Spitting Carbs, two possibles? Opinions please........

[berni29]

Hi There

I have just put the head back on my +2, and the back carb is spitting nastily when the engine is reving. The exhaust has new gaskets. Could it be an exhaust air leak (as I remember Keith mentioning as a cause of spitting) or could it be arcing between the plug leads (which I think might be happening because I can hear a slight clicking every so often at low speeds). The leads are the old copper ones. I always seem to get a bigger spark with them.

Any opinions or other likely culprits?

Mnay thanks

Berni

[ppnelan]

Berni,

I assume you checked the valve timing, and the valve clearances are correct?

If you think the sparks are 'leaking' out of the HT leads (I've had this), try putting something (e.g. bits of hose) around them to space them away from any metal parts of the engine (especially bits with sharp corners). And don't go for huge plug gaps which will increase the HT lead voltage - I was told (by your friend Mr. K.F.) that the SIZE of spark is irrelevant.

Matthew

[lotuselan2]

Berni
I do not believe my recent "scenario" is the same as yours but after seeking advice in all the right places including this board I did not uncover to problem. My problem was that ALL four carb throats were spitting equally. It would idle smooth but any small amount of throttle and they would spit like crazy and engine would die if not returned to idel immediately. After checking all the "normal" and recommended items (valve timing, ignition timing, bad gas(?), etc. I was close to giving up. I found that my enclosed SuperTrap exhaust had come undone and was blocking 99% of my exhaust outlet, just enough gas would pass to idle. I had the "old potatoe up the exhaust pipe" trick.

Since you have only one cyclinder at issue your problem is not the same as mine. But do have a look to be sure the exhaust valve on the bad cycliner is opening properly.
Ken

[ppnelan]

P.S. Make sure there aren't any paper or cloth plugs still in the ports or exhaust manifold - you know, the ones you put in to keep dirt out while it's all in pieces?! Believe me, they don't help with smooth running...

Matthew

[lotuselan2]

Matthew
That brings back an old memory. My FF2000 motor had just come back from an engine rebuild and managed to overheat in about 30 seconds. After the second attempt I found a plastic plug shoved into the water pump outlet to keep the dirt out (and water too)! So Murphy does rule sometimes.
Ken

[schroeder]

Is it spiiting at revs or on overun between revs? One cylinder or two? Could it just be carbs are out of balance? Check flow rates on each. I have experienced a bent throttle spindle that made it imposssible to balance. Unless you have messed with the carbs it is unlikely to be the spindle but it could be the balance between carbs is off.

If 1 cylinder is it 1 or 4. If 1 perhaps a headlamp vacuum leak. If 4 perhaps a servo vacuum leak.

Stu

[berni29]

Hi

Many thanks for the suggestions. I am pretty sure that there is nothing in the exhaust, and the valve clearances should be correct. I will measure them again now that it is all running. All the spitting is on Nos 3 & 4. It happens mainly on constant revs. Not on blipping the throttle or so much on overrun. Fire comes out a bit so its pretty impressive. The carbs are balanced OK with one of those synchrometers.

Thanks again

Berni

[rgh0]

Berni

Go to the files section in yahoo group "Sidedraft central" and get a copy of Keiths tuning white paper. Then follow its steps and you will identify the cause and cure your problems. Otherwise go to an experienced twin cam dyno tuning place at pay for them to do it using their equipment

Rohan

[berni29]

Hi Rohan

I have not been on the Yahoo for many a month, but off I go! Many thanks for the suggestion.

Berni