Very weird problem.

[Captjacksparrow]

I'm working on a 98 Cat, ZL500. It runs beautiful, pulls normal and starts normal warm. It will not start cold unless you put fuel down the plug holes, then fires up first pull and runs normal with choke for a bit then stays idling and running normal. Been through the carbs and choke system, every thing is clean and good and choke set right. I thought I would just put a primer on it if it can't pull enough fuel cold through the choke. So I put a primer on it. Weirdly enough, makes no difference, 3, 4 or more primer shots, still won't start. put gas in the plug hole, fires right up. The primer is injecting the gas right at the carb book like every other primer machine. Never seen anything like it, just won't seem to pull gas from the intake unless it's primed through the plugs. I'm lost, reeds look ok too. It starts normal when in the warm garage too.........Anyone ever seen this before?

 

[gedakbx]

Bad pulse line to the fuel pump?

 

[JMCX]

The primer thing is certainly baffling. You know the choke lever has two positions? Kind of flipped over centre is full choke and straight up is half choke/high idle. The choke cable is adjustable as well.

 

[moyiesledhead]

The primer thing is certainly baffling. You know the choke lever has two positions? Kind of flipped over centre is full choke and straight up is half choke/high idle. The choke cable is adjustable as well.

Not so baffling. A primer on an old school piston ported engine injects the fuel more or less directly into the cylinder. On this case reed inducted engine however it needs to get past the reeds, then through the crankcase, then the transfer ports to get into the cylinder. Nowhere near comparable to dumping it down the spark plug hole. You're basically dumping raw fuel into the crankcase.

 

[greenthumb]

Not so baffling. A primer on an old school piston ported engine injects the fuel more or less directly into the cylinder. On this case reed inducted engine however it needs to get past the reeds, then through the crankcase, then the transfer ports to get into the cylinder. Nowhere near comparable to dumping it down the spark plug hole. You're basically dumping raw fuel into the crankcase.

Piston ported still goes through the crankcase. If the cylinder bore was ever exposed directly to the intake side, it would become an exhaust.


I know nothing about that engine, but the op's problem is definitely puzzling. The vapor alone from the primer fuel is pretty flammable. I assume you confirmed that the primer is actually working?

 

[MP Kid]

Reed valve motor is a one way air pump… any dribble of fuel on the intake/reeds/crankcase should eventually make it to the spark plug….
Why it won’t fire is a good question?

 

[moyiesledhead]

Piston ported still goes through the crankcase. If the cylinder bore was ever exposed directly to the intake side, it would also become an exhaust.

And there would be zero lubrication to the crank bearings too. Brain fart! Don't know what I was thinking.

 

[rsaint]

What is compression like?

 

[Ghfalls]

Fuel lines are good with no cracks?

 

[gedakbx]

Weak crank seals

 

[TROLLCAT]

compression test me is thinking its low

 

[Kibbels]

Weak crank seals

That’s what I was thinking.. Dan check that out!

 

[Captjacksparrow]

Already checked the crank seals, compression is 117 each, fuel pulse line is good as is the fuel lines, choke is set up as tight as it can be and still have the plungers close. I am using full choke. Primer is new and pumping full stroke. I put the primer on because it wouldn't start if it sat outside for more than 3 hours without putting gas in the cyl. I checked the pistons for a skirt missing, both look good. (If a skirt is missing it will loose some crankcase compression so may not pull as hard on the carbs.) If it's sitting there idling, I can stroke the primer 4 times, full stroke and the engine barely even idles down. I've checked with the carbs off and it's definitely pumping right into the bore of the carb outside the reeds. Reeds are good also. I've been doing this for 45 years and never seen anything like this before, LOL.

 

[Trashy]

Have you tried checking/replacing your floats in the carbs? I know they can over fuel running or not. Wondering if they could under fuel, until the engine was running?

 

[gedakbx]

Fuel pump good, not dried out or float levels low in the carbs. Sounds like fun LOL

 

[rusty]

pull the air box and try it again.and is the primer ports on the engine side of the carb?

 

[Captjacksparrow]

pull the air box and try it again.and is the primer ports on the engine side of the carb?

Yup, I put the primer ports right in the reed boss of the case. The floats are good and set right and carbs are clean and everything there is good. It starts good if it's warmer. like it can sit for 2 days in a shop and fire right up. If it sits outside for a day but it's warm, above freezing, it will fire up on choke. But if it's close to freezing or below, not a chance unless you put gas down the plugs. Then it fires right up and stays running. It runs great and pulls good otherwise. I can't imaging it being ignition related? Like a weak coil? If so it shouldn't start when givin gas though? I'm lost on this one.

 

[Rhodesie]

for some reason fuel is not being forced from the case to the cylinders. i know you said you checked seals but when its cold they could be sucking air. try having someone pull it over and give it a shot of ether in the carbs and see if it will light. in the cold the seals must be leaking. you have covered pretty much everything else.

 

[97summit]

Could it be the fuel pump? Maybe it has a leak and is draining the fuel out of the carb??

 

[mclean]

Kinked line that is more flexible when warm and stiff and pinched when cold