Issues with twin Mikuni carbs on 1965 Suzuki 150

[hotrodjohn71]

I am pretty sure its my twin Mikuni carbs on my old Suzuki are the thing giving me trouble.

The machine is a 1965 Suzuki S32-2. Its 149cc twin cylinder with a dynamo charge/ start system non reed valve style 2 stroke motor. The
Carbs:Twin Mikuni S32D (book says VM20SH).

Tried to rule everything else out.
I have about 100 pounds compression on each cylinder, getting spark and just changed the points, condensors and timed the machine at about .110 stroke distance btdc with a dial indicator in the spark plug holes. These motors run a timing of 25 degrees btdc.

I did a quickie crankcase pressure test at 7 psi pressure but I didn't watch for the whole five minutes. It held for about a minute then I ended the test. No oil in the dynamo compartment to indicate a bad crankcase seal.

The bike will start and run but the problem is there's no response off idle and up unless I blip the choke then it will rev. When I try to rev it without the choke, it just falls flat and dies. When I pull the throttle cable (at idle) individually on each carb, there is no response, just the sound of the carb bore opening but no increase whatsoever. This is puzzling to me because this says to me that there is a simultaneous problem with both of these carbs at the same time since pulling the throttle individually on a carb should rev the motor atleast a little but nothing from each. both respond the same.

I've taken these carbs off and cleaned and inspected them at least ten times. I've blown all of the passages and pinned the holes and jets and passages several times. They seem to be clean as a whistle. I can see light through the main jet passage and idle jet passages. I set the floats according to this old Clymer manual I have for the bike which is about the only info I can find about tuning it anywhere.

It had been rideable for a couple days, then this issue happened and I can't seem to figure out why. As I said, it will idle, but no response when twisting the throttle and it just dies. No response from the mixture screws, and when I twist the throttle there's nothing and it just dies. No response.

Is there a trick to these Mikunis? I'd really like to ride this bike some more if I can just get this figured out.
Thanks, John

[Craig380]

Hi, this is just a guess but my feeling is that those carbs have a choke / throat size that is WAY too big for the bike. Are they 20mm throat size? Were they bodged on by a previous owner? The cut-away mounting point suggests so.

My guess is that the cylinders just don't create enough 'suck' through the carb throats to pull fuel through the jets at smaller throttle openings. Also the jet sizes may be all wrong

I've no idea how easy / difficult it would be finding a set of correct carbs for the bike but that's what you might need to do to get it running right.

[jabcb]

The carb specs are listed in Service Bulletin Specification-11
http://www.ozebook.com/compendium/techb ... 010-14.pdf

Per the specs, carbs are 20mm.
Some flange-mounted Mikuni carbs have the cutout on one side, so the cutout doesn’t suggest mods by a PO.