Engine Surging - why?

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Mark

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I bet this question has been asked here before so it will probably be an easy one for you guys.

75 Mk3 850. Just tinkering with my bike, I pulled off the 34mm. Mikuni and Put on a 36mm.
Now the engine surges under load.

I have a collection of pilot jets and have swapped it out a couple of times, going both up and down in size. No noticable change. In my experience, surging is a lean condition - right?
Partially clogged jet - I have ruled that out.

What am I missing?
Intake leak? Float height? clogged carb? Is the 36mm. just too big?

Please help a brother out with your experiences and hopefully save me some time and frustration excercising this demon.
 
Jets the same in the bigger carb? You lost some velocity going to a bigger carb @ lower rpm. Do a plug chop at a given throttle setting.
 
I think its running too rich, but where in the throttle range is not known...if you are above 3,000ft sea level, the main jet may need to be smaller to compensate. A 36mm Mikuni is going to have less gas velocity over the main & needle jets, so possibly if the surging is happening at mid throttle (please confirm) then a possible cure is to lower the needle first then after a test ride, consider smaller needle jet and main jet at the same time. The pilot jet could probably be at 35 maybe 30 if you are at high altitude which I believe Colarado is at. You could try the parts out of the 34 that was working OK, but keep the 36 slide. Also as others mentioned, check the float height.

Mick
 
Pilot jets have little to do with it after 1/4 throttle so no wonder it didn't change things.
I'd be looking at needle and main jet sizes.
Just curious, how was it running with the 34 and , if good, why ya switching it out? :D
david
 
Thanks for the replies, To answer some of your questions:

It surges off idle on up - I would say.... 1/4 to 1/2 throttle.
(I've got the idle circuit dialed in pretty good)

I set the needle clip in the middle
( just like the other carb.)

I live at 7000 ft. elevation and ride in between 5000 ft. and 10000 ft.
(Rocky Mountain High)

I switched just because it is in new condition
(and just experimenting / tinkering)
 
Surging is a result of rich mixture. Get a new set of plugs one range hotter and go down one notch on the needle. How is bike at full throttle? Try one setting change at a time.
 
My 72 750 has a Combat head and a Big Bore exhaust system.
6DH3 needle clip in center
#159 type Q2 needle jet
The Main jet is 300 (with everythig else proper, mains can go up or down a little only effecting fuel economy and upper end responsiveness) and pilot jet 40.
My top end is fresh which I believe with the 1 1/2" exhaust, produces the optimal venturi for the larger 36mmVM.

I had an issue a while back where my rpm's took a hesitation to return to normal, racing enough to make shifting awkward. I went crazy trying to figure it out, re-routing the cables and double checking everything I could think of.

Finally I took out the needle and rolled it on a flat. It had a wobble, not much, but just enough to cause a slight drag on the needle jet. I trued it up (delicate work) and all was well. These needle are soft and getting them in and out, as you well know, can be a feat in its self.

I do not know if this will help but it is an other ithem to concider.
 
Don't know anything about Mikunis in this application (replacing Amals) but at 7k feet you definitely need smaller main jets and less fuel overall. The fact that your bike came with 2 32mm Amals indicates that the problem is not with the carb size per se, since its max flow capacity is much less than a pair of 32mm Amals. It's strictly as air/fuel ratio issue that you need to sort out. On my 850 here in Mexico City - 7400 ft - I had to drop the Amal mains to a 240 from a standard main of 260. I don't know if that's any help and/or how that ratio would convert to Mikuni main jet sizes.
 
But it is not sucking thru both 32mm carbs at the same time so you can't really compare it like that.
 
"But it is not sucking thru both 32mm carbs at the same time so you can't really compare it like that."

You are right - that's a good point. With that in mind I'd say the carb is too large for the engine and you've lost too much velocity.
 
I'm running a 40 mm Lectron with no problems, I would think a 36 mm would be OK.
I still think you have fuel level issues.
 
I run a 36mm Mikuni VM single carb on my '74 850 with no problems. Even considering the altitude, I don't think carb size is the issue. If you can get it to idle, but can't get it to run smoothly once you actually start lifting the needle, it points to something clogged, or possibly a float level problem as previously mentioned.
 
Thanks for all of the replies and insights.
I have got all this weekend to play with it, So I'll pull the carb off tomorrow and dig into it.

1. Check float height
2. Switch out main jet (230 in it now, I kinda figured that was on the small side already but I have jets all the way down to 205, so I do have options)
3. Drop the needle

I'm going to shoot for leaning it out even more and see what happens. I will report back with my findings.
 
I think your main jet is waaayyy small. Bring that up to no less than 280. Remember main jetting is for WOT, so forget about the main for now.
All else will be getting the needle and needle jet just right. That is the real trick with the Mikuni's. 100's of variables. When it's right, it's righteous.
What needle and needle jet do you have in there now and what is in the 34mm? Although clip position does matter, it is for fine tuning, so let's just look at the needle and needle jet.
A stuck needle and seat in the float bowl doesn't seem right cause you would have gas pouring out of the tubes and onto the floor. Float heights don't change much by accident. I suspect that is ok, but should be checked.
 
make sure your battery is in good condition, low battery voltage will make a boyer surge.
 
Ok, So I couldn't wait until tomorrow - I took off early from work and here's where I'm at with it right now.

35 Pilot jet
260 Main Jet
6DH3 needle second notch from the top
Float level looks good

RESULT: bad stumble off idle , plugs fouled, still surging (it doesn't seem as bad but I didn't really go far ).

Next step,
I'm letting it cool down some before I dig back in.
(the woman is bitchin' at me because I now have the house smelling like gas and exhaust fumes - :mrgreen: )

35 Pilot jet
280 Main jet
6DH3 needle - top notch

We'll see what happens.

P.S. I do have a Boyer , The bike is always on a battery tender and charged up.
 
Unless you are going to start lookiing at this one point at a time you may as well take the carb off and start hitting it with a big hammer, it will give you more satisfaction.
Why are you putting a bigger main jet in then leaning the needle when the black plugs should already tell you you are to rich.
Try turning the lights on and see if it makes any difference
 
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