CMB 90 and BVM Needle Settings

[Doug Babcock]

I just put a 90 CMB Gold Head and a BVM needle in my 19 year old Sprint Cat 80/90. I bought the motor used and just broken in from Allan Yuen, and thanks also to Stu Barr for the CMB pipe, header and remote needle fuel inlet, Walt Barney for the tank and needle, John Otto for the 40% nitro fuel and Mark Sholund for starting work on a prop for it.

I ran the motor on the hose and it starts and runs fine. I set the hose pressure so that the head just gets warm to the touch. I set the BVM needle at 3 turns out and the idle needle has been adjusted so that blipping the throttle with no load has it hesitating slightly, then reving up. This has the idle needle about .070" out of the main fuel barrel when the throttle is wide open. If I crank the idle needle in, at some point the engine revs right up and goes real lean without any throttle input, which I'd expect. I don't leave it there. When it's too rich, blipping the throttle just kills the motor.

The pipe length is set at 12 1/2" from the cylinder bore centerline to the apex of the divergent cone. Pipe pressure appears to be fine.

I slapped on a balanced, slightly thinned and de-toungued X465 that I worked on myself. I'm no prop guy, but it should at least work to get the boat going. Mark Sholund will be working with me on good props.

I think that all these parts are fine by themselves, but when we throw the boat in the water it dies when I give it throttle immediately as it's being launched.

I can surmise that the high speed BVM needle isn't even in the picture under no-load conditions, so it may be too lean, but I have no feel for the BVM needle or the motor adjustments in this application.

Anyone have any ideas?

Thanks to all that I've talked to and read about on this great forum.

Regards,

Doug

 

[Bob Morton]

Doug,

Pulstech is right they run rich. If you have a or access to a Boris Meter set it at 40. Start with a small prop (X455/3) then work up to a larger prop. Run it at the lake,drive way tuning never works. jMO.

Nails

 

[Joe Warren]

I just put a 90 CMB Gold Head and a BVM needle in my 19 year old Sprint Cat 80/90. I bought the motor used and just broken in from Allan Yuen, and thanks also to Stu Barr for the CMB pipe, header and remote needle fuel inlet, Walt Barney for the tank and needle, John Otto for the 40% nitro fuel and Mark Sholund for starting work on a prop for it.

I ran the motor on the hose and it starts and runs fine. I set the hose pressure so that the head just gets warm to the touch. I set the BVM needle at 3 turns out and the idle needle has been adjusted so that blipping the throttle with no load has it hesitating slightly, then reving up. This has the idle needle about .070" out of the main fuel barrel when the throttle is wide open. If I crank the idle needle in, at some point the engine revs right up and goes real lean without any throttle input, which I'd expect. I don't leave it there. When it's too rich, blipping the throttle just kills the motor.

The pipe length is set at 12 1/2" from the cylinder bore centerline to the apex of the divergent cone. Pipe pressure appears to be fine.

I slapped on a balanced, slightly thinned and de-toungued X465 that I worked on myself. I'm no prop guy, but it should at least work to get the boat going. Mark Sholund will be working with me on good props.

I think that all these parts are fine by themselves, but when we throw the boat in the water it dies when I give it throttle immediately as it's being launched.

I can surmise that the high speed BVM needle isn't even in the picture under no-load conditions, so it may be too lean, but I have no feel for the BVM needle or the motor adjustments in this application.

Anyone have any ideas?

Thanks to all that I've talked to and read about on this great forum.

Regards,

Doug

Doug you are NOT going to have any luck running 2 High speed needles in line. Both low/high Needles need to be cut out & plugged if you are using a bob violet. Then open the carb barrel valve approx 30 percent so the boat will crank & maintain a high idle before you launch. Are you running a Pipe pressure line to the tank?

 

[Doug Babcock]

Thanks, guys.

I don't think any water is getting in the carb at launch as the cowl protects it very well. I'll check that, however.

I've resolved myself to the fact that there's probably a Boris Mazur flow meter in my future at some time, but I also suspect that this thing can be made to run initially without relying on one.

Yes, I'm running pipe pressure.

As far as the needles go, the main, high speed needle that came attached to the engine has been removed and a straight-in fuel nipple with no needle has been fitted in its place.

I'll try backing the BVM and idle needles out when I get back up to the lake. I also have an X460/3 and an X465 that's been cut down to about 55mm in diameter.

Thanks for the info!

Regards,

Doug

 

[Joe Warren]

Thanks, guys.

I don't think any water is getting in the carb at launch as the cowl protects it very well. I'll check that, however.

I've resolved myself to the fact that there's probably a Boris Mazur flow meter in my future at some time, but I also suspect that this thing can be made to run initially without relying on one.

Yes, I'm running pipe pressure.

As far as the needles go, the main, high speed needle that came attached to the engine has been removed and a straight-in fuel nipple with no needle has been fitted in its place.

I'll try backing the BVM and idle needles out when I get back up to the lake. I also have an X460/3 and an X465 that's been cut down to about 55mm in diameter.

Thanks for the info!

Regards,

Doug

Doug you are in the ball park on prop selection. After you get the engine carb problems resolved. I think Both needles are going to have to be removed. What does the fuel spray bar look like? Is is cut off Or is it 2 pieces telescoping into each other. You should only see 1/2 of a spray bar. is your strut up in the tunnel or is the strut below the rear sponsons? You base line strut set up should be Level witht the rear sponsons to maybe up in the tunnel just a little

 

[Doug Babcock]

Hi Joe,

The original high speed needle/fuel inlet had a notch in the end where the new non-needle fuel inlet has no notch.

Attached is a picture of the original stock fuel inlet and needle that shows the notch. The fuel inlet that I replaced it with has the fuel inlet going straight in where the needle is on the original and no notch. The spray bar is the same length on both. I don't have a picture handy of the new fuel inlet, but there is no high speed needle in it. The fuel has no restriction beyond the BVM needle at wide open throttle. The only restriction presented to the fuel is when the idle needle enters the spray bar when the throttle is closed.

I should have brought the boat home from the lake yesterday so I could take a picture of both the original fuel inlet/needle and the new fuel inlet with no needle. Maybe I'll just fly up tonight and get it if I have the time.

Attached are pictures of the original fuel inlet/needle and looking straight into the carb throat with the throttle wide open. The notch faces the intake disc.

The gap in between the end of the idle needle and the end of the spray bar in this picture is indicative of the idle needle being backed out so the the outside of the outer O-ring on the idle needle is even with the outside edge of the carb. I guess what your saying is that even this distance between the idle needle and the spray bar end is too small and that in order to not have to back the idle needle out beyond where it's outer O-ring becomes ineffective, the idle needle needs to be ground off?

The only other issue is the notch that exists in the end of the original where there's no on the needleless fuel inlet.

Regards,

Doug

 

[Joe Warren]

Hi Joe,

The original high speed needle/fuel inlet had a notch in the end where the new non-needle fuel inlet has no notch.

Attached is a picture of the original stock fuel inlet and needle that shows the notch. The fuel inlet that I replaced it with has the fuel inlet going straight in where the needle is on the original and no notch. The spray bar is the same length on both. I don't have a picture handy of the new fuel inlet, but there is no high speed needle in it. The fuel has no restriction beyond the BVM needle at wide open throttle. The only restriction presented to the fuel is when the idle needle enters the spray bar when the throttle is closed.

I should have brought the boat home from the lake yesterday so I could take a picture of both the original fuel inlet/needle and the new fuel inlet with no needle. Maybe I'll just fly up tonight and get it if I have the time.

Attached are pictures of the original fuel inlet/needle and looking straight into the carb throat with the throttle wide open. The notch faces the intake disc.

The gap in between the end of the idle needle and the end of the spray bar in this picture is indicative of the idle needle being backed out so the the outside of the outer O-ring on the idle needle is even with the outside edge of the carb. I guess what your saying is that even this distance between the idle needle and the spray bar end is too small and that in order to not have to back the idle needle out beyond where it's outer O-ring becomes ineffective, the idle needle needs to be ground off?

The only other issue is the notch that exists in the end of the original where there's no on the needleless fuel inlet.

Regards,

Doug

Spray bar is good. Screw both low/high needles out & grind the needles (point) off with a belt sander you can screw them back into the holes to plug the holes up. the o`rings on the needles help to keep them from screwing themselves back out. Adjust the barrel valve to about a 30% opening @ idle. Crank the boat if it tries to run away? Open the needle on the bob violet. You did not say if you had it (bob violet) hook to third channel servo? if you do roll it open to increase fuel flow. This will slow the engine rpm`s down. I think you wil be happier setting the carb up like this. it works much easier. When it is set up correctly. You should be able to turn on the boat & radio & fire the boat up without holding the radio in your hand & it maintain a medium rpm waiting to be launched.Cut the points off completely. Just leave a clean thread to screw back into the carb hsg.p.s. (that entire taper is part of the needle)

 

[Doug Babcock]

So you're talking about cutting off the whole tapered part of the needles beyond the threads. I get that.

One thing that I guess I haven't explained properly is that in place of the main needle and fuel inlet piece on the carb, I have installed a straight fuel inlet that was designed to be used with a remote needle. I got this from Stu Barr. There is no main needle on the engine. The fuel just flows right in with no needle in the way. All of the high speed mixture setting is done with the remote BVM, and yes, I have the BVM on the 3rd channel.

The only reason I mention this is the only difference between the original spray bar with the needle and the one that's in there now without the needle is that the original one has that notch in the end where the fuel exits into the barrel. The new one has no notch, so the fuel isn't directed into the engine as the notch would tend to do. It squirts out the end and vacuum bends it into the engine.

Should I make a notch in the new one?

Thanks,

Doug

 

[Joe Warren]

So you're talking about cutting off the whole tapered part of the needles beyond the threads. I get that.

One thing that I guess I haven't explained properly is that in place of the main needle and fuel inlet piece on the carb, I have installed a straight fuel inlet that was designed to be used with a remote needle. I got this from Stu Barr. There is no main needle on the engine. The fuel just flows right in with no needle in the way. All of the high speed mixture setting is done with the remote BVM, and yes, I have the BVM on the 3rd channel.

The only reason I mention this is the only difference between the original spray bar with the needle and the one that's in there now without the needle is that the original one has that notch in the end where the fuel exits into the barrel. The new one has no notch, so the fuel isn't directed into the engine as the notch would tend to do. It squirts out the end and vacuum bends it into the engine.

Should I make a notch in the new one?

Thanks,

Doug

Dougf, I would just smooth the notch off, I Have raced many carbs with the spray bar blunt. As close to the middle of the venturi as possible is great. it will run with the notch too if you must run it this way. I dont have a pic of the carb mod you have made witht the new inlet. But you can use the existing stk fuel inlet if you cut the taper off & just screw the threaded part back into the hole to plug it.

 

[Doug Babcock]

OK. That's what I'll do then.

I'll cut the taper off the idle needle and screw it all the way in. That takes it completely out of the barrel.

The straight fuel inlet protrudes the same distance into the barrel as the very end of the pictured needled, notched inlet does. That's fine then.

I understand about the engine not being able to take throttle very well under no load as I've experienced this. I'll richen up the BVM some more manually, launch, then lean it with the 3rd channel. At some point, it has to keep running.

Fortunately, I have two CMB 90's. The other one has a dual plug head and has never been run.

Just received a new prop from Mark yesterday. Quite a bit better than my attempt at prop mods by a long shot.

Strut depth is about 3/8" from the bottom of the sponsons to the shaft centerline when the boat is lying on a flat surface. Strut angle has the shaft parallel with the bottom of the boat; ie: no up or down angle.

I'll try it again this coming weekend.

Thanks to all,

Doug

 

[Joe Warren]

OK. That's what I'll do then.

I'll cut the taper off the idle needle and screw it all the way in. That takes it completely out of the barrel.

The straight fuel inlet protrudes the same distance into the barrel as the very end of the pictured needled, notched inlet does. That's fine then.

I understand about the engine not being able to take throttle very well under no load as I've experienced this. I'll richen up the BVM some more manually, launch, then lean it with the 3rd channel. At some point, it has to keep running.

Fortunately, I have two CMB 90's. The other one has a dual plug head and has never been run.

Just received a new prop from Mark yesterday. Quite a bit better than my attempt at prop mods by a long shot.

Strut depth is about 3/8" from the bottom of the sponsons to the shaft centerline when the boat is lying on a flat surface. Strut angle has the shaft parallel with the bottom of the boat; ie: no up or down angle.

I'll try it again this coming weekend.

Thanks to all,

Doug

Doug If you are 3/8 below the rear sponsons. Most basic cat set ups are set the boat on a level surface & push the strut down to the tabel this gets your Strut bottom level with the rear sponsons & No angle on the strut. This is a Good starting point. Also remember the engine becomes leaner when the prop hits the water under load. When you get the needle in the ball park for a launch the engine will sounds rich with fuel & be smoking a little. If it is screaming lean & Quick to accelerate you need to richen up the needle. If you have a flow meter a Number of 40 or less depending on the air would be a good place to fire it up at.......Most modern flow meters today use a Blood pressure cuff type indicator

 

[Doug Babcock]

I should have been more clear. The shaft centerline is in fact 3/8" ABOVE the bottom of the sponsons, which makes the bottom of the strut exactly flat on the table with no angle, just as you have indicated it should be. I had adjusted the strut height by doing exactly what you said. The fact that I measured the shaft centerline above the sponson bottoms was merely incidental.

Thanks for all your help. I'm sure I'll be able to get it going this weekend and I'll let you know.

Doug

 

[Joe Warren]

I should have been more clear. The shaft centerline is in fact 3/8" ABOVE the bottom of the sponsons, which makes the bottom of the strut exactly flat on the table with no angle, just as you have indicated it should be. I had adjusted the strut height by doing exactly what you said. The fact that I measured the shaft centerline above the sponson bottoms was merely incidental.

Thanks for all your help. I'm sure I'll be able to get it going this weekend and I'll let you know.

Doug

your in the ball park with prop & strut set up..... get the carb issues resolved & you will be running.

 

[Doug Babcock]

Thanks, Joe.

My God, you have a lot of boats!

What's your opinion on the 101 versus the 91?

I guess I might also ask what your opinion is about the 91 versus my stock 90 Gold Head from the mid 1990's.

How about single versus dual plug heads? I have both, but am starting out with the single.

 

[Joe Warren]

Thanks, Joe.

My God, you have a lot of boats!

What's your opinion on the 101 versus the 91?

I guess I might also ask what your opinion is about the 91 versus my stock 90 Gold Head from the mid 1990's.

How about single versus dual plug heads? I have both, but am starting out with the single.

RS101 Is THE Engine if you must have All Out Power. I have seen Nothing out run them if they are set up properly! RS91 very Good Engine, But as you know less Bore Means a little less Power. But still a great Engine. & you can buy replacement parts for Both

 

[Doug Babcock]

Well, I finally got a chance to try the boat this weekend and unfortunately it still won't run.

Here's what I did:

1. Cut off the idle needle taper just beyond the threads so there is no taper that gets inserted into the spray bar at any time. I screwed the idle needle in so that it's O-ring seals the shaft, and no part of it extends into the barrel.

2. I installed the high speed needle/spray bar assembly back into the carb with the high speed needle taper cut off back to the threads, then screwed the needle all the way in and checked that it didn't get in the way of fuel flowing into the engine. It doesn't.

3. Opened up the BVM needle 3, 4, 5 and 6 turns during the testing. Obviously, the main needle has virtually nothing to do with the idle as expected.

4. Throttle trim is set to about 1/3 open.

First off, the engine won't even idle with the cut off idle needle. If I put my other idle needle in (it still has the taper), I can get the engine to idle fine by screwing it out maybe 3 turns from all the way in. As I screw that idle needle out, the engine runs richer and richer, spitting fuel out the carb and eventually the engine won't run without the glow plug battery attached. Real rich well before the needle is even close to the richness that the cutoff idle needle presents. The engine simply won't run when the idle needle that I cut the taper off is inserted.

No matter where the BVM is set, the instant it's thrown it in the water it dies.

Throttle response when unloaded is interesting in that I have to be real careful when I momentarily blip it to full throttle or the engine will die.

I also notice that I'm getting some pretty big air bubbles flowing through the fuel line after I start it. The Walt Barney tank is brand new and was near full. I run pipe pressure from a CMB muffled pipe.

Any ideas?

Thanks,

Doug

 

[dwilfong]

Well, I finally got a chance to try the boat this weekend and unfortunately it still won't run.

Here's what I did:

1. Cut off the idle needle taper just beyond the threads so there is no taper that gets inserted into the spray bar at any time. I screwed the idle needle in so that it's O-ring seals the shaft, and no part of it extends into the barrel.

2. I installed the high speed needle/spray bar assembly back into the carb with the high speed needle taper cut off back to the threads, then screwed the needle all the way in and checked that it didn't get in the way of fuel flowing into the engine. It doesn't.

3. Opened up the BVM needle 3, 4, 5 and 6 turns during the testing. Obviously, the main needle has virtually nothing to do with the idle as expected.

4. Throttle trim is set to about 1/3 open.

First off, the engine won't even idle with the cut off idle needle. If I put my other idle needle in (it still has the taper), I can get the engine to idle fine by screwing it out maybe 3 turns from all the way in. As I screw that idle needle out, the engine runs richer and richer, spitting fuel out the carb and eventually the engine won't run without the glow plug battery attached. Real rich well before the needle is even close to the richness that the cutoff idle needle presents. The engine simply won't run when the idle needle that I cut the taper off is inserted.

No matter where the BVM is set, the instant it's thrown it in the water it dies.

Throttle response when unloaded is interesting in that I have to be real careful when I momentarily blip it to full throttle or the engine will die.

I also notice that I'm getting some pretty big air bubbles flowing through the fuel line after I start it. The Walt Barney tank is brand new and was near full. I run pipe pressure from a CMB muffled pipe.

Any ideas?

Thanks,

Doug

have you ever pulled the head of the eng and checked it out?

 

[Doug Babcock]

Yeah, I did pull the head and it looks fine in there. The O-ring for the water jacket is in good shape. The piston looks good as well as the bore.

I just thought of something although I don't think it's an issue. When I first got the engine, I removed the rear housing that the carb attaches to and the intake disc fell out on the bench. Not knowing which way it went back in, I checked my other never-been-run engine to make sure I put it in the right way. I was careful to put it in the same direction as the other motor so I think it's correct, but I'll check it again tomorrow. If I happened to put it in backwards and there is no mechanical interference that would stop it from moving freely if I did so, that could really mess up the intake timing.

As I said, I don't think that's the issue, but I'll check it just to be sure.

Doug

 

[Joe Warren]

Doug the old Cmb 90 dish was a .030 thick & smooth on both sides. You could easly flip it over & not know it. Put the piston @ Bottom dead center. (all the way down) Put the rotor on the end of the crank So the Intake window is fully closed @ piston @ bottom dead center. This would be the proper location for the Disk. Now if you rotate the engine in running rotation You should notice the intake rotor closes about 62 Degrees After the piston reaches Top Dead Center & contuine to rotate the engine & the rotor will still be closed @ Bottom dead Center. Just after bottom dead center the Intake window begans to open again,. This is what you want & this would get he disk located on the crank pin properly.......p.s Is your needle on the Fuel Pick up side of Walt`s tank? it should have had a Small piece of fuel tubing slid onto the pick up tube when you got it.

 

[Doug Babcock]

Hi Joe,

I just went out and took the motor out of the boat and removed the rear cover. As the disc fell out again, I was ready for it and caught it so I could put it back the same way it came out. Looks like it was backwards! When it fell out before, the way I checked it with my other motor was by looking into the carb barrel of the other motor and spinning the flywheel. This casual inspection would have not enabled me to see the rougly 20 degrees of intake timing difference that reversing the disc would have caused since you can't see the intake port edges looking into the carb barrel. I wrongly assumed that a reversed disc would be way out of whack rather than just 20 degrees. Based on degreeing it as you said, the way I had it was more like 80 degrees ATDC at full closure rather than the correct 62.

Now I'm going to think about what this kind of intake timing retarding would do. Nothing good, I'm sure!

Hopefully, this is the problem.

Fuel tank inlet and outlet are verified and fine.

Thanks,

Doug