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It's a hassle, but swapping around the carbs to see if the problem moves with them is useful to see the problem is carb related or not.

 

If it is, it's going to be a little work to sort out. Lots of trial and error.

 

One of my Mikunis was misbehaving once, same issue as you - fine on heavy throttle, but not firing at idle. I tried to be methodical in my approach and nothing worked. What did work was stripping the carb to the body, submerging it in water, then blowing air from a compressor through each passage. The thought here being that the water in the passages will grab more detritus while being blown out. In this case I rebuilt the carb afterwards and it worked perfectly. Presumably some dirt was blocking one of the pilot jet passages.

 

Good luck!

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It's a hassle, but swapping around the carbs to see if the problem moves with them is useful to see the problem is carb related or not.

 

If it is, it's going to be a little work to sort out. Lots of trial and error.

 

One of my Mikunis was misbehaving once, same issue as you - fine on heavy throttle, but not firing at idle. I tried to be methodical in my approach and nothing worked. What did work was stripping the carb to the body, submerging it in water, then blowing air from a compressor through each passage. The thought here being that the water in the passages will grab more detritus while being blown out. In this case I rebuilt the carb afterwards and it worked perfectly. Presumably some dirt was blocking one of the pilot jet passages.

 

Good luck!

 

I fear perhaps I might have to go down this route. i also don't have filters on my carbs yet - could dirt and crap have gotten in and clogged some bits up? Thanks for the advice!

 

I feel like I'm always taking two steps forward and one step back, haha

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I fear perhaps I might have to go down this route. i also don't have filters on my carbs yet - could dirt and crap have gotten in and clogged some bits up? Thanks for the advice!

 

I feel like I'm always taking two steps forward and one step back, haha

At lest you have 66% of your engine running, mine is at 0%.

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So it looks like it's just Cyl5 now, so it idles on 5cyl.

 

Removing the spark plug does nothing on this cyl. I replaced all my plugs and issue persists. When I removed plug 5, I noticed that it smelt of fuel and that it was a bit wet around the ring.

 

Does this mean it gets fuel, but no spark?

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So it looks like it's just Cyl5 now, so it idles on 5cyl.

 

Removing the spark plug does nothing on this cyl. I replaced all my plugs and issue persists. When I removed plug 5, I noticed that it smelt of fuel and that it was a bit wet around the ring.

 

Does this mean it gets fuel, but no spark?

 

There is an easy way for checking spark. Pull the plug and and attach it to the wire. Start the engine and ground the plug against the valve cover or chassis. If you see a spark between the center electrode and strap than you have spark. Be careful no to get too close to the spark, getting shocked won't be a lot of fun.

if you are not getting spark check your gap. Also what plugs are you running? Good luck! I have been following this thread for a while. Love the build! 

 

https://youtu.be/mUOmsGiirTU

Edited by theczechone
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There is an easy way for checking spark. Pull the plug and and attach it to the wire. Start the engine and ground the plug against the valve cover or chassis. If you see a spark between the center electrode and strap than you have spark. Be careful no to get too close to the spark, getting shocked won't be a lot of fun.

if you are not getting spark check your gap. Also what plugs are you running? Good luck! I have been following this thread for a while. Love the build! 

 

https://youtu.be/mUOmsGiirTU

 

I checked that and I get spark. The Cyl5 plug is wet and covered in fuel, whereas the others aren't. 

Still, unplugging Cyl5 does nothing to the engine and it still idles with 5 cylinders. I'm thinking maybe something is stuck (like the idle jet maybe?) and it springs to life once the mains kick on. Not sure why the plug would be wet, though.

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Hi, Just a thought. If you are getting a spark at the plug and fuel is present, Check that the valve adjustment is correct. If the valves are not closing all the way you will have a miss on that cylinder. If the valve clearances are correct do a compression test in that cylinder to see if you have a bent valve that is not closing all the way.

David

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If you swap the plug and wire to another cylinder, does the problem move with it? That should at least rule that out.

 

Any chance it could be the distributor? I'm inexperienced with these things so I might not be of much help here.

 

I've tried swapping plugs and it's only that cylinder. Haven't tried swapping wires, but the wires are brand new MSD spark plug wires. 

It could be the dizzy, but I don' think the issue is there once the car is under load above 3500-4k RPM, which is weird. The cap looks good, but I'm gonna try replacing that next.

 

Hi, Just a thought. If you are getting a spark at the plug and fuel is present, Check that the valve adjustment is correct. If the valves are not closing all the way you will have a miss on that cylinder. If the valve clearances are correct do a compression test in that cylinder to see if you have a bent valve that is not closing all the way.

David

 

Compression check comes out healthy and strong. If the valves were an issue, I'd imagine the issue would be more noticeable at higher RPM, but it seems to only be an idle issue. It actually runs much better at higher RPM, which makes me think it's an issue with the idle in the carb since the mains take over right about that mark.

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I've tried swapping plugs and it's only that cylinder. Haven't tried swapping wires, but the wires are brand new MSD spark plug wires.

It could be the dizzy, but I don' think the issue is there once the car is under load above 3500-4k RPM, which is weird. The cap looks good, but I'm gonna try replacing that next.

Just because the leads are new doesn't mean they can't be faulty.

An easy one to test by swapping 2 leads around

You could also try turning the dizzy around by 60degrees and moving all the leads around 1 hole so that cylinder is on a different post. Will be a quick way to test the cap and rotor for misalignment.

 

Semi-related story, I had an issue with an rb engine that would foul cylinder 5 all the time. Tried everything I could think of.

Turns out the fuel pressure regulator was leaking through the diaphragm and the vacuum fitting was opposite cylinder 5 so the fuel was just running in while it was idling.

Edited by fraz
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Or you can test the ignition leads by grabbing your multimeter and put one probe on each end of the lead to see what the ohm resistance is - if it's infinite or much higher than the other ignition leads, then you know it's faulty.

 

I wouldn't be shy about swapping one carb over with another though, and seeing if the problem moves with it. It's not a lot of work and will let you know whether the issue is air/fuel related, or spark related. Even brand new Webers have been known to have issues - quality control isn't what it once was...

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I checked that and I get spark. The Cyl5 plug is wet and covered in fuel, whereas the others aren't. 

Still, unplugging Cyl5 does nothing to the engine and it still idles with 5 cylinders. I'm thinking maybe something is stuck (like the idle jet maybe?) and it springs to life once the mains kick on. Not sure why the plug would be wet, though.

 

IMHO,  the Red wording tells you the issue. Your low speed circuit on the carb is likely plugged. Either the Low speed jet or a bit of debris in the low speed circuit. Plug is wet because Ignition can't fire the Lean mixture. There is still fuel in the mixture, but too lean to initiate combustion at idle. Hence... a wet plug. 

 

You have already cheeked spark and compression on #5. You reported both are good. That leaves fuel as the culprit.

Edited by Chickenman
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Just another thought. Have you considered a defective plug on  Cylinder #5?  Rare but it happens. You may have a carbon trace on the outside of the plug. Check it carefully. A plug can fire at atmospheric pressure, but fail when put under combustion pressures. Throw a fresh plug in or switch from another cylinder. 

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  • 2 months later...

Ok, it's been a while since I gave an update, so here it goes!

Every wire in the car, from front to back, has been redone, replaced or discarded. The car runs on what is needed. No heater, no fan controls, no radio, no antenna switch - not even the fuel light was spared. The car runs on only what's needed, nothing more and nothing less. 

ijncejTl.jpg

 

The issue with the idle and pilot jets is resolved, too. Idles and runs on all six cylinders now! I'm not sure what did it, but I replaced my filter, cleaned my tank, cleaned out the carbs as best as I could and messed around with the idle mixture - and eventually it worked!

 

I've spent many hours now diving into every piece of literature I can find on DCOE carburetors and I've surely learned a lot to the point where I feel very comfortable working on these guys. What I couldn't find much on was what parts of the DCOE family (Weber, Mikuni, OER, Solex, etc.) is interchangeable - specifically talking about jets.

What I learned is that Weber idle jets work as long as you get the appropriate seats for them. Mikuni Air Correctors work on these carburetors, too, and fit right into the emulsion tube. The mains, however, only seem to fit with OER ones. I went through a lot of different setups to find something that works for this engine and so far the perfect recipe seems to be:

Mains: 135

Air: 160

Idle: 65(OER)/65F9(Weber)

I am currently running 62.5 pilots, but it's a tad bit too lean when cruising. i've tried 70F8's and 70F9's and they're both too rich. My guess is that 65's should fit in perfectly, but they're still on their way from Japan since I'm going with the OER one. The F9 designation on Webers is the same as the OER.

ALwwi81l.jpg

 

I also got a wideband hooked up now so I can get proper readings on this thing. At idle I'm currently sitting at 12.7-13 AFR, it goes up to 14 when cruising under 2kRPM (should richen up a bit with the 65's) and I get in the mid 12's at WOT, which seems to be the sweetspot for this engine.

UteyEAKl.jpg

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  • 4 weeks later...

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