is the pulsating gadget not pulsating enough and if that's it what would cause that??? Fun fun!!:wacko:

A weak pulse would be caused by a faulty internal crank seal between the two cylinders.

retiredpop... yes there is an external impulse line from the crank to the carb. It is clear as stated above I pulled it off the crankcase and sucked on it while I had the engine running and it seemed to help the engine run better, drawing fuel to carb while hold throttle 3/4 to full, it wouldn't run prior. So are you saying if I flipped the block around between the carb and intake so all holes will line up that it too serves as an impulse to carb?? You also stated a leak in the crankcase seal could cause low impulse? that means pulling the heads and jugs etc??

Thanks for you help!!

Actually I meant is it clear into the engine crankcase, not into the carburetor. What I'm wondering is if there is some sort of blockage in the crankcase fitting.

If you flipped the block around I really doubt that you would get an impulse for the carb because the carb probably does not have the internal passages for that application. If it did have they probably would not have used the external pulse source.

Unfortunately if you have a leak in the inner seal you need to dismantle the engine which includes pulling the heads, jugs, pistons, and splitting the crankcase.

If you have a vacuum gauge you may be able to T it into the impulse line to see if you have a pulse there. You should see it vibrate back and forth while it is running. At least that would give you an indication that there is a pulse present.

Just curious what carb. do you have on that engine. My 71 399 had a HR76A with internal pulse for fuel pump. If yours is different carb. or engine may have been changed at one time.

Just curious what carb. do you have on that engine. My 71 399 had a HR76A with internal pulse for fuel pump. If yours is different carb. or engine may have been changed at one time.

Good point!

This 1971 399 has the same, a HR76A but it has the external pulse line. The manual I have shows the set up I have similar to the Elan, tank and all rather than the Olympic. This machine didn't have a brass check valve and filter on the pick up line in the tank, someone suggested it should have one. I put one on but the manual doesn't show it either. I am wondering if that is restricting the fuel in anyway. I am going to remove it and see what happens. My carb has a filter on the bottom and the previous owner put a primer bulb in the fuel line and heavy black rubber fuel line. You guys could be right this may not be the right carb who knows. The idler boggies were installed upside down when I bought the machine. There were a few other things that made me think some hack was working on the machine. However if there is a pulse fitting on the crank wouldn't that indicate the carb should match?? There is a pulse there when the machine is idling and when i pull the line off it dies.

Thanks guys for the input, back to the drawing board. I really don't want to rip this engine down, I don't think it's worth it. If I do I will rebuild it obviously.

Geo

Carb number is located on top of carb where it attaches to the manifold if it is a 76A then some one changed the fuel pump stack along the way or you could have a 73' carb and engine any way I think it it still should work.

The check valve should stay it keeps fuel in the line so the pump can pick it up when you pull the starter if you remove it gas will drain back to tank and you will need to pull several times to get the gas up to the carb.

I would remove the bulb primer it is just a source of air leaks in the fuel lines and run new line from tank to carb (this could be your problem.) The previous owner must of did this because of no check valve in tank get rid of it they leak and cause problems.

Boggie sets can flip if the track is to loose.

However if there is a pulse fitting on the crank wouldn't that indicate the carb should match?? There is a pulse there when the machine is idling and when i pull the line off it dies.

Geo

I think you are making the right assumption about the carb being the right one because of the fitting on the crankcase.

I have a question about the return fuel line to the tank. When the sled is running do you have fuel pumping back to the tank? That is a good indication if it is pumping enough fuel.

Yes there is fuel returning to the tank in the return line. I think I will remove the primer bulb for starters and replace the heavy rubber fuel line with the plastic type. I will also try removing the check valve/filter from the tank just to see if it makes a difference. I think end of day I agree it should be there to keep the line from loosing fuel back to the tank when sitting. I see no place where there would have been a primer, no spot on the dash?? I am wondering if this model didn't have one for some odd reason.

Cheers!
George

If it has a choke it dosen't have a primer. Don't think tilly carbs ever had primers. You can put one on aftermarket but you don't need it. I tried one on my Oly. but it was all most more trouble than it was worth.

The manual I borrowed from the library showed a couple models with Tilly carbs and primers but you are right it likely doesn't need it. I am thinking the problem is as retiredpop stated aways back is the seal in the crankcase.

You can spray ether around crank behind clutch while the engine is idling to see about seal leak if the engine speeds up you have a leak. You will have to pull flywheel to check pto seal look for leaking oil around crank case and base gasket on jug.

Problem solved, I sold the sled. Thanks for all the ideas and suggestions guys.

Take care and maybe I'll be back with a different sled and different problems, haha.

George