GS550 - mechanical slide carb and CV carb interchangeability.

[Guest]

Hey, just curious if anyone knows the compatibility risks of putting the older mechanical GS550 carbs on a later CV carb GS550.

I have an 81 GS 550 with CV carbs. I'd like to experiment with the earlier carbs on my bike. I know I'll need the correct carb holders/boots for the smaller mechanical carbs. I ordered a set but they're in the mail. I have a good working set of both styles of carb.

Given that I'm using the correct carb holders, will either style of carb fit on my bike? Is there any difference between the early and late GS550 heads that will make it impossible to "retrofit" the carbs?

In terms of tuning, I'm game for cleaning, syncing, and swapping jets. That will be done and isn't intimidating to me.

The bike has no airbox, and predictably runs sort of rough with the CV carbs and pods. I'm hoping the mechanical slide carbs will be more tunable for the situation I'm dealing with.

I understand the ideal scenario would be the factory correct airbox with CV carbs, but that's not the question I'm asking.

Thanks in advance.

[Nessism]

One issue you may have is getting the smaller carb boots to bolt-up to the later cylinder head. There will be a step where the carbs boots meet the head too, but I don't think this will cause any major issues in of itself though.

[Steve]

Exactly what are you trying to accomplish with this experiment?

For street use, the CV-type carbs that you already have on there are MUCH more friendly.

However, if you plan on racing, where more of your time will be spent with the throttle wide open, the VM carbs <might> be better.

It does not take all that much to get the CV-type carbs to run without an airbox, although that is the better route.

.

[Ira]

Hi Steve. To be honest, I'm really bad at tuning the CV carbs. Not for lack of effort either. I've have the bike several years and got the main/WOT dialed in, acceleration off the line is really good, and idle is a bit high, but acceptable. However, changing throttle positions when riding feels choppy and unpredictable. Closing the throttle results in backfiring, and any time I decelerate without completely closing the throttle (like coming to a complete stop) I get slow response, and uneven surging power.

I have a couple other bikes with mechanical slide carbs, and I've never had these problems. I'm pretty good at tuning nice crisp throttle response, even after changing elements of exhaust and intake. So while this is fully related to my skills as a tuner, I'm just frustrated with the CV carbs and would like to try something I'm familiar with.

[Steve]

Fair enough, but the size mis-match might bring you more headaches to sort through.

You say you have proper-size boots coming. No doubt they will match the carbs, but there is no guarantee that the mounting bolts are the same distance apart.

Assuming that they do, indeed, bolt into place, there will be a lip where the smaller carb boot meets the larger port on the head. That will likely introduce a fair amount of turbulence in the air stream. Yeah, some turbulence is good for keeping the mixture mixed, but I would have questions about that sudden 10mm increase in diameter, making a 5mm lip.

.

[scratch]

The bolt holes are significantly off… and the rack spacing would be a total miss-match.
GS550 VM on left, CV right

[scratch]

Best way is a head-swap. The good news? It's easy & 550 heads are plentiful...

(the better way is an airbox, tuned CV's on a 550 trump VMs 9 days out of 10. My gal and I have five 550's... VM's for show, CV's for go)

[gs7.11]

Put the Airbox back on.

[Ira]

Hey guys, thanks for the useful info. Swapping heads is a bit drastic for an experiment, so I'm going to keep at the tuning. Anyone need a great set of mechanical carbs from a low-mile bike? They're going on ebay next week, but hit me up if you want them!

As I mentioned previously, "put the airbox back on" isn't helpful. Yes, that would be easy, but it's been hashed over in a bunch of threads and I've read them. I have several stock bikes with airboxes to ride - this one is for making mistakes and learning a few things. I appreciate the help avoiding this particular dead-end.

Thanks!