4 Strike Engine Refuses to Start

Timbone

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#1
You can see my project under the thread “New project idea”. I have pretty much finished the motorcycle/ motorbike. I should be doing my first rounds of road testing by now. But my engine won’t kick! I’m gonna need a really good idea.

The engine is a 140cc Briggs that I converted to horizontal use. I’m using the stock carb that I converted to run at 90*. I built a custom throttle body that converted 100% butterfly valve movement to equal full throttle input turn. No problems there. I have the fuel tank on and I have good fuel line.

Spark is powerful. Compression is good.

After my first set of tries (including starting fluid), I got nothing. No pop. No smoke. Engine feels right when I pull the starter cord. I’m using choke. Not using choke. Trying every throttle position. Nada.

I removed the valve cover. The rocker arm had unseated from the push rod on the exhaust valve! That’s gotta be it! I reseated everything, replaced the cover, tried again. Nothing.

So I removed the spark plug (checked for spark) and removed the valve cover. This engine is pristine! Slowly I spun the crank by hand, starting at TDC and working thru all four strokes. Just after TDC, exhaust valve opens then closes - back to TDC and slightly beyond when the intake valve opens and closes - both closed for TDC. Sure seems perfect.

I’m just using a tiny bit of gas - from my premix stock. I figure it should pop regardless, a little 2 stroke oil won’t matter much. There’s plenty of gas in the float bowl and I have blasted starter fluid into the carb throat.

Am I missing something?

Thanks in advance!
 


darwin

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#2
Tough one, do you have strong suction through the carb throat when pulling on the rope? Can tell you a lot about timing if the valves are functioning properly. Just an idea.
 

Timbone

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#3
Good idea. I’ll check that. My valve cam could be out phase though I thought I checked that before I closed the case. If intake suction is weak, yes, I have a more serious problem.
 
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#4
By your other post, I'll go as stock as I can with the motor (carb, exhaust, etc) then when you have it running, then start putting your desired parts back one by one being the carb the last one... the NT carburetor on the 4-stroke is known to be a pain in the ass... Good luck! I'm sure you will figure it out.
 

Timbone

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#5
That’s good advice. I removed the NT.

I’m thinking the timing cam is off 180 degrees. I’ll have to remove the motor from the bike, break the case, flip the cam, then remake a new gasket and seal it up again.

I’ll reapply oil after the gaskets set, then I’ll recheck the valves before attempting to start.

I’m gonna get this bike running!

Thanks for the ideas!
 

Timbone

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#6
Tough one, do you have strong suction through the carb throat when pulling on the rope? Can tell you a lot about timing if the valves are functioning properly. Just an idea.
Yes, I have great suction on the carb throat. In fact, while testing, I got the motor to pop three or four times. Still, to be sure, I cracked the case and triple checked the cam setting. Perfect!

I have to wait 24 hours for the gasket to set up. I’m thinking that the perfect combo of choke and throttle setting will get it going.

Tim
 

Timbone

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#7
As I noted in the project thread, I was able to start the engine a few times and it ran well. But it is still crazy hard to start!

I rigged up a 15/16” drill start and, though it has worked, the compression pushes my little drill to the limit. I’ve started the motor via pull, too, but I am batting something like 3 out of 500.

I’m using the stock carb and it seems to be flooding the cylinder with gas. I’ve ordered a new carb - something with some options for tuning - and we’ll see what that brings.
 
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#8
As I noted in the project thread, I was able to start the engine a few times and it ran well. But it is still crazy hard to start!

I rigged up a 15/16” drill start and, though it has worked, the compression pushes my little drill to the limit. I’ve started the motor via pull, too, but I am batting something like 3 out of 500.

I’m using the stock carb and it seems to be flooding the cylinder with gas. I’ve ordered a new carb - something with some options for tuning - and we’ll see what that brings.
What about the exhaust? have you checked it?
 

Timbone

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#9
Exhaust is fine. Clean as a whistle! This motor is brand new - the few times I started it it runs great. Not very loud either.

I have a new 4 stroke carb coming soon. I’ll properly hook up the throttle and we’ll see if things are different. My torque converter is on - I’m ready for road tests!
 

Timbone

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#10
I got the engine running today and, for the most part, it ran well. Couldn’t really get a good solid idle, though.

I bought a real adjustable 4 stroke carb and I welded together a pretty nifty adapter to fix that thing to the intake free of any air leakage.

Yesterday, after hooking up the throttle cable and fuel line, I added some fuel and took some pills. Backfiring through the carb? It was dark and I had no air filter attached so it looked pretty cool. A few pulls later and - whoa! - fire coming out of the carb! I freaked, but the fire didn’t last long. Still, I think my heart rate spiked to about 200 bpm!

So today, I removed the float bowl and examined the float. Absolutely perfect.

But the backfiring thru the carb told me something. At firing, TDC, air fuel mix had to be running back thru the intake valve. Valve adjustment time.

I had to take the entire rocker assembly apart just to be absolutely sure what was happening. Both valve clearances were super loose. I worked thru the cycle, stopping the piston about 1/4” down past the compression stroke. By eye, I narrowed the clearances.

Using starter fluid, the bike started and ran well. No backfiring at all! But I can’t get a start without starter fluid. In fact, I can’t start at all now - seems like compression is way up. Maybe the valve settings were changed. I’ll buy some feeler gauges and set my clearances properly tomorrow.
 

Timbone

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#12
Well, I was prepared for my first road tests this morning. Thoroughly inspected my carb (float bowl gasket is a bit off) but overall looks great.

Removed the valve cover and, with feeler gauges, I set the intake clearance at .005 and the exhaust at .007. Perfect.

Still no start. Can’t even get a start with starter fluid. It makes no sense.

I dug out my spark tester. Again, touching the block and pull starting I see good spark. BUT, the spark tester shows very weak spark! Tomorrow I’ll check my magneto gap (should be .012) and see if I get better spark. This would explain everything. Just a few minutes ago I ordered a replacement magneto.
 

Timbone

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#13
After several checks and fixes, I still cannot get the engine to start. So frustrating.

Kill switch disconnected.

As for my spark, my plug gap was only .019 - I regapped to .030.

Valve clearances set to spec.

Removed magneto and tested it on my multimeter. 4.6 ohms - right where it should be. There was ZERO clearance between the armature and the magnet. I set the clearance to .011.

I see spark on every pull with plug removed. Plug tester shows about 17kVolts, much lower than what I am used to seeing on China motor CDIs.

I ordered another magneto just in case.

On removal, plug looks dry. My carb is good, with excellent throttle slide movement. Good suction on intake.

The closest it comes to running seems to be with throttle wide open and no choke. I know that compression can suppress good spark but the engine feels right, and sounds right, when pulled. It’s popping one or twice, then dead. I can smell a faint stink of combustion fumes.

If I replace the magneto with new and get the same result, I don’t know what I’ll do.
 
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#14
My 3hp flathead briggs is giving problems too it’s on a rupp minibike. Ran fine when I first got it but then refused to run, it has an updraft carb I cleaned it out it back on and zilch. Motor is 30 years old but like I said it was running.
4E274BD9-E59D-4F18-BC7C-E2E8CD3C23EE.jpeg
FD962D1E-EAA5-4BD9-B1E6-5974ED5976C8.jpeg
 
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#15
After several checks and fixes, I still cannot get the engine to start. So frustrating.

Kill switch disconnected.

As for my spark, my plug gap was only .019 - I regapped to .030.

Valve clearances set to spec.

Removed magneto and tested it on my multimeter. 4.6 ohms - right where it should be. There was ZERO clearance between the armature and the magnet. I set the clearance to .011.

I see spark on every pull with plug removed. Plug tester shows about 17kVolts, much lower than what I am used to seeing on China motor CDIs.

I ordered another magneto just in case.

On removal, plug looks dry. My carb is good, with excellent throttle slide movement. Good suction on intake.

The closest it comes to running seems to be with throttle wide open and no choke. I know that compression can suppress good spark but the engine feels right, and sounds right, when pulled. It’s popping one or twice, then dead. I can smell a faint stink of combustion fumes.

If I replace the magneto with new and get the same result, I don’t know what I’ll do.
I know that sounds silly... but did you check for cracks on the cylinder? any kind of leak on it?
 

curtisfox

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#16
Well, I was prepared for my first road tests this morning. Thoroughly inspected my carb (float bowl gasket is a bit off) but overall looks great.

Removed the valve cover and, with feeler gauges, I set the intake clearance at .005 and the exhaust at .007. Perfect.

Still no start. Can’t even get a start with starter fluid. It makes no sense.

I dug out my spark tester. Again, touching the block and pull starting I see good spark. BUT, the spark tester shows very weak spark! Tomorrow I’ll check my magneto gap (should be .012) and see if I get better spark. This would explain everything. Just a few minutes ago I ordered a replacement magneto.
Isn't the mag gap suppose to be 20, we used to take a match book cover or a 3 x 5 recipe card and set the gap. Have you also checked the flywheel key? ............Curt
 

FNTPuck

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#17
Is this the original carb?

Between at first only starting with starting fluid, and now plug being dry after many failed starts, it leads me directly to the carb. Should have been flooding out if you were pulling it like crazy trying to start it.

Pull the bowl on the carb and see if its dry - you may have a stuck float or seat needle that is blocking fuel.
 

Timbone

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#18
Thanks to all for the suggestions. Had to do a brake job on my car last night so I’ll be back at this problem this evening. Very helpful!
 
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#19
In my mind, if it won't start on starter fluid then its not a fuel problem. Unless maybe its flooded already. I'd dry out the carb and cylinder, then try running it on starter fluid with out the fuel even hooked up to the carb. If that doesn't make it fire for a few seconds then I would stop thinking about fuel problems. Perhaps get a compression tester and check that. Sounds like you do your own car work too, a compression tester is a worthy investment.
 

curtisfox

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#20
Well, I was prepared for my first road tests this morning. Thoroughly inspected my carb (float bowl gasket is a bit off) but overall looks great.

Removed the valve cover and, with feeler gauges, I set the intake clearance at .005 and the exhaust at .007. Perfect.

Still no start. Can’t even get a start with starter fluid. It makes no sense.

I dug out my spark tester. Again, touching the block and pull starting I see good spark. BUT, the spark tester shows very weak spark! Tomorrow I’ll check my magneto gap (should be .012) and see if I get better spark. This would explain everything. Just a few minutes ago I ordered a replacement magneto.
Reading over again i think the valves should be .007 and exhaust should be .009, that's a fairly old engine. The old cast iron blocks was .008 and .0010. You may have to pull the flywheel and change the points, and condenser, points should be .0020
 

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