Predator 420 Carb Help

cml1of4

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So I recently upgraded my non hemi 420:

Arc rod
34mm mikuni kit from GPS
TWISTER 250 CAM
50lb Valve Springs
Header
40 series CVT

I have it geared and setup so it won't rev past 4500. Once I go to the next stage up, I will get a flywheel.

I think my issue is with the carb. I've been messing with jetting and no matter how lean I go, it still appears to be rich. I'm currently at a 190 main and 12.5 pilot jet. I ordered a 10 pilot but it isn't here yet.

I've seen people run these on stock engines so I'm very confused as to why I am having soany problems with it. The plug always comes out black, it has black smoke at idle. It falls over and bogs at WOT usually. The other day, I had a 195 main jet with the same pilot and it was running the best it had. I could actually get some WOT in.

Additionally, partial throttle-between. 1/4 and 3/4 feels amazing. I can feel the increase in power from the modifications and it pulls like a freight train.

I also tried a Chinese 32mm I got off Amazon. It was better at idle but I couldn't get much on the high end. Carb seemed a bit small for the engine.

I've checked valve lash, confirmed timing of the cam, changed to a fresh plug, checked the clearance between the coil and flywheel. I'm really at a loss right now as to what to do.
 
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RLS_Underground

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So I recently upgraded my non hemi 420:

Arc rod
34mm mikuni kit from GPS
TWISTER 250 CAM
50lb Valve Springs
Header
49 series CVT

I have it geared and setup so it won't rev past 4500. Once I go to the next stage up, I will get a flywheel.

I think my issue is with the carb. I've been messing with jetting and no matter how lean I go, it still appears to be rich. I'm currently at a 190 main and 12.5 pilot jet. I ordered a 10 pilot but it isn't here yet.

I've seen people run these on stock engines so I'm very confused as to why I am having soany problems with it. The plug always comes out black, it has black smoke at idle.
Definitely an excessively rich pilot, the extent suggests both the jet and fuel air screw are not right
It falls over and bogs at WOT usually. The other day, I had a 195 main jet with the same pilot and it was running the best it had. I could actually get some WOT in.
That's a lean pilot according to the symptoms
Additionally, partial throttle-between. 1/4 and 3/4 feels amazing. I can feel the increase in power from the modifications and it pulls like a freight train.
This says confirms the main is lean by maybe 2 sizes (#200)
The only reason youre not glasizing the plug grey/white is because an excessively rich pilot circuit will cause the main to be too rich at no fault of its own, and it masks the main being too lean the in the same sense.
I also tried a Chinese 32mm I got off Amazon. It was better at idle but I couldn't get much on the high end. Carb seemed a bit small for the engine.
A formula that has proven accurate tim and time again regardless if its for 2t or 4t is this
√(D)×2=Y
D being displacement solves for Y (optimum carb size)
SOOO
√(420)×2=40.98 So Realistically You should be using a 38mm carb minimum
I've checked valve lash, confirmed timing of the cam, changed to a fresh plug, checked the clearance between the coil and flywheel. I'm really at a loss right now as to what to do.
The issue with using a too small carb on an engine is that the intake signal at idle will be too strong leading to needing an abnormal and excessively small jet size to counteract the fact the engine is attempting to suck the bowl dry through the pilot... if the carb is far enough undersized it will do the same to the main circuit leading to an abnormally small jet and dropping the needle to delay the WOT jet response.

It's pretty much all the same but in reverse for a too big carb, the signal will be too weak and crazy large jetting will be needed to make the carb more sensitive to fact the carb isn't making enough vacuum to properly fuel itself.

It's all a balancing act between volume, velocity, and available vacuum.

The carb on my 208 is a bit too small (24mm should be 28mm), but its not so much that using a longer intake runner was ineffective.

A longer intake can weaken the intake signal as well as add a tunnel ram effect (yes that thing Chrysler developed 60's/70's)

A shorter one can also be used to boost the signal however it is far less effective than getting the correct size carb
 

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cml1of4

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Definitely an excessively rich pilot, the extent suggests both the jet and fuel air screw are not right

That's a lean pilot according to the symptoms

This says confirms the main is lean by maybe 2 sizes (#200)
The only reason youre not glasizing the plug grey/white is because an excessively rich pilot circuit will cause the main to be too rich at no fault of its own, and it masks the main being too lean the in the same sense.

A formula that has proven accurate tim and time again regardless if its for 2t or 4t is this
√(D)×2=Y
D being displacement solves for Y (optimum carb size)
SOOO
√(420)×2=40.98 So Realistically You should be using a 38mm carb minimum

The issue with using a too small carb on an engine is that the intake signal at idle will be too strong leading to needing an abnormal and excessively small jet size to counteract the fact the engine is attempting to suck the bowl dry through the pilot... if the carb is far enough undersized it will do the same to the main circuit leading to an abnormally small jet and dropping the needle to delay the WOT jet response.

It's pretty much all the same but in reverse for a too big carb, the signal will be too weak and crazy large jetting will be needed to make the carb more sensitive to fact the carb isn't making enough vacuum to properly fuel itself.

It's all a balancing act between volume, velocity, and available vacuum.

The carb on my 208 is a bit too small (24mm should be 28mm), but its not so much that using a longer intake runner was ineffective.

A longer intake can weaken the intake signal as well as add a tunnel ram effect (yes that thing Chrysler developed 60's/70's)

A shorter one can also be used to boost the signal however it is far less effective than getting the correct size carb
So is your overall assumption my carb is too small and that is causing my issues?
 

RLS_Underground

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As far as anything else I'm going g to guess you left the air screw at what ever generally random setting it was at...

Turn it all the way in to a soft seat and back it out 2 full turns... this is the default starting point.

You want an already up to temp engine, set the idle screw at 2-3 full turns out from soft seat for the tuning start point default.

While the engine is running you will turn the air screw out 1 turn every 30-45 seconds until the engine stops responding (it should increase in rpm each turn)
Keep count of additional turns, once it stops responding, you use the number of extra turn to jet down or up because the manufacturer tuning for most carbs (Dellorto, Keihin, Mikuni, etc) is the have the air screw set between 1¾-2¾ turns out, and not even an eighth more or less
 

RLS_Underground

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So is your overall assumption my carb is too small and that is causing my issues?
It seems to be the most likely considering you're talking about 10 and 12.5 pilot jets... thats way crazy small even after considering that Mikuni references their jets differently than Keihin does.

I can say I've ever heard of a Mikuni needing a pilot smaller than 15 tbh
 

cml1of4

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As far as anything else I'm going g to guess you left the air screw at what ever generally random setting it was at...

Turn it all the way in to a soft seat and back it out 2 full turns... this is the default starting point.

You want an already up to temp engine, set the idle screw at 2-3 full turns out from soft seat for the tuning start point default.

While the engine is running you will turn the air screw out 1 turn every 30-45 seconds until the engine stops responding (it should increase in rpm each turn)
Keep count of additional turns, once it stops responding, you use the number of extra turn to jet down or up because the manufacturer tuning for most carbs (Dellorto, Keihin, Mikuni, etc) is the have the air screw set between 1¾-2¾ turns out, and not even an eighth more or less
I should have mentioned. I've played with that air screw a bit. It hasn't solved the rich idle. I can get a slightly better idle, but still black smoke.
 

cml1of4

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It seems to be the most likely considering you're talking about 10 and 12.5 pilot jets... thats way crazy small even after considering that Mikuni references their jets differently than Keihin does
So why would the 32mm idle so much better than the 34mm? It seemed much cleaner and didn't die as much.
 

RLS_Underground

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So why would the 32mm idle so much better than the 34mm? It seemed much cleaner and didn't die as much.
There's always an exception to any given rule.

The formula is less rule and more a widely accepted suggestion that works well enough to say it hasn't really been proven wrong more than randomly by finicky engines (in our cases off label application)

Something else to consider is that round slide carbs (VM is round) are inefficient due to flow and pressure turbulence created by the pocket they all have in the cutaway which is also a potential issue. A low cut away will pull harder on the pilot than a higher one (I may have that backwards)
Cutaway tuning is totally a thing some advanced tuners do.

Flat and D-Slides (PWK and TM are D slide) create less turbulence making them not only more efficient, but they tend to have a much crisper throttle response
 

cml1of4

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There's always an exception to any given rule.

The formula is less rule and more a widely accepted suggestion that works well enough to say it hasn't really been proven wrong more than randomly by finicky engines (in our cases off label application)

Something else to consider is that round slide carbs (VM is round) are inefficient due to flow and pressure turbulence created by the pocket they all have in the cutaway which is also a potential issue. A low cut away will pull harder on the pilot than a higher one (I may have that backwards)
Cutaway tuning is totally a thing some advanced tuners do.

Flat and D-Slides (PWK and TM are D slide) create less turbulence making them not only more efficient, but they tend to have a much crisper throttle response
So what would you recommend as a solution? I'd like something cheaper to test with first before ordering another mikuni.
 

RLS_Underground

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So what would you recommend as a solution? I'd like something cheaper to test with first before ordering another mikuni.
Cheapest thing I can think of (assuming you have a welder) is making your own intake manifold to fit the carb and the port (mild taper will likely be required) it should help or hurt by enough either way to point you the right direction.
If it gets better you're running too small but you should probably expect to need to jet the pilot up a bit.

I'd say something around 6" (linear) with a 45 to 90° bend to assist getting a good swirl going.

I also forgot to point out (it was only implied) that you cannot accurately jet the main until you've gone through the carb procedurally starting with the pilot jet and air screw, and finishing with the needle clip position if that's even necessary
 

cml1of4

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Cheapest thing I can think of (assuming you have a welder) is making your own intake manifold to fit the carb and the port (mild taper will likely be required) it should help or hurt by enough either way to point you the right direction.
If it gets better you're running too small but you should probably expect to need to jet the pilot up a bit.

I'd say something around 6" (linear) with a 45 to 90° bend to assist getting a good swirl going.

I also forgot to point out (it was only implied) that you cannot accurately jet the main until you've gone through the carb procedurally starting with the pilot jet and air screw, and finishing with the needle clip position if that's even necessary
I'm going to make an intake like you described but I went back to the 32mm today and got it 95% there. It still bogs and dies if I mash WOT from a stop but as long as I'm rolling, I can go WOT. Its usually fine, but sometimes the revs dont climb immediatly and it feels a little boggy and then it pushes through. I leaned out the pilot jet and it idles great now, no black smoke. It also is reurning to idle nicely weras with the 34, i had to feather the throttle after driving. Sometimes, I see a little black smoke if I'm just on the throttle and putting around at under 5mph. It appears to me that it's still a bit rich at bottom and a bit lean in the mid range. That sound right to you? Also, the 32mm is a flat slide. I'm thinking it prolly just needs the needle moved up to richen up the mid range and maybe one size smaller pilot jet.
 

RLS_Underground

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I'm going to make an intake like you described but I went back to the 32mm today and got it 95% there. It still bogs and dies if I mash WOT from a stop but as long as I'm rolling, I can go WOT. Its usually fine, but sometimes the revs dont climb immediatly and it feels a little boggy and then it pushes through. I leaned out the pilot jet and it idles great now, no black smoke. It also is reurning to idle nicely weras with the 34, i had to feather the throttle after driving. Sometimes, I see a little black smoke if I'm just on the throttle and putting around at under 5mph. It appears to me that it's still a bit rich at bottom and a bit lean in the mid range. That sound right to you? Also, the 32mm is a flat slide. I'm thinking it prolly just needs the needle moved up to richen up the mid range and maybe one size smaller pilot jet.
Smashing it WOT is going to hesitate regardless, your main is too small based on the indicated behavior so leave the needle alone for now in favor of jumping a jet size or two up on the main.
 

cml1of4

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Smashing it WOT is going to hesitate regardless, your main is too small based on the indicated behavior so leave the needle alone for now in favor of jumping a jet size or two up on the main.
Ty for all the help so far.

I went up for the main jet and left the pilot. It's idling great and goes WOT mostly fine but it feels down on power. No smoke, backfires, or anything obvious like that. It does black smoke from about 1/4 to 3/4 throttle. I'm thinking I prolly want to lean out the needle setting. Any ideas on lack of power up top?
 

RLS_Underground

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Ty for all the help so far.

I went up for the main jet and left the pilot. It's idling great and goes WOT mostly fine but it feels down on power. No smoke, backfires, or anything obvious like that. It does black smoke from about 1/4 to 3/4 throttle. I'm thinking I prolly want to lean out the needle setting. Any ideas on lack of power up top?
Black smoke = rich period
Needle is responsible for the middle 3rd of the range, once the needle is clear of the jet it's all main nozzle.
 

cml1of4

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Black smoke = rich period
Needle is responsible for the middle 3rd of the range, once the needle is clear of the jet it's all main nozzle.
Ok. That's what I was thinking. I'll lean the needle out one step at a time and see what happens. Any ideas on WOT? It feels down on power but revs out fine. I'm not seeing any black smoke, no backfires, isn't bogging. I'm thinking it's maybe a little rich on the high end.
 

RLS_Underground

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Ok. That's what I was thinking. I'll lean the needle out one step at a time and see what happens. Any ideas on WOT? It feels down on power but revs out fine. I'm not seeing any black smoke, no backfires, isn't bogging. I'm thinking it's maybe a little rich on the high end.
Power drop is generally caused by being rich or lean.

If you increased the main, how many sizes up did you jump?
 

cml1of4

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Power drop is generally caused by being rich or lean.

If you increased the main, how many sizes up did you jump?
Ty again for the help. I had gone from a 130 to a 135 main. I just went to a 138 and it seems better. I'm going to mess with some other small changes but i think i have it from here. Im a little concerned the 32mm is too small. Im getting a drop in power at the high end but ill keep messing with jets. You've been most helpful. I also ordered a 38mm carb and am in the process of fabricating an intake.
 

RLS_Underground

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Ty again for the help. I had gone from a 130 to a 135 main. I just went to a 138 and it seems better. I'm going to mess with some other small changes but i think i have it from here. Im a little concerned the 32mm is too small. Im getting a drop in power at the high end but ill keep messing with jets. You've been most helpful. I also ordered a 38mm carb and am in the process of fabricating an intake.
I've ran into this before with cheap China copies.

Remove the bowl, remove the main jet, and then remove the Emulsion tube (aka main nozzle) and take a few CLEAR pics of it... I think I know exactly what the problem is with the 32mm carb specifically.

If you wanna try guessing what I'm thinking, take the Emulsion tube out of the Genuine Mikuni, take pics and also compare them to each other for yourself.
 

cml1of4

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I've ran into this before with cheap China copies.

Remove the bowl, remove the main jet, and then remove the Emulsion tube (aka main nozzle) and take a few CLEAR pics of it... I think I know exactly what the problem is with the 32mm carb specifically.

If you wanna try guessing what I'm thinking, take the Emulsion tube out of the Genuine Mikuni, take pics and also compare them to each other for yourself.
Will do. I'll take pics next time I'm in there. Prolly tomorrow.
 

cml1of4

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I've ran into this before with cheap China copies.

Remove the bowl, remove the main jet, and then remove the Emulsion tube (aka main nozzle) and take a few CLEAR pics of it... I think I know exactly what the problem is with the 32mm carb specifically.

If you wanna try guessing what I'm thinking, take the Emulsion tube out of the Genuine Mikuni, take pics and also compare them to each other for yourself.
 
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