PHF 30 jetting for RT 450

[Don450]

Hello to you all, this is my first posting. I would like to try fitting a PHF 30 carby from a 350 Vento to a newly acquired 450 R/T with unknown history. Does anyone know what numbers would work please ie slide, needle, needle jet, idle jet, main jet etc. This tryout is due to very hard starting from cold with two different VHB 29 AD carbies which were very clean before the starting attempt. The engine did start, but only after a spray of ether. It revved, but something like an idle could only be achieved with the slide screw lifting the slide to maximum - the mixture screw had almost no effect even though the idle circuit passages all blew air through when cleaned with carby cleaner and compressor. Ignition is standard two wire R/T with no battery, and have not put a degree wheel on yet to check timing.
Settings for the VHB 29's
slide 60
needle v7 - new and set on 2
needle jet 265T
idle jet 50
main jet 138

no choke cable
open pipe no muffler

[DewCatTea-Bob]

[quote= Don450 ...
" I would like to try fitting a PHF 30 carby from a 350 Vento to a newly acquired 450 R/T
Does anyone know what numbers would work please ie slide, needle, needle jet, idle jet, main jet etc. "

____ The 350's carb ought-to be jetted good enough to go, you just might prefer to raise the needle-valve a notch and increase the main-jet size a couple increments.



" This tryout is due to very hard starting from cold with two different VHB 29 AD carbies which were very clean before the starting attempt. The engine did start, but only after a spray of ether. "

____ Do you understand how to properly use the stock-carb's cold-start circuit ? _ (It's not an actual 'choke' arrangement !)
The internal plug-block must be raised-UP to it's lifted-position, (only for cold-engine starting).
Once the cyl.head is at-least fairly warm, then further use of the carb's cold-start circuit should no-longer be useful.



" It revved, but something like an idle could only be achieved with the slide screw lifting the slide to maximum - the mixture screw had almost no effect "

____ The mix.screw is only expected to be most effective just when the throttle-slide is allowed to be almost shut.



" no choke cable "

____ Then how are you able to make any use of the cold-start circuit ?
If you've been ignoring that quite useful advantage while trying to start a cold motor, then it's really no wonder at all that you've been having troubles getting the engine started !


Hopeful-Cheers,
-Bob

[Ventodue]

Welcome Don.

I agree with Bob - whichever type of carb you fit, VHB or PHF, you're gonna struggle getting the bike started without using the choke - which, as Bob said, is not a choke, but an enricher.

Your set-up for the VHBs is pretty much correct, btw. If it were me, before trying the PHF, I'd get the choke circuit on one of the VHBs working - and try again.

Good Luck!

Craig

[Don450]

Thank you both. I will attach a cable to get the enricher circuit working on the vhb, check the timing and eventually try the PDF with the changes Bob suggested. My other 450's start with no vhb enricher needed so I made a wrong assumption with this one.
Thank you, Don.

[DewCatTea-Bob]

[quote= Don450 ...
" and eventually try the PDF with the changes Bob suggested. "

____ Not that those-changes are really needed to simply run the engine, and the particular main-jet size doesn't much matter unless you intend to run full-throttle extensively.
And for permanently tuned use on a 450, the needle-jet & throttle-slide could likely use some size-fussing.
Otherwise, for test-use,, you could simply go-ahead & bolt it onto the 450 just as it was tuned for the 350. _ As such intended temp.trial-swapping has often been done, only to become ignored and end-up left as-is for a full season or longer, (without notable consequence).



" My other 450's start with no vhb enricher needed "

____ I really would-NOT expect ANY cold engine (equipped with a VHB.square-slide carb) to be able to easily start-up (with just one or two kicks), unless the enrichener-circuit doesn't happen to be completely closed-off ALL the way ! _ (As it's certainly possible for hot engines to run well enough left that way.)
So, I'd much suspect that your 450-engines which start-up okay (with their stock VHB-carb) without the cold-start circuit being fully activated, must most likely have their stater-jet's air circuit-way not quite completely closed-off all the way, (possibly due-to miss-adjusted choke-cables).


Duke-Cheers,
-Bob

[Nick]

"Carb cleaner" usually won't clean a carb properly. Best way is to completely disassemble, then soak all metal parts including jets in lacquer thinner for an hour or so, then blast everything out with compressed air.

[Don450]

The bike is now starting, revving and idling, using the VHB 29.
Things:
1. The idle screw was missing the washer and o ring - missed this because the parts diagram did not show it.
2. The addition of an enricher cable now lets the engine start from cold without resorting to starting ether.
3. After trying size 50, 55, 62 and 65 idle jets, the 65 produced best idle and performance at low throttle when revving ( idle screw outward about half a turn ) When the 50 was in the carby, there was some spit back indicating a lean mixture, which is why the idle jet eventually ended up at a surprising 65
- notes:
the compression on this 450 seems nowhere near as much as others
open pipe - no muffler
small MCS PF2 filter pod attached directly to carby body
checked the other 450's enricher cables and they have slack in the cable - the plunger is seated home at the bottom.

Thank you Bob, Craig and Nick for helpful comments.

Cheers, Don.

[Ventodue]

Good, Don - congratulations! Coupla comments, FWIW:

1. Agree with you about the O-ring and washer on the idle screw. Some diagrams DO show them, e.g:
http://www.dellorto.co.uk/images/parts/VHBdiag.pdf

... while others don’t, e.g the Ducati parts manual . That leads me to speculate that it may have been something that showed up as a problem after the carb had been in production for a while. Certainly the replacement series for the VHBS, the PHFs, had them from the start.

2. I too have had to increase the idle jet slightly on my 450 SCR to stop a tendency to pop back on the over-run - previously, I chased a possible air-leak for ages without success. It seems to work .

Just for the record: using that large an idle jet with that small an opening of the mixture screw means that the carb is feeding a relatively small amount of a relatively rich mixture. I can’t see any particular harm in so doing - after all, what works, works! And it may be related to your air filter and silencer set-up.

Ciao

Craig

[Don450]

Craig, just curious:
1. Size of the idle jet in your SCR please?
2. Do you think the lower compression in my 450 R/T is an influencing factor to increased idle jet size ?
Cheers, Don

[Ventodue]

Glad you asked Don, 'cos it made me go look and I was bit surprised to find that I ended up with a 60! So bigger than I would have guessed ... The Amici recommend a 50, btw.

Can't say about possible influence of compression - I don't know enuf 'bout engines and things . But I reckon Eldert or Nigel might know ...

Ciao

Craig