Interesting difference with new diaphragms

[Venturous Randy]

When people have talked about the low end power on these 1200 motors, I felt like mine seemed to have more power (torque) than most. Mine at 25mph in 5th gear on flat ground would pull surprisingly strong compared to what other described about having to keep their bike above 3,000 rpm's. I also felt like at about 6,500 rpm's that the power was starting to taper off in that it was not pulling as hard.

I know this bike pretty well as I have put 100,000 miles on it since I have had it for 13 years. I have patched the diaphragms a couple of times and it did seem to make it run a little better.

With the new diaphragms, it is just opposite. Now, it feels a bit less torquey at low rpm's, but as I go over 3,000 rpm's in 1st gear at wide open throttle it now is almost scarey how quick it will pull to 8,000 rpm's. It now seems like just a few seconds in 1st and 2nd to 70 mph or more.

I am wondering if I will get better gas mileage than before as I was getting from about 38 to 43, according to the road and my speeds. Now, if I am going to get anywhere near that good I am going to have to crank her down some as I am now catching myself looking for a place to crank it on. It's almost addictive how much pull it has as it is winding up and at 8,000 rpm's it is still pulling hard.

I was just wondering if anyone has experienced this change with new diaphragms?

RandyA

[mbrood]

I would also like to see some feedback on the "replacement diaphragms". I always heard that the specific compound was fairly critical in the CVC design (constant velocity carb). The old type were a fabric embedded in thin rubber and these new ones seem to be just rubber. My 86 also has some old ones and a couple spots coated with a thin wipe of Yamabond #4 and she pulls from low rpm quite well. Funny, I really don't know how hard she pulls at 7g or above... guess I'm treating her too much like a baby?

[Thom]

Randy , funny feeling is it , hitting 8Gs ? the same thing happened

to me , makes on ramps a lot of fun !

Thom

[Dano]

I think I'm gonna change out to my new Sirius diaphragms sometime this week or next, mines crap at lower rpms till about 2750-3k. She'll buck and snort at ya till she gets up where she likes it, then hang on!

 

Dan

[mraf]

When I changed mine the bike definitely ran better overall making the bike seem more nimble. But it didn't change the mileage much until I finally bit the bullet and installed Iridium spark plugs. They along with the new diaphragms now have my bike regularly getting 45-46 MPG at 55. Mine was still very touchy on how to go about starting it in the mornings even with the new diaphragms. The new Iridium plugs have taken the guesswork out of how to start this bike when it is stone cold. Put on choke, hit starter, immediate results with no stumbling or plug fouling. The bike goes to fast idle and runs perfect. With the old standard NGKs plug fouling was a given if you didn't hold your tongue just right.

[Oldgold]

When I changed mine, the source for diaphragms only wasn't available so I ended up with the factory slide diaphragm minus new needle. What they sell doesn't work for 83's as the

lg. spring won't fit into the slide well so I had to get the later springs which are a smaller diameter and clear the small step inside the new slides. Anyway all this did change the power characteristics and the mileage of my 83, more or less what you noted, had more low end before and actually better mileage as long as I drove moderately. I think this may be due to the different springs as the original lg. diameters seem to be a lot softer than the later springs and probably allowed the needles to start opening at lower rpm??

[Gearhead]

I've heard that leaky carb diaphragms cause top-end power loss and that was my experience, although not that dramatic. I wonder if your low-end felt enhanced simply because the top-end was subpar. It would have come on gradually as the diaphragms wore out.

 

Jeremy

[jasonm.]

I replaced my slide diaphragm assemblies with OEMs some 6 years ago because 2 originals had holes. Yes, top end improved and the acceleration was to spec. Smooth at all RPMs above 2500 when whacked open. Just recently I replaced the small orings on the needle jet shafts and cleaned the jet blocks. The bike is totally changed. Only needs choke to start well below 60F. Otherwise turn the key when cold and push the button results in instant 700rpm. Will pull cleanly in 5th from 2000 or less.Runs EFI like. All 4 low speed screws set to 2.5 ccw per my digital tach confirmed this is best for slightly rich. 2.25ccw was peak. Right now I am trying non-resistor DP8EA-9 plugs. Non-resistor plugs did not affect the AM-FM. But I do hear a slight hum on the CB when I put my ear to the speaker. But the hum could have always been there. I have the stock 10k caps. I may go to iridiums.

[Gearhead]

Just recently I replaced the small orings on the needle jet shafts

 

Do you mean the pilot screws, or are you referring to something else?

 

Jeremy

[skydoc_17]

I replaced the Diaphragms only with the Ebay ones last August and about 10,000 miles ago. For the the first 3000 miles I noticed that the new Diaphragms were what I would call a "little stiff". It didn't seem like the bike was as quick off the line but pulled much harder in the higher RPMS. I can now say that this is not a problem. The throttle response is excellent in the lower RPMS now that the Diaphragms have "broken in". I pulled all 4 Diaphragms at 8900 miles right before Freebird's MD to do an adjustment on the needle mod and found all 4 to be in excellent condition. Another thing I wanted to mention is the fact that with the stock Diaphragms (woven material, rubber coated) the Diaphragm actually had folds in it, and this is where the holes developed, with the molded rubber replacement Diaphragms the rubber functions much like a speaker cone and does not fold in on itself. I still have no Idea how long the replacement Diaphragms will last but at 1/4 the cost of the stock ones I am willing to wait and find out. I really didn't see much of an improvement in gas mileage when I replaced my Diaphragms but I did see an improvement in the way the bike started and ran. The needle mod was where I found a substantial improvement in the gas mileage, after trying 5 different shim thicknesses I have settled on -.040 over the stock nylon shim. I am not getting the best mileage (42 to 45 mpg) but I was able to get the plugs to "tan up" where as when I went to -.050 over the stock shim I got 48 to 52 mpg but the plug was as white as a ghost. Also I noticed a bit of power loss at top end with the -.050 shim, mind you it was a very slight loss of power. I also adjusted the idle mixture screws with a tach. and the 21/4 turn setting netted me the best starting and smoothest idle. As I put more miles on this engine I will keep you posted as to the life of the replacement Diaphragms and any other tricks I find as I try to squeak the best performance and gas mileage out of my 87'VR's 1300 engine.

Earl