What are the limits to mounting a carb tilted?

Brad William /

...as in not perpendicular. As in axially tilted. The carb throat remaining level, with the carb body tilted along that axis.

Maybe I'm not using the right search terms, but I can't find much on line about this.

The CV bings on my gravity fed BMW 650 are mounted with significant tilt on both axis. Its necessary because of their unusual placement (to say the least:((). Evidently it's not forbidden.

I just put a new carb on an e50. Its a little 16.5 mm carb designed as a Yamaha dt50 two stroke replacement. The yam is close enough in requirements and displacement to the e50 that I just mounted it with the needle and jetting out of the box, ran it, and adjusted from there. Very good so far. Plug chop good. Running temps fine. Idles fine. Check. Check. But it's fuel starving at WOT. Odd since I've experimented with half a dozen carbs on e50s in the past and never had this problem. It was necessary to tilt the carb a bit to shoehorn it in. But not more than 5 degrees.

I raised the float (sits lower with the carb held upside down) and it helped significantly, but it still starved at WOT. Maybe the float seat needs to be a little larger?

Its an OEM piston, cylinder. Its unlikely a delivery problem from the tank, petcock, line, since the previous carb ran fine ( but needed replacement). Ran with gas cap off. Still starved. Seems clear that the fuel level in the bowl just can't keep up with delivery. I'd expect that, perhaps, if the displacement had increased, but this is the OEM e50.(edited)

Re: What are the limits to mounting a carb tilted?

Brad William /

This is the carb...

https://www.jmpbparts.com/16mm-carburetor-yamaha-dt-rd.html

58 main jet sounds too small. But it seems to run fine with it. A larger main wouldn't have any effect on what I'm describing would it? Seems like it would be draining the bowl even faster, no?(edited)

Re: What are the limits to mounting a carb tilted?

Brandon Weiss (Detective brandon to you) /

I doubt that a 5 degree tilt would cause an issue. It's likely fuel flow. Have you tested flow with the fuel line off the carb?

Re: What are the limits to mounting a carb tilted?

Dirty30 Dillon /

Yeah, you're "fuel-starved" because your main jet is too small.

Re: What are the limits to mounting a carb tilted?

Overpriced Parts /

Ya check fuel flow but I know my mikuni carbs on my magnum Gila builds are basically flat, just little angle and the floats had to be adjusted to the point where they almost leaked or they were fuel starved at hi speed but they are tm24s on big 74cc 8 petal reed cylinders that have 157.5-160 main jets in them

I got fuel flow Galore, I run a huge lawn tractor fuel filter and I actually have a few pounds of pressure due to the tank height to carb distance so could be float if you got good fuel flow,

Re: What are the limits to mounting a carb tilted?

Brad William /

I measured the main jet on this new carb.

.033 in / 84 to 85mm approx.

The carb its replacing ran best with a .029 in / 74 to 75mm approx jet.

So it appears as if I'm in a reasonable range of jet and shouldn't go bigger.

The prev carb was a pz19, better suited/designed to 4 stroke I believe, but it served me pretty well for a while.

I was really hoping that this more two stroke designed carb would be a big improvement and hope to use it on my airsal 44mm kit this winter.

Re: What are the limits to mounting a carb tilted?

Brad William /

Mikuni has their own formula for jet sizing. When discussing main jet sizing on MA, 60, 65, 70, 75, 80 etc, is everyone just talking millimeters? Should I try downjetting this?

Re: What are the limits to mounting a carb tilted?

Overpriced Parts /

A mikuni let you get about 80% of the tune and you think it’s great whereas a dellorto PHPG won’t let you get no where near that until you’re completely tuned!

I love the 21 mm dellorto carburetor but I also like the Mikuni 24 TM as well as the dellorto phva 14-17.5 mm carbs,

Shame carb tuning on a 2t is so easy yet so hard!

Re: What are the limits to mounting a carb tilted?

> Brad William Wrote:

> -------------------------------------------------------

> Mikuni has their own formula for jet sizing. When discussing main jet

> sizing on MA, 60, 65, 70, 75, 80 etc, is everyone just talking

> millimeters? Should I try downjetting this?

Previously you ran a 19mm carb that had a '75' jet , which ran OK .

Now you're running a 16mm carb with a '85' jet .

Just a quick deduction should tell you that you need to down jet the 16mm to be somewhere close . Less air requires less fuel . ;)

Re: What are the limits to mounting a carb tilted?

Brad William /

The Chinese pz19 is a keihin clone, and measures between 16 and 17 at the throat (it's oval so it's hard to be exact).

But yeah. I'll try jetting down a bit and see what's what.

Re: What are the limits to mounting a carb tilted?

> Overpriced Parts Wrote:

> -------------------------------------------------------

> I got fuel flow Galore, I run a huge lawn tractor fuel filter and I

> I actually have a few pounds of pressure due to the tank height to carb

1psi of gas requires a bit over 3' of head.

id be surprised if your tank was >6' higher than your carb...

;)

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