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Swapping CV to VM?

  • Thread starter Thread starter Coldbroke
  • Start date Start date
C

Coldbroke

Guest
Hey guys pretty new to the forum still but I have a question I can't really find myself.

I have a 81 gs1000g, I'm trying to finish up alot of maintenance and everything but the carbs are giving me slight issues. Nothing to serious but just little annoying things. Mine are the CV (bs34ss) carbs and I'm running pods on them ( yes I know the whole shabang about that) they run good it's all re jetted good power all the way through the rpms but the response isint fully there and it kinda struggles to idle most of the time. I have turned the idle screw up and itll idle fine but once I ride and come to a stop the idle will hang around 2100 for about 4 seconds before it starts to slowly fall back down to 1000 dont think I have any leaks I bought new intake boots due to the old ones being torn, i also have fully torn apart the carbs and cleaned them.
Now the big question, can I run a vm series carb? I'm pretty sure the 79 has the vm26ss flattop. Mind you the bike doesn't have alot of factory stuff left so I dont have an air box and all that. So if possible I'd rather just swap the carbs and call it a day IF it's that easy. I'm not sure if everything will line up and mount correctly right off the bat.

If anyone has done this or knows about a swap other than the RS36 setup I would greatly appreciate the help.
 
Try something that is a LOT easier than a carb swap. :-k

Change the o-rings in the intake tubes.

If you are running your bike on the street, with "normal" street riding, the CV-type carbs are much better for throttle response. They are designed to reduce, if not eliminate, the bogging that happens on sudden application of throttle. If you are using the bike for racing (on the track), where you are mostly at full throttle, except for corner approaches and shifting, the VM carbs would be better, as the slide in the CV-type carbs takes a bit of time (a fraction of a second) to respond. On the street, that fraction of a second delay will make the bike quicker, because it won't bog down, requiring correction. On the track, that fraction of a second will be multiplied by many shifts and will cost you a couple of seconds per lap. Where do YOU ride? :-k

.
 
Where are the pilot screws set and did you remove them before cleaning? You might want to open them to 3 or 3.5 turns open to see if that helps. Carb vacuum sync? It's critical to vacuum sync.
 
Steve said:
Try something that is a LOT easier than a carb swap. :-k

Change the o-rings in the intake tubes.

If you are running your bike on the street, with "normal" street riding, the CV-type carbs are much better for throttle response. They are designed to reduce, if not eliminate, the bogging that happens on sudden application of throttle. If you are using the bike for racing (on the track), where you are mostly at full throttle, except for corner approaches and shifting, the VM carbs would be better, as the slide in the CV-type carbs takes a bit of time (a fraction of a second) to respond. On the street, that fraction of a second delay will make the bike quicker, because it won't bog down, requiring correction. On the track, that fraction of a second will be multiplied by many shifts and will cost you a couple of seconds per lap. Where do YOU ride? :-k

.
Click to expand...

Hey Steve thanks for the response, I did get new OEM O-rings when I bought the new intake tubes. Although they were ever so slightly smaller in the groove they sat in when I installed them.
I ride purely street
 
Last edited:
I'm sorry I didn't mean the pilots but the actual knob to adjust the idle. But I didn't remove the pilot screws due to them being stuck. I've sprayed penetrating oil on top of the screws to try and break them free but I dont want to strip out the tops.
Yes the carbs have also been synced did them at the end of the season.
 
Coldbroke said:
I'm sorry I didn't mean the pilots but nthe actual knob to adjust the idle. But I didn't remove the pilot screws due to them being stuck. I've sprayed penetrating oil on top of the screws to try and break them free but I dont want to strip out the tops.
Yes the carbs have also been synced did them at the end of the season.
Click to expand...

Spent a good part of all last summer trying to chase down a similar issue on my bs34ss carbs on my 450. They where completely stripped and cleaned, new o-rings, new intake o-rings, the works. One of my pilot screws was feeling odd when turned and sure enough, the o-ring which was replaced last winter had torn in half. They didn't have a washer on them and the parts fische doesn't show it since it's supposed to be plugged at the factory. Hit an auto parts store for an o-ring that was close enough and luckily found some washers that would fit at the hardware store in the little 'burg I was in at the time. That did the trick! I'd highly suggest getting those pilot screws straightened out.
 
Coldbroke said:
I'm sorry I didn't mean the pilots but the actual knob to adjust the idle. But I didn't remove the pilot screws due to them being stuck. I've sprayed penetrating oil on top of the screws to try and break them free but I dont want to strip out the tops.
Yes the carbs have also been synced did them at the end of the season.
Click to expand...

The passages through the pilot circuit are very small and likely partly blocked. The only way to properly clean them is to remove the pilot screws. Not to mention, you can't tune the idle mixture if the screws are seized.
 
LAB3 said:
Spent a good part of all last summer trying to chase down a similar issue on my bs34ss carbs on my 450. They where completely stripped and cleaned, new o-rings, new intake o-rings, the works. One of my pilot screws was feeling odd when turned and sure enough, the o-ring which was replaced last winter had torn in half. They didn't have a washer on them and the parts fische doesn't show it since it's supposed to be plugged at the factory. Hit an auto parts store for an o-ring that was close enough and luckily found some washers that would fit at the hardware store in the little 'burg I was in at the time. That did the trick! I'd highly suggest getting those pilot screws straightened out.
Click to expand...

Nessism said:
The passages through the pilot circuit are very small and likely partly blocked. The only way to properly clean them is to remove the pilot screws. Not to mention, you can't tune the idle mixture if the screws are seized.
Click to expand...

Hmmm then it looks like I'll have to finally tackle those screws. I'll spray a little more oil on the tops of them to hopefully help break them loose. I had a hell of a time trying to get one free but I lost that battle. Any tips on getting them out safely?
 
Heat and a tight fitting screwdriver. Grind the tip of a fatter screwdriver down to size until it fits tight. And use a propane torch on the carb body to break the chemical bond (after filling the tower with PB Blaster or similar). Worst case, if the slot gets stripped, is to cut down the tower and cut a new slot with a dremel cut off tool. Turn the screw both ways back and forth and don't try to force anything. Once you get it cracked loose you can get it out as long as you don't get impatient.
 
Coldbroke said:
Any tips on getting them out safely?
Click to expand...
Use a reinforced cut-off wheel in a Dremel to cut a slot down the surrounding tower and enhance the slot in the top of the screw.

IMG_4081.jpg


.
 
Im wondering if the diaphrams are getting stiff from age and not flexing easy enough to let the slides drop faster.
 
Nessism said:
Heat and a tight fitting screwdriver. Grind the tip of a fatter screwdriver down to size until it fits tight. And use a propane torch on the carb body to break the chemical bond (after filling the tower with PB Blaster or similar). Worst case, if the slot gets stripped, is to cut down the tower and cut a new slot with a dremel cut off tool. Turn the screw both ways back and forth and don't try to force anything. Once you get it cracked loose you can get it out as long as you don't get impatient.
Click to expand...

Steve said:
Use a reinforced cut-off wheel in a Dremel to cut a slot down the surrounding tower and enhance the slot in the top of the screw.

IMG_4081.jpg


.
Click to expand...

Steve said:
The slides do not control engine speed.
dunno.gif


.
Click to expand...

Alright I'll attempt this next week when the weather warms up, currently trying to time my new dyna s ignition. 2&3 gives no mark other than F. And with the current timing its blowing smoke out the exhaust.
 
Alright well luckily I got 1 out but the other 3 are really stuck and 2 of them are now stripped. Heat didn't really work sadly so I might use the dremel to get them but I'm leaning toward a reverse tap. I'll see what my options are tomorrow, if a shop can do it for cheap I might just have them do it.
 
Coldbroke said:
Alright well luckily I got 1 out but the other 3 are really stuck and 2 of them are now stripped. Heat didn't really work sadly so I might use the dremel to get them but I'm leaning toward a reverse tap. I'll see what my options are tomorrow, if a shop can do it for cheap I might just have them do it.
Click to expand...

I would try left handed drill bit and spiral fluted screw extractor first before I dremeled through the tower. Just drill slowly and make sure you use the correct size bit, and make sure to use a hand tap with the screw extractor, not the drill.
 
Welp just a small update on everything so the post isint left in the dark.
I managed to get the second screw out but the other 2 are really bad, luckily a good friend of mine asked his dad if he could take a look and get them out hes been a machinist for years so hopefully all is well. I really wasn't confident on drilling them and possibly screwing myself out of some carbs. He should have them done by this Wednesday, so ill toss another update here on how it all runs with them properly tuned.
 
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