Issues with carbs

Lorenzo · Oct 17, 2013

My bike is a 750 Roadster Year 1973 and I mount a 32 mm Amal setup which I believe was pretty original for 750s in 1973 (please take into consideration my bike is not a combat)

I have my specialized mechanic trying to set up the carbs and he is having issues with the mid range which seems not as it should be, but has some flat spot..
The bike idles and does the high end very well though. Plug color is excellent. But my mechanic is saying the 32 mm setup was just for the 850s and this could be the reason for its less than perfect performance.

These are my Specs
32mm
3.5 slide
Top clip position
260 mains
106 needle jet
850 style spray tube

What could be that loss in the mid range?

Dkt26 · Oct 17, 2013

Didn't the 750's use 30mm amals with 220 mains and a 3.0 slide?

Larso1 · Oct 17, 2013

I have dual 34mm Mikunis on my 850 Roadster and they work very well with no flat spots, so 32mm Amals on your 750 should not be an issue. I used the Colortune sparkplug to read the color of the spark. This worked very well to assess the correct needle and jet sizes for proper mixture.

Lorenzo · Oct 17, 2013

my undestanding is that the 30 mm were for the machines until 1972 and in 1973 both the 850 and the 750 had the 32 mm

Dkt26 said:
Didn't the 750's use 30mm amals with 220 mains and a 3.0 slide?

pierodn · Oct 18, 2013

We just talking pm about, 30 or 32mm Amals on your 750 is not that the issue.
Piero

Keith1069 · Oct 18, 2013

3.5 slide

I'd say that the slide is too weak and would give a flat spot on opening throttle as it comes into play directly the throttle is opened from idle ............As one other poster said, a 3 is normal, a little richer. 260 mains are for 850's. 220/230 would be better but that end of the carburation is not an issue it seems.

Matt Spencer · Oct 18, 2013

Theres a few ' differant ' needles , and a few 32 set ups .

you NEED the Viton Tipped needles , the shakes can throw the nylon neddles nutty .

Coustom groovers customise needles & slide cut outs .

you need to do search , for a few off the good amal tuneing sites . Or look at the top here for referances .

pete.v · Oct 18, 2013

Sounds like 850 carbs, all right.
That does not mean that they won't work for your 750.
You may need to make some changes like
1. the main jet to a 220 or 230,
2. the slide from a 3.5 to a 3.0,
3. possibly the spray tube if notched
4. and the needle.
Other than that you should be good to go.

Thomasdunstall · Oct 18, 2013

Hi Lorenzo,
New 230 main and 3 slide
new slide is the key...seams to be a wearing problem...

Nortiboy · Oct 18, 2013

Can you give your engine number. MKV 750 (made 1973) had 30 mm carbs.

john robert bould · Oct 18, 2013

Slide cut-away's are important and often over-looked. my bonnie would no respond to upping-lowering the needle...i lowered the slide cut-away by maching 2mm off the base, picked up the needle and ...wow! it rev's clean!

Keith1069 said:
3.5 slide

Click to expand...

I'd say that the slide is too weak and would give a flat spot on opening throttle as it comes into play directly the throttle is opened from idle ............As one other poster said, a 3 is normal, a little richer. 260 mains are for 850's. 220/230 would be better but that end of the carburation is not an issue it seems.

Lorenzo · Oct 18, 2013

Nortiboy said:
Can you give your engine number. MKV 750 (made 1973) had 30 mm carbs.

Thank you. My VIN numbers are: 750RS2208XX and it is dated April 1973

Lorenzo · Oct 18, 2013

I checked my factory shop manual and in the section Carburetor it clearly says:
"Type 930 Amal concentric carburetors are fitted to early 750's. Type 932 Amal concentrics of 32 mm bore size are fitted to Combat engine 750's, other late 750's, and 850's"

Dkt26 said:
Didn't the 750's use 30mm amals with 220 mains and a 3.0 slide?

htown16 · Oct 18, 2013

I have a very late 72 750 bike that is basically 73 spec with 32 MM carbs. No problems with them, I think all 73 750's had the 32's regardless of whether standard compression head or higher compression rh6 head. I had to change mine to the #3 slides to get it to run right. Also, needle in the lowest groove or richest setting. 220 main jets. I have the stepped spray tubes, no issues with that. This is with peashooters and K&N airfilter.

Nortiboy · Oct 19, 2013

htown16 said:
I think all 73 750's had the 32's

All 750's made in 1973 had 30 mm carbs. Mine and all the others I have seen have 30 mm carbs with the old 28.5mm inlet head.

Issues with carbs

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