Coughing carbs

[SteveP]

I have Weber 45s on my 1700 xflow which also has a Kent 244 cam. Since I bought it 6 months ago, the carbs have tended to 'cough' at low revs i.e 2 - 2.5K. I had my head in the engine bay at the weekend and swear I saw what looked like a fine mist being ejected from the where the (left) carb joins the inlet manifold.

 

It is quite sporadic and doesn't feel like a misfire, indeed get the thing upto 4500 - 6000 revs and it positively sings with no hint of coughing.

 

It was recently rolling roaded and they commented that the timing and carburation were all fine.

 

Is this characteristic of a slightly wilder cam than the 234 standard, carbs out of balance, or something else?

 

SteveP

[John]

I used to have this on my x/f and it was cured by changing the air correctors to more suitable sizes at a rolling road. I suggest you try using another rolling road.

 

John

[philwaters]

I've always found that spitting back/coughing carbs being down to the balance. Certainly, that has always seemed to sort my 40's out - that and the popping over-run (which is a shame really!)

 

Phil Waters

2000 Zetec is in, waiting to start it up wink.gif

[Casbar]

There shouldn't be anything leaking from around the manifold. One way of checking is to spray some WD40 around where the carbs join the manifold, if the revs rise then you have an air leak, and should tighten the carbs up until it goes away.

 

Spitting at low revs can be caused by the slow running screws being slightly out. So re-set the slow running screws, these can also effect the carbs upto around 2500 rpm.

 

Fuel can be effected by cold and heat, so if its very cold and damp, the mixture can change, this could also cause a missfire.

 

Also check the balance.

[cdg]

I wouldn't worry: the symptoms you describe are very typical. It's all to do with a fairly aggressive cam which isn't designed to run at lowish rpm, and the effects of a fixed -jet carb. with big chokes. You could spend a small fortune trying to eliminate what is a minor problem, so my advice would be to keep it above 2500rpm and forget about it!

 

Chris.

 

 

[Simos]

When my car was RR'd by RK (and if memory serves) he pointed out (after jets had been drilled out to the exact sizes required) that the top end was a smidge too rich w.r.t the low end.

 

So you've a choice, tune the bottom end correctly, or, the top end. I chose the top

and the best correction was leaning the slow jets so the change carries through the rev range).

 

Whilst fixing the top end, it gives a tendency to spit at 1800-2300 rpm.

 

When it spits, I change gear. I figure if RK gives the choice of to spit or not to spit, those are the options !

 

Per the fumes, and i'm guessing, it's not from the crank case breather ?

 

(45's on a BD by the way)

[cdg]

There you go!

 

What did I tell you?!

 

Chris

[Roger King]

What size are your slow run jets? If this is occuring only on very light throttle, it may be due to weak mixture at this point. Light throttle mixture setting is more black art than science.

 

45s come as standard on slow run jets that are a little weak for the cam you are running.

 

It might also be that you could do with an extra progression hole. This is likely to be the case if the problem only occurs on VERY light throttle. 45s often need this mod, whereas 40s are generally OK in this respect.

 

Certainly out of balance carbs can only make things worse.

[SteveP]

Many thanks for all the replies.

 

Roger, unfortunately I have no idea what size my slow run jets are but yes, the spitting does tend to occur on a light throttle, floor it and there is no coughing at all, so it sounds like you've hit the nail on the head.

 

I'll check the balance of the carbs for sure and invest in some tinkering if I find it becomes an excessive problem.

 

SteveP