D ports and jetting

T

TBW

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Just installed JS inlet and exhaust D ports to my ‘72 Combat (210xxx) during a rebuild. Running mk2 Amals (106 needle jet, mid groove and 260 mains ) , PW 3, Surefire, JS rods n pistons .

Be interested in the wise n knowledgeable on the forum ref any jetting changes.
Txs much.
 
I can't really help at all with Amals. Won't use them. So how is it running since putting the restrictive sleeves in the ports? I'm guessing not better. I wanted to see what the exhaust D-port hype was about, but the JS exhaust D-ports produced a detuning result for me. Terrible bottom end that was flawless before installing them. I actually threw the parts in the trash. I use 35mm FCRs, a SS cam, the JS long rods and short skirt high CR pistons and a Molnar crank and my engine is considerably better across the board from idle to WOT at high revs with the unrestricted exhaust ports flowing into a 2 into 1 exhaust. I won't put the JS intake sleeves in my engine either. I don't have the time to waste starting over with a tune that may never be as good as what I have already accomplished. Just saying.

I figure there will be a rash of people praising the JS parts. Hopefully the guys that think the parts are super duper power producing upgrades can help you with the Amal jetting.
 
Schwany,
I thought I should throw in my thoughts on your post. I have a stock 850 with Premiers and the d-port inserts on the exhaust. I don’t know if they are a plus or not at the high end above 6K but my bike could not run better as a daily rider in my opinion.
Mike
 
I tried the d exhaust ports on my 850. No jetting change but had to reduce idle speed. They do something but hard to say if they increase power.
George
 
YING said:
Schwany,
I thought I should throw in my thoughts on your post. I have a stock 850 with Premiers and the d-port inserts on the exhaust. I don’t know if they are a plus or not at the high end above 6K but my bike could not run better as a daily rider in my opinion.
Mike
Click to expand...
That's good. I think they are meant for stock engines, not engines with head port work and other additional personal tuning touches. If they had worked, I would say so. I was going to give them away to another member here that thinks they are fantastic, but went looking for them couldn't find them. I must have had one of those I'll never use these again and junked them moments. Kind of rare for me to toss out anything actually. Also a shame, cuz I know I cleaned them up after trying them.

Test fitting while I still had them:
D ports and jetting


As far as TBW's situation is concerned during the initial discussion of the JS inserts the intake inserts would more than likely require some carburetor tuning changes. The exhaust inserts were supposed to be plug and play more or less.
 
Here's the D port by Jim Comstock I believe that later became the FA ex port.
D ports and jetting


Here's the ex D port of the FA head
D ports and jetting




All of these D ports were made to improve HP and efficiency.



The JS ex D port.
D ports and jetting

Most customers report and improvement in performance. One customer said "Its the biggest bang for the buck" in increased Norton performance. If you want to go a step further for a noticable increase in power with the JS D ports then you can raise the port roof since you have already raised the bottom of the port.



The Harley XR750 ex D port - the highest HP twin valve 750 on the planet.
D ports and jetting
 
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If the XR750 was so good how come when I came across Alan Cathcart on his restored ex HD works XR bike in a Classic F750 race at Brands, started alongside him on the grid, out accelerated him and finished 2nd to another Norton while the XR was way back in , I think, 6th?
 
Hybridracer - If you out accellerated an XR750 then it must have been having problems or maybe it was an earlier version with round ports. Maybe you've got some incredible engine. Either way you beat him - congrats. Maybe try it again against any good running XR750 with 100 HP and the Ken Augustine designed raised floor cobrahead ports. I'd like to see someone put the cobra head shaped ports into their Norton racer for some serious HP beyond what is available with round ports.

D ports and jetting
 
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Is anyone ever going to help TBW with the jetting in his Amals attached to his combat head using his cam with the complete set of sleeves installed?

Will re-jetting actually produce good results, or do the roofs on both the intake and exhaust side need to be raised?
 
Here it is-

So not the same engine as the later race bikes.
 

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I can’t include attachments any more but google’s AI gives a comprehensive answer which satisfies me that Alan’s XR was an ex works racer with hot motor not an inferior single carb, cast iron boat anchor.
 
All the write ups I read on Alan’s bike had it with a 750 bottom end and 900cc top end off of a Sportster. He took out to bonneville at least once and it wasn’t that fast…

But he owned a lot of bikes, and there is certainly no way for me to know if the one above is the one you raced.
 
TBW said:
Just installed JS inlet and exhaust D ports to my ‘72 Combat (210xxx) during a rebuild. Running mk2 Amals (106 needle jet, mid groove and 260 mains ) , PW 3, Surefire, JS rods n pistons .

Be interested in the wise n knowledgeable on the forum ref any jetting changes.
Txs much.
Click to expand...
Sorry, I can’t help much with you Mk2 Amal question but 260 main jets seem a bit large.
I presume you have already changed the old 32mm manifolds for 32 - 30 tapered ones.
 
All I can say on this subject is :
Very happy with the reduction intake tract sleeves . Perfect fit and 30 mm. performance attained .
The D-Port exhaust kit , not so much . Mostly the fit . A bit sloppy fit and then carbon built up between the floor insert sleeve area , so I will figure out a better way to make it work . Back to stock on that one and now happy . Blasted about today . All good .
 

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