850 Interstate warmup issues

Hi all

I have a small issue with my Interstate 850 on startup that I hope you can advise me with.

The bike has always started 1st or 2nd kick no matter how long it’s been laid up.

From cold of course you have to start it with the choke, however it won’t run without near full choke until its well and truly warmed up (hot). Problem really is that if you let the bike sit like that or even just keep giving her throttle, then the plugs just soot up and she stops. So the procedure I’ve lived with for the past 2 years is to be ready to ride, start her up, blip throttle for 15 seconds, into 1st and go…..try not to stop for anything until she’s hot and running off choke. Once she’s happy off choke she runs beautifully.

I read a few posts recently on this forum and the Norton Owners forum, and what I’m experiencing here is not normal apparently.

So my thoughts are that maybe the carby is setup too lean and thus can’t run without choke until hot.

I pulled the carby off and this is what I found:-

Carby is Mikuni VM36 (marking punched in the side is 36S / 4

Float set to 18.3mm (spec is 17mm – 19 mm, so slightly high but OK)

P2 needle jet

Needle is 6DH4 --- but was set on highest notch….ie as lean as it would go

Pilot jet is a 20

Main jet is a 240

Posts I have read for commando 850 with Mikuni suggest:-

Main 300

Pilot 30

I have adjusted the airscrew as per recommendations for setting this up whilst bike was properly warmed up and idle set (1000 rpm), it does respond to adjustment but at the extremes of adjustment, which I read suggests wrong size pilot jet.

So, what I am planning on doing is ordering a few more pilot jets in higher sizes to try out.

I,m worried about the size of the main as well, the runs I,ve done with plug chops suggest it is rather on the lean side, so I’ll order a couple of larger mains as well. The needle will go back to middle position as well.

Sorry about the long ramble. Am I on the right track? What do you think?

Cheers

Norm

I know nothing about Mikuni pilot jet sizes but your symptoms are really close to mine when I fitted Premiers to my T120.
Richer pilots solved it.

But your issue could simply be a partially clogged pilot jet. Mikunis are susceptible to this.

Do Mikunis have a choke?
Clogged pilot jet or an air leak at the manifold.

maylar,
Mikuni carburetors have a Choke. CHOKE is stamped on the choke lever. It's an enrichment circuit.

Norm ,
Do what you plan on doing. It is the logical path. Maybe hold off on changing the main until you feel the cold idle issue is as you would like it.

I have dual FCR Keihin carburetion, but the bike won't idle stone cold unless I turn up the idle screw for a few minutes. In my case the inability for the bike to idle stone cold is intentional. I always tune for a warm engine, not a cold one. Keihin FCRs are not designed with a Choke. If it idled stone cold, it would be too rich for my tuning style.

If I am right the Mikuni has an enricher not a conventional choke like the mk2 Amal and you can get different jets for these as it sounds to rich , I had the same problem ,it sorted mine until I went with twin Amal’s.

The idle jet is too small, try a #40. The main jet is close, but try a 260. Needle is also close, try a 6DH3 in the middle groove (position 3). Needle jet is on the large side, try an O-8.

Float level is correct when the float arms (carb upside down) are parallel to the bowel seating surface.

Best.

maylar said:

Do Mikunis have a choke?
Clogged pilot jet or an air leak at the manifold.

Click to expand...

Mikuni apparently did not understand the term choke. They labeled the enhancer lever "choke". AFAIK, it's just like an Amal MK2. The enhancer uncovers a hole that adds suction to the pilot circuit - I think of it as a auto-tickler.

Air screw in too far/pilot jet too small/air leak/enhancer seal bad.

For cold starts the "choke" circuit enriches the the mixture by adding extra fuel, not blocking in coming air with is properly called a choke.

When the engine is dialed in you turn on the enricheners and shouldn't need any throttle. It is wise to get your engine off the enrichener/choke because you will be washing your bores.

Best.

Thanks for the replies guys, some good advice there. Yeah...I procrastinated a bit in original post about whether to call it a choke or enrichener....it is afterall just a plunger that opens up the enrichening circuit.
Pilot jets aren't that pricey so I'll buy a few .....25, 30, 35, 40. Get a few mains and the needle that was suggested. I enjoy messing with the bike and being retired have the time to do it, so all good.

Thanks for the input again guys.....invaluable advice.

Cheers

Norm