JRC PWK issue

Are ya having a problem with your rig? We'll try to help. Share your tech tips and experiences here. Dr. Billy Glaser, author of the "Unofficial 750 Ural Service Manual" site myural.com, is moderating this section.
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GSPearson
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JRC PWK issue

Post by GSPearson » Thu Mar 14, 2019 10:20 am

Reposted here because I’m a dumbass and posed in the wrong section.

Howdy Y’all,

So, I got some JRC’s. Tapped the right side so I can use my balancer. Currently got them running 40 pilot, which looks good on the colortune plug, and the air screw setting is within the correct range according to the JRC tuning guide. It came with a 127 (if I remember correctly), which was far too lean. Moved up 135, and it runs fairly well. If I barely blip throttle, it’s as responsive as the CVK’s. If I slowly bring it up, it’s also fine; however, if I quickly go from 0-1/4 or 1/2 throttle, I continually get a bog and popping. So I richened up the needle length, no change. Checked for air leaks, have found none. I figure I’m barely getting into my main jet, so I move up to 140’s; same problem.

The valve clearances have been adjusted within the past 1000km, but I plan on checking them again.

I’ve had the notion that it might be an ignition timing problem. My thought is: since these slide carbs react much more quickly than the diaphragm carbs, is there a chance that my fuel delivery is somehow not matching up with my ignition timing? If so, wouldn’t that mean you’d have slightly retarded timing in relation to the carbs. However, if that were so, wouldn’t I be getting too much fuel when I open the throttle that much and that quickly? Does this train of thought jive with reality?

I’m going to go ahead and check the ignition timing anyway, but I thought I’d pick y’all’s brains for a possible cause.

Anyway, thanks in advance (pun) for the help!!
2007 Gear-up arctic
JRC PWK pilot 40, main 140
K&N air filter

Newer style front timing cover and oil filter

jeffsaline
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Re: JRC PWK issue

Post by jeffsaline » Thu Mar 14, 2019 11:01 am

I've got no experience with the fuel setup you are asking about... but that sounds like the idle mixture is a bit lean. Maybe try to richen each air/fuel mixture screw about the thickness of the notch of the screwdriver slot and then test ride it. Continue until the bog is gone.

Best,

Jeff
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GSPearson
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Re: JRC PWK issue

Post by GSPearson » Thu Mar 14, 2019 11:23 am

jeffsaline wrote:
Thu Mar 14, 2019 11:01 am
I've got no experience with the fuel setup you are asking about... but that sounds like the idle mixture is a bit lean. Maybe try to richen each air/fuel mixture screw about the thickness of the notch of the screwdriver slot and then test ride it. Continue until the bog is gone.

Best,

Jeff
Yes I would think so as well, but the colortune shows it to be perfect if not a touch rich. But I will try your suggestion.
2007 Gear-up arctic
JRC PWK pilot 40, main 140
K&N air filter

Newer style front timing cover and oil filter

rivers
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Re: JRC PWK issue

Post by rivers » Thu Mar 14, 2019 11:52 am

I'm not familiar with your carbs either. My basic rules for trouble shooting carbs is first "insure buy checking" valves(cold motor) and ign are in spec. As in don't assume/guess/hope they are are cause they were 1000km ago. Trying to tune out a valve/ign issue with the carbs just results in seriously whacked out carbs. Now you have 2 problems to sort out. Also fine tuning carbs needs to be done on a hot/fully warmed to normal op temp motor. Best of luck, keep at it.
Joe
014 GU
"I'm lost but I'm making good time."

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Msblu79
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Re: JRC PWK issue

Post by Msblu79 » Thu Mar 14, 2019 12:44 pm

The JRC carbs may be to "quick" and the intake ports & cam timing can't deliver air fast enough. The CVK carbs react much slower and there is time for air flow to match the fuel delivery. You may have to "live" with it and remember to open the throttle slower as its possible there may not be any carb adjustments to help the intake/air flow situation. :(
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Modtop airbox, 2013 branch pipes, Perch rings, Drilled slides, Gossie needles, 145 mains, 45 pilots, insulated manifolds, Modtop pushrods, Dual FMF exhaust, PA Multi-spark system w/ VOES, Spin on filter conversion, 33mpg, Royal Purple fluids.

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Re: JRC PWK issue

Post by cateyetech » Thu Mar 14, 2019 6:48 pm

GSPearson wrote:
Thu Mar 14, 2019 10:20 am
Reposted here because I’m a dumbass and posed in the wrong section.

Howdy Y’all,

So, I got some JRC’s. Tapped the right side so I can use my balancer. Currently got them running 40 pilot, which looks good on the colortune plug, and the air screw setting is within the correct range according to the JRC tuning guide. It came with a 127 (if I remember correctly), which was far too lean. Moved up 135, and it runs fairly well. If I barely blip throttle, it’s as responsive as the CVK’s. If I slowly bring it up, it’s also fine; however, if I quickly go from 0-1/4 or 1/2 throttle, I continually get a bog and popping. So I richened up the needle length, no change. Checked for air leaks, have found none. I figure I’m barely getting into my main jet, so I move up to 140’s; same problem.

The valve clearances have been adjusted within the past 1000km, but I plan on checking them again.

I’ve had the notion that it might be an ignition timing problem. My thought is: since these slide carbs react much more quickly than the diaphragm carbs, is there a chance that my fuel delivery is somehow not matching up with my ignition timing? If so, wouldn’t that mean you’d have slightly retarded timing in relation to the carbs. However, if that were so, wouldn’t I be getting too much fuel when I open the throttle that much and that quickly? Does this train of thought jive with reality?

I’m going to go ahead and check the ignition timing anyway, but I thought I’d pick y’all’s brains for a possible cause.

Anyway, thanks in advance (pun) for the help!!
Hey GSP
Did you try what suggested on the other thread?
(you know the mods can move a thread, you don't have to delete it :) )
If so did it help?

:foilhead:
Charlie ╭∩╮(-_-)╭∩╮
2003 Gear-Up Ромашка Мзй
765cc - Mikuni TM33 carburetors - Modified stock airbox with Uni foam filter
2-1 exhaust - Hand worked cylinder heads - Type V ignition system
A Warn XT17 witch that works from any angle - More handmade parts than I can list :foilhead:

The only Ural to post a second gear wheelie :evil:

2000 Patrol Teh Урал
Thanks DaveO, I'll try to ride it like you, I'll try :foilhead:

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Eric N
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Re: JRC PWK issue

Post by Eric N » Thu Mar 14, 2019 8:03 pm

Not sure the advice you got in the other thread. Flat slide carbs that don't have an accelerator pump will have a lean condition if they're quickly opened. They're generally race applications where the bike's gonna be and stay at wide open throttle. When you open them up, they let in great air, but the vacuum takes a moment before it starts pulling the proper fuel. That's why carbs like a TM33 pump a squirt of fuel in that scenario.

I'm not aware of any adjustments that will solve the fast throttle lean spot on a non-pumper carb. You could put a smaller pilot and a different needle in, shift the fuel curve up a bit onto the needle, but increase in air without instant matching increase in fuel = temporary lean.
2018 Gear Up
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GSPearson
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Re: JRC PWK issue

Post by GSPearson » Sat Mar 16, 2019 4:47 pm

cateyetech wrote:
Thu Mar 14, 2019 6:48 pm
GSPearson wrote:
Thu Mar 14, 2019 10:20 am
Reposted here because I’m a dumbass and posed in the wrong section.

Howdy Y’all,

So, I got some JRC’s. Tapped the right side so I can use my balancer. Currently got them running 40 pilot, which looks good on the colortune plug, and the air screw setting is within the correct range according to the JRC tuning guide. It came with a 127 (if I remember correctly), which was far too lean. Moved up 135, and it runs fairly well. If I barely blip throttle, it’s as responsive as the CVK’s. If I slowly bring it up, it’s also fine; however, if I quickly go from 0-1/4 or 1/2 throttle, I continually get a bog and popping. So I richened up the needle length, no change. Checked for air leaks, have found none. I figure I’m barely getting into my main jet, so I move up to 140’s; same problem.

The valve clearances have been adjusted within the past 1000km, but I plan on checking them again.

I’ve had the notion that it might be an ignition timing problem. My thought is: since these slide carbs react much more quickly than the diaphragm carbs, is there a chance that my fuel delivery is somehow not matching up with my ignition timing? If so, wouldn’t that mean you’d have slightly retarded timing in relation to the carbs. However, if that were so, wouldn’t I be getting too much fuel when I open the throttle that much and that quickly? Does this train of thought jive with reality?

I’m going to go ahead and check the ignition timing anyway, but I thought I’d pick y’all’s brains for a possible cause.

Anyway, thanks in advance (pun) for the help!!
Hey GSP
Did you try what suggested on the other thread?
(you know the mods can move a thread, you don't have to delete it :) )
If so did it help?

:foilhead:

Yes I tried richening the pilot mixture. I’ve been busy the past few days. Still plan on checking valves and ignition timing, but I’ve now had not only a couple folks on here mention that a flat slide needs an accelerator pump but also a couple mechanic friends. I will report back. There’s not a whole lot of tuning info on here with regard to the PWK.
2007 Gear-up arctic
JRC PWK pilot 40, main 140
K&N air filter

Newer style front timing cover and oil filter

rivers
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Re: JRC PWK issue

Post by rivers » Sat Mar 16, 2019 6:02 pm

As Eric mentioned previously, flatslides are pretty much for WOT racing applications. Not designed for street use where you run at various throttle openings. Do able with small slow throttle moves but large/quick will be miserable. Why on an 07 750 did you decide to go with these carbs? Hopefully not thinking you'll make a Ural faster. :( Best you can do with a Uralis fine tune it for smooth streetability.
Would have been a LOT easier dialing in the stock CVK's. If you still have the stock carbs I'd put my time and labor into those.
Joe
014 GU
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cateyetech
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Re: JRC PWK issue

Post by cateyetech » Sat Mar 16, 2019 10:50 pm

What I posted, before it deleted :(
Try turning the pilot air screws in 1/8 turn
check it and try it again
These type of carburetors with out accelerator pumps
will require a slower pull


If richening the mix helps and the main jet is the correct size (or very close)
then changing the main jet air bleed may required (that's why it's not a fixed orifice)

All mechanical carbs require appropriate modulation, epically high flow ones
even round slide carbs.
That's why at the shop I recommend a performance kit on the stock Keihin CVK 40mm
carbs vs S&S replacement (butterfly type carb w/ accelerator pump)
You can overmodulate :lol: the throttle :shock:
If you jerk it wide open then roll off you will feel the bikes power increase :?
With CVK's you just request more throttle, like vacuum secondary car carbs vs mechanical ones


:rant:
FLAT SLIDE CARBURATORS ARE FOR RACING/ WIDE OPEN USE

What type of racing is WOT?
Drag racing and (maybe) flat track (they hit the kill switch to set the bike)

Motocross / Supercross / Road Tracks / Hair scramble (woods racing) require a LOT of throttle modulation
and flat slide carbs are on almost all of these bikes

Screenshot (34).png
Screenshot (35).png
I guess all these were RACE RIGS :lol:

All the disinformation :( very sad, very sad :shock:

Contrary to popular opinion you can make a Ural run
faster or really stronger, HP is what limits you top speed :P

Hey GSP

Are you going to leave us hanging on this thread like you did on the engine noise thread?
just saying

:foilhead:
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Charlie ╭∩╮(-_-)╭∩╮
2003 Gear-Up Ромашка Мзй
765cc - Mikuni TM33 carburetors - Modified stock airbox with Uni foam filter
2-1 exhaust - Hand worked cylinder heads - Type V ignition system
A Warn XT17 witch that works from any angle - More handmade parts than I can list :foilhead:

The only Ural to post a second gear wheelie :evil:

2000 Patrol Teh Урал
Thanks DaveO, I'll try to ride it like you, I'll try :foilhead:

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cateyetech
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Re: JRC PWK issue

Post by cateyetech » Sat Mar 16, 2019 10:52 pm

Oh yea
None of those Soviet carbs have a accelerator pump :)

:foilhead:
Charlie ╭∩╮(-_-)╭∩╮
2003 Gear-Up Ромашка Мзй
765cc - Mikuni TM33 carburetors - Modified stock airbox with Uni foam filter
2-1 exhaust - Hand worked cylinder heads - Type V ignition system
A Warn XT17 witch that works from any angle - More handmade parts than I can list :foilhead:

The only Ural to post a second gear wheelie :evil:

2000 Patrol Teh Урал
Thanks DaveO, I'll try to ride it like you, I'll try :foilhead:

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Re: JRC PWK issue

Post by Eric N » Sun Mar 17, 2019 1:20 am

GSPearson,

You might try going down a carb size and see if that reduces your issue. 32mm carbs don't all have the exact same flow qualities, flatslide will flow more than the CV's. Your 32mm flat slide might be too large airflow-wise for an otherwise stock setup. The bigger = better isn't always true and oversized carbs will be more temperamental than correct sized carbs. A smaller size will have less air volume for the lean pocket. You may get better throttle response/ tunability with no loss in performance.

I feel the 32mm CV's that came with my 2001 were too large for the 650 engine. They're ok for the 750, and I think they'll be better once I finish the 825cc conversion.

You're using a 32mm carb for 375cc displacement (2 carbs, 750cc). My Enfield uses a 28mm round slide carb for 500cc displacement, and while that's not a "performance carb" it's also very well behaved.
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GSPearson
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Re: JRC PWK issue

Post by GSPearson » Wed Mar 20, 2019 12:25 am

rivers wrote:
Sat Mar 16, 2019 6:02 pm
As Eric mentioned previously, flatslides are pretty much for WOT racing applications. Not designed for street use where you run at various throttle openings. Do able with small slow throttle moves but large/quick will be miserable. Why on an 07 750 did you decide to go with these carbs? Hopefully not thinking you'll make a Ural faster. :( Best you can do with a Uralis fine tune it for smooth streetability.
Would have been a LOT easier dialing in the stock CVK's. If you still have the stock carbs I'd put my time and labor into those.
My diaphragms were in pretty bad shape. I got the PWK’s at the suggestion of Robby at raceway.
2007 Gear-up arctic
JRC PWK pilot 40, main 140
K&N air filter

Newer style front timing cover and oil filter

GSPearson
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Re: JRC PWK issue

Post by GSPearson » Wed Mar 20, 2019 12:27 am

cateyetech wrote:
Sat Mar 16, 2019 10:50 pm
What I posted, before it deleted :(
Try turning the pilot air screws in 1/8 turn
check it and try it again
These type of carburetors with out accelerator pumps
will require a slower pull


If richening the mix helps and the main jet is the correct size (or very close)
then changing the main jet air bleed may required (that's why it's not a fixed orifice)

All mechanical carbs require appropriate modulation, epically high flow ones
even round slide carbs.
That's why at the shop I recommend a performance kit on the stock Keihin CVK 40mm
carbs vs S&S replacement (butterfly type carb w/ accelerator pump)
You can overmodulate :lol: the throttle :shock:
If you jerk it wide open then roll off you will feel the bikes power increase :?
With CVK's you just request more throttle, like vacuum secondary car carbs vs mechanical ones


:rant:
FLAT SLIDE CARBURATORS ARE FOR RACING/ WIDE OPEN USE

What type of racing is WOT?
Drag racing and (maybe) flat track (they hit the kill switch to set the bike)

Motocross / Supercross / Road Tracks / Hair scramble (woods racing) require a LOT of throttle modulation
and flat slide carbs are on almost all of these bikes


Screenshot (34).png

Screenshot (35).png

I guess all these were RACE RIGS :lol:

All the disinformation :( very sad, very sad :shock:

Contrary to popular opinion you can make a Ural run
faster or really stronger, HP is what limits you top speed :P

Hey GSP

Are you going to leave us hanging on this thread like you did on the engine noise thread?
just saying

:foilhead:

Damn, I didn’t realize I hadn’t mentioned what ended up happening with that. It was a loose valve guide. Would only get loose enough when them heads heated up.

My apologies for that. I’m insanely busy, traveling all over for work all the time.
2007 Gear-up arctic
JRC PWK pilot 40, main 140
K&N air filter

Newer style front timing cover and oil filter

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cateyetech
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Re: JRC PWK issue

Post by cateyetech » Sat Mar 23, 2019 10:47 pm

Hey GSP

Hope you have had a little time to ride your rig :moto:

The flat slide carbs don't require a accelerator pump, only 1 of the
Mikuni TM series of carbs has a pump.

I have been running flat slide carbs for over 2 years now,
yes they have pumps but that's not the only reason I went them.

viewtopic.php?f=11&t=47247&p=591156#p591156

I looked at the JRC's when I was carb shopping :)

I prefer carbs with a Idle fuel screw instead of a pilot air screw
WHY?
When adjusted for best idle the 1/6-1/8 throttle (pilot circuit) is too lean
When adjusted for best 1/6-1/8 throttle the idle circuit is too rich :shock:
These type of carbs are harder to tune :x

There have been a few members who have ran the JRC's
Here is one report on them :shock:
https://sovietsteeds.com/forums/viewtopic.php?t=16765

Tuning a carb is not always just jetting :wink:
There are also air bleeds and slides with different cutaways :!:
Different engine types have different requirements.

Here is a good guide
http://iwt.com.au/mikunicarb.htm

Good Luck :cheers:
and keep us posted :)

:foilhead:
Charlie ╭∩╮(-_-)╭∩╮
2003 Gear-Up Ромашка Мзй
765cc - Mikuni TM33 carburetors - Modified stock airbox with Uni foam filter
2-1 exhaust - Hand worked cylinder heads - Type V ignition system
A Warn XT17 witch that works from any angle - More handmade parts than I can list :foilhead:

The only Ural to post a second gear wheelie :evil:

2000 Patrol Teh Урал
Thanks DaveO, I'll try to ride it like you, I'll try :foilhead:

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