LCGB Forums

What causes excessive spit back? My son and I have exactly the same kits on our scooters. 200 Ali barrel Rayspeed kits with 30mm Dellorto PHBH carbs. My sons has horrendous spit back. The carb has 2 small holes in the bell mouth, the one that is in the middle goes through to the needle area. When the engine is at fast tick over fuel is almost running out of this hole. We built our engines 12 months ago and all has been well until quite recently. He had a problem with dirt getting through from a different fuel tank that he fitted, but the carb and everything was cleaned out. We have swapped carbs but the problem remains.
Cheers Andy

Possible his rings are worn? Just a guess.
Ian

I think port timings are a major factor in carb' spit back. Reed vales are the solution.

I suspect the issue has been compounded by retarding the ignition. 16/17 degree seems to be where most are setting theirs , which is quite a bit different from the 20+ mark where they were designed to run at.

Well worth looking at the rings, I had a similar scenario after a welded tank let welding residue wear the piston and rings
Horrendous spit back and flooding with a breathe sweet.
Changed piston and rings and it was all good.

We'll checked timing and that is ok, swapped carb for one off mine (again) new plug still spitting back. So as per the replies earlier I checked the rings. 95 thou ring gap!!
So tonight I have just fitted new rings and fired her up. Better but still spitting back quite badly.
So my next assumption is it must be the barrel.
Any thoughts?

Have you considered that the atmospherics have changed quite a lot recently with the warm weather and you may need to adjust your jetting?
I've had a similar issue.

Over advanced inlet timing, has the piston skirt been trimmed ?

As I run exactly the same set up on my scooter built at the same time and almost the same mileage I think I have a pretty good comparison. I don't see how the atmospherics would only effect one scooter. No the piston skirt has not been trimmed. I think I am going to try a rebore and see what happens. I will update as soon as I do it.
Cheers Andy

Apologies... I thought it conceivable that two engines, even though they are the same spec, could possibly require separate jetting or have reacted differently to atmospherics.
This obviously isn't the case.

I know you're running a bigger carb etc but I had the same problem with my Muggy using a Jetex carb. It was an existing setup that all of a sudden started to be hard or impossible to start when warm. Found loads of fuel in the carb rubber and then obviously the spit back that was horrendous on tickover. I had the same suggestions re rings, Carb, float height, piston bore clearance. Timing? I could not understand how the difference between say 19 or 17 could make it spit fuel out. Port timing could but nothing had been changed on a standard Muggy setup. All of the above was complicated by the use of an M Tech unit. In the end The M tech came off and I went back to basics after changing rings piston, different thicknesses of base gaskets, all to no avail, I was losing the will to live.
I ended up changing the carb for a Dellorto 26mm. From the first start no spitback.
These carbs have been a steep learning curve for me as now I have a load more variables but I have no sign of any spitback. I have been able to refit the M Tech and am very impressed with the Dellorto being a modern carb. I was a big fan of the Jetex but never again. I'm still tweaking my setup but the low down power out of corners all the way through the rev range is so much better. The Jetex was very crude tickover or full blast.
Not a lot of help in your case as you already had these carbs setup. I have found that float height makes a hell of a difference on these carbs, lowering the float height may improve it also the angle that the carb sits, some manifolds go for a very steep slop back, made worse it the carb is not vertical.
Just a thought about the kit a mate had a steel lined ali kit (I don't know if yours is the same) but the liner was moving up and down inside the barrel. it ticked ever so slightly but he also had power loss due to the port timings. He had the liner pinned in the end.
Sorry if its a bit of post good luck