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VHB27 carb (stock jets) on a stock motor but with sacsche iginition (I've tried points as well) and stock exhaust.

It cranks up easily with choke but I need to immediately shut off choke as it starts popping. Once choke is off, it will idle perfectly. The throttle hangs and takes a while to come back down to idle, which I thought would mean I need to richen up the idle circuit. Backed the mixture screw all the way out, and it go better....ok, so I went up a few sizes on the idle jet and it was fine for a while but I'm still backed all the way out on the idle mixture screw and throttle is still hanging a bit. Also getting small hiccups when holding throttle and riding.
I broke out the colortune and I can see I'm running super rich through the whole range. Getting black smoke from the exhaust when revving as well.
Went to adjust the valve lash, just incase, and the intake valve had basically no clearance so I fixed that. Same fuel/air issues persist.
Oh, and the idle speed drops quite a bit as I ride and warm the bike up.
Am I going crazy? The bike definitely runs rich but the throttle still hangs and gets way worse if I lean in the idle circuit.

Side note: I had a new flip lever choke on it and went back to the stock one, just incase that was a factor. It was not.

Afaik there never was a VHB 27 DellOrto on any 350 w/c Scramblers. All had a VHB 29. Solely the 250 SCR had a different size, VHB 26.

Hans

ducwiz wrote:Afaik there never was a VHB 27 DellOrto on any 350 w/c Scramblers. All had a VHB 29. Solely the 250 SCR had a different size, VHB 26.

Hans

I have a SS1 29D laying around from my Mark 3 350 D...that should work until I source a VHB 29, I suppose. Bummer, I wonder why mine had a 27 bolted up. I do have a 250 Monza that it might work on.

Just for reference: http://www.micapeak.com/vintage/carb.html

Extracted from there:


Hans

ducwiz wrote:Just for reference: http://www.micapeak.com/vintage/carb.html

Extracted from there:

Code: Select all

Model           Year      Dell'Orto type   Size   Needle   Main   Pilot   Slide   Needle pos.   Needle jet
350 Scrambler   1968-69   SS1 29D          29     M14      125    50      100     2             265

Hans

You were right. A local friend had a VHB29 laying around for me. Got it closely jetted to the stock numbers and it already runs almost exactly as it should. No longer running overly rich with weird characteristics! Bonus, it's a pumper variant and that works pretty dang well!
Thanks for pointing that out, Hans. Was scratching my head over this one.

Went for a ride yesterday and it's running well through the rev range and idles great. Still have a pretty bad hanging throttle when I come back down to idle. I bumped the pilot jet to a 50 and the hanging throttle still persists for a few seconds before it settles down to idle. Now the idle is rich, of course.
Just double-checked the exhaust valve lash. It's a little tight but I can get a .010 feeler gauge. I have a green/white painted cam, so that number matches. I also replaced the intake rubber between the carb and head but I'll double-check for vacuum leaks again.

Oh fun. There's a vacuum leak on the new intake rubber. That'll explain the issue.

Hi Blaine

a known problem . most of times a worped intake manifold from over tightening .

Eldert

Eldert wrote:Hi Blaine

a known problem . most of times a worped intake manifold from over tightening .

Eldert

Yup. You're correct. It was a warped intake. Had a few spares and got one bolted up. I'm able to get down to a 45 pilot jet now and the throttle returns great. Running great on the bench. I'll be able to test ride tomorrow but it was running fine, minus the returning idle on yesterday's ride so I expect it will be just fine.
That was a BIG ol leak. Brake clean would kill the motor instantly.