Lighter Carburettor Springs

gpacs

gpacs

Full member
Location
Melbourne Australia
Greg Parish said:
On  all our triples, racers, and the Land Speed Record Laverda 500 we used these light springs but also put a second extension spring along the frame of the 500 to make the pull even lighter lighter without compromising the spring force that pushes the slide down.
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I just wanted to clarify this shortening of Carb Springs again because the danger of a wide open sticking throttle with a single pull cable has been known to happen after cutting slide springs in Dellorto carbs. The sticking of the throttle can be caused by a dirty/kinked cable etc. as Piet has pointed out on another thread but the real danger is that at wide open throttle the vacuum and volume of air under the slide can keep it open or slow the start of it's return.
Mikuni, Keihin and many Japanese manufacturers use pull/pull cables to negate this affect. i.e. One cable to open/one to close.

Having said that, there are many riders whose experience has been that they have cut the springs and not had any problem. I don't dispute this, but the warning of the danger should be acknowledged and taken into consideration before recomending the cutting of springs too short chasing the lighter pull.
For the sake a few dollars it's better to purchase the correct springs or install a counterspring.
 
I ordered some from Greg to ease my old wrists on the ride down to Breganze in 2 weeks.
My next of kin will let you know if I fly off an Alpine pass.

Left hand wrist is taken care of by the excellent DAM easy clutch which I've now quite extensively tested.

Paul
 
I think I used some from a Mikuni carb once. Worked fine. They were long but had softer tension. Later I renewed my work 36mm carbs by boring the slide bores and making heavy bronze slides which were then hard chromed and ground to fit. Unfortunately I made the cutaways 70, which is a bit lean for most - worked ok with my SF1 cam and even later with a 5C SFC cam. I may make removable adaptors to allow alteration of the cutaway and then offer those 36ers for sale - no accel pumps, so they were effectively PHBs.
 
Each bike will be different of course. There is indeed a risk. On my Jota I' took a progressive approach to reducing the length of my springs. After making sure I had the lightest springs (from a Guzzi dealer - they have the same issue), I started with chopping a straight third off. No problems. I then took a coil off every time the carbs needed some TLC - intending to continue until a closing problem surfaced. I got down to just over half original length and called it quits at that - no problems came along. I tested the closing at WOT and full over-run (draw), and all at various different ambient temps. All was OK. Of course that cannot be a guide for everyone. Cable type / condition / run, as well as things like carby arm pivot alignment, wear and roughness of the slides (plus a multitude of other things) will all contribute to holding the carbs open when you don't want them to be.  That's got to be scary and is why I stopped when the throttle became manageable for my wrists (despite many teenage years dedicated to improving the muscle structure in that region).

 
Piranha Brother 2 said:
I think I used some from a Mikuni carb once. Worked fine. They were long but had softer tension. Later I renewed my work 36mm carbs by boring the slide bores and making heavy bronze slides which were then hard chromed and ground to fit. Unfortunately I made the cutaways 70, which is a bit lean for most - worked ok with my SF1 cam and even later with a 5C SFC cam. I may make removable adaptors to allow alteration of the cutaway and then offer those 36ers for sale - no accel pumps, so they were effectively PHBs.
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Hi Quentin - You only need to take 1mm off the backdrop of the slide to make a "70" into a "60" cutaway  - Regards - Phil
 
I tried this home made mod as an experiment and it works well. The fleshy part of the palm leads the rotation of the throttle which reduces the work the fingers need to do, allowing you to get to full throttle quickly. Will see how it goes at the track next weekend but great so far.

IMG_20190609_124515.jpg


The inspiration was Ron's ovulating throttle and Vince's suggestion of the throttle rocker.
 
cbertozz said:
I tried this home made mod as an experiment and it works well. The fleshy part of the palm leads the rotation of the throttle which reduces the work the fingers need to do, allowing you to get to full throttle quickly. Will see how it goes at the track next weekend but great so far.

IMG_20190609_124515.jpg


The inspiration was Ron's ovulating throttle and Vince's suggestion of the throttle rocker.
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i use a paddle from scottoiler its great as you just rest your palm on it.
 
phil37 said:
Hi Quentin - You only need to take 1mm off the backdrop of the slide to make a "70" into a "60" cutaway  - Regards - Phil
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Correct me if I'm wrong, but I think it's the other way around, Phil. 70s have 7mm of cutaway and 60s have 6mm, etc - so I need to add material. I'll be running FCR 39mm carbs on both SFs so the Dell work is way back on the bottom paddock burner  :D
 
Phil was talking about taking a bit off the bottom of the slide, not the cutaway. Will work provided your custom made slide is high enough so the top edge isn't exposed when the throttle is closed. You'll also need to raise the needle by the same amount.
 
Piranha Brother 2 said:
Correct me if I'm wrong, but I think it's the other way around, Phil. 70s have 7mm of cutaway and 60s have 6mm, etc - so I need to add material. I'll be running FCR 39mm carbs on both SFs so the Dell work is way back on the bottom paddock burner  :D
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No Quentin - The "70" has a cutaway of 7mm higher than the backdrop of the slide - removing 1mm from the underneath of the slide would reduce cutaway height to 6mm. - made me think though! When there was a glut of "50" slides in the Uk that came from the Jaramas in the US  - Slaters rightly took them out and replaced them with "60's" - they dumped the "50's" in a skip at the back of their premises - had a lovely "Dumpster diving" session - made lots of "55" slides for the 600 monty's we were doing by machining (filing) the cutaway up a bit! - Regards - Phil
 
Ah, now that is something else again - I too have increased cutaway by making the cutaway wider, but hadn't thought about taking 1 or 2mm off the slide baseline. Worth a try for sure. Might have to take same off the idle stop area as well ... and fit  spacer under the needle clip.
 
Previous mechanic at the shop im working at had carbs jam open on a yammie xs1100, 2 nd gear, spent a month in hospital.  Single cable. Wedged under a parked car, ouch.....
 
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