Replacing the CV carb sync screw+locknut with something better

[t3rmin]

I'm redirecting a discussion which was taking place in the Tips and Tricks section since it's become more relevant to the Tech section and so we can hopefully get some more input. The original thread is here:

http://www.thegsresources.com/_forum/showthread.php?t=98272

Everyone knows and hates the sync screw+locknut setup on the CV carbs. Even if you have the special "hollow 8mm socket with long screwdriver through the middle" tool, it's still a pain since tightening the locknut turns the screw slightly. You've got to anticipate and account for this when turning the screw which makes the whole thing a big pain in the neck.

Nert has come up with a great alternative (see above thread link) which eliminates this hassle, but involves dismounting and drilling into the carbs. He said he originally wanted to employ a simpler modification but couldn't find the proper size screw. Here's a rundown of the simpler mod idea courtesy of Terry:

I don't want to disparage "Nert"s idea at all, but it seems to me that replacing the set screw and locknut with a pan head screw with a stiff spring underneath (same principle as your idle adjustment screw, mixture screws etc) would be a lot easier, with no requirement to remove the carbs, or drill any holes? The spring would stop the screws from moving under vibration, and would be a little less "fiddly", I reckon? Cheers, Terry.

Sounds *great* to me: no drilling, easy install, makes life easier for everyone. IMHO this would be a great asset to the GS community.

So, GSers, lets figure it out! We need:
1) The sizing on the existing screw: thread width, pitch, and length. I'll measure it when I get home today if nobody has their bike handy.
2) A new screw with a head with a flat bottom suitable for retaining a little spring. This is the most difficult part. The existing screw has extra fine threads, so it'll be hard to find. It is possible to have screws custom made, but it's expensive, especially for small quantities. Surely somebody here knows of a place that stocks non-standard fine thread metric fasteners at a reasonable price?
3) A spring to put the screw through to keep it from vibrating out of adjustment, as described above. I don't think this'll be terribly hard to find. We just need something the screw will fit through that's within a reasonable range of length and stiffness -- there's some leway here. My local hardware store has a good assortment...

Maybe sombody can put together kits and sell 'em (like Robert Barr sells o-rings) if we can find these items; and/or if we have to have a fastener custom made, we might be able to make a group buy to defray the costs.

[bwringer]

Thanks for posting this!

One alternative would be to simply re-tap the existing hole in the adjuster. However, the ultrafine threads are there for a reason -- those sync screws are already incredibly sensitive to extremely small movements, and a courser thread would make this sensitivity even worse. Even if you didn't have to muck with a locknut, it would be very difficult to set things precisely.

I've gone through some pretty large spring selections in hardware and supply stores, but I haven't found anything small enough in diameter that is also stiff enough. (We'll leave out length, since it's easy enough to cut the springs.)

It might be simplest of all to find other carb models (like a late-model Kawasaki or something) that use spring loaded screws, which we might then be able to order from BikeBandit or wherever.

[terry]

Thanks Mate. Metric screws are relatively easy to find here in "Metrificated" Oz, I'm thinking that a nice set of stainless steel "Allen head" screws might be the go, I think standard 5mm screws are .75 pitch, and fine are .50, but I'm guessing that they're .75.

I'll check it out tonight, I think I've got a spare set of carbs in a box somewhere? As far as springs go, I was just going to pull an old set of carbs apart and take the springs off the idle mixture screws? Cheers, Terry.

[nert]

Thanks for posting this!

One alternative would be to simply re-tap the existing hole in the adjuster. However, the ultrafine threads are there for a reason -- those sync screws are already incredibly sensitive to extremely small movements, and a courser thread would make this sensitivity even worse. Even if you didn't have to muck with a locknut, it would be very difficult to set things precisely.

I've gone through some pretty large spring selections in hardware and supply stores, but I haven't found anything small enough in diameter that is also stiff enough. (We'll leave out length, since it's easy enough to cut the springs.)

It might be simplest of all to find other carb models (like a late-model Kawasaki or something) that use spring loaded screws, which we might then be able to order from BikeBandit or wherever.

bwringer hit the nail on the head, (or the screw in the pitch). The adjustment screw is small in diameter and has extremly fine threads to provide precision adjustment. I think everyone is thinking 5mm as in the ADAPTER thread for the vacuum tubing. I don't think screws like we need are standard anywhere. Especially extra long and custom head. But, iam on the other side of the pond.
bwringer second option (late model, already in use and available) is a very good idea, if it already exists. Some motorcycle mechanic who has access to many brands, models and configurations may have the answer.

[t3rmin]

Ok the stock screw is 5mm in diameter (well 4.90mm, but is that what a 5.0mm screw actually measures???), 1.5cm long, and has 0.50mm pitch threads.

So we need a M5x0.50 pan head (or other flat bottom head) screw that's, say, 2cm long (extra room for spring). DOES ANYONE KNOW WHERE TO BUY SUCH A SCREW??

The best I've been able to find so far is M5x0.50 threaded rod and M5x0.50 nuts. The rod could be cut in 2cm lengths and the nuts glued permanently on one end, but that would be awful and I'd really like to find a proper screw.

[txironhead]

All of the M5's I'm finding are .8 pitch. Love the idea, though. You should be able to find out for sure what size it is by taking the locknut off and taking that to a specialty fastener store and seeing if they have a screw that fits it. I like this idea so much that that's what I'm going to do locally. If I find a screw that fits and is approximately 20mm in length, I'll post it here and also how much they are.

Just for the record (provided I CAN find one), would you guys prefer slotted, Philips, or allen head? I think I'd prefer allen for mine.

[nert]

allen would be great. may make it a little more difficult to find a spring, or a washer would need to be added. allen would allow use of a long "T" handle allen wrench with the ball end that allows angles.

[staschman]

I've tried to find a replacement screw with the same fine threads, and was unable to. Would love to find a source for these.

[t3rmin]

Yeah standard pitch for M5 is 0.80 and they don't generally do "fine" threads in smaller diameter bolts/screws, so this will be very hard to find. If anyone locates a supplier, please do tell!

I'd prefer phillips since I think you'd have to find an abnormally long "T" handle allen wrench, and if it were a slot head the screwdriver would keep slipping off. Lately I've frowned at phillips heads since they're all over my bike in freakin' innapropriate places, but for this application I think it'd be best (don't need to put much torque on it and it's not going to seize).

I'm going to email some custom fasteners companies, but having these fabicated on a small scale is likely to be really expensive. Lets hope we can find somebody that stocks them already.

[t3rmin]

I have run across a few M5x0.50 dies, so if anyone knows if it'd be possible to find "blanks" and make these ourselves, that's an option.

[t3rmin]

Ok, I've sent emails to around 30 different fastener suppliers/distributors/manufacturers asking whether they stock a screw that size and if not, if they can fabricate it. We'll see...

[nert]

\
I'd prefer phillips since I think you'd have to find an abnormally long "T" handle allen wrench, and if it were a slot head the screwdriver would keep slipping off.

I have a set of 10" plus or minus "T" handle allen wrenches. Not hard to come by. But in any case, philly is fine.

\Ok, I've sent emails to around 30 different fastener suppliers/distributors/manufacturers asking whether they stock a screw that size and if not, if they can fabricate it. We'll see...

you need to be commended. you are trully on a mission, and pumped.
Your efforts if successful (not due to effort) will be a great help to gs guys all over the world. applaus...

[t3rmin]

I have a set of 10" plus or minus "T" handle allen wrenches. Not hard to come by. But in any case, philly is fine.

Well maybe I over-estimate the required length and/or avilability of long allen wrenches. ;-) But I know I can grab a long phillips head screwdriver for nuthin' at any hardware store. One other thing is allen can be slightly harder to get the driver seated in the head versus phillips, especially considering how many times you move around between the three adjusting points. I'll go ahead and pursue phillips, but I'll take freakin' torx if that's the only thing available! ;-) I desperately want to get this done, it's such a good solution...

[bwringer]

I have run across a few M5x0.50 dies, so if anyone knows if it'd be possible to find "blanks" and make these ourselves, that's an option.

You can get 5mm rod in a variety of materials from McMaster-Carr ( http://mcmaster.com ) . However, that wouldn't do much good for forming the heads.

Have we verified that the diameter and thread is 5mm X .50mm?

[t3rmin]

Have we verified that the diameter and thread is 5mm X .50mm?

Yep. 4.90mm by the digital caliper for diameter, but I assume that's still classified as a 5mm fastener. Threads are definitely .50; I have a .50 pitch tap from another project and they mate up perfectly.

Like I said, I did locate some M5x0.50 threaded rod and some matching nuts, but whatever ya did with that would be ugly. ;-)

I pulled the locknuts off and put a drop of blue loctite on the threads tonight. While syncing, the screws felt like they had enough resistance to keep from vibrating around. Somebody already mentioned that was their complete solution. I must confess that if I can't find a cheap source for these screws I may wimp out and just give the loctite a try. So much for my crusade! ;-) Boy was it easy syncing without those locknuts! Didn't have to remove the choke bracket either, since a screwdriver alone slips in there just fine (the Motion Pro tool can't get to the middle adjuster without removing the choke cable bracket on my carbs).