JS exhaust port fillers

C

Carl H

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Dec 13, 2017
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I recently purchased the JS Port fillers I had seen mentioned on this site, They arrived quickly and are easy to install about 1-1/2 hrs total. I was surprised that when I did a quick check for fit in 4 heads I had on the bench and the fillers all fit nicely and contacted the port floor as they should. So now I put them in the hopped up MK3 with the crossover pipes, I did have Dunstall Decibel mufflers on it as I was testing how they would act compared to the black caps I normally use. After a good test drive, the idle went from about 1100, to over 2000 RPMs and it would not come down. Another person had mentioned the high idle too.

So I did some adjusting and did some more rides, You really do feel a better kick in the pants when you give it a slight twist of throttle and I could probably richen the needle up , but It ran well and I got decent spark plug chops. It's a little hard to gauge as my MK3 is a very fast bike and is tuned very well for fast acceleration as it is/ was. But I will buy more for my other bikes. I will try them in my Combat with the cross over pipes and the Dunstall mufflers to see if it wakes up the power it should have.
 
I do not usually richen the mixture up. I start rich and then lean off. I lower the needles until I get the cough then raise them one. With petrol, it is more difficult than with methanol to get the mixture lean enough to get best power, because the jets flow half as much. A tuning error with petrol has twice the effect. It is probably the main reason old race bikes are faster when methanol fuel is used instead of petrol. And the reason fuel injection makes modern bikes faster.
 
Schwany said:
Are you using Amal carburetion or something else on your hopped up 850?
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I've got 932 Amals with 270 mains, I have it set up lean for the Blackcaps on the needle and pilot jet. I do have stay up floats with about an 18 thou pilot jet The pressed in type. All my bikes use Amals. Also Web Cam and RH10 head with a skimmed head and a good valve job. My original black cap mufflers baffles are pretty much rusted out and seem to perform pretty good.
 
acotrel said:
I do not usually richen the mixture up. I start rich and then lean off. I lower the needles until I get the cough then raise them one. With petrol, it is more difficult than with methanol to get the mixture lean enough to get best power, because the jets flow half as much. A tuning error with petrol has twice the effect. It is probably the main reason old race bikes are faster when methanol fuel is used instead of petrol. And the reason fuel injection makes modern bikes faster.
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All the racers who rode my equipment loved the way they carburate, It made them easy bikes to race. I do a lot of my own stuff to the Amal. Norton 932s are tricky to get leaned down to make them good.
 
Oh goodness I forgot the title as I read the thread.... as you were.

But I think my confusion stems from the reference to 1-1/2 hours to install? I would've thought loosening the muffler bolts, remove the roses and move the headers aside and slip them in. Tighten 'er back up. Am I missing something or just misremembering the process?
 
Hi Carl, will be interested to hear how they work in your Combat as been reading the RH4 thread and wondering about RH6 head ; I’ve Dunstall 2:1:2 & “silencers” , Mk 2 Amal 932.
Txs .
 
acotrel said:
I do not usually richen the mixture up. I start rich and then lean off. I lower the needles until I get the cough then raise them one. With petrol, it is more difficult than with methanol to get the mixture lean enough to get best power, because the jets flow half as much. A tuning error with petrol has twice the effect. It is probably the main reason old race bikes are faster when methanol fuel is used instead of petrol. And the reason fuel injection makes modern bikes faster.
Click to expand...
Methonal fuelled engines can run at a higher cr than petrol engines and thermodynamics laws dictate that the higher the cr, the higher the power.
 
Carl H said:
All the racers who rode my equipment loved the way they carburate, It made them easy bikes to race. I do a lot of my own stuff to the Amal. Norton 932s are tricky to get leaned down to make them good.
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Have you read the pilot circuit mods made by David Drew in NOC Victoria Tech Article at their web site?
 
BrianK said:
Oh goodness I forgot the title as I read the thread.... as you were.

But I think my confusion stems from the reference to 1-1/2 hours to install? I would've thought loosening the muffler bolts, remove the roses and move the headers aside and slip them in. Tighten 'er back up. Am I missing something or just misremembering the process?
Click to expand...
I was doing a muffler change and had to clean up threads in the mufflers and find parts and clean the exhaust system up, drill and re safety wire the roses. And a few other things. I guess it could be done in 1/2 hr., just not in my shop.
 
hpshooter said:
Which Web cam are you running and how much did you take off the head?
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I got the WEB Cam 312 "bolt in type" But with the head skim of about 40 thou, I had to flycut valve pockets in the 850 pistons. When you cc heads , I found the compression ratio is lower than Norton claims. I ended up with 8.6 to 1 with no base gasket and the skim. I have 185 compression. The bike pulls hard from ALL rpm ranges, that is what I want for street use.
 
hybridracer said:
Methonal fuelled engines can run at a higher cr than petrol engines and thermodynamics laws dictate that the higher the cr, the higher the power.
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Combustion conditions depend on a balance between compression ratio, ignition advance and mixture.
Methanol has unlimited antiknock, so can run at very high compression ratios. When it runs at higher compression, a richer mixture is often used or more ignition advance than at lower compression so the balance is restored. In many motors, higher compression is achieved by using high crown pistons. The same result can be achieved by using low crown pistons and adjusting the jetting. Methanol has about 0.8 times the calorific value of petrol, but usually twice as much of it is used to get similar power. It's main benefit is it hides-up the tuning errors. With methanol, the jetting can be fine enough to get the best power. Petrol needle jets do not come in quarter of a thou increments. To get the carburation right with methanol, half thou increments are necessary. The two recommended sizes for Commandos on petrol, I think are 0.106 and 0.107 inch. If they were in half thou increments, they would still not be fine enough.
I was surprised with my bike, when slower taper needles made a difference to acceleration. It is like riding a two-stroke - wind the throttle on slower. If the mixture richens too fast the bike accelerates slower.
If you tried riding a good two-stroke, the way most guys ride a Commando, it would probably stop.
 
There are a few other variables, Cam type for a particular use or track and cam timing. Good Cam builders make their products better over the years and refine them and accurately duplicate their cam's profiles. Usually, the spec sheets say what you want to hear, but the cams actually degree in with number that make their cam do what you need. That is my experience with the 5 cams I've gotten from WEB Cams.
 
The only things which cams seem to change, is their reaction with the exhaust system which can affect the shape of the power band, and where it appears in the rev range. The gearing is usually altered to suit it. With a Commando , it can be very deceiving. Because of the heavy crank, you don't know what you have, until you start changing the gearing. My bike is set-up for racing on short tight circuits. If I took it to race at Phillip Island, I would need to be careful to ensure it still self-steered in the correct direction when powered through high speed bends. I would probably need to go there twice to become competitive.
 
Carl H said:
I was doing a muffler change and had to clean up threads in the mufflers and find parts and clean the exhaust system up, drill and re safety wire the roses. And a few other things. I guess it could be done in 1/2 hr., just not in my shop.
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That would do it! I had to chase the threads on one of my exhaust ports myself last trip in there. Thankfully I had an old set of 850 roses (I use 750 ones now, with aftermarket non-crossovered pipes) so I was able to cut slots in the threads of one of those to make a tool. It now sits in my "thread tools" box instead of my "Norton parts" box.
 
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