Board index Flathead Power-Technical Questions, Answers, and Suggestions 45 Flatties Guess the main jet!

Guess the main jet!

Post Mon May 30, 2011 8:51 am

Posts: 15
My 45 runs rich.
I've all the usual hop ups, like bigger stroke 4-5/8", bigger carb 38mm mikuni, milled heads 6,5:1, correct squish, ported, relived and bigger valves.
The main jet is nr 310 in the carb and i feel like the bike is reluctant to rev and it smokes black

Is there anyone with an similar setup who is willing to share his mainjet size, so i can get in the right ballpark at once?
I'm in the break in process and i dont like to break in my bike by running 30 small runs down my road trying diffrent main jet sizes :(

Post Wed Jun 01, 2011 9:21 pm
GuS

Posts: 369
Location: Bergen, Norway
I have gone the same way, 4 7/16 stroke, 36 mm Mik. Not similar to what you have, but here is where I am now after playing around with different settings:
Pilot 40
Slide 2,5
Needle jet P-6
Jet needle 6DP 17
Clip position no4 (towards rich)
Main Jet 300
Air jet 0.6

Still not too happy with the low speed response. It helps when I use the 5 th Clip position.
Next try will be either 2.0 Slide and or needle where first taper starts earlier.
Also a little rich, maybe a Needle jet of P-4 is something I’ll try later.
Not being an expert on the Carbs, I believe your problem is not the main Jet. Main Jet have the influence on upper range of throttle, breaking in I believe you’re not there yet.
GuS

Post Wed Jun 22, 2011 3:56 pm

Posts: 15
Thank you for answering! I forgot to look for answers after a while.

I've remeasured the carb and its not an 38mm it's an 36mm like your's. And i bought 4 main jets ranging from 300 to 270 and the 6dp17 needle. i have made dual tapered coned exahaust from 38mm to 60mm over an length of 600mm. I think they can have big influence over the carb. i also have the wldr replika cams.

i'am also a beliver in the need of a little burnpressure to help the rings too seat correctly in to the cylinderwall. so i break in my engines with a couple of heat cycles and then calm riding with gradually more and more trottle mixed with enginebreaking to lube my walls. But with my engine running too rich, the gas wash the oilcoated cylinderwalls clean. therefore pushing the engine in to premature failure :(

Post Wed Jun 22, 2011 4:57 pm

Posts: 332
Location: north central Ma.
Find your self a "color tune" spark plug there made in the UK by Gunson its a clear glass plug so you see the burn and adjust for bluish color. they work great in dark bat cave garages we all work in...
They even sell adapters for the biger plug hole.

Check with Seven Ent or any british car parts house.

44
"Smok'in the competition NOT Tobacco"
"Transplant organs, Don't bury them!"
Why dwarf? 5/8 scale race cars! http://www.dwarfcarracing.com

Post Sun Jun 26, 2011 10:00 am
GuS

Posts: 369
Location: Bergen, Norway
Still maintain, rich reading when you’re breaking in and are running with less than ¾ throttle is the culprit of to “rich” needle/clip position, needle jet and pilot.
I went with 300 based on information I found in Panics “Mikuni tuning booklet” and will look into fine tuning the main and air jet when am finished with breaking in.
Which setting do you have on the on the low circuit jets, which slide do you use?
Interested to hear others experience with jetting their stroked 45s
GuS

Post Fri Jul 22, 2011 2:20 am

Posts: 15
GuS wrote:
Still maintain, rich reading when you’re breaking in and are running with less than ¾ throttle is the culprit of to “rich” needle/clip position, needle jet and pilot.
I went with 300 based on information I found in Panics “Mikuni tuning booklet” and will look into fine tuning the main and air jet when am finished with breaking in.
Which setting do you have on the on the low circuit jets, which slide do you use?
Interested to hear others experience with jetting their stroked 45s
GuS

Sorry for the late response!
....house rebuilding, newborn kid etc....

How ever, im now running an 290 main jet, 6dh3(?) needle, 2.5 cut away trottle and a 35 pilot jet.
It suffers from an rich spot at 1/4 at constant held throttle. Give the engine any load and it cleans up. I went from an 40 pilot jet to a 35 and it improved some, but not completly.

im also running a magneto set at 30 degrees for starting and 45 for running. But my engine likes more like 50+degrees. is that a bad thing or shouldn't i worry?

Post Fri Aug 19, 2011 10:12 am

Posts: 15
I'm now running #320 main and a #25 needle jet. topend improved a lot but that bad spot at 1/4 is still there. I've a 3.0 trottle that i have'nt tested yet.

Im sending a couple of pictures of the thing . let's see if i figured out how to attach the darn things.
You do not have the required permissions to view the files attached to this post.

Post Sun Aug 21, 2011 10:06 am

Posts: 304
Location: Jonesville, Louisiana, USA
That is one cool looking scooter, Elffors !


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