Tuning Troubles

['72-750]

Hi all,

I have been reading for the last 12 months whilst rebuilding my 1972 Honda CB750 and have found some great advice on here. However, now I am after something more specific to what I am experiencing...

I recently fired up my hog after a full rebuild (engine incl. bearings, pistons and rings, valves etc, full carbs with seals, jets, needles etc). I have out pod filters and a 4into2 exhaust system onto it. I have put #40, and #110 jets in, set the needles to the middle, i have set the floats to 26mm.

What i am experiencing is smoke out of the left hand side. #1 cylinder has been putting out a small amount of a black liquid on the plugs, i dont think its oil but more sooty fuel (i hope).  I took it for a ride though a think this is starting to clear but it is still producing excessive smoke (seems to be white...?).

It idles ok at low rpm but fluctuates in rpm so i havent been able to set it to its 'happy place'. When I took it for a ride it rode good at low and mid rpm but when I tried to get up her is just bogged down, carried on and lost all power...

Not sure if this is all related but thats what ive got. I sync'd the carbs with a manometer and statically set the timing. I dont have a timing light or a compression test kit so i can't go much farther here...

I would like to get it on a dyno at some stage but for now I would love to just get it running good enough so I can run it in and get a feel for it...

Any help is appreciated.

thanks

tim

[Re-run]

Well black smoke indicates a rich condition. How far are your air screws turned out?  

[zedx]

Don't mean to steal the thread but I have white smoke coming out of mine does this indicate a lean condition?

['72-750]

I have tried running them one turn out, to 2-1/2 turns out with mixed results. It doesnt seem consistent to point to this to me...

How much impact would the needle adjustment have here?

[Re-run]

Well first off, white smoke generally means condensation is being burned off. Not uncommon and should stop usually after a couple minutes but can always happen again. Depends on the moisture level where you live.

The manual states the default setting for the air screw. It is best to set it to that.
When you worked on the carbs, were they bench synced? If the sync is fairly of, that could explain the lumpy idle.

If it will stay running, you might want to hook up some vacuum gauges and see where each carb is at.

The needle basically controls the transition to the main jets. You could lower that by raising the clip 1 space.

How are the other 3 cylinders doing?

[TOOLS1]

On your running problems it sounds like you have three things going on (I am assuming you have the roundtop carbs) First the 110 main is too lean for pods, and aftermarket exhaust, try a 120. Second, sync the carbs!!! Third, you might have a vacuum leak. Re tighten the clamps on the carb boots when the engine is hot, sometimes this will help.
TOOLS

[Re-run]

Well, he was getting black smoke out of #1. now, it is possible that the plug was fuel fouled and he just got a couple puffs of black. He should check his plugs though to be sure if he is lean of rich.

It sounds like most of the issue he is describing is taking place at idle or lower rpm, so mains wouldn't be an issue yet. They are probably too small once he gets closed to wot though.

['72-750]

So,

Main jets changed to #120. Needles lowered one notch, all plugs cleaned, mixture screw set at 1.5 turns out and Carbs sync'd again.

Result, black soot is not showing on cylinders 2-4 now which is good. Seems to idle a little better.

However, #1 cylinder is producing some thick black oil type liquid all the way through the exhaust. It is still blowing smoke accross low to mid rpm range (havent opened it right up yet), and i have changed my mind and decided that the smoke is not white at all... Its black and isnt overly thick but is pretty consistent...

Im going to go and grab a compression tester and hope its not piston rings letting oil through. If it is oil, wouldnt it burn it off rather than distibute it through the pipes as an oily film? Is there anything else that could be causing this? The plug looked a lot better this time though, I'll take it for a ride and see if it isnt just residual oil still in the pipes from previous...

['72-750]

So,

The compression is consistent accross all cylinders at about 110psi which is low. I am at a bit of a loss here. #1 cylinder does not seem to be firing as it is obvious the left exhaust is not as hot or putting out as much exhaust as the right (running a 4 into 2 system). And there is no heat discoloration on the #1 exhuast header where as the other three are showing signs of heat. #1 is also still blowing smoke, but if it doesnt seem to be coming from oil past the rings (as the compressions are all consistent), where could this be coming from?

I have re-checked timing, valve tapet clearances which are all ok so i am not sure what else could be contributing to the low compression. The pistons and rings were changed, and the cylinders honed and I have only put approximately 60 miles on it since rebuild....

[rich]

It takes a while for the rings to seat so you could have a little blow by for some time. And if you are using an automotive type gauge to read the compression it could be giving you a lower reading because of the diameter of the lines and fittings.

I wouldn't fault the compression readings at this point. They're all the same, which is good.

How about the plug wire and plug cap on #1? Can you swap the coils around to see if the problem moves to another cylinder?

Have you been using the same plug all this time? Move it to another cylinder and see if the problem moves with it.

All that's left is the carb and carb boot.