Question for the gurus on intake manifold issues!

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mountain80

Member
Aug 8, 2008
260
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Red Deer, Alberta
Hi all,
I have a reallllllly interesting question so lets get started.
First of all the specs:
66cc with a puch head, recut the chamber to knock compression down to 150 psi or so
Sbp pipe at approximately the beginning primary length, decoked and the fibreglass in the pipe was plugged so repacked with stainless scrub pads and re-riveted back together
Dellorto phbg 16mm carb and free breathing filter
adjustable cdi graciously built by Ivan
Intake and exhaust port have been opened up mildly, with nice radiuses in the corners.
Transfer ports have been blended nicely as well.
Intake side of piston matched to intake port.
Crank has been rebalanced for higher rpm/balance factor.

Custom intake manifold built by myself with a 16 mm through bore that mimics the stock manifold and is only perhaps 1/4''-3/8'' longer max.
It has a bung welded onto it for the barb that has the tube to a custom aluminum boost bottle.

The manifold is where problems are occurring. Last year I was running exact same setup (but using stock oem manifold)and at about 6500 rpm the bike would sorta 4 stroke then rev like a rape date up to 7800 rpm and more if I let it go, did not want to go higher for more than a brief spurt. Now I realize the pipe action may have caused this weird sorta double carburating rpm range on info provided from other forum members.
The new intake mimics the stock oem shape as I have very little space in my frame and the curve is smooth and the flange that bolts to the port is beautifully hand blended and after all was done I hit the inside with a bead blaster to keep the air fuel mixture tumbling.
After starting rich this season and working leaner 1 jet at a time the mixture is nice and tan even if you look rt down at the bottom of the porcelain in a bh7s plug.
NOW the bike does not want to rev more than 6200 rpm, lots of power and torque all the way but wont rev out. Starts easily, idles like a champ.

Anybody have any answers or areas on where to start??? This is very strange!
 
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Davezilla

Davezilla

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Can you post pics of your setup? Mainly the pipe and the intake system...
Usually when I see the power come on really strong then everything just stops at a certain rpm where it refuses to go over it sounds like the baffle cone may be at too steep of an angle. Also, what about the stinger length and diameter? there's another common place for restrictions when you open up the engine to breathe better.

The 16mm carb and manifold should get you 8k rpm easily unless there's something restricting the flow like a sharp bend that wasn't blended out or a gasket hanging into the intake stream, it could be your boost bottle bung if it protrudes into the stream, I know you say you welded it in but I'm just throwing it out there as another possibility. I'm not a big fan of boost bottles but if they work for you then keep it, have you tried blocking off the line to the boost bottle to see if your rpm's come back without the boost bottle?
How about porting and port timing? have you done any porting or changes to your port timing by raising the exhaust port roof or intake port floor, or index the piston? A port timing mismatch can also give you the type of performance you mentioned where it has decent power, then a weak spot, then comes on strong again, but that can also be a mis match with the intake and exhaust.

It definitely sounds like a restriction somewhere in the intake or exhaust system or a mis match between the intake and exhaust, but it would be much easier if I could see pics of what you're running, otherwise, I can just throw out ideas that may or may not help.
 
Davezilla

Davezilla

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Ok... thanks for the pics and scratch that last part of my first post... I'm studying the pics and see a few possibilities...
The barb for the boost bottle... do the threads protrude into the intake tract?
and the copper adapters coming off the header may be adding some restriction not from the bends but from the step ups and step downs causing a lot of extra turbulance right there at a certain rpm preventing it from reving further.
I'm still looking at the pics and trying to see if I can see anything else that jumps out at me, but all in all it looks like you did a good job on everything
 
Davezilla

Davezilla

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Got any pics of the whole exhaust?
That intake looks great, about the only thing I would have done different was put the barb for the boost bottle on the underside so it could send the blowback to the bottle more efficiently, where it's at right now may be even causing a vacuum in the bottle because the air can slip past the hole and back into the carb instead of going into the bottle until the next intake stroke, but that's all I see there...
 
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mountain80

Member
Aug 8, 2008
260
4
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Red Deer, Alberta
I don't believe that their is more than 1or 2 thread in intake tract whatsoever but I can pull it off and reblend it to the intake in a pinch. Porting is basically stock ports widened out maybe 2mm at most but roofs and floor stayed within a mm or so, just cleaned and blended nicely with 1/4'' radiuses in all corners to keep rings from hanging up. Bung was welded externally. I have not tried plugging off boost bottle yet but it is on the list. I will go and snap a pic of complete exhaust.
 
Davezilla

Davezilla

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Ok.. I would start by blending out any threads that protrude into the intake tract, just use a dremel or pencil grinder if you got one and leave the barb installed if you can, if not, then unscrew it and cut out however many threads stick into the intake tract. It won't hurt it if that isn't the problem, but it'll flow a bit better. It coiuld be a combination of that and maybe some exhaust restriction.
It looks like you're running a temp sensor on your head? if so, how are your cylinder head temps at max power?
 
Davezilla

Davezilla

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I don't see anything wrong with the exhaust system except what I mentioned before about the copper tubing adapters at the header. You could use a piece of 1" emt tubing and weld that in place to eliminate those step ups and step downs from the adapters. your Cylinder head temp is in good range which is telling me that the exhaust is flowing enough at max power and the stinger is big enough for your setup. you're good with the CHT as long as you can keep it under 400, but the cooler the better. can you measure your fuel mix at max power? maybe it's going rich at max power and restricting it from reving higher, you can try jetting down 1 size but watch your head temp.

Also a dumb question but did you put in new rings or is that a new jug when you did the engine? I also see that you said you cut on the piston skirt, how high up the intake side did you go? I try to stay under 5mm there to prevent low end torque loss, but you may need to cut some off the piston top at the exhaust and transfers to keep things matched up, cutting 1mm at the exhaust port will make a noticeable difference, you can cut more off but leave at least 2mm of metal above the ring
 
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mountain80

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Aug 8, 2008
260
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Red Deer, Alberta
Yes, new rings,jug, and piston. The intake of piston skirt was blended so that at TDC the piston skirt matched the roof of the intake port only. I can eventually fab and weld up a new exhaust section to replace the copper fittings. No I cannot measure my fuel mixture at max power unfortuneately but I have a long consistent climbing hill by my house where I can can keep the engine at full load and do a proper plug chop, I monitor my cht and when its hot and revving good it will hit about 360F.
 
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Davezilla

Davezilla

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360 at max power is still in safe range... I was just asking about the new rings because it might not be reving to max rpm just yet because it needs to be run in a bit more... everything else I see looks good. have you run a full gallon of gas thru it since the new build?
 
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mountain80

Member
Aug 8, 2008
260
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Red Deer, Alberta
Yes 1000 miles I guess. Last year with the oem manifold it would do the 4stroke like stuttering at 6500rpm and after about say 10 or 15 seconds she would get through it and go like mad. shutting off the boost bottle might be my first thing to try as it is simple.
 
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Davezilla

Davezilla

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that's more than enough for a full run in so that's obviously not the problem... I'd say try cleaning up the inside of the intake manifold so the threads don't protrude into the intake tract, make sure you're not running too rich at max power, and maybe weld in a straight section of pipe on the exhaust.
The only other thing that could restrict it would be the ignition timing curve. You mentioned your CDI is adjustable? Try and experiment with the timing at high rpm
 
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mountain80

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Aug 8, 2008
260
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Red Deer, Alberta
Due to the size of that frame I would have to mimic the copper pipe section but can do it with a nicely bent tubing. I can also experiment with the cdi, I will have to open it up but I do not believe that I have a lot of retard at high rpm, I will get on this tomorrow for sure.
 
Davezilla

Davezilla

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I agree... try the easy stuff first like the timing, then the intake blending, then the exhaust last if nothing else fixes it. For the exhaust, it's fairly easy to find some tubing that has the bends where you need them... scrap handlebars work great for this as most larger motorcycles such as Harleys use 1" diameter bars, and lots of custom builds use larger diameters like 1 1/4 and 1 1/2"... Something like this, may have the bend you need, just cut out the section you can use and weld it in... http://www.ebay.com/itm/Harley-stoc...classic-touring-handlebars-bars-/131150558973 Also, save this seller if you have a Harley or other motorcycle since he sells a lot of stuff that's either obsolete or ready for the junkyard, you can usually find stuff that's fixable or easily re purposed on the cheap.

Another good source of pre bent piping is off 4 stroke dirtbike exhaust pipes since they're usually between 3/4" and 1 1/2" the full length with penty of bends in them for cutting out and re purposing.
 
headtrama

headtrama

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Jul 8, 2010
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california
What I have learned from messing around with these motors is they don't like bends in the intake track . Try a short straight shot manifold and better air filter .
 
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mountain80

Member
Aug 8, 2008
260
4
18
Red Deer, Alberta
Well' it is not the boost bottle. Disconnecting it only made the carb want a half turn more fuel on the pilot screw. Went for a run, it is not the jetting judging by the plug. That filter has little to no restriction whatsoever but I will try it off. Realize that the SAME SETUP with a OEM manifold had the burble at 6500 then took off to 7800 rpm. The only thing changed this year was the custom manifold. Their is some very light scratches on the bore, running 30:1 oil/fuel mixture. Is the sbp exhaust manifold too restrictive?????
 
Davezilla

Davezilla

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Was it able to rev out to 7800 rpm with the SBP pipe before? or were you using a different pipe last year too? I keep going back and looking at the SBP pipe and it looks like it should be able to rev... One thing I'll bring up about expansion chambers also is that they can have all kinds of crazy bends in then to fit just about any frame, but one little pea size dent in the wrong place will really screw up or change the way they perform. Check to be sure there are no dents, especially in the baffle cone area if all else fails.

Let's try this aproach first... Since you're saying the only thing that's different this time around is the intake manifold, let's put the old one back on and take things from there so we can better figure out if the new one is where the problem is or not. if that pipe is also new for this year, put last year's pipe back on... basically we want to restore the engine to it's last known good running setup then put the new components back on one at a time then test run it so we can better pinpoint where the problem really is. Your engine looks great and that setup should be capable of giving you the same or better rpm range, but sometimes there's one component in the mix that just won't work the way it should either because of a mis match or the engine just simply don't like a certain component in the system, 2 strokes can be very picky sometimes about the intake or exhaust length, size, shape, etc. But if we go back to square 1 we should be able to figure it out in a day or so...
 
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mountain80

Member
Aug 8, 2008
260
4
18
Red Deer, Alberta
Ok, well except the intake manifold, everything including the pipe is the same. I worked very late so I did not have a chance to machine a carb adapter for the oem manifold today but I will asap hopefully tomorrow. The old one was hooped trying to remove it from the carb. Their is a very small dent in the pipe, I will get a pic of the outline later tonite before bed showing length etc with a ruler. Something came to mind, when I changed the balance factor I had drilled several holes in the crank web does this have any strange effect I am not aware of??? Also for these higher revs is more advance or less required as well as retard?? Or is it trial and error??? Running off to the garage in a bit for the pipe pic.
TIA to everyone who is helping out with my problem Especially Davezilla.
Cheers.
 
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