Jet problems....

[Bickers]

Mornin All...

I'm trying to track down an over fuelling problem on my 1973 Honda 500/4.

I have found that the main jets are 98's and the Clymer says the standard bike had 100's.

My question is would the differance in the jet sizes make a huge differance to the amount of fuel going through, or just a slight one?!?!?

I took the carbs off because after changing the plugs and doing a 15 minute journey, the new plugs were covered in soot. I have adjusted the floats and returned the mixture screws back to factory standard, but before I rebuild them I though I'd check if it would make much of a differance and whether its worth fitting the 100 jets back in.

Cheers

Bickers

[Bickers]

I know its not a performance bike, but it was in 1973....

[cagimaha]

The main jet does not affect the running much unless you're at full throttle so I doubt it is the cause of the symptoms you describe. Also the larger the jet size number, the more fuel it flows so the 100's would only exacerbate the problem. It's way before my time but IIRC the 500/4 runs a bank of 4 keihin roundslides? If so there are a couple of things I would check. First check is that the air filter is in decent condition, over-oiled filters will cause a rich mixture as well as dirty or blocked filters. Next check none of the airways are blocked, either spend some time with a can of carb cleaner and some plastic bristles off a brush checking all the orifices you can find or shell out to get the carbs ultrasonically cleaned. It's probably worth checking the following points before shelling out for the ultrasonics in case it's another problem. Next check that the needles aren't worn by biting them gently with your front teeth and sliding the needle between them (I'd remove any petrol traces first, they don't taste so good) and if you can feel even the slightest of notches then you need new needles. The next thing to check is that the emulsion tubes aren't worn which is usually evidenced by a mark where the needle seats in them. Generally though if the needles are all right then the emulsion tubes will be, this assumes that the needles haven't been changed at some point though... The emulsion tubes are usually pressed into the carb body so can sometimes be a bit of a pain to remove and because they're brass they are easily damaged so go gently.

If the carbs are CV type ones then all the above points still apply but you should also check that any pressure balance hoses aren't blocked and that the diaphragms are in good condition. Check you can lift each slide by hand and it should return smoothly. Hope this helps, Ben.

[cagimaha]

Just another thought... It is possible that the problem is ignition based rather than fuelling. A weak spark would cause symptoms similar to the ones you describe, sluggish performance and fouled plugs. It's worth a check anyways.

[Tennis]

How was it running before the plug change?

[Bickers]

More like a triple than a four, stuttering between three and four cylinders, when I first put the new ignition in it ran like a classic dream, untill about fifteen minutes when the plugs got coated up and it went back to running like a pig.

Thats the only reason that I checked the carbs before thinking the timing was out... I have changed the plugs, coils, leads and caps. It had recently (100 miles ago, but 15 years ago as its been stored for a while) had electronic igniotion fitted aswell.

The slides run freely and they are a bank of 4 keihin roundslides... 10 points to that man!

At least that rules out the jets and leaves me a few things to check before I rebuild them.

Also I reset the float levels as the 4th cylinder was dropping all of the fuel in the float chamber everytime I stopped, all four were well out, but that wouldnt cause bad running would it?

Thanks again, not only does the advice I get on here usually sort out my problems, but I learn a hell of a lot in one paragraph.

Cheers

[cagimaha]

The float height can actually be pretty crucial on some older carb designs. Basically if the float needle valve lets too much fuel in some of it will come out of the overflow but some of it will also be forced up through the emulsion tube due to the pressure of all the fuel in the tank pushing down on it causing an over-rich mixture. I'd give the carbs a quick once over with some carb cleaner but if the float heights were well out then I suspect that will be the issue. Fingers crossed for the easy fix! Glad I could return the favour and help someone, Ben.

[lorenzo]

Just as avery basic rule of thumb, your pilot jets will do starting and the first 2000 rpm, needles and float heights will sort your mid range then main jet size will effect your wide open throttle top end. So yes, float height is pretty crucial in the grand scheme of things.