Help & FAQs    Search BCF    Members    Groups    Events Calendar
Resend my activation email : Register : Log in 
BCF: Bike Chat Forums


Jetting PHBH28 on premix

 Reply to topic
Bike Chat Forums Index -> The Workshop
View previous topic : View next topic  
Author Message

B5234FT
Brolly Dolly



Joined: 28 Sep 2009
Karma :
PostPosted: 20:27 - 06 Jun 2013    Post subject: Jetting PHBH28 on premix Reply with quote
I bought a Husqvarna SM125 this time last year as a non runner (first bike) and I've rebuilt the engine. I got it to start but couldn't get it to idle (it just revved very high to the point I shut it off), so I pulled the carb apart and checked all the numbers and it seemed to be a carb off another bike as all the jets and the throttle slide etc were different from stock.

I then bought a stock carb and fitted that, and the issue is somewhat improved. it will now idle at a sensible speed, but unless you lift the choke every minute or so, it eventually dies off.

When I rebuilt it, I removed the injection pump and premixed the fuel (I'm not planning on doing too many miles and don't trust them, been premixing for outboards and chainsaws for years) and it's been suggested the jet sizes may need altered to cope with the change in fuel viscosity?

Has anyone got any experience of jetting on premix, or suggestions why I can't get a steady idle?

Cheers
 Back to top
View user's profile Send private message You must be logged in to rate posts

Paddy Blake
World Chat Champion



Joined: 29 Jun 2006
Karma :
PostPosted: 21:25 - 06 Jun 2013    Post subject: Reply with quote
You sound like you know what you are talking about so I don't mean to patronize you.

Is their not a screw on the carburetor the adjust the air/fuel mixture.

This will effect tick over.

Is there a chance of an air leak between the carb and head.
 Back to top
View user's profile Send private message Send e-mail You must be logged in to rate posts

B5234FT
Brolly Dolly



Joined: 28 Sep 2009
Karma :
PostPosted: 21:29 - 06 Jun 2013    Post subject: Reply with quote
Haha don't worry about that, I've done my time with cars but bikes are new territory!

There is indeed, and adjusting that brought the revs back to something sensible but it won't continue to idle without needing a little choke even when hot.

An air leak is possible I guess, but its a flanged rubber tube, which is bolted to the crank case (not been off since I got it) and fastens to the carb with a jubilee clip. Is it ok to test with carb cleaner or something?

Cheers for the reply
 Back to top
View user's profile Send private message You must be logged in to rate posts

Paddy Blake
World Chat Champion



Joined: 29 Jun 2006
Karma :
PostPosted: 21:34 - 06 Jun 2013    Post subject: Reply with quote
B5234FT wrote:
Haha don't worry about that, I've done my time with cars but bikes are new territory!

There is indeed, and adjusting that brought the revs back to something sensible but it won't continue to idle without needing a little choke even when hot.

An air leak is possible I guess, but its a flanged rubber tube, which is bolted to the crank case (not been off since I got it) and fastens to the carb with a jubilee clip. Is it ok to test with carb cleaner or something?

Cheers for the reply


Yes,that is the best thing to do.Any change in the way it revs will till you there is a leak.
 Back to top
View user's profile Send private message Send e-mail You must be logged in to rate posts

B5234FT
Brolly Dolly



Joined: 28 Sep 2009
Karma :
PostPosted: 19:39 - 02 Jul 2013    Post subject: Reply with quote
Hi

Much messing around later I found I'd reassembled the carb with the main needle circlip above the retaining plate instead of below.

I can get it to start, warm it up on a fraction of choke and then get it to sit at idle without stalling.

With this achieved I set about trying to set up the idle circuit and found this:

https://justkdx.dirtrider.net/printcarbtuning.html


Quote:
The Pilot Circuit, Tuning from Idle to ¼ Throttle -You can use the air screw to help determine if your pilot jet is appropriately sized. Take your bike for a short ride letting the engine come up to normal operating temperature. With the engine stopped, transmission in neutral and the bike on its stand turn the air screw clockwise until it just seats, gentle now it’s delicate and you don’t need to torque it down just gently seat it. Now turn the air screw a quarter of a turn out so the engine will fire and start it. Slowly turn the air screw counter clockwise ( out ) until the point where the engine just reaches the maximum obtainable rpm and continuing to turn the air screw beyond this point wont increase the engine speed (rpms) any further. I find it’s easier to hear the rpm increasing if you set the idle at its lowest possible position without the engine stalling. You’ll want to repeat this procedure a couple times until you’re confident that you’ve found the right spot and that the result is reproducible. When you’re comfortable count the number of turns ( 360° revolutions ) you’ve backed the air screw out to reach this point. The normal operating range is between 1 and 1.5 turns out so if you find the ideal setting is less that 0.75 turns out consider installing the next richer pilot jet (larger number ). If you find the ideal setting is more than 2 turns out consider installing the next leaner pilot jet ( smaller number ).


I did that, with the revs slowly increasing with each 1/8 turn until it took off. The revs skyrocketed after about 1.5 turns, I shut off the key and it carried on running. I dropped it into first and stalled it, but I'm curious whats going on? Is it possible I unwound the screw so gradually that I leaned it out for long enough to heat the plug electrode enough to keep it running?

As always cheers for any advice!
 Back to top
View user's profile Send private message You must be logged in to rate posts

The other Paul Rudd
Scooby Slapper



Joined: 06 May 2013
Karma :
PostPosted: 20:11 - 02 Jul 2013    Post subject: Reply with quote
Could possibly be a dodgy crank seal if the inlet is leak free and it's jetted correctly.

Also dismantle the carb completely and check there are no extra gauze filters after the fuel spigot and before the float valve. I was chasing a fuel blockage for ages on my Ariel until, looking over an exploded view, I happened to notice an extra gauze filter in a banjo fitting.
 Back to top
View user's profile Send private message You must be logged in to rate posts
Old Thread Alert!

The last post was made 12 years, 179 days ago. Instead of replying here, would creating a new thread be more useful?
  Display posts from previous:   
This page may contain affiliate links, which means we may earn a small commission if a visitor clicks through and makes a purchase. By clicking on an affiliate link, you accept that third-party cookies will be set.

Post new topic   Reply to topic    Bike Chat Forums Index -> The Workshop All times are GMT
Page 1 of 1

 
 You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum

Read the Terms of Use! - Powered by phpBB © phpBB Group
 
Debug Mode: ON - Server: birks (www) - Page Generation Time: 0.07 Sec - Server Load: 0.54 - MySQL Queries: 14 - Page Size: 49.93 Kb