Help with updated master cylinder...and stiff throttle response

[RPGR90s]

My buddy converted the cable actuated MC on his 1976 R90s to an updated setup, with the MC on the lever. I was helping him with another problem and he mentioned that he loves the increased braking power (stock rotors, stock ATe calipers) but the throttle pull is a bear.

I tried it and you need a gorilla wrist to turn it. First thing I noticed is that it has one cable coming out of the lever assy, and then splits into two before going into the stock Dellorto carbs. So I noted that as a possible issue, but wanted to check the carb springs first. My old 1993 R100RT had a 1 cable split into two setup as I recall.

Both carbs have the light Dellorto springs installed (based on total spring length and number of winds). Since we pulled the top off of one carb and the slide dis-assembled, I noticed that twisting the grip was way easier as it was now only pulling one carb assy. Could that be the problem? Cables are new, and the cam assy. is properly greased at the handlebar.

I know many Airhead owners have done the upgrade but I'm guessing a lot of those used the twin cables coming out of the lever assy, as opposed to the one cable and then the splitter. Also, I'm guessing most conversions are pulling stock Bing butterfly style carbs as opposed to slide type Dellorto's.

So I'm testing a theory that the two into one setup he went to, AND the ratio of the twistgrip and cable cam (1/4 turn) is causing the excessive tension by pulling two slides instead of twisting two butterflies.

1. Would going to a two cable setup (with the same MC mounted at the lever) alleviate the issue?

Thanks so much,

RPGR90s

 

[20774]

I added a bit more to the title since the issue not all about the master cylinder.

I'll have to think about this but why would opening butterflies versus the Del slides be such a mechanical difference? In stock situations the butterfly opens quite easily and you have experience with Del Ortos and how they feel. There has to be some other cause.

 

[AntonLargiader]

There are several different throttle cams, and the more lift they have the more work they are doing to lift the slides. In order to convert to handlebar-mounted MC he had to select a throttle cam. Does he have a part number for the cam he used?

 

[RPGR90s]

There are several different throttle cams, and the more lift they have the more work they are doing to lift the slides. In order to convert to handlebar-mounted MC he had to select a throttle cam. Does he have a part number for the cam he used?

He didn't add the Handlebar MC, as he bought the bike that way. I'll mention this and see if we can get more information on what cam he has. Thanks very much.

 

[AntonLargiader]

There’s a page on my website that’s all about throttle parts. The cams are shown.

 

[Lendog]

My buddy converted the cable actuated MC on his 1976 R90s to an updated setup, with the MC on the lever. I was helping him with another problem and he mentioned that he loves the increased braking power (stock rotors, stock ATe calipers) but the throttle pull is a bear.

I tried it and you need a gorilla wrist to turn it. First thing I noticed is that it has one cable coming out of the lever assy, and then splits into two before going into the stock Dellorto carbs. So I noted that as a possible issue, but wanted to check the carb springs first. My old 1993 R100RT had a 1 cable split into two setup as I recall.

Both carbs have the light Dellorto springs installed (based on total spring length and number of winds). Since we pulled the top off of one carb and the slide dis-assembled, I noticed that twisting the grip was way easier as it was now only pulling one carb assy. Could that be the problem? Cables are new, and the cam assy. is properly greased at the handlebar.

I know many Airhead owners have done the upgrade but I'm guessing a lot of those used the twin cables coming out of the lever assy, as opposed to the one cable and then the splitter. Also, I'm guessing most conversions are pulling stock Bing butterfly style carbs as opposed to slide type Dellorto's.

So I'm testing a theory that the two into one setup he went to, AND the ratio of the twistgrip and cable cam (1/4 turn) is causing the excessive tension by pulling two slides instead of twisting two butterflies.

1. Would going to a two cable setup (with the same MC mounted at the lever) alleviate the issue?

Thanks so much,

RPGR90s

Del-orto carbs have stiff springs, even the “soft” ones, cable routing is important, Magura makes a nice 1/4 turn throttle, two cables will not change the tension,
remove the cables, hang vertical and lubricate, disassemble the cable junction and check for binding

 

[RPGR90s]

Del-orto carbs have stiff springs, even the “soft” ones, cable routing is important, Magura makes a nice 1/4 turn throttle, two cables will not change the tension,
remove the cables, hang vertical and lubricate, disassemble the cable junction and check for binding

Thanks for the info.
Actually, there are three designated types. At least there are now these days. Not 100% sure what was installed back at the factory, but these three types are still available from Bevel Heaven, and some other suppliers:

Light - springs 1 mm wire diameter, 127mm total length, 17 coils
Med - springs 1.1 mm wire diameter, 132mm total length, 17 coils
Heavy - springs 1.1 mm wire diameter, 135mm total length, 15 coils

I will check and re-route the cables if necessary but since they're nylon lined, it's not a good idea to lubricate them.

 

[Lendog]

Thanks for the info.
Actually, there are three designated types. At least there are now these days. Not 100% sure what was installed back at the factory, but these three types are still available from Bevel Heaven, and some other suppliers:

Light - springs 1 mm wire diameter, 127mm total length, 17 coils
Med - springs 1.1 mm wire diameter, 132mm total length, 17 coils
Heavy - springs 1.1 mm wire diameter, 135mm total length, 15 coils

I will check and re-route the cables if necessary but since they're nylon lined, it's not a good idea to lubricate them.

Of course if the cable manufacturer recommends not to lube, don’t do it, nylon lined cables installed in a bind will cause a groove in the nylon and cause pinching, if it is pinching for very long the cable is ruined on the inside and nice on the outside