Auto Technologies Head swap frustration

Bruce MacGregor said:

May check your muffles / exhaust system for blockage. I tried a set of new mufflers on my bike and it would not run very well at all untill I knocked the baffels out of them.

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If the valve spring thing checks out this answers strikes a chord with me, i have one set of mufflers that my bike won't go over 65 - 70 MPH with and another (cheaper and less baffled) set that it is quite happy with. If you are in an area where you can get away with it i would try a short run without mufflers and see what happens.

A story oft' repeated.

Guy buys high end engine part and installs it, loses power over previous OEM bit.

In frustration, he replaces OEM Amal carbs with single mikuni, replaces points with Boyer, and becomes far more frustrated when improvement is not found.

Turns out it was a bad head gasket, spark plug or cam timing was off one tooth.

SO MANY THINGS that can go wrong!

comnoz said:

You can get a close enough measurement of the valve spring installed height to know if it is the problem by using a 6mm wide strip cut from a tin can about 4 inches in length. The object is to find out exactly how long the spring is when it is in place on the head with the valve fully closed. Slide the strip down along side the valve spring so the end of the strip is resting in the lower spring seat. Now take your pocket knife and make a small cut in the side of the strip at the point where the top of the spring meets the upper spring retainer. Now remove the strip and measure the distance with a good rule or caliper. Or compare the reading with the old head if they are using the same valve springs. If the measurement is within 1.5 mm of the spec then this is not likely to be the problem although for maximum performance this measurement should be within .5 MM of the spec. Micrometers are available that replace the spring temporarily to measure this height. Jim

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My machinist explained this to me 20 years ago; if you are within 1/16", easy enough to measure with a chunk of coke can, move on to another potential trouble spot.

grandpaul said:

A story oft' repeated.

Guy buys high end engine part and installs it, loses power over previous OEM bit.

In frustration, he replaces OEM Amal carbs with single mikuni, replaces points with Boyer, and becomes far more frustrated when improvement is not found.

Turns out it was a bad head gasket, spark plug or cam timing was off one tooth.

SO MANY THINGS that can go wrong!

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And it is so easy to overthink a problem and spend piles of time and money on exotic repairs and about the time you are ready to pull that last hair you realize it really didn't need modified or updated but just a basic repair.
There are many great modifications one can do that make small improvements in operation or durability or just to make it yours but in reality the bike was a pretty good performer when it was built or it never would have survived. Jim

Amen To that Jim. Phil

OK guys as usual it has been worth it to sort through this thread as I am learning stuff I never knew I would want to know. It is rather off topic, but there is a comment early on that some suggest that not polishing intake manifolds is preferred as the rough surface and turbulence improves flow? I don't get out much, but this is contrary to everything I thought I knew.

Russ

Aries...stick with it, I am looking forward to a report on that head when you get it sorted.

I hear the above thoughts on modifications loud and clear, blame it on the Aussies!! Took the bike out today for a trial run having checked timing and running the carbs with a 107 needle jet and returned ( having already tried smaller and larger sizes) to a 240 main jet.
Same problem, faulters and hesitates. Starts about 3500 rpm in all gears. Have found though that if I drop a gear or rev it through the lower gears, by the time it reluctantly reaches 4500 the engine smooths out and accelerates as it should. So I guess that negates valve float then? New plugs make no difference.
Out of curiosity, I noticed the manifold connecting to the carbs is 32mm but narrows to 30mm connecting to a 32mm cylinder head inlet. Im guessing this okay as fuel flow isnt impeded.
Thats it really, mystery continues. Any further thoughts out there?

Aries

rvich said:

OK guys as usual it has been worth it to sort through this thread as I am learning stuff I never knew I would want to know. It is rather off topic, but there is a comment early on that some suggest that not polishing intake manifolds is preferred as the rough surface and turbulence improves flow? I don't get out much, but this is contrary to everything I thought I knew.

Russ

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Read this http://www.mototuneusa.com/homework.htm about half way down the page "Why Not Polish The Ports ??"

Out of curiosity, I noticed the manifold connecting to the carbs is 32mm but narrows to 30mm connecting to a 32mm cylinder head inlet. Im guessing this okay as fuel flow isnt impeded.
Thats it really, mystery continues. Any further thoughts out there?

A Combat would have come with 32mm to 32mm manifolds, the later 850's used 32mm to 30mm manifolds as the head was 30mm inlet which helped at low revs. If you have 32mm inlets on your head then you need 32mm-32mm manifolds.

aries said:

I hear the above thoughts on modifications loud and clear, blame it on the Aussies!! Took the bike out today for a trial run having checked timing and running the carbs with a 107 needle jet and returned ( having already tried smaller and larger sizes) to a 240 main jet.
Same problem, faulters and hesitates. Starts about 3500 rpm in all gears. Have found though that if I drop a gear or rev it through the lower gears, by the time it reluctantly reaches 4500 the engine smooths out and accelerates as it should. So I guess that negates valve float then? New plugs make no difference.
Out of curiosity, I noticed the manifold connecting to the carbs is 32mm but narrows to 30mm connecting to a 32mm cylinder head inlet. Im guessing this okay as fuel flow isnt impeded.
Thats it really, mystery continues. Any further thoughts out there?

Aries

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Aries
I don't think the tapered manifolds would cause the problems you are seeing although I can not say that combination has been tested.
Since the engine reves freely over 4500 I would agree that valve float is unlikely.

It really sounds like a carburetion problem. So chances are its ignition. :?

What plugs are you running?

After you ride the bike for a half a km in the problem area, stop the motor, coast to a stop and look at a plug. Is it black and fouled or clean and white? Do both plugs look the same?

What # needle jets have been tried. Are you at low altitude? And what effect did the changes have.

What do you have for ignition coils. I am asking because the porting on the new head will raise cylinder pressure through the midrange and will require a bit more ignition voltage than before. You might want to reduce the plug gap to .015 and see if that makes a difference. Have you checked your plug wires with an ohmmeter?

It sounds like there is a simple basic problem. Jetting should not be this big an issue because the needed jetting changes I have seen so far have been small. Jim

Aries, It sounds like you have the later 32mm to 30mm manifolds. I do not see a real problem and the way some are talking, this setup may be desirable. That being said, it sounds like you could trade even up with a couple people close to this thread for a set of 32x32's.

Unboosted intake only has up to 14.7 PSI to push in mixture while exhaust has many 100's PSI. Needs differ for most flow. Fuel tends to sling out of mixture as it makes turns fuel being heavier molecules that air component and then tends to condense or collect on surfaces so less to burn for heating the Nitrogen that does the work. Rough textured intakes help vapor flow similar to golf ball dimples that give energy to surface turbulence so fluids can flow to equalize pressures with smaller energy sucking votrex-eddies that also tends to re-mix fuel back into the flow.

Plus this streamlining principle in subsonic flows below mach 1 vs super sonic flows about mach 1, blasting out open exhaust valve.

Its just about laminar flow across the surface. Slightly textured surface causes the first layer of flow (boundary layer) to hug the surface. This reduces flow friction at subsonic speeds, and allows the flow layers above the boundary layer to 'slip' more easily and reduce turbulence. Turbulence at subsonic speeds causes flow velocity to suffer.

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I had to check into this using 750 size head/ports/valves, in 20 % bigger engine with 10+ PSI pushing it in as did not want to hit super sonic nozzle choking, which still flows like crazy but does not increase much even with some extra push. It'd take a Lot extra push w/o upping the diameters. Up to sonic pressure wave shock fronts piling up, it helps mix inertia to ram more in.

I've had two engines that woke up dramatically by funneling down intake with a trip up lip in the way, one a 25 hp 2 cycle outboard that suddenly over rev'd and lifted me and hull out of the water to ride its cavitation plate and then the prop as blades slapped down with a strong pull accelerating another 6-7 mph.
The other one was a 45 cid 4 stroke you wouldn't believe how surprised it made me.

http://en.wikipedia.org/wiki/Cylinder_head_porting
http://theamcpages.com/engine-design-va ... -ports.htm
http://www.idav.ucdavis.edu/~garth/pdfs/vis06.pdf
http://old.vrvis.at/scivis/design/

With this in mind Lotus has developed an innovative port design (figure 2) which has shown improvements in flow coefficient of about 8%. Vortices are generated that draw the flow back towards the port floor and re-energize the boundary layer to offset the flow separation which causes the tumble, the flow attaches more to the inner radius of the port with the modified geometry. The modified geometry shows a similar effect at lower speed and consequently reduces the tumble also. However, the reduction in tumble at low speeds is small and is more than compensated for by the increased flow."

http://speedtalk.com/forum/viewtopic.php?t=4785

I'm with Comnoz here - this head is really a bolt-on-and-go piece. There shouldn't be any radical changes necessary to get you down the road (quickly!).

The fact that you can rev through the problem sounds like a mid-range jetting issue.
But it also rings of an electrical/ignition issue too.
Is it RPM related, or is it throttle position related?


Whatever it is, it's a little thing. We've all heard this before a million times, but it really is worth repeating: When you hear hoofbeats, look for horses, not zebras.

Aries, does you carbs have a notched spray tube (3rd one from left) and maybe a needle with a 4 groove identifier? If so, you may want to get "622/074 SPRAY (CHOKE) TUBE" (1st one) and "622/124 Needle - 4 Stroke"(2 groove identifier).

And look in the carb to make sure both spray tubes are the same height. They can come loose and slide down or turn. Jim

hi aries,am i right in thinking that your motor is a 72 combat or a 72 std motor with a combat head,and that the only thing you originally changed was the head

chris plant said:

hi aries,am i right in thinking that your motor is a 72 combat or a 72 std motor with a combat head,and that the only thing you originally changed was the head

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And new mark 1's.

Is it possible the taller head made push rod length too short to fully lift valves?

It might be worth checking cam chain tension, if too slack can advance spark too quick or too much as rev's rise then tops out rpm power increase no matter the gear-engine loading.

I've had something similar, good power up to a point then dropped off misfiring, turned out wires vibrating to short out/disconnect at some threshold but not below it. Check in darkness might reveal something.

Needle clips can come loose but likely you got that covered.

Spark plug gap to big to keep good spark at rpms?

Timing light to verify ignition is doing its job all the way up rpm scale?

hobot said:

Is it possible the taller head made push rod length too short to fully lift valves?

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No, because it rockets off at 4500.

After you ride the bike for a half a km in the problem area, stop the motor, coast to a stop and look at a plug. Is it black and fouled or clean and white? Do both plugs look the same?

Jim has a great idea that may help shorten the time it takes to find out whats going on. Plugs will give you a very good indication of whats going on at the time it's not running good. Also what Homeslice says too. I found with my rebuild that I was a bit rich in the mid range, And it was running just like what you seem to have going on, Once I dropped the needle down it started to clear up nicely. That gave me a starting point as to which at least I could stop chasing all the other things, Good luck.