Posted Friday, January 29, 2010 @ 03:55 AM  

I think you all know both my RF4s have 1400cc motors with Zenith 28RXZ carburetors. Because of developing my inverted running mod among other things, I seem to have had more than my ‘fair’ share of carb related problems, so here is some info for others so cursed.

I have experienced the following symptoms:

If the engine starts and runs fine at idle, but will not give any power when you open the throttle even a little; it just coughs and splutters and strangles. That’s almost certainly a serious blockage in your main jet.

Turn off the fuel cock in the cockpit.
Remove the carb’s front cover / filter housing / plenum. That exposes your main jet, which is the brass thing with a screw head at bottom front right of the carb (bottom left as you are looking rearwards at it). Cut the locking wire (if fitted) and, holding a wide-topped glass container under it to catch the 20cc of fuel from the float bowl, unscrew the main jet.

Standard main jet size is 1.25 mm diameter, although many of them removed from our three carbs have turned out to be bigger than that in reality. So, gently insert the butt of a 1.25mm or smaller drill bit and see if something appears. If so, problem solved.

The next set of symptoms is that you can start, idle and taxi out, but when you open the throttle fully for take-off the engine immediately stutters and stumbles. That’s still a blocked main jet, but the blockage does not completely obstruct the jet. Procedure as before.

Next symptoms: you can start, idle, taxi out, and when you open the throttle fully for take-off the engine gives full power, but after two seconds (count them) the engine surges, farts, burps etc. If you throttle back to half throttle the engine seems to run fine at reduced power, but as soon as you open the taps again it falters.

That is not a main jet blockage, but a restriction of flow from the bottom of the float bowl into the main jet’s small chamber below it.

Initial procedure as before, but now you need to undo the two screws holding the float bowl to the top of the carburetor. You do NOT need to undo anything else, and you certainly do not need to remove the carb. Once you’ve undone those screws, you can lower the float bowl and look inside it. Remove the float, its pivot and the needle valve it operates and you should see the obstruction. If not, blow compressed air in through where the main jet screws in. Reassemble. Test fly. Job done.

Final symptoms: you start your take-off, but after five, ten or so seconds the power starts to fade, eventually with the engine stumbling as before. This is a restricted fuel flow into the carburetor float bowl, which holds 20cc – I thought I’d calculated that as theoretically enough for 13 seconds at full power, but now I can’t do the sum.

I hope that is of use to you all for further trouble-shooting.

Posted Friday, January 29, 2010 @ 08:37 AM  

Here's some more trouble-shooting advice from British Fournier expert, Dave Bland.

Hi Bob

With your spluttering, have you tried the obvious? Touch the exhaust pipes and see if one is cold (dead plug) then try again with a new plug fitted.

It sounds similar to a problem that I recently sorted. This guy’s carb had obviously been bugger'd about with by someone, as he too had thought it a fuel problem when it was in fact his HT leads and magneto.

If you have not tried it already, run the engine to the point that all the coughing and farting starts then simultaniously kill the mag and shut the fuel valve. Drop the float bowl and see what you find, if it is empty your engine is drawing more fuel than the inlet (‘needle’) valve can supply and you will need a bigger one or to adjust the float.

Someone had put a huge needle in this Stromberg CD150 carb, but left the standard 1.75 inlet valve and hence the float bowl was emptying through the proverbial 1" pipe while filling from a 1/2" pipe and surprise surprise it sucked more fuel into the engine than could get into the float bowl and at 2700rpm it coughed and farted.

Although I only found this after replacing the plug leads that were breaking down and the condenser in the mag which was also failing which caused similar symptoms but at 2000rpm.

As nothing had changed with the carb, I don't know why he spent 2 days messing up the carb and charging lots of money before suspecting the ignition.
However, you say that you have enlarged your main jet so I would suspect that you have upset your carb in a similar way, re my suggestion.

If you do find the float bowl dry I'm afraid I can't help as I don't know the standard Zenith 28RXZ carb.

If the float bowl is full and the filters etc are clean then it’s time to check the ignition, but be patient. Remove, clean, gap etc plugs and mark them 1234/ABCD whatever, and refit. Run up until missing occurs and let it fart for a minute or two, then shut down and go home. (If you take the plugs out hot you will bugger up the threads in the head.)

Next day remove the plugs and look for any that are unusually sooty and record the result, clean the plugs and refit them in different positions and repeat the procedure. If you find the plug in the same pot/s sooty suspect that lead of
breaking down under load.