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Message started by Bobster on 05/07/06 at 14:05:18

Title: Carb diagnoisis?
Post by Bobster on 05/07/06 at 14:05:18

Im hoping someone that knows carbs well can help. My mods are as follows. Main jet is 155, pilot is 55, mixture screw is out about 2.5 turns, harley muffler, cone filter, half spacer. When the engine is cold, it fires right up without choke and idles fine. After letting idle for a few minutes, when i start to accelerate, she bogs bad for about 5 mins. then runs great. If i pull the choke half way while im riding she runs great until i come to a stop, or pull in clutch. She wont idle with any choke pulled. What part of the carb needs adjusting? Thanks

Title: Re: Carb diagnoisis?
Post by lancer on 05/07/06 at 15:22:06

You have gone up 4 sizes on the main jet, which may be Ok, and one size larger on the pilot with a 2.5 turn out setting........you are running very rich.

It fires up and runs fine when cold because it is already rich...does not need more choke. After it warms up it bogs down...classic over rich. When you are riding at speed and pull the choke out it does not make a difference because at speed that little bit of extra fuel is a small % of the total coming through the needle jet/main jet...but when you then come to a stop and go back to idle then it becomes a much bigger issue...and the engine dies from an over rich condition..

Turn your pilot adjusting screw back to "1 turn out" (the engine needs to be warmed up when you do the adjustments) and see what happens. If it is still acting like it is too rich the back down to the #52.5 jet.
Remember, every thing from idle to 1/4 throttle (not rpm or mph...it is throttle position) is all about the pilot jet size and the pilot adjusting screw setting (turns out). When the correct pilot jet size is in the carb then the adjusting screw will find happiness (max rpm at idle) in the 1-2 turns out position. If it is less than 1 turn then go one size smaller on the pilot jet...if it is more than 2 turns out then go to the next larger jet.

For the mid-range (1/4 to 3/4 throttle setting) you want the white spacer to be as small as possible so that it gives you the hardest pull/acceleration without cutting out from being over-rich.

The main jet should be the largest jet that will give you the highest possible speed at full throttle, but without cutting out due to being over-rich.

When you have the settings at what seems like the right place, then check the spark plug to make sure you have it right...it makes a great backup check.

Check out this link to the Mikuni website, this is their tuning guide and they probably know what they are doing. There is a lot of good info about all aspects of tuning carbs. They reference Harley's frequently but it is all basic stuff and applies to all 4-stroke engine tuning. This is just one part of what they have, so spend some time there and learn a lot.

http://www.mikuni.com/fs-tuning_guide.html

Bobster wrote:

Title: Re: Carb diagnoisis?
Post by Bobster on 05/07/06 at 18:35:42

I appreciate the effort in your answer. I have a good idea what to do. Thanks

Title: Re: Carb diagnoisis?
Post by Brewbrother on 05/07/06 at 20:24:57

Bobster wrote:

I appreciate the effort in your answer. I have a good idea what to do. Thanks

lancer nailed it. Its all about balance between air, fuel and exhaust.

Title: Re: Carb diagnoisis?
Post by Bobster on 05/08/06 at 12:49:55

My mixture screw was at 1.5 turns out. I thought it was 2.5 turns. I tried it cold this morning at .5 turn out and it was better. I can see that my pilot 55 is too large.

Title: Re: Carb diagnoisis?
Post by lancer on 05/08/06 at 19:37:41

There you go, keep working the "12 step plan"...well it may not be 12 exactly, but you get it. And study the Mikuni site instructions. If you do not have a Mikuni carb book, that would be a good resource to have around. You can find them on ebay. ;D

Title: Re: Carb diagnoisis?
Post by Savage_Rob on 05/09/06 at 13:12:22

http://www.mikuni.com/fs-tuning_guide.html