Dellorto carbs not opening fully

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bob

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Here's an old chestnut...
I was peeking down the throat of the carbs on my racer tonight [as you do] and was perturbed to notice that on WOT the lower edge of the front of the slide was poking down into the airstream about 3mm. Phil used to do a spacer for PHFs to help eliminate this and I wondered if has anybody got any of these lying around which they want to part with to save me the hassle of making some up? 





Here's a pic of Phil's motor installed in my spaceframe , you can see the spacers undereneath the carb tops
 

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Bob,
I recall reading on LP that Phil had a number of lifter kits available, so I should imagine that Keith may now has these.

Scrumpy
 
Bob,

Does your twistgrip have enough travel to completely lift the slides?  Do you have a cable splitter fitted?

Seem to remember you fitted 36mm carbs, the stock cable splitter is at its' limit at around 33-34mm travel. 

With the throttle pulled to full extent, can you still lift the slides through the carb choke?

piet
 
Piet, I'm on a twin cable set up with no splitter.  With the throttle fully open there is no lift left in the slides so I'm fairly confident that the problem lies with the carbs [which are 36s]. I've got a few scrap carb bodies and tops knocking about which might provide an opportunity to make up a suitable spacer on a miller.
 
Hi Phil,
I'm guessing you developed the spacer to accomodate the extra lift needed on your smoothbored carbs. My carbs are brand new 36mms and the slider is definitely stopping short of max travel. I'll remove the throttle spring and see if I can get full travel on the slide.
I've just remembered I fitted some Japanese carb springs last year and I'm wondering if they're becoming coil bound, I reckon I already know the answer..... :-[ I shall report back later!
Bob
 
Nick, believe it or not the pumper operating arm came under suspicion this morning!
Order is now restored and the culprit was the Yamaha RD250 springs I'd fitted. They worked really well and actually had a lower compressed stack height than the Dellorto springs but unfortunately they didn't seat quite fully in the carb top and were 'fouling on the clearance' as engineers say. I've refitted a set of lightweight Dellorto springs [still heavier than the Yam] and WOT is now restored, albeit with a slightly heavier throttle action. How do you Jota jockeys hold three of the buggers open?
Just goes to show that it pays to check and then check again.
 
Bob,
Glad you have sorted it! If you disconnect the pumpers, you lighten the action! Though you make it heavier with a quick action throttle...then you make it lighter again by fitting the  lighter springs which are a stock option :D. Honestly, all this buggering about...I must get a bike with fuel injection ;)
Nick7
 
Two words:  race regulations :D :D I'd like to try a set of Miks on my bike but they just ain't allowed...
 
Hi Bob,

you can  cut the springs a bit shorter, which will give a lighter twisting of the grip. Also what helped on my bike is a thin sheet of teflon under the twist-grip, routing the cables accurately (and take some with a teflon tube inside if available), spraying the moving parts in the dome of the carb and the linkages with WD 40 etc. etc.

But at the end, it will never reach the light action of a "Desmodromically" actuated carb-set just like the mikunis... What about CR's? Aren't they allowed, as they are round-slides?

Cheers

Lothar
 
Another thing that can help reduce friction is to replace the bronze bushes in the carb top extensions with modern teflon-impregnated 'DU' type bushes - cheap and easy to find.  I recently did this and it also helped with consistency of vacuum guage readings by removing slop from the linkages.
 
Thanks for all the suggestions.
My bike hasn't got a particularly heavy throttle action with the Dellorto springs installed, it's just that it had a really nice action with the Jap springs so I've been spoilt. I don't like shortening springs because of course shortening them increases the spring rate.
The Teflon bush idea is good except a twin doesn't have the carb extensions!

I'm going to be scouring the autojumbles for some more Japanese springs to see if I can get it back to how it was. I've just remembered  they weren't RD 250 springs, they were X7. I'll keep looking.
 
Bristol and Bath bike show at the Bath and West Showground, Shepton Mallet this weekend. There will be an autojumble. Hopefully I will be going so happy to look out for some if you wish. Anybody else planning on visiting?

Scrumpy 
 
Just man up and flex those muscles Bob.  :D

I have 3 full length springs and working pumps in my Dellortos. Throttle action is OK.

I cut the springs down in my brother's 3C at his request. Throttle is lighter, but not by much.

Cheers,
Cam
 
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