Rpms slow to drop back to idle

The bike specs-
11.5:1 compression head
920cc
newer single Dell'orto PHF36 with velocity stack
Stock exhaust
Joe Hunt magneto

I'm sure I know the answer to this, but I just wanna check.

I'm rejetting the carb due to going from big bore exhaust to the stock exhaust(its quieter) but I'm having a problem with the rpms not dropping back to idle quickly, they hang up 2500-4000 rpms when I blip the throttle or even on the road just riding and let off the throttle. The cable is good, manifold boot is good.

I'm confused cause it ran good with the big bore jetting sizes, but as a couple of days riding with the stock exhaust on it started having this problem and it got worse an worse. I started with a 50 pilot worked up to a 58 pilot and it still does this, I ordered 60-70 jets

does this sound like a lean pilot jet problem?

Cheers

Since you say its getting worse, I'd be looking for an air leak somewhere ?
Carb manifold, exhaust fit into the head, slip joints on the pipes anywhere.
Tappet clearances OK ?
Might be worth doing a quick compression test too, to see that the valves are seating OK.
Make absolutely certain is not the throttle cable sticking - or the carb sticking though.

Usually if you go to a quieter exhaust then the jetting needs to be smaller, so unless the carb is new then something is a bit odd....? This is a new carb, though ?
Might be masking the symptoms, and going in the wrong direction, but lifting the needle a notch sounds like its called for ??

Opethiselps.

It is a newer carb, it's only a couple months old. I've moved the clip. I thought I should have leaned it out too..... I'll do a compression check

I've had this happen with sticky mechanical advance getting stuck to advanced. Might be worth double checking it applied to your magneto. But if evolving worse I'd vote air leak too.

Shoot some carb cleaner around the manifolds at idle and see if the rpm changes. Good way to check for vacuum leaks.

You didn't happen to replace the spring in the carb did you? Reason I ask is that I replaced carb springs in my Dell Orto VHB30 carbs on my Guzzi 850T with lighter ones (common mod on Guzzis as dual throttle pull with flat side carbs can be tough on the wrist with standard springs). I bought new lighter springs from a Guzzi dealer but didn't check them before installing them. They were supposed to be a Dell Orto factory part. They seemed to work fine until the bike warmed up then I started having exactly the same symptoms as you describe, bike was slow to return to idle. Only thing I could figure was that it must have been due to the springs. When I pulled the springs back out and compared them to the original stronger ones, they were about 6 mm shorter. So I think they were just not seating the slide back to the bottom of the carb very well. I am going to try stretching the lighter springs out a bit to the same length and see if that solves the problem. I am thinking that if you replaced the spring, it may also be slightly shorter than stock and not seating the slide as effectively as it should. Another non air-related possibility is some sort of restriction in the throttle cable or the throttle assembly itself.

That's a good one too. Check your cables for binding/pinch or tight bend.

No, it doesn't sound like a lean pilot jet. It sounds like an overly rich one. Which makes sense.

You installed a more restrictive exhaust, meaning all other things equal you're flowing less air and your mixture is therefore richer, and you're trying to cure that with BIGGER pilot jets? You're going in the wrong direction, IMHO.

Why not reinstall your original pilot jet, see if that helps, and if so, try even smaller ones?

FWIW. YMMV!

If your bike ran good with the previous carb settings, why not revert back to them? Fitting different parts such as exhausts may require new jetting, but this isnt simply a matter of guessing the sizes and fitting them, but is going to need proper setting up on a rolling road, or getting some non alcohol fuel and setting up using plug colour readings.

I had a similar problem on my 850 with 36 single Mikuni, a bit frighteniing when it stuck wide open when heading at a rock wall!
I put it down to the vacuum holding the carbie slide in the bore, the solution was to add approx 1 teaspoon of two stroke oil to the tank when topping up to keep the slide lubricated.

Tpeever- I did not replace the spring, but thanks for the info i'll keep that in mind when I do replace it.

Briank- I was wondering if I was going the wrong way, I thought stock exhaust ment I had to use smaller jets, I have one step down piolt and main. I'll put them in today and see if that helps.

Carbonfibre- when I had the stock setting I had to drop the needle to the lowest point, so I figured I had to lean it out, till.... this whole high rpms thing started to happen, which then just really confused me cause that seems to be a classic sign of a lean condition.

Fredful- I did pull my velocity stack off and saw that my slide was not dropping as fast. But then when I shut the bike off and manually lifted and dropped the slide it was very quick to drop.

4 hours later, 6 pilot jets, 4 main jets, 15ish road trips, I figured it out.

Starting stock Dell'orto PHF36 specs
130 main
50 pilot

-Brand new NGK bp6 (cooler plugs)
-Pilot jet was richen to 52
-Main jet was left at 130
-Dropped needle clip all the way down(raised the needle) I could put a bigger jet in
-Air mixture screw 2 1/2 turns out
- Idle rpms 1000-1100

Last test ride went like this
Secondary roads- no hesitation from 0- throttle-1/4 - 1/2 -3/4 Some very light back firing once and a while when letting off throttle
Highway- 105 mph very smooth........ till helmet started choking me.

Performed plug chop (not at 105)
Black on the rim, coffee with cream in the center.

Next test will be tomorrow see how it rides after a cold night of sitting.
Thanks for everyone's help!
Cheers

Still a bit lean at 3/4 throttle ? Have you got a matching pair of needles... I used a piece of 800 wet and dry to tune my needles in the Pits one day. (TZ250 Yammy in a Hagon Long Track frame) Didn't want to nip up on the shut off, at 100 mph.It worked...AC.

Yetiman said:

Briank- I was wondering if I was going the wrong way, I thought stock exhaust ment I had to use smaller jets, I have one step down piolt and main. I'll put them in today and see if that helps.

Click to expand...

Yes, exactly. Maybe I misread your post - I thought you were RICHENING in response to having gone back to the stock exhausts from free-er flowing ones. Sorry!

Seems weird that you've fattened the needle setting and apparently had good results. Although all setting are interrelated, so maybe the additional gas from the pilot....oh, I give up!

Anyway, you seem to be on the right track, so congratulations and keep up the fine-tuning!