carb help

[wkoepke]

will the carbs from a 1200 work on my 1300?

[Dano]

Squeeze? RandyA? Condor? One of you guys gotta know! Come on, fess up!

[Yammer Dan]

There is a different vacum hookup on the 83 but not sure about the others.

[pegscraper]

Yes, they will all interchange. That vacuum hookup on the '83s should be connected to manifold vacuum anyway and the carb connection plugged off.

[GaryZ]

Pegs,

 

Can you tell me how to route the vacuum lines on a non-California VR?

I want to remove the California vapor recovery system . . .

[pegscraper]

I don't know enough about the 1st gens to tell you much specific. Remove it and plug any open vacuum ports. The boost sensor unit that manages the spark advance should be connected to manifold vacuum and not the source above the throttle plates that was used on the '83s. (Boy I can't wait until a certain well known moderator reads this one. Be assured, he'll jump up and down and throw a fit. Expect it, and ignore it.)

 

wkoepke, if you don't already have the 1200 carbs, you might look for a set of VMX carbs. You'll get a 35mm throttle bore instead of the stock 34mm one. The choke linkage will be a little different, but will also interchange.

[Condor]

Yes, they will all interchange. That vacuum hookup on the '83s should be connected to manifold vacuum anyway and the carb connection plugged off.

 

This is the first time I've ever heard of this. I know the pressure sensors are the same for all years, but the TCI's which control the advance are probably programed differently between the 83 and 84.. I'm ready to learn something. Could you explain???

[pegscraper]

Sorry, I've been busy and been gone. Does anyone know what specifically is different between the '83 and '84 TCIs? Spark advance curves can be the only answer. They don't do anything else.

 

Have you heard the term "ported vacuum" as opposed to manifold vacuum? This stunt started being done on cars in about the late '60s. Manifold vacuum level gives a good read of engine load, and the vacuum advance unit on the side of a distributor or the pressure sensor on this bike will vary the spark advance based on engine load. Ported vacuum is much the same as manifold vacuum, except its source is above the throttle plates. When the throttle plates are open, the vacuum levels between the two are much the same. But at idle when the throttle plates are closed, the manifold vacuum source remains high while the ported vacuum source drops to zero. Less vacuum retards the timing, so at idle, with a ported vacuum source, you have no load based spark advance. With manifold vacuum controlling the thing, you have maximum spark advance at idle. So the difference between ported vacuum and manifold vacuum is - what do you want the spark timing to do at idle speed?

 

At idle speed or constant cruise speed, ie. low engine load, the throttle plates are closed or nearly so, air/fuel density is low which burns so slowly that it requires the spark to be ignited earlier to actually burn at the proper time in the crankshaft rotation. Under a high engine load, the throttle plates are open further and air/fuel density is high which burns much quicker and does not require near so much spark advance to burn at the proper crankshaft position. Under heavy throttle, spark timing should be retarded. Under light throttle, constant cruise or idle, spark timing should be more advanced.

 

The idea behind using a ported vacuum source for vacuum advance was to retard the spark timing at idle, which would increase the exhaust temperature at idle speeds which supposedly makes the emissions cleaner at idle, but this method of trying to do that actually makes emissions worse anyway. Also, retarded spark timing makes the engine burn gas less efficiently which makes the engine run hot and hurts in town gas mileage where you spend time at idle speed. I say do the engine a favor and let it run better at idle. Connect the pressure sensor to the manifold vacuum source and plug off the ported vacuum source. And I also say do this to any '60s or '70s or '80s car that was set up this way. Clear as mud?

[GaryZ]

Connect the pressure sensor to the manifold vacuum source and plug off the ported vacuum source. And I also say do this to any '60s or '70s or '80s car that was set up this way. Clear as mud?

 

I have played with mild hot rod Chevy's since the early 1970's and always prefer manifold vacuum. The possible problem with a fixed ignition like my VR would be detonation. On Chevy's I can adjust initial timing to minimize detonation . . .