Carby Dax RT carb problems- 4-stroking with 65 main jet

[Blue Bandit]

Hi everybody! I have just installed my new Dax RT carb on my daily commuter bike. I used to run the stock carb until I was fed up with the inconsistent performance. I ride ten miles a day to work and ten miles back so I need reliability and prefer low maintenance. I'm not quite sure that it is correct grammar to use the words low maintenance and motorized bicycle in the same sentence. Anyways, I have freed up the baffles in the muffler some time ago as well as put on the new cone filter that came with the RT carb. I started out with a .75 jet and found it to four-stroke immediately and all through the powerband as well as severely limiting the top speed. I moved down to the .70 and it showed minor improvement in the knocking department and I started to gain more top-end. I now moved even further down to a .65 and I am satisfied with the low-end power and top speed. It now four-strokes from a little bit over half power to the top. It four-strokes only when I let off of the gas. I did a plug chop and the color seems fine. I am worried that moving to .60 might be on the lean side. The only thing I can think of is this new high performance filter is too restrictive. I do not have intake leaks. Everything is sealed with red high-temp silicone.
How can I stop this four stroking? It sounds like a woodpecker when I let off the gas or hold my speed down a hill.

-Another thing is that the idle screw has to be screwed almost all the way in to keep it idling.

Any help would be appreciated.

-Blue Bandit

 

[Purple Haze]

Try setting the float lower, sometimes they are too high from the factory. Also, make sure the carb is close to level, this can effect mixture if it is tilted too much.

 

[Blue Bandit]

The carb is very close to level and I just looked at the plug after 2 days (40 miles) of riding and its quite dark and oily.

I am assuming that if the float level is set too high then the bowl will fill excessively and cause the richness I am experiencing? I'll take a look at that.

 

[HeadSmess]

I am assuming that if the float level is set too high then the bowl will fill excessively and cause the richness I am experiencing? I'll take a look at that.

100% correct. venturi in carb lowers pressure in throat. fuel is LIFTED against gravity from the bowl. closer the fuel is to the top, the less "suck" needed. same suck as before = more fuel. (not completely accurate description but thats the basics of it)
it doesnt make a huge diffence unless the bowl is extremely deep.

it will leak and flood and spray fuel everywhere before it runs you too rich.

to me...run a hotter plug, by one number, run a smaller jet.


i usually run a 55-60 for best results.

dont worry about lean. the things wont start/wont run when their bordering on perfect. they always run slightly rich. have never had a lean sieze. on other engines, yes, but never on a ht.

most two strokes should compression brake almost silently, with just the odd pop....pop pop... here or there. no woodpeckers except up the hills, thankyou!

 

[Blue Bandit]

That is exactly what I needed to hear. The smallest Jet in my collection is a 60. It should be perfect I suspect. What a great answer. On a side-note, I have seized this very engine when it was almost brand new. I failed to keep checking the head nuts and every other bolt which seals the engine as a matter of fact. I ran it for a good 7 miles at full bore until it locked itself up. The new head and piston was cheap and I have checked the nuts religiously with a deflecting torque wrench. It also gave me a chance to put a needle bearing on the piston end of the con-rod instead of the bushing that it came with. Long story short....I have attached pictures of the damage for good measure. I now have 750 miles on that same bottom end with a new top.

What a treat!

 

[HeadSmess]

now thats an impressive looking piston bit of detonation, bit of mechanical damage...quality

it actually almost looks like it had a void there in the middle, or it was thinner than normal... cus i can see about 1-2mm that was deifinitely going through the "slushy" stage, but that lip under it just doesnt look normal... the inside of the piston doesnt have any strange steps that yours looks to have... it should be the full thickness there at the crown.

does your cam have macro setting?

be interesting to see inside the orig and a newbie side by side...

 

[Blue Bandit]

I will search for some better pictures I may have stored on my computer. The piston seemed to have melted a hole in the crown and then dripped onto the crank while some melted alluminum fused itself to the side of the cylinder wall. I opened the engine case and picked out the pieces. In the last second before it froze up I bet that molten alluminum was slinging everywhere. I swear the plug base was glowing red. So you think I should run a hotter plug? I have a B6HS. What's the next one up? B7 or B5?

 

[Blue Bandit]

I will search for some better pictures I may have stored on my computer. The piston seemed to have melted a hole in the crown and then dripped onto the crank while some melted alluminum fused itself to the side of the cylinder wall. I opened the engine case and picked out the pieces. In the last second before it froze up I bet that molten alluminum was slinging everywhere. I swear the plug base was glowing red. So you think I should run a hotter plug? I have a B6HS. What's the next one up? B7 or B5?