'79 850 vm main jet question

[raistian77]

doing a second strip and dip (found some trash in the lines)

Noticed that the main jet is set to the 2nd slot down, is this normal? When I did the original strip and dip the PO had jacked all the mix screw positions up trying to fight a lean condition caused by bad boot o'rings. Did he move this also?

[srsupertrap]

You mean needle jet position, the main jet just screws into the breather pipe which screws in the carb body

[raistian77]

You mean needle jet position, the main jet just screws into the breather pipe which screws in the carb body

yes, needle position seems like there are 5 positions avalible all 4 are set to the 2nd down.

[Big T]

doing a second strip and dip (found some trash in the lines)

Noticed that the main jet is set to the 2nd slot down, is this normal? When I did the original strip and dip the PO had jacked all the mix screw positions up trying to fight a lean condition caused by bad boot o'rings. Did he move this also?

Yes, he moved the needle setting and in the WRONG direction

Having the clip lower on the needle richens the mixture

[49er]

Yes, he moved the needle setting and in the WRONG direction

Having the clip lower on the needle richens the mixture

Not so. The '79 GS850 VM carbs had 5DL36 -2 needles. The -2 means that they were positioned 2 slots from the end of the needle. This is slightly leaner than in the middle position as the needle is sitting lower into the main jet at idle.

You should find they are fine in that position once the pilot circuit passages have been cleared of gunk and you have no air leaks anywhere in the intake system.

[raistian77]

Not so. The '79 GS850 VM carbs had 5DL36 -2 needles. The -2 neans that they were positioned 2 slots from the end of the needle. This is slightly leaner than in the middle position as the needle is sitting lower into the main jet at idle.

You should find they are fine in that position once the pilot circuit passages have been cleared of gunk and you have no air leaks anywhere in the intake system.

Thanks, Thats a relief. On the first strip and dip the pilot screws were 4 turns out and the pilot air screws were all bottomed.

Bike has been running great, easy start and such. But I have a vibration that comes in at about 3,500 rpm. Thinking its an imbalance somewhere in the carbs.

[raistian77]

I have noticed that on other forums the main complaint about the GS VM carbs is they are so lean. Is this accurate?

[49er]

I have noticed that on other forums the main complaint about the GS VM carbs is they are so lean. Is this accurate?

Yes, the '79 VM's were deliberately set lean from the factory to meet more demanding global emission laws. These laws became so strict in some countries, that plugs were fitted to the CV carbs covering the fuel adjustment screws of the idle circuit. Service manuals had sentences such as "under no circumstances must these screws be altered", touting suggestions that the factory had a magician finely tuning each primary circuit. This occurred on the CV carbs from '80 onwards.

When you richen the stock VM needles by moving the clip to the 3 rd position, you will loose some crispness in the mid range. You can shim an intermediate position ( I used a .016" washer) to gain the desired Air Fuel Ratio for your midrange performance. You need to do the appropriate plug chops, or use a dyno to confirm this.

[raistian77]

Yes, the '79 VM's were deliberately set lean from the factory to meet more demanding global emission laws. These laws became so strict in some countries, that plugs were fitted to the CV carbs covering the fuel adjustment screws of the idle circuit. Service manuals had sentences such as "under no circumstances must these screws be altered", touting suggestions that the factory had a magician finely tuning each primary circuit. This occurred on the CV carbs from '80 onwards.

When you richen the stock VM needles by moving the clip to the 3 rd position, you will loose some crispness in the mid range. You can shim an intermediate position ( I used a .016" washer) to gain the desired Air Fuel Ratio for your midrange performance. You need to do the appropriate plug chops, or use a dyno to confirm this.

good deal, I'll run by radioshack tommorow and get the washer assortment to shim the needle a hair richer.

[Steve]

Thanks, Thats a relief. On the first strip and dip the pilot screws were 4 turns out and the pilot air screws were all bottomed.

Where do you have them set now?

In stock trim (airbox and exhaust), the fuel screws (the ones on the bottom) should be about 3/4 turn out and the air screws (the ones on the side) should be about double that, or 1 1/2 turns. When the bike is warmed up, tweak the air screws for highest idle speed.

.

[raistian77]

Where do you have them set now?

In stock trim (airbox and exhaust), the fuel screws (the ones on the bottom) should be about 3/4 turn out and the air screws (the ones on the side) should be about double that, or 1 1/2 turns. When the bike is warmed up, tweak the air screws for highest idle speed.

.

When I got the bike, my local dealer mechanic gave me the service manual for the bike. it states the pilot is 5/8 out and the pilot air is 1 1/4 out

[Steve]

When I got the bike, my local dealer mechanic gave me the service manual for the bike. it states the pilot is 5/8 out and the pilot air is 1 1/4 out

And those settings were to satisfy the EPA, and the bike ran like crap.

It works much better if you go to 3/4 and 1 1/2.

.

[SqDancerLynn1]

Adjust the air screws for highest idle. Then do a carb sync. It will amaze you
how much smother the bike runs.
Those washers Radio Shack pn 64-3022

[raistian77]

Adjust the air screws for highest idle. Then do a carb sync. It will amaze you
how much smother the bike runs.
Those washers Radio Shack pn 64-3022

thanks, gonna grab some this afternoon

Ok Steve I'll try the 3/4 and 1 1/2 instead

[tas850g]

thanks, gonna grab some this afternoon

Ok Steve I'll try the 3/4 and 1 1/2 instead

Those turns out will be a very good starting point.