TOPIC:

I know its been covered before on the forum, but my Z13 is again refusing to start after she's been stood for a few days, but after a few minutes of hot air on the carbs from the wife's turbo hair drier, she quickly fires up on 2, then 4, then 6 - then settles into a normal tickover and runs perfectly: well, until she's left again for a few days

Pretty sure one of the wise sages from these hallowed pages (maybe Scotch or Kawboy) did provide an explanation previously, but I cant seem to locate it. Any advise genuinely appreciated as its really annoying and I would love to banish the gremlin once and for all. Oddly, if I remember to start her every 2 or 3 days it never happens, and once started the bike runs pefectly thru the entitre rev range.

Cheers Tim

My 2 cents- There's a setup problem with your choke linkage. I say this because there are 3 separate carbs but the common to all three is the linkage. If you had reported that it would fire on 2 or 4 cylinders and then the rest would eventually fire up, then I would suspect faulty (blocked) choke circuits. Something is causing the choke plungers to not open up enough to provide the enrichened fuel mixture. In a slimplified explanation, the choke circuit on these carbs is a little mini carb with rich jets. You either open them up or they are closed On a majority of "other" carbs, there's a butterfly valve that closes down the main throttle bore which causes higher vacuum on the main and idle jets richening the mixture. Keihin have always gone with the choke plunger type.
The other thing that can happen in situations as you''ve described, is too much air flow through the throttle bores which will starve the air going to the choke circuit. This could happen if the idle circuit has issues and in order to keep the bike running at idle, the throttle plates are open more than they should be.

The reason the hair dryer trick works supports the reason we have choke circuits in carburetors. Cool/cold air and cold fuel will not mix properly and the only way we can get this cold mixture to combust is to give it soo much gas and hope that some of it ignites. Once the air intake runners and the carb bodies start to heat up along with the cylinder walls and combustion chamber then the fuel mixture will heat up and homogenize and then total combustion can happen.

One test you could do to confirm the problem in the choke circuit is to get the engine up to running temperature and then with the engine at idle, open the choke circuit full open . If it doesn't flood, studder and almost die, then you know the choke circuit is the problem.

Last thing that comes to mind is the fuel source for the choke circuit. As I remember it ( and it has been a couple of years ince I delved into the carbs) is the fuel pick up point for the choke circuit comes from a port in the bottom of the float bowl and up through an enrichment tube in carb body. See this post Of my rebuild specifically post #6097. www.kz1300.com/index.php/forum/carburato...carb-issues?start=12 I found the port in the float bowl blocked with crud which I believe is caused by the ethanol in the fuel, mixing with water and settling in the bottom of the float bowls and then the aluminum in the float bowls oxidizing causing the white powder like build up. In this post you can also see the whitish residue on the choke circuit fuel pick up tube/jet.

John, thanks that helps to explains a lot. I had an issue ages ago with fuel contamination (caused by very fine blasting media in my tank not - introduced by me - long story)
Struck her up today and she started first touch today (as its only 1 day since last run) - got her fully warm and as suggested opened the choke, and she spluttered and flooded. But I'll re-check the choke circuit, I thought my carbs were spotless inside as she runs perfectly thru the entire rev range (well after she has started anyway), but I've probably missed a tiny blockage of crud and its causing the issue.

Cheers Tim

An additional comment regarding the Choke; Hard start can also be due to the choke cam-roller not dropping fully into the "valley" on the end of the cam. If it doesn't, the butterflies won't close enough to create sufficient vacuum to draw fuel into the choke circuit. Typically this can be done with an adjustment to the fast idle screw but there are 3 things to look for.
1) The roller is not touching the cam when the choke is OFF,
2) The roller is not riding partially "on the slope" when full choke is on. With the choke full-on the roller should be at the very bottom of the slope which ensures the butterflies are closing fully, and
3) If these two parameters can not be accounted for then check that the fast-idle cam is positioned correctly on the choke-shaft. On almost every set of carbs I've rebuilt I find the fast idle cam (and the Choke-plunger lifters) not aligned correctly because of multiple "divots" in the "V" of the choke rod. The pointed set-screw then goes into the closest "divot" which is not necessarily the correct one. This will position the choke-cam slightly high or low relative to the plungers and the other parts of the choke/fast idle set-up and can make the correct set-up difficult if not impossible. hope this helps

Try adding some injector/carb cleaner fuel additive to your next tankful of petrol. I always do this after my bike has been stored for the winter.

Hi Tim,

After finishing off the bike at the weekend the final job was to balance carbs and the usual tweaks. Well, the carbs refused to balance with all kinds of popping and back firing and the middle carburettor refusing to stop overflowing fuel into the air box. The carbs were on and off 3 or 4 times on Saturday with me finally noticing that the left carburettor throttle slide was sticking and not returning to the bottom position.

Cleaned throttle slides and bores, reassembled and bench set carbs by using Scotch’s paper strip method. Started bike up and couldn’t believe how close they are. Only the left carb was about 3 hg different to the other two carbs. Adjusted the left carb and the bike was very responsive and seemed to run perfectly. Tightened everything up and restarted bike. Ran ok for a few minutes and then a misfire occurred. Noticed cylinder number 5 was not running as hot as the other cylinders. Checked the plugs and it’s oviously running rich. Cleaned plugs and restarted and once again gran perfectly. Think I’ll order a colortune to carry out the final adjustments.

A friend called round today to have a look at the bike and obviously wanted to hear it running. It refused to start, choke on or off, it didn’t even cough. Will try to have another look later on this week as have it booked in for an MOT on Saturday.

Regards

Martin.