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TOPIC: Dealing with the hard start issue

Dealing with the hard start issue 11 months 4 weeks ago #20241

  • scotch
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My ‘80A can also be difficult to start from time to time, when it's sat for many days. We think everything is set-up perfectly and yet, "it's not perfect". If it was - they would start quickly every time after sitting for any length of time. This is one of the most common problems with 1300’s that until recently I couldn't understand, despite having some experience with these carbs.
We know the fuel-bowls dry-out quickly. We also know that all the fuel galleries that deliver fuel/mixing-air are all above the fuel-level and will be void also. This includes the choke circuit.
Vacuum is required to "pull" the fuel up and through the delivery galleries and to the venturi.
So with a well charged battery providing adequate cranking rotation speed, “Why are we not getting fuel to the engine”?
Specifically - Why isn't the choke circuit delivering "starting fuel"?
Assuming the carbs have been thoroughly flushed (Cleaning Tool), the idle mix-screws are set, float-level is correct (Brass Floats anyone?), choke plungers, fast-idle cam synchronized correctly, fast-idle (cam-roller) is sitting correctly, ignition and valve tolerances within spec. and carbs are sync'd ......then what the hell is the problem?
There’s only one conclusion I can rationalize: Insufficient vacuum to lift the fuel - unless you're jump-starting and spinning the engine like crazy! …….and even then it can take considerable time to get some flame!
So if it's a lack of vacuum; where is the vacuum break? (Note: I’m not saying vacuum leak. There’s a difference!)
There are the idle-air and high-speed mixing-air inlets. (Front of carb.) Then there are the "enrichment" valves which supply additional metered air to the fuel galleries. (Unless closed by high vacuum upon deceleration). These are all metered internally but non-the-less are a vacuum-break.
Then there’s our choke circuit! When the plungers are lifted they are supposed to allow a greater amount of fuel (relative to the idle circuit) to pass into the large delivery port on the engine side of the throttle-plates. It's a logical assumption that this should be a High Vacuum area! If there is no choke fuel being drawn up efficiently then where’s the vacuum that should be providing the fuel lift? And let’s keep in mind that most 1300’s have lots of miles, have known ring-sealing issues and for many other reasons will not be “factory-tight”
If you look into the bottom of the choke plunger-well, you'll see three large ports. Two (facing the engine side) that deliver choke-fuel to the intake manifold. The other (facing the air-filter side) is the same diameter and height above the choke plunger seat. This port sources air from the (atmospheric) underside of the slide-diaphragm. With the throttle plates closed at full-choke- the need for some additional mixing air seems logical. I'm calling this port the “Choke Mixing-Air” port.
However: When the plungers are lifted they expose these 3 ports the same amount, simultaneously. And these ports are huge! This is a major vacuum-break and it defies logic – to me.
To see if my theory had any validity, I restricted the 3 “Choke Mixing-Air” ports. The photos are self-explanatory. Silicone and brass tube. This is a 75% reduction in size so it makes sense that the vacuum applied to the choke circuit from the Intake stroke should be significantly increased by the reduction in the size of the “Choke Mixing-Air” port.
Before re-installing the carbs I spun the engine over about 10-12 seconds to clear any residual fuel/air remaining in the cylinders. The bowls and carbs were already dry but compressed air made certain that there was no fuel/ vapor remaining in any of the delivery runners, and then the carbs were reinstalled with the fuel supply left OFF.
The bike then sat completely fuel-dry for almost 3 full days.
I would normally expect to have to do some serious cranking to get it to start.
Turned on the fuel – listened to the bowls fill and when the “gurgling” stopped I applied FULL choke and went for the start. The engine started immediately – In less than two revolutions at which point I reduced the choke and the engine settled into a smooth 1200 RPM warm-up idle No “carb. Adjustments” were required FUCK, YA !
I’ll keep y’all posted !


1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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Last edit: by scotch. Reason: clarification detail regarding the ports referred to

Dealing with the hard start issue 11 months 4 weeks ago #20242

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The German specialist in 6 cylinders starts the reluctant bikes by turning the idle screw down, using normal choke. When engine starts, he just turns the idle up again. This supports the lack of vacuum you have discovered. I am plain lazy and just give my lady a squirt of quick start.
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Last edit: by Kawboy.

Dealing with the hard start issue 11 months 4 weeks ago #20243

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Scotch d'you think they deliberately erred on the side of caution with this enrichening, in order to minimise risk of flooding on cold-starts?

I'm thinking (from a manufacturers' perspective) that grumbles about slow starts & batteries may be a better risk than hydro-locks and bendy rods.

Must say though, excellent deductive reasoning Mr. Holmes!
1979 KZ1300 A1
1999 K1200LT - now sold, I like them however was nearly as big as my X5 but no faster.
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Dealing with the hard start issue 11 months 3 weeks ago #20248

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Thanks for the feedback Lare & RC. Certainly going to be monitoring this. Admittedly - one "test" start does not definitively prove/guarantee the proof of concept but the first result given the scenario/explanation indicates it worked. I was not exaggerating in any way about getting the immediate start. AND; I was VERY reluctant in even posting this, as such. But "Holy Cow" (no offense to anyone), I was truly and purely amazed! And it takes a lot these days !
R/C - Hydro-locking is a result of fuel leaking past a faulty valve, and loading a cylinder. With or without this modification the choke system can not create a H/L situation.
A) Remember; ALL the fuel feeds are ABOVE the fuel level even if the bowl is puking fuel !
B) The mod is delivering choke-fuel adequate for starting but can only do so when the engine is being cranked-over which precludes the concern.
But a very legitimate concern on your part - you' are thinking !
LareN - Perhaps someone on your side of the pond is in a position where they could also try this and provide some feedback. Don't need to explain further about it's simplicity, it's non-destructive and easily reversible ! You have a large contingent "over there" and are obviously very adept at maintaining these machines.
This would be very valuable to me personally and to the idea, itself.
For what it's worth - It took me 1.5 hours to re and re the carbs to do this - plus a sync-check. So it is NOT complicated. And once on the bench the lids can be removed without separating the carbs. Just remove the choke lever tension screw and the idle adjust "knob".

I have no theory's further to what I have explained regarding the intended design specs for "The Port".
I'm not concerned about having it too rich to start because the moment the (my) engine starts with or without - I have to reduce the full choke to a partial to control the engine speed, which as mentioned, settles into a warm-up idle of 1200 RPM. At this point my choke-plungers are closed with the "roller" just about to leave the fast-idle cam. By the time the temp. gauge starts to move I have removed that last "bit" of choke lever and the Idle is back to being controlled by the idle-adjust "knob"

I feel like "Scrooge" on Christmas morning !
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !

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Last edit: by scotch.

Dealing with the hard start issue 11 months 3 weeks ago #20257

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I'm new to the reviving of old kz1300s. I owned my first one in 1982 and gave it up in 1985. Now, I'm much older and have 4 1300s with only one sort of running. It starts fairly well when cold. Not great but it starts and once started it runs great. However, once the engine temperature gets up and I shut it off, it won't start. I've pulled the plugs and they are dry. It isn't getting fuel. The bike is a 1982 (manufactured 12/81) with 4,200 miles. It could be vacuum or a faulty fuel solenoid (the solenoid is clicking but that doesn't necessarily mean that it is working correctly. Any thoughts?

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Dealing with the hard start issue 11 months 3 weeks ago #20259

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Hey Moto. Start by removing the fuel solenoid. Don't feel compelled to reinstall it ! I doubt that's the problem. If it's getting sufficient fuel to to start and run until you park it ? then won't start....It's likely a problem with how the carbs are set-up. That's a small Novel in itself.
Start reading. There's been lots discussed here - you just have to take some time to find the topics.
Try to avoid multiple "Adjustments" With out a lot of experience trying to adjust "everything" at once will be a gong-show. One at a time is best or you'll never understand the specific cause of the problem, may be simply masking it and creating another issue elsewhere. That can be frustrating down the road - so to speak.
At this point - not knowing anything about your bike it's impossible to narrow things down further then this. Let us know what you've done and the results. In short time we'll have enough info. to start really helping. Relevant pictures are always useful. cheers
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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Last edit: by scotch.
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