Anyone use #300 main jets on a MK3?

jimbo

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On my newly rebuilt engine, and with twin 32MM amals, I seem to have a lean condition on wide open throttle. I started with 260's and now have 280s. Ordered 300's . Has anyone used #300 main jets on a MK3? Seems huuuge!
 
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Fuel starvation problem? Remove the carb drain plugs, put a cup underneath, open the taps. After one minute you should have about 300 cc's from each carb. Fuel level in the float bowls? Check with utube.
 

Fast Eddie

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I have a mk2 with a stage one type motor and 32mm Premiers. On the previously lean side am running a 280, and am in the process of installing one of the Maney reproduction 2EX1 exhausts, so will be running a 300 on that side to start with.

This guy seems to have run 300s on his with that same exhaust, and they were too small!

https://www.accessnorton.com/Norton...tw-tour-850-barrels-needed.25460/#post-379279
When I tried a Maney type exhaust I had to WEAKEN the needle on my FCRs compared to running stock pipes and peashooters, main jet stayed the same.

You’re probably safest trying the 300s, just don’t be surprised if that changes.
 

gortnipper

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When I tried a Maney type exhaust I had to WEAKEN the needle on my FCRs compared to running stock pipes and peashooters, main jet stayed the same.

You’re probably safest trying the 300s, just don’t be surprised if that changes.
Yeah, I know wherever I start out probably wont be where I end up!
 

gtiller

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220 mains, 106 needle jet and number 19 pilot jets over here.

The only thing non-standard is the pre-mk3 ham can instead of the plastic airbox

(with the plastic airbox they were jetted at 200/105)


300s seem waay over the top!
 
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I too, would suspect fuel delivery. Have you made the cut in the fuel inlet boss of the float bowls?
 
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I agree with Danno. Nortons need the modified float bowls. The huge main jet won't properly correct this problem
 

jimbo

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I can do that, but I never seemed to need it for decades
 
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illf8ed

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Hi Jimbo

Before cutting the fuel bowls I’d follow htowns suggestion to see if the fuel is flowing to the bowl. What petcocks are you using? The Taiwan copies have a problem with the rubber seal inserts rotating and restricting gas flow.
 
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jimbo

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Hi Jimbo

Before cutting the fuel bowls I’d follow htowns suggestion to see if the fuel is flowing to the bowl. What petcocks are you using? The Taiwan copies have a problem with the rubber seal inserts rotating and restricting gas flow.
I checked the pingel(s) they flow fine. tank cap breather is clear. Just wondered if 300's were unusual
 

Deets55

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I'd be interested to know if anyone has done this on a commando and the difference it made?
Cheers
Baz,
I my case I was chasing a problem something like Jimbo. Everything indicated I was having a fuel flow problem at high RPM, high load. The slot did not solve the problem. The issue wound up being in my Boyer MKIII ignition box. Bike ran great except for top gear, high RPM acceleration.
Pete
 

jimbo

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Baz,
I my case I was chasing a problem something like Jimbo. Everything indicated I was having a fuel flow problem at high RPM, high load. The slot did not solve the problem. The issue wound up being in my Boyer MKIII ignition box. Bike ran great except for top gear, high RPM acceleration.
Pete
wow, how do you explain that?
When I had the bike WOT ,then closed the throttle just a little , it seemed to pull harder, that's why I think I need more jet
 

Deets55

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Jimbo,
Maybe the transistors were breaking down under load? Intermittent internal connection?
I have no idea. Only found the problem about a month later. Bike stopped running when I pulled into a parking spot. I ripped everything apart in the parking lot and couldn’t get it to go. I was getting ready to call for the hook and decided to tap the ignition box with a screwdriver handle. Bike started and I drove home. Replaced the box with a MKIV. Bike ran fine and my “fueling” problem was gone. That’s also when I found out cheap DVM are terrible for resistance readings.....but that’s another story
Pete
 
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