crazy carbs, explain this?

[psycoreefer]

OK, so I have two 11's. An 81, which runs great but is new to me, and a 79 which I've had for 3ish years and has a stumble around 5k. I've been chasing that stumble for most of those 3 years, and now that I've got another set of carbs I though I could solve the carb vs "other" issue question on that 5k stumble.

I swapped the carbs, 79 <-> 81 no other changes. both carbs have stock (for that year) jetting, with the exception of the 79 needles raised one position. (I raised the needles and it improved the stumble at 5k so I left them.)

The 79 runs great with the 81 carbs, no stumble so I guess it answered my question, its gotta be the carbs. Now here is the crazy part, the 81 won't run past 5k, I mean literally won't, with the 79 carbs. Pulled the plugs and they are super black = way to rich.

Now I understand that each engine is a bit different, and the carbs may need to be tweaked to account for that but this seems to be way way off.

How is it that my 79 has slightly dark but "ok" plugs and gets 35mpg with the 79 carbs, and the 81 fouls plugs and won't even run with the 79 carbs? And yet the 79 runs perfectly with the 81 carbs? What am I missing?

I know the solution is to swap the carbs back, and continue chasing my 5K stumble on the 79.....but inquiring minds want to know.....

[fredintoon]

- - - The 79 runs great with the 81 carbs, no stumble so I guess it answered my question, its gotta be the carbs. Now here is the crazy part, the 81 won't run past 5k, I mean literally won't, with the 79 carbs. Pulled the plugs and they are super black = way to rich. - - -

Hi psyco,
I dunno neither but what I'd do is put the '81 carbs back on the '81 to double-check the super-rich thing. Then get another set of '81 carbs to put on your '81.
Or, a stray thought:-
Do the '79 carbs have those cross-drilled passages from the main jets to the slow running jets? If so, the pilot jet installation holes need to have those rubber bungs in them or the '79 carbs will run super rich.

[dbeardslee]

I was working on another members 80SG last year, and it was like it hit a brick wall at 5k rpms. In diddling around I found that it had the 'bastardized' carbs that require a plug over the pilot jet tunnel. Plugs were missing and that's why it wouldn't go past 5k. I put some plugs in the towers and all of a sudden it had the top end back. On the '79 carbs there should be a bolt in the pilot jet tunnel with a washer underneath it. If you look at where the orifice goes across from the pilot tower to the main tower, without the washer the length of the bolt is just about long enough to partially cover the orifice, so you want to make sure the washers are in place. Not sure that would cause the problem, but it's something to check.

Dang you type fast, Fred .

[79XS11F]

OK, so I have two 11's. An 81, which runs great but is new to me, and a 79 which I've had for 3ish years and has a stumble around 5k. I've been chasing that stumble for most of those 3 years, and now that I've got another set of carbs I though I could solve the carb vs "other" issue question on that 5k stumble.

I swapped the carbs, 79 <-> 81 no other changes. both carbs have stock (for that year) jetting, with the exception of the 79 needles raised one position. (I raised the needles and it improved the stumble at 5k so I left them.)

The 79 runs great with the 81 carbs, no stumble so I guess it answered my question, its gotta be the carbs. Now here is the crazy part, the 81 won't run past 5k, I mean literally won't, with the 79 carbs. Pulled the plugs and they are super black = way to rich.

Now I understand that each engine is a bit different, and the carbs may need to be tweaked to account for that but this seems to be way way off.

How is it that my 79 has slightly dark but "ok" plugs and gets 35mpg with the 79 carbs, and the 81 fouls plugs and won't even run with the 79 carbs? And yet the 79 runs perfectly with the 81 carbs? What am I missing?

I know the solution is to swap the carbs back, and continue chasing my 5K stumble on the 79.....but inquiring minds want to know.....

You may want to try using 140 mains with the K&N with the pipes. Put in the bigger mains and return the needle jet to its normal position. If the needle needs adjustment after that do so in small increments by using thin small washers under the clip. If you can get access to some color tune plugs they will tell you what's going on.
Rob

[psycoreefer]

hey guys, I appreciate the carb advise, but your missing the point, I'm confused as to how the 79 carbs can function well (I won't say perfectly because of the stumble) on one bike and horribly on the other. I'd mark it up to differences in the engine except that the 81 carbs work on both bikes....and if the engines were that different I'd expect the 81 carbs to not work on the 79.

as for the other suggestions:
Yes - the 79 carbs have both the washer and the screw in the pilot tower.
No - the 81 carbs are not the "bastard" version, I checked during the rebuild of them.

79xs11f - if your interested I'll dig up the thread where I covered all the different setups I tried to erase that stumble on the 79, long story short, I tried 140 mains, as well as different pilots, different needles, different float heights, etc...really I tried everything and the only thing that helped the stumble without hurting either WOT or off idle was the needle height, which improved it but didn't eliminate it...I've actually been considering trying raising it one more position, but I've been to lazy to wrestle with those super deep snap rings in the slides. I'd try a color tune but I'm cheap and I don't think I could see it while riding down the road

[79XS11F]

Well, the engines in both bike are very similar but if I am not mistaken they have different timing curves, different size valves and different cams. anything that changes the way the engine breaths will affect carb jetting and function. There's no doubt that ether set of carbs could be jetted to work well on ether engine with a lot of on and off testing.

Have you drilled the air box? 2 1 inches in the bottom center will increase air flow.
Rob

[John]

You didn't mention what you did for the bowl vents when you installed the 79 carbs on the 81 engine. How did you vent them?

[Ken Talbot]

John has it - the 79 float bowls are vented to atmosphere via the hose coming from the upper Ts between the carbs. They normally connect to nipples on the airbox that are drilled.

The nipples on the airbox of an 80 or 81 are not drilled, so they do not allow the required venting.

[psycoreefer]

left the vent tubes off the air box, just tucked them up on top of the 81 box, so they are open to the air, but good idea...

[foreda]

Vacuum

What kind of vacuum difference are you getting between the sets?

[psycoreefer]

sorry, I don't understand your question? Do you mean the reading with the gauge on the synch port?

[foreda]

Vacuum

Yes.. When you installed, I assume you sync'd? I'm reaching here..