Spitting back through carburettors at 4.5k rpm

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Spitting back through carburettors at 4.5k rpm

Postby simonnorthroad » 23 May 2017 21:43

Im in the process of jetting up a non standard Morini (501 engine in a 350 frame, L cam, gutted airbox, fruity pipes) and it's getting there. Except, at a steady or closing throttle, it spits back audibly through the carbs up under the tank at 4,500rpm ie almost exactly at half-revs.
Is this likely to be weakness in the needles? Ive done a plug-chop for main jet and that's spot-on
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Re: Spitting back through carburettors at 4.5k rpm

Postby MickeyMoto » 24 May 2017 15:44

Try carb balance first. Do you have the balance tube fitted? Removed mine.
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Re: Spitting back through carburettors at 4.5k rpm

Postby harrymuffin » 24 May 2017 18:52

A main jet plug chop is meaningless now lead has been removed from petrol. If you want the carb settings for my 5speed 507cc engine, but with 2-1 exhaust, K&Ns 28mm carbs radiuses valve seats 12:1 compression, give me a PM.
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Re: Spitting back through carburettors at 4.5k rpm

Postby norbert » 25 May 2017 00:39

But why has it to be a PM :?:

I still do believe that we have servived because of "institucions" like ATG, La Strega, this forum and ital-web.de and stuff like that, without hiding secrets.

The carburacion of a 501 in a "street"-frame , what means öriginal air fillter(boxes) of a 350 0 500 or type K+N is of comon interest. It surely is worth an own thread. I´ve spend years in trying to colect opinions about that theme, but it´s a hard job :roll:

Don´t have any idea of what is a main jet plug job (looking the color ot the plug at certain rph to choose the main jet?).

My experience:
I´m pleased for years with a 501 from a Camel, suposed L5 cam, with orignal filter box of a 350 K-frame (empty or with worn filters inside) with PHBH 28:
slide 50
needle X18 (third grovve from top)
atomizer 268T
main jet 130-140
idle 50 (might be better a 45)
choke 50-
This works realy fine with the old 350 Lafrancnis from the 350 (opened a bit to fit the 500 tubes).

I´ve tried to start with the same configuracion with my new bike (Dulcinea) with K+K and a self designed 2in2exhaust without conecting pipe (althoug planed with it :roll: ). Didn´t work at all. Now Ivé got nearly the original 501 setting (apart from the main jet, that is changed a lot because of the other air-filter system)
slide 40
needle X18 (third groove from the top)
atomizer 264 BC
main 130-140
idle 45
choke 50
Still don´t know how it works above 5000 rph, running in the new build motor :wink:

norbert
Last edited by norbert on 25 May 2017 17:49, edited 2 times in total.
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Re: Spitting back through carburettors at 4.5k rpm

Postby simonnorthroad » 25 May 2017 10:06

Thanks for your replies guys. Very helpful.
Norbert, Id 'collected' your old jetting settings before from this Forum and saved them, but you had a 50 slide! My 28mm Dellortos have a more-common 40 slide so this latest posting is very useful..
Isnt yours and my Main Jet size MASSIVELY bigger than standard New York's tiny 76?! Mine is running 132 main and seemed OK with a plug-chop - for what that's worth.... Ill recheck my needles and needle positions tonight.

I am off to the Pyrenees in a couple of weeks so my mixture is likely to get richer rather than weaker....
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Re: Spitting back through carburettors at 4.5k rpm

Postby simonnorthroad » 25 May 2017 10:10

Currently Im running:

'Fruity' exhausts, K&Ns mounted up under the tank on the airbox rubbers, L cam
and 28mm PHBH Dellortos:

40 slide,
X18 needle
136 main (not, 132 ignore previous posting!)
42 pilot
60 starter jet
BC264 Atomiser
200 Float needle jet
11.3gr float

and it's not far off, apart from the mid-range spitting-back
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Re: Spitting back through carburettors at 4.5k rpm

Postby norbert » 25 May 2017 17:38

That´s nearly the same as I`m trying now:
40 slide
x18 needle (third groove from the top)
132 main jet (what means in reality it´s nearly 140)
45 pilot
50 starter jet
264 BC atomizer
float needle jet and floats I guess I also have the same (serie)

Pedro tried the other configuracion that I have in Rocinante (50 slide and atomizer 268T) and it seems to work well for him.
I don´t know why it does not work with Dulcinea. At least not with the motor I mounted to pass the Tüv and for the trip to the danish meeting at Sörens garden. Last week I changed the motor and this one is a bit pimped (roling bearing on the right side, and taking of some weight from the crank, connecting rod, belt drive, primary, clutch and valve tappet). I have to run it first carefully, so still don´t know how it works above of 5000 revs. When the motor is free, i guess I´ll try it again with the the setting of Rocinante because this works realy nice. Maybe with the 40 slide and 264BC it works better at lower revs. But normaly I don´t use my bikes in city traffic and move the 501 motor between 5000 and 6500 travelling for country roads.

During the last years my experience with original brand new Dellorto jets ist that normaly they are wider than stamped. You always can pass a 130 gauge in a jet declared as 125. In the 132 I´ve put right now, I can even pass a 140 gauge.

norbert
Last edited by norbert on 25 May 2017 20:48, edited 1 time in total.
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Re: Spitting back through carburettors at 4.5k rpm

Postby MickeyMoto » 25 May 2017 18:40

Hi Norbert,

Is there a correlation between jet size stamped and the gauge size? I thought it was a 'flow' rate?

I am running 115s in mine but will not rev above 6,500 in top, although the gearing is higher than a standard Excalibur (140kmh at 6,500 rpm, Excalibur was 130kmh)). I am trying a two teeth larger rear sprocket first to bring it to Excalibur gearing (when I can ride again). If not then I will try larger jets! However, keeping it at 6k for 160 kms on a few of occasions over Easter did not hole a piston and the economy was about 4.5l / 100km... however it does pull from 2,000 rpm, so maybe the 31/2 air filter is the problem with the Excalibur cam?

Apart feom Main Jet, the jetting etc is standard Excalibur RLX.

Never easy... :) but it is still a nice bike to ride, just needs the top end sorting.
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Re: Spitting back through carburettors at 4.5k rpm

Postby norbert » 25 May 2017 20:40

As far as I know, the Dellorto numeracion is for 1/100 mm not for flow (like some other carburaters).

I´m wearing 14/39 teeth and the lying Veglia speedo (allways linear 10 km/h more than reality) of Rocinante gives me about 140 at 6000. I barely do more than 7000 revs, there is no need for, I´m not going on circuits. All the 501 that I know that have been on a bank, gave some hp more than the oficial 42. The 501 and 500 are suposed to have the same 42 hp, what definitly is not true!

I don´t think that the Excalibur cam should be a problem. In the end every bike has it´s own demands in some aspects :wink:
Mickey, I would try a larger main jet. Although a 501 normaly never will turn like a 350 sport at the top!
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Re: Spitting back through carburettors at 4.5k rpm

Postby MickeyMoto » 25 May 2017 21:01

Danke schon, Norbert.

The Excalibur pulled to 160kmh @ 8,000. I would expect a 160+ top speed. My speed is from my Sat Nav. I'll let you know how it goes, although I do need to look at the small throttle opening fuelling.
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Re: Spitting back through carburettors at 4.5k rpm

Postby 'It must be a .....' » 25 May 2017 21:19

Hi, I'm using a 501 Coguaru engine (28mm PHBH) in a 500 Camel with standard air box/filters and a free flowing silencer (adjustable).
Initially it wouldn't rev over 3500 under load.
As I'd ridden 501 Camels in the past with different exhausts which all ran well, I focused on the intake/filters and mainly sticking to the standard 501 carb settings; but the problem wasn't solved until I replaced the 76 main jets with larger ones.
It now revs freely and if anything is slightly rich on these settings:

40 slide,
X18 needle (2nd)
128 main jet
42 pilot
50 starter jet
BC264 atomiser
250 float valve

Could your 200 float valves (26mm PHBH?) be the problem?

Hope this helps, regards Ian
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Re: Spitting back through carburettors at 4.5k rpm

Postby danomar » 25 May 2017 22:38

Did you investigate the ignition system? Spitting at moderate revs sounds like what used to be a points problem: On the Morini, there might be a fault in the ignition rotor, stator, or one of the coils.
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Re: Spitting back through carburettors at 4.5k rpm

Postby harrymuffin » 26 May 2017 08:26

According to GOD -P.E. Irving, who was development engineer at Velocette in their most successful years, designed the Vincent engines and then developed the Buick engine into the Repco Brabham, and quoting from his bible 'Tuning for Speed', you would need at least 44bhp at the crankshaft (old hp) to get a unstreamlined heavy weight bike (bike plus rider) up to 110mph and over 50bhp to do 120mph, which correlates pretty accurately from experience with most of the bikes I have owned in the past and is about right with the two Morinis I am now down to. So after transmission losses you may have around 38BHP at the back wheel. Also, remember Italian horses are always bigger than other countries horses as Ferrari was eventually forced to admit after overstating his GP engine outputs for years. You can double check his estimations by looking at the speeds of al the bikes that competed in the IOM and the speed traps on the Sulby Straight e.g. Honda fours 75BHP streamlined medium weight top speed 155-160mph. present day Hondas 200bhp+ heavy weight streamlined 185mph.
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Re: Spitting back through carburettors at 4.5k rpm

Postby simonnorthroad » 26 May 2017 11:13

Danomar I havent yet investigated this. The very specific revs that it happens at (4500rpm) could well be ignition? Ive raised the needles last night to see if it's fueling but leaking fork-seals stopped play
It Must Be... I tried 250 float valves and a lower needle but every time I opened the throttle the bike just died - wouldnt run at all. All the info Ive collated over the years from others' jetting of 28mms seems to point to 200s being the best size and I believe std for the 501 New Yorks
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Re: Spitting back through carburettors at 4.5k rpm

Postby 72degrees » 28 May 2017 10:27

The dreaded mid-range flat spot as big as a house. I don't know about 501s, but PHBH28s , L/L5 cam and a 2:1 on a 350 or 375 can make getting carburation right tricky when whacked open at 4 - 5000 rpm. From personal experience, I had to go leaner on the mains to get the project 350 to run properly under load in such conditions. Down from 132 on the 375/L5 to 125 on the 350/L setup.
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