New Carb Diaphragms and Clutch, compression

[Gearhead]

Both of these subjects have been under a lot of discussion lately, and my bike needed both. Saturday was the big day, and here is my story.

 

I'll start with the clutch. What happens to these things that makes them slip without wearing? All discs measured exactly the same as the brand-new discs - no measurable wear whatsoever. Spring measured within spec. Yet I had mild slippage sometimes under full throttle (not very bad, but annoying). I put the new discs in anyway. Didn't touch the half-disc in the "damper" mechanism as the manual says not to unless there is "severe chatter". I didn't sand the plates or replace the spring. Now after about 40 miles of riding and quite a bit of full throttle. I have experienced no slip - hooray.

 

I've experienced this before. I've had (3) 87 Yahamas. I've had a Virago for a long time, and WAY back I had exactly the same problem with the clutch. I recall measuring the old discs to be well withing spec, just like this time, replaced them anyway, and I've never had that clutch slip again in 80,000 more miles. The mileage on it at the time was probably in the 50k's, same as the Venture now. I also had a Radian for awhile with less than 20k miles on it, and it's clutch slipped on occasion. I never worked on it and then sold it. What's up with that???

 

While I had the cover off I removed the nasty old remains of clear coat and polished - looks way cool but now the other case covers look very bad comparision - they feel embarassed, too. Worst part of the job was removing the old, rock-hard gasket from the cover.

 

Replacing the slides went about as expected. I didn't want to remove the carbs so I haven't looked at the coasting diaphragms, just the slides. I had pulled #3 and it was quite cracked, so I ordered (4) of them. I found that #2 had been replaced sometime, don't know when but it was at least 2 years ago, probably longer. The diaphragm was good on it. At $55 a shot I wish I had known that before ordering, but the rubber was a little harder on it than on the new ones so I just put in the whole matched set and will keep the "new-old" one around for a spare.

 

This profoundly affected the tuning of the bike. The carbs, which I just synced a couple k-miles ago, were way out, and the pilot screws which I had also adjusted needed to be leaned out. This gives me hope that this will answer my poor MPG question.

 

Everybody said bad diaphragms kills the top end power, so I was looking forward to knocking my socks off on the first ride, although it ran pretty good before. Well, my socks are still on, but I think top-end is up. It sure gets up and goes. For one thing, it seemed to be running out of steam at 100 before, and this morning I saw 110 mph with a little still left (ran out of room). OTOH, it may have been clutch slippage or aprehension of a weave that kept me below 100 before, so I'm not totally sure if my perception is accurate. It tachs out to 8k rpm with no problem. I'll let you know about the MPG, although all this WOT ain't gonna be too good on that!

 

Finally, I had gotten some weird compression readings before which made me wonder if something inside the engine was about smoked. I preferred to deny it rather than think much about it. On Saturday I ran compression again with the engine HOT and new schrader valves in my gage hoses (I tried two gages). I got 190 to 200 psi in all cylinders - wow! I concluded the following:

 

1) A hot engine can make a difference in reading consistency. It also makes a difference in burned fingers.

2) There is a schrader valve in the end of the gage hose that need replacing on occasion. It is special, so don't install a regular tire valve core (it looks the same) - get it from a tool store. It has a profound effect on readings!

3) Different gages sometimes read differently, so use the same gage for your whole test.

4) Even the different hoses made 10psi difference. Both had new schrader valves. All I can figure is that the valves were slightly different, but I didn't take the time to swap the new valve cores and retest.

5) They say to open the throttle all the way, but on a CV carb that leaves the slide blocking most of the intake path. I removed the air filter and used a dowel to hold the slide open for the test (as well as opening throttle). This seemed to make about a 5psi difference, and allowed the compression to build a little faster. I wouldn't say it's a critical part of the test, but it did affect the test.

 

Jeremy

[Dano]

Could diaphragms be part of my high-idle (2k) that I experience after warm-up? Just wondering.......

Dan

[Gearhead]

Dan, I didn't experience that symptom. What happens if you turn the idle speed screw down?

 

Jeremy