R75/5 w/Mikuni question

[8 Gauge]

First post on your forums. Glad to be here and thanks for your advice; in advance!

A little history on my machine: I inherited a '70 r75/5 from my grand father. Have been doing some minor tinkering with it for years and finally got her on the road last year. I rebuilt my Bings and had her running decently last year but kept having carb issues. Was pretty sure my enricheners weren't built properly and have since replaced them with Mikuni vm34s.

My understanding is that VM32s are the carb of choice and that jetting info is available (I have seen the info from Stans floating around)

My VM34s have the following: 210 Main 35 Pilot P-0 Needle Jet and 6DH4 Needle with a #2.5 slide

Bike starts easily (kick starter) but revs seem to stay high longer when throttle is let off and seems to sound a bit tinny. This makes me think that she is running lean. I think that I need to run a richer needle jet. Maybe go with a P-4?

One of the main reasons for swapping out my Bings is that I have much more experience with Jap carbs because most of my wrenching experience comes from 2 stroke dirt bikes.

Any insight on 34mm vs 32mm carbs and jetting would be greatly appreciated.

Thank you!

[8 Gauge]

Adding a bit of information as I scour the Internet for more knowledge. My right cylinder seemed off with my Bings and also seems off a bit with the MIkunis as well. With the filters off she coughs a bit of fuel/air out of the right side carb a bit. Usually only after i rev her up a bit and the revs come down. Nothing major here but enough to think i may have a different problem.

Im thinking that checking valve specs is the next step before i attempt to tune? Valvetrain noise is very quiet. The consensus that I'm getting is that a nice "tik tik tik" sound from the top end is normal. The process of adjusting valve clearance seems pretty straightforward.

Does my logic seem sound on this?

Thanks!

[Major Softie]

Adding a bit of information as I scour the Internet for more knowledge. My right cylinder seemed off with my Bings and also seems off a bit with the MIkunis as well. With the filters off she coughs a bit of fuel/air out of the right side carb a bit. Usually only after i rev her up a bit and the revs come down. Nothing major here but enough to think i may have a different problem.

Im thinking that checking valve specs is the next step before i attempt to tune? Valvetrain noise is very quiet. The consensus that I'm getting is that a nice "tik tik tik" sound from the top end is normal. The process of adjusting valve clearance seems pretty straightforward.

Does my logic seem sound on this?

Thanks!

Yes.

There's not a lot of knowledge here about Mikunis on Airheads, but you may get lucky and have one or two of the gang familiar with your setup. Then again, maybe no one.

The fact that you are still having a similar problem with the same cylinder definitely points to a cause other than the carbs. Checking your valves is always a good idea, but even before I did that, I'd switch your plug wires and see if the problem moved. If the problem switches sides, then you know it is in the ignition. If it does not, I'd still replace the plugs before I'd be certain the problem was not ignition related.

[Duane Ausherman]

The first year of those carbs (#1 & #2) and the next sets (#3 & #4) were problematic. They wouldn't stay adjusted for more than a few seconds. They would seem to have a gremlin inside. There is a mod that greatly improved them, but the carbs were replaced by tame ones in 1972 with the #9 & #10 variety.

I have no idea where you got information that the Mikunis are better, but the Bings are stable and reliable. I never understood the advantages of the Mikunis.

[Major Softie]

I have no idea where you got information that the Mikunis are better, but the Bings are stable and reliable. I never understood the advantages of the Mikunis.

If you read his post carefully, Duane, you'd see that he went for the Mikunis only because he had a lot of tuning experience with Mikunis, and none with Bings. There is something to be said for that.

[Motorhead]

Main 200
pilot 35
Needle jet P2
Needle 6DH3
2.5 slide

For 32mm so even with a 34 you'd be closer with this

[8 Gauge]

Thank you all for providing feedback!

Before I dive into tuning I am going go check valve specs and then move on to ignition timing. I have a hunch that my ignition system is on due to her running nicely for part of last year. I was pleasantly surprised that the bike had a ton of torque that came on real smooth. Plugs are new and i have spare bosch and NGKs laying around for the bike as well.

After that I'll richen up my needle jet and see what happens. Cross your fingers!

Will post an update after I dive in this weekend.

Thanks again!

[Motorhead]

YES set the valves check the static time setting have the carb cables as close as you can

when the carbs are even the motor will turn 3/10 a second faster than a Bing

I'm useing the 32MM VM's on my Home built 900cc /5 mix is good and bike run's great

Duane............ Mickuni's are cheap to replace over the bing and they are decent for the Money and set and forget
I alway try to make a better Beemer if one gets stuck and can't restore

No "O" rings to miss/fail or mistry to fix especally the choke...............SIMPLE

[8 Gauge]

Curiosity got the best of me and I pulled the valve covers. Didn't get the time to get out the feeler gauges, but by "eyeballing it" all valve clearances seemed reasonable. I was able to spin all the push rods and everything seemed tight and nothing seemed to be adjusted very far off from another. Will do a more scientific evaluation and make sure but I have a feeling that this is not my problem. Will a small amount of misadjustment cause a very lean condition?

I found further evidence of my hypothesis due to the plug on the offending cylinder showing signs of white on the electrode. Barely visable but the plugs were new and I didn't run it very long (nor put the engine under a load)

Next step checking ingition specs?

Thanks!

[Motorhead]

When ever I found a miss match in plugs with mixing it was carb balance that pointed to the problem

without a place to plug into the stream of vacume its hard for most backyard Mechanic to get them even

make it idle the same each.......... side should help the other

with the motor warm get one side to tick faster, just off idle! find the slow one and adjust the slow cable to come to the point of Motor comming to balance, then roll off and let it idle then when you roll the throttle on the motor should just roll to speed no coff or hiccup